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Title: The Principles of Biology, Volume 1 (of 2)
Author: Spencer, Herbert
Language: English
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* * * * *
THE PRINCIPLES OF
BIOLOGY
BY
HERBERT SPENCER
[Illustration]
IN TWO VOLUMES
VOLUME I
NEW YORK AND LONDON
D. APPLETON AND COMPANY
1910
COPYRIGHT, 1866, 1898,
BY D. APPLETON AND COMPANY.
PREFACE
TO THE REVISED AND ENLARGED EDITION.
Rapid in all directions, scientific progress has during the last generation
been more rapid in the direction of Biology than in any other; and had this
work been one dealing with Biology at large, the hope of bringing it up to
date could not have been rationally entertained. But it is a work on the
_Principles_ of Biology; and to bring an exposition of these up to date,
seemed not impossible with such small remnant of energy as is left me.
Slowly, and often interrupted by ill-health, I have in the course of the
last two years, completed this first volume of the final edition.
Numerous additions have proved needful. What was originally said about
vital changes of matter has been supplemented by a chapter on "Metabolism."
Under the title "The Dynamic Element in Life," I have added a chapter which
renders less inadequate the conception of Life previously expressed. A gap
in preceding editions, which should have been occupied by some pages on
"Structure," is now filled up. Those astonishing actions in cell-nuclei
which the microscope has of late revealed, will be found briefly set forth
under the head of "Cell-Life and Cell-Multiplication." Further evidence and
further thought have resulted in a supplementary chapter on "Genesis,
Heredity, and Variation"; in which certain views enunciated in the first
edition are qualified and developed. Various modern ideas are considered
under the title "Recent Criticisms and Hypotheses." And the chapter on "The
Arguments from Embryology" has been mainly rewritten. Smaller increments
have taken the shape of new sections incorporated in pre-existing chapters.
They are distinguished by the following section-marks:--§ 8a, § 46a, § 87a,
§ 100a, § 113a, § 127a, §§ 130a-130d. There should also be mentioned a
number of foot-notes of some significance not present in preceding
editions. Of the three additional appendices the two longer ones have
already seen the light in other shapes.
After these chief changes have now to be named the changes necessitated by
revision. In making them assistance has been needful. Though many of the
amendments have resulted from further thought and inquiry, a much larger
number have been consequent on criticisms received from gentlemen whose aid
I have been fortunate enough to obtain: each of them having taken a
division falling within the range of his special studies. The part
concerned with Organic Chemistry and its derived subjects, has been looked
through by Mr. W. H. Perkin, Ph.D., F.R.S., Professor of Organic Chemistry,
Owens College, Manchester. Plant Morphology and Physiology have been
overseen by Mr. A. G. Tansley, M.A., F.L.S., Assistant Professor of Botany,
University College, London. Criticisms upon parts dealing with Animal
Morphology, I owe to Mr. E. W. MacBride, M.A., Fellow of St. John's
College, Cambridge, Professor of Zoology in the McGill University,
Montreal, and Mr. J. T. Cunningham, M.A., late Fellow of University
College, Oxford. And the statements included under Animal Physiology have
been checked by Mr. W. B. Hardy, M.A., Fellow of Gonville and Caius
College, Cambridge, Demonstrator of Physiology in the University. Where the
discoveries made since 1864 have rendered it needful to change the text,
either by omissions or qualifications or in some cases by additions, these
gentlemen have furnished me with the requisite information.
Save in the case of the preliminary portion, bristling with the
technicalities of Organic Chemistry (including the pages on "Metabolism"),
I have not submitted the proofs, either of the new chapters or of the
revised chapters, to the gentlemen above named. The abstention has resulted
partly from reluctance to trespass on their time to a greater extent than
was originally arranged, and partly from the desire to avoid complicating
my own work. During the interval occupied in the preparation of this volume
the printers have kept pace with me, and I have feared adding to the
entailed attention the further attention which correspondence and
discussion would have absorbed: feeling that it was better to risk minor
inaccuracies than to leave the volume unfinished: an event which at one
time appeared probable. I make this statement because, in its absence, one
or other of these gentlemen might be held responsible for some error which
is not his but mine.
Yet another explanation is called for. Beyond the exposition of those
general truths constituting the Principles of Biology as commonly accepted,
the original edition of this work contained sundry views for which
biological opinion did not furnish any authority. Some of these have since
obtained a certain currency; either in their original forms or in modified
forms. Misinterpretations are likely to result. Readers who have met with
them in other works may, in the absence of warning, suppose, to my
disadvantage, that I have adopted them without acknowledgment. Hence it
must be understood that where no indication to the contrary is given the
substance is unchanged. Beyond the corrections which have been made in the
original text, there are, in some cases, additions to the evidence or
amplifications of the argument; but in all sections not marked as new, the
essential ideas set forth are the same as they were in the original edition
of 1864.
BRIGHTON,
_August, 1898_.
PREFACE.
The aim of this work is to set forth the general truths of Biology, as
illustrative of, and as interpreted by, the laws of Evolution: the special
truths being introduced only so far as is needful for elucidation of the
general truths.
For aid in executing it, I owe many thanks to Prof. Huxley and Dr. Hooker.
They have supplied me with information where my own was deficient;[1] and,
in looking through the proof-sheets, have pointed out errors of detail into
which I had fallen. By having kindly rendered me this valuable assistance,
they must not, however, be held committed to any of the enunciated
doctrines that are not among the recognized truths of Biology.
The successive instalments which compose this volume, were issued to the
subscribers at the following dates:--No. 7 (pp. 1-80) in January, 1863; No.
8 (pp. 81-160) in April, 1863; No. 9 (pp. 161-240) in July, 1863; No. 10
(pp. 241-320) in January, 1864; No. 11 (pp. 321-400) in May, 1864; and No.
12 (pp. 401-476) in October, 1864.
_London, September 29th, 1864._
CONTENTS OF VOL. I.
----
CHAPTER PAGE
I. ORGANIC MATTER 3
II. THE ACTIONS OF FORCES ON ORGANIC MATTER 27
III. THE RE-ACTIONS OF ORGANIC MATTER ON FORCES 45
III^A. METABOLISM 62
IV. PROXIMATE CONCEPTION OF LIFE 78
V. THE CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES 91
VI. THE DEGREE OF LIFE VARIES AS THE DEGREE OF 101
CORRESPONDENCE
VI^A. THE DYNAMIC ELEMENT IN LIFE 111
VII. THE SCOPE OF BIOLOGY 124
PART II.--THE INDUCTIONS OF BIOLOGY.
I. GROWTH 135
II. DEVELOPMENT 162
II^A. STRUCTURE 181
III. FUNCTION 197
IV. WASTE AND REPAIR 213
V. ADAPTATION 227
VI. INDIVIDUALITY 244
VI^A. CELL-LIFE AND CELL-MULTIPLICATION 252
VII. GENESIS 269
VIII. HEREDITY 301
IX. VARIATION 320
X. GENESIS, HEREDITY, AND VARIATION 336
X^A. GENESIS, HEREDITY, AND VARIATION--_Concluded_ 356
XI. CLASSIFICATION 374
XII. DISTRIBUTION 395
PART III.--THE EVOLUTION OF LIFE.
I. PRELIMINARY 415
II. GENERAL ASPECTS OF THE SPECIAL-CREATION-HYPOTHESIS 417
III. GENERAL ASPECTS OF THE EVOLUTION-HYPOTHESIS 431
IV. THE ARGUMENTS FROM CLASSIFICATION 441
V. THE ARGUMENTS FROM EMBRYOLOGY 450
VI. THE ARGUMENTS FROM MORPHOLOGY 468
VII. THE ARGUMENTS FROM DISTRIBUTION 476
VIII. HOW IS ORGANIC EVOLUTION CAUSED? 490
IX. EXTERNAL FACTORS 499
X. INTERNAL FACTORS 508
XI. DIRECT EQUILIBRATION 519
XII. INDIRECT EQUILIBRATION 529
XIII. THE CO-OPERATION OF THE FACTORS 549
XIV. THE CONVERGENCE OF THE EVIDENCES 554
XIV^A. RECENT CRITICISMS AND HYPOTHESES 559
APPENDICES.
A. THE GENERAL LAW OF ANIMAL FERTILITY 577
B. THE INADEQUACY OF NATURAL SELECTION, ETC. 602
C. THE INHERITANCE OF FUNCTIONALLY-WROUGHT MODIFICATIONS: 692
A SUMMARY
D. ON ALLEGED "SPONTANEOUS GENERATION" AND ON THE 696
HYPOTHESIS OF PHYSIOLOGICAL UNITS
PART I.
THE DATA OF BIOLOGY.
CHAPTER I.
ORGANIC MATTER.
§ 1. Of the four chief elements which, in various combinations, make up
living bodies, three are gaseous under all ordinary conditions and the
fourth is a solid. Oxygen, hydrogen, and nitrogen are gases which for many
years defied all attempts to liquefy them, and carbon is a solid except
perhaps at the extremely high temperature of the electric arc. Only by
intense pressures joined with extreme refrigerations have the three gases
been reduced to the liquid form.[2] There is much significance in this.
When we remember how those redistributions of Matter and Motion which
constitute Evolution, structural and functional, imply motions in the units
that are redistributed; we shall see a probable meaning in the fact that
organic bodies, which exhibit the phenomena of Evolution in so high a
degree, are mainly composed of ultimate units having extreme mobility. The
properties of substances, though destroyed to sense by combination, are not
destroyed in reality. It follows from the persistence of force, that the
properties of a compound are _resultants_ of the properties of its
components--_resultants_ in which the properties of the components are
severally in full action, though mutually obscured. One of the leading
properties of each substance is its degree of molecular mobility; and its
degree of molecular mobility more or less sensibly affects the molecular
mobilities of the various compounds into which it enters. Hence we may
infer some relation between the gaseous form of three out of the four chief
organic elements, and that comparative readiness displayed by organic
matters to undergo those changes in the arrangement of parts which we call
development, and those transformations of motion which we call function.
Considering them chemically instead of physically, it is to be remarked
that three out of these four main components of organic matter, have
affinities which are narrow in their range and low in their intensity.
Hydrogen, it is true, may be made to combine with a considerable number of
other elements; but the chemical energy which it shows is scarcely at all
shown within the limits of the organic temperatures. Of carbon it may
similarly be said that it is totally inert at ordinary heats; that the
number of substances with which it unites is not great; and that in most
cases its tendency to unite with them is but feeble. Lastly, this chemical
indifference is shown in the highest degree by nitrogen--an element which,
as we shall hereafter see, plays the leading part in organic changes.
Among the organic elements (including under the title not only the four
chief ones, but also the less conspicuous remainder), that capability of
assuming different states called allotropism, is frequent. Carbon presents
itself in the three unlike conditions of diamond, graphite, and charcoal.
Under certain circumstances, oxygen takes on the form in which it is called
ozone. Sulphur and phosphorus (both, in small proportions, essential
constituents of organic matter) have allotropic modifications. Silicon,
too, is allotropic; while its oxide, silica, which is an indispensable
constituent of many lower organisms, exhibits the analogue of
allotropism--isomerism. No other interpretation being possible we are
obliged to regard allotropic change as some change of molecular
arrangement. Hence this frequency of its occurrence among the components of
organic matter is significant as implying a further kind of molecular
mobility.
One more fact, that is here of great interest for us, must be set down.
These four elements of which organisms are almost wholly composed, exhibit
certain extreme unlikenesses. While between two of them we have an
unsurpassed contrast in chemical activity; between one of them and the
other three, we have an unsurpassed contrast in molecular mobility. While
carbon, until lately supposed to be infusible and now volatilized only in
the electric arc, shows us a degree of atomic cohesion greater than that of
any other known element, hydrogen, oxygen, and nitrogen show the least
atomic cohesion of all elements. And while oxygen displays, alike in the
range and intensity of its affinities, a chemical energy exceeding that of
any other substance (unless fluorine be considered an exception), nitrogen
displays the greatest chemical inactivity. Now on calling to mind one of
the general truths arrived at when analyzing the process of Evolution, the
probable significance of this double difference will be seen. It was shown
(_First Principles_, § 163) that, other things equal, unlike units are more
easily separated by incident forces than like units are--that an incident
force falling on units that are but little dissimilar does not readily
segregate them; but that it readily segregates them if they are widely
dissimilar. Thus, the substances presenting these two extreme contrasts,
the one between physical mobilities, and the other between chemical
activities, fulfil, in the highest degree, a certain further condition to
facility of differentiation and integration.
§ 2. Among the diatomic combinations of the three elements, hydrogen,
nitrogen and oxygen, we find a molecular mobility much less than that of
these elements themselves; at the same time that it is much greater than
that of diatomic compounds in general. Of the two products formed by the
union of oxygen with carbon, the first, called carbonic oxide, which
contains one atom[3] of carbon to one of oxygen (expressed by the symbol
CO) is a gas condensible only with great difficulty; and the second,
carbonic acid, containing an additional atom of oxygen (CO_{2}) assumes a
liquid form also only under a pressure of about forty atmospheres. The
several compounds of oxygen with nitrogen, present us with an instructive
gradation. Nitrous oxide (N_{2}O), is a gas condensible only under a
pressure of some fifty atmospheres; nitric oxide (NO) is a gas which
although it has been liquefied does not condense under a pressure of 270
atmospheres at 46.4° F. (8° C.): the molecular mobility remaining
undiminished in consequence of the volume of the united gases remaining
unchanged. Nitrogen trioxide (N_{2}O_{3}) is gaseous at ordinary
temperatures, but condenses into a very volatile liquid at the zero of
Fahrenheit; nitrogen tetroxide (N_{2}O_{4}) is liquid at ordinary
temperatures and becomes solid at the zero of Fahrenheit; while nitrogen
pentoxide (N_{2}O_{5}) may be obtained in crystals which melt at 85° and
boil at 113°. In this series we see, though not with complete uniformity, a
decrease of molecular mobility as the weights of the compound molecules are
increased. The hydro-carbons illustrate the same general truth still
better. One series of them will suffice. Marsh gas (CH_{4}) is gaseous
except under great pressure and at very low temperatures. Olefiant gas
(C_{2}H_{4}) and ethane (C_{2}H_{6}) may be readily liquefied by pressure.
Propane (C_{3}H_{8}) becomes liquid without pressure at the zero of
Fahrenheit. Hexane (C_{5}H_{12}) is a liquid which boils at 160°. And the
successively higher multiples, heptane (C_{7}H_{16}), octane (C_{8}H_{18}),
and nonane (C_{9}H_{20}) are liquids which boil respectively at 210°, 257°,
and 302°. Pentadecan (C_{15}H_{32}) is a liquid which boils at 270°, while
paraffin-wax, which contains the still higher multiples, is solid. There
are three compounds of hydrogen and nitrogen that have been obtained in a
free state--ammonia (NH_{3}) is gaseous, but liquefiable by pressure, or by
reducing its temperature to -40° F., and it solidifies at -112° F.;
hydrazine (NH_{2}--NH_{2}) is liquid at ordinary temperatures, but
hydrozoic acid (N_{3}H) has so far only been obtained in the form of a
highly explosive gas. In cyanogen, which is composed of carbon and
nitrogen, (CN)_{2}, we have a gas that becomes liquid at a pressure of four
atmospheres and solid at -30° F. And in paracyanogen, formed of the same
proportions of these elements in higher multiples, we have a solid which
does not fuse or volatilize at ordinary temperatures. Lastly, in the most
important member of this group, water (H_{2}O), we have a compound of two
difficultly-condensible gases which assumes both the fluid state and the
solid state within ordinary ranges of temperature; while its molecular
mobility is still such that its fluid or solid masses are continually
passing into the form of vapour, though not with great rapidity until the
temperature is raised to 212°.
Considering them chemically, it is to be remarked of these diatomic
compounds of the four chief organic elements, that they are, on the
average, less stable than diatomic compounds in general. Water, carbonic
oxide, and carbonic acid, are, it is true, difficult to decompose. But
omitting these, the usual strength of union among the elements of the
above-named substances is low considering the simplicity of the substances.
With the exception of acetylene and possibly marsh gas, the various
hydro-carbons are not producible by directly combining their elements; and
the elements of most of them are readily separable by heat without the aid
of any antagonistic affinity. Nitrogen and hydrogen do not unite with each
other immediately save under very exceptional circumstances; and the
ammonia which results from their union, though it resists heat, yields to
the electric spark. Cyanogen is stable: not being resolved into its
components below a bright red heat. Much less stable, however, are several
of the oxides of nitrogen. Nitrous oxide, it is true, does not yield up its
elements below a red heat; but nitrogen tetroxide cannot exist if water be
added to it; nitrous acid is decomposed by water; and nitric acid not only
readily parts with its oxygen to many metals, but when anhydrous,
spontaneously decomposes. Here it will be well to note, as having a bearing
on what is to follow, how characteristic of most nitrogenous compounds is
this special instability. In all the familiar cases of sudden and violent
decomposition, the change is due to the presence of nitrogen. The explosion
of gunpowder results from the readiness with which the nitrogen contained
in the nitrate of potash, yields up the oxygen combined with it. The
explosion of gun-cotton, which also contains nitrogen, is a substantially
parallel phenomenon. The various fulminating salts are all formed by the
union with metals of a certain nitrogenous acid called fulminic acid; which
is so unstable that it cannot be obtained in a separate state.
Explosiveness is a property of nitro-mannite, and also of nitro-glycerin.
Iodide of nitrogen detonates on the slightest touch, and often without any
assignable cause. And the bodies which explode with the most tremendous
violence of any known, are the chloride of nitrogen (NCl_{3}) and hydrazoic
acid (N_{3}H). Thus these easy and rapid decompositions, due to the
chemical indifference of nitrogen, are characteristic. When we come
hereafter to observe the part which nitrogen plays in organic actions, we
shall see the significance of this extreme readiness shown by its compounds
to undergo changes. Returning from these facts parenthetically introduced,
we have next to note that though among the diatomic compounds of the four
chief organic elements, there are a few active ones, yet the majority of
them display a smaller degree of chemical energy than the average of
diatomic compounds. Water is the most neutral of bodies: usually producing
little chemical alteration in the substances with which it combines; and
being expelled from most of its combinations by a moderate heat. Carbonic
acid is a relatively feeble acid: the carbonates being decomposed by the
majority of other acids and by ignition. The various hydro-carbons are but
narrow in the range of their comparatively weak affinities. The compounds
formed by ammonia have not much stability: they are readily destroyed by
heat, and by the other alkalies. The affinities of cyanogen are tolerably
strong, though they yield to those of the chief acids. Of the several
oxides of nitrogen, it is to be remarked that, while those containing the
smaller proportions of oxygen are chemically inert, the one containing the
greatest proportion of oxygen (nitric acid) though chemically active, in
consequence of the readiness with which one part of it gives up its oxygen
to oxidize a base with which the rest combines, is nevertheless driven from
all its combinations by a red heat.
These diatomic compounds, like their elements, are to a considerable degree
characterized by the prevalence among them of allotropism; or, as it is
more usually called when displayed by compound bodies--isomerism. Professor
Graham finds reason for thinking that a change in atomic arrangements of
this nature, takes place in water, at or near the melting point of ice. In
the various series of hydro-carbons, differing from each other only in the
ratios in which the elements are united, we find not simply isomerism but
polymerism occurring to an almost infinite extent. In some series of
hydro-carbons, as, for example, the terpenes, we find isomerism and at the
same time a great tendency to undergo polymerisation. And the relation
between cyanogen and paracyanogen is, as we saw, a polymeric one.
There is one further fact respecting these diatomic compounds of the chief
organic elements, which must not be overlooked. Those of them which form
parts of the living tissues of plants and animals (excluding water which
has a mechanical function, and carbonic acid which is a product of
decomposition) belong for the most part to one group--the
carbo-hydrates.[4] And of this group, which is on the average characterized
by comparative instability and inertness, these carbo-hydrates found in
living tissues are among the most unstable and inert.
§ 3. Passing now to the substances which contain three of these chief
organic elements, we have first to note that along with the greater atomic
weight which mostly accompanies their increased complexity, there is, on
the average, a further marked decrease of molecular mobility. Scarcely any
of them maintain a gaseous state at ordinary temperatures. One class of
them only, the alcohols and their derivatives, evaporate under the usual
atmospheric pressure; but not rapidly unless heated. The fixed oils, though
they show that molecular mobility implied by an habitually liquid state,
show this in a lower degree than the alcoholic compounds; and they cannot
be reduced to the gaseous state without decomposition. In their allies, the
fats, which are solid unless heated, the loss of molecular mobility is
still more marked. And throughout the whole series of the fatty acids, in
which to a fixed proportion of oxygen there are successively added higher
equimultiples of carbon and hydrogen, we see how the molecular mobility
decreases with the increasing sizes of the molecules. In the amylaceous and
sugar-group of compounds, solidity is the habitual state: such of them as
can assume the liquid form, doing so only when heated to 300° or 400° F.;
and decomposing when further heated, rather than become gaseous. Resins and
gums exhibit general physical properties of like character and meaning.
In chemical stability these triatomic compounds, considered as a group, are
in a marked degree below the diatomic ones. The various sugars and kindred
bodies, decompose at no very high temperatures. The oils and fats also are
readily carbonized by heat. Resinous and gummy substances are easily made
to render up some of their constituents. And the alcohols, with their
allies, have no great power of resisting decomposition. These bodies,
formed by the union of oxygen, hydrogen, and carbon, are also, as a class,
chemically inactive. Formic and acetic are doubtless energetic acids; but
the higher members of the fatty-acid series are easily separated from the
bases with which they combine. Saccharic acid, too, is an acid of
considerable power; and sundry of the vegetable acids possess a certain
activity, though an activity far less than that of the mineral acids. But
throughout the rest of the group, there is shown but a small tendency to
combine with other bodies; and such combinations as are formed have usually
little permanence.
The phenomena of isomerism and polymerism are of frequent occurrence in
these triatomic compounds. Starch and dextrine are probably polymeric.
Fruit-sugar and grape-sugar, mannite and sorbite, cane-sugar and
milk-sugar, are isomeric. Sundry of the vegetal acids exhibit similar
modifications. And among the resins and gums, with their derivatives,
molecular re-arrangements of this kind are not uncommon.
One further fact respecting these compounds of carbon, oxygen and hydrogen,
should be mentioned; namely, that they are divisible into two classes--the
one consisting of substances that result from the destructive decomposition
of organic matter, and the other consisting of substances that exist as
such in organic matter. These two classes of substances exhibit, in
different degrees, the properties to which we have been directing our
attention. The lower alcohols, their allies and derivatives, which possess
greater molecular mobility and chemical stability than the rest of these
triatomic compounds, are rarely found in animal or vegetal bodies. While
the sugars and amylaceous substances, the fixed oils and fats, the gums and
resins, which have all of them much less molecular mobility, and are,
chemically considered, more unstable and inert, are components of the
living tissues of plants and animals.
§ 4. Among compounds containing all the four chief organic elements, a
division analogous to that just named may be made. There are some which
result from the decomposition of living tissues; there are others which
make parts of living tissues in their state of integrity; and these two
groups are contrasted in their properties in the same way as are the
parallel groups of triatomic compounds.
Of the first division, certain products found in the animal excretions are
the most important, and the only ones that need be noted; such, namely, as
urea, kreatine, kreatinine. These animal-bases exhibit much less molecular
mobility than the average of the substances treated of in the last section:
being solid at ordinary temperatures, fusing, where fusible at all, at
temperatures above that of boiling water, and having no power to assume a
gaseous state. Chemically considered, their stability is low, and their
activity but small, in comparison with the stabilities and activities of
the simpler compounds.
It is, however, the nitrogenous constituents of living tissues, that
display most markedly those characteristics of which we have been tracing
the growth. Albumen, fibrin, casein, and their allies, are bodies in which
that molecular mobility exhibited by three of their components in so high a
degree is reduced to a minimum. These substances are known only in the
solid state. That is to say, when deprived of the water usually mixed with
them, they do not admit of fusion, much less of volatilization. To which
add, that they have not even that molecular mobility which solution in
water implies; since, though they form viscid mixtures with water, they do
not dissolve in the same perfect way as do inorganic compounds. The
chemical characteristics of these substances are instability and inertness
carried to the extreme. How rapidly albumenoid matters decompose under
ordinary conditions, is daily seen: the difficulty of every housewife being
to prevent them from decomposing. It is true that when desiccated and kept
from contact with air, they may be preserved unchanged for long periods;
but the fact that they can be only thus preserved, proves their great
instability. It is true, also, that these most complex nitrogenous
principles are not absolutely inert, since they enter into combinations
with some bases; but their unions are very feeble.
It should be noted, too, of these bodies, that though they exhibit in the
lowest degree that kind of molecular mobility which implies facile
vibration of the molecules as wholes, they exhibit in high degrees that
kind of molecular mobility resulting in isomerism, which implies permanent
changes in the positions of adjacent atoms with respect to each other. Each
of them has a soluble and an insoluble form. In some cases there are
indications of more than two such forms. And it appears that their
metamorphoses take place under very slight changes of conditions.
In these most unstable and inert organic compounds, we find that the
molecular complexity reaches a maximum: not only since the four chief
organic elements are here united with small proportions of sulphur and
sometimes phosphorus; but also since they are united in high multiples. The
peculiarity which we found characterized even diatomic compounds of the
organic elements, that their molecules are formed not of single equivalents
of each component, but of two, three, four, and more equivalents, is
carried to the greatest extreme in these compounds, which take the leading
part in organic actions. According to Lieberkühn, the formula of albumen is
C_{72}H_{112}SN_{18}O_{22}. That is to say, with the sulphur there are
united seventy-two atoms of carbon, one hundred and twelve of hydrogen,
eighteen of nitrogen, and twenty-two of oxygen: the molecule being thus
made up of more than two hundred ultimate atoms.
§ 5. Did space permit, it would be useful here to consider in detail the
interpretations that may be given of the peculiarities we have been
tracing: bringing to their solution, the general mechanical principles
which are now found to hold true of molecules as of masses. But it must
suffice briefly to indicate the conclusions which such an inquiry promises
to bring out.
Proceeding on these principles, it may be argued that the molecular
mobility of a substance must depend partly on the inertia of its molecules;
partly on the intensity of their mutual polarities; partly on their mutual
pressures, as determined by the density of their aggregation; and (where
the molecules are compound) partly on the molecular mobilities of their
component molecules. Whence it is to be inferred that any three of these
remaining constant, the molecular mobility will vary as the fourth. Other
things equal, therefore, the molecular mobility of molecules must decrease
as their masses increase; and so there must result that progression we have
traced, from the high molecular mobility of the uncombined organic
elements, to the low molecular mobility of those large-moleculed substances
into which they are ultimately compounded.
Applying to molecules the mechanical law which holds of masses, that since
inertia and gravity increase as the cubes of the dimensions while cohesion
increases as their squares, the self-sustaining power of a body becomes
relatively smaller as its bulk becomes greater; it might be argued that
these large, aggregate molecules which constitute organic substances, are
mechanically weak--are less able than simpler molecules to bear, without
alteration, the forces falling on them. That very massiveness which renders
them less mobile, enables the physical forces acting on them more readily
to change the relative positions of their component atoms; and so to
produce what we know as re-arrangements and decompositions.
Further, it seems a not improbable conclusion, that this formation of large
aggregates of elementary atoms and resulting diminution of self-sustaining
power, must be accompanied by a decrease of those dimensional contrasts to
which polarity is ascribable. A sphere is the figure of equilibrium which
any aggregate of units tends to assume, under the influence of simple
mutual attraction. Where the number of units is small and their mutual
polarities are decided, this proclivity towards spherical grouping will be
overcome by the tendency towards some more special form, determined by
their mutual polarities. But it is manifest that in proportion as an
aggregate molecule becomes larger, the effects of simple mutual attraction
must become relatively greater; and so must tend to mask the effects of
polar attraction. There will consequently be apt to result in highly
compound molecules like these organic ones, containing hundreds of
elementary atoms, such approximation to the spherical form as must involve
a less distinct polarity than in simpler molecules. If this inference be
correct, it supplies us with an explanation both of the chemical inertness
of these most complex organic substances, and of their inability to
crystallize.
§ 6. Here we are naturally introduced to another aspect of our subject--an
aspect of great interest. Professor Graham has published a series of
important researches, which promise to throw much light on the constitution
and changes of organic matter. He shows that solid substances exist under
two forms of aggregation--the _colloid_ or jelly-like, and the
_crystalloid_ or crystal-like. Examples of the last are too familiar to
need specifying. Of the first may be named such instances as "hydrated
silicic acid, hydrated alumina, and other metallic peroxides of the
aluminous class, when they exist in the soluble form; with starch, dextrine
and the gums, caramel, tannin, albumen, gelatine, vegetable and animal
extractive matters." Describing the properties of colloids, Professor
Graham says:--"Although often largely soluble in water, they are held in
solution by a most feeble force. They appear singularly inert in the
capacity of acids and bases, and in all the ordinary chemical relations." *
* * "Although chemically inert in the ordinary sense, colloids possess a
compensating activity of their own arising out of their physical
properties. While the rigidity of the crystalline structure shuts out
external impressions, the softness of the gelatinous colloid partakes of
fluidity, and enables the colloid to become a medium of liquid diffusion,
like water itself." * * * "Hence a wide sensibility on the part of colloids
to external agents. Another and eminently characteristic quality of
colloids is their mutability." * * * "The solution of hydrated silicic
acid, for instance, is easily obtained in a state of purity, but it cannot
be preserved. It may remain fluid for days or weeks in a sealed tube, but
is sure to gelatinize and become insoluble at last. Nor does the change of
this colloid appear to stop at that point; for the mineral forms of silicic
acid, deposited from water, such as flint, are often found to have passed,
during the geological ages of their existence, from the vitreous or
colloidal into the crystalline condition (H. Rose). The colloid is, in
fact, a dynamical state of matter, the crystalloidal being the statical
condition. The colloid possesses _energia_. It may be looked upon as the
primary source of the force appearing in the phenomena of vitality. To the
gradual manner in which colloidal changes take place (for they always
demand time as an element) may the characteristic protraction of
chemico-organic changes also be referred."
The class of colloids includes not only all those most complex nitrogenous
compounds characteristic of organic tissues, and sundry of the
carbo-hydrates found along with them; but, significantly enough, it
includes several of those substances classed as inorganic, which enter into
organized structures. Thus silica, which is a component of many plants, and
constitutes the spicules of sponges as well as the shells of many
foraminifera and infusoria, has a colloid, as well as a crystalloid,
condition. A solution of hydrated silicic acid passes in the course of a
few days into a solid jelly that is no longer soluble in water; and it may
be suddenly thus coagulated by a minute portion of an alkaline carbonate,
as well as by gelatine, alumina, and peroxide of iron. This last-named
substance, too--peroxide of iron--which is an ingredient in the blood of
mammals and composes the shells of certain _Protozoa_, has a colloid
condition. "Water containing about one per cent. of hydrated peroxide of
iron in solution, has the dark red colour of venous blood." * * * "The red
solution is coagulated in the cold by traces of sulphuric acid, alkalies,
alkaline carbonates, sulphates, and neutral salts in general." * * * "The
coagulum is a deep red-coloured jelly, resembling the clot of blood, but
more transparent. Indeed, the coagulum of this colloid is highly suggestive
of that of blood, from the feeble agencies which suffice to effect the
change in question, as well as from the appearance of the product." The
jelly thus formed soon becomes, like the last, insoluble in water. Lime
also, which is so important a mineral element in living bodies, animal and
vegetal, enters into a compound belonging to this class. "The well-known
solution of lime in sugar forms a solid coagulum when heated. It is
probably, at a high temperature, entirely colloidal."
Generalizing some of the facts which he gives, Professor Graham says:--"The
equivalent of a colloid appears to be always high, although the ratio
between the elements of the substance may be simple. Gummic acid, for
instance, may be represented by C^{12}H^{22}O^{11}; but, judging from the
small proportions of lime and potash which suffice to neutralize this acid,
the true numbers of its formula must be several times greater. It is
difficult to avoid associating the inertness of colloids with their high
equivalents, particularly where the high number appears to be attained by
the repetition of a small number. The inquiry suggests itself whether the
colloid molecule may not be constituted by the grouping together of a
number of smaller crystalloid molecules, and whether the basis of
colloidality may not really be this composite character of the molecule."
§ 7. A further contrast between colloids and crystalloids is equally
significant in its relations to vital phenomena. Professor Graham points
out that the marked differences in volatility displayed by different
bodies, are paralleled by differences in the rates of diffusion of
different bodies through liquids. As alcohol and ether at ordinary
temperatures, and various other substances at higher temperatures, diffuse
themselves in a gaseous form through the air; so, a substance in aqueous
solution, when placed in contact with a mass of water (in such way as to
avoid mixture by circulating currents) diffuses itself through this mass of
water. And just as there are various degrees of rapidity in evaporation, so
there are various degrees of rapidity in diffusion: "the range also in the
degree of diffusive mobility exhibited by different substances appears to
be as wide as the scale of vapour-tensions." This parallelism is what might
have been looked for; since the tendency to assume a gaseous state, and the
tendency to spread in solution through a liquid, are both consequences of
molecular mobility. It also turns out, as was to be expected, that
diffusibility, like volatility, has, other things equal, a relation to
molecular weight--other things equal, we must say, because molecular
mobility must, as pointed out in § 5, be affected by other properties of
atoms, besides their inertia. Thus the substance most rapidly diffused of
any on which Professor Graham experimented, was hydrochloric acid--a
compound which is of low molecular weight, is gaseous save under a pressure
of forty atmospheres, and ordinarily exists as a liquid, only in
combination with water. Again, "hydrate of potash may be said to possess
double the velocity of diffusion of sulphate of potash, and sulphate of
potash again double the velocity of sugar, alcohol, and sulphate of
magnesia,"--differences which have a general correspondence with
differences in the massiveness of their molecules.
But the fact of chief interest to us here, is that the relatively
small-moleculed crystalloids have immensely greater diffusive power than
the relatively large-moleculed colloids. Among the crystalloids themselves
there are marked differences of diffusibility; and among the colloids
themselves there are parallel differences, though less marked ones. But
these differences are small compared with that between the diffusibility of
the crystalloids as a class, and the diffusibility of the colloids as a
class. Hydrochloric acid is seven times as diffusible as sulphate of
magnesia; but it is fifty times as diffusible as albumen, and a hundred
times as diffusible as caramel.
These differences of diffusibility manifest themselves with nearly equal
distinctness, when a permeable septum is placed between the solution and
the water. The result is that when a solution contains substances of
different diffusibilities, the process of dialysis, as Professor Graham
calls it, becomes a means of separating the mixed substances: especially
when such mixed substances are partly crystalloids and partly colloids. The
bearing of this fact on the interpretation of organic processes will be
obvious. Still more obvious will its bearing be, on joining with it the
remarkable fact that while crystalloids can diffuse themselves through
colloids nearly as rapidly as through water, colloids can scarcely diffuse
themselves at all through other colloids. From a mass of jelly containing
salt, into an adjoining mass of jelly containing no salt, the salt spread
more in eight days than it spread through water in seven days; while the
spread of "caramel through the jelly appeared scarcely to have begun after
eight days had elapsed." So that we must regard the colloidal compounds of
which organisms are built, as having, by their physical nature, the ability
to separate colloids from crystalloids, and to let the crystalloids pass
through them with scarcely any resistance.
One other result of these researches on the relative diffusibilities of
different substances has a meaning for us. Professor Graham finds that not
only does there take place, by dialysis, a separation of _mixed_ substances
which are unlike in their molecular mobilities; but also that _combined_
substances between which the affinities are feeble, will separate on the
dialyzer, if their molecular mobilities are strongly contrasted. Speaking
of the hydrochloride of peroxide of iron, he says, "such a compound
possesses an element of instability in the extremely unequal diffusibility
of its constituents;" and he points out that when dialyzed, the
hydrochloric acid gradually diffuses away, leaving the colloidal peroxide
of iron behind. Similarly, he remarks of the peracetate of iron, that it
"may be made a source of soluble peroxide, as the salt referred to is
itself decomposed to a great extent by diffusion on the dialyzer." Now this
tendency to separate displayed by substances which differ widely in their
molecular mobilities, though usually so far antagonized by their affinities
as not to produce spontaneous decomposition, must, in all cases, induce a
certain readiness to change which would not else exist. The unequal
mobilities of the combined atoms must give disturbing forces a greater
power to work transformations than they would otherwise have. Hence the
probable significance of a fact named at the outset, that while three of
the chief organic elements have the greatest atomic mobilities of any
elements known, the fourth, carbon, has the least atomic mobility of known
elements. Though, in its simple compounds, the affinities of carbon for the
rest are strong enough to prevent the effects of this great difference from
clearly showing themselves; yet there seems reason to think that in those
complex compounds composing organic bodies--compounds in which there are
various cross affinities leading to a state of chemical tension--this
extreme difference in the molecular mobilities must be an important aid to
molecular re-arrangements. In short, we are here led by concrete evidence
to the conclusion which we before drew from first principles, that this
great unlikeness among the combined units must facilitate differentiations.
§ 8. A portion of organic matter in a state to exhibit those phenomena
which the biologist deals with, is, however, something far more complex
than the separate organic matters we have been studying; since a portion of
organic matter in its integrity, contains several of these.
In the first place no one of those colloids which make up the mass of a
living body, appears capable of carrying on vital changes by itself: it is
always associated with other colloids. A portion of animal-tissue, however
minute, almost always contains more than one form of protein-substance:
different chemical modifications of albumen and gelatine are present
together, as well as, probably, a soluble and insoluble modification of
each; and there is usually more or less of fatty matter. In a single
vegetal cell, the minute quantity of nitrogenous colloid present, is
imbedded in colloids of the non-nitrogenous class. And the microscope makes
it at once manifest, that even the smallest and simplest organic forms are
not absolutely homogeneous.
Further, we have to contemplate organic tissue, formed of mingled colloids
in both soluble and insoluble states, as permeated throughout by
crystalloids. Some of these crystalloids, as oxygen,[5] water, and perhaps
certain salts, are agents of decomposition; some, as the saccharine and
fatty matters, are probably materials for decomposition; and some, as
carbonic acid, water, urea, kreatine, and kreatinine, are products of
decomposition. Into the mass of mingled colloids, mostly insoluble and
where soluble of very low molecular mobility or diffusive power, we have
constantly passing, crystalloids of high molecular mobility or diffusive
power, that are capable of decomposing these complex colloids, or of
facilitating decompositions otherwise caused; and from these complex
colloids, when decomposed, there result other crystalloids (the two chief
ones extremely simple and mobile, and the rest comparatively so) which
diffuse away as rapidly as they are formed.
And now we may clearly see the necessity for that peculiar composition
which we find in organic matter. On the one hand, were it not for the
extreme molecular mobility possessed by three out of the four of its chief
elements; and were it not for the consequently high molecular mobility of
their simpler compounds; there could not be this quick escape of the waste
products of organic action; and there could not be that continuously active
change of matter which vitality implies. On the other hand, were it not for
the union of these extremely mobile elements into immensely complex
compounds, having relatively vast molecules which are made comparatively
immobile by their inertia, there could not result that mechanical fixity
which prevents the components of living tissue from diffusing away along
with the effete matters produced by decomposition.
§ 8a. Let us not omit here to note the ways in which the genesis of these
traits distinguishing organic matter conforms to the laws of evolution as
expressed in its general formula.
In pursuance of the belief now widely entertained by chemists that the
so-called elements are not elements, but are composed of simpler matters
and probably of one ultimate form of matter (for which the name "protyle"
has been suggested by Sir W. Crookes), it is to be concluded that the
formation of the elements, in common with the formation of all those
compounds of them which Nature presents, took place in the course of Cosmic
Evolution. Various reasons for this inference the reader will find set
forth in the Addenda to an essay on "The Nebular Hypothesis" (see _Essays_,
vol. I, p. 155). On tracing out the process of compounding and
re-compounding by which, hypothetically, the elements themselves and
afterwards their compounds and re-compounds have arisen, certain cardinal
facts become manifest.
1. Considered as masses, the units of the elements are the smallest, though
larger than the units of the primordial matter. Later than these, since
they are composed of them, and since they cannot exist at temperatures so
high as those at which the elements can exist, come the diatomic
compounds--oxides, chlorides, and the rest--necessarily larger in their
molecules. Above these in massiveness come the molecules of the
multitudinous salts and kindred bodies. When associated, as these commonly
are, with molecules of water, there again results in each case increase of
mass; and unable as they are to bear such high temperatures, these
molecules are necessarily later in origin than those of the anhydrous
diatomic compounds. Within the general class of triatomic compounds, more
composite still, come the carbohydrates, which, being able to unite in
multiples, form still larger molecules than other triatomic compounds.
Decomposing as they do at relatively low temperatures, these are still more
recent in the course of chemical evolution; and with the genesis of them
the way is prepared for the genesis of organic matter strictly so called.
This includes the various forms of protein-substance, containing four chief
elements with two minor ones, and having relatively vast molecules.
Unstable as these are in presence of heat and surrounding affinities, they
became possible only at a late stage in the genesis of the Earth. Here,
then, in that chemical evolution which preceded the evolution of life, we
see displayed that process of integration which is the primary trait of
evolution at large.
2. Along with increasing integration has gone progress in heterogeneity.
The elements, regarding them as compound, are severally more heterogeneous
than "protyle." Diatomic molecules are more heterogeneous than these
elements; triatomic more heterogeneous than diatomic; and the molecules
containing four elements more heterogeneous than those containing three:
the most heterogeneous of them being the proteids, which contain two other
elements. The hydrated forms of all these compounds are more heterogeneous
than are the anhydrous forms. And most heterogeneous of all are the
molecules which, besides containing three, four, or more elements, also
exhibit the isomerism and polymerism which imply unions in multiples.
3. This formation of molecules more and more heterogeneous during
terrestrial evolution, has been accompanied by increasing heterogeneity in
the aggregate of compounds of each kind, as well as an increasing number of
kinds; and this increasing heterogeneity is exemplified in an extreme
degree in the compounds, non-nitrogenous and nitrogenous, out of which
organisms are built. So that the classes, orders, genera, and species of
chemical substances, gradually increasing as the Earth has assumed its
present form, increased in a transcendent degree during that stage which
preceded the origin of life.
§ 9. Returning now from these partially-parenthetic observations, and
summing up the contents of the preceding pages, we have to remark that in
the substances of which organisms are composed, the conditions necessary to
that re-distribution of Matter and Motion which constitutes Evolution, are
fulfilled in a far higher degree than at first appears.
The mutual affinities of the chief organic elements are not active within
the limits of those temperatures at which organic actions take place; and
one of these elements is especially characterized by its chemical
indifference. The compounds formed by these elements in ascending grades of
complexity, become progressively less stable. And those most complex
compounds into which all these four elements enter, together with small
proportions of two other elements which very readily oxidize, have an
instability so great that decomposition ensues under ordinary atmospheric
conditions.
Among these elements out of which living bodies are built, there is an
unusual tendency to unite in multiples; and so to form groups of products
which have the same chemical elements in the same proportions, but,
differing in their modes of aggregation, possess different properties. This
prevalence among them of isomerism and polymerism, shows, in another way,
the special fitness of organic substances for undergoing re-distributions
of their components.
In those most complex compounds that are instrumental to vital actions,
there exists a kind and degree of molecular mobility which constitutes the
plastic quality fitting them for organization. Instead of the extreme
molecular mobility possessed by three out of the four organic elements in
their separate states--instead of the diminished, but still great,
molecular mobility possessed by their simpler combinations, the gaseous and
liquid characters of which unfit them for showing to any extent the process
of Evolution--instead of the physical properties of their less simple
combinations, which, when not made unduly mobile by heat, assume the unduly
rigid form of crystals; we have in these colloids, of which organisms are
mainly composed, just the required compromise between fluidity and
solidity. They cannot be reduced to the unduly mobile conditions of liquid
and gas; and yet they do not assume the unduly fixed condition usually
characterizing solids. The absence of power to unite together in polar
arrangement, leaves their molecules with a certain freedom of relative
movement, which makes them sensitive to small forces, and produces
plasticity in the aggregates composed of them.
While the relatively great inertia of these large and complex organic
molecules renders them comparatively incapable of being set in motion by
the ethereal undulations, and so reduced to less coherent forms of
aggregation, this same inertia facilitates changes of arrangement among
their constituent molecules or atoms; since, in proportion as an incident
force impresses but little motion on a mass, it is the better able to
impress motion on the parts of the mass in relation to one another. And it
is further probable that the extreme contrasts in molecular mobilities
among the components of these highly complex molecules, aid in producing
modifiability of arrangement among them.
Lastly, the great difference in diffusibility between colloids and
crystalloids, makes possible in the tissues of organisms a specially rapid
re-distribution of matter and motion; both because colloids, being easily
permeable by crystalloids, can be chemically acted on throughout their
whole masses, instead of only on their surfaces; and because the products
of decomposition, being also crystalloids, can escape as fast as they are
produced: leaving room for further transformations. So that while the
composite molecules of which organic tissues are built up, possess that low
molecular mobility fitting them for plastic purposes, it results from the
extreme molecular mobilities of their ultimate constituents, that the waste
products of vital activity escape as fast as they are formed.
To all which add that the state of warmth, or increased molecular
vibration, in which all the higher organisms are kept, increases these
various facilities for re-distribution: not only as aiding chemical
changes, but as accelerating the diffusion of crystalloid substances.
CHAPTER II.
THE ACTIONS OF FORCES ON ORGANIC MATTER.
§ 10. To some extent, the parts of every body are changed in their
arrangement by any incident mechanical force. But in organic bodies, and
especially in animal bodies, the changes of arrangement produced by
mechanical forces are usually conspicuous. It is a distinctive mark of
colloids that they readily yield to pressures and tensions, and that they
recover, more or less completely, their original shapes, when the pressures
or tensions cease. Evidently without this pliability and elasticity, most
organic actions would be impossible. Not only temporary but also permanent
alterations of form are facilitated by this colloid character of organic
matter. Continued pressure on living tissue, by modifying the processes
going on in it (perhaps retarding the absorption of new material to replace
the old that has decomposed and diffused away), gradually diminishes and
finally destroys its power of resuming the outline it had at first. Thus,
generally speaking, the substances composing organisms are modifiable by
arrested momentum or by continuous strain, in far greater degrees than are
inorganic substances.
§ 11. Sensitiveness to certain forces which are quasi-mechanical, if not
mechanical in the usual sense, is seen in two closely-related peculiarities
displayed by organic matter as well as other matter which assumes the same
state of molecular aggregation.
Colloids take up by a power called "capillary affinity," a large quantity
of water: undergoing at the same time great increase of bulk with change of
form. Conversely, with like readiness, they give up this water by
evaporation; resuming, partially or completely, their original states.
Whether resulting from capillarity, or from the relatively great
diffusibility of water, or from both, these changes are to be here noted as
showing another mode in which the arrangements of parts in organic bodies
are affected by mechanical actions.
In what is termed osmose, we have a further mode of an allied kind. When on
opposite sides of a permeable septum, and especially a septum of colloidal
substance, are placed miscible solutions of different densities, a double
transfer takes place: a large quantity of the less dense solution finds its
way through the septum into the more dense solution; and a small quantity
of the more dense finds its way into the less dense--one result being a
considerable increase in the bulk of the more dense at the expense of the
less dense. This process, which appears to depend on several conditions, is
not yet fully understood. But be the explanation what it may, the process
is one that tends continually to work alterations in organic bodies.
Through the surfaces of plants and animals, transfers of this kind are ever
taking place. Many of the conspicuous changes of form undergone by organic
germs, are due mainly to the permeation of their limiting membranes by the
surrounding liquids.
It should be added that besides the direct alterations which the imbibition
and transmission of water and watery solutions by colloids produce in
organic matter, they produce indirect alterations. Being instrumental in
conveying into the tissues the agents of chemical change, and conveying out
of them the products of chemical change, they aid in carrying on other
re-distributions.
§ 12. As elsewhere shown (_First Principles_, § 100) heat, or a raised
state of molecular vibration, enables incident forces more easily to
produce changes of molecular arrangement in organic matter. But besides
this, it conduces to certain vital changes in so direct a way as to become
their chief cause.
The power of the organic colloids to imbibe water, and to bring along with
it into their substance the materials which work transformations, would not
be continuously operative if the water imbibed were to remain. It is
because it escapes, and is replaced by more water containing more
materials, that the succession of changes is maintained. Among the higher
animals and higher plants its escape is facilitated by evaporation. And the
rate of evaporation is, other things equal, determined by heat. Though the
current of sap in a tree is partly dependent on some action, probably
osmotic, that goes on in the roots; yet the loss of water from the surfaces
of the leaves, and the consequent absorption of more sap into the leaves by
capillary attraction, must be a chief cause of the circulation. The
drooping of a plant when exposed to the sunshine while the earth round its
roots is dry, shows us how evaporation empties the sap-vessels; and the
quickness with which a withered slip revives on being placed in water,
shows us the part which capillary action plays. In so far, then, as the
evaporation from a plant's surface helps to produce currents of sap through
the plant, we must regard the heat which produces this evaporation as a
part-cause of those re-distributions of matter which these currents effect.
In terrestrial animals, heat, by its indirect action as well as by its
direct action, similarly aids the changes that are going on. The exhalation
of vapour from the lungs and the surface of the skin, forming the chief
escape of the water that is swallowed, conduces to the maintenance of those
currents through the tissues without which the functions would cease. For
though the vascular system distributes nutritive liquids in ramified
channels through the body; yet the absorption of these liquids into
tissues, partly depends on the escape of liquids which the tissues already
contain. Hence, to the extent that such escape is facilitated by
evaporation, and this evaporation facilitated by heat, heat becomes an
agent of re-distribution in the animal organism.[6]
§ 13. Light, which is now known to modify many inorganic compounds--light,
which works those chemical changes utilized in photography, causes the
combinations of certain gases, alters the molecular arrangements of many
crystals, and leaves traces of its action even on substances that are
extremely stable,--may be expected to produce marked effects on substances
so complex and unstable as those which make up organic bodies. It does
produce such effects; and some of them are among the most important that
organic matter undergoes.
The molecular changes wrought by light in animals are of but secondary
moment. There is the darkening of the skin that follows exposure to the
Sun's rays. There are those alterations in the retina which cause in us
sensations of colours. And on certain eyeless creatures that are
semi-transparent, the light permeating their substance works some effects
evinced by movements. But speaking generally, the opacity of animals limits
the action of light to their surfaces; and so renders its direct
physiological influence but small.[7] On plants, however, the solar rays
that produce in us the impression of yellow, are the immediate agents of
those molecular changes through which are hourly accumulated the materials
for further growth. Experiments have shown that when the Sun shines on
living leaves, they begin to exhale oxygen and to accumulate carbon and
hydrogen--results which are traced to the decomposition, by the solar rays,
of the carbonic acid and water absorbed. It is now an accepted conclusion
that, by the help of certain classes of the ethereal undulations
penetrating their leaves, plants are enabled to separate from the
associated oxygen those two elements of which their tissues are chiefly
built up.
This transformation of ethereal undulations into certain molecular
re-arrangements of an unstable kind, on the overthrow of which the
stored-up forces are liberated in new forms, is a process that underlies
all organic phenomena. It will therefore be well if we pause a moment to
consider whether any proximate interpretation of it is possible. Researches
in molecular physics give us some clue to its nature.
The elements of the problem are these:--The atoms[8] of several ponderable
matters exist in combination: those which are combined having strong
affinities, but having also affinities less strong for some of the
surrounding atoms that are otherwise combined. The atoms thus united, and
thus mixed among others with which they are capable of uniting, are exposed
to the undulations of a medium that is so rare as to seem imponderable.
These undulations are of numerous kinds: they differ greatly in their
lengths, or in the frequency with which they recur at any given point. And
under the influence of undulations of a certain frequency, some of these
atoms are transferred from atoms for which they have a stronger affinity,
to atoms for which they have a weaker affinity. That is to say, particular
orders of waves of a relatively imponderable matter, remove particular
atoms of ponderable matter from their attachments, and carry them within
reach of other attachments. Now the discoveries of Bunsen and Kirchoff
respecting the absorption of particular luminiferous undulations by the
vapours of particular substances, joined with Prof. Tyndall's discoveries
respecting the absorption of heat by gases, show very clearly that the
atoms of each substance have a rate of vibration in harmony with ethereal
waves of a certain length, or rapidity of recurrence. Every special kind of
atom can be made to oscillate by a special order of ethereal waves, which
are absorbed in producing its oscillations; and can by its oscillations
generate this same order of ethereal waves. Whence it appears that immense
as is the difference in density between ether and ponderable matter, the
waves of the one can set the atoms of the other in motion, when the
successive impacts of the waves are so timed as to correspond with the
oscillations of the atoms. The effects of the waves are, in such case,
cumulative; and each atom gradually acquires a momentum made up of
countless infinitesimal momenta. Note, further, that unless the members of
a chemically-compound molecule are so bound up as to be incapable of any
relative movements (a supposition at variance with the conceptions of
modern science) we must conceive them as severally able to vibrate in
unison or harmony with those same classes of ethereal waves that affect
them in their uncombined states. While the compound molecule as a whole
will have some new rate of oscillation determined by its attributes as a
whole; its components will retain their original rates of oscillation,
subject only to modifications by mutual influence. Such being the
circumstances of the case we may partially understand how the Sun's rays
can effect chemical decompositions. If the members of a diatomic molecule
stand so related to the undulations falling on them, that one is thrown
into a state of increased oscillation and the other not; it is manifest
that there must arise a tendency towards the dislocation of the two--a
tendency which may or may not take effect, according to the weakness or
strength of their union, and according to the presence or absence of
collateral affinities. This inference is in harmony with several
significant facts. Dr. Draper remarks that "among metallic substances
(compounds) those first detected to be changed by light, such as silver,
gold, mercury, lead, have all high atomic weights; and such as sodium and
potassium, the atomic weights of which are low, appeared to be less
changeable." As here interpreted, the fact specified amounts to this; that
the compounds most readily decomposed by light, are those in which there is
a marked contrast between the atomic weights of the constituents, and
probably therefore a marked contrast between the rapidities of their
vibrations. The circumstance, too, that different chemical compounds are
decomposed or modified in different parts of the spectrum, implies that
there is a relation between special orders of undulations and special
orders of molecules--doubtless a correspondence between the rates of these
undulations and the rates of oscillation which some of the components of
such molecules will assume. Strong confirmation of this view may be drawn
from the decomposing actions of those longer ethereal waves which we
perceive as heat. On contemplating the whole series of diatomic compounds,
we see that the elements which are most remote in their atomic weights, as
hydrogen and the noble metals generally, will not combine at all, or do so
with great difficulty: their vibrations are so unlike that they cannot keep
together under any conditions of temperature. If, again, we look at a
smaller group, as the metallic oxides, we see that whereas those metals
which have atoms nearest in weight to the atoms of oxygen, cannot be
separated from oxygen by heat, even when it is joined by a powerful
collateral affinity; those metals which differ more widely from oxygen in
their atomic weights, can be de-oxidized by carbon at high temperatures;
and those which differ from it most widely combine with it very
reluctantly, and yield it up if exposed to thermal undulations of moderate
intensity. Here indeed, remembering the relations among the atomic weights
in the two cases, may we not suspect a close analogy between the
de-oxidation of a metallic oxide by carbon under the influence of the
longer ethereal waves, and the de-carbonization of carbonic acid by
hydrogen under the influence of the shorter ethereal waves?
These conceptions help us to some dim notion of the mode in which changes
are wrought in light in the leaves of plants. Among the several elements
concerned, there are wide differences in molecular mobility, and probably
in the rates of molecular vibration. Each is combined with one of the
others, but is capable of forming various combinations with the rest. And
they are severally in presence of a complex compound into which they all
enter, and which is ready to assimilate with itself the new compound
molecules they form. Certain of the ethereal waves falling on them when
thus arranged, cause a detachment of some of the combined atoms and a union
of the rest. And the conclusion suggested is that the induced vibrations
among the various atoms as at first arranged, are so incongruous as to
produce instability, and to give collateral affinities the power to work a
rearrangement which, though less stable under other conditions, is more
stable in the presence of these particular undulations. There seems,
indeed, no choice but to conceive the matter thus. An atom united with one
for which it has a strong affinity, has to be transferred to another for
which it has a weaker affinity. This transfer implies motion. The motion is
given by the waves of a medium that is relatively imponderable. No one wave
of this imponderable medium can give the requisite motion to this atom of
ponderable matter: especially as the atom is held by a positive force
besides its inertia. The motion required can hence be given only by
successive waves; and that these may not destroy each other's effects, it
is needful that each shall strike the atom just when it has completed the
recoil produced by the impact of previous ones. That is, the ethereal
undulations must coincide in rate with the oscillations of the atom,
determined by its inertia and the forces acting on it. It is also requisite
that the rate of oscillation of the atom to be detached, shall differ from
that of the atom with which it is united; since if the two oscillated in
unison the ethereal waves would not tend to separate them. And, finally,
the successive impacts of the ethereal waves must be accumulated until the
resulting oscillations have become so wide in their sweep as greatly to
weaken the cohesion of the united atoms, at the same time that they bring
one of them within reach of other atoms with which it will combine. In this
way only does it seem possible for such a force to produce such a transfer.
Moreover, while we are thus enabled to conceive how light may work these
molecular changes, we also gain an insight into the method by which the
insensible motions propagated to us from the Sun, are treasured up in such
ways as afterwards to generate sensible motions. By the accumulation of
infinitesimal impacts, atoms of ponderable matter are made to oscillate.
The quantity of motion which each of them eventually acquires, effects its
transfer to a position of unstable equilibrium, from which it can
afterwards be readily dislodged. And when so dislodged, along with other
atoms similarly and simultaneously affected, there is suddenly given out
all the motion which had been before impressed on it.
Speculation aside, however, that which it concerns us to notice is the
broad fact that light is an all-important agent of molecular changes in
organic substances. It is not here necessary for us to ascertain _how_
light produces these compositions and decompositions. It is necessary only
for us to observe that it _does_ produce them. That the characteristic
matter called chlorophyll, which gives the green colour to leaves, makes
its appearance whenever the blanched shoots of plants are exposed to the
Sun; that the petals of flowers, uncoloured while in the bud, acquire their
bright tints as they unfold; and that on the outer surfaces of animals,
analogous changes are induced; are wide inductions which are enough for our
present purpose.
§ 14. We come next to the agency of chief importance among those that work
changes in organic matter; namely, chemical affinity. How readily vegetal
and animal substances are modified by other substances put in contact with
them, we see daily illustrated. Besides the many compounds which cause the
death of an organism into which they are put, we have the much greater
number of compounds which work those milder effects termed
medicinal--effects implying, like the others, molecular re-arrangements.
Indeed, most soluble chemical compounds, natural and artificial, produce,
when taken into the body, alterations that are more or less manifest in
their results.
After what was shown in the last chapter, it will be manifest that this
extreme modifiability of organic matter by chemical agencies, is the chief
cause of that active molecular re-arrangement which organisms, and
especially animal organisms, display. In the two fundamental functions of
nutrition and respiration, we have the means by which the supply of
materials for this active molecular re-arrangement is maintained.
The process of animal nutrition consists partly in the absorption of those
complex substances which are thus highly capable of being chemically
altered, and partly in the absorption of simpler substances capable of
chemically altering them. The tissues always contain small quantities of
alkaline and earthy salts, which enter the system in one form and are
excreted in another. Though we do not know specifically the parts which
these salts play, yet from their universal presence, and from the
transformations which they undergo in the body, it may be safely inferred
that their chemical affinities are instrumental in working some of the
metamorphoses ever going on.
The inorganic substance, however, on which mainly depend these
metamorphoses in organic matter, is not swallowed along with the solid and
liquid food, but is absorbed from the surrounding medium--air or water, as
the case may be. Whether the oxygen taken in, either, as by the lowest
animals, through the general surface, or, as by the higher animals, through
respiratory organs, is the immediate cause of those molecular changes which
are ever going on throughout the living tissues; or whether the oxygen,
playing the part of scavenger, merely aids these changes by carrying away
the products of decompositions otherwise caused; it equally remains true
that these changes are maintained by its instrumentality. Whether the
oxygen absorbed and diffused through the system effects a direct oxidation
of the organic colloids which it permeates, or whether it first leads to
the formation of simpler and more oxidized compounds, which are afterwards
further oxidized and reduced to still simpler forms, matters not, in so far
as the general result is concerned. In any case it holds good that the
substances of which the animal body is built up, enter it in either an
unoxidized or in a but slightly oxidized and highly unstable state; while
the great mass of them leave it in a fully oxidized and stable state. It
follows, therefore, that, whatever the special changes gone through, the
general process is a falling from a state of unstable chemical equilibrium
to a state of stable chemical equilibrium. Whether this process be direct
or indirect, the total molecular re-arrangement and the total motion given
out in effecting it, must be the same.
§ 15. There is another species of re-distribution among the component
matters of organisms, which is not immediately effected by the affinities
of the matters concerned, but is mediately effected by other affinities;
and there is reason to think that the re-distribution thus caused is
important in amount, if not indeed the most important. In ordinary cases of
chemical action, the two or more substances concerned themselves undergo
changes of molecular arrangement; and the changes are confined to the
substances themselves. But there are other cases in which the chemical
action going on does not end with the substances at first concerned, but
sets up chemical actions, or changes of molecular arrangement, among
surrounding substances that would else have remained quiescent. And there
are yet further cases in which mere contact with a substance that is itself
quiescent, will cause other substances to undergo rapid metamorphoses. In
what we call fermentation, the first species of this communicated chemical
action is exemplified. One part of yeast, while itself undergoing molecular
change, will convert 100 parts of sugar into alcohol and carbonic acid; and
during its own decomposition, one part of diastase "is able to effect the
transformation of more than 1000 times its weight of starch into sugar." As
illustrations of the second species, may be mentioned those changes which
are suddenly produced in many colloids by minute portions of various
substances added to them--substances that are not undergoing manifest
transformations, and suffer no appreciable effects from the contact. The
nature of the first of these two kinds of communicated molecular change,
which here chiefly concerns us, may be rudely represented by certain
visible changes communicated from mass to mass, when a series of masses has
been arranged in a special way. The simplest example is that furnished by
the child's play of setting bricks on end in a row, in such positions that
when the first is overthrown it overthrows the second, the second the
third, the third the fourth, and so on to the end of the row. Here we have
a number of units severally placed in unstable equilibrium, and in such
relative positions that each, while falling into a state of stable
equilibrium, gives an impulse to the next sufficient to make the next,
also, fall from unstable to stable equilibrium. Now since, among mingled
compound molecules, no one can undergo change in the arrangement of its
parts without a molecular motion that must cause some disturbance all
round; and since an adjacent molecule disturbed by this communicated
motion, may have the arrangement of its constituent atoms altered, if it is
not a stable arrangement; and since we know, both that the molecules which
are changed by this so-called catalysis _are_ unstable, and that the
molecules resulting from their changes are _more_ stable; it seems probable
that the transformation is really analogous, in principle, to the familiar
one named. Whether thus interpretable or not, however, there is good reason
for thinking that to this kind of action is due a large amount of vital
metamorphosis. Let us contemplate the several groups of facts which point
to this conclusion.[9]
In the last chapter (§ 2) we incidentally noted the extreme instability of
nitrogenous compounds in general. We saw that sundry of them are liable to
explode on the slightest incentive--sometimes without any apparent cause;
and that of the rest, the great majority are very easily decomposed by
heat, and by various substances. We shall perceive much significance in
this general characteristic when we join it with the fact that the
substances capable of setting up extensive molecular changes in the way
above described are all nitrogenous ones. Yeast consists of vegetal cells
containing nitrogen,--cells that grow by assimilating the nitrogenous
matter contained in wort. Similarly, the "vinegar-plant," which greatly
facilitates the formation of acetic acid from alcohol, is a fungoid growth
that is doubtless, like others of its class, rich in nitrogenous compounds.
Diastase, by which the transformation of starch into sugar is effected
during the process of malting, is also a nitrogenous body. So too is a
substance called synaptase--an albumenous principle contained in almonds,
which has the power of working several metamorphoses in the matters
associated with it. These nitrogenized compounds, like the rest of their
family, are remarkable for the rapidity with which they decompose; and the
extensive changes produced by them in the accompanying carbo-hydrates, are
found to vary in their kinds according as the decompositions of the
ferments vary in their stages. We have next to note, as having here a
meaning for us, the chemical contrasts between those organisms which carry
on their functions by the help of external forces, and those which carry on
their functions by forces evolved from within. If we compare animals and
plants, we see that whereas plants, characterized as a class by containing
but little nitrogen, are dependent on the solar rays for their vital
activities; animals, the vital activities of which are not thus dependent,
mainly consist of nitrogenous substances. There is one marked exception to
this broad distinction, however; and this exception is specially
instructive. Among plants there is a considerable group--the Fungi--many
members of which, if not all, can live and grow in the dark; and it is
their peculiarity that they are very much more nitrogenous than other
plants. Yet a third class of facts of like significance is disclosed when
we compare different portions of the same organism. The seed of a plant
contains nitrogenous substance in a far higher ratio than the rest of the
plant; and the seed differs from the rest of the plant in its ability to
initiate, in the absence of light, extensive vital changes--the changes
constituting germination. Similarly in the bodies of animals, those parts
which carry on active functions are nitrogenous; while parts that are
non-nitrogenous--as the deposits of fat--carry on no active functions. And
we even find that the appearance of non-nitrogenous matter throughout
tissues normally composed almost wholly of nitrogenous matter, is
accompanied by loss of activity: what is called fatty degeneration being
the concomitant of failing vitality. One more fact, which serves to make
still clearer the meaning of the foregoing ones, remains--the fact, namely,
that in no part of any organism where vital changes are going on, is
nitrogenous matter wholly absent. It is common to speak of plants--or at
least all parts of plants but the seeds--as non-nitrogenous. But they are
only relatively so; not absolutely. The quantity of albumenoid substance in
the tissues of plants, is extremely small compared with the quantity
contained in the tissues of animals; but all plant-tissues which are
discharging active functions have some albumenoid substance. In every
living vegetal cell there is a certain part that includes nitrogen as a
component. This part initiates those changes which constitute the
development of the cell. And if it cannot be said that it is the worker of
all subsequent changes undergone by the cell, it nevertheless continues to
be the part in which the independent activity is most marked.
Looking at the evidence thus brought together, do we not get an insight
into the actions of nitrogenous matter as a worker of organic changes? We
see that nitrogenous compounds in general are extremely prone to decompose:
their decomposition often involving a sudden and great evolution of energy.
We see that the substances classed as ferments, which, during their own
molecular changes, set up molecular changes in the accompanying
carbo-hydrates, are all nitrogenous. We see that among classes of
organisms, and among the parts of each organism, there is a relation
between the amount of nitrogenous matter present and the amount of
independent activity. And we see that even in organisms and parts of
organisms where the activity is least, such changes as do take place are
initiated by a substance containing nitrogen. Does it not seem probable,
then, that these extremely unstable compounds have everywhere the effect of
communicating to the less unstable compounds associated with them,
molecular movements towards a stable state, like those they are themselves
undergoing? The changes which we thus suppose nitrogenous matter to produce
in the body, are clearly analogous to those which we see it produce out of
the body. Out of the body, certain carbo-hydrates in continued contact with
nitrogenous matter, are transformed into carbonic acid and alcohol, and
unless prevented the alcohol is transformed into acetic acid: the
substances formed being thus more highly oxidized and more stable than the
substances destroyed. In the body, these same carbo-hydrates, in continued
contact with nitrogenous matter, are transformed into carbonic acid and
water: substances which are also more highly oxidized and more stable than
those from which they result. And since acetic acid is itself resolved by
further oxidation into carbonic acid and water; we see that the chief
difference between the two cases is, that the process is more completely
effected in the body than it is out of the body. Thus, to carry further the
simile used above, the molecules of carbo-hydrates contained in the tissues
are, like bricks on end, not in the stablest equilibrium; but still in an
equilibrium so stable, that they cannot be overthrown by the chemical and
thermal forces which the body brings to bear on them. On the other hand,
being like similarly-placed bricks that have very narrow ends, the
nitrogenous molecules contained in the tissues are in so unstable an
equilibrium that they cannot withstand these forces. And when these
delicately-poised nitrogenous molecules fall into stable arrangements, they
give impulses to the more firmly-poised non-nitrogenous molecules, which
cause them also to fall into stable arrangements. It is a curious and
significant fact that in the arts, we not only utilize this same principle
of initiating extensive changes among comparatively stable compounds, by
the help of compounds much less stable, but we employ for the purpose
compounds of the same general class. Our modern method of firing a gun is
to place in close proximity with the gunpowder which we wish to decompose
or explode, a small portion of fulminating powder, which is decomposed or
exploded with extreme facility, and which, on decomposing, communicates the
consequent molecular disturbance to the less-easily decomposed gunpowder.
When we ask what this fulminating powder is composed of, we find that it is
a nitrogenous salt.[10]
Thus, besides the molecular re-arrangements produced in organic matter by
direct chemical action, there are others of kindred importance produced by
indirect chemical action. Indeed, the inference that some of the leading
transformations occurring in the animal organism, are due to this so-called
catalysis, appears necessitated by the general aspect of the facts, apart
from any such detailed interpretations as the foregoing. We know that
various amylaceous and saccharine matters taken as food do not appear in
the excreta, and must therefore be decomposed in their course through the
body. We know that these matters do not become components of the tissues,
but only of the contained liquids and solids; and that thus their
metamorphosis is not a direct result of tissue-change. We know that their
stability is such that the thermal and chemical forces to which they are
exposed in the body, cannot alone decompose them. The only explanation open
to us, therefore, is that the transformation of these carbo-hydrates into
carbonic acid and water, is due to communicated chemical action.
§ 16. This chapter will have served its purpose if it has given a
conception of the extreme modifiability of organic matter by surrounding
agencies. Even were it possible, it would be needless to describe in detail
the immensely varied and complicated changes which the forces from moment
to moment acting on them, work in living bodies. Dealing with biology in
its general principles, it concerns us only to notice how specially
sensitive are the substances of which organisms are built up to the varied
influences that act upon organisms. Their special sensitiveness has been
made sufficiently manifest in the several foregoing sections.
CHAPTER III.
THE RE-ACTIONS OF ORGANIC MATTER ON FORCES.
§ 17. Re-distributions of Matter imply concomitant re-distributions of
Motion. That which under one of its aspects we contemplate as an alteration
of arrangement among the parts of a body, is, under a correlative aspect,
an alteration of arrangement among certain momenta, whereby these parts are
impelled to their new positions. At the same time that a force, acting
differently on the different units of an aggregate, changes their relations
to one another; these units, reacting differently on the different parts of
the force, work equivalent changes in the relations of these to one
another. Inseparably connected as they are, these two orders of phenomena
are liable to be confounded together. It is very needful, however, to
distinguish between them. In the last chapter we took a rapid survey of the
re-distributions which forces produce in organic matter; and here we must
take a like survey of the simultaneous re-distributions undergone by the
forces.
At the outset we are met by a difficulty. The parts of an inorganic mass
undergoing re-arrangement by an incident force, are in most cases
passive--do not complicate those necessary re-actions that result from
their inertia, by other forces which they themselves originate. But in
organic matter the re-arranged parts do not re-act in virtue of their
inertia only. They are so constituted that an incident force usually sets
up in them other actions which are much more important. Indeed, what we may
call the indirect reactions thus caused, are so great in their amounts
compared with the direct re-actions, that they quite obscure them.
The impossibility of separating these two kinds of reaction compels us to
disregard the distinction between them. Under the above general title, we
must include both the immediate re-actions and those re-actions mediately
produced, which are among the most conspicuous of vital phenomena.
§ 18. From organic matter, as from all other matter, incident forces call
forth that re-action which we know as heat. More or less of molecular
vibration necessarily results when, to the forces at work among the
molecules of any aggregate, other forces are added. Experiment abundantly
demonstrates this in the case of inorganic masses; and it must equally hold
in the case of organic masses. In both cases the force which, more
markedly than any other, produces this thermal re-action, is that which
ends in the union of different substances. Though inanimate bodies admit of
being greatly heated by pressure and by the electric current, yet the
evolutions of heat, thus induced are neither so common, nor in most cases
so conspicuous, as those resulting from chemical combination. And though in
animate bodies there are certain amounts of heat generated by other
actions, yet these are secondary to the heat generated by the action of
oxygen on the substances composing the tissues and the substances contained
in them. Here, however, we see one of the characteristic distinctions
between inanimate and animate bodies. Among the first there are but few
which ordinarily exist in a condition to evolve the heat caused by chemical
combination; and such as are in this condition soon cease to be so when
chemical combination and genesis of heat once begin in them. Whereas, among
the second there universally exists the ability, more or less decided, thus
to evolve heat; and the evolution of heat, in some cases very slight and in
no cases very great, continues as long as they remain animate bodies.
The relation between active change of matter and re-active genesis of
molecular vibration, is clearly shown by the contrasts between different
organisms, and between different states and parts of the same organism. In
plants the genesis of heat is extremely small, in correspondence with their
extremely small production of carbonic acid: those portions only, as
flowers and germinating seeds, in which considerable oxidation is going on,
having decidedly raised temperatures. Among animals we see that the
hot-blooded are those which expend much force and respire actively. Though
insects are scarcely at all warmer than the surrounding air when they are
still, they rise several degrees above it when they exert themselves; and
in mammals, which habitually maintain a temperature much higher than that
of their medium, exertion is accompanied by an additional production of
heat.
This molecular agitation accompanies the falls from unstable to stable
molecular combinations; whether they be those from the most complex to the
less complex compounds, or whether they be those ultimate falls which end
in fully oxidized and relatively simple compounds; and whether they be
those of the nitrogenous matters composing the tissues or those of the
non-nitrogenous matters diffused through them. In the one case as in the
other, the heat must be regarded as a concomitant. Whether the
distinction, originally made by Liebig, between nitrogenous substances as
tissue-food and non-nitrogenous substances as heat-food, be true or not in
a narrower sense, it cannot be accepted in the sense that tissue-food is
not also heat-food. Indeed he does not himself assert it in this sense. The
ability of carnivorous animals to live and generate heat while consuming
matter that is almost exclusively nitrogenous, suffices to prove that the
nitrogenous compounds forming the tissues are heat-producers, as well as
the non-nitrogenous compounds circulating among and through the tissues: a
conclusion which is indeed justified by the fact that nitrogenous
substances out of the body yield heat, though not a large amount, during
combustion. But most likely this antithesis is not true even in the more
restricted sense. The probability is that the hydrocarbons and
carbo-hydrates which, in traversing the system, are transformed by
communicated chemical action, evolve, during their transformation, not heat
alone but also other kinds of force. It may be that as the nitrogenous
matter, while falling into more stable molecular arrangements, generates
both that molecular agitation called heat and such other molecular
movements as are resolved into forces expended by the organism; so, too,
does the non-nitrogenous matter. Or perhaps the concomitants of this
metamorphosis of non-nitrogenous matter vary with the conditions. Heat
alone may result when it is transformed while in the circulating fluids,
but partly heat and partly another force when it is transformed in some
active tissue that has absorbed it; just as coal, though producing little
else but heat as ordinarily burnt, has its heat partially transformed into
mechanical motion if burnt in a steam-engine furnace. In such case the
antithesis of Liebig would be reduced to this--that whereas nitrogenous
substance is tissue-food _both_ as material for building-up tissue and as
material for its function; non-nitrogenous substance is tissue-food _only_
as material for function.
There can be no doubt that this thermal re-action which chemical action
from moment to moment produces in the body, is from moment to moment an aid
to further chemical action. We before saw (_First Principles_, § 100) that
a state of raised molecular vibration is favourable to those
re-distributions of matter and motion which constitute Evolution. We saw
that in organisms distinguished by the amount and rapidity of such
re-distributions, this raised state of molecular vibration is conspicuous.
And we here see that this raised state of molecular vibration is itself a
continuous consequence of the continuous molecular re-distributions it
facilitates. The heat generated by each increment of chemical change makes
possible the succeeding increment of chemical change. In the body this
connexion of phenomena is the same as we see it to be out of the body. Just
as in a burning piece of wood, the heat given out by the portion actually
combining with oxygen, raises the adjacent portion to a temperature at
which it also can combine with oxygen; so, in a living animal, the heat
produced by oxidation of each portion of organized or unorganized
substance, maintains the temperature at which the unoxidized portions can
be readily oxidized.
§ 19. Among the forces called forth from organisms by re-action against the
actions to which they are subject, is Light. Phosphorescence is in some few
cases displayed by plants--especially by certain fungi. Among animals it is
comparatively common. All know that there are several kinds of luminous
insects; and many are familiar with the fact that luminosity is a
characteristic of various marine creatures.
Much of the evidence is supposed to imply that this evolution of light,
like the evolution of heat, is consequent on oxidation of the tissues or of
matters contained in them. Light, like heat, is the expression of a raised
state of molecular vibration: the difference between them being a
difference in the rates of vibration. Hence it seems inferable that by
chemical action on substances contained in the organism, heat or light may
be produced, according to the character of the resulting molecular
vibrations. Some experimental evidence supports this view. In
phosphorescent insects, the continuance of the light is found to depend on
the continuance of respiration; and any exertion which renders respiration
more active, increases the brilliancy of the light. Moreover, by separating
the luminous matter, Prof. Matteucci has shown that its emission of light
is accompanied by absorption of oxygen and escape of carbonic acid. The
phosphorescence of marine animals has been referred to other causes than
oxidation; but it may perhaps be explicable without assuming any more
special agency. Considering that in creatures of the genus _Noctiluca_, for
example, to which the phosphorescence most commonly seen on our own coasts
is due, there is no means of keeping up a constant circulation, we may
infer that the movements of aerated fluids through their tissues, must be
greatly affected by impulses received from without. Hence it may be that
the sparkles visible at night when the waves break gently on the beach, or
when an oar is dipped into the water, are called forth from these creatures
by the concussion, not because of any unknown influence it excites, but
because, being propagated through their delicate tissues, it produces a
sudden movement of the fluids and a sudden increase of chemical action.
Nevertheless, in other phosphorescent animals inhabiting the sea, as in the
_Pyrosoma_ and in certain _Annelida_, light seems to be produced otherwise
than by direct re-action on the action of oxygen. Indeed, it needs but to
recall the now familiar fact that certain substances become luminous in the
dark after exposure to sunlight, to see that there are other causes of
light-emission.
§ 20. The re-distributions of inanimate matter are habitually accompanied
by electrical disturbances; and there is abundant evidence that electricity
is generated during those re-distributions of matter that are ever taking
place in organisms. Experiments have shown "that the skin and most of the
internal membranes are in opposite electrical states;" and also that
between different internal organs, as the liver and the stomach, there are
electrical contrasts: such contrasts being greatest where the processes
going on in the compared parts are most unlike. It has been proved by du
Bois-Reymond that when any point in the longitudinal section of a muscle is
connected by a conductor with any point in its transverse section, an
electric current is established; and further, that like results occur when
nerves are substituted for muscles. The special causes of these phenomena
have not yet been determined. Considering that the electric contrasts are
most marked where active secretions are going on--considering, too, that
they are difficult to detect where there are no appreciable movements of
liquids--considering, also, that even when muscles are made to contract
after removal from the body, the contraction inevitably causes movements of
the liquids still contained in its tissues; it may be that they are due
simply to the friction of heterogeneous substances, which is universally a
cause of electric disturbance. But whatever be the interpretation, the fact
remains the same:--there is throughout the living organism, an unceasing
production of differences between the electric states of different parts;
and, consequently, an unceasing restoration of electric equilibrium by the
establishment of currents among these parts.
Besides these general, and not conspicuous, electrical phenomena common to
all organisms, vegetal as well as animal, there are certain special and
strongly marked ones. I refer, of course, to those which have made the
_Torpedo_ and the _Gymnotus_ objects of so much interest. In these
creatures we have a genesis of electricity which is not incidental on the
performance of their different functions by the different organs; but one
which is itself a function, having an organ appropriate to it. The
character of this organ in both these fishes, and its largely-developed
connexions with the nervous centres, have raised in some minds the
suspicion that in it there takes place a transformation of what we call
nerve-force into the force known as electricity. Perhaps, however, the true
interpretation may rather be that by nervous stimulation there is set up in
these animal-batteries that particular transformation of molecular motion
which it is their function to produce.
But whether general or special, and in whatever manner produced, these
evolutions of electricity are among the reactions of organic matter called
forth by the actions to which it is subject. Though these re-actions are
not direct, but seem to be remote consequences of changes wrought by
external agencies on the organism, they are yet incidents in that general
re-distribution of motion which these external agencies initiate; and as
such must here be noticed.
§ 21. To these known modes of motion, has next to be added an unknown one.
Heat, Light, and Electricity are emitted by inorganic matter when
undergoing changes, as well as by organic matter. But there is manifested
in some classes of living bodies a kind of force which we cannot identify
with any of the forces manifested by bodies that are not alive,--a force
which is thus unknown, in the sense that it cannot be assimilated to any
otherwise-recognized class. I allude to what is called nerve-force.
This is habitually generated in all animals, save the lowest, by incident
forces of every kind. The gentle and violent mechanical contacts, which in
ourselves produce sensations of touch and pressure--the additions and
abstractions of molecular vibration, which in ourselves produce sensations
of heat and cold, produce in all creatures that have nervous systems,
certain nervous disturbances: disturbances which, as in ourselves, are
either communicated to the chief nervous centre, and there arouse
consciousness, or else result in mere physical processes set going
elsewhere in the organism. In special parts distinguished as organs of
sense, other external actions bring about other nervous re-actions, that
show themselves either as special sensations or as excitements which,
without the intermediation of distinct consciousness, beget actions in
muscles or other organs. Besides neural discharges following the direct
incidence of external forces, others are ever being caused by the incidence
of forces which, though originally external, have become internal by
absorption into the organism of the agents exerting them. For thus may be
classed those neural discharges which result from modifications of the
tissues wrought by substances carried to them in the blood. That the
unceasing change of matter which oxygen and other agents produce throughout
the system, is accompanied by production of nerve-force, is shown by
various facts;--by the fact that nerve-force is no longer generated if
oxygen be withheld or the blood prevented from circulating; by the fact
that when the chemical transformation is diminished, as during sleep with
its slow respiration and circulation, there is a diminution in the quantity
of nerve-force; by the fact that an excessive expenditure of nerve-force
involves excessive respiration and circulation, and excessive waste of
tissue. To these proofs that nerve-force is evolved in greater or less
quantity, according as the conditions to rapid molecular change throughout
the body are well or ill fulfilled, may be added proofs that certain
special molecular actions are the causes of these special re-actions. The
effects of the vegeto-alkalies put beyond doubt the inference that the
overthrow of molecular equilibrium by chemical affinity, when it occurs in
certain parts, causes excitement in the nerves proceeding from those parts.
Indeed, looked at from this point of view, the two classes of nervous
changes--the one initiated from without and the other from within--are seen
to merge into one class. Both of them may be traced to metamorphosis of
tissue. The sensations of touch and pressure are doubtless consequent on
accelerated changes of matter, produced by mechanical disturbance of the
mingled fluids and solids composing the parts affected. There is abundant
evidence that the gustatory sensation is due to the chemical actions set up
by particles which find their way through the membrane covering the nerves
of taste; for, as Prof. Graham points out, sapid substances belong to the
class of crystalloids, which are able rapidly to permeate animal tissue,
while the colloids which cannot pass through animal tissue are insipid.
Similarly with the sense of smell. Substances which excite this sense are
necessarily more or less volatile; and their volatility being the result of
their molecular mobility, implies that they have, in a high degree, the
power of getting at the olfactory nerves by penetrating their mucous
investment. Again, the facts which photography has familiarized us with,
show that those nervous impressions called colours, are primarily due to
certain changes wrought by light in the substance of the retina. And
though, in the case of hearing, we cannot so clearly trace the connexion of
cause and effect, yet as we see that the auditory apparatus is one fitted
to intensify those vibrations constituting sound, and to convey them to a
receptacle containing liquid in which nerves are immersed, it can scarcely
be doubted that the sensation of sound proximately results from molecular
re-arrangements caused in these nerves by the vibrations of the liquid:
knowing, as we do, that the re-arrangement of molecules is in all cases
aided by agitation. Perhaps, however, the best proof that nerve-force,
whether peripheral or central in origin, results from chemical change, lies
in the fact that most of the chemical agents which powerfully affect the
nervous system, affect it whether applied at the centre or at the
periphery. Various mineral acids are tonics--the stronger ones being
usually the stronger tonics; and this which we call their acidity implies a
power in them of acting on the nerves of taste, while the tingling or pain
following their absorption through the skin, implies that the nerves of the
skin are acted on by them. Similarly with certain vegeto-alkalies which are
peculiarly bitter. By their bitterness these show that they affect the
extremities of the nerves, while, by their tonic properties, they show that
they affect the nervous centres: the most intensely bitter among them,
strychnia, being the most powerful nervous stimulant.[11] However true it
may be that this relation is not a regular one, since opium, hashish, and
some other drugs, which work marked effects on the brain, are not
remarkably sapid--however true it may be that there are relations between
particular substances and particular parts of the nervous system; yet such
instances do but qualify, without negativing, the general proposition. The
truth of this proposition can scarcely be doubted when, to the facts above
given, is added the fact that various condiments and aromatic drugs act as
nervous stimulants; and the fact that anæsthetics, besides the general
effects they produce when inhaled or swallowed, produce local effects of
like kind--first stimulant and then sedative--when absorbed through the
skin; and the fact that ammonia, which in consequence of its extreme
molecular mobility so quickly and so violently excites the nerves beneath
the skin, as well as those of the tongue and the nose, is a rapidly-acting
stimulant when taken internally.
Whether a nerve is merely a conductor, which delivers at one of its
extremities an impulse received at the other, or whether, as some now
think, it is itself a generator of force which is initiated at one
extremity and accumulates in its course to the other extremity, are
questions which cannot yet be answered. All we know is that agencies
capable of working molecular changes in nerves are capable of calling forth
from them manifestations of activity. And our evidence that nerve-force is
thus originated, consists not only of such facts as the above, but also of
more conclusive facts established by direct experiments on
nerves--experiments which show that nerve-force results when the cut end of
a nerve is either mechanically irritated, or acted on by some chemical
agent, or subject to the galvanic current--experiments which prove that
nerve-force is generated by whatever disturbs the molecular equilibrium of
nerve-substance.
§ 22. The most important of the re-actions called forth from organisms by
surrounding actions, remains to be noticed. To the various forms of
insensible motion thus caused, we have to add sensible motion. On the
production of this mode of force more especially depends the possibility of
all vital phenomena. It is, indeed, usual to regard the power of generating
sensible motion as confined to one out of the two organic sub-kingdoms; or,
at any rate, as possessed by but few members of the other. On looking
closer into the matter, however, we see that plant-life as well as
animal-life, is universally accompanied by certain manifestations of this
power; and that plant-life could not otherwise continue.
Through the humblest, as well as through the highest, vegetal organisms,
there are ever going on certain re-distributions of matter. In Protophytes
the microscope shows us an internal transposition of parts, which, when not
immediately visible, is proved to exist by the changes of arrangement that
become manifest in the course of hours and days. In the individual cells of
many higher plants, an active movement among the contained granules may be
witnessed. And well-developed cryptogams, in common with all phanerogams,
exhibit this genesis of mechanical motion still more conspicuously in the
circulation of sap. It might, indeed, be concluded _a priori_, that through
plants displaying much differentiation of parts, an internal movement must
be going on; since, without it, the mutual dependence of organs having
unlike functions would be impossible. Besides keeping up these motions of
liquids internally, plants, especially of the lower orders, move their
external parts in relation to each other, and also move about from place to
place. There are countless such illustrations as the active locomotion of
the zoospores of many _Algæ_, the rhythmical bendings of the _Oscillatoræ_,
the rambling progression of the _Diatomaceæ_. In fact many of these
smallest vegetals, and many of the larger ones in their early stages,
display a mechanical activity not distinguishable from that of the simplest
animals. Among well-organized plants, which are never locomotive in their
adult states, we still not unfrequently meet with relative motions of
parts. To such familiar cases as those of the Sensitive plant and the
Venus' fly-trap, many others may be added. When its base is irritated the
stamen of the Berberry flower leans over and touches the pistil. If the
stamens of the wild _Cistus_ be gently brushed with the finger, they spread
themselves: bending away from the seed-vessel. And some of the
orchid-flowers, as Mr. Darwin has shown, shoot out masses of pollen on to
the entering bee, when its trunk is thrust down in search of honey.
Though the power of moving is not, as we see, a characteristic of animals
alone, yet in them, considered as a class, it is manifested to an extent so
marked as practically to become their most distinctive trait. For it is by
their immensely greater ability to generate mechanical motion, that animals
are enabled to perform those actions which constitute their visible lives;
and it is by their immensely greater ability to generate mechanical motion,
that the higher orders of animals are most obviously distinguished from the
lower orders. Though, on remembering the seemingly active movements of
infusoria, some will perhaps question this last-named contrast, yet, on
comparing the quantities of matter propelled through given spaces in given
times, they will see that the momentum evolved is far less in the
_Protozoa_ than in the _Metazoa_. These sensible motions of animals are
effected in sundry ways. In the humblest forms, and even in some of the
more developed forms which inhabit the water, locomotion results from the
oscillations of whip-like appendages, single or double, or from the
oscillations of cilia: the contractility resides in these waving hairs that
grow from the surface. In many _Coelenterata_ certain elongations or tails
of ectodermal or endodermal cells shorten when stimulated, and by these
rudimentary contractile organs the movements are effected. In all the
higher animals, however, and to a smaller degree in many of the lower,
sensible motion is generated by a special tissue, under a special
excitement. Though it is not strictly true that such animals show no
sensible motions otherwise caused, since all of them have certain ciliated
membranes, and since the circulation of liquids in them is partially due to
osmotic and capillary actions; yet, generally speaking, we may say that
their movements are effected solely by muscles which contract solely
through the agency of nerves.
What special transformations of force generate these various mechanical
changes, we do not, in most cases, know. Those re-distributions of liquid,
with the alterations of form sometimes caused by them, that result from
osmose, are not, indeed, incomprehensible. Certain motions of plants which,
like those of the "animated oat," follow contact with water, are easily
interpreted; as are also such other vegetal motions as those of the
Touch-me-not, the Squirting Cucumber, and the _Carpobolus_. But we are
ignorant of the mode in which molecular movement is transformed into the
movement of masses, in animals. We cannot refer to known causes the
rhythmical action of a Medusa's disc, or that slow decrease of bulk which
spreads throughout the mass of an _Alcyonium_ when one of its component
individuals has been irritated. Nor are we any better able to say how the
insensible motion transmitted through a nerve, gives rise to sensitive
motion in a muscle. It is true that Science has given to Art several
methods of changing insensible into sensible motion. By applying heat to
water we vaporize it, and the movement of its expanding vapour we transfer
to solid matter; but evidently the genesis of muscular movement is in no
way analogous to this. The force evolved in a galvanic battery or by a
dynamo, we communicate to a soft iron magnet through a wire coiled round
it; and it would be possible, by placing near to each other several magnets
thus excited, to obtain, through the attraction of each for its neighbours,
an accumulated movement made up of their separate movements, and thus
mechanically to imitate a muscular contraction. But from what we know of
organic matter there is no reason to suppose that anything analogous to
this takes place in it. We can, however, through one kind of molecular
change, produce sensible changes of aggregation such as possibly might,
when occurring in organic substance, cause sensible motion in it. I refer
to change that is allotropic or isomeric. Sulphur, for example, assumes
different crystalline and non-crystalline forms at different temperatures,
and may be made to pass backwards and forwards from one form to another, by
slight variations of temperature: undergoing each time an alteration of
bulk. We know that this allotropism, or rather its analogue isomerism,
prevails among colloids--inorganic and organic. We also know that some of
these metamorphoses among colloids are accompanied by visible
re-arrangements: instance hydrated silicic acid, which, after passing from
its soluble state to the state of an insoluble jelly, begins, in a few
days, to contract and to give out part of its contained water. Now
considering that such isomeric changes of organic as well as inorganic
colloids, are often rapidly produced by very slight causes--a trace of a
neutral salt or a degree or two rise of temperature--it seems not
impossible that some of the colloids constituting muscle may be thus
changed by a nervous discharge: resuming their previous condition when the
discharge ceases. And it is conceivable that by structural arrangements,
minute sensible motions so caused may be accumulated into large sensible
motions.
§ 23. But the truths which it is here our business especially to note, are
independent of hypotheses or interpretations. It is sufficient for the ends
in view, to observe that organic matter _does_ exhibit these several
conspicuous reactions when acted on by incident forces. It is not requisite
that we should know _how_ these re-actions originate.
In the last chapter were set forth the several modes in which incident
forces cause re-distributions of organic matter; and in this chapter have
been set forth the several modes in which is manifested the motion
accompanying this re-distribution. There we contemplated, under its several
aspects, the general fact that, in consequence of its extreme instability,
organic matter undergoes extensive molecular re-arrangements on very slight
changes of conditions. And here we have contemplated, under its several
aspects, the correlative general fact that, during these extensive
molecular re-arrangements, there are evolved large amounts of energy. In
the one case the components of organic matter are regarded as falling from
positions of unstable equilibrium to positions of stable equilibrium; and
in the other case they are regarded as giving out in their falls certain
momenta--momenta that may be manifested as heat, light, electricity,
nerve-force, or mechanical motion, according as the conditions determine.
I will add only that these evolutions of energy are rigorously dependent on
these changes of matter. It is a corollary from the primordial truth which,
as we have seen, underlies all other truths, (_First Principles_, §§ 62,
189,) that whatever amount of power an organism expends in any shape, is
the correlate and equivalent of a power which was taken into it from
without. On the one hand, it follows from the persistence of force that
each portion of mechanical or other energy which an organism exerts,
implies the transformation of as much organic matter as contained this
energy in a latent state. And on the other hand, it follows from the
persistence of force that no such transformation of organic matter
containing this latent energy can take place, without the energy being in
one shape or other manifested.
CHAPTER III^{A.}
METABOLISM.
§ 23a. In the early forties the French chemist Dumas pointed out the
opposed actions of the vegetal and animal kingdoms: the one having for its
chief chemical effect the decomposition of carbon-dioxide, with
accompanying assimilation of its carbon and liberation of its oxygen, and
the other having for its chief chemical effect the oxidation of carbon and
production of carbon-dioxide. Omitting those plants which contain no
chlorophyll, all others de-oxidize carbon; while all animals, save the few
which contain chlorophyll, re-oxidize carbon. This is not, indeed, a
complete account of the general relation; since it represents animals as
wholly dependent on plants, either directly or indirectly through other
animals, while plants are represented as wholly independent of animals; and
this last representation though mainly true, since plants can obtain direct
from the inorganic world certain other constituents they need, is in some
measure not true, since many with greater facility obtain these materials
from the decaying bodies of animals or from their _excreta_. But after
noting this qualification the broad antithesis remains as alleged.
How are these transformations brought about? The carbon contained in
carbon-dioxide does not at a bound become incorporated in the plant, nor
does the substance appropriated by the animal from the plant become at a
bound carbon-dioxide. It is through two complex sets of changes that these
two ultimate results are brought about. The materials forming the tissues
of plants as well as the materials contained in them, are progressively
elaborated from the inorganic substances; and the resulting compounds,
eaten and some of them assimilated by animals, pass through successive
changes which are, on the average, of an opposite character: the two sets
being constructive and destructive. To express changes of both these
natures the term "metabolism" is used; and such of the metabolic changes as
result in building up from simple to compound are distinguished as
"anabolic," while those which result in the falling down from compound to
simple are distinguished as "katabolic." These antithetical names do not
indeed cover all the molecular transformations going on. Many of them,
known as isomeric, imply neither building up nor falling down: they imply
re-arrangement only. But those which here chiefly concern us are the two
opposed kinds described.
A qualification is needful. These antithetic changes must be understood as
characterizing plant-life and animal-life in general ways rather than in
special ways--as expressing the transformations in their totalities but not
in their details. For there are katabolic processes in plants, though they
bear but a small ratio to the anabolic ones; and there are anabolic
processes in animals, though they bear but a small ratio to the katabolic
ones.
From the chemico-physical aspect of these changes we pass to those
distinguished as vital; for metabolic changes can be dealt with only as
changes effected by that living substance called protoplasm.
§ 23b. On the evolution-hypothesis we are obliged to assume that the
earliest living things--probably minute units of protoplasm smaller than
any the microscope reveals to us--had the ability to appropriate directly
from the inorganic world both the nitrogen and the materials for
carbo-hydrates without both of which protoplasm cannot be formed; since in
the absence of preceding organic matter there was no other source. The
general law of evolution as well as the observed actions of _Protozoa_ and
_Protophyta_, suggest that these primordial types simultaneously displayed
animal-life and plant-life. For whereas the developed animal-type cannot
form from its inorganic surroundings either nitrogenous compounds or
carbo-hydrates; and whereas the developed plant-type, able to form
carbo-hydrates from its inorganic surroundings, depends for the formation
of its protoplasm mainly, although indirectly, on the nitrogenous compounds
derived from preceding organisms, as do also most of the plants devoid of
chlorophyll--the fungi; we are obliged to assume that in the beginning,
along with the expending activities characterizing the animal-type, there
went the accumulating activities characterizing both of the vegetal
types--forms of activity by-and-by differentiated.
Though the successive steps in the artificial formation of organic
compounds have now gone so far that substances simulating proteids, if not
identical with them, have been produced, yet we have no clue to the
conditions under which proteids arose; and still less have we a clue to the
conditions under which inert proteids became so combined as to form active
protoplasm. The essential fact to be recognized is that living matter,
originated as we must assume during a long stage of progressive cooling in
which the infinitely varied parts of the Earth's surface were slowly
passing through appropriate physical conditions, possessed from the outset
the power of assimilating to itself the materials from which more living
matter was formed; and that since then all living matter has arisen from
its self-increasing action. But now, leaving speculation concerning these
anabolic changes as they commenced in the remote past, let us contemplate
them as they are carried on now--first directing our attention to those
presented in the vegetal world.
§ 23c. The decomposition of carbon-dioxide (§ 13)--the separation of its
carbon from the combined oxygen so that it may enter into one or other form
of carbo-hydrate,--is not now ordinarily effected, as we must assume it
once was, by the undifferentiated protoplasm; but is effected by a
specialized substance, chlorophyll, imbedded in the protoplasm and
operating by its instrumentality. The chlorophyll-grain is not simply
immersed in protoplasm but is permeated throughout its substance by a
protoplasmic network or sponge-work apparently continuous with the
protoplasm around; or, according to Sachs, consists of protoplasm holding
chlorophyll-particles in suspension: the mechanical arrangement
facilitating the chemical function. The resulting abstraction of carbon
from carbon-dioxide, by the aid of certain ethereal undulations, appears to
be the first step in the building up of organic compounds--the first step
in the primary anabolic process. We are not here concerned with details.
Two subsequent sets of changes only need here to be noted--the genesis of
the passive materials out of which plant-structure is built up, and the
genesis of the active materials by which these are produced and the
building up effected.
The hydrated carbon which protoplasm, having the chlorophyll-grain as its
implement, produces from carbonic acid and water, appears not to be of one
kind only. The possible carbo-hydrates are almost infinite in number.
Multitudes of them have been artificially made, and numerous kinds are made
naturally by plants. Though perhaps the first step in the reduction of the
carbon from its dioxide may be always the same, yet it is held probable
that in different types of plants different types of carbo-hydrates
forthwith arise, and give differential characters to the compounds
subsequently formed by such types: sundry of the changes being katabolic
rather than anabolic. Of leading members in the group may be named dextrin,
starch, and the various sugars characteristic of various plants, as well as
the cellulose elaborated by further anabolism. Considered as the kind of
carbo-hydrate in which the products of activity are first stored up, to be
subsequently modified for divers purposes, starch is the most important of
these; and the process of storage is suggested by the structure of the
starch-grain. This consists of superposed layers, implying intermittent
deposits: the probability being that the variations of light and heat
accompanying day and night are associated now with arrest of the deposit
and now with recommencement of it. Like in composition as this stored-up
starch is with sugar of one or other kind, and capable of being deposited
from sugar and again assuming the sugar form, this substance passes, by
further metabolism, here into the cellulose which envelopes each of the
multitudinous units of protoplasm, there into the spiral fibres, annuli, or
fenestrated tubes which, in early stages of tissue-growth, form channels
for the sap, and elsewhere into other components of the general structure.
The many changes implied are effected in various ways: now by that simple
re-arrangement of components known as isomeric change; now by that taking
from a compound one of its elements and inserting one of another kind,
which is known as substitution; and now by oxidation, as when the
oxy-cellulose which constitutes wood-fibre, is produced.
Besides elaborating building materials, the protoplasm elaborates
itself--that is, elaborates more of itself. It is chemically distinguished
from the building materials by the presence of nitrogen. Derived from
atmospheric ammonia, or from decaying or excreted organic matter, or from
the products of certain fungi and microbes at its roots, the nitrogen in
one or other combination is brought into a plant by the upward current; and
by some unknown process (not dependent on light, since it goes on equally
well if not better in darkness) the protoplasm dissociates and appropriates
this combined nitrogen and unites it with a carbo-hydrate to form one or
other proteid--albumen, gluten, or some isomer; appropriating at the same
time from certain of the earth-salts the requisite amount of sulphur and in
some cases phosphorus. The ultimate step, as we must suppose, is the
formation of living protoplasm out of these non-living proteids. A cardinal
fact is that proteids admit of multitudinous transformations; and it seems
not improbable that in protoplasm various isomeric proteids are mingled. If
so, we must conclude that protoplasm admits of almost infinite variations
in nature. Of course _pari passu_ with this dual process--augmentation of
protoplasm and accompanying production of carbo-hydrates--there goes
extension of plant-structure and plant-life.
To these essential metabolic processes have to be added certain ancillary
and non-essential ones, ending in the formation of colouring matters,
odours, essential oils, acrid secretions, bitter compounds and poisons:
some serving to attract animals and others to repel them. Sundry of these
appear to be excretions--useless matters cast out, and are doubtless
katabolic.
The relation of these facts here sketched in rude outline to the doctrine
of Evolution at large should be observed. Already we have seen how (§ 8a),
in the course of terrestrial evolution, there has been an increasingly
heterogeneous assemblage of increasing heterogeneous compounds, preparing
the way for organic life. And here we may see that during the development
of plant-life from its lowest algoid and fungoid forms up to those forms
which constitute the chief vegetal world, there has been an increasing
number of complex organic compounds formed; displayed at once in the
diversity of them contained in the same plant and in the still greater
diversity displayed in the vast aggregate of species, genera, orders, and
classes of plants.
§ 23d. On passing to the metabolism characterizing animal life, which, as
already indicated, is in the main a process of decomposition undoing the
process of composition characterizing vegetal life, we may fitly note at
the outset that it must have wide limits of variation, alike in different
classes of animals and even in the same animal.
If we take, on the one hand, a carnivore living on muscular tissue (for
wild carnivores preying upon herbivores which can rarely become fat obtain
scarcely any carbo-hydrates) and observe that its food is almost
exclusively nitrogenous; and if, on the other hand, we take a graminivorous
animal the food of which (save when it eats seeds) contains comparatively
little nitrogenous matter; we seem obliged to suppose that the parts played
in the organic processes by the proteids and the carbo-hydrates can in
considerable measures replace one another. It is true that the quantity of
food and the required alimentary system in the last case, are very much
greater than in the first case. But this difference is mainly due to the
circumstance that the food of the graminivorous animal consists chiefly of
waste-matter--ligneous fibre, cellulose, chlorophyll--and that could the
starch, sugar, and protoplasm be obtained without the waste-matter, the
required bulks of the two kinds of food would be by no means so strongly
contrasted. This becomes manifest on comparing flesh-eating and
grain-eating birds--say a hawk and a pigeon. In powers of flight these do
not greatly differ, nor is the size of the alimentary system conspicuously
greater in the last than in the first; though probably the amount of food
consumed is greater. Still it seems clear that the supply of energy
obtained by a pigeon from carbo-hydrates with a moderate proportion of
proteids is not widely unlike that obtained by a hawk from proteids alone.
Even from the traits of men differently fed a like inference may be drawn.
On the one hand we have the Masai who, during their warrior-days, eat flesh
exclusively; and on the other hand we have the Hindus, feeding almost
wholly on vegetable food. Doubtless the quantities required in these cases
differ much; but the difference between the rations of the flesh-eater and
the grain-eater is not so immense as it would be were there no substitution
in the physiological uses of the materials.
Concerning the special aspects of animal-metabolism, we have first to note
those various minor transformations that are auxiliary to the general
transformation by which force is obtained from food. For many of the vital
activities merely subserve the elaboration of materials for activity at
large, and the getting rid of waste products. From blood passing through
the salivary glands is prepared in large quantity a secretion containing
among other matters a nitrogenous ferment, ptyaline, which, mixed with food
during mastication, furthers the change of its starch into sugar. Then in
the stomach come the more or less varying secretions known in combination
as gastric juice. Besides certain salts and hydrochloric acid, this
contains another nitrogenous ferment, pepsin, which is instrumental in
dissolving the proteids swallowed. To these two metabolic products aiding
solution of the various ingested solids, is presently added that product of
metabolism in the pancreas which, added to the chyme, effects certain other
molecular changes--notably that of such amylaceous matters as are yet
unaltered, into saccharine matters to be presently absorbed. And let us
note the significant fact that the preparation of food-materials in the
alimentary canal, again shows us that unstable nitrogenous compounds are
the agents which, while themselves changing, set up changes in the
carbo-hydrates and proteids around: the nitrogen plays the same part here
as elsewhere. It does the like in yet another viscus. Blood which passes
through the spleen on its way to the liver, is exposed to the action of "a
special proteid of the nature of alkali-albumin, holding iron in some way
peculiarly associated with it." Lastly we come to that all-important organ
the liver, at once a factory and a storehouse. Here several metabolisms are
simultaneously carried on. There is that which until recent years was
supposed to be the sole hepatic process--the formation of bile. In some
liver-cells are masses of oil-globules, which seem to imply a carbo-hydrate
metamorphosis. And then, of leading importance, comes the extensive
production of that animal-starch known as glycogen--a substance which, in
each of the cells generating it, is contained in a plexus of protoplasmic
threads: again a nitrogenous body diffused through a mass which is now
formed out of sugar and is now dissolved again into sugar. For it appears
that this soluble form of carbo-hydrate, taken into the liver from the
intestine, is there, when not immediately needed, stored up in the form of
glycogen, ready to be re-dissolved and carried into the system either for
immediate use or for re-deposit as glycogen at the places where it is
presently to be consumed: the great deposit in the liver and the minor
deposits in the muscles being, to use the simile of Prof. Michael Foster,
analogous in their functions to a central bank and branch banks.
An instructive parallelism may be noted between these processes carried on
in the animal organism and those carried on in the vegetal organism. For
the carbo-hydrates named, easily made to assume the soluble or the
insoluble form by the addition or subtraction of a molecule of water, and
thus fitted sometimes for distribution and sometimes for accumulation, are
similarly dealt with in the two cases. As the animal-starch, glycogen, is
now stored up in the liver or elsewhere and now changed into glucose to be
transferred, perhaps for consumption and perhaps for re-deposit; so the
vegetal starch, made to alternate between soluble and insoluble states, is
now carried to growing parts where by metabolic change it becomes cellulose
or other component of tissue and now carried to some place where, changed
back into starch, it is laid aside for future use; as it is in the turgid
inside leaves of a cabbage, the root of a turnip, or the swollen
underground stem we know as a potato: the matter which in the animal is
used up in generating movement and heat, being in the plant used up in
generating structures. Nor is the parallelism even now exhausted; for, as
by a plant starch is stored up in each seed for the subsequent use of the
embryo, so in an embryo-animal glycogen is stored up in the developing
muscles for subsequent use in the completion of their structures.
§ 23e. We come now to the supreme and all-pervading metabolism which has
for its effects the conspicuous manifestations of life--the nervous and
muscular activities. Here comes up afresh a question discussed in the
edition of 1864--a question to be reconsidered in the light of recent
knowledge--the question what particular metabolic changes are they by which
in muscle the energy existing under the form of molecular motion is
transformed into the energy manifested as molar motion?
There are two views respecting the nature of this transformation. One is
that the carbo-hydrate present in muscle must, by further metabolism, be
raised into the form of a nitrogenous compound or compounds before it can
be made to undergo that sudden decomposition which initiates muscular
contraction. The other is the view set forth in § 15, and there reinforced
by further illustrations which have occurred to me while preparing this
revised edition--the view that the carbo-hydrate in muscle, everywhere in
contact with unstable nitrogenous substance, is, by the shock of a small
molecular change in this, made to undergo an extensive molecular change,
resulting in the oxidation of its carbon and consequent liberation of much
molecular motion. Both of these are at present only hypotheses, in support
of which respectively the probabilities have to be weighed. Let us compare
them and observe on which side the evidence preponderates.
We are obliged to conclude that in carnivorous animals the katabolic
process is congruous with the first of these views, in so far that the
evolution of energy must in some way result solely from the fall of complex
nitrogenous compounds into those simpler matters which make their
appearance as waste; for, practically, the carnivorous animal has no
carbo-hydrates out of which otherwise to evolve force. To this admission,
however, it should be added that possibly out of the exclusively
nitrogenous food, glycogen or sugar has to be obtained by partial
decomposition before muscular action can take place. But when we pass to
animals having food consisting mainly of carbo-hydrates, several
difficulties stand in the way of the hypothesis that, by further
compounding, proteids must be formed from the carbo-hydrates before
muscular energy can be evolved. In the first place the anabolic change
through which, by the addition of nitrogen, &c., a proteid is formed from a
carbo-hydrate, must absorb an energy equal to a moiety of that which is
given out in the subsequent katabolic change. There can be no dynamic
profit on such part of the transaction as effects the composition and
subsequent decomposition of the proteid, but only on such part of the
transaction as effects the decomposition of the carbo-hydrate. In the
second place there arises the question--whence comes the nitrogen required
for the compounding of the carbo-hydrates into proteids? There is none save
that contained in the serum-albumen or other proteid which the blood
brings; and there can be no gain in robbing this proteid of nitrogen for
the purpose of forming another proteid. Hence the nitrogenizing of the
surplus carbo-hydrates is not accounted for. One more difficulty remains.
If the energy given out by a muscle results from the katabolic consumption
of its proteids, then the quantity of nitrogenous waste matters formed
should be proportionate to the quantity of work done. But experiments have
proved that this is not the case. Long ago it was shown that the amount of
urea excreted does not increase in anything like proportion to the amount
of muscular energy expended; and recently this has been again shown.
On this statement a criticism has been made to the following
effect:--Considering that muscle will contract when deprived of oxygen and
blood and must therefore contain matter from which the energy is derived;
and considering that since carbonic acid is given out the required carbon
and oxygen must be derived from some component of muscle; it results that
the energy must be obtained by decomposition of a nitrogenous body. To this
reasoning it may be objected, in the first place, that the conditions
specified are abnormal, and that it is dangerous to assume that what takes
place under abnormal conditions takes place also under normal ones. In
presence of blood and oxygen the process may possibly, or even probably, be
unlike that which arises in their absence: the muscular substance may begin
consuming itself when it has not the usual materials to consume. Then, in
the second place, and chiefly, it may be replied that the difficulty raised
in the foregoing argument is not escaped but merely obscured. If, as is
alleged, the carbon and oxygen from which carbonic acid is produced, form,
under the conditions stated, parts of a complex nitrogenous substance
contained in muscle, then the abstraction of the carbon and oxygen must
cause decomposition of this nitrogenous substance; and in that case the
excretion of nitrogenous waste must be proportionate to the amount of work
done, which it is not. This difficulty is evaded by supposing that the
"stored complex explosive substance must be, in living muscle, of such
nature" that after explosion it leaves a "nitrogenous residue available for
re-combination with fresh portions of carbon and oxygen derived from the
blood and thereby the re-constitution of the explosive substance." This
implies that a molecule of the explosive substance consists of a complex
nitrogenous molecule united with a molecule of carbo-hydrate, and that time
after time it suddenly decomposes this carbo-hydrate molecule and thereupon
takes up another such from the blood. That the carbon is abstracted from
the carbo-hydrate molecule can scarcely be said, since the feebler
affinities of the nitrogenous molecule can hardly be supposed to overcome
the stronger affinities of the carbo-hydrate molecule. The carbo-hydrate
molecule must therefore be incorporated bodily. What is the implication?
The carbo-hydrate part of the compound is relatively stable, while the
nitrogenous part is relatively unstable. Hence the hypothesis implies that,
time after time, the unstable nitrogenous part overthrows the stable
carbo-hydrate part, without being itself overthrown. This conclusion, to
say the least of it, does not appear very probable.
The alternative hypothesis, indirectly supported as we saw by proofs that
outside the body small amounts of change in nitrogenous compounds initiate
large amounts of change in carbonaceous compounds, may in the first place
be here supported by some further indirect evidences of kindred natures. A
haystack prematurely put together supplies one. Enough water having been
left in the hay to permit chemical action, the decomposing proteids forming
the dead protoplasm in each cell, set up decomposition of the
carbo-hydrates with accompanying oxidation of the carbon and genesis of
heat; even to the extent of producing fire. Again, as shown above, this
relation between these two classes of compounds is exemplified in the
alimentary canal; where, alike in the saliva and in the pancreatic
secretion, minute quantities of unstable nitrogenous bodies transform great
quantities of stable carbo-hydrates. Thus we find indirect reinforcements
of the belief that the katabolic change generating muscular energy is one
in which a large decomposition of a carbo-hydrate is set up by a small
decomposition of a proteid.[12]
§ 23f. A certain general trait of animal organization may fitly be named
because its relevance, though still more indirect, is very significant.
Under one of its aspects an animal is an apparatus for the multiplication
of energies--a set of appliances by means of which a minute amount of
motion initiates a larger amount of motion, and this again a still larger
amount. There are structures which do this mechanically and others which do
it chemically.
Associated with the peripheral ends of the nerves of touch are certain
small bodies--_corpuscula tactus_--each of which, when disturbed by
something in contact with the skin, presses on the adjacent fibre more
strongly than soft tissue would do, and thus multiplies the force producing
sensation. While serving the further purpose of touching at a distance, the
_vibrissæ_ or whiskers of a feline animal achieve a like end in a more
effectual way. The external portion of each bristle acts as the long arm of
a lever, and the internal portion as the short arm. The result is that a
slight touch at the outer end of the bristle produces a considerable
pressure of the inner end on the nerve-terminal: so intensifying the
impression. In the hearing organs of various inferior types of animals, the
otolites in contact with the auditory nerves, when they are struck by
sound-waves, give to the nerves much stronger impressions than these would
have were they simply immersed in loose tissue; and in the ears of
developed creatures there exist more elaborate appliances for augmenting
the effects of aerial vibrations. From this multiplication of molar actions
let us pass to the multiplication of molecular actions. The retina is made
up of minute rods and cones, so packed together side by side that they can
be separately affected by the separate parts of the images of objects. As
each of them is but 1/10,000th of an inch in diameter, the ethereal
undulations falling upon it can produce an amount of change almost
infinitesimal--an amount probably incapable of exciting a nerve-centre, or
indeed of overcoming the molecular inertia of the nerve leading to it. But
in close proximity are layers of granules into which the rods and cones
send fibres, and beyond these, about 1/100th of an inch from the retinal
layer, lie ganglion-cells, in each of which a minute disturbance may
readily evolve a larger disturbance; so that by multiplication, single or
perhaps double, there is produced a force sufficient to excite the fibre
connected with the centre of vision. Such, at least, judging from the
requirement and the structure, seems to me the probable interpretation of
the visual process; though whether it is the accepted one I do not know.
But now, carrying with us the conception made clear by the first cases and
suggested by the last, we shall appreciate the extent to which this general
physiological method, as we may call it, is employed. The convulsive action
caused by tickling shows it conspicuously. An extremely small amount of
molecular change in the nerve-endings produces an immense amount of
molecular change, and resulting molar motion, in the muscles. Especially is
this seen in one whose spinal cord has been so injured that it no longer
conveys sensations from the lower limbs to the brain; and in whom,
nevertheless, tickling of the feet produces convulsive actions of the legs
more violent even than result when sensation exists: clearly proving that
since the minute molecular change produced by the tickling in the
nerve-terminals cannot be equivalent in quantity to the amount implied by
the muscular contraction, there must be a multiplication of it in those
parts of the spinal cord whence issue the reflex stimuli to the muscles.
Returning now to the question of metabolism, we may see that the processes
of multiplication above supposed to take place in muscle, are analogous in
their general nature to various other physiological processes. Carrying
somewhat further the simile used in § 15 and going back to the days when
detonators, though used for small arms, were not used for artillery, we may
compare the metabolic process in muscle to that which would take place if a
pistol were fired against the touch-hole of a loaded cannon: the cap
exploding the pistol and the pistol the cannon. For in the case of the
muscle, the implication is that a nervous discharge works in certain
unstable proteids through which the nerve-endings are distributed, a small
amount of molecular change; that the shock of this causes a much larger
amount of molecular change in the inter-diffused carbo-hydrate, with
accompanying oxidation of its carbon; and that the heat liberated sets up a
transformation, probably isomeric, in the contractile substance of the
muscular fibre: an interpretation supported by cases in which small rises
and falls of temperature cause alternating isomeric changes; as instance
Mensel's salt.
Ending here this exposition, somewhat too speculative and running into
details inappropriate to a work of this kind, it suffices to note the most
general facts concerning metabolism. Regarded as a whole it includes, in
the first place, those anabolic or building-up processes specially
characterizing plants, during which the impacts of ethereal undulations are
stored up in compound molecules of unstable kinds; and it includes, in the
second place, those katabolic or tumbling-down changes specially
characterizing animals, during which this accumulated molecular motion
(contained in the food directly or indirectly supplied by plants), is in
large measure changed into those molar motions constituting animal
activities. There are multitudinous metabolic changes of minor kinds which
are ancillary to these--many katabolic changes in plants and many anabolic
changes in animals--but these are the essential ones.[13]
CHAPTER IV.[14]
PROXIMATE CONCEPTION OF LIFE.
§ 24. To those who accept the general doctrine of Evolution, it need
scarcely be pointed out that classifications are subjective conceptions,
which have no absolute demarcations in Nature corresponding to them. They
are appliances by which we limit and arrange the matters under
investigation; and so facilitate our thinking. Consequently, when we
attempt to define anything complex, or make a generalization of facts other
than the most simple, we can scarcely ever avoid including more than we
intended, or leaving out something which should be taken in. Thus it
happens that on seeking a definite idea of Life, we have great difficulty
in finding one that is neither more nor less than sufficient. Let us look
at a few of the most tenable definitions that have been given. While
recognizing the respects in which they are defective, we shall see what
requirements a more satisfactory one must fulfil.
Schelling said that Life is the tendency to individuation. This formula,
until studied, conveys little meaning. But we need only consider it as
illustrated by the facts of development, or by the contrast between lower
and higher forms of life, to recognize its significance; especially in
respect of comprehensiveness. As before shown, however (_First Principles_,
§ 56), it is objectionable; partly on the ground that it refers not so much
to the functional changes constituting Life, as to the structural changes
of those aggregates of matter which manifest Life; and partly on the ground
that it includes under the idea Life, much that we usually exclude from it:
for instance--crystallization.
The definition of Richerand,--"Life is a collection of phenomena which
succeed each other during a limited time in an organized body,"--is liable
to the fatal criticism, that it equally applies to the decay which goes on
after death. For this, too, is "a collection of phenomena which succeed
each other during a limited time in an organized body."
"Life," according to De Blainville, "is the two-fold internal movement of
composition and decomposition, at once general and continuous." This
conception is in some respects too narrow, and in other respects too wide.
On the one hand, while it expresses what physiologists distinguish as
vegetative life, it does not indicate those nervous and muscular functions
which form the most conspicuous and distinctive classes of vital phenomena.
On the other hand, it describes not only the integrating and disintegrating
process going on in a living body, but it equally well describes those
going on in a galvanic battery; which also exhibits a "two-fold internal
movement of composition and decomposition, at once general and continuous."
Elsewhere, I have myself proposed to define Life as "the co-ordination of
actions."[15] This definition has some advantages. It includes all organic
changes, alike of the viscera, the limbs, and the brain. It excludes the
great mass of inorganic changes; which display little or no co-ordination.
By making co-ordination the specific character of vitality, it involves the
truths, that an arrest of co-ordination is death, and that imperfect
co-ordination is disease. Moreover, it harmonizes with our ordinary ideas
of life in its different grades; seeing that the organisms which we rank as
low in their degrees of life, are those which display but little
co-ordination of actions; and seeing that from these up to man, the
recognized increase in degree of life corresponds with an increase in the
extent and complexity of co-ordinations. But, like the others, this
definition includes too much. It may be said of the Solar System, with its
regularly-recurring movements and its self-balancing perturbations, that
it, also, exhibits co-ordination of actions. And however plausibly it may
be argued that, in the abstract, the motions of the planets and satellites
are as properly comprehended in the idea of life as the changes going on in
a motionless, unsensitive seed: yet, it must be admitted that they are
foreign to that idea as commonly received, and as here to be formulated.
It remains to add the definition since suggested by Mr. G. H. Lewes--"Life
is a series of definite and successive changes, both of structure and
composition, which take place within an individual without destroying its
identity." The last fact which this statement brings into view--the
persistence of a living organism as a whole, in spite of the continuous
removal and replacement of its parts--is important. But otherwise it may be
argued that, since changes of structure and composition, though
concomitants of muscular and nervous actions, are not the muscular and
nervous actions themselves, the definite excludes the more visible
movements with which our idea of life is most associated; and further that,
in describing vital changes as _a series_, it scarcely includes the fact
that many of them, as Nutrition, Circulation, Respiration, and Secretion,
in their many subdivisions, go on simultaneously.
Thus, however well each of these definitions expresses the phenomena of
life under some of its aspects, no one of them is more than approximately
true. It may turn out that to find a formula which will bear every test is
impossible. Meanwhile, it is possible to frame a more adequate formula than
any of the foregoing. As we shall presently find, these all omit an
essential peculiarity of vital changes in general--a peculiarity which,
perhaps more than any other, distinguishes them from non-vital changes.
Before specifying this peculiarity, however, it will be well to trace our
way, step by step, to as complete an idea of Life as may be reached from
our present stand-point; by doing which we shall both see the necessity for
each limitation as it is made, and ultimately be led to feel the need for a
further limitation.
And here, as the best mode of determining what are the traits which
distinguish vitality from non-vitality, we shall do well to compare the two
most unlike kinds of vitality, and see in what they agree. Manifestly, that
which is essential to Life must be that which is common to Life of all
orders. And manifestly, that which is common to all forms of Life, will
most readily be seen on contrasting those forms of Life which have the
least in common, or are the most unlike.[16]
§ 25. Choosing assimilation, then, for our example of bodily life, and
reasoning for our example of that life known as intelligence; it is first
to be observed, that they are both processes of change. Without change,
food cannot be taken into the blood nor transformed into tissue; without
change, there can be no getting from premisses to conclusion. And it is
this conspicuous display of changes which forms the substratum of our idea
of Life in general. Doubtless we see innumerable changes to which no notion
of vitality attaches. Inorganic bodies are ever undergoing changes of
temperature, changes of colour, changes of aggregation; and decaying
organic bodies also. But it will be admitted that the great majority of the
phenomena displayed by inanimate bodies, are statical and not dynamical;
that the modifications of inanimate bodies are mostly slow and unobtrusive;
that on the one hand, when we see sudden movements in inanimate bodies, we
are apt to assume living agency, and on the other hand, when we see no
movements in living bodies, we are apt to assume death. Manifestly then, be
the requisite qualifications what they may, a true idea of Life must be an
idea of some kind of change or changes.
On further comparing assimilation and reasoning, with a view of seeing in
what respect the changes displayed in both differs from non-vital changes,
we find that they differ in being not simple changes; in each case there
are _successive_ changes. The transformation of food into tissue involves
mastication, deglutition, chymification, chylification, absorption, and
those various actions gone through after the lacteal ducts have poured
their contents into the blood. Carrying on an argument necessitates a long
chain of states of consciousness; each implying a change of the preceding
state. Inorganic changes, however, do not in any considerable degree
exhibit this peculiarity. It is true that from meteorologic causes,
inanimate objects are daily, sometimes hourly, undergoing modifications of
temperature, of bulk, of hygrometric and electric condition. Not only,
however, do these modifications lack that conspicuousness and that rapidity
of succession which vital ones possess, but vital ones form an _additional_
series. Living as well as not-living bodies are affected by atmospheric
influences; and beyond the changes which these produce, living bodies
exhibit other changes, more numerous and more marked. So that though
organic change is not rigorously distinguished from inorganic change by
presenting successive phases; yet vital change so greatly exceeds other
change in this respect, that we may consider it as a distinctive character.
Life, then, as thus roughly differentiated, may be regarded as change
presenting successive phases; or otherwise, as a series of changes. And it
should be observed, as a fact in harmony with this conception, that the
higher the life the more conspicuous the variations. On comparing inferior
with superior organisms, these last will be seen to display more rapid
changes, or a more lengthened series of them, or both.
On contemplating afresh our two typical phenomena, we may see that vital
change is further distinguished from non-vital change, by being made up of
many _simultaneous_ changes. Nutrition is not simply a series of actions,
but includes many actions going on together. During mastication the stomach
is busy with food already swallowed, on which it is pouring out solvent
fluids and expending muscular efforts. While the stomach is still active,
the intestines are performing their secretive, contractile, and absorbent
functions; and at the same time that one meal is being digested, the
nutriment obtained from a previous meal is undergoing transformation into
tissue. So too is it, in a certain sense, with mental changes. Though the
states of consciousness which make up an argument occur in series, yet, as
each of them is complex, a number of simultaneous changes have taken place
in establishing it. Here as before, however, it must be admitted that the
distinction between animate and inanimate is not precise. No mass of dead
matter can have its temperature altered, without at the same time
undergoing an alteration in bulk, and sometimes also in hygrometric state.
An inorganic body cannot be compressed, without being at the same time
changed in form, atomic arrangement, temperature, and electric condition.
And in a vast and mobile aggregate like the sea, the simultaneous as well
as the successive changes outnumber those going on in an animal.
Nevertheless, speaking generally, a living thing is distinguished from a
dead thing by the multiplicity of the changes at any moment taking place in
it. Moreover, by this peculiarity, as by the previous one, not only is the
vital more or less clearly marked off from the non-vital; but creatures
possessing high vitality are marked off from those possessing low vitality.
It needs but to contrast the many organs cooperating in a mammal, with the
few in a polype, to see that the actions which are progressing together in
the body of the first, as much exceed in number the actions progressing
together in the body of the last, as these do those in a stone. As at
present conceived, then, Life consists of simultaneous and successive
changes.
Continuance of the comparison shows that vital changes, both visceral and
cerebral, differ from other changes in their _heterogeneity_. Neither the
simultaneous acts nor the serial acts, which together constitute the
process of digestion, are alike. The states of consciousness comprised in
any ratiocination are not repetitions one of another, either in composition
or in modes of dependence. Inorganic processes, on the other hand, even
when like organic ones in the number of the simultaneous and successive
changes they involve, are unlike them in the relative homogeneity of these
changes. In the case of the sea, just referred to, it is observable that
countless as are the actions at any moment going on, they are mostly
mechanical actions that are to a great degree similar; and in this respect
differ widely from the actions at any moment taking place in an organism.
Even where life is nearly simulated, as by the working of a steam-engine,
we see that considerable as is the number of simultaneous changes, and
rapid as are the successive ones, the regularity with which they soon recur
in the same order and degree, renders them unlike those varied changes
exhibited by a living creature. Still, this peculiarity, like the
foregoing ones, does not divide the two classes of changes with precision;
since there are inanimate things presenting considerable heterogeneity of
change: for instance, a cloud. The variations of state which this
undergoes, both simultaneous and successive, are many and quick; and they
differ widely from one another both in quality and quantity. At the same
instant there may occur change of position, change of form, change of size,
change of density, change of colour, change of temperature, change of
electric state; and these several kinds of change are continuously
displayed in different degrees and combinations. Yet when we observe that
very few inorganic objects manifest heterogeneity of change comparable to
that manifested by organic objects, and further, that in ascending from low
to high forms of life, we meet with an increasing variety in the kinds of
changes displayed; we see that there is here a further leading distinction
between vital and non-vital actions. According to this modified conception,
then, Life is made up of heterogeneous changes both simultaneous and
successive.
If, now, we look for some trait common to the nutritive and logical
processes, by which they are distinguished from those inorganic processes
that are most like them in the heterogeneity of the simultaneous and
successive changes they comprise, we discover that they are distinguished
by the _combination_ among their constituent changes. The acts which make
up digestion are mutually dependent. Those composing a train of reasoning
are in close connection. And, generally, it is to be remarked of vital
changes, that each is made possible by all, and all are affected by each.
Respiration, circulation, absorption, secretion, in their many
sub-divisions, are bound up together. Muscular contraction involves
chemical change, change of temperature, and change in the excretions.
Active thought influences the operations of the stomach, of the heart, of
the kidneys. But we miss this union among non-vital activities. Life-like
as may seem the action of a volcano in respect of the heterogeneity of its
many simultaneous and successive changes, it is not life-like in respect of
their combination. Though the chemical, mechanical, thermal, and electric
phenomena exhibited have some inter-dependence, yet the emissions of
stones, mud, lava, flame, ashes, smoke, steam, take place irregularly in
quantity, order, intervals, and mode of conjunction. Even here, however,
it cannot be said that inanimate things present no parallels to animate
ones. A glacier may be instanced as showing nearly as much combination in
its change as a plant of the lowest organization. It is ever growing and
ever decaying; and the rates of its composition and decomposition preserve
a tolerably constant ratio. It moves; and its motion is in immediate
dependence on its thawing. It emits a torrent of water, which, in common
with its motion, undergoes annual variations as plants do. During part of
the year the surface melts and freezes alternately; and on these changes
depend the variations in movement, and in efflux of water. Thus we have
growth, decay, changes of temperature, changes of consistence, changes of
velocity, changes of excretion, all going on in connexion; and it may be as
truly said of a glacier as of an animal, that by ceaseless integration and
disintegration it gradually undergoes an entire change of substance without
losing its individuality. This exceptional instance, however, will scarcely
be held to obscure that broad distinction from inorganic processes which
organic processes derive from the combination among their constituent
changes. And the reality of this distinction becomes yet more manifest when
we find that, in common with previous ones, it not only marks off the
living from the not-living, but also things which live little from things
which live much. For while the changes going on in a plant or a zoophyte
are so imperfectly combined that they can continue after it has been
divided into two or more pieces, the combination among the changes going on
in a mammal is so close that no part cut off from the rest can live, and
any considerable disturbance of one chief function causes a cessation of
the others. Hence, as we now regard it, Life is a combination of
heterogeneous changes, both simultaneous and successive.
When we once more look for a character common to these two kinds of vital
action, we perceive that the combinations of heterogeneous changes which
constitute them, differ from the few combinations which they otherwise
resemble, in respect of _definiteness_. The associated changes going on in
a glacier, admit of indefinite variation. Under a conceivable alteration of
climate, its thawing and its progression may be stopped for a million
years, without disabling it from again displaying these phenomena under
appropriate conditions. By a geological convulsion, its motion may be
arrested without an arrest of its thawing; or by an increase in the
inclination of the surface it slides over, its motion may be accelerated
without accelerating its rate of dissolution. Other things remaining the
same, a more rapid deposit of snow may cause great increase of bulk; or,
conversely, the accretion may entirely cease, and yet all the other actions
continue until the mass disappears. Here, then, the combination has none of
that definiteness which, in a plant, marks the mutual dependence of
respiration, assimilation, and circulation; much less has it that
definiteness seen in the mutual dependence of the chief animal functions;
no one of which can be varied without varying the rest; no one of which can
go on unless the rest go on. Moreover, this definiteness of combination
distinguishes the changes occurring in a living body from those occurring
in a dead one. Decomposition exhibits both simultaneous and successive
changes, which are to some extent heterogeneous, and in a sense combined;
but they are not combined in a definite manner. They vary according as the
surrounding medium is air, water, or earth. They alter in nature with the
temperature. If the local conditions are unlike, they progress differently
in different parts of the mass, without mutual influence. They may end in
producing gases, or adipocire, or the dry substance of which mummies
consist. They may occupy a few days or thousands of years. Thus, neither in
their simultaneous nor in their successive changes, do dead bodies display
that definiteness of combination which characterizes living ones. It is
true that in some inferior creatures the cycle of successive changes admits
of a certain indefiniteness--that it may be suspended for a long period by
desiccation or freezing, and may afterwards go on as though there had been
no breach in its continuity. But the circumstance that only a low order of
life can have its changes thus modified, serves but to suggest that, like
the previous characteristics, this characteristic of definiteness in its
combined changes, distinguishes high vitality from low vitality, as it
distinguishes low vitality from inorganic processes. Hence, our formula as
further amended reads thus:--Life is a definite combination of heterogenous
changes, both simultaneous and successive.
Finally, we shall still better express the facts if, instead of saying _a_
definite combination of heterogeneous changes, we say _the_ definite
combination of heterogeneous changes. As it at present stands, the
definition is defective both in allowing that there may be _other_ definite
combinations of heterogeneous changes, and in directing attention to the
heterogeneous changes rather than to the definiteness of their combination.
Just as it is not so much its chemical elements which constitute an
organism, as it is the arrangement of them into special tissues and organs;
so it is not so much its heterogeneous changes which constitute Life, as it
is the co-ordination of them. Observe what it is that ceases when life
ceases. In a dead body there are going on heterogeneous changes, both
simultaneous and successive. What then has disappeared? The definite
combination has disappeared. Mark, too, that however heterogeneous the
simultaneous and successive changes exhibited by such an inorganic object
as a volcano, we much less tend to think of it as living than we do a watch
or a steam-engine, which, though displaying changes that, serially
contemplated, are largely homogeneous, displays them definitely combined.
So dominant an element is this in our idea of Life, that even when an
object is motionless, yet, if its parts be definitely combined, we conclude
either that it has had life, or has been made by something having life.
Thus, then, we conclude that Life is--_the_ definite combination of
heterogeneous changes, both simultaneous and successive.
§ 26. Such is the conception at which we arrive without changing our
stand-point. It is, however, an incomplete conception. This ultimate
formula (which is to a considerable extent identical with one above
given--"the co-ordination of actions;" seeing that "definite combination"
is synonymous with "co-ordination," and "changes both simultaneous and
successive" are comprehended under the term "actions;" but which differs
from it in specifying the fact, that the actions or changes are
"heterogeneous")--this ultimate formula, I say, is after all but a rude
approximation. It is true that it does not fail by including the growth of
a crystal; for the successive changes this implies cannot be called
heterogeneous. It is true that the action of a galvanic battery is not
comprised in it; since here, too, heterogeneity is not exhibited by the
successive changes. It is true that by this same qualification the motions
of the Solar System are excluded, as are also those of a watch and a
steam-engine. It is true, moreover, that while, in virtue of their
heterogeneity, the actions going on in a cloud, in a volcano, in a glacier,
fulfil the definition; they fall short of it in lacking definiteness of
combination. It is further true that this definiteness of combination
distinguishes the changes taking place in an organism during life from
those which commence at death. And beyond all this it is true that, as well
as serving to mark off, more or less clearly, organic actions from
inorganic actions, each member of the definition serves to mark off the
actions constituting high vitality from those constituting low vitality;
seeing that life is high in proportion to the number of successive changes
occurring between birth and death; in proportion to the number of
simultaneous changes; in proportion to the heterogeneity of the changes; in
proportion to the combination subsisting among the changes; and in
proportion to the definiteness of their combination. Nevertheless,
answering though it does to so many requirements, this definition is
essentially defective. _The definite combination of heterogeneous changes,
both simultaneous and successive_, is a formula which fails to call up an
adequate conception. And it fails from omitting the most distinctive
peculiarity--the peculiarity of which we have the most familiar experience,
and with which our notion of Life is, more than with any other, associated.
It remains now to supplement the conception by the addition of this
peculiarity.
CHAPTER V.
THE CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES.
§ 27. We habitually distinguish between a live object and a dead one, by
observing whether a change which we make in the surrounding conditions, or
one which Nature makes in them, is or is not followed by some perceptible
change in the object. By discovering that certain things shrink when
touched, or fly away when approached, or start when a noise is made, the
child first roughly discriminates between the living and the not-living;
and the man when in doubt whether an animal he is looking at is dead or
not, stirs it with his stick; or if it be at a distance, shouts, or throws
a stone at it. Vegetal and animal life are alike primarily recognized by
this process. The tree that puts out leaves when the spring brings increase
of temperature, the flower which opens and closes with the rising and
setting of the sun, the plant that droops when the soil is dry and
re-erects itself when watered, are considered alive because of these
induced changes; in common with the acorn-shell which contracts when a
shadow suddenly falls on it, the worm that comes to the surface when the
ground is continuously shaken, and the hedgehog that rolls itself up when
attacked.
Not only, however, do we look for some response when an external stimulus
is applied to a living organism, but we expect a fitness in the response.
Dead as well as living things display changes under certain changes of
condition: instance, a lump of carbonate of soda that effervesces when
dropped into sulphuric acid; a cord that contracts when wetted; a piece of
bread that turns brown when held near the fire. But in these cases, we do
not see a connexion between the changes undergone and the preservation of
the things that undergo them; or, to avoid any teleological
implication--the changes have no apparent relations to future events which
are sure or likely to take place. In vital changes, however, such relations
are manifest. Light being necessary to vegetal life, we see in the action
of a plant which, when much shaded, grows towards the unshaded side, an
appropriateness which we should not see did it grow otherwise. Evidently
the proceedings of a spider which rushes out when its web is gently shaken
and stays within when the shaking is violent, conduce better to the
obtainment of food and the avoidance of danger than were they reversed. The
fact that we feel surprise when, as in the case of a bird fascinated by a
snake, the conduct tends towards self-destruction, at once shows how
generally we have observed an adaptation of living changes to changes in
surrounding circumstances.
A kindred truth, rendered so familiar by infinite repetition that we forget
its significance, must be named. There is invariably, and necessarily, a
conformity between the vital functions of any organism and the conditions
in which it is placed--between the processes going on inside of it and the
processes going on outside of it. We know that a fish cannot live long in
air, or a man under water. An oak growing in the ocean and a seaweed on the
top of a hill, are incredible combinations of ideas. We find that each kind
of animal is limited to a certain range of climate; each kind of plant to
certain zones of latitude and elevation. Of the marine flora and fauna,
each species is found only between such and such depths. Some blind
creatures flourish in dark caves; the limpet where it is alternately
covered and uncovered by the tide; the red-snow alga rarely elsewhere than
in the arctic regions or among alpine peaks.
Grouping together the cases first named, in which a particular change in
the circumstances of an organism is followed by a particular change in it,
and the cases last named, in which the constant actions occurring within an
organism imply some constant actions occurring without it; we see that in
both, the changes or processes displayed by a living body are specially
related to the changes or processes in its environment. And here we have
the needful supplement to our conception of Life. Adding this all-important
characteristic, our conception of Life becomes--The definite combination of
heterogeneous changes, both simultaneous and successive, _in correspondence
with external co-existences and sequences_. That the full significance of
this addition may be seen, it will be necessary to glance at the
correspondence under some of its leading aspects.[17]
§ 28. Neglecting minor requirements, the actions going on in a plant
pre-suppose a surrounding medium containing at least carbonic acid and
water, together with a due supply of light and a certain temperature.
Within the leaves carbon is being appropriated and oxygen given off;
without them, is the gas from which the carbon is taken, and the
imponderable agents that aid the abstraction. Be the nature of the process
what it may, it is clear that there are external elements prone to undergo
special re-arrangements under special conditions. It is clear that the
plant in sunshine presents these conditions and so effects these
re-arrangements. And thus it is clear that the changes which primarily
constitute the plant's life, are in correspondence with co-existences in
its environment.
If, again, we ask respecting the lowest protozoon how it lives; the answer
is, that while on the one hand its substance is undergoing disintegration,
it is on the other hand absorbing nutriment; and that it may continue to
exist, the one process must keep pace with, or exceed, the other. If
further we ask under what circumstances these combined changes are
possible, there is the reply that the medium in which the protozoon is
placed, must contain oxygen and food--oxygen in such quantity as to produce
some disintegration; food in such quantity as to permit that disintegration
to be made good. In other words--the two antagonistic processes taking
place internally, imply the presence externally of materials having
affinities that can give rise to them.
Leaving those lowest animal forms which simply take in through their
surfaces the nutriment and oxygenated fluids coming in contact with them,
we pass to those somewhat higher forms which have their tissues slightly
specialized. In these we see a correspondence between certain actions in
the digestive sac, and the properties of certain surrounding bodies. That a
creature of this order may continue to live, it is necessary not only that
there be masses of substance in the environment capable of transformation
into its own tissue, but also that the introduction of these masses into
its stomach, shall be followed by the secretion of a solvent fluid which
will reduce them to a fit state for absorption. Special outer properties
must be met by special inner properties.
When, from the process by which food is digested, we turn to the process by
which it is seized, the same general truth faces us. The stinging and
contractile power of a polype's tentacle, correspond to the sensitiveness
and strength of the creatures serving it for prey. Unless that external
change which brings one of these creatures in contact with the tentacle,
were quickly followed by those internal changes which result in the coiling
and drawing up of the tentacle, the polype would die of inanition. The
fundamental processes of integration and disintegration within it, would
get out of correspondence with the agencies and processes without it, and
the life would cease.
Similarly, when the creature becomes so large that its tissue cannot be
efficiently supplied with nutriment by mere absorption through its lining
membrane, or duly oxygenated by contact with the fluid bathing its surface,
there arises a need for a distributing system by which nutriment and oxygen
may be carried throughout the mass; and the functions of this system, being
subsidiary to the two primary functions, form links in the correspondence
between internal and external actions. The like is obviously true of all
those subordinate functions, secretory and excretory, that facilitate
oxidation and assimilation.
Ascending from visceral actions to muscular and nervous actions, we find
the correspondence displayed in a manner still more obvious. Every act of
locomotion implies the expenditure of certain internal forces, adapted in
amounts and directions to balance or out-balance certain external forces.
The recognition of an object is impossible without a harmony between the
changes constituting perception, and particular properties co-existing in
the environment. Escape from enemies implies motions within the organism,
related in kind and rapidity to motions without it. Destruction of prey
requires a special combination of subjective actions, fitted in degree and
succession to overcome a group of objective ones. And so with those
countless automatic processes constituting instincts.
In the highest order of vital changes the same fact is equally manifest.
The empirical generalization that guides the farmer in his rotation of
crops, serves to bring his actions into concord with certain of the actions
going on in plants and soil. The rational deductions of the educated
navigator who calculates his position at sea, form a series of mental acts
by which his proceedings are conformed to surrounding circumstances. Alike
in the simplest inferences of the child and the most complex ones of the
man of science, we find a correspondence between simultaneous and
successive changes in the organism, and co-existences and sequences in its
environment.
§ 29. This general formula which thus includes the lowest vegetal processes
along with the highest manifestations of human intelligence, will perhaps
call forth some criticisms which it is desirable here to meet.
It may be thought that there are still a few inorganic actions included in
the definition; as, for example, that displayed by the mis-named
storm-glass. The feathery crystallization which, on a certain change of
temperature, takes place in its contained solution, and which afterwards
dissolves to reappear in new forms under new conditions, may be held to
present simultaneous and successive changes that are to some extent
heterogeneous, that occur with some definiteness of combination, and, above
all, occur in apparent correspondence with external changes. In this case
vegetal life is simulated to a considerable extent; but it is _merely_
simulated. The relation between the phenomena occurring in the storm-glass
and in the atmosphere respectively, is not a correspondence at all, in the
proper sense of the word. Outside there is a thermal change; inside there
is a change of atomic arrangement. Outside there is another thermal change;
inside there is another change of atomic arrangement. But subtle as is the
dependence of each internal upon each external change, the connexion
between them does not, in the abstract, differ from the connexion between
the motion of a straw and the motion of the wind that disturbs it. In
either case a change produces a change, and there it ends. The alteration
wrought by some environing agency on this or any other inanimate object,
does not tend to induce in it a secondary alteration which anticipates some
secondary alteration in the environment. But in every living body there is
a tendency towards secondary alterations of this nature; and it is in their
production that the correspondence consists. The difference may be best
expressed by symbols. Let A be a change in the environment, and B some
resulting change in an inorganic mass. Then A having produced B, the action
ceases. Though the change A in the environment is followed by some
consequent change _a_ in it; no parallel sequence in the inorganic mass
simultaneously generates in it some change _b_ that has reference to the
change _a_. But if we take a living body of the requisite organization, and
let the change A impress on it some change C; then, while in the
environment A is occasioning _a_, in the living body C will be occasioning
_c_; of which _a_ and _c_ will show a certain concord in time, place, or
intensity. And while it is _in_ the continuous production of such concords
or correspondences that Life consists, it is _by_ the continuous production
of them that Life is maintained.
The further criticism to be expected concerns certain verbal imperfections
in the definition, which it seems impossible to avoid. It may fairly be
urged that the word _correspondence_ will not include, without straining,
the various relations to be expressed by it. It may be asked:--How can the
continuous _processes_ of assimilation and respiration correspond with the
_co-existence_ of food and oxygen in the environment? or again:--How can
the act of secreting some defensive fluid correspond with some external
danger which may never occur? or again:--How can the _dynamical_ phenomena
constituting perception correspond with the _statical_ phenomena of the
solid body perceived? The only reply is, that we have no word sufficiently
general to comprehend all forms of this relation between the organism and
its medium, and yet sufficiently specific to convey an adequate idea of the
relation; and that the word _correspondence_ seems the least objectionable.
The fact to be expressed in all cases is that certain changes, continuous
or discontinuous, in the organism, are connected after such a manner that
in their amounts, or variations, or periods of occurrence, or modes of
succession, they have a reference to external actions, constant or serial,
actual or potential--a reference such that a definite relation among any
members of the one group, implies a definite relation among certain members
of the other group.
§ 30. The presentation of the phenomena under this general form, suggests
that our conception of Life may be reduced to its most abstract shape by
regarding its elements as relations only. If a creature's rate of
assimilation is increased in consequence of a decrease of temperature in
the environment, it is that the relation between the food consumed and the
heat produced, is so re-adjusted by multiplying both its members, that the
altered relation in the environment between the quantity of heat absorbed
from, and radiated to, bodies of a given temperature, is counterbalanced.
If a sound or a scent wafted to it on the breeze prompts the stag to dart
away from the deer-stalker, it is that there exists in its neighbourhood a
relation between a certain sensible property and certain actions dangerous
to the stag, while in its body there exists an adapted relation between the
impression this sensible property produces, and the actions by which danger
may be escaped. If inquiry has led the chemist to a law, enabling him to
tell how much of any one element will combine with so much of another, it
is that there has been established in him specific mental relations, which
accord with specific chemical relations in the things around. Seeing, then,
that in all cases we may consider the external phenomena as simply in
relation, and the internal phenomena also as simply in relation; our
conception of Life under its most abstract aspect will be--_The continuous
adjustment of internal relations to external relations_.[18]
While it is simpler, this formula has the further advantage of being
somewhat more comprehensive. To say that it includes not only those
definite combinations of simultaneous and successive changes in an
organism, which correspond to co-existences and sequences in the
environment, but also those structural arrangements which _enable_ the
organism to adapt its actions to actions in the environment, is going too
far; for though these structural arrangements present internal relations
adjusted to external relations, yet the _continuous adjustment_ of
relations cannot be held to include a _fixed adjustment_ already made.
Life, which is made up of _dynamical_ phenomena, cannot be described in
terms that shall at the same time describe the apparatus manifesting it,
which presents only _statical_ phenomena. But while this antithesis serves
to remind us that the distinction between the organism and its actions is
as wide as that between Matter and Motion, it at the same time draws
attention to the fact that, if the structural arrangements of the adult are
not properly included in the definition, yet the developmental processes by
which those arrangements were established, are included. For that process
of evolution during which the organs of the embryo are fitted to their
prospective functions, is the gradual or continuous adjustment of internal
relations to external relations. Moreover, those structural modifications
of the adult organism which, under change of climate, change of occupation,
change of food, bring about some re-arrangement in the organic balance, may
similarly be regarded as progressive or continuous adjustments of internal
relations to external relations. So that not only does the definition, as
thus expressed, comprehend all those activities, bodily and mental, which
constitute our ordinary idea of Life; but it also comprehends both those
processes of development by which the organism is brought into general
fitness for such activities, and those after-processes of adaptation by
which it is specially fitted to its special activities.
Nevertheless, so abstract a formula as this is scarcely fitted for our
present purpose. Reserving it for use where specially appropriate, it will
be best commonly to employ its more concrete equivalent--to consider the
internal relations as "definite combinations of simultaneous and successive
changes;" the external relations as "co-existences and sequences;" and the
connexion between them as a "correspondence."
CHAPTER VI.
THE DEGREE OF LIFE VARIES AS THE DEGREE OF CORRESPONDENCE.
§ 31. Already it has been shown respecting each other component of the
foregoing definition, that the life is high in proportion as that component
is conspicuous; and it is now to be remarked, that the same thing is
especially true respecting this last component--the correspondence between
internal and external relations. It is manifest, _a priori_, that since
changes in the physical state of the environment, as also of those
mechanical actions and those variations of available food which occur in
it, are liable to stop the processes going on in the organism; and since
the adaptive changes in the organism have the effects of directly or
indirectly counter-balancing these changes in the environment; it follows
that the life of the organism will be short or long, low or high, according
to the extent to which changes in the environment are met by corresponding
changes in the organism. Allowing a margin for perturbations, the life will
continue only while the correspondence continues; the completeness of the
life will be proportionate to the completeness of the correspondence; and
the life will be perfect only when the correspondence is perfect. Not to
dwell in general statements, however, let us contemplate this truth under
its concrete aspects.
§ 32. In life of the lowest order we find that only the most prevalent
co-existences and sequences in the environment, have any simultaneous and
successive changes answering to them in the organism. A plant's vital
processes display adjustment solely to the continuous co-existence of
certain elements and forces surrounding its roots and leaves; and vary only
with the variations produced in these elements and forces by the Sun--are
unaffected by the countless mechanical movements and contacts occurring
around; save when accidentally arrested by these. The life of a worm is
made up of actions referring to little else than the tangible properties of
adjacent things. All those visible and audible changes which happen near
it, and are connected with other changes that may presently destroy it,
pass unrecognized--produce in it no adapted changes: its only adjustment of
internal relations to external relations of this order, being seen when it
escapes to the surface on feeling the vibrations produced by an approaching
mole. Adjusted as are the proceedings of a bird to a far greater number of
co-existences and sequences in the environment, cognizable by sight,
hearing, scent, and their combinations: and numerous as are the dangers it
shuns and the needs it fulfils in virtue of this extensive correspondence;
it exhibits no such actions as those by which a human being counterbalances
variations in temperature and supply of food, consequent on the seasons.
And when we see the plant eaten, the worm trodden on, the bird dead from
starvation; we see alike that the death is an arrest of such correspondence
as existed, that it occurred when there was some change in the environment
to which the organism made no answering change, and that thus, both in
shortness and simplicity, the life was incomplete in proportion as the
correspondence was incomplete. Progress towards more prolonged and higher
life, evidently implies ability to respond to less general co-existences
and sequences. Each step upwards must consist in adding to the
previously-adjusted relations of actions or structures which the organism
exhibits, some further relation parallel to a further relation in the
environment. And the greater correspondence thus established, must, other
things equal, show itself both in greater complexity of life, and greater
length of life: a truth which will be fully perceived on remembering the
enormous mortality which prevails among lowly-organized creatures, and the
gradual increase of longevity and diminution of fertility which we meet
with on ascending to creatures of higher and higher developments.
It must be remarked, however, that while length and complexity of life are,
to a great extent, associated--while a more extended correspondence in the
successive changes commonly implies increased correspondence in the
simultaneous changes; yet it is not uniformly so. Between the two great
divisions of life--animal and vegetal--this contrast by no means holds. A
tree may live a thousand years, though the simultaneous changes going on in
it answer only to the few chemical affinities in the air and the earth, and
though its serial changes answer only to those of day and night, of the
weather and the seasons. A tortoise, which exhibits in a given time nothing
like the number of internal actions adjusted to external ones that are
exhibited by a dog, yet lives far longer. The tree by its massive trunk and
the tortoise by its hard carapace, are saved the necessity of responding to
those many surrounding mechanical actions which organisms not thus
protected must respond to or die; or rather--the tree and the tortoise
display in their structures, certain simple statical relations adapted to
meet countless dynamical relations external to them. But notwithstanding
the qualifications suggested by such cases, it needs but to compare a
microscopic fungus with an oak, an animalcule with a shark, a mouse with a
man, to recognize the fact that this increasing correspondence of its
changes with those of the environment which characterizes progressing life,
habitually shows itself at the same time in continuity and in complication.
Even were not the connexion between length of life and complexity of life
thus conspicuous, it would still be true that the life is great in
proportion as the correspondence is great. For if the lengthened existence
of a tree be looked upon as tantamount to a considerable amount of life;
then it must be admitted that its lengthened display of correspondence is
tantamount to a considerable amount of correspondence. If, otherwise, it be
held that notwithstanding its much shorter existence, a dog must rank above
a tortoise in degree of life because of its superior activity; then it is
implied that its life is higher because its simultaneous and successive
changes are more complex and more rapid--because the correspondence is
greater. And since we regard as the highest life that which, like our own,
shows great complexity in the correspondences, great rapidity in the
succession of them, and great length in the series of them; the equivalence
between degree of life and degree of correspondence is unquestionable.
§ 33. In further elucidation of this general truth, and especially in
explanation of the irregularities just referred to, it must be pointed out
that as the life becomes higher the environment itself becomes more
complex. Though, literally, the environment means all surrounding space
with the co-existences and sequences contained in it: yet, practically, it
often means but a small part of this. The environment of an entozoon can
scarcely be said to extend beyond the body of the animal in which the
entozoon lives. That of a freshwater alga is virtually limited to the ditch
inhabited by the alga. And, understanding the term in this restricted
sense, we shall see that the superior organisms inhabit the more
complicated environments.
Thus, contrasted with the life found on land, the lower life is that found
in the sea; and it has the simpler environment. Marine creatures are
affected by fewer co-existences and sequences than terrestrial ones. Being
very nearly of the same specific gravity as the surrounding medium, they
have to contend with less various mechanical actions. The sea-anemone fixed
to a stone, and the acalephe borne along in the current, need to undergo no
internal changes such as those by which the caterpillar meets the varying
effects of gravitation, while creeping over and under the leaves. Again,
the sea is liable to none of those extreme and rapid alterations of
temperature which the air suffers. Night and day produce no appreciable
modifications in it; and it is comparatively little affected by the
seasons. Thus its contained fauna show no marked correspondences similar to
those by which air-breathing creatures counterbalance thermal changes.
Further, in respect to the supply of nutriment, the conditions are more
simple. The lower tribes of animals inhabiting the water, like the plants
inhabiting the air, have their food brought to them. The same current which
brings oxygen to the oyster, also brings it the microscopic organisms on
which it lives: the disintegrating matter and the matter to be integrated,
co-exist under the simplest relation. It is otherwise with land animals.
The oxygen is everywhere, but the sustenance is not everywhere: it has to
be sought; and the conditions under which it is to be obtained are more or
less complex. So too with that liquid by the agency of which the vital
processes are carried on. To marine creatures water is ever present, and by
the lowest is passively absorbed; but to most creatures living on the earth
and in the air, it is made available only through those nervous changes
constituting perception, and those muscular ones by which drinking is
effected. Similarly, after tracing upwards from the _Amphibia_ the widening
extent and complexity which the environment, as practically considered,
assumes--after observing further how increasing heterogeneity in the flora
and fauna of the globe, itself progressively complicates the environment of
each species of organism--it might finally be shown that the same general
truth is displayed in the history of mankind, who, in the course of their
progress, have been adding to their physical environment a social
environment that has been growing ever more involved. Thus, speaking
generally, it is clear that those relations in the environment to which
relations in the organism must correspond, themselves increase in number
and intricacy as the life assumes a higher form.
§ 34. To make yet more manifest the fact that the degree of life varies as
the degree of correspondence, let me here point out, that those other
distinctions successively noted when contrasting vital changes with
non-vital changes, are all implied in this last distinction--their
correspondence with external co-existences and sequences; and further, that
the increasing fulfilment of those other distinctions which we found to
accompany increasing life, is involved in the increasing fulfilment of this
last distinction. We saw that living organisms are characterized by
successive changes, and that as the life becomes higher, the successive
changes become more numerous. Well, the environment is full of successive
changes, and the greater the correspondence, the greater must be the number
of successive changes in the organism. We saw that life presents
simultaneous changes, and that the more elevated it is, the more marked the
multiplicity of them. Well, besides countless co-existences in the
environment, there are often many changes occurring in it at the same
moment; and hence increased correspondence with it implies in the organism
an increased display of simultaneous changes. Similarly with the
heterogeneity of the changes. In the environment the relations are very
varied in their kinds, and hence, as the organic actions come more and more
into correspondence with them, they too must become very varied in their
kinds. So again is it even with definiteness of combination. As the most
important surrounding changes with which each animal has to deal, are the
definitely-combined changes exhibited by other animals, whether prey or
enemies, it results that definiteness of combination must be a general
characteristic of the internal ones which have to correspond with them. So
that throughout, the correspondence of the internal relations with the
external ones is the essential thing; and all the special characteristics
of the internal relations, are but the collateral results of this
correspondence.
§§ 35, 36. Before closing the chapter, it will be useful to compare the
definition of Life here set forth, with the definition of Evolution set
forth in _First Principles_. Living bodies being bodies which display in
the highest degree the structural changes constituting Evolution; and Life
being made up of the functional changes accompanying these structural
changes; we ought to find a certain harmony between the definitions of
Evolution and of Life. Such a harmony is not wanting.
The first distinction we noted between the kind of change shown in Life,
and other kinds of change, was its serial character. We saw that vital
change is substantially unlike non-vital change, in being made up of
_successive_ changes. Now since organic bodies display so much more than
inorganic bodies those continuous differentiations and integrations which
constitute Evolution; and since the re-distributions of matter thus carried
so far in a comparatively short period, imply concomitant re-distributions
of motion; it is clear that in a given time, organic bodies must undergo
changes so comparatively numerous as to render the successiveness of their
changes a marked characteristic. And it will follow _a priori_, as we found
it to do _a posteriori_, that the organisms exhibiting Evolution in the
highest degree, exhibit the longest or the most rapid successions of
changes, or both. Again, it was shown that vital change is distinguished
from non-vital change by being made up of many _simultaneous_ changes; and
also that creatures possessing high vitality are marked off from those
possessing low vitality, by the far greater number of their simultaneous
changes. Here, too, there is entire congruity. In _First Principles_,
§ 156, we reached the conclusion that a force falling on any aggregate is
divided into several forces; that when the aggregate consists of parts that
are unlike, each part becomes a centre of unlike differentiations of the
incident force; and that thus the multiplicity of such differentiations
must increase with the multiplicity of the unlike parts. Consequently
organic aggregates, which as a class are distinguished from inorganic
aggregates by the greater number of their unlike parts, must be also
distinguished from them by the greater number of simultaneous changes they
display; and, further, that the higher organic aggregates, having more
numerous unlike parts than the lower, must undergo more numerous
simultaneous changes. We next found that the changes occurring in living
bodies are contrasted with those occurring in other bodies, as being much
more _heterogeneous_; and that the changes occurring in the superior living
bodies are similarly contrasted with those occurring in inferior ones.
Well, heterogeneity of function is the correlate of heterogeneity of
structure; and heterogeneity of structure is the leading distinction
between organic and inorganic aggregates, as well as between the more
highly organized and the more lowly organized. By reaction, an incident
force must be rendered multiform in proportion to the multiformity of the
aggregate on which it falls; and hence those most multi-form aggregates
which display in the highest degree the phenomena of Evolution structurally
considered, must also display in the highest degree the multiform actions
which constitute Evolution functionally considered. These heterogeneous
changes, exhibited simultaneously and in succession by a living organism,
prove, on further inquiry, to be distinguished by their _combination_ from
certain non-vital changes which simulate them. Here, too, the parallelism
is maintained. It was shown in _First Principles_, Chap. XIV, that an
essential characteristic of Evolution is the integration of parts, which
accompanies their differentiation--an integration shown both in the
consolidation of each part, and in the union of all the parts into a whole.
Hence, animate bodies having greater co-ordination of parts than inanimate
ones must exhibit greater co-ordination of changes; and this greater
co-ordination of their changes must not only distinguish organic from
inorganic aggregates, but must, for the same reason, distinguish higher
organisms from lower ones, as we found that it did. Once more, it was
pointed out that the changes constituting Life differ from other changes in
the _definiteness_ of their combination, and that a distinction like in
kind though less in degree, holds between the vital changes of superior
creatures and those of inferior creatures. These, also, are contrasts in
harmony with the contrasts disclosed by the analysis of Evolution. We saw
(_First Principles_, §§ 129-137) that during Evolution there is an increase
of definiteness as well as an increase of heterogeneity. We saw that the
integration accompanying differentiation has necessarily the effect of
increasing the distinctness with which the parts are marked off from each
other, and that so, out of the incoherent and indefinite there arises the
coherent and definite. But a coherent whole made up of definite parts
definitely combined, must exhibit more definitely combined changes than a
whole made up of parts that are neither definite in themselves nor in their
combination. Hence, if living bodies display more than other bodies this
structural definiteness, then definiteness of combination must be a
characteristic of the changes constituting Life, and must also distinguish
the vital changes of higher organisms from those of lower organisms.
Finally, we discovered that all these peculiarities are subordinate to the
fundamental peculiarity, that vital changes take place in correspondence
with external co-existences and sequences, and that the highest Life is
reached, when there is some inner relation of actions fitted to meet every
outer relation of actions by which the organism can be affected. But this
conception of the highest Life, is in harmony with the conception, before
arrived at, of the limit of Evolution. When treating of equilibration as
exhibited in organisms (_First Principles_, §§ 173, 174), it was pointed
out that the tendency is towards the establishment of a balance between
inner and outer changes. It was shown that "the final structural
arrangements must be such as will meet all the forces acting on the
aggregate, by equivalent antagonistic forces," and that "the maintenance of
such a moving equilibrium" as an organism displays, "requires the habitual
genesis of internal forces corresponding in number, directions, and
amounts, to the external incident forces--as many inner functions, single
or combined, as there are single or combined outer actions to be met." It
was shown, too, that the relations among ideas are ever in progress towards
a better adjustment between mental actions and those actions in the
environment to which conduct must be adjusted. So that this continuous
correspondence between inner and outer relations which constitutes Life,
and the perfection of which is the perfection of Life, answers completely
to that state of organic moving equilibrium which we saw arises in the
course of Evolution and tends ever to become more complete.
CHAPTER VI^A.
THE DYNAMIC ELEMENT IN LIFE.
§ 36a. A critical comparison of the foregoing formula with the facts proves
it to be deficient in more ways than one. Let us first look at vital
phenomena which are not covered by it.
Some irritant left by an insect's ovipositor, sets up on a plant the morbid
growth named a gall. The processes in the gall do not correspond with any
external co-existences or sequences relevant to the plant's life--show no
internal relations adjusted to external relations. Yet we cannot deny that
the gall is alive. So, too, is it with a cancer in or upon an animal's
body. The actions going on in it have no reference, direct or indirect, to
actions in the environment. Nevertheless we are obliged to say that they
are vital; since it grows and after a time dies and decomposes.
A kindred lesson meets us when from pathological evidence we turn to
physiological evidence. The functions of some important organs may still be
carried on for a time apart from those of the body as a whole. An excised
liver, kept at a fit temperature and duly supplied with blood, secretes
bile. Still more striking is the independent action of the heart. If
belonging to a cold-blooded animal, as a frog, the heart, when detached,
continues to beat, even until its integuments have become so dry that they
crackle. Now though under such conditions its pulsations, which ordinarily
form an essential part of the linked processes by which the correspondence
between inner and outer actions is maintained, no longer form part of such
processes, we must admit that the continuance of them implies a vital
activity.
Embryological changes force the same truth upon us. What are we to say of
the repeated cell-fissions by which in some types a blastula, or
mulberry-mass, is formed, and in other types a blastoderm? Neither these
processes nor the structures immediately resulting from them, show any
correspondences with co-existences and sequences in the environment; though
they are first steps towards the organization which is to carry on such
correspondences. Even this extremely small fulfilment of the definition is
absent in the cases of rudimentary organs, and especially those rudimentary
organs which after being partly formed are absorbed. No adjustment can be
alleged between the inner relations which these present and any outer
relations. The outer relations they refer to ceased millions of years ago.
Yet unquestionably the changes which bring about the production and
absorption of these futile structures are vital changes.
Take another class of exceptions. What are we to say of a laugh? No
correspondence, or part of a correspondence, by which inner actions are
made to balance outer actions, can be seen in it. Or again, if, while
working, an artisan whistles, the making of the sounds and the
co-ordination of ideas controlling them, cannot be said to exhibit
adjustment between certain relations of thoughts, and certain relations of
things. Such kinds of vital activities lie wholly outside of the definition
given.
But perhaps the clearest and simplest proof is yielded by contrasting
voluntary and involuntary muscular actions. Here is a hawk adapting its
changing motions to the changing motions of a pigeon, so as eventually to
strike it: the adjustment of inner relations to outer relations is
manifest. Here is a boy in an epileptic fit. Between his struggles and the
co-existences and sequences around him there is no correspondence whatever.
Yet his movements betray vitality just as much as do the movements of the
hawk. Both exhibit that principle of _activity_ which constitutes the
essential element in our conception of life.
§ 36b. Evidently, then, the preceding chapters recognize only the _form_ of
our conception of life and ignore the _body_ of it. Partly sufficing as
does the definition reached to express the one, it fails entirely to
express the other. Life displays itself in ways which conform to the
definition; but it also displays itself in many other ways. We are obliged
to admit that the element which is common to the two groups of ways is the
essential element. The essential element, then, is that special kind of
energy seen alike in the usual classes of vital actions and in those
unusual classes instanced above.
Otherwise presenting the contrast, we may say that due attention has been
paid to the connexions among the manifestations, while no attention has
been paid to that which is manifested. When it is said that life is "the
definite correspondence of heterogeneous changes, both simultaneous and
successive, in correspondence with external co-existences and sequences,"
there arises the question--Changes of what? Within the body there go on
many changes, mechanical, chemical, thermal, no one of which is the kind of
change in question; and if we combine in thought so far as we can these
kinds of changes, in such wise that each maintains its character as
mechanical, chemical, or thermal, we cannot get out of them the idea of
Life. Still more clearly do we see this insufficiency when we take the more
abstract definition--"the continuous adjustment of internal relations to
external relations." Relations between what things? is the question then to
be asked. A relation of which the terms are unspecified does not connote a
thought but merely the blank form of a thought. Its value is comparable to
that of a cheque on which no amount is written. If it be said that the
terms cannot be specified because so many heterogeneous kinds of them have
to be included, then there comes the reply that under cover of this
inability to make a specification of terms that shall be adequately
comprehensive, there is concealed the inability to conceive the required
terms in any way.
Thus a critical testing of the definition brings us, in another way, to the
conclusion reached above, that that which gives the substance to our idea
of Life is a certain unspecified principle of activity. The dynamic element
in life is its essential element.
§ 36c. Under what form are we to conceive this dynamic element? Is this
principle of activity inherent in organic matter, or is it something
superadded? Of these alternative suppositions let us begin with the last.
As I have remarked, in another place, the worth of an hypothesis may be
judged from its genealogy; and so judged the hypothesis of an independent
vital principal does not commend itself. Its history carries us back to the
ghost-theory of the savage. Suggested by experiences of dreams, there
arises belief in a double--a second self which wanders away during sleep
and has adventures but comes back on waking; which deserts the body during
abnormal insensibility of one or other kind; and which is absent for a long
period at death, though even then is expected eventually to return. This
indwelling other-self, which can leave the body at will, is by-and-by
regarded as able to enter the bodies of fellow men or of animals; or again,
by implication, as liable to have its place usurped by the intruding
doubles of fellow men, living or dead, which cause fits or other ills.
Along with these developments its quality changes. At first thought of as
quite material it is gradually de-materialized, and in advanced times comes
to be regarded as spirit or breath; as we see in ancient religious books,
where "giving up the ghost" is shown by the emergence of a small floating
figure from the mouth of a dying man. This indwelling second self, more and
more conceived as the real self which uses the body for its purposes, is,
with the advance of intelligence, still further divested of its definite
characters; and, coming in mediæval days to be spoken of as "animal
spirits," ends in later days in being called a vital principle.
Entirely without assignable attributes, this something occurs in thought
not as an idea but as a pseud-idea (_First Principles_, Chap. II). It is
assumed to be representable while really unrepresentable. We need only
insist on answers to certain questions to see that it is simply a name for
an alleged existence which has not been conceived and cannot be conceived.
1. Is there one kind of vital principle for all kinds of organisms, or is
there a separate kind for each? To affirm the first alternative is to say
that there is the same vital principle for a microbe as for a whale, for a
tape-worm as for the person it inhabits, for a protococcus as for an oak;
nay more--is to assert community of vital principle in the thinking man and
the unthinking plant. Moreover, asserting unity of the vital principle for
all organisms, is reducing it to a force having the same unindividualized
character as one of the physical forces. If, on the other hand, different
kinds of organisms have different kinds of vital principles, these must be
in some way distinguished from one another. How distinguished? Manifestly
by attributes. Do they differ in extension? Evidently; since otherwise that
which animates the vast _Sequoia_ can be no larger than that which animates
a yeast-plant, and to carry on the life of an elephant requires a quantity
of vital principle no greater than that required for a microscopic monad.
Do they differ otherwise than in amount? Certainly; since otherwise we
revert to the preceding alternative, which implies that the same quality of
vital principle serves for all organisms, simple and complex: the vital
principle is a uniform force like heat or electricity. Hence, then, we have
to suppose that every species of animal and plant has a vital principle
peculiar to itself--a principle adapted to use the particular set of
structures in which it is contained. But dare anyone assert this
multiplication of vital principles, duplicating not only all existing
plants and animals but all past ones, and amounting in the aggregate to
some millions?
2. How are we to conceive that genesis of a vital principle which must go
along with the genesis of an organism? Here is a pollen-grain which,
through the pistil, sends its nucleus to unite with the nucleus of the
ovule; or here are the nuclei of spermatozoon and ovum, which, becoming
fused, initiate a new animal: in either case failure of union being
followed by decomposition of the proteid materials, while union is followed
by development. Whence comes that vital principle which determines the
organizing process? Is it created afresh for every plant and animal? or, if
not, where and how did it pre-exist? Take a simpler form of this problem. A
protophyte or protozoon, having grown to a certain size, undergoes a series
of complex changes ending in fission. In its undivided state it had a vital
principle. What of its divided state? The parts severally swim away, each
fully alive, each ready to grow and presently to subdivide, and so on and
so on, until millions are soon formed. That is to say, there is a
multiplication of vital principles as of the protozoa animated by them. A
vital principle, then, both divides and grows. But growth implies
incorporation of something. What does the vital principle incorporate? Is
it some other vital principle external to it, or some materials out of
which more vital principle is formed? And how, in either case, can the
vital principle be conceived as other than a material something, which in
its growth and multiplication behaves just as visible matter behaves?
3. Equally unanswerable is the question which arises in presence of life
that has become latent. Passing over the alleged case of the mummy wheat,
the validity of which is denied, there is experimental proof that seeds
may, under conditions unfavourable to germination, retain for ten, twenty,
and some even for thirty years, the power to germinate when due moisture
and warmth are supplied. (_Cf._ Kerner's _Nat. Hist. of Plants_, i, 51-2).
Under what form has the vital principle existed during these long
intervals? It is a principle of activity. In this case, then, the principle
of activity becomes inactive. But how can we conceive an inactive activity?
If it is a something which though inactive may be rendered active when
conditions favour, we are introduced to the idea of a vital principle of
which the vitality may become latent, which is absurd. What shall we say of
the desiccated rotifer which for years has seemed to be nothing more than a
particle of dust, but which now, when water is supplied, absorbs it, swells
up, and resumes those ciliary motions by which it draws in nutriment? Was
the vital principle elsewhere during these years of absolute quiescence? If
so, why did it come back at the right moment? Was it all along present in
the rotifer though asleep? How happened it then to awaken at the time when
the supply of water enabled the tissues to resume their functions? How
happened the physical agent to act not only on the material substance of
the rotifer, but also on this something which is not a material substance
but an immaterial source of activity? Evidently neither alternative is
thinkable.
Thus, the alleged vital principle exists in the minds of those who allege
it only as a verbal form, not as an idea; since it is impossible to bring
together in consciousness the terms required to constitute an idea. It is
not even "a figment of imagination," for that implies something imaginable,
but the supposed vital principle cannot even be imagined.
§ 36d. When, passing to the alternative, we propose to regard life as
inherent in the substances of the organisms displaying it, we meet with
difficulties different in kind but scarcely less in degree. The processes
which go on in living things are incomprehensible as results of any
physical actions known to us.
Consider one of the simplest--that presented by an ordinary vegetal cell
forming part of a leaf or other plant-structure. Its limiting membrane,
originally made polyhedral by pressure of adjacent cells, is gradually
moulded "into one of cylindrical, fibrous, or tabular shape, and
strengthening its walls with pilasters, borders, ridges, hooks, bands, and
panels of various kinds" (Kerner, i, 43): small openings into adjacent
cells being either left or subsequently made. Consisting of
non-nitrogenous, inactive matters, these structures are formed by the
inclosed protoplast. How formed? Is it by the agency of the nucleus? But
the nucleus, even had it characters conceivably adapting it to this
function, is irregularly placed; and that it should work the same effects
upon the cell-wall whether seated in the middle, at one end, or one side,
is incomprehensible. Is the protoplasm then the active agent? But this is
arranged into a network of strands and threads utterly irregular in
distribution and perpetually altering their shapes and connexions. Exercise
of fit directive action by the protoplasm is unimaginable.
Another instance:--Consider the reproductive changes exhibited by the
_Spirogyra_. The delicate threads which, in this low type of Alga, are
constituted of single elongated cells joined end to end, are here and there
adjacent to one another; and from a cell of one thread and a cell of
another at fit distance, grow out prominences which, meeting in the
interspace and forming a channel by the dissolution of their adjoined
cell-walls, empty through it the endochrome of the one cell into the other:
forming by fusion of the two a zygote or reproductive body. Under what
influence is this action initiated and guided? There is no conceivable
directive agency in either cell by which, when conditions are fit, a
papilla is so formed as to meet an opposite papilla.
Or again, contemplate the still more marvellous transformation occurring in
_Hydrodictyon utriculosum_. United with others to form a cylindrical
network, each sausage-shaped cell of this Alga contains, when fully
developed, a lining chromatophore made of nucleated protoplasm with
immersed chlorophyll-grains. This, when the cell is adult, divides into
multitudinous zoospores, which presently join their ends in such ways as to
form a network with meshes mostly hexagonal, minute in size, but like in
arrangement to the network of which the parent cell formed a part.
Eventually escaping from the mother-cell, this network grows and presently
becomes as large as the parent network. Under what play of forces do these
zoospores arrange themselves into this strange structure?
Kindred insoluble problems are presented by animal organisms of all grades.
Of microscopic types instance the Coccospheres and Rhabdospheres found in
the upper strata of sea-water. Each is a fragment of protoplasm less than
one-thousandth of an inch in diameter, shielded by the elaborate protective
structures it has formed. The elliptic coccoliths of the first, severally
having a definite pattern, unite to form by overlapping an imbricated
covering; and of the other the covering consists of numerous
trumpet-mouthed processes radiating on all sides. To the question--How does
this particle of granular protoplasm, without organs or definite structure,
make for itself this complicated calcareous armour? there is no conceivable
answer.
Like these _Protozoa_, the lowest _Metazoa_ do things which are quite
incomprehensible. Here is a sponge formed of classes of monads having among
them no internuncial appliances by which in higher types cooperation is
carried on--flagellate cells that produce the permeating currents of water,
flattened cells forming protective membranes, and amoeboid cells lying free
in the gelatinous mesoderm. These, without apparent concert, build up not
only the horny network constituting the chief mass of their habitation, but
also embodied spicules, having remarkable symmetrical forms. By what
combined influences the needful processes are effected, it is impossible to
imagine.
If we turn to higher types of _Metazoa_ in which, by the agency of a
nervous system, many cooperations of parts are achieved in ways that are
superficially comprehensible, we still meet with various actions of which
the causation cannot be represented in thought. Lacking other calcareous
matter, a hen picks up and swallows bits of broken egg-shells; and,
occasionally, a cow in calf may be seen mumbling a bone she has
found--evidently scraping off with her teeth some of its mass. These
proceedings have reference to constitutional needs; but how are they
prompted? What generates in the cow a desire to bite a substance so unlike
in character to her ordinary food? If it be replied that the blood has
become poor in certain calcareous salts and that hence arises the appetite
for things containing them, there remains the question--How does this
deficiency so act on the nervous system as to generate this vague desire
and cause the movements which satisfy it? By no effort can we figure to
ourselves the implied causal processes.
In brief, then, we are obliged to confess that Life in its essence cannot
be conceived in physico-chemical terms. The required principle of activity,
which we found cannot be represented as an independent vital principle, we
now find cannot be represented as a principle inherent in living matter.
If, by assuming its inherence, we think the facts are accounted for, we do
but cheat ourselves with pseud-ideas.
§ 36e. What then are we to say--what are we to think? Simply that in this
direction, as in all other directions, our explanations finally bring us
face to face with the inexplicable. The Ultimate Reality behind this
manifestation, as behind all other manifestations, transcends conception.
It needs but to observe how even simple forms of existence are in their
ultimate natures incomprehensible, to see that this most complex form of
existence is in a sense doubly incomprehensible.
For the actions of that which the ignorant contemptuously call brute
matter, cannot in the last resort be understood in their genesis. Were it
not that familiarity blinds us, the fall of a stone would afford matter for
wonder. Neither Newton nor anyone since his day has been able to conceive
how the molecules of matter in the stone are affected not only by the
molecules of matter in the adjacent part of the Earth but by those forming
parts of its mass 8,000 miles off which severally exercise their influence
without impediment from intervening molecules; and still less has there
been any conceivable interpretation of the mode in which every molecule of
matter in the Sun, 92 millions of miles away, has a share in controlling
the movements of the Earth. What goes on in the space between a magnet and
the piece of iron drawn towards it, or how on repeatedly passing a magnet
along a steel needle this, by some change of molecular state as we must
suppose, becomes itself a magnet and when balanced places its poles in
fixed directions, we do not know. And still less can we fathom the physical
process by which an ordered series of electric pulses sent through a
telegraph wire may be made to excite a corresponding series of pulses in a
parallel wire many miles off.
Turn to another class of cases. Consider the action of a surface of glass
struck by a cathode current and which thereupon generates an order of rays
able to pass through solid matters impermeable to light. Or contemplate the
power possessed by uranium and other metals of emitting rays imperceptible
by our eyes as light but which yet, in what appears to us absolute
darkness, will, if passed through a camera, produce photographs. Even the
actions of one kind of matter on another are sufficiently remarkable. Here
is a mass of gold which, after the addition of 1-500th part of bismuth, has
only 1-28th of the tensile strength it previously had; and here is a mass
of brass, ordinarily ductile and malleable, but which, on the addition of
1-10,000th part of antimony, loses its character. More remarkable still are
the influences of certain medicines. One-hundredth of a grain of
nitro-glycerine is a sufficient dose. Taking an average man's weight as 150
pounds, it results that his body is appreciably affected in its state by
the 115-millionth part of its weight of this nitrogenous compound.
In presence of such powers displayed by matter of simple kinds we shall see
how impossible it is even to imagine those processes going on in organic
matter out of which emerges the dynamic element in Life. As no separate
form of proteid possesses vitality, we seem obliged to assume that the
molecule of protoplasm contains many molecules of proteids, probably in
various isomeric states, all capable of ready change and therefore
producing great instability of the aggregate they form. As before pointed
out (§ 4), a proteid-molecule includes more than 220 equivalents of several
so-called elements. Each of these undecomposed substances is now recognized
by chemists as almost certainly consisting of several kinds of components.
Hence the implication is that a proteid-molecule contains thousands of
units, of which the different classes have their respective rates of
inconceivably rapid oscillation, while each unit, receiving and emitting
ethereal undulations, affects others of its kind in its own and adjacent
molecules: an immensely complex structure having immensely complex
activities. And this complexity, material and dynamic, in the
proteid-molecule we must regard as raised to a far higher degree in the
unit of protoplasm. Here as elsewhere alternative impossibilities of
thought present themselves. We find it impossible to think of Life as
imported into the unit of protoplasm from without; and yet we find it
impossible to conceive it as emerging from the cooperation of the
components.
§ 36f. But now, having confessed that Life as a principle of activity is
unknown and unknowable--that while its phenomena are accessible to thought
the implied noumenon is inaccessible--that only the manifestations come
within the range of our intelligence while that which is manifested lies
beyond it; we may resume the conclusions reached in the preceding chapters.
Our surface knowledge continues to be a knowledge valid of its kind, after
recognizing the truth that it is only a surface knowledge.
For the conclusions we lately reached and the definition emerging from
them, concern the _order_ existing among the actions which living things
exhibit; and this order remains the same whether we know or do not know the
nature of that from which the actions originate. We found a distinguishing
trait of Life to be that its changes display a correspondence with
co-existences and sequences in the environment; and this remains a
distinguishing trait, though the thing which changes remains inscrutable.
The statement that the continuous adjustment of internal relations to
external relations constitutes Life as cognizable by us, is not invalidated
by the admission that the reality in which these relations inhere is
incognizable.
Hence, then, after duly recognizing the fact that, as pointed out above,
Life, even phenomenally considered, is not entirely covered by the
definition, since there are various abnormal manifestations of life which
it does not include, we may safely accept it as covering the normal
manifestations--those manifestations which here concern us. Carrying with
us the definition, therefore we may hereafter use it for guidance through
all those regions of inquiry upon which we now enter.
CHAPTER VII.
THE SCOPE OF BIOLOGY.
§ 37. As ordinarily conceived, the science of Biology falls into two great
divisions, the one dealing with animal life, called Zoology, and the other
dealing with vegetal life, called Botany, or more properly to be called
Phytology. But convenient as is this division, it is not that which arises
if we follow the scientific method of including in one group all the
phenomena of fundamentally the same order and putting separately in another
group all the phenomena of a fundamentally different order. For animals and
plants are alike in having structures; and animals and plants are alike in
having functions performed by these structures; and the distinction between
structures and functions transcends the difference between any one
structure and any other or between any one function and any other--is,
indeed, an absolute distinction, like that between Matter and Motion.
Recognizing, then, the logic of the division thus indicated, we must group
the parts of Biology thus:--
1. An account of the structural phenomena presented by organisms. This
subdivides into:--
_a._ The established structural phenomena presented by individual
organisms.
_b._ The changing structural phenomena presented by successions of
organisms.
2. An account of the functional phenomena which organisms present. This,
too, admits of subdivision into:--
_a._ The established functional phenomena of individual organisms.
_b._ The changing functional phenomena of successions of organisms.
3. An account of the actions of Structures on Functions and the re-actions
of Functions on Structures. Like the others, this is divisible into:--
_a._ The actions and re-actions as exhibited in individual organisms.
_b._ The actions and re-actions as exhibited in successions of
organisms.
4. An account of the phenomena attending the production of successions of
organisms: in other words--the phenomena of Genesis.
Of course, for purposes of exploration and teaching, the division into
Zoology and Botany, founded on contrasts so marked and numerous, must
always be retained. But here recognizing this familiar distinction only as
much as convenience obliges us to do, let us now pass on to consider, more
in detail, the classification of biologic phenomena above set down in its
leading outlines.
§ 38. The facts of structure shown in an individual organism, are of two
chief kinds. In order of conspicuousness, though not in order of time,
there come first those arrangements of parts which characterize the mature
organism; an account of which, originally called Anatomy, is now called
Morphology. Then come those successive modifications through which the
organism passes in its progress from the germ to the developed form; an
account of which is called Embryology.
The structural changes which any series of individual organisms exhibits,
admit of similar classification. On the one hand, we have those inner and
outer differences of shape, that arise between the adult members of
successive generations descended from a common stock--differences which,
though usually not marked between adjacent generations, become great in
course of multitudinous generations. On the other hand, we have those
developmental modifications, seen in the embryos, through which such
modifications of the descended forms are reached.
Interpretation of the structures of individual organisms and successions of
organisms, is aided by two subsidiary divisions of biologic inquiry, named
Comparative Anatomy (properly Comparative Morphology) and Comparative
Embryology. These cannot be regarded as in themselves parts of Biology;
since the facts embraced under them are not substantive phenomena, but are
simply incidental to substantive phenomena. All the truths of structural
Biology are comprehended under the two foregoing subdivisions; and the
comparison of these truths as presented in different classes of organisms,
is simply a _method_ of interpreting them.
Nevertheless, though Comparative Morphology and Comparative Embryology do
not disclose additional concrete facts, they lead to the establishment of
certain abstract facts. By them it is made manifest that underneath the
superficial differences of groups and classes and types of organisms, there
are hidden fundamental similarities; and that the courses of development in
such groups and classes and types, though in many respects divergent, are
in some essential respects, coincident. The wide truths thus disclosed,
come under the heads of General Morphology and General Embryology.
By contrasting organisms there is also achieved that grouping of the like
and separation of the unlike, called Classification. First by observation
of external characters; second by observation of internal characters; and
third by observation of the phases of development; it is ascertained what
organisms are most similar in all respects; what organisms otherwise unlike
are like in important traits; what organisms though apparently unallied
have common primordial characters. Whence there results such an
arrangement of organisms, that if certain structural attributes of any one
be given, its other structural attributes may be _empirically_ predicted;
and which prepares the way for that interpretation of their relations and
genesis, which forms an important part of _rational_ Biology.
§ 39. The second main division of Biology, above described as embracing the
functional phenomena of organisms, is that which is in part signified by
Physiology: the remainder being distinguishable as Objective Psychology.
Both of these fall into subdivisions that may best be treated separately.
That part of Physiology which is concerned with the molecular changes going
on in organisms, is known as Organic Chemistry. An account of the modes in
which the force generated in organisms by chemical change, is transformed
into other forces, and made to work the various organs that carry on the
functions of Life, comes under the head of Organic Physics. Psychology,
which is mainly concerned with the adjustment of vital actions to actions
in the environment (in contrast with Physiology, which is mainly concerned
with vital actions apart from actions in the environment) consists of two
quite distinct portions. Objective Psychology deals with those functions of
the nervo-muscular apparatus by which such organisms as possess it are
enabled to adjust inner to outer relations; and includes also the study of
the same functions as externally manifested in conduct. Subjective
Psychology deals with the sensations, perceptions, ideas, emotions, and
volitions that are the direct or indirect concomitants of this visible
adjustment of inner to outer relations. Consciousness under its different
modes and forms, being a subject-matter radically distinct in nature from
the subject-matter of Biology in general; and the method of self-analysis,
by which alone the laws of dependence among changes of consciousness can be
found, being a method unparalleled by anything in the rest of Biology; we
are obliged to regard Subjective Psychology as a separate study. And since
it would be very inconvenient wholly to dissociate Objective Psychology
from Subjective Psychology, we are practically compelled to deal with the
two as forming an independent science.
Obviously, the functional phenomena presented in successions of organisms,
similarly divide into physiological and psychological. Under the
physiological come the modifications of bodily actions that arise in the
course of generations, as concomitants of structural modifications; and
these may be modifications, qualitative or quantitative, in the molecular
changes classed as chemical, or in the organic actions classed as physical,
or in both. Under the psychological come the qualitative and quantitative
modifications of instincts, feelings, conceptions, and mental processes in
general, which occur in creatures having more or less intelligence, when
certain of their conditions are changed. This, like the preceding
department of Psychology, has in the abstract two different aspects--the
objective and the subjective. Practically, however, the objective, which
deals with these mental modifications as exhibited in the changing habits
and abilities of successive generations of creatures, is the only one
admitting of investigation; since the corresponding alterations in
consciousness cannot be immediately known to any but the subjects of them.
Evidently, convenience requires us to join this part of Psychology along
with the other parts as components of a distinct sub-science.
Light is thrown on functions, as well as on structures, by comparing
organisms of different kinds. Comparative Physiology and Comparative
Psychology, are the names given to those collections of facts respecting
the homologies and analogies, bodily and mental, disclosed by this kind of
inquiry. These classified observations concerning likenesses and
differences of functions, are helpers to interpret functions in their
essential natures and relations. Hence Comparative Physiology and
Comparative Psychology are names of methods rather than names of true
subdivisions of Biology.
Here, however, as before, comparison of special truths, besides
facilitating their interpretation, brings to light certain general truths.
Contrasting functions bodily and mental as exhibited in various kinds of
organisms, shows that there exists, more or less extensively, a community
of processes and methods. Hence result two groups of propositions
constituting General Physiology and General Psychology.
§ 40. In these divisions and subdivisions of the first two great
departments of Biology, facts of Structure are considered separately from
facts of Function, so far as separate treatment of them is possible. The
third great department of Biology deals with them in their necessary
connexions. It comprehends the determination of functions by structures,
and the determination of structures by functions.
As displayed in individual organisms, the effects of structures on
functions are to be studied, not only in the broad fact that the general
kind of life an organism leads is necessitated by the main characters of
its organization, but in the more special and less conspicuous fact, that
between members of the same species, minor differences of structure lead to
minor differences of power to perform certain actions, and of tendencies to
perform such actions. Conversely, under the reactions of functions on
structures in individual organisms, come the facts showing that functions,
when fulfilled to their normal extents, maintain integrity of structure in
their respective organs; and that within certain limits increases of
functions are followed by such structural changes in their respective
organs, as enable them to discharge better their extra functions.
Inquiry into the influence of structure on function as seen in successions
of organisms, introduces us to such phenomena as Mr. Darwin's _Origin of
Species_ deals with. In this category come all proofs of the general truth,
that when an individual is enabled by a certain structural peculiarity to
perform better than others of its species some advantageous action; and
when it bequeaths more or less of its structural peculiarity to
descendants, among whom those which have it most markedly are best able to
thrive and propagate; there arises a visibly modified type of structure,
having a more or less distinct function. In the correlative class of facts
(by some asserted and by others denied), which come under the category of
reactions of function on structure as exhibited in successions of
organisms, are to be placed all those modifications of structure which
arise in races, when changes of conditions entail changes in the balance of
their functions--when altered function externally necessitated, produces
altered structure, and continues doing this through successive generations.
§ 41. The fourth great division of Biology, comprehending the phenomena of
Genesis, may be conveniently separated into three subdivisions.
Under the first, comes a description of all the special modes whereby the
multiplication of organisms is carried on; which modes range themselves
under the two chief heads of sexual and asexual. An account of Sexual
Multiplication includes the various processes by which germs and ova are
fertilized, and by which, after fertilization, they are furnished with the
materials, and maintained in the conditions, needful for their development.
An account of Asexual Multiplication includes the various processes by
which, from the same fertilized germ or ovum, there are produced many
organisms partially or totally independent of one another.
The second of these subdivisions deals with the phenomena of Genesis in the
abstract. It takes for its subject-matter such general questions as--What
is the end subserved by the union of sperm-cell and germ-cell? Why cannot
all multiplication be carried on after the asexual method? What are the
laws of hereditary transmission? What are the causes of variation?
The third subdivision is devoted to still more abstract aspects of the
subject. Recognizing the general facts of multiplication, without reference
to their modes or immediate causes, it concerns itself simply with the
different rates of multiplication in different kinds of organisms and
different individuals of the same kind. Generalizing the numerous contrasts
and variations of fertility, it seeks a rationale of them in their
relations to other organic phenomena.
§ 42. Such appears to be the natural arrangement of divisions and
subdivisions which Biology presents. It is, however, a classification of
the parts of the science when fully developed; rather than a classification
of them as they now stand. Some of the subdivisions above named have no
recognized existence, and some of the others are in quite rudimentary
states. It is impossible now to fill in, even in the roughest way, more
than a part of the outlines here sketched.
Our course of inquiry being thus in great measure determined by the present
state of knowledge, we are compelled to follow an order widely different
from this ideal one. It will be necessary first to give an account of those
empirical generalizations which naturalists and physiologists have
established: appending to those which admit of it, such deductive
interpretations as _First Principles_ furnishes us with. Having done this,
we shall be the better prepared for dealing with the leading truths of
Biology in connexion with the doctrine of Evolution.
PART II.
THE INDUCTIONS OF BIOLOGY.
CHAPTER I.
GROWTH.
§ 43. Perhaps the widest and most familiar induction of Biology, is that
organisms grow. While, however, this is a characteristic so uniformly and
markedly displayed by plants and animals, as to be carelessly thought
peculiar to them, it is really not so. Under appropriate conditions,
increase of size takes place in inorganic aggregates, as well as in organic
aggregates. Crystals grow; and often far more rapidly than living bodies.
Where the requisite materials are supplied in the requisite forms, growth
may be witnessed in non-crystalline masses: instance the fungous-like
accumulation of carbon that takes place on the wick of an unsnuffed candle.
On an immensely larger scale, we have growth in geologic formations: the
slow accumulation of deposited sediment into a stratum, is not
distinguishable from growth in its widest acceptation. And if we go back to
the genesis of celestial bodies, assuming them to have arisen by Evolution,
these, too, must have gradually passed into their concrete shapes through
processes of growth. Growth is, indeed, as being an integration of matter,
the primary trait of Evolution; and if Evolution of one kind or other is
universal, growth is universal--universal, that is, in the sense that all
aggregates display it in some way at some period.
The essential community of nature between organic growth and inorganic
growth, is, however, most clearly seen on observing that they both result
in the same way. The segregation of different kinds of detritus from each
other, as well as from the water carrying them, and their aggregation into
distinct strata, is but an instance of a universal tendency towards the
union of like units and the parting of unlike units (_First Principles_,
§ 163). The deposit of a crystal from a solution is a differentiation of
the previously mixed molecules; and an integration of one class of
molecules into a solid body, and the other class into a liquid solvent. Is
not the growth of an organism an essentially similar process? Around a
plant there exist certain elements like the elements which form its
substance; and its increase of size is effected by continually integrating
these surrounding like elements with itself. Nor does the animal
fundamentally differ in this respect from the plant or the crystal. Its
food is a portion of the environing matter that contains some compound
atoms like some of the compound atoms constituting its tissues; and either
through simple imbibition or through digestion, the animal eventually
integrates with itself, units like those of which it is built up, and
leaves behind the unlike units. To prevent misconception, it may be well to
point out that growth, as here defined, must be distinguished from certain
apparent and real augmentations of bulk which simulate it. Thus, the long,
white potato-shoots thrown out in the dark, are produced at the expense of
the substances which the tuber contains: they illustrate not the
accumulation of organic matter, but simply its re-composition and
re-arrangement. Certain animal-embryos, again, during their early stages,
increase considerably in size without assimilating any solids from the
environment; and they do this by absorbing the surrounding water. Even in
the highest organisms, as in children, there appears sometimes to occur a
rapid gain in dimensions which does not truly measure the added quantity of
organic matter; but is in part due to changes analogous to those just
named. Alterations of this kind must not be confounded with that growth,
properly so called, of which we have here to treat.
The next general fact to be noted respecting organic growth, is, that it
has limits. Here there appears to be a distinction between organic and
inorganic growth; but this distinction is by no means definite. Though that
aggregation of inanimate matter which simple attraction produces, may go on
without end; yet there appears to be an end to that more definite kind of
aggregation which results from polar attraction. Different elements and
compounds habitually form crystals more or less unlike in their sizes; and
each seems to have a size that is not usually exceeded without a tendency
arising to form new crystals rather than to increase the old. On looking
at the organic kingdom as a whole, we see that the limits between which
growth ranges are very wide apart. At the one extreme we have monads so
minute as to be rendered but imperfectly visible by microscopes of the
highest power; and at the other extreme we have trees of 400 to 500 feet
high and animals of 100 feet long. It is true that though in one sense this
contrast may be legitimately drawn, yet in another sense it may not; since
these largest organisms arise by the combination of units which are
individually like the smallest. A single plant of the genus _Protococcus_,
is of the same essential structure as one of the many cells united to form
the thallus of some higher Alga, or the leaf of a phænogam. Each separate
shoot of a phænogam is usually the bearer of many leaves. And a tree is an
assemblage of numerous united shoots. One of these great teleophytes is
thus an aggregate of aggregates of aggregates of units, which severally
resemble protophytes in their sizes and structures; and a like building up
is traceable throughout a considerable part of the animal kingdom. Even,
however, when we bear in mind this qualification, and make our comparisons
between organisms of the same degree of composition, we still find the
limit of growth to have a great range. The smallest branched flowering
plant is extremely insignificant by the side of a forest tree; and there is
an enormous difference in bulk between the least and the greatest mammal.
But on comparing members of the same species, we discover the limit of
growth to be much less variable. Among the _Protozoa_ and _Protophyta_,
each kind has a tolerably constant adult size; and among the most complex
organisms the differences between those of the same kind which have reached
maturity, are usually not very great. The compound plants do, indeed,
sometimes present marked contrasts between stunted and well-grown
individuals; but the higher animals diverge but inconsiderably from the
average standards of their species.
On surveying the facts with a view of empirically generalizing the causes
of these differences, we are soon made aware that by variously combining
and conflicting with one another, these causes produce great irregularities
of result. It becomes manifest that no one of them can be traced to its
consequences, unqualified by the rest. Hence the several statements
contained in the following paragraphs must be taken as subject to mutual
modification.
Let us consider first the connexion between degree of growth and complexity
of structure. This connexion, being involved with many others, becomes
apparent only on so averaging the comparisons as to eliminate differences
among the rest. Nor does it hold at all where the conditions are radically
dissimilar, as between plants and animals. But bearing in mind these
qualifications, we shall see that organization has a determining influence
on increase of mass. Of plants the lowest, classed as Thallophytes, usually
attain no considerable size. Algæ, Fungi, and the Lichens formed by
association of them count among their numbers but few bulky species: the
largest, such as certain Algæ found in antarctic seas, not serving greatly
to raise the average; and these gigantic seaweeds possess a considerable
complexity of histological organization very markedly exceeding that of
their smaller allies. Though among Bryophytes and Pteridophytes there are
some, as the Tree-ferns, which attain a considerable height, the majority
are but of humble growth. The Monocotyledons, including at one extreme
small grasses and at the other tall palms, show us an average and a maximum
greater than that reached by the Pteridophytes. And the Monocotyledons are
exceeded by the Dicotyledons; among which are found the monarchs of the
vegetal kingdom. Passing to animals, we meet the fact that the size
attained by _Vertebrata_ is usually much greater than the size attained by
_Invertebrata_. Of invertebrate animals the smallest, classed as
_Protozoa_, are also the simplest; and the largest, belonging to the
_Annulosa_ and _Mollusca_, are among the most complex of their respective
types. Of vertebrate animals we see that the greatest are Mammals, and that
though, in past epochs, there were Reptiles of vast bulks, their bulks did
not equal that of the whale: the great Dinosaurs, though as long, being
nothing like as massive. Between reptiles and birds, and between
land-vertebrates and water-vertebrates, the relation does not hold: the
conditions of existence being in these cases widely different. But among
fishes as a class, and among reptiles as a class, it is observable that,
speaking generally, the larger species are framed on the higher types. The
critical reader, who has mentally checked these statements in passing them,
has doubtless already seen that this relation is not a dependence of
organization on growth but a dependence of growth on organization. The
majority of Dicotyledons are smaller than some Monocotyledons; many
Monocotyledons are exceeded in size by certain Pteridophytes; and even
among Thallophytes, the least developed among compound plants, there are
kinds of a size which many plants of the highest order do not reach.
Similarly among animals. There are plenty of Crustaceans less than
_Actiniæ_; numerous reptiles are smaller than some fish; the majority of
mammals are inferior in bulk to the largest reptiles; and in the contrast
between a mouse and a well-grown _Medusa_, we see a creature that is
elevated in type of structure exceeded in mass by one that is extremely
low. Clearly then, it cannot be held that high organization is habitually
accompanied by great size. The proposition here illustrated is the converse
one, that great size is habitually accompanied by high organization. The
conspicuous facts that the largest species of both animals and vegetals
belong to the highest classes, and that throughout their various
sub-classes the higher usually contain the more bulky forms, show this
connexion as clearly as we can expect it to be shown, amid so many
modifying causes and conditions.
The relation between growth and supply of available nutriment, is too
familiar a relation to need proving. There are, however, some aspects of it
that must be contemplated before its implications can be fully appreciated.
Among plants, which are all constantly in contact with the gaseous, liquid,
and solid matters to be incorporated with their tissues, and which, in the
same locality, receive not very unlike amounts of light and heat,
differences in the supplies of available nutriment have but a subordinate
connexion with differences of growth. Though in a cluster of herbs
springing up from the seeds let fall by a parent, the greater sizes of some
than of others is doubtless due to better nutrition, consequent on
accidental advantages; yet no such interpretation can be given of the
contrast in size between these herbs and an adjacent tree. Other conditions
here come into play: one of the most important being, an absence in the one
case, and presence in the other, of an ability to secrete such a quantity
of ligneous fibre as will produce a stem capable of supporting a large
growth. Among animals, however, which (excepting some _Entozoa_) differ
from plants in this, that instead of bathing their surfaces the matters
they subsist on are dispersed, and have to be obtained, the relation
between available food and growth is shown with more regularity. The
_Protozoa_, living on microscopic fragments of organic matter contained in
the surrounding water, are unable, during their brief lives, to accumulate
any considerable quantity of nutriment. _Polyzoa_, having for food these
scarcely visible members of the animal kingdom, are, though large compared
with their prey, small as measured by other standards; even when aggregated
into groups of many individuals, which severally catch food for the common
weal, they are often so inconspicuous as readily to be passed over by the
unobservant. And if from this point upwards we survey the successive grades
of animals, it becomes manifest that, in proportion as the size is great,
the masses of nutriment are either large, or, what is practically the same
thing, are so abundant and so grouped that large quantities may be readily
taken in. Though, for example, the greatest of mammals, the arctic whale,
feeds on such comparatively small creatures as the acalephes and molluscs
floating in the seas it inhabits, its method of gulping in whole shoals of
them and filtering away the accompanying water, enables it to secure great
quantities of food. We may then with safety say that, other things equal,
the growth of an animal depends on the abundance and sizes of the masses of
nutriment which its powers enable it to appropriate. Perhaps it may be
needful to add that, in interpreting this statement, the proportion of
competitors must be taken into account. Clearly, not the absolute, but the
relative, abundance of fit food is the point; and this relative abundance
very much depends on the number of individuals competing for the food. Thus
all who have had experience in fishing in Highland lochs, know that where
the trout are numerous they are small, and that where they are
comparatively large they are comparatively few.
What is the relation between growth and expenditure of energy? is a
question which next presents itself. Though there is reason to believe such
a relation exists, it is not very readily traced: involved as it is with so
many other relations. Some contrasts, however, may be pointed out that
appear to give evidence of it. Passing over the vegetal kingdom, throughout
which the expenditure of force is too small to allow of such a relation
being visible, let us seek in the animal kingdom, some case where classes
otherwise allied, are contrasted in their locomotive activities. Let us
compare birds on the one hand, with reptiles and mammals on the other. It
is an accepted doctrine that birds are organized on a type closely allied
to the reptilian type, but superior to it; and though in some respects the
organization of birds is inferior to that of mammals, yet in other
respects, as in the greater heterogeneity and integration of the skeleton,
the more complex development of the respiratory system, and the higher
temperature of the blood, it may be held that birds stand above mammals.
Hence were growth dependent only on organization, we might infer that the
limit of growth among birds should not be much short of that among mammals;
and that the bird-type should admit of a larger growth than the
reptile-type. Again, we see no manifest disadvantages under which birds
labour in obtaining food, but from which reptiles and mammals are free. On
the contrary, birds are able to get at food that is fixed beyond the reach
of reptiles and mammals; and can catch food that is too swift of movement
to be ordinarily caught by reptiles and mammals. Nevertheless, the limit of
growth in birds falls far below that reached by reptiles and mammals. With
what other contrast between these classes, is this contrast connected? May
we not suspect that it is connected (partially though not wholly) with the
contrast between their amounts of locomotive exertion? Whereas mammals
(excepting bats, which are small), are during all their movements supported
by solid surfaces or dense liquids; and whereas reptiles (excepting the
ancient pterodactyles, which were not very large), are similarly restricted
in their spheres of movement; the majority of birds move more or less
habitually through a rare medium, in which they cannot support themselves
without relatively great efforts. And this general fact may be joined with
the special fact, that those members of the class _Aves_, as the _Dinornis_
and _Epiornis_, which approached in size to the larger _Mammalia_ and
_Reptilia_, were creatures incapable of flight--creatures which did not
expend this excess of force in locomotion. But as implied above, and as
will presently be shown, another factor of importance comes into play; so
that perhaps the safest evidence that there is an antagonism between the
increase of bulk and the quantity of motion evolved is that supplied by the
general experience, that human beings and domestic animals, when overworked
while growing, are prevented from attaining the ordinary dimensions.
One other general truth concerning degrees of growth, must be set down. It
is a rule, having exceptions of no great importance, that large organisms
commence their separate existences as masses of organic matter more or less
considerable in size, and commonly with organizations more or less
advanced; and that throughout each organic sub-kingdom, there is a certain
general, though irregular, relation between the initial and the final
bulks. Vegetals exhibit this relation less manifestly than animals. Yet
though, among the plants that begin life as minute spores, there are some
which, by the aid of an intermediate form, grow to large sizes, the immense
majority of them remain small. While, conversely, the great Monocotyledons
and Dicotyledons, when thrown off from their parents, have already the
formed organs of young plants, to which are attached stores of highly
nutritive matter. That is to say, where the young plant consists merely of
a centre of development, the ultimate growth is commonly insignificant; but
where the growth is to become great, there exists to start with, a
developed embryo and a stock of assimilable matter. Throughout the animal
kingdom this relation is tolerably manifest though by no means uniform.
Save among classes that escape the ordinary requirements of animal life,
small germs or eggs do not in most cases give rise to bulky creatures.
Where great bulk is to be reached, the young proceeds from an egg of
considerable bulk, or is born of considerable bulk ready-organized and
partially active. In the class Fishes, or in such of them as are subject to
similar conditions of life, some proportion usually obtains between the
sizes of the ova and the sizes of the adult individuals; though in the
cases of the sturgeon and the tunny there are exceptions, probably
determined by the circumstances of oviposition and those of juvenile life.
Reptiles have eggs that are smaller in number, and relatively greater in
mass, than those of fishes; and throughout this class, too, there is a
general congruity between the bulk of the egg and the bulk of the adult
creature. As a group, birds show us further limitations in the numbers of
their eggs as well as farther increase in their relative sizes; and from
the minute eggs of the humming-bird up to the immense ones of the
_Epiornis_, holding several quarts, we see that, speaking generally, the
greater the eggs the greater the birds., Finally, among mammals (omitting
the marsupials) the young are born, not only of comparatively large sizes,
but with advanced organizations; and throughout this sub-division of the
_Vertebrata_, as throughout the others, there is a manifest connexion
between the sizes at birth and the sizes at maturity. As having a kindred
meaning, there must finally be noted the fact that the young of these
highest animals, besides starting in life with bodies of considerable
sizes, almost fully organized, are, during subsequent periods of greater or
less length, supplied with nutriment--in birds by feeding and in mammals by
suckling and afterwards by feeding. So that beyond the mass and
organization directly bequeathed, a bird or mammal obtains a further large
mass at but little cost to itself.
Were exhaustive treatment of the topic intended, it would be needful to
give a paragraph to each of the incidental circumstances by which growth
may be aided or restricted:--such facts as that an entozoon is limited by
the size of the creature, or even the organ, in which it thrives; that an
epizoon, though getting abundant nutriment without appreciable exertion, is
restricted to that small bulk at which it escapes ready detection by the
animal it infests; that sometimes, as in the weazel, smallness is a
condition to successful pursuit of the animals preyed upon; and that in
some cases, the advantage of resembling certain other creatures, and so
deceiving enemies or prey, becomes an indirect cause of restricted size.
But the present purpose is simply to set down those most general relations
between growth and other organic traits, which induction leads us to.
Having done this, let us go on to inquire whether these general relations
can be deductively established.
§ 44. That there must exist a certain dependence of growth on organization,
may be shown _a priori_. When we consider the phenomena of Life, either by
themselves or in their relations to surrounding phenomena, we see that,
other things equal, the larger the aggregate the greater is the needful
complexity of structure.
In plants, even of the highest type, there is a comparatively small mutual
dependence of parts: a gathered flower-bud will unfold and flourish for
days if its stem be immersed in water; and a shoot cut off from its
parent-tree and stuck in the ground will grow. The respective parts having
vital activities that are not widely unlike, it is possible for great bulk
to be reached without that structural complexity required for combining the
actions of parts. Even here, however, we see that for the attainment of
great bulk there requires such a degree of organization as shall
co-ordinate the functions of roots and branches--we see that such a size as
is reached by trees, is not possible without a vascular system enabling the
remote organs to utilize each other's products. And we see that such a
co-existence of large growth with comparatively low organization as occurs
in some of the marine _Algæ_, occurs where the conditions of existence do
not necessitate any considerable mutual dependence of parts--where the near
approach of the plant to its medium in specific gravity precludes the need
of a well-developed stem, and where all the materials of growth being
derived from the water by each portion of the thallus, there requires no
apparatus for transferring the crude food materials from part to part.
Among animals which, with but few exceptions, are, by the conditions of
their existence, required to absorb nutriment through one specialized part
of the body, it is clear that there must be a means whereby other parts of
the body, to be supported by this nutriment, must have it conveyed to them.
It is clear that for an equally efficient maintenance of their nutrition,
the parts of a large mass must have a more elaborate propelling and
conducting apparatus; and that in proportion as these parts undergo greater
waste, a yet higher development of the vascular system is necessitated.
Similarly with the prerequisites to those mechanical motions which animals
are required to perform. The parts of a mass cannot be made to move, and
have their movements so co-ordinated as to produce locomotive and other
actions, without certain structural arrangements; and, other things equal,
a given amount of such activity requires more involved structural
arrangements in a large mass than in a small one. There must at least be a
co-ordinating apparatus presenting greater contrasts in its central and
peripheral parts.
The qualified dependence of growth on organization, is equally implied when
we study it in connexion with that adjustment of inner to outer relations
which constitutes Life as phenomenally known to us. In plants this is less
striking than in animals, because the adjustment of inner to outer
relations does not involve conspicuous motions. Still, it is visible in the
fact that the condition on which alone a plant can grow to a great size,
is, that it shall, by the development of a massive trunk, present inner
relations of forces fitted to counterbalance those outer relations of
forces which tend continually, and others which tend occasionally, to
overthrow it; and this formation of a core of regularly-arranged woody
fibres is an advance in organization. Throughout the animal kingdom this
connexion of phenomena is manifest. To obtain materials for growth; to
avoid injuries which interfere with growth; and to escape those enemies
which bring growth to a sudden end; implies in the organism the means of
fitting its movements to meet numerous external co-existences and
sequences--implies such various structural arrangements as shall make
possible these variously-adapted actions. It cannot be questioned that,
everything else remaining constant, a more complex animal, capable of
adjusting its conduct to a greater number of surrounding contingencies,
will be the better able to secure food and evade damage, and so to increase
bulk. And evidently, without any qualification, we may say that a large
animal, living under such complex conditions of existence as everywhere
obtain, is not possible without comparatively high organization.
While, then, this relation is traversed and obscured by sundry other
relations, it cannot but exist. Deductively we see that it must be
modified, as inductively we saw that it is modified, by the circumstances
amid which each kind of organism is placed, but that it is always a factor
in determining the result.
§ 45. That growth is, _cæteris paribus_, dependent on the supply of
assimilable matter, is a proposition so continually illustrated by special
experience, as well as so obvious from general experience, that it would
scarcely need stating, were it not requisite to notice the qualifications
with which it must be taken.
The materials which each organism requires for building itself up, are not
of one kind but of several kinds. As a vehicle for transferring matter
through their structures, all organisms require water as well as solid
constituents; and however abundant the solid constituents there can be no
growth in the absence of water. Among the solids supplied, there must be a
proportion ranging within certain limits. A plant round which carbonic
acid, water, and ammonia exist in the right quantities, may yet be arrested
in its growth by a deficiency of potassium. The total absence of lime from
its food may stop the formation of a mammal's skeleton: thus dwarfing, if
not eventually destroying, the mammal; and this no matter what quantities
of other needful colloids and crystalloids are furnished.
Again, the truth that, other things equal, growth varies according to the
supply of nutriment, has to be qualified by the condition that the supply
shall not exceed the ability to appropriate it. In the vegetal kingdom, the
assimilating surface being external and admitting of rapid expansion by the
formation of new roots, shoots, and leaves, the effect of this limitation
is not conspicuous. By artificially supplying plants with those materials
which they have usually the most difficulty in obtaining, we can greatly
facilitate their growth; and so can produce striking differences of size in
the same species. Even here, however, the effect is confined within the
limits of the ability to appropriate; since in the absence of that solar
light and heat by the help of which the chief appropriation is carried on,
the additional materials for growth are useless. In the animal kingdom this
restriction is rigorous. The absorbent surface being, in the great majority
of cases, internal; having a comparatively small area, which cannot be
greatly enlarged without reconstruction of the whole body; and being in
connexion with a vascular system which also must be re-constructed before
any considerable increase of nutriment can be made available; it is clear
that beyond a certain point, very soon reached, increase of nutriment will
not cause increase of growth. On the contrary, if the quantity of food
taken in is greatly beyond the digestive and absorbent power, the excess,
becoming an obstacle to the regular working of the organism, may retard
growth rather than advance it.
While then it is certain, _a priori_, that there cannot be growth in the
absence of such substances as those of which an organism consists; and
while it is equally certain that the amount of growth must primarily be
governed by the supply of these substances; it is not less certain that
extra supply will not produce extra growth, beyond a point very soon
reached. Deduction shows to be necessary, as induction makes familiar, the
truths that the value of food for purposes of growth depends not on the
quantity of the various organizable materials it contains, but on the
quantity of the material most needed; that given a right proportion of
materials, the pre-existing structure of the organism limits their
availability; and that the higher the structure, the sooner is this limit
reached.
§ 46. But why should the growth of every organism be finally arrested?
Though the rate of increase may, in each case, be necessarily restricted
within a narrow range of variation--though the increment that is possible
in a given time, cannot exceed a certain amount; yet why should the
increments decrease and finally become insensible? Why should not all
organisms, when supplied with sufficient materials, continue to grow as
long as they live? To find an answer to this question we must revert to the
nature and functions of organic matter.
In the first three chapters of Part I, it was shown that plants and animals
mainly consist of substances in states of unstable equilibrium--substances
which have been raised to this unstable equilibrium by the expenditure of
the forces we know as solar radiations, and which give out these forces in
other forms on falling into states of stable equilibrium. Leaving out the
water, which serves as a vehicle for these materials and a medium for their
changes; and excluding those mineral matters that play either passive or
subsidiary parts; organisms are built up of compounds which are stores of
force. Thus complex colloids and crystalloids which, as united together,
form organized bodies, are the same colloids and crystalloids which give
out, on their decomposition, the forces expended by organized bodies. Thus
these nitrogenous and carbonaceous substances, being at once the materials
for organic growth and the sources of organic energy, it results that as
much of them as is used up for the genesis of energy is taken away from the
means of growth, and as much as is economized by diminishing the genesis of
energy, is available for growth. Given that limited quantity of nutritive
matter which the pre-existing structure of an organism enables it to
absorb; and it is a necessary corollary from the persistence of force, that
the matter accumulated as growth cannot exceed that surplus which remains
undecomposed after the production of the required amounts of sensible and
insensible motion. This, which would be rigorously true under all
conditions if exactly the same substances were used in exactly the same
proportions for the production of force and for the formation of tissue,
requires, however, to be taken with the qualification that some of the
force-evolving substances are not constituents of tissue; and that thus
there may be a genesis of force which is not at the expense of potential
growth. But since organisms (or at least animal organisms, with which we
are here chiefly concerned) have a certain power of selective absorption,
which, partially in an individual and more completely in a race, adapts the
proportions of the substances absorbed to the needs of the system; then if
a certain habitual expenditure of force leads to a certain habitual
absorption of force-evolving matters that are not available for growth; and
if, were there less need for such matters, the ability to absorb matters
available for growth would be increased to an equivalent extent; it follows
that the antagonism described does, in the long run, hold even without this
qualification. Hence, growth is substantially equivalent to the absorbed
nutriment, minus the nutriment used up in action.
This, however, is no answer to the question--why has individual growth a
limit?--why do the increments of growth bear decreasing ratios to the mass
and finally come to an end? The question is involved. There are more causes
than one why the excess of absorbed nutriment over expended nutriment must,
other things equal, become less as the size of the animal becomes greater.
In similarly-shaped bodies the masses, and therefore the weights, vary as
the cubes of the dimensions; whereas the powers of bearing the stresses
imposed by the weights vary as the squares of the dimensions. Suppose a
creature which a year ago was one foot high, has now become two feet high,
while it is unchanged in proportions and structure; what are the necessary
concomitant changes? It is eight times as heavy; that is to say, it has to
resist eight times the strain which gravitation puts upon certain of its
parts; and when there occurs sudden arrest of motion or sudden genesis of
motion, eight times the strain is put upon the muscles employed. Meanwhile
the muscles and bones have severally increased their abilities to bear
strains in proportion to the areas of their transverse sections, and hence
have severally only four times the tenacity they had. This relative
decrease in the power of bearing stress does not imply a relative decrease
in the power of generating energy and moving the body; for in the case
supposed the muscles have not only increased four times in their transverse
sections but have become twice as long, and will therefore generate an
amount of energy proportionate to their bulk. The implication is simply
that each muscle has only half the power to withstand those shocks and
strains which the creature's movements entail; and that consequently the
creature must be either less able to bear these, or must have muscles and
bones having relatively greater transverse dimensions: the result being
that greater cost of nutrition is inevitably caused and therefore a
correlative tendency to limit growth. This necessity will be seen still
more clearly if we leave out the motor apparatus, and consider only the
forces required and the means of supplying them. For since, in similar
bodies, the areas vary as the squares of the dimensions, and the masses
vary as the cubes; it follows that the absorbing surface has become four
times as great, while the weight to be moved by the matter absorbed has
become eight times as great. If then, a year ago, the absorbing surface
could take up twice as much nutriment as was needed for expenditure, thus
leaving one-half for growth, it is now able only just to meet expenditure,
and can provide nothing for growth. However great the excess of
assimilation over waste may be during the early life of an active organism,
we see that because a series of numbers increasing as the cubes, overtakes
a series increasing as the squares, even though starting from a much
smaller number, there must be reached, if the organism lives long enough, a
point at which the surplus assimilation is brought down to nothing--a point
at which expenditure balances nutrition--a state of moving equilibrium. The
only way in which the difficulty can be met is by gradual re-organization
of the alimentary system; and, in the first place, this entails direct cost
upon the organism, and, in the second place, indirect cost from the
carrying of greater weight: both tending towards limitation. There are two
other varying relations between degrees of growth and amounts of expended
force; one of which conspires with the last, while the other conflicts with
it. Consider, in the first place, the cost at which nutriment is
distributed through the body and effete matters removed from it. Each
increment of growth being added at the periphery of the organism, the force
expended in the transfer of matter must increase in a rapid progression--a
progression more rapid than that of the mass. But as the dynamic expense of
distribution is small compared with the dynamic value of the materials
distributed, this item in the calculation is unimportant. Now consider, in
the second place, the changing proportion between production and loss of
heat. In similar organisms the quantities of heat generated by similar
actions going on throughout their substance, must increase as the masses,
or as the cubes of the dimensions. Meanwhile, the surfaces from which loss
of heat takes place, increase only as the squares of the dimensions. Though
the loss of heat does not therefore increase only as the squares of the
dimensions, it certainly increases at a smaller rate than the cubes. And to
the extent that augmentation of mass results in a greater retention of
heat, it effects an economization of force. This advantage is not, however,
so important as at first appears. Organic heat is a concomitant of organic
action, and is so abundantly produced during action that the loss of it is
then usually of no consequence: indeed the loss is often not rapid enough
to keep the supply from rising to an inconvenient excess. It is chiefly in
respect of that maintenance of heat which is needful during quiescence,
that large organisms have an advantage over small ones in this relatively
diminished loss. Thus these two subsidiary relations between degrees of
growth and amounts of expended force, being in antagonism, we may conclude
that their differential result does not greatly modify the result of the
chief relation.
Comparisons of these deductions with the facts appear in some cases to
verify them and in other cases not to do so. Throughout the vegetal
kingdom, there are no distinct limits to growth except those which death
entails. Passing over a large proportion of plants which never exceed a
comparatively small size, because they wholly or partially die down at the
end of the year, and looking only at trees that annually send forth new
shoots, even when their trunks are hollowed by decay; we may ask--How does
growth happen here to be unlimited? The answer is, that plants are only
accumulators: they are in no very appreciable degree expenders. As they do
not undergo waste there is no reason why their growth should be arrested by
the equilibration of assimilation and waste. Again, among animals there
are sufficient reasons why the correspondence cannot be more than
approximate. Besides the fact above noted, that there are other varying
relations which complicate the chief one. We must bear in mind that the
bodies compared are not truly similar: the proportions of trunk to limbs
and trunk to head, vary considerably. The comparison is still more
seriously vitiated by the inconstant ratio between the constituents of
which the body is composed. In the flesh of adult mammalia, water forms
from 68 to 71 per cent., organic substance from 24 to 28 per cent., and
inorganic substance from 3 to 5 per cent.; whereas in the foetal state, the
water amounts to 87 per cent., and the solid organic constituents to only
11 per cent. Clearly this change from a state in which the force-evolving
matter forms one-tenth of the whole, to a state in which it forms two and a
half tenths, must greatly interfere with the parallelism between the actual
and the theoretical progression. Yet another difficulty may come under
notice. The crocodile is said to grow as long as it lives; and there
appears reason to think that some predaceous fishes, such as the pike, do
the same. That these animals of comparatively high organization have no
definite limits of growth, is, however, an exceptional fact due to the
exceptional non-fulfilment of those conditions which entail limitation.
What kind of life does a crocodile lead? It is a cold-blooded, or almost
cold-blooded, creature; that is, it expends very little for the maintenance
of heat. It is habitually inert: not usually chasing prey but lying in wait
for it; and undergoes considerable exertion only during its occasional
brief contests with prey. Such other exertion as is, at intervals, needful
for moving from place to place, is rendered small by the small difference
between the animal's specific gravity and that of water. Thus the crocodile
expends in muscular action an amount of force that is insignificant
compared with the force commonly expended by land-animals. Hence its
habitual assimilation is diminished much less than usual by habitual waste;
and beginning with an excessive disproportion between the two, it is quite
possible for the one never quite to lose its advance over the other while
life continues. On looking closer into such cases as this and that of the
pike, which is similarly cold-blooded, similarly lies in wait, and is
similarly able to obtain larger and larger kinds of prey as it increases in
size; we discover a further reason for this absence of a definite limit. To
overcome gravitative force the creature has not to expend a muscular power
that is large at the outset, and increases as the cubes of its dimensions:
its dense medium supports it. The exceptional continuance of growth
observed in creatures so circumstanced, is therefore perfectly explicable.
§ 46a. If we go back upon the conclusions set forth in the preceding
section, we find that from some of them may be drawn instructive
corollaries respecting the limiting sizes of creatures inhabiting different
media. More especially I refer to those varying proportions between mass
and stress from which, as we have seen, there results, along with
increasing size, a diminishing power of mechanical self-support: a relation
illustrated in its simplest form by the contrast between a dew-drop, which
can retain its spheroidal form, and the spread-out mass of water which
results when many dew-drops run together. The largest bird that flies (the
argument excludes birds which do not fly) is the Condor, which reaches a
weight of from 30 to 40 lbs. Why does there not exist a bird of the size of
an elephant? Supposing its habits to be carnivorous, it would have many
advantages in obtaining prey: mammals would be at its mercy. Evidently the
reason is one which has been pointed out--the reason that while the weight
to be raised and kept in the air by a bird increases as the cubes of its
dimensions, the ability of its bones and muscles to resist the strains
which flight necessitates, increases only as the squares of the dimensions.
Though, could the muscles withstand any tensile strain they were subject
to, the power like the weight might increase with the cubes, yet since the
texture of muscle is such that beyond a certain strain it tears, it results
that there is soon reached a size at which flight becomes impossible: the
structures must give way. In a preceding paragraph the limit to the size of
flying creatures was ascribed to the greater physiological cost of the
energy required; but it seems probable that the mechanical obstacle here
pointed out has a larger share in determining the limit.
In a kindred manner there results a limitation of growth in a land-animal,
which does not exist for an animal living in the water. If, after comparing
the agile movements of a dog with those of a cow, the great weight of which
obviously prevents agility; or if, after observing the swaying flesh of an
elephant as it walks along, we consider what would happen could there be
formed a land-animal equal in mass to the whale (the long Dinosaurs were
not proportionately massive) it needs no argument to show that such a
creature could not stand, much less move about. But in the water the strain
put upon its structures by the weights of its various parts is almost if
not quite taken away. Probably limitation in the quantity of food
obtainable becomes now the chief, if not the sole, restraint.
And here we may note, before leaving the topic, something like a converse
influence which comes into play among creatures inhabiting the water. Up to
the point at which muscles tear from over-strain, larger and smaller
creatures otherwise alike, remain upon a par in respect of the relative
amounts of energy they can evolve. Had they to encounter no resistance from
their medium, the implication would be that neither would have an advantage
over the other in respect of speed. But resistance of the medium comes into
play; and this, other things equal, gives to the larger creature an
advantage. It has been found, experimentally, that the forces to be
overcome by vessels moving through the water, built as they are with
immersed hinder parts which taper as fish taper, are mainly due to what is
called "skin-friction." Now in two fish unlike in size but otherwise
similar skin-friction bears to the energy that can be generated, a smaller
proportion in the larger than in the smaller; and the larger can therefore
acquire a greater velocity. Hence the reason why large fish, such as the
shark, become possible. In a habitat where there is no ambush (save in
exceptional cases like that of the _Lophius_ or Angler) everything depends
on speed; and if, other things equal, a larger fish had no mechanical
advantage over a smaller, a larger fish could not exist--could not catch
the requisite amount of prey.
§ 47. Obviously this antagonism between accumulation and expenditure, must
be a leading cause of the contrasts in size between allied organisms that
are in many respects similarly conditioned. The life followed by each kind
of animal is one involving a certain average amount of exertion for the
obtainment of a given amount of nutriment--an exertion, part of which goes
to the gathering or catching of food, part to the tearing and mastication
of it, and part to the after-processes requisite for separating the
nutritive molecules--an exertion which therefore varies according as the
food is abundant or scarce, fixed or moving, according as it is
mechanically easy or difficult to deal with when secured, and according as
it is, or is not, readily soluble. Hence, while among animals of the same
species having the same mode of life, there will be a tolerably constant
ratio between accumulation and expenditure, and therefore a tolerably
constant limit of growth, there is every reason to expect that different
species, following different modes of life, will have unlike ratios between
accumulation and expenditure, and therefore unlike limits of growth.
Though the facts as inductively established, show a general harmony with
this deduction, we cannot usually trace it in any specific way; since the
conflicting and conspiring factors which affect growth are so numerous.
§ 48. One of the chief causes, if not the chief cause, of the differences
between the sizes of organisms, has yet to be considered. We are introduced
to it by pushing the above inquiry a little further. Small animals have
been shown to possess an advantage over large ones in the greater ratio
which, other things equal, assimilation bears to expenditure; and we have
seen that hence small animals in becoming large ones, gradually lose that
surplus of assimilative power which they had, and eventually cannot
assimilate more than is required to balance waste. But how come these
animals while young and small to have surplus assimilative powers? Have all
animals equal surpluses of assimilative powers? And if not, how far do
differences between the surpluses determine differences between the limits
of growth? We shall find, in the answers to these questions, the
interpretation of many marked contrasts in growth that are not due to any
of the causes above assigned. For example, an ox immensely exceeds a sheep
in mass. Yet the two live from generation to generation in the same fields,
eat the same grass, obtain these aliments with the same small expenditure
of energy, and differ scarcely at all in their degrees of organization.
Whence arises, then, their striking unlikeness of bulk?
We noted when studying the phenomena of growth inductively, that organisms
of the larger and higher types commence their separate existences as masses
of organic matter having tolerable magnitudes. Speaking generally, we saw
that throughout each organic sub-kingdom the acquirement of great bulk
occurs only where the incipient bulk and organization are considerable; and
that they are the more considerable in proportion to the complexity of the
life which the organism is to lead.
The deductive interpretation of this induction may best be commenced by an
analogy. A street orange-vendor makes but a trifling profit on each
transaction; and unless more than ordinarily fortunate, he is unable to
realize during the day a larger amount than will meet his wants; leaving
him to start on the morrow in the same condition as before. The trade of
the huxter in ounces of tea and half-pounds of sugar, is one similarly
entailing much labour for small returns. Beginning with a capital of a few
pounds, he cannot have a shop large enough, or goods sufficiently abundant
and various, to permit an extensive business. He must be content with the
half-pence and pence which he makes by little sales to poor people; and if,
avoiding bad debts, he is able by strict economy to accumulate anything, it
can be but a trifle. A large retail trader is obliged to lay out much money
in fitting up an adequate establishment; he must invest a still greater sum
in stock; and he must have a further floating capital to meet the charges
that fall due before his returns come in. Setting out, however, with means
enough for these purposes, he is able to make many and large sales; and so
to get greater and more numerous increments of profit. Similarly, to get
returns in thousands merchants and manufacturers must make their
investments in tens of thousands. In brief, the rate at which a man's
wealth accumulates is measured by the surplus of income over expenditure;
and this, save in exceptionably favourable cases, is determined by the
capital with which he begins business. Now applying the analogy, we may
trace in the transactions of an organism, the same three ultimate elements.
There is the expenditure required for the obtainment and digestion of food;
there is the gross return in the shape of nutriment assimilated or fit for
assimilation; and there is the difference between this gross return of
nutriment and the nutriment that was used up in the labour of securing
it--a difference which may be a profit or a loss. Clearly, however, a
surplus implies that the force expended is less than the force latent in
the assimilated food. Clearly, too, the increment of growth is limited to
the amount of this surplus of income over expenditure; so that large growth
implies both that the excess of nutrition over waste shall be relatively
considerable, and that the waste and nutrition shall be on extensive
scales. And clearly, the ability of an organism to expend largely and
assimilate largely, so as to make a large surplus, presupposes a large
physiological capital in the shape of organic matter more or less developed
in its structural arrangements.
Throughout the vegetal kingdom, the illustrations of this truth are not
conspicuous and regular: the obvious reason being that since plants are
accumulators and in so small a degree expenders, the premises of the above
argument are but very partially fulfilled. The food of plants (excepting
Fungi and certain parasites) being in great measure the same for all, and
bathing all so that it can be absorbed without effort, their vital
processes result almost entirely in profit. Once fairly rooted in a fit
place, a plant may thus from the outset add a very large proportion of its
entire returns to capital; and may soon be able to carry on its processes
on a large scale, though it does not at first do so. When, however, plants
are expenders, namely, during their germination and first stages of growth,
their degrees of growth _are_ determined by their amounts of vital capital.
It is because the young tree commences life with a ready-formed embryo and
store of food sufficient to last for some time, that it is enabled to
strike root and lift its head above the surrounding herbage. Throughout the
animal kingdom, however, the necessity of this relation is everywhere
obvious. The small carnivore preying on small herbivores, can increase in
size only by small increments: its organization unfitting it to digest
larger creatures, even if it can kill them, it cannot profit by amounts of
nutriment exceeding a narrow limit; and its possible increments of growth
being small to set out with, and rapidly decreasing, must come to an end
before any considerable size is attained. Manifestly the young lion, born
of tolerable bulk, suckled until much bigger, and fed until half-grown, is
enabled by the power and organization which he thus gets _gratis_, to catch
and kill animals big enough to give him the supply of nutriment needed to
meet his large expenditure and yet leave a large surplus for growth. Thus,
then, is explained the above-named contrast between the ox and the sheep. A
calf and a lamb commence their physiological transactions on widely
different scales; their first increments of growth are similarly contrasted
in their amounts; and the two diminishing series of such increments end at
similarly-contrasted limits.
§ 49. Such are the several conditions by which the phenomena of growth are
determined. Conspiring and conflicting in endless unlike ways and degrees,
they in every case qualify more or less differently each other's effects.
Hence it happens that we are obliged to state each generalization as true
on the average, or to make the proviso--other things equal.
Understood in this qualified form, our conclusions are these. First, that
growth being an integration with the organism of such environing matters as
are of like natures with the matters composing the organism, its growth is
dependent on the available supply of them. Second, that the available
supply of assimilable matter being the same, and other conditions not
dissimilar, the degree of growth varies according to the surplus of
nutrition over expenditure--a generalization which is illustrated in some
of the broader contrasts between different divisions of organisms. Third,
that in the same organism the surplus of nutrition over expenditure differs
at different stages; and that growth is unlimited or has a definite limit,
according as the surplus does or does not rapidly decrease. This
proposition we found exemplified by the almost unceasing growth of
organisms that expend relatively little energy; and by the definitely
limited growth of organisms that expend much energy. Fourth, that among
organisms which are large expenders of force, the size ultimately attained
is, other things equal, determined by the initial size: in proof of which
conclusion we have abundant facts, as well as the _a priori_ necessity that
the sum-totals of analogous diminishing series, must depend upon the
amounts of their initial terms. Fifth, that where the likeness of other
circumstances permits a comparison, the possible extent of growth depends
on the degree of organization; an inference testified to by the larger
forms among the various divisions and sub-divisions of organisms.
CHAPTER II.
DEVELOPMENT.[19]
§ 50. Certain general aspects of Development may be studied apart from any
examination of internal structures. These fundamental contrasts between the
modes of arrangement of parts, originating, as they do, the leading
external distinctions among the various forms of organization, will be best
dealt with at the outset. If all organisms have arisen by Evolution, it is
of course not to be expected that such several modes of development can be
absolutely demarcated: we are sure to find them united by transitional
modes. But premising that a classification of modes can but approximately
represent the facts, we shall find our general conceptions of Development
aided by one.
Development is primarily _central_. All organic forms of which the entire
history is known, set out with a symmetrical arrangement of parts round a
centre. In organisms of the lowest grade no other mode of arrangement is
ever definitely established; and in the highest organisms central
development, though subordinate to another mode of development, continues
to be habitually shown in the changes of minute structure. Let us glance at
these propositions in the concrete. Practically every plant and every
animal in its earliest stage is a portion of protoplasm, in the great
majority of cases approximately spherical but sometimes elongated,
containing a rounded body consisting of specially modified protoplasm,
which is called a nucleus; and the first changes that occur in the germ
thus constituted, are changes that take place in this nucleus, followed by
changes round the centres produced by division of this original centre.
From this type of structure, the simplest organisms do not depart; or
depart in no definite or conspicuous ways. Among plants, many of the
simplest _Algæ_ and _Fungi_ permanently maintain such a central
distribution; while among animals it is permanently maintained by creatures
like the _Gregarina_, and in a different manner by the _Amoeba_,
_Actinophrys_, and their allies: the irregularities which are many and
great do not destroy this general relation of parts. In larger organisms,
made up chiefly of units that are analogous to these simplest organisms,
the formation of units ever continues to take place round nuclei; though
usually the nuclei soon cease to be centrally placed.
Central development may be distinguished into _unicentral_ and
_multicentral_; according as the product of the original germ develops more
or less symmetrically round one centre, or develops without subordination
to one centre--develops, that is, in subordination to many centres.
Unicentral development, as displayed not in the formation of single cells
but in the formation of aggregates, is not common. The animal kingdom shows
it only in some of the small group of colonial _Radiolaria_. It is feebly
represented in the vegetal kingdom by a few members of the _Volvocineæ_. On
the other hand, multicentral development, or development round
insubordinate centres, is variously exemplified in both divisions of the
organic world. It is exemplified in two distinct ways, according as the
insubordination among the centres of development is partial or total. We
may most conveniently consider it under the heads hence arising.
Total insubordination among the centres of development, is shown where the
units or cells, as fast as they are severally formed, part company and lead
independent lives. This, in the vegetal kingdom, habitually occurs among
the _Protophyta_, and in the animal kingdom, among the _Protozoa_. Partial
insubordination is seen in those somewhat advanced organisms, that consist
of units which, though they have not separated, have so little mutual
dependence that the aggregate they form is irregular. Among plants, the
Thallophytes very generally exemplify this mode of development. Lichens,
spreading with flat or corrugated edges in this or that direction as the
conditions determine, have no manifest co-ordination of parts. In the
_Algæ_ the Nostocs and various other forms similarly show us an
unsymmetrical structure. Of _Fungi_ we may say that creeping kinds display
no further dependence of one part on another than is implied by their
cohesion. And even in such better-organized plants as the _Marchantia_, the
general arrangement shows no reference to a directive centre. Among animals
many of the Sponges in their adult forms may be cited as devoid of that
co-ordination implied by symmetry: the units composing them, though they
have some subordination to local centres, have no subordination to a
general centre. To distinguish that kind of development in which the whole
product of a germ coheres in one mass, from that kind of development in
which it does not, Professor Huxley has introduced the words "_continuous_"
and "_discontinuous_;" and these seem the best fitted for the purpose.
Multicentral development, then, is divisible into continuous and
discontinuous.
From central development we pass insensibly to that higher kind of
development for which _axial_ seems the most appropriate name. A tendency
towards this is vaguely manifested almost everywhere. The great majority
even of _Protophyta_ and _Protozoa_ have different longitudinal and
transverse dimensions--have an obscure if not a distinct axial structure.
The originally spheroidal and polyhedral units out of which higher
organisms are mainly built, usually pass into shapes that are subordinated
to lines rather than to points. And in the higher organisms, considered as
wholes, an arrangement of parts in relation to an axis is distinct and
nearly universal. We see it in the superior orders of Thallophytes; and in
all the cormophytic plants. With few exceptions the _Coelenterata_ clearly
exhibit it; it is traceable, though less conspicuously, throughout the
_Mollusca_; and the _Annelida_, _Arthropoda_, and _Vertebrata_ uniformly
show it with perfect definiteness.
This kind of development, like the first kind, is of two orders. The whole
germ-product may arrange itself round a single axis, or it may arrange
itself round many axes: the structure may be _uniaxial_ or _multiaxial_.
Each division of the organic kingdom furnishes examples of both these
orders. In such _Fungi_ as exhibit axial development at all, we commonly
see development round a single axis. Some of the _Algæ_, as the common
tangle, show us this arrangement. And of the higher plants, many
Monocotyledons and small Dicotyledons are uniaxial. Of animals, the
advanced are without exception in this category. There is no known
vertebrate in which the whole of the germ-product is not subordinated to a
single axis. In the _Arthropoda_, the like is universal; as it is also in
the superior orders of _Mollusca_. Multiaxial development occurs in most of
the plants we are familiar with--every branch of a shrub or tree being an
independent axis. But while in the vegetal kingdom multiaxial development
prevails among the highest types, in the animal kingdom it prevails only
among the lowest types. It is extremely general, if not universal, among
the _Coelenterata_; it is characteristic of the _Polyzoa_; the compound
Ascidians exhibit it; and it is seen, though under another form, in certain
of the inferior Annelids.
Development that is axial, like development that is central, may be either
continuous or discontinuous: the parts having different axes may continue
united, or they may separate. Instances of each alternative are supplied by
both plants and animals. Continuous multiaxial development is that which
plants usually display, and need not be illustrated further than by
reference to every garden. As cases of it in animals may be named all the
compound _Hydrozoa_ and _Actinozoa_; and such ascidian forms as the
_Botryllidæ_. Of multiaxial development that is discontinuous, a familiar
instance among plants exists in the common strawberry. This sends out over
the neighbouring surface, long slender shoots, bearing at their extremities
buds that presently strike roots and become new individuals; and these by
and by lose their connexions with the original axis. Other plants there are
that produce certain specialized buds called bulbils, which separating
themselves and falling to the ground, grow into independent plants. Among
animals the fresh-water polype very clearly shows this mode of development:
the young polypes, budding out from its surface, severally arrange their
parts around distinct axes, and eventually detaching themselves, lead
separate lives, and produce other polypes after the same fashion. By some
of the lower _Annelida_, this multiplication of axes from an original axis,
is carried on after a different manner: the string of segments
spontaneously divides; and after further growth, division recurs in one or
both of the halves. Moreover in the _Syllis ramosa_, there occurs lateral
branching also.
Grouping together its several modes as above delineated, we see that
{ Unicentral
{ Central { or { Continuous
{ { Multicentral { or
{ { Discontinuous
DEVELOPMENT is { or
{
{ { Uniaxial
{ Axial { or { Continuous
{ Multiaxial { or
{ Discontinuous
Any one well acquainted with the facts, may readily raise objections to
this arrangement. He may name forms which do not obviously come under any
of these heads. He may point to plants that are for a time multicentral but
afterwards develop axially. And from lower types of animals he may choose
many in which the continuous and discontinuous modes are both displayed.
But, as already hinted, an arrangement free from such anomalies must be
impossible, if the various kinds of organization have arisen by Evolution.
The one above sketched out is to be regarded as a rough grouping of the
facts, which helps us to a conception of them in their totality; and, so
regarded, it will be of service when we come to treat of Individuality and
Reproduction.
§ 51. From these most general external aspects of organic development, let
us now turn to its internal and more special aspects. When treating of
Evolution as a universal process of things, a rude outline of the course of
structural changes in organisms was given (_First Principles_, §§ 110, 119,
132). Here it will be proper to describe these changes more fully.
The bud of any common flowering plant in its earliest stage, consists of a
small hemispherical or sub-conical projection. While it increases most
rapidly at the apex, this presently develops on one side of its base, a
smaller projection of like general shape with itself. Here is the rudiment
of a leaf, which presently spreads more or less round the base of the
central hemisphere or main axis. At the same time that the central
hemisphere rises higher, this lateral prominence, also increasing, gives
rise to subordinate prominences or lobes. These are the rudiments of
stipules, where the leaves are stipulated. Meanwhile, towards the other
side of the main axis and somewhat higher up, another lateral prominence
arising marks the origin of a second leaf. By the time that the first leaf
has produced another pair of lobes, and the second leaf has produced its
primary pair, the central hemisphere, still increasing at its apex,
exhibits the rudiment of a third leaf. Similarly throughout. While the germ
of each succeeding leaf thus arises, the germs of the previous leaves, in
the order of their priority, are changing their rude nodulated shapes into
flattened-out expansions; which slowly put on those sharp outlines they
show when unfolded. Thus from that extremely indefinite figure, a rounded
lump, giving off from time to time lateral lumps, which severally becoming
symmetrically lobed gradually assume specific and involved forms, we pass
little by little to that comparatively complex thing--a leaf-bearing shoot.
Internally, a bud undergoes analogous changes; as witness this
account:--"The general mass of thin-walled parenchymatous cells which
occupies the apical region, and forms the _growing point_ of the shoot, is
covered by a single external layer of similar cells, which increase in
number by the formation of new walls in one direction only, perpendicular
to the surface of the shoot, and thus give rise only to the _epidermis_ or
single layer of cells covering the whole surface of the shoot. Meanwhile
the general mass below grows as a whole, its constituent cells dividing in
all directions. Of the new cells so formed, those removed by these
processes of growth and division from the actual apex, begin, at a greater
or less distance from it, to show signs of the differentiation which will
ultimately lead to the formation of the various tissues enclosed by the
epidermis of the shoot. First the pith, then the vascular bundles, and then
the cortex of the shoot, begin to take on their special characters."
Similarly with secondary structures, as the lateral buds whence leaves
arise. In the, at first, unorganized mass of cells constituting the
rudimentary leaf, there are formed vascular bundles which eventually become
the veins of the leaf; and _pari passu_ with these are formed the other
tissues of the leaf. Nor do we fail to find an essentially parallel set of
changes, when we trace the histories of the individual cells. While the
tissues they compose are separating, the cells are growing step by step
more unlike. Some become flat, some polyhedral, some cylindrical, some
prismatic, some spindle-shaped. These develop spiral thickenings in their
interiors; and those, reticulate thickenings. Here a number of cells unite
together to form a tube: and there they become almost solid by the internal
deposition of woody or other substance. Through such changes, too numerous
and involved to be here detailed, the originally uniform cells go on
diverging and rediverging until there are produced various forms that seem
to have very little in common.
The arm of a man makes its first appearance in as simple a way as does the
shoot of a plant. According to Bischoff, it buds-out from the side of the
embryo as a little tongue-shaped projection, presenting no differences of
parts; and it might serve for the rudiment of some one of the various other
organs that also arise as buds. Continuing to lengthen, it presently
becomes somewhat enlarged at its end; and is then described as a pedicle
bearing a flattened, round-edged lump. This lump is the representative of
the future hand, and the pedicle of the future arm. By and by, at the edges
of this flattened lump, there appear four clefts, dividing from each other
the buds of the future fingers; and the hand as a whole grows a little more
distinguishable from the arm. Up to this time the pedicle has remained one
continuous piece, but it now begins to show a bend at its centre, which
indicates the division into arm and forearm. The distinctions thus rudely
indicated gradually increase: the fingers elongate and become jointed, and
the proportions of all the parts, originally very unlike those of the
complete limb, slowly approximate to them. During its bud-like stage, the
rudimentary arm consists only of partially-differentiated tissues. By the
diverse changes these gradually undergo they are transformed into bones,
muscles, blood-vessels, and nerves. The extreme softness and delicacy of
these primary tissues, renders it difficult to trace the initial stages of
the differentiations. In consequence of the colour of their contents, the
blood-vessels are the first parts to become distinct. Afterwards the
cartilaginous parts, which are the bases of the future bones, become marked
out by the denser aggregation of their constituent cells, and by the
production between these of a hyaline substance which unites them into a
translucent mass. When first perceptible, the muscles are gelatinous, pale,
yellowish, transparent, and indistinguishable from their tendons. The
various other tissues of which the arm consists, beginning with very
faintly-marked differences, become day by day more definite in their
qualitative appearances. In like manner the units composing these tissues
severally assume increasingly-specific characters. The fibres of muscle, at
first made visible in the midst of their gelatinous matrix only by
immersion in alcohol, grow more numerous and distinct; and by and by they
begin to exhibit transverse stripes. The bone-cells put on by degrees their
curious structure of branching canals. And so in their respective ways with
the units of skin and the rest.
Thus in each of the organic sub-kingdoms, we see this change from an
incoherent, indefinite homogeneity to a coherent, definite heterogeneity,
illustrated in a quadruple way. The originally-like units called cells,
become unlike in various ways, and in ways more numerous and marked as the
development goes on. The several tissues which these several classes of
cells form by aggregation, grow little by little distinct from each other;
and little by little put on those structural complexities that arise from
differentiations among their component units. In the shoot, as in the limb,
the external form, originally very simple, and having much in common with
simple forms in general, gradually acquires an increasing complexity, and
an increasing unlikeness to other forms. Meanwhile, the remaining parts of
the organism to which the shoot or limb belongs, having been severally
assuming structures divergent from one another and from that of this
particular shoot or limb, there has arisen a greater heterogeneity in the
organism as a whole.
§ 52. One of the most remarkable inductions of embryology comes next in
order. And here we find illustrated the general truth that in mental
evolution as in bodily evolution the progress is from the indefinite and
inexact to the definite and exact. For the first statement of this
induction was but an adumbration of the correct statement.
As a result of his examinations von Baer alleged that in its earliest stage
every organism has the greatest number of characters in common with all
other organisms in their earliest stages; that at a stage somewhat later
its structure is like the structures displayed at corresponding phases by a
less extensive assemblage of organisms; that at each subsequent stage
traits are acquired which successively distinguish the developing embryo
from groups of embryos that it previously resembled--thus step by step
diminishing the group of embryos which it still resembles; and that thus
the class of similar forms is finally narrowed to the species of which it
is a member. This abstract proposition will perhaps not be fully
comprehended by the general reader. It will be best to re-state it in a
concrete shape. Supposing the germs of all kinds of organisms to be
simultaneously developing, we may say that all members of the vast
multitude take their first steps in the same direction; that at the second
step one-half of this vast multitude diverges from the other half, and
thereafter follows a different course of development; that the immense
assemblage contained in either of these divisions very soon again shows a
tendency to take two or more routes of development; that each of the two or
more minor assemblages thus resulting, shows for a time but small
divergences among its members, but presently again divides into groups
which separate ever more widely as they progress; and so on until each
organism, when nearly complete, is accompanied in its further modifications
only by organisms of the same species; and last of all, assumes the
peculiarities which distinguish it as an individual--diverges to a slight
extent to the organisms it is most like.
But, as above said, this statement is only an adumbration. The order of
Nature is habitually more complex than our generalizations represent it as
being--refuses to be fully expressed in simple formulæ; and we are obliged
to limit them by various qualifications. It is thus here. Since von Baer's
day the careful observations of numerous observers have shown his
allegation to be but approximately true. Hereafter, when discussing the
embryological evidence of Evolution, the causes of deviations will be
discussed. For the present it suffices to recognize as unquestionable the
fact that whereas the germs of organisms are extremely similar, they
gradually diverge widely, in modes now regular and now irregular, until in
place of a multitude of forms practically alike we finally have a multitude
of forms most of which are extremely unlike. Thus, in conformity with the
law of evolution, not only do the parts of each organism advance from
indefinite homogeneity to definite heterogeneity, but the assemblage of all
organisms does the same: a truth already indicated in _First Principles_.
§ 53. This comparison between the course of development, in any creature,
and the course of development in all other creatures--this arrival at the
conclusion that the course of development in each, at first the same as in
all others, becomes stage by stage differentiated from the courses in all
others, brings us within view of an allied conclusion. If we contemplate
the successive stages passed through by any higher organism, and observe
the relation between it and its environment at each of these stages; we
shall see that this relation is modified in a way analogous to that in
which the relation between the organism and its environment is modified, as
we advance from the lowest to the highest grades. Along with the
progressing differentiation of each organism from others, we find a
progressing differentiation of it from its environment; like that
progressing differentiation from the environment which we meet with in the
ascending forms of life. Let us first glance at the way in which the
ascending forms of life exhibit this progressing differentiation from the
environment.
In the first place, it is illustrated in _structure_. Advance from the
homogeneous to the heterogeneous, itself involves an increasing distinction
from the inorganic world. Passing over the _Protozoa_, of which the
simplest probably disappeared during the earliest stages of organic
evolution, and limiting our comparison to the _Metazoa_, we see that low
types of these, as the _Coelenterata_, are relatively simple in their
organization; and the ascent to organisms of greater and greater complexity
of structure, is an ascent to organisms which are in that respect more
strongly contrasted with the structureless environment. In _form_, again,
we see the same truth. An ordinary characteristic of inorganic matter is
its indefiniteness of form; and this is also a characteristic of the lower
organisms, as compared with the higher. Speaking generally, plants are less
definite than animals, both in shape and size--admit of greater
modifications from variations of position and nutrition. Among animals, the
simplest Rhizopods may almost be called amorphous: the form is never
specific, and is constantly changing. Of the organisms resulting from the
aggregation of such creatures, we see that while some, as the
_Foraminifera_, assume a certain definiteness of form, in their shells at
least, others, as the Sponges, are very irregular. The Zoophytes and the
_Polyzoa_ are compound organisms, most of which have a mode of growth not
more determinate than that of plants. But among the higher animals, we find
not only that the mature shape of each species is very definite, but that
the individuals of each species differ little in size. A parallel increase
of contrast is seen in _chemical composition_. With but few exceptions, and
those only partial ones, the lowest animal and vegetal forms are
inhabitants of the water; and water is almost their sole constituent.
Desiccated _Protophyta_ and _Protozoa_ shrink into mere dust; and among the
Acalephes we find but a few grains of solid matter to a pound of water. The
higher aquatic plants, in common with the higher aquatic animals,
possessing as they do increased tenacity of substance, also contain a
greater proportion of the organic elements; further they show us a greater
variety of composition in their different parts; and thus in both ways are
chemically more unlike their medium. And when we pass to the superior
classes of organisms--land-plants and land-animals--we see that, chemically
considered, they have little in common either with the earth on which they
stand or the air which surrounds them. In _specific gravity_ too, we may
note a like truth. The simplest forms, in common with the spores and
gemmules of higher ones, are as nearly as may be of the same specific
gravity as the water in which they float; and though it cannot be said that
among aquatic creatures, superior specific gravity is a standard of general
superiority, yet we may fairly say that the higher orders of them, when
divested of the appliances by which their specific gravity is regulated,
differ more from water in their relative weights than do the lowest. In
terrestrial organisms, the contrast becomes marked. Trees and plants, in
common with insects, reptiles, mammals, birds, are all of a specific
gravity considerably less than that of the earth and immensely greater than
that of the air. Yet further, we see the law fulfilled in respect of
_temperature_. Plants generate but extremely small quantities of heat,
which are to be detected only by delicate experiments; and practically they
may be considered as having the same temperature as their environment. The
temperature of aquatic animals is very little above that of the surrounding
water: that of the invertebrata being mostly less than a degree above it,
and that of fishes not exceeding it by more than two or three degrees; save
in the case of some large red-blooded fishes, as the tunny, which exceed it
in temperature by nearly ten degrees. Among insects the range is from two
to ten degrees above that of the air: the excess varying according to their
activity. The heat of reptiles is from four to fifteen degrees more than
the heat of their medium. While mammals and birds maintain a heat which
continues almost unaffected by external variations, and is often greater
than that of the air by seventy, eighty, ninety, and even a hundred
degrees. Once more, in greater _self-mobility_ a progressive
differentiation is traceable. The chief characteristic by which we
distinguish dead matter is its inertness: some form of independent motion
is our most familiar proof of life. Passing over the indefinite border-land
between the animal and vegetal kingdoms, we may roughly class plants as
organisms which, while they exhibit that kind of motion implied in growth,
are not only devoid of locomotive power, but with some unimportant
exceptions are devoid of the power of moving their parts in relation to
each other; and thus are less differentiated from the inorganic world than
animals. Though in those microscopic _Protophyta_ and _Protozoa_ inhabiting
the water we see locomotion produced by ciliary action; yet this
locomotion, while rapid relatively to the sizes of their bodies, is
absolutely slow. Of the _Coelenterata_ a great part are either permanently
rooted or habitually stationary; and so have scarcely any self-mobility but
that implied in the relative movements of parts; while the rest, of which
the common jelly-fish serves as a sample, have mostly but little ability to
move themselves through the water. Among the higher aquatic
_Invertebrata_,--cuttlefishes and lobsters, for instance,--there is a very
considerable power of locomotion; and the aquatic _Vertebrata_ are,
considered as a class, much more active in their movements than the other
inhabitants of the water. But it is only when we come to air-breathing
creatures that we find the vital characteristics of self-mobility
manifested in the highest degree. Flying insects, mammals, birds, travel
with velocities far exceeding those attained by any of the lower classes of
animals. Thus, on contemplating the various grades of organisms in their
ascending order, we find them more and more distinguished from their
inanimate media, in _structure_, in _form_, in _chemical composition_, in
_specific gravity_, in _temperature_, in _self-mobility_. It is true that
this generalization does not hold with complete regularity. Organisms which
are in some respects the most strongly contrasted with the environing
inorganic world, are in other respects less contrasted than inferior
organisms. As a class, mammals are higher than birds; and yet they are of
lower temperature and have smaller powers of locomotion. The stationary
oyster is of higher organization than the free-swimming medusa; and the
cold-blooded and less heterogeneous fish is quicker in its movements than
the warm-blooded and more heterogeneous sloth. But the admission that the
several aspects under which this increasing contrast shows itself, bear
variable ratios to each other, does not conflict with the general truth
that as we ascend in the hierarchy of organisms, we meet with not only an
increasing differentiation of parts but also an increasing differentiation
from the surrounding medium in sundry other physical attributes. It would
seem that this trait has some necessary connexion with superior vital
manifestations. One of those lowly gelatinous forms, so transparent and
colourless as to be with difficulty distinguished from the water it floats
in, is not more like its medium in chemical, mechanical, optical, thermal,
and other properties, than it is in the passivity with which it submits to
all the influences and actions brought to bear upon it; while the mammal
does not more widely differ from inanimate things in these properties, than
it does in the activity with which it meets surrounding changes by
compensating changes in itself. And between these extremes, these two kinds
of contrast vary together. So that in proportion as an organism is
physically like its environment it remains a passive partaker of the
changes going on in its environment; while in proportion as it is endowed
with powers of counteracting such changes, it exhibits greater unlikeness
to its environment.[20]
If now, from this same point of view, we consider the relation borne to its
environment by any superior organism in its successive stages, we find an
analogous series of contrasts. Of course in respect of degrees of
_structure_ the parallelism is complete. The difference, at first small,
between the little-structured germ and the little-structured inorganic
world, necessarily becomes greater, step by step, as the differentiations
of the germ become more numerous and definite. How of _form_ the like
holds is equally manifest. The sphere, which is the point of departure
common to all organisms, is the most generalized of figures; and one that
is, under various circumstances, assumed by inorganic matter. But as it
develops it loses all likeness to inorganic objects in the environment; and
eventually becomes distinct even from nearly all organic objects in its
environment. In _specific gravity_ the alteration, though not very marked,
is still in the same direction. Development being habitually accompanied by
a relative decrease in the quantity of water and an increase in the
quantity of constituents that are heavier than water, there results a small
augmentation of relative weight. In power of maintaining a _temperature_
above that of surrounding things, the differentiation from the environment
that accompanies development is marked. All ova are absolutely dependent
for their heat on external sources. The mammalian young one is, during its
uterine life, dependent on the maternal heat; and at birth has but a
partial power of making good the loss by radiation. But as it advances in
development it gains an ability to maintain a constant temperature above
that of surrounding things: so becoming markedly unlike them. Lastly, in
_self-mobility_ this increasing contrast is no less decided. Save in a few
aberrant tribes, chiefly parasitic, we find the general fact to be that the
locomotive power, totally absent or very small at the outset, increases
with the advance towards maturity. The more highly developed the organism
becomes, the stronger grows the contrast between its activity and the
inertness of the objects amid which it moves.
Thus we may say that the development of an individual organism, is at the
same time a differentiation of its parts from each other, and a
differentiation of the consolidated whole from the environment; and that in
the last as in the first respect, there is a general analogy between the
progression of an individual organism and the progression from the lowest
orders of organisms to the highest orders. It may be remarked that some
kinship seems to exist between these generalizations and the doctrine of
Schelling, that Life is the tendency to individuation. For evidently, in
becoming more distinct from one another and from their environment,
organisms acquire more marked individualities. As far as I can gather from
outlines of his philosophy, however, Schelling entertained this conception
in a general and transcendental sense, rather than in a special and
scientific one.
§ 54. Deductive interpretations of these general facts of development, in
so far as they are possible, must be postponed until we arrive at the
fourth and fifth divisions of this work. There are, however, one or two
general aspects of these inductions which may be here conveniently dealt
with deductively.
Grant that each organism is at the outset relatively homogeneous and that
when complete it is relatively heterogeneous, and it necessarily follows
that development is a change from the homogeneous to the heterogeneous--a
change during which there must be gone through all the gradations of
heterogeneity that lie between these extremes. If, again, there is at first
indefiniteness and at last definiteness, the transition cannot but be from
the one to the other of these through all intermediate degrees of
definiteness. Further, if the parts, originally incoherent or uncombined,
eventually become relatively coherent or combined, there must be a
continuous increase of coherence or combination. Hence the general truth
that development is a change from incoherent, indefinite homogeneity, to
coherent, definite heterogeneity, becomes a self-evident one when
observation has shown us the state in which organisms begin and the state
in which they end.
Just in the same way that the growth of an entire organism is carried on by
abstracting from the environment substances like those composing the
organism; so the production of each organ within the organism is carried on
by abstracting from the substances contained in the organism, those
required by this particular organ. Each organ at the expense of the
organism as a whole, integrates with itself certain kinds and proportions
of the matters circulating around it; in the same way that the organism as
a whole, integrates with itself certain kinds and proportions of matters at
the expense of the environment as a whole. So that the organs are
qualitatively differentiated from each other, in a way analogous to that by
which the entire organism is qualitatively differentiated from things
around it. Evidently this selective assimilation illustrates the general
truth, set forth and illustrated in _First Principles_, that like units
tend to segregate. It illustrates, moreover, the further aspect of this
general truth, that the pre-existence of a mass of certain units produces a
tendency for diffused units of the same kind to aggregate with this mass
rather than elsewhere. It has been shown of particular salts, A and B,
co-existing in a solution not sufficiently concentrated to crystallize,
that if a crystal of the salt A be put into the solution, it will increase
by uniting with itself the dissolved atoms of the salt A; and that
similarly, though there otherwise takes place no deposition of the salt B,
yet if a crystal of the salt B is placed in the solution, it will exercise
a coercive force on the diffused atoms of this salt, and grow at their
expense. Probably much organic assimilation occurs in the same way.
Particular parts of the organism are composed of special units or have the
function of secreting special units, which are ever present in them in
large quantities. The fluids circulating through the body contain special
units of this same order. And these diffused units are continually being
deposited along with the groups of like units that already exist. How
purely physical are the causes of this selective assimilation, is, indeed,
shown by the fact that abnormal constituents of the blood are segregated in
the same way. The chalky deposits of gout beginning at certain points,
collect more and more around those points. And similarly in numerous
pustular diseases. Where the component units of an organ, or some of them,
do not exist as such in the circulating fluids, but are formed out of
elements or compounds that exist separately in the circulating fluids, the
process of differential assimilation must be of a more complex kind. Still,
however, it seems not impossible that it is carried on in an analogous way.
If there be an aggregate of compound atoms, each of which contains the
constituents A, B, C; and if round this aggregate the constituents A and B
and C are diffused in uncombined states; it may be suspected that the
coercive force of these aggregated compound atoms A, B, C, may not only
bring into union with themselves adjacent compound atoms A, B, C, but may
cause the adjacent constituents A and B and C to unite into such compound
atoms, and then aggregate with the mass.
CHAPTER II^A.
STRUCTURE.[21]
§ 54a. As, in the course of evolution, we rise from the smallest to the
largest aggregates by a process of integration, so do we rise by a process
of differentiation from the simplest to the most complex aggregates. The
initial types of life are at once extremely small and almost structureless.
Passing over those which swarm in the air, the water, and the soil, and are
now some of them found to be causes of diseases, we may set out with those
ordinarily called _Protozoa_ and _Protophyta_: the lowest of which,
however, are either at once plants and animals, or are now one and now the
other.
That the first living things were minute portions of simple protoplasm is
implied by the general theory of Evolution; but we have no evidence that
such portions exist now. Even admitting that there are protoplasts (using
this word to include plant and animal types) which are without nuclei,
still they are not homogeneous--they are granular. Whether a nucleus is
always present is a question still undecided; but in any case the types
from which it is absent are extremely exceptional. Thus the most general
structural traits of protoplasts are--the possession of an internal part,
morphologically central though often not centrally situated, a general mass
of protoplasm surrounding it, and an inclosing differentiated portion in
contact with the environment. These essential elements are severally
subject to various complications.
In some simple types the limiting layer or cortical substance can scarcely
be said to exist as a separate element. The exoplasm, distinguished from
the endoplasm by absence or paucity of granules, is continually changing
places with it by the sending out of pseudopodia which are presently drawn
back into the general mass: the inner and outer, being unsettled in
position, are not permanently differentiated. Then we have types,
exemplified by _Lithamoeba_, constituted of protoplasm covered by a
distinct pellicle, which in sundry groups becomes an outer shell of various
structure: now jelly-like, now of cellulose, now siliceous or calcareous.
While here this envelope has a single opening, there it is perforated all
over--a fenestrated shell. In some cases an external layer is formed of
agglutinated sand-particles; in others of imbricated plates, as in
Coccospheres; and in many others radiating spicules stand out on all sides.
Throughout sundry classes the exoplasm develops cilia, by the wavings of
which the creatures are propelled through the water--cilia which may be
either general or local. And then this cortical layer, instead of being
spherical or spheroidal, may become plano-spiral, cyclical, crosier-shaped,
and often many-chambered; whence there is a transition to colonies.
Meanwhile the inclosed protoplasm, at first little more than a network or
foamwork containing granules and made irregular by objects drawn in as
nutriment, becomes variously complicated. In some low types its continuity
is broken by motionless, vacant spaces, but in higher types there are
contractile vacuoles slowly pulsing, and, as we may suppose, moving the
contained liquid hither and thither; while there are types having many
passive vacuoles along with a few active ones. In some varieties the
protruded parts, or pseudopodia, into which the protoplasm continually
shapes itself, are comparatively short and club-shaped; in others they are
long and fine filaments which anastomose, so forming a network running here
and there into little pools of protoplasm. Then there are kinds in which
the protoplasm streams up and down the protruding spicules: sometimes
inside of them, sometimes outside. Always, too, there is included in the
protoplasm a small body known as a centrosome.
Lastly, we have the innermost element, considered the essential
element--the nucleus. According to Prof. Lankester, it is absent from
_Archerina_, and there are types in which it is made visible only by the
aid of special reagents. Ordinarily it is marked off from the surrounding
protoplasm by a delicate membrane, just as the protoplasm itself is marked
off by the exoplasm from the environment. Most commonly there is a single
nucleus, but occasionally there are many, and sometimes there is a chief
one with minor ones. Moreover, within the nucleus itself there have of late
years been discovered remarkable structural elements which undergo
complicated changes.
These brief statements indicate only the most general traits of an immense
variety of structures--so immense a variety that Prof. Lankester, in
distinguishing the classes, sub-classes, orders, and genera in the briefest
way, occupies 37 quarto pages of small type. And to give a corresponding
account of _Protophyta_ would require probably something like equal space.
Thus these living things, so minute that unaided vision fails to disclose
them, constitute a world exhibiting varieties of structure which it
requires the devotion of a life to become fully acquainted with.
§ 54b. If higher forms of life have arisen from lower forms by evolution,
the implication is that there must once have existed, if there do not still
exist, transitional forms; and there follows the comment that there _do_
still exist transitional forms. Both in the plant-world and in the
animal-world there are types in which we see little more than simple
assemblages of _Protophyta_ or of _Protozoa_--types in which the units,
though coherent, are not differentiated but constitute a uniform mass. In
treating of structure we are not here concerned with these unstructured
types, but may pass on to those aggregates of protoplasts which show us
differentiated parts--_Metaphyta_ and _Metazoa_: economizing space by
limiting our attention chiefly to the last.
When, half a century ago, some currency was given to the statement that all
kinds of organisms, plant and animal, which our unaided eyes disclose, are
severally composed of myriads of living units, some of them partially, if
not completely, independent, and that thus a man is a vast nation of minute
individuals of which some are relatively passive and others relatively
active, the statement met, here with incredulity and there with a shudder.
But what was then thought a preposterous assertion has now come to be an
accepted truth.
Along with gradual establishment of this truth has gone gradual
modification in the form under which it was originally asserted. If some
inhabitant of another sphere were to describe one of our towns as composed
exclusively of houses, saying nothing of the contained beings who had built
them and lived in them, we should say that he had made a profound error in
recognizing only the inanimate elements of the town and disregarding the
animate elements. Early histologists made an analogous error. Plants and
animals were found to consist of minute members, each of which appeared to
be simply a wall inclosing a cavity--a cell. But further investigation
proved that the content of the cell, presently distinguished as protoplasm,
is its essential living part, and that the cell-wall, when present, is
produced by it. Thus the unit of composition is a protoplast, usually
enclosed, with its contained nucleus and centrosome.
§ 54c. As above implied, the individualities of the units are not wholly
lost in the individuality of the aggregate, but continue, some of them, to
be displayed in various degrees: the great majority of them losing their
individualities more and more as the type of the aggregate becomes higher.
In a slightly organized Metazoon like the sponge, the subordination is but
small. Only those members of the aggregate which, flattened and united
together, form the outer layer and those which become metamorphosed into
spicules, have entirely lost their original activities. Of the rest nearly
all, lining the channels which permeate the mass, and driving onwards the
contained sea-water by the motions of their whip-like appendages,
substantially retain their separate lives; and beyond these there exist in
the gelatinous substance lying between the inner and outer layers, which is
regarded as homologous with a mesoderm, amoeba-form protoplasts which move
about from place to place.
Relations between the aggregate and the units which are in this case
permanent, are in other cases temporary: characterizing early stages of
embryonic development. For example, drawings of Echinoderm larvæ at an
early stage, show us the potential independence of all the cells forming
the blastosphere; for in the course of further development some of these
resume the primitive amoeboid state, migrate through the internal space,
and presently unite to form certain parts of the growing structures. But
with the progress of organization independence of this kind diminishes.
Converse facts are presented after development has been completed; for with
the commencement of reproduction we everywhere see more or less resumption
of individual life among the units, or some of them. It is a trait of
transitional types between _Protozoa_ and _Metazoa_ to lead an aggregate
life as a plasmodium, and then for this to break up into its members, which
for a time lead individual lives as generative agents; and sundry low kinds
of plants possessing small amounts of structure, have generative
elements--zoospores and spermatozoids--which show us a return to unit life.
Nor, indeed, are we shown this only in the lowest plants; for it has
recently been found that in certain of the higher plants--even in
Phænogams--spermatozoids are produced. That is to say, the units resume
active lives at places where the controlling influence of the aggregate is
failing; for, as we shall hereafter see, places at which generation
commences answer to this description.
These different kinds of evidence jointly imply that the individual lives
of the units are subordinate to the general life in proportion as this is
high. Where the organism is very inferior in type the unit-life remains
permanently conspicuous. In some superior types there is a display of
unit-life during embryonic stages in which the co-ordinating action of the
aggregate is but incipient. With the advance of development the unit-life
diminishes; but still, in plants, recommences where the disintegrating
process which initiates generation shows the coercive power of the
organization to have become small.
Even in the highest types, however, and even when they are fully developed,
unit-life does not wholly disappear: it is clearly shown in ourselves. I do
not refer simply to the fact that, as throughout the animal kingdom at
large and a considerable part of the vegetal kingdom, the male generative
elements are units which have resumed the primitive independent life, but I
refer to a much more general fact. In that part of the organism which,
being fundamentally an aqueous medium, is in so far like the aqueous medium
in which ordinary protozoon life is carried on, we find an essentially
protozoon life. I refer of course to the blood. Whether the tendency of the
red corpuscles (which are originally developed from amoeba-like cells) to
aggregate into _rouleaux_ is to be taken as showing life in them, may be
left an open question. It suffices that the white corpuscles or leucocytes,
retaining the primitive amoeboid character, exhibit individual activities:
send out prolongations like pseudopodia, take in organic particles as food,
and are independently locomotive. Though far less numerous than the red
corpuscles, yet, as ten thousand are contained in a cubic millimetre of
blood--a mass less than a pin's head--it results that the human body is
pervaded throughout all its blood-vessels by billions of these separately
living units. In the lymph, too, which also fulfils the requirements of
liquidity, these amoeboid units are found. Then we have the curious
transitional stage in which units partially imbedded and partially free
display a partial unit-life. These are the ciliated epithelium-cells,
lining the air-passages and covering sundry of the mucous membranes which
have more remote connexions with the environment, and covering also the
lining membranes of certain main canals and chambers in the nervous system.
The inner parts of these unite with their fellows to form an epithelium,
and the outer parts of them, immersed either in liquid or semi-liquid
(mucus), bear cilia that are in constant motion and "produce a current of
fluid over the surface they cover:" thus simulating in their positions and
actions the cells lining the passages ramifying through a sponge. The
partially independent lives of these units is further seen in the fact that
after being detached they swim about in water for a time by the aid of
their cilia.
§ 54d. But in the _Metazoa_ and _Metaphyta_ at large, the associated units
are, with the exceptions just indicated, completely subordinated. The
unit-life is so far lost in the aggregate life that neither locomotion nor
the relative motion of parts remains; and neither in shape nor composition
is there resemblance to protozoa. Though in many cases the internal
protoplasm continues to carry on vital processes subserving the needs of
the aggregate, in others vital processes of an independent kind appear to
cease.
It will naturally be supposed that after recognizing this fundamental trait
common to all types of organisms above the _Protozoa_ and _Protophyta_, the
next step in an account of structure must be a description of their organs,
variously formed and combined--if not in detail yet in their general
characters. This, however, is an error. There are certain truths of
structure higher in generality than any which can be alleged of organs. We
shall see this if we compare organs with one another.
Here is a finger stiffened by its small bones and yet made flexible by the
uniting joints. There is a femur which helps its fellow to support the
weight of the body; and there again is a rib which, along with others,
forms a protective box for certain of the viscera. Dissection reveals a set
of muscles serving to straighten and bend the fingers, certain other
muscles that move the legs, and some inconspicuous muscles which,
contracting every two or three seconds, slightly raise the ribs and aid in
inflating the lungs. That is to say, fingers, legs, and chest possess
certain structures in common. There is in each case a dense substance
capable of resisting stress and a contractile substance capable of moving
the dense substance to which it is attached. Hence, then, we have first to
give an account of these and other chief elements which, variously joined
together, form the different organs: we have to observe the general
characters of _tissues_.
On going back to the time when the organism begins with a single cell, then
becomes a spherical cluster of cells, and then exhibits differences in the
modes of aggregation of these cells, the first conspicuous rise of
structure (limiting ourselves to animals) is the formation of three layers.
Of these the first is, at the outset and always, the superficial layer in
direct contact with the environment. The second, being originally a part of
the first, is also in primitive types in contact with the environment, but,
being presently introverted, forms the rudiment of the food-cavity; or,
otherwise arising in higher types, is in contact with the yelk or food
provided by the parent. And the third, presently formed between these two,
consists at the outset of cells derived from them imbedded in an
intercellular substance of jelly-like consistence. Hence originate the
great groups classed as epithelium-tissue, connective tissue (including
osseous tissue), muscular tissue, nervous tissue. These severally contain
sub-kinds, each of which is a complex of differentiated cells. Being brief,
and therefore fitted for the present purposes, the sub-classification given
by Prof. R. Hertwig may here be quoted;--
"The physiological character of epithelia is given in the fact that they
cover the surfaces of the body, their morphological character in that
they consist of closely compressed cells united only by a cementing
substance.
"According to their further functional character epithelia are divided
into glandular epithelia (unicellular and multicellular glands), sensory,
germinal, and pavement epithelia.
"According to the structure are distinguished one-layered (cubical,
cylindrical, pavement epithelia) and many-layered epithelia, ciliated and
flagellated epithelia, epithelia with or without cuticle.
"The physiological character of the connective tissues rests upon the
fact that they fill up spaces between other tissues in the interior of
the body.
"The morphological character depends upon the presence of the
intercellular substance.
"According to the quantity and the structure of the intercellular
substance the connective substances are divided into (1) cellular (with
little intercellular substance); (2) homogeneous; (3) fibrillar
connective tissue; (4) cartilage; (5) bone.
"The physiological character of muscular tissue is contained in the
increased capacity for contraction.
"The morphological character is found in the fact that the cells have
secreted muscle-substance.
"According to the nature of the muscle-substance are distinguished smooth
and cross-striated muscle-fibres.
"According to the character and derivation of the cells
(muscle-corpuscles) the musculature is divided into epithelial
(epithelial muscle-cells, primary bundles) and connective-tissue muscle
cells (contractile fibre-cells).
"The physiological character of nervous tissue rests upon the
transmission of sensory stimuli and voluntary impulses, and upon the
co-ordination of these into unified psychic activity.
"The conduction takes place by means of nerve-fibres (non-medullated and
medullated fibrils and bundles of fibrils); the co-ordination of stimuli
by means of ganglion-cells (bipolar, multipolar ganglion-cells)."
(_General Principles of Zoology_, pp. 117-8.)
But now concerning cells out of which, variously modified, obscured, and
sometimes obliterated, tissues are formed, we have to note a fact of much
significance. Along with the cell-doctrine as at first held, when attention
was given to the cell itself rather than to its contents, there went the
belief that each of these morphological units is structurally separate from
its neighbours. But since establishment of the modern view that the
essential element is the contained protoplasm, histologists have discovered
that there are protoplasmic connexions between the contents of adjacent
cells. Though cursorily observed at earlier dates, it was not until some
twenty years ago that in plant-tissues these were clearly shown to pass
through openings in the cell-walls. It is said that in some cases the
openings are made, and the junctions established, by a secondary process;
but the implication is that usually these living links are left between
multiplying protoplasts; so that from the outset the protoplasm pervading
the whole plant maintains its continuity. More recently sundry zoologists
have alleged that a like continuity exists in animals. Especially has this
been maintained by Mr. Adam Sedgwick. Numerous observations made on
developing ova of fishes have led him to assert that in no case do the
multiplying cells so-called--blastomeres and their progeny--become entirely
separate. Their fission is in all cases incomplete. A like continuity has
been found in the embryos of many Arthropods, and more recently in the
segmenting eggs and blastulæ of Echinoderms. The _syncytium_ thus formed is
held by Mr. Sedgwick to be maintained in adult life, and in this belief he
is in agreement with sundry others. Bridges of protoplasm have been seen
between epithelium-cells, and it is maintained that cartilage-cells,
connective tissue cells, the cells forming muscle-fibres, as well as
nerve-cells, have protoplasmic unions. Nay, some even assert that an ovum
preserves a protoplasmic connexion with the matrix in which it develops.
A corollary of great significance may here be drawn. It has been observed
that within a vegetal cell the strands of protoplasm stretched in this or
that direction contain moving granules, showing that the strands carry
currents. It has also been observed that when the fission of a protozoon is
so nearly complete that its two halves remain connected only by a thread,
currents of protoplasm move through this thread, now one way now the other.
The inference fairly to be drawn is that such currents pass also through
the strands which unite the protoplasts forming a tissue. What must happen?
So long as adjacent cells with their contents are subject to equal
pressures no tendency to redistribution of the protoplasm exists, and there
may then occur the action sometimes observed inside the strands within a
cell: currents with their contained granules moving in opposite directions.
But if the cells forming a portion of tissue are subject to greater
pressure than the cells around, their contained protoplasm must be forced
through the connecting threads into these surrounding cells. Every change
of pressure at every point must cause movements and counter-movements of
this kind. Now in the _Metazoa_ at large, or at least in all exhibiting
relative motions of parts, and especially in all which are capable of rapid
locomotion, such changes of pressure are everywhere and always taking
place. The contraction of a muscle, besides compressing its components,
compresses neighbouring tissues; and every instant contractions and
relaxations of muscles go on throughout the limbs and body during active
exertion. Moreover, each attitude--standing, sitting, lying down, turning
over--entails a different set of pressures, both of the parts on one
another and on the ground; and those partial arrests of motion which result
from sitting down the feet alternately when running, send jolts or waves of
varying pressure through the body. The vital actions, too, have kindred
effects. An inspiration alters the stress on the tissues throughout a
considerable part of the trunk, and a heart-beat propels, down to the
smallest arteries, waves which slightly strain the tissues at large. The
component cells, thus subject to mechanical disturbances, small and great,
perpetual and occasional, are ever having protoplasm forced into them and
forced out of them. There are gurgitations and regurgitations which, if
they do not constitute a circulation properly so called, at least imply an
unceasing redistribution. And the implication is that in the course of
days, weeks, months, years, each portion of protoplasm visits every part of
the body.
Without here stating specifically the bearings of these inferences upon the
problems of heredity, it will be manifest that certain difficulties they
present are in a considerable degree diminished.
§ 54e. Returning from this parenthetical discussion to the subject of
structure, we have to observe that besides facts presented by tissues and
facts presented by organs, there are certain facts, less general than the
one and more general than the other, which must now be noted. In the order
of decreasing generality an account of organs should be preceded by an
account of systems of organs. Some of these, as the muscular system and the
osseous system, are co-extensive with tissues, but others of them are not.
The nervous system, for example, contains more than one kind of tissue and
is constituted of many different structures: besides afferent and efferent
nerves there are the ganglia immediately controlling the viscera, and there
are the spinal and cerebral masses, the last of which is divisible into
numerous unlike parts. Then we have the vascular system made up of the
heart, arteries, veins, and capillaries. The lymphatic system, too, with
its scattered glands and ramifying channels has to be named. And then, not
forgetting the respiratory system with its ancillary appliances, we have
the highly heterogeneous alimentary system; including a great number of
variously-constructed organs which work together. On contemplating these
systems we see their common character to be that while as wholes they
cooperate for the carrying on of the total life, each of them consists of
cooperative parts: there is cooperation within cooperation.
There is another general aspect under which structures must be
contemplated. They are divisible into the universal and the
particular--those which are everywhere present and those which occupy
special places. The blood which a scratch brings out shows us that the
vascular system sends branches into each spot. The sensation accompanying a
scratch proves that the nervous system, too, has there some of its ultimate
fibrils. Unobtrusive, and yet to be found at every point, are the ducts of
the lymphatic system. And in all parts exists the connective tissue--an
inert tough substance which, running through interspaces, wraps up and
binds together the other tissues. As is implied by this description, these
structures stand in contrast with local structures. Here is a bone, there
is a muscle, in this place a gland, in that a sense-organ. Each has a
limited extent and a particular duty. But through every one of them ramify
branches of these universal structures. Every one of them has its arteries
and veins and capillaries, its nerves, its lymphatics, its connective
tissue.
Recognition of this truth introduces what little has here to be said
concerning organs; for of course in a work limited to principles no
detailed account of these can be entered upon. This remainder truth is
that, different as they may be in the rest of their structures, all organs
are alike in certain of their structures. All are furnished with these
appliances for nutrition, depuration and excitation: they have all to be
sustained, all to be stimulated, all to be kept clean. It has finally to be
remarked that the general structures which pervade all the special
structures at the same time pervade one another. The universal nervous
system has everywhere ramifying through it the universal vascular system
which feeds it; and the universal vascular system is followed throughout
all its ramifications by special nerves which control it. The lymphatics
forming a drainage-system run throughout the other systems; and in each of
these universal systems is present the connective tissue holding their
parts in position.
§ 54f. So vast and varied a subject as organic structure, even though the
treatment of it is limited to the enunciation of principles, cannot, of
course, be dealt with in the space here assigned. Next to nothing has been
said about plant-structures, and in setting forth the leading traits of
animal-structures the illustrations given have been mostly taken from
highly-developed creatures. In large measure adumbration rather than
exposition is the descriptive word to be applied.
Nevertheless the reader may carry away certain truths which, exemplified in
a few cases, are exemplified more or less fully in all cases. There is the
fundamental fact that the plants and animals with which we are
familiar--_Metaphyta_ and _Metazoa_--are formed by the aggregation of units
homologous with _Protozoa_. These units, often conspicuously showing their
homology in early embryonic stages, continue some of them to show it
throughout the lives of the highest type of _Metazoa_, which contain
billions of units carrying on a protozoon life. Of the protoplasts not thus
active the great mass, comparatively little transformed in low organisms,
become more and more transformed as the ascent to high organisms goes on;
so that, undergoing numerous kinds of metamorphoses, they lose all likeness
to their free homologues, both in shape and composition. The cell-contained
protoplasts thus variously changed are fused together into tissues in which
their individualities are practically lost; but they nevertheless remain
connected throughout by permeable strands of protoplasm. Arising by
complication of the outer and inner layers of the embryo and growing more
unlike as their units become more obscured, these tissues are formed into
systems, which develop into sets of organs. Some of the resulting
structures are localized and special but others are everywhere interfused.
While the first named of these facts are displayed in every _Metazoon_, and
while the last named are visible only in _Metazoa_ of considerably
developed structures, a gradual transition is shown in intermediate kinds
of _Metazoa_. Of this transition it remains to say that it is effected by
the progressive development of auxiliary appliances. For example, the
primitive foot-cavity is a sac with one opening only; then comes a second
opening through which the waste-matter of the food is expelled. The
alimentary canal between these openings is at first practically uniform;
afterwards in a certain part of its wall arise numerous bile-cells; these
accumulating form a hollow prominence; and this, enlarging, becomes in
higher types a liver, while the hollow becomes its duct. In other gradual
ways are formed other appended glands. Meanwhile the canal itself has its
parts differentiated: one being limited to swallowing, another to
triturating, another to adding various solvents, another to absorbing the
prepared nutriment, another to ejecting the residue. Take again the visual
organ. The earliest form of it is a mere pigment-speck below the surface.
From this (saying nothing here of multiple eyes) we rise by successive
complications to a retina formed of multitudinous sensory elements, lenses
for throwing images upon it, a curtain for shutting out more or less light,
muscles for moving the apparatus about, others for adjusting its focus;
and, finally, added to these, either a nictitating membrane or eyelids for
perpetually wiping its surface, and a set of eyelashes giving notice when a
foreign body is dangerously near. This process of elaborating organs so as
to meet additional requirements by additional parts, is the process pursued
throughout the body at large.
Of plant-structures, concerning which so little has been said, it may here
be remarked that their relative simplicity is due to the simplicity of
their relations to food. The food of plants is universally distributed,
while that of animals is dispersed. The immediate consequences are that in
the one case motion and locomotion are superfluous, while in the other case
they are necessary: the differences in the degrees of structure being
consequences. Recognizing the locomotive powers of minute _Algæ_ and the
motions of such other _Algæ_ as _Oscillatoria_, as well as those movements
of leaves and fructifying organs seen in some Phænogams, we may say,
generally, that plants are motionless; but that they can nevertheless carry
on their lives because they are bathed by the required nutriment in the air
and in the soil. Contrariwise, the nutriment animals require is distributed
through space in portions: in some cases near one another and in other
cases wide apart. Hence motion and locomotion are necessitated; and the
implication is that animals must have organs which render them possible. In
the first place there must be either limbs or such structures as those
which in fish, snakes, and worms move the body along. In the second place,
since action implies waste, there must be a set of channels to bring
repairing materials to the moving parts. In the third place there must be
an alimentary system for taking in and preparing these materials. In the
fourth place there must be organs for separating and excreting
waste-products. All these appliances must be more highly developed in
proportion as the required activity is greater. Then there must be an
apparatus for directing the motions and locomotions--a nervous system; and
as fast as these become rapid and complex the nervous system must be
largely developed, ending in great nervous centres--seats of intelligence
by which the activities at large are regulated. Lastly, underlying all the
structural contrasts between plants and animals thus originating, there is
the chemical contrast; since the necessity for that highly nitrogenous
matter of which animals are formed, is entailed by the necessity for
rapidly evolving the energy producing motion. So that, strange as it seems,
those chemical, physical, and mental characters of animals which so
profoundly distinguish them from plants, are all remote results of the
circumstance that their food is dispersed instead of being everywhere
present.
CHAPTER III.
FUNCTION.
§ 55. Does Structure originate Function, or does Function originate
Structure? is a question about which there has been disagreement. Using the
word Function in its widest signification, as the totality of all vital
actions, the question amounts to this--does Life produce Organization, or
does Organization produce Life?
To answer this question is not easy, since we habitually find the two so
associated that neither seems possible without the other; and they appear
uniformly to increase and decrease together. If it be said that the
arrangement of organic substances in particular forms, cannot be the
ultimate cause of vital changes, which must depend on the properties of
such substances; it may be replied that, in the absence of structural
arrangements, the forces evolved cannot be so directed and combined as to
secure that correspondence between inner and outer actions which
constitutes Life. Again, to the allegation that the vital activity of every
germ whence an organism arises, is obviously antecedent to the development
of its structures, there is the answer that such germ is not absolutely
structureless.
But in truth this question is not determinable by any evidence now
accessible to us. The very simplest forms of life known (even the
non-nucleated, if there are any) consist of granulated protoplasm; and
granulation implies structure. Moreover since each kind of protozoon, even
the lowest, has its specific mode of development and specific
activity--even down to bacteria, some kinds of which, otherwise
indistinguishable, are distinguishable by their different reactions on
their media--we are obliged to conclude that there must be constitutional
differences between the protoplasms they consist of, and this implies
structural differences. It seems that structure and function must have
advanced _pari passu_: some difference of function, primarily determined by
some difference of relation to the environment, initiating a slight
difference of structure, and this again leading to a more pronounced
difference of function; and so on through continuous actions and reactions.
§ 56. Function falls into divisions of several kinds according to our point
of view. Let us take these divisions in the order of their simplicity.
Under Function in its widest sense, are included both the statical and the
dynamical distributions of force which an organism opposes to the forces
brought to bear on it. In a tree the woody core of trunk and branches, and
in an animal the skeleton, internal or external, may be regarded as
passively resisting the gravity and momentum which tend habitually or
occasionally to derange the requisite relations between the organism and
its environment; and since they resist these forces simply by their
cohesion, their functions may be classed as _statical_. Conversely, the
leaves and sap-vessels in a tree, and those organs which in an animal
similarly carry on nutrition and circulation, as well as those which
generate and direct muscular motion, must be considered as _dynamical_ in
their actions. From another point of view Function is divisible into the
_accumulation of energy_ (latent in food); the _expenditure of energy_
(latent in the tissues and certain matters absorbed by them); and the
_transfer of energy_ (latent in the prepared nutriment or blood) from the
parts which accumulate to the parts which expend. In plants we see little
beyond the first of these: expenditure being comparatively slight, and
transfer required mainly to facilitate accumulation. In animals the
function of _accumulation_ comprehends those processes by which the
materials containing latent energy are taken in, digested, and separated
from other materials; the function of _transfer_ comprehends those
processes by which these materials, and such others as are needful to
liberate the energies they contain, are conveyed throughout the organism;
and the function of _expenditure_ comprehends those processes by which the
energy is liberated from these materials and transformed into properly
co-ordinated motions. Each of these three most general divisions includes
several more special divisions. The accumulation of energy may be separated
into _alimentation_ and _aeration_; of which the first is again separable
into the various acts gone through between prehension of food and the
transformation of part of it into blood. By the transfer of energy is to be
understood what we call _circulation_; if the meaning of circulation be
extended to embrace the duties of both the vascular system and the
lymphatics. Under the head of expenditure of energy come _nervous actions_
and _muscular actions_: though not absolutely co-extensive with expenditure
these are almost so. Lastly, there are the subsidiary functions which do
not properly fall within any of these general functions, but subserve them
by removing the obstacles to their performance: those, namely, of
_excretion_ and _exhalation_, whereby waste products are got rid of. Again,
disregarding their purposes and considering them analytically, the general
physiologist may consider functions in their widest sense as the
correlatives of tissues--the actions of epidermic tissue, cartilaginous
tissue, elastic tissue, connective tissue, osseous tissue, muscular tissue,
nervous tissue, glandular tissue. Once more, physiology in its concrete
interpretations recognizes special functions as the ends of special
organs--regards the teeth as having the office of mastication; the heart as
an apparatus to propel blood; this gland as fitted to produce one requisite
secretion and that to produce another; each muscle as the agent of a
particular motion; each nerve as the vehicle of a special sensation or a
special motor impulse.
It is clear that dealing with Biology only in its larger aspects,
specialities of function do not concern us; except in so far as they serve
to illustrate, or to qualify, its generalities.
§ 57. The first induction to be here set down is a familiar and obvious
one; the induction, namely, that complexity of function is the correlative
of complexity of structure. The leading aspects of this truth must be
briefly noted.
Where there are no distinctions of structure there are no distinctions of
function. A Rhizopod will serve as an illustration. From the outside of
this creature, which has not even a limiting membrane, there are protruded
numerous processes. Originating from any point of the surface, each of
these may contract again and disappear, or it may touch some fragment of
nutriment which it draws with it, when contracting, into the general
mass--thus serving as hand and mouth; or it may come in contact with its
fellow-processes at a distance from the body and become confluent with
them; or it may attach itself to an adjacent fixed object, and help by its
contraction to draw the body into a new position. In brief, this speck of
animated jelly is at once all stomach, all skin, all mouth, all limb, and
doubtless, too, all lung. In organisms having a fixed distribution of parts
there is a concomitant fixed distribution of actions. Among plants we see
that when, instead of a uniform tissue like that of many _Algæ_, everywhere
devoted to the same process of assimilation, there arise, as in the higher
plants, root and stem and leaves, there arise correspondingly unlike
processes. Still more conspicuously among animals do there result varieties
of function when the originally homogeneous mass is replaced by
heterogeneous organs; since, both singly and by their combinations,
modified parts generate modified changes. Up to the highest organic types
this dependence continues manifest; and it may be traced not only under
this most general form, but also under the more special form that in
animals having one set of functions developed to more than usual
heterogeneity there is a correspondingly heterogeneous apparatus devoted to
them. Thus among birds, which have more varied locomotive powers than
mammals, the limbs are more widely differentiated; while the higher
mammals, which rise to more numerous and more involved adjustments of inner
to outer relations than birds, have more complex nervous systems.
§ 58. It is a generalization almost equally obvious with the last, that
functions, like structures, arise by progressive differentiations. Just as
an organ is first an indefinite rudiment, having nothing but some most
general characteristic in common with the form it is ultimately to take; so
a function begins as a kind of action that is like the kind of action it
will eventually become, only in a very vague way. And in functional
development, as in structural development, the leading trait thus early
manifested is followed successively by traits of less and less importance.
This holds equally throughout the ascending grades of organisms and
throughout the stages of each organism. Let us look at cases: confining our
attention to animals, in which functional development is better displayed
than in plants.
The first differentiation established separates the two
fundamentally-opposed functions above named--the accumulation of energy and
the expenditure of energy. Passing over the _Protozoa_ (among which,
however, such tribes as present fixed distributions of parts show us
substantially the same thing), and commencing with the lowest
_Coelenterata_, where definite tissues make their appearance, we observe
that the only large functional distinction is between the endoderm, which
absorbs nutriment, and the ectoderm which, by its own contractions and
those of the tentacles it bears, produces motion: the contractility being
however to some extent shared by the endoderm. That the functions of
accumulation and expenditure are here very incompletely distinguished, may
be admitted without affecting the position that this is the first
specialization which begins to appear. These two most general and most
radically-opposed functions become in the _Polyzoa_, much more clearly
marked-off from each other: at the same time that each of them becomes
partially divided into subordinate functions. The endoderm and ectoderm are
no longer merely the inner and outer walls of the same simple sac into
which the food is drawn: but the endoderm forms a true alimentary canal,
separated from the ectoderm by a peri-visceral cavity, containing the
nutritive matters absorbed from the food. That is to say, the function of
accumulating force is exercised by a part distinctly divided from the part
mainly occupied in expending force: the structure between them, full of
absorbed nutriment, effecting in a vague way that transfer of force which,
at a higher stage of evolution, becomes a third leading function.
Meanwhile, the endoderm no longer discharges the accumulative function in
the same way throughout its whole extent; but its different portions,
oesophagus, stomach and intestine, perform different portions of this
function. And instead of a contractility uniformly diffused through the
ectoderm, there have arisen in the intermediate mesoderm some parts which
have the office of contracting (muscles), and some parts which have the
office of making them contract (nerves and ganglia). As we pass upwards,
the transfer of force, hitherto effected quite incidentally, comes to have
a special organ. In the ascidian, circulation is produced by a muscular
tube, open at both ends, which, by a wave of contraction passing along it,
sends out at one end the nutrient fluid drawn in at the other; and which,
having thus propelled the fluid for a time in one direction, reverses its
movement and propels it in the opposite direction. By such means does this
rudimentary heart generate alternating currents in the nutriment occupying
the peri-visceral cavity. How the function of transferring energy, thus
vaguely indicated in these inferior forms, comes afterwards to be the
definitely-separated office of a complicated apparatus made up of many
parts, each of which has a particular portion of the general duty, need not
be described. It is sufficiently manifest that this general function
becomes more clearly marked-off from the others, at the same time that it
becomes itself parted into subordinate functions.
In a developing embryo, the functions or more strictly the structures which
are to perform them, arise in the same general order. A like primary
distinction very early appears between the endoderm and the ectoderm--the
part which has the office of accumulating energy, and the part out of which
grow those organs that are the great expenders of energy. Between these two
there presently arises the mesoderm in which becomes visible the rudiment
of that vascular system, which has to fulfil the intermediate duty of
transferring energy. Of these three general functions, that of accumulating
energy is carried on from the outset: the endoderm, even while yet
incompletely differentiated from the ectoderm, absorbs nutritive matters
from the subjacent yelk. The transfer of energy is also to some extent
effected by the rudimentary vascular system, as soon as its central cavity
and attached vessels are sketched out. But the expenditure of energy (in
the higher animals at least) is not appreciably displayed by those
ectodermic and mesodermic structures that are afterwards to be mainly
devoted to it: there is no sphere for the actions of these parts. Similarly
with the chief subdivisions of these fundamental functions. The distinction
first established separates the office of transforming other energy into
mechanical motion, from the office of liberating the energy to be so
transformed. While in the layer between endoderm and ectoderm are arising
the rudiments of the muscular system, there is marked out in the ectoderm
the rudiment of the nervous system. This indication of structures which are
to share between them the general duty of expending energy, is soon
followed by changes that foreshadow further specializations of this general
duty. In the incipient nervous system there begins to arise that contrast
between the cerebral mass and the spinal cord, which, in the main, answers
to the division of nervous actions into directive and executive; and, at
the same time, the appearance of vertebral laminæ foreshadows the
separation of the osseous system, which has to resist the strains of
muscular action, from the muscular system, which, in generating motion,
entails these strains. Simultaneously there have been going on similar
actual and potential specializations in the functions of accumulating
energy and transferring energy. And throughout all subsequent phases the
method is substantially the same.
This progress from general, indefinite, and simple kinds of action to
special, definite, and complex kinds of action, has been aptly termed by
Milne-Edwards, "the physiological division of labour." Perhaps no metaphor
can more truly express the nature of this advance from vital activity in
its lowest forms to vital activity in its highest forms. And probably the
general reader cannot in any other way obtain so clear a conception of
functional development in organisms, as he can by tracing out functional
development in societies: noting how there first comes a distinction
between the governing class and the governed class; how while in the
governing class there slowly grow up such differences of duty as the civil,
military, and ecclesiastical, there arise in the governed class fundamental
industrial differences like those between agriculturists and artizans; and
how there is a continual multiplication of such specialized occupations and
specialized shares of each occupation.
§ 59. Fully to understand this change from homogeneity of function to
heterogeneity of function, which accompanies the change from homogeneity of
structure to heterogeneity of structure, it is needful to contemplate it
under a converse aspect. Standing alone, the above exposition conveys an
idea that is both inadequate and erroneous. The divisions and subdivisions
of function, becoming definite as they become multiplied, do not lead to a
more and more complete independence of functions; as they would do were the
process nothing beyond that just described; but by a simultaneous process
they are rendered more mutually dependent. While in one respect they are
separating from each other, they are in another respect combining with each
other. At the same time that they are being differentiated they are also
being integrated. Some illustrations will make this plain.
In animals which display little beyond the primary differentiation of
functions, the activity of that part which absorbs nutriment or accumulates
energy, is not immediately bound up with the activity of that part which,
in producing motion, expends energy. In the higher animals, however, the
performance of the alimentary functions depends on the performance of
various muscular and nervous functions. Mastication and swallowing are
nervo-muscular acts; the rhythmical contractions of the stomach and the
allied vermicular motions of the intestines, result from the reflex
stimulation of certain muscular coats caused by food; the secretion of the
several digestive fluids by their respective glands, is due to nervous
excitation of them; and digestion, besides requiring these special aids, is
not properly performed in the absence of a continuous discharge of energy
from the great nervous centres. Again, the function of transferring
nutriment or latent energy, from part to part, though at first not closely
connected with the other functions, eventually becomes so. The short
contractile tube which propels backwards and forwards the blood contained
in the peri-visceral cavity of an ascidian, is neither structurally nor
functionally much entangled with the creature's other organs. But on
passing upwards through higher types, in which this simple tube is replaced
by a system of branched tubes, that deliver their contents through their
open ends into the tissues at distant parts; and on coming to those
advanced types which have closed arterial and venous systems, ramifying
minutely in every corner of every organ; we find that the vascular
apparatus, while it has become structurally interwoven with the whole body,
has become unable properly to fulfil its office without the help of offices
that are quite separated from its own. The heart, though mainly automatic
in its actions, is controlled by the nervous system, which takes a share in
regulating the contractions both of the heart and the arteries. On the due
discharge of the respiratory function, too, the function of circulation is
directly dependent: if the aeration of the blood is impeded the vascular
activity is lowered; and arrest of the one very soon causes stoppage of the
other. Similarly with the duties of the nervo-muscular system. Animals of
low organization, in which the differentiation and integration of the vital
actions have not been carried far, will move about for a considerable time
after being eviscerated, or deprived of those appliances by which energy is
accumulated and transferred. But animals of high organization are instantly
killed by the removal of these appliances, and even by the injury of minor
parts of them: a dog's movements are suddenly brought to an end, by cutting
one of the main canals along which the materials that evolve movements are
conveyed. Thus while in well-developed creatures the distinction of
functions is very marked, the combination of functions is very close. From
instant to instant the aeration of blood implies that certain respiratory
muscles are being made to contract by nervous impulses passing along
certain nerves; and that the heart is duly propelling the blood to be
aerated. From instant to instant digestion proceeds only on condition that
there is a supply of aerated blood, and a due current of nervous energy
through the digestive organs. That the heart of a mammal may act, its
muscle substance must be continuously fed with an abundant supply of
arterial blood.
It is not easy to find an adequate expression for this double
re-distribution of functions. It is not easy to realize a transformation
through which the functions thus become in one sense separated and in
another sense combined, or even interfused. Here, however, as before, an
analogy drawn from social organization helps us. If we observe how the
increasing division of labour in societies is accompanied by a closer
co-operation; and how the agencies of different social actions, while
becoming in one respect more distinct, become in another respect more
minutely ramified through one another; we shall understand better the
increasing physiological co-operation that accompanies increasing
physiological division of labour. Note, for example, that while local
divisions and classes of the community have been growing unlike in their
several occupations, the carrying on of their several occupations has been
growing dependent on the due activity of that vast organization by which
sustenance is collected and diffused. During the early stages of social
development, every small group of people, and often every family, obtained
separately its own necessaries; but now, for each necessary, and for each
superfluity, there exists a combined body of wholesale and retail
distributors, which brings its branched channels of supply within reach of
all. While each citizen is pursuing a business that does not immediately
aim at the satisfaction of his personal wants, his personal wants are
satisfied by a general agency which brings from all places commodities for
him and his fellow-citizens--an agency which could not cease its special
duties for a few days, without bringing to an end his own special duties
and those of most others. Consider, again, how each of these differentiated
functions is everywhere pervaded by certain other differentiated functions.
Merchants, manufacturers, wholesale distributors of their several species,
together with lawyers, bankers, &c., all employ clerks. In clerks we have a
specialized class dispersed through various other classes; and having its
function fused with the different functions of these various other classes.
Similarly commercial travellers, though having in one sense a separate
occupation, have in another sense an occupation forming part of each of the
many occupations which it aids. As it is here with the sociological
division of labour, so is it with the physiological division of labour
above described. Just as we see in an advanced community, that while the
magisterial, the clerical, the medical, the legal, the manufacturing, and
the commercial activities, have grown distinct, they have yet their
agencies mingled together in every locality; so in a developed organism, we
see that while the general functions of circulation, secretion, absorption,
excretion, contraction, excitation, &c., have become differentiated, yet
through the ramifications of the systems apportioned to them, they are
closely combined with one another in every organ.
§ 60. The physiological division of labour is usually not carried so far as
wholly to destroy the primary physiological community of labour. As in
societies the adaptation of special classes to special duties, does not
entirely disable these classes from performing one another's duties on an
emergency; so in organisms, tissues and structures that have become fitted
to the particular offices they have ordinarily to discharge, often remain
partially able to discharge other offices. It has been pointed out by Dr.
Carpenter, that "in cases where the different functions are highly
specialized, the general structure retains, more or less, the primitive
community of function which originally characterized it." A few instances
will bring home this generalization.
The roots and leaves of plants are widely differentiated in their
functions: by the roots, water and mineral substances are absorbed; while
the leaves take in, and decompose, carbonic acid. Nevertheless, by many
botanists it is held that some leaves, or parts of them, can absorb water;
and in what are popularly called "air-plants," or at any rate in some kinds
of them, the absorption of water is mainly and in some cases wholly carried
on by them and by the stems. Conversely, the underground parts can
partially assume the functions of leaves. The exposed tuber of a potato
develops chlorophyll on its surface, and in other cases, as in that of the
turnip, roots, properly so called, do the like. In trees the trunks, which
have in great measure ceased to produce buds, recommence producing them if
the branches are cut off; sometimes aerial branches send down roots to the
earth; and under some circumstances the roots, though not in the habit of
developing leaf-bearing organs, send up numerous suckers. When the
excretion of bile is arrested, part goes to the skin and some to the
kidneys, which presently suffer under their new task. Various examples of
vicarious functions may be found among animals. The excretion of carbonic
acid and absorption of oxygen are mainly performed by the lungs, in
creatures which have lungs; but in such creatures there continues a certain
amount of cutaneous respiration, and in soft-skinned batrachians like the
frog, this cutaneous respiration is important. Again, when the kidneys are
not discharging their duties a notable quantity of urea is got rid of by
perspiration. Other instances are supplied by the higher functions. In man
the limbs, which among lower vertebrates are almost wholly organs of
locomotion, are specialized into organs of locomotion and organs of
manipulation. Nevertheless, the human arms and legs do, when needful,
fulfil, to some extent, each other's offices. Not only in childhood and old
age are the arms used for purposes of support, but on occasions of
emergency, as when mountaineering, they are used by men in full vigour. And
that legs are to a considerable degree capable of performing the duties of
arms, is proved by the great amount of manipulatory skill reached by them
when the arms are absent. Among the perceptions, too, there are examples of
partial substitution. The deaf Dr. Kitto described himself as having become
excessively sensitive to vibrations propagated through the body; and as so
having gained the power of perceiving, through his general sensations,
those neighbouring concussions of which the ears ordinarily give notice.
Blind people make hearing perform, in part, the office of vision. Instead
of identifying the positions and sizes of neighbouring objects by the
reflection of light from their surfaces, they do this in a rude way by the
reflection of sound from their surfaces.
We see, as we might expect to see, that this power of performing more
general functions, is great in proportion as the organs have been but
little adapted to their special functions. Those parts of plants which show
so considerable an ability to discharge each others' offices, are not
widely unlike in their minute structures. And the tissues which in animals
are to some extent mutually vicarious, are tissues in which the original
cellular composition is still conspicuous. But we do not find evidence that
the muscular, nervous, or osseous tissues are able in any degree to perform
those processes which the less differentiated tissues perform. Nor have we
any proof that nerve can partially fulfil the duty of muscle, or muscle
that of nerve. We must say, therefore, that the ability to resume the
primordial community of function, varies inversely as the established
specialization of function; and that it disappears when the specialization
of function becomes great.
§ 61. Something approaching to _a priori_ reasons may be given for the
conclusions thus reached _a posteriori_. They must be accepted for as much
as they seem worth.
It may be argued that on the hypothesis of Evolution, Life necessarily
comes before organization. On this hypothesis, organic matter in a state of
homogeneous aggregation must precede organic matter in a state of
heterogeneous aggregation. But since the passing from a structureless state
to a structured state, is itself a vital process, it follows that vital
activity must have existed while there was yet no structure: structure
could not else arise. That function takes precedence of structure, seems
also implied in the definition of Life. If Life is shown by inner actions
so adjusted as to balance outer actions--if the implied energy is the
_substance_ of Life while the adjustment of the actions constitutes its
_form_; then may we not say that the actions to be formed must come before
that which forms them--that the continuous change which is the basis of
function, must come before the structure which brings function into shape?
Or again, since in all phases of Life up to the highest, every advance is
the effecting of some better adjustment of inner to outer actions; and
since the accompanying new complexity of structure is simply a means of
making possible this better adjustment; it follows that the achievement of
function is, throughout, that for which structure arises. Not only is this
manifestly true where the modification of structure results by reaction
from modification of function; but it is also true where a modification of
structure otherwise produced, apparently initiates a modification of
function. For it is only when such so-called spontaneous modification of
structure subserves some advantageous action, that it is permanently
established. If it is a structural modification that happens to facilitate
the vital activities, "natural selection" retains and increases it; but if
not, it disappears.
The connexion which we noted between heterogeneity of structure and
heterogeneity of function--a connexion made so familiar by experience as to
appear scarcely worth specifying--is clearly a necessary one. It follows
from the general truth that in proportion to the heterogeneity of any
aggregate, is the heterogeneity it will produce in any incident force
(_First Principles_, § 156). The energy continually liberated in the
organism by decomposition, is here the incident force; the functions are
the variously modified forms produced in its divisions by the organs they
pass through; and the more multiform the organs the more multiform must be
the differentiations of the force passing through them.
It follows obviously from this, that if structure progresses from the
homogeneous, indefinite, and incoherent, to the heterogeneous, definite,
and coherent, so too must function. If the number of different parts in an
aggregate must determine the number of differentiations produced in the
energies passing through it--if the distinctness of these parts from one
another, must involve distinctness in their reactions, and therefore
distinctness between the divisions of the differentiated energy; there
cannot but be a complete parallelism between the development of structure
and the development of function. If structure advances from the simple and
general to the complex and special, function must do the same.
CHAPTER IV.
WASTE AND REPAIR.
§ 62. Throughout the vegetal kingdom, the processes of Waste and Repair are
comparatively insignificant in their amounts. Though all parts of plants
save the leaves, or other parts which are green, give out carbonic acid;
yet this carbonic acid, assuming it to indicate consumption of tissue, or
rather of the protoplasm contained in the tissue, indicates but a small
consumption. Of course if there is little waste there can be but little
repair--that is, little of the interstitial repair which restores the
integrity of parts worn by functional activity. Nor, indeed, is there
displayed by plants in any considerable degree, if at all, that other
species of repair which consists in the restoration of lost or injured
organs. Torn leaves and the shoots that are shortened by the pruner, do not
reproduce their missing parts; and though when the branch of a tree is cut
off close to the trunk, the place is in course of years covered over, it is
not by any reparative action in the wounded surface but by the lateral
growth of the adjacent bark. Hence, without saying that Waste and Repair do
not go on at all in plants, we may fitly pass them over as of no
importance.
There are but slight indications of waste in those lower orders of animals
which, by their comparative inactivity, show themselves least removed from
vegetal life. Actiniæ kept in an aquarium, do not appreciably diminish in
bulk from prolonged abstinence. Even fish, though much more active than
most other aquatic creatures, appear to undergo but little loss of
substance when kept unfed during considerable periods. Reptiles, too,
maintaining no great temperature, and passing their lives mostly in a state
of torpor, suffer but little diminution of mass by waste. When, however, we
turn to those higher orders of animals which are active and hot-blooded, we
see that waste is rapid: producing, when unchecked, a notable decrease in
bulk and weight, ending very shortly in death. Besides finding that waste
is inconsiderable in creatures which produce but little insensible and
sensible motion, and that it becomes conspicuous in creatures which produce
much insensible and sensible motion; we find that in the same creatures
there is most waste when most motion is generated. This is clearly proved
by hybernating animals. "Valentin found that the waking marmot excreted in
the average 75 times more carbonic acid, and inhaled 41 times more oxygen
than the same animal in the most complete state of hybernation. The stages
between waking and most profound hybernation yielded intermediate figures.
A waking hedgehog yielded about 20.5 times more carbonic acid, and consumed
18.4 times more oxygen than one in the state of hybernation."[22] If we
take these quantities of absorbed oxygen and excreted carbonic acid, as
indicating something like the relative amounts of consumed organic
substance, we see that there is a striking contrast between the waste
accompanying the ordinary state of activity, and the waste accompanying
complete quiescence and reduced temperature. This difference is still more
definitely shown by the fact, that the mean daily loss from starvation in
rabbits and guinea-pigs, bears to that from hybernation, the proportion of
18.3:1. Among men and domestic animals, the relation between degree of
waste and amount of expended energy, though one respecting which there is
little doubt, is less distinctly demonstrable; since waste is not allowed
to go on uninterfered with. We have, however, in the lingering lives of
invalids who are able to take scarcely any nutriment but are kept warm and
still, an illustration of the extent to which waste diminishes as the
expenditure of energy declines.
Besides the connexion between the waste of the organism as a whole and the
production of sensible and insensible motion by the organism as a whole,
there is a traceable connexion between the waste of special parts and the
activities of such special parts. Experiments have shown that "the starving
pigeon daily consumes in the average 40 times more muscular substance that
the marmot in the state of torpor, and only 11 times more fat, 33 times
more of the tissue of the alimentary canal, 18.3 times more liver, 15 times
more lung, 5 times more skin." That is to say, in the hybernating animal
the parts least consumed are the almost totally quiescent motor-organs, and
the part most consumed is the hydro-carbonaceous deposit serving as a store
of energy; whereas in the pigeon, similarly unsupplied with food but awake
and active, the greatest loss takes place in the motor-organs. The
relation between special activity and special waste, is illustrated, too,
in the daily experiences of all: not indeed in the amount of decrease of
the active parts in bulk or weight, for this we have no means of
ascertaining; but in the diminished ability of such parts to perform their
functions. That legs exerted for many hours in walking and arms long
strained in rowing, lose their powers--that eyes become enfeebled by
reading or writing without intermission--that concentrated attention,
unbroken by rest, so prostrates the brain as to incapacitate it for
thinking; are familiar truths. And though we have no direct evidence to
this effect, there is little danger in concluding that muscles exercised
until they ache or become stiff, and nerves of sense rendered weary or
obtuse by work, are organs so much wasted by action as to be partially
incompetent.
Repair is everywhere and always making up for waste. Though the two
processes vary in their relative rates both are constantly going on. Though
during the active, waking state of an animal waste is in excess of repair,
yet repair is in progress; and though during sleep repair is in excess of
waste, yet some waste is necessitated by the carrying on of certain
never-ceasing functions. The organs of these never-ceasing functions
furnish, indeed, the most conclusive proofs of the simultaneity of repair
and waste. Day and night the heart never stops beating, but only varies in
the rapidity and vigour of its beats; and hence the loss of substance which
its contractions from moment to moment entail, must from moment to moment
be made good. Day and night the lungs dilate and collapse; and the muscles
which make them do this must therefore be kept in a state of integrity by a
repair which keeps pace with waste, or which alternately falls behind and
gets in advance of it to a very slight extent.
On a survey of the facts we see, as we might expect to see, that the
progress of repair is most rapid when activity is most reduced. Assuming
that the organs which absorb and circulate nutriment are in proper order,
the restoration of the body to a state of integrity, after the
disintegration consequent on expenditure of energy, is proportionate to the
diminution in expenditure of energy. Thus we all know that those who are in
health, feel the greatest return of vigour after profound sleep--after
complete cessation of motion. We know that a night during which the
quiescence, bodily and mental, has been less decided, is usually not
followed by that spontaneous overflow of energy which indicates a high
state of efficiency throughout the organism. We know, again, that
long-continued recumbency, even with wakefulness (providing the wakefulness
is not the result of disorder), is followed by a certain renewal of
strength; though a renewal less than that which would have followed the
greater inactivity of slumber. We know, too, that when exhausted by labour,
sitting brings a partial return of vigour. And we also know that after the
violent exertion of running, a lapse into the less violent exertion of
walking, results in a gradual disappearance of that prostration which the
running produced. This series of illustrations conclusively proves that the
rebuilding of the organism is ever making up for the pulling down of it
caused by action; and that the effect of this rebuilding becomes more
manifest, in proportion as the pulling down is less rapid. From each
digested meal there is every few hours absorbed into the mass of prepared
nutriment circulating through the body, a fresh supply of the needful
organic compounds; and from the blood, thus occasionally re-enriched, the
organs through which it passes are ever taking up materials to replace the
materials used up in the discharge of functions. During activity the
reintegration falls in arrear of the disintegration; until, as a
consequence, there presently comes a general state of functional languor;
ending, at length, in a quiescence which permits the reintegration to
exceed the disintegration, and restore the parts to their state of
integrity. Here, as wherever there are antagonistic actions, we see
rhythmical divergences on opposite sides of the medium state--changes which
equilibrate each other by their alternate excesses. (_First Principles_,
§§ 85, 173.)
Illustrations are not wanting of special repair that is similarly ever in
progress, and similarly has intervals during which it falls below waste and
rises above it. Every one knows that a muscle, or a set of muscles,
continuously strained, as by holding out a weight at arm's length, soon
loses its power; and that it recovers its power more or less fully after a
short rest. The several organs of the special sensations yield us like
experiences. Strong tastes, powerful odours, loud sounds, temporarily unfit
the nerves impressed by them for appreciating faint tastes, odours, or
sounds; but these incapacities are remedied by brief intervals of repose.
Vision still better illustrates this simultaneity of waste and repair.
Looking at the Sun so affects the eyes that, for a short time, they cannot
perceive the things around with the usual clearness. After gazing at a
bright light of a particular colour, we see, on turning the eyes to
adjacent objects, an image of the complementary colour; showing that the
retina has, for the moment, lost the power to feel small amounts of those
rays which have strongly affected it. Such inabilities disappear in a few
seconds or a few minutes, according to circumstances. And here, indeed, we
are introduced to a conclusive proof that special repair is ever
neutralizing special waste. For the rapidity with which the eyes recover
their sensitiveness, varies with the reparative power of the individual. In
youth the visual apparatus is so quickly restored to its state of
integrity, that many of these _photogenes_, as they are called, cannot be
perceived. When sitting on the far side of a room, and gazing out of the
window against a light sky, a person who is debilitated by disease or
advancing years, perceives, on transferring the gaze to the adjacent wall,
a momentary negative image of the window--the sash-bars appearing light and
the squares dark; but a young and healthy person has no such experience.
With a rich blood and vigorous circulation, the repair of the visual nerves
after impressions of moderate intensity, is nearly instantaneous.
Function carried to excess may produce waste so great that repair cannot
make up for it during the ordinary daily periods of rest; and there may
result incapacities of the over-taxed organs, lasting for considerable
periods. We know that eyes strained by long-continued minute work lose
their power for months or years: perhaps suffering an injury from which
they never wholly recover. Brains, too, are often so unduly worked that
permanent relaxation fails to restore them to vigour. Even of the motor
organs the like holds. The most frequent cause of what is called "wasting
palsy," or atrophy of the muscles, is habitual excess of exertion: the
proof being that the disease occurs most frequently among those engaged in
laborious handicrafts, and usually attacks first the muscles which have
been most worked.
There has yet to be noticed another kind of repair--that, namely, by which
injured or lost parts are restored. Among the _Hydrozoa_ it is common for
any portion of the body to reproduce the rest; even though the rest to be
so reproduced is the greater part of the whole. In the more
highly-organized _Actinozoa_ the half of an individual will grow into a
complete individual. Some of the lower Annelids, as the _Nais_, may be cut
into thirty or forty pieces and each piece will eventually become a perfect
animal. As we ascend to higher forms we find this reparative power much
diminished, though still considerable. The reproduction of a lost claw by a
lobster or crab, is a familiar instance. Some of the inferior _Vertebrata_
also, as lizards, can develop new limbs or new tails, in place of those
which have been cut off; and can even do this several times over, though
with decreasing completeness. The highest animals, however, thus repair
themselves to but a very small extent. Mammals and birds do it only in the
healing of wounds; and very often but imperfectly even in this. For in
muscular and glandular organs the tissues destroyed are not properly
reproduced, but are replaced by tissue of an irregular kind which serves to
hold the parts together. So that the power of reproducing lost parts is
greatest where the organization is lowest; and almost disappears where the
organization is highest. And though we cannot say that in the intermediate
stages there is a constant inverse relation between reparative power and
degree of organization; yet we may say that there is some approach to such
a relation.
§ 63. There is an obvious and complete harmony between the first of the
above inductions and the deduction which follows immediately from first
principles. We have already seen (§ 23) "that whatever amount of power an
organism expends in any shape, is the correlate and equivalent of a power
that was taken into it from without." Motion, sensible or insensible,
generated by an organism, is insensible motion which was absorbed in
producing certain chemical compounds appropriated by the organism under the
form of food. As much energy as was required to raise the elements of these
complex atoms to their state of unstable equilibrium, is given out in their
falls to a state of stable equilibrium; and having fallen to a state of
stable equilibrium they can give out no further energy, but have to be got
rid of as inert and useless. It is an inevitable corollary "from the
persistence of force, that each portion of mechanical or other energy which
an organism exerts, implies the transformation of as much organic matter as
contained this energy in a latent state;" and that this organic matter in
yielding up its latent energy, loses its value for the purposes of life,
and becomes waste matter needing to be excreted. The loss of these complex
unstable substances must hence be proportionate to the quantity of expended
force. Here, then, is the rationale of certain general facts lately
indicated. Plants do not waste to any considerable degree, for the obvious
reason that the sensible and insensible motions they generate are
inconsiderable. Between the small waste, small activity, and low
temperature of the inferior animals, the relation is similarly one
admitting of _a priori_ establishment. Conversely, the rapid waste of
energetic, hot-blooded animals might be foreseen with equal certainty. And
not less manifestly necessary is the variation in waste which, in the same
organism, attends the variation in the heat and mechanical motion produced.
Between the activity of a special part and the waste of that part, a like
relation may be deductively inferred; though it cannot be inferred that
this relation is equally definite. Were the activity of every organ quite
independent of the activities of other organs, we might expect to trace out
this relation distinctly; but since increased activity in any organ or
group of organs, as the muscles, necessarily entails increased activity in
other organs, as in the heart, lungs, and nervous system, it is clear that
special waste and general waste are too much entangled to admit of a
definite relation being established between special waste and special
activity. We may fairly say, however, that this relation is quite as
manifest as we can reasonably anticipate.
§ 64. Deductive interpretation of the phenomena of Repair, is by no means
so easy. The tendency displayed by an animal organism, as well as by each
of its organs, to return to a state of integrity by the assimilation of new
matter, when it has undergone the waste consequent on activity, is a
tendency which is not manifestly deducible from first principles; though it
appears to be in harmony with them. If in the blood there existed
ready-formed units exactly like in kind to those of which each organ
consists, the sorting of these units, ending in the union of each kind with
already existing groups of the same kind, would be merely a good example of
Segregation (_First Principles_, § 163). It would be analogous to the
process by which, from a mixed solution of salts, there are, after an
interval, deposited separate masses of these salts in the shape of
different crystals. But as already said (§ 54), though the selective
assimilation by which the repair of organs is effected, may result in part
from an action of this kind, the facts cannot be thus wholly accounted for;
since organs are in part made up of units which do not exist as such in the
circulating fluids. We must suppose that, as suggested in § 54, groups of
compound units have a certain power of moulding adjacent fit materials into
units of their own form. Let us see whether there is not reason to think
such a power exists.
"The poison of small-pox or of scarlatina," remarks Mr. (now Sir James)
Paget, "being once added to the blood, presently affects the composition of
the whole: the disease pursues its course, and, if recovery ensue, the
blood will seem to have returned to its previous condition: yet it is not
as it was before; for now the same poison may be added to it with
impunity." ... "The change once effected, may be maintained through life.
And herein seems to be a proof of the assimilative force in the blood: for
there seems no other mode of explaining these cases than by admitting that
the altered particles have the power of assimilating to themselves all
those by which they are being replaced: in other words, all the blood that
is formed after such a disease deviates from the natural composition, so
far as to acquire the peculiarity engendered by the disease: it is formed
according to the altered model." Now if the compound molecules of the
blood, or of an organism considered in the aggregate, have the power of
moulding into their own type the matters which they absorb as nutriment;
and if they have the power when their type has been changed by disease, of
moulding materials afterwards received into the modified type; may we not
reasonably suspect that the more or less specialized molecules of each
organ have, in like manner, the power of moulding the materials which the
blood brings to them into similarly specialized molecules? The one
conclusion seems to be a corollary from the other. Such a power cannot be
claimed for the component units of the blood without being conceded to the
component units of every tissue. Indeed the assertion of this power is
little more than an assertion of the fact that organs composed of
specialized units _are_ capable of resuming their structural integrity
after they have been wasted by function. For if they do this, they must do
it by forming from the materials brought to them, certain specialized units
like in kind to those of which they are composed; and to say that they do
this, is to say that their component units have the power of moulding fit
materials into other units of the same order.
§ 65. What must we say of the ability an organism has to re-complete itself
when one of its parts has been cut off? Is it of the same order as the
ability of an injured crystal to re-complete itself. In either case new
matter is so deposited as to restore the original outline. And if in the
case of the crystal we say that the whole aggregate exerts over its parts a
force which constrains the newly-integrated molecules to take a certain
definite form, we seem obliged, in the case of the organism, to assume an
analogous force. If when the leg of a lizard has been amputated there
presently buds out the germ of a new one, which, passing through phases of
development like those of the original leg, eventually assumes a like shape
and structure, we assert only what we see, when we assert that the entire
organism, or the adjacent part of it, exercises such power over the forming
limb as makes it a repetition of its predecessor. If a leg is reproduced,
where there was a leg, and a tail where there was a tail, there seems no
alternative but to conclude that the forces around it control the formative
processes going on in each part. And on contemplating these facts in
connexion with various kindred ones, there is suggested the hypothesis,
that the form of each species of organism is determined by a peculiarity in
the constitution of its units--that these have a special structure in which
they tend to arrange themselves; just as have the simpler units of
inorganic matter. Let us glance at the evidences which more especially
thrust this conclusion upon us.
A fragment of a Begonia-leaf imbedded in fit soil and kept at an
appropriate temperature, will develop a young Begonia; and so small is the
fragment which is thus capable of originating a complete plant, that
something like a hundred plants may be produced from a single leaf. The
friend to whom I owe this observation, tells me that various succulent
plants have like powers of multiplication. Illustrating a similar power
among animals, we have the often-cited experiments of Trembley on the
common polype. Each of the four pieces into which one of these creatures
was cut, grew into a perfect individual. In each of these, again, bisection
and tri-section were followed by like results. And so with their segments,
similarly produced, until as many as fifty polypes had resulted from the
original one. Bodies when cut off regenerated heads; heads regenerated
bodies; and when a polype had been divided into as many pieces as was
practicable, nearly every piece survived and became a complete animal.
What, now, is the implication? We cannot say that in each portion of a
Begonia-leaf, and in every fragment of a Hydra's body, there exists a
ready-formed model of the entire organism. Even were there warrant for the
doctrine that the germ of every organism contains the perfect organism in
miniature, it still could not be contended that each considerable part of
the perfect organism resulting from such a germ, contains another such
miniature. Indeed the one hypothesis negatives the other. The implication
seems, therefore, to be that the living particles composing one of these
fragments, have an innate tendency to arrange themselves into the shape of
the organism to which they belong. We must infer that the active units
composing a plant or animal of any species have an intrinsic aptitude to
aggregate into the form of that species. It seems difficult to conceive
that this can be so; but we see that it _is_ so. Groups of units taken from
an organism (providing they are of a certain bulk and not much
differentiated into special structures) _have_ this power of re-arranging
themselves. Manifestly, too, if we are thus to interpret the reproduction
of an organism from one of its amorphous fragments, we must thus interpret
the reproduction of any minor portion of an organism by the remainder. When
in place of its lost claw a lobster puts forth a cellular mass which, while
increasing in bulk, assumes the form and structure of the original claw, we
cannot avoid ascribing this result to a play of forces like that which
moulds the materials contained in a piece of Begonia-leaf into the shape of
a young Begonia.
§ 66. As we shall have frequent occasion hereafter to refer to these units
which possess the property of arranging themselves into the special
structures of the organisms to which they belong; it will be well here to
ask by what name they may be most fitly called.
On the one hand, it cannot be in those chemical compounds characterizing
organic bodies that this specific property dwells. It cannot be that the
molecules of albumin, or fibrin, or gelatine, or other proteid, possess
this power of aggregating into these specific shapes; for in such case
there would be nothing to account for the unlikenesses of different
organisms. If the proclivities of proteid molecules determined the forms of
the organisms built up of them or by them, the occurrence of such endlessly
varied forms would be inexplicable. Hence what we may call the _chemical
units_ are clearly not the possessors of this property.
On the other hand, this property cannot reside in what may be roughly
distinguished as the _morphological units_. The germ of every organism is a
minute portion of encased protoplasm commonly called a cell. It is by
multiplication of cells that all the early developmental changes are
effected. The various tissues which successively arise in the unfolding
organism, are primarily cellular; and in many of them the formation of
cells continues to be, throughout life, the process by which repair is
carried on. But though cells are so generally the ultimate visible
components of organisms, that they may with some show of reason be called
the morphological units; yet we cannot say that this tendency to aggregate
into special forms dwells in them. In many cases a fibrous tissue arises
out of a nucleated blastema, without cell-formation; and in such cases
cells cannot be regarded as units possessing the structural proclivity. But
the conclusive proof that the morphological units are not the building
factors in an organism composed of them, is yielded by their independent
homologues the so-called unicellular organisms. For each of these displays
the power to assume its specific structure. Clearly, if the ability of a
multicellular organism to assume its specific structure resulted from the
cooperation of its component cells, then a single cell, or the independent
homologue of a single cell, having no other to cooperate with, could
exhibit no structural traits. Not only, however, do single-celled organisms
exhibit structural traits, but these, even among the simplest, are so
distinct as to originate classification into orders, genera, and species;
and they are so constant as to remain the same from generation to
generation.
If, then, this organic polarity (as we might figuratively call this
proclivity towards a specific structural arrangement) can be possessed
neither by the chemical units nor the morphological units, we must conceive
it as possessed by certain intermediate units, which we may term
_physiological_. There seems no alternative but to suppose that the
chemical units combine into units immensely more complex than themselves,
complex as they are; and that in each organism the physiological units
produced by this further compounding of highly compound molecules, have a
more or less distinctive character. We must conclude that in each case some
difference of composition in the units, or of arrangement in their
components, leading to some difference in their mutual play of forces,
produces a difference in the form which the aggregate of them assumes.
The facts contained in this chapter form but a small part of the evidence
which thrusts this assumption upon us. We shall hereafter find various
reasons for inferring that such physiological units exist, and that to
their specific properties, more or less unlike in each plant and animal,
various organic phenomena are due.
CHAPTER V.
ADAPTATION.
§ 67. In plants waste and repair being scarcely appreciable, there are not
likely to arise appreciable changes in the proportions of already-formed
parts. The only divergences from the average structures of a species, which
we may expect particular conditions to produce, are those producible by the
action of these conditions on parts in course of formation; and such
divergences we do find. We know that a tree which, standing alone in an
exposed position, has a short and thick stem, has a tall and slender stem
when it grows in a wood; and that also its branches then take a different
inclination. We know that potato-sprouts which, on reaching the light,
develop into foliage, will, in the absence of light, grow to a length of
several feet without foliage. And every in-door plant furnishes proof that
shoots and leaves, by habitually turning themselves to the light, exhibit a
certain adaptation--an adaptation due, as we must suppose; to the special
effects of the special conditions on the still growing parts. In animals,
however, besides analogous structural changes wrought during the period of
growth, by subjection to circumstances unlike the ordinary circumstances,
there are structural changes similarly wrought after maturity has been
reached. Organs that have arrived at their full sizes possess a certain
modifiability; so that while the organism as a whole retains pretty nearly
the same bulk, the proportions of its parts may be considerably varied.
Their variations, here treated of under the title Adaptation, depend on
specialities of individual action. In the last chapter we saw that the
actions of organisms entail re-actions on them; and that specialities of
action entail specialities of re-action. Here it remains to be pointed out
that these special actions and re-actions do not end with temporary
changes, but work permanent changes.
If, in an adult animal, the waste and repair in all parts were exactly
balanced--if each organ daily gained by nutrition exactly as much as it
lost daily by the discharge of its function--if excess of function were
followed only by such excess of nutrition as balanced the extra waste; it
is clear that there would occur no change in the relative sizes of organs.
But there is no such exact balance. If the excess of function, and
consequent excess of waste, is moderate, it is not simply compensated by
repair but more than compensated--there is a certain increase of bulk. This
is true to some degree of the organism as a whole, when the organism is
framed for activity. A considerable waste giving considerable power of
assimilation, is more favourable to accumulation of tissue than is
quiescence with its comparatively feeble assimilation: whence results a
certain adaptation of the whole organism to its requirements. But it is
more especially true of the parts of an organism in relation to one
another. The illustrations fall into several groups. The growth of muscles
exercised to an unusual degree is a matter of common observation. In the
often-cited blacksmith's arm, the dancer's legs and the jockey's crural
adductors, we have marked examples of a modifiability which almost every
one has to some extent experienced. It is needless to multiply proofs. The
occurrence of changes in the structure of the skin, where the skin is
exposed to unusual stress of function, is also familiar. That thickening of
the epidermis on a labourer's palm results from continual pressure and
friction, is certain. Those who have not before exerted their hands, find
that such an exercise as rowing soon begins to produce a like thickening.
This relation of cause and effect is still better shown by the marked
indurations at the ends of a violinist's fingers. Even in mucous membrane,
which ordinarily is not subject to mechanical forces of any intensity,
similar modifications are possible: witness the callosity of the gums which
arises in those who have lost their teeth, and have to masticate without
teeth. The vascular system furnishes good instances of the increased growth
that follows increased function. When, because of some permanent
obstruction to the circulation, the heart has to exert a greater
contractile force on the mass of blood which it propels at each pulsation,
and when there results the laboured action known as palpitation, there
usually occurs dilatation, or hypertrophy, or a mixture of the two: the
dilatation, which is a yielding of the heart's structure under the
increased strain, implying a failure to meet the emergency; but the
hypertrophy, which consists in a thickening of the heart's muscular walls,
being an adaptation of it to the additional effort required. Again, when an
aneurism in some considerable artery has been obliterated, either
artifically or by a natural inflammatory process; and when this artery has
consequently ceased to be a channel for the blood; some of the adjacent
arteries which anastomose with it become enlarged, so as to carry the
needful quantity of blood to the parts supplied. Though we have no direct
proof of analogous modifications in nervous structures, yet indirect proof
is given by the greater efficiency that follows greater activity. This is
manifested alike in the senses and the intellect. The palate may be
cultivated into extreme sensitiveness, as in professional tea-tasters. An
orchestral conductor gains, by continual practice, an unusually great
ability to discriminate differences of sound. In the finger-reading of the
blind we have evidence that the sense of touch may be brought by exercise
to a far higher capability than is ordinary.[23] The increase of power
which habitual exertion gives to mental faculties needs no illustration:
every person of education has personal experience of it. Even from the
osseous structures evidence may be drawn. The bones of men accustomed to
great muscular action are more massive, and have more strongly marked
processes for the attachment of muscles, than the bones of men who lead
sedentary lives; and a like contrast holds between the bones of wild and
tame animals of the same species. Adaptations of another order, in which
there is a qualitative rather than a quantitative modification, arise after
certain accidents to which the skeleton is liable. When the hip-joint has
been dislocated, and long delay has made it impossible to restore the parts
to their proper places, the head of the thigh-bone, imbedded in the
surrounding muscles, becomes fixed in its new position by attachments of
fibrous tissue, which afford support enough to permit a halting walk. But
the most remarkable modification of this order occurs in united ends of
fractured bones. "False joints" are often formed--joints which rudely
simulate the hinge structure or the ball-and-socket structure, according as
the muscles tend to produce a motion of flexion and extension or a motion
of rotation. In the one case, according to Rokitansky, the two ends of the
broken bone become smooth and covered with periosteum and fibrous tissue,
and are attached by ligaments that allow a certain backward and forward
motion; and in the other case the ends, similarly clothed with the
appropriate membranes, become the one convex and the other concave, are
inclosed in a capsule, and are even occasionally supplied with synovial
fluid!
The general truth that extra function is followed by extra growth, must be
supplemented by the equally general truth, that beyond a limit, usually
soon reached, very little, if any, further modification can be produced.
The experiences which we colligate into the one induction thrust the other
upon us. After a time no training makes the pugilist or the athlete any
stronger. The adult gymnast at last acquires the power to perform certain
difficult feats; but certain more difficult feats no additional practice
enables him to perform. Years of discipline give the singer a particular
loudness and range of voice, beyond which further discipline does not give
greater loudness or wider range: on the contrary, increased vocal exercise,
causing a waste in excess of repair, is often followed by decrease of
power. In the exaltation of the perceptions we see similar limits. The
culture which raises the susceptibility of the ear to the intervals and
harmonies of notes, will not turn a bad ear into a good one. Lifelong
effort fails to make this artist a correct draftsman or that a fine
colourist: each does better than he did at first, but each falls short of
the power attained by some other artists. Nor is this truth less clearly
illustrated among the more complex mental powers. A man may have a
mathematical faculty, a poetical faculty, or an oratorical faculty, which
special education improves to a certain extent. But unless he is unusually
endowed in one of those directions, no amount of education will make him a
first-rate mathematician, a first-rate poet, or a first-rate orator. Thus
the general fact appears to be that while in each individual certain
changes in the proportions of parts may be caused by variations of
functions, the congenital structure of each individual puts a limit to the
modifiability of every part. Nor is this true of individuals only: it
holds, in a sense, of species. Leaving open the question whether, in
indefinite times, indefinite modifications may not be produced by
inheritance of functionally wrought adaptations; experience proves that
within assigned times, the changes wrought in races of organisms by changes
of conditions fall within narrow limits. Though by discipline, aided by
selective breeding, one variety of horse has had its locomotive power
increased considerably beyond the locomotive powers of other varieties; yet
further increase takes place, if at all, at an inappreciable rate. The
different kinds of dogs, too, in which different forms and capacities have
been established, do not now show aptitudes for diverging in the same
directions at considerable rates. In domestic animals generally, certain
accessions of intelligence have been produced by culture; but accessions
beyond these are inconspicuous. It seems that in each species of organism
there is a margin for functional oscillations on all sides of a mean state,
and a consequent margin for structural variations; that it is possible
rapidly to push functional and structural changes towards the extreme of
this margin in any direction, both in an individual and in a race; but that
to push these changes further in any direction, and so to alter the
organism as to bring its mean state up to the extreme of the margin in that
direction, is a comparatively slow process.[24]
We also have to note that the limited increase of size produced in any
organ by a limited increase of its function, is not maintained unless the
increase of function is permanent. A mature man or other animal, led by
circumstances into exerting particular members in unusual degrees, and
acquiring extra sizes in these members, begins to lose such extra sizes on
ceasing to exert the members; and eventually lapses more or less nearly
into the original state. Legs strengthened by a pedestrian tour, become
relatively weak again after a prolonged return to sedentary life. The
acquired ability to perform feats of skill disappears in course of time, if
the performance of them be given up. For comparative failure in executing a
piece of music, in playing a game at chess, or in anything requiring
special culture, the being out of practice is a reason which every one
recognizes as valid. It is observable, too, that the rapidity and
completeness with which an artificial power is lost, is proportionate to
the shortness of the cultivation which evoked it. One who has for many
years persevered in habits which exercise special muscles or special
faculties of mind, retains the extra capacity produced, to a very
considerable degree, even after a long period of desistance; but one who
has persevered in such habits for but a short time has, at the end of a
like period, scarcely any of the facility he had gained. Here too, as
before, successions of organisms present an analogous fact. A species in
which domestication continued through many generations, has organized
certain peculiarities; and which afterwards, escaping domestic discipline,
returns to something like its original habits; soon loses, in great
measure, such peculiarities. Though it is not true, as alleged, that it
resumes completely the structure it had before domestication, yet it
approximates to that structure. The Dingo, or wild dog of Australia, is one
of the instances given of this; and the wild horse of South America is
another. Mankind, too, supplies us with instances. In the Australian bush
and in the backwoods of America, the Anglo-Saxon race, in which
civilization has developed the higher feelings to a considerable degree,
rapidly lapses into comparative barbarism: adopting the moral code, and
sometimes the habits, of savages.
§ 68. It is important to reach, if possible, some rationale of these
general truths--especially of the last two. A right understanding of these
laws of organic modification underlies a right understanding of the great
question of species. While, as before hinted (§ 40), the action of
structure on function is one of the factors in that process of
differentiation by which unlike forms of plants and animals are produced,
the reaction of function on structure is another factor. Hence, it is well
worth while inquiring how far these inductions are deductively
interpretable.
The first of them is the most difficult to deal with. Why an organ exerted
somewhat beyond its wont should presently grow, and thus meet increase of
demand by increase of supply, is not obvious. We know, indeed, (_First
Principles_, §§ 85, 173,) that of necessity, the rhythmical changes
produced by antagonistic organic actions cannot any of them be carried to
an excess in one direction, without there being produced an equivalent
excess in the opposite direction. It is a corollary from the persistence of
force, that any deviation effected by a disturbing cause, acting on some
member of a moving equilibrium, must (unless it altogether destroys the
moving equilibrium) be eventually followed by a compensating deviation.
Hence, that excess of repair should succeed excess of waste, is to be
expected. But how happens the mean state of the organ to be changed? If
daily extra waste naturally brings about daily extra repair only to an
equivalent extent, the mean state of the organ should remain constant. How
then comes the organ to augment in size and power?
Such answer to this question as we may hope to find, must be looked for in
the effects wrought on the organism as a whole by increased function in one
of its parts. For since the discharge of its function by any part is
possible only on condition that those various other functions on which its
own is immediately dependent are also discharged, it follows that excess in
its function presupposes some excess in their functions. Additional work
given to a muscle implies additional work given to the branch arteries
which bring it blood, and additional work, smaller in proportion, to the
arteries from which these branch arteries come. Similarly, the smaller and
larger veins which take away the blood, as well as those structures which
deal with effete products, must have more to do. And yet further, on the
nervous centres which excite the muscle a certain extra duty must fall. But
excess of waste will entail excess of repair, in these parts as well as in
the muscle. The several appliances by which the nutrition and excitation of
an organ are carried on, must also be influenced by this rhythm of action
and reaction; and therefore, after losing more than usual by the
destructive process they must gain more than usual by the constructive
process. But temporarily-increased efficiency in these appliances by which
blood and nervous force are brought to an organ, will cause extra
assimilation in the organ, beyond that required to balance its extra
expenditure. Regarding the functions as constituting a moving equilibrium,
we may say that divergence of any function in the direction of increase,
causes the functions with which it is bound up to diverge in the same
direction; that these, again, cause the functions which they are bound up
with, also to diverge in the same direction; and that these divergences of
the connected functions allow the specially-affected function to be carried
further in this direction than it could otherwise be--further than the
perturbing force could carry it if it had a fixed basis.
It must be admitted that this is but a vague explanation. Among actions so
involved as these, we can scarcely expect to do more than dimly discern a
harmony with first principles. That the facts are to be interpreted in some
such way, may, however, be inferred from the circumstance that an extra
supply of blood continues for some time to be sent to an organ that has
been unusually exercised; and that when unusual exercise is long continued
a permanent increase of vascularity results.
§ 69. Answers to the questions--Why do these adaptive modifications in an
individual animal soon reach a limit? and why, in the descendants of such
animal, similarly conditioned, is this limit very slowly extended?--are to
be found in the same direction as was the answer to the last question. And
here the connexion of cause and consequence is more manifest.
Since the function of any organ is dependent on the functions of the organs
which supply it with materials and stimuli; and since the functions of
these subsidiary organs are dependent on the functions of organs which
supply them with materials and stimuli; it follows that before any great
extra power of discharging its function can be gained by a
specially-exercised organ, a considerable extra power must be gained by a
series of immediately-subservient organs, and some extra power by a
secondary series of remotely-subservient organs. Thus there are required
numerous and wide-spread modifications. Before the artery which feeds a
hard-worked muscle can permanently furnish a large additional quantity of
blood, it must increase in diameter; and that its increase of diameter may
be of use, the main artery from which it diverges must also be so far
modified as to bring this additional quantity of blood to the branch
artery. Similarly with the veins; similarly with the structures which
remove waste-products; similarly with the nerves. And when we ask what
these subsidiary changes imply, we are forced to conclude that there must
be an analogous group of more numerous changes ramifying throughout the
system. The growth of the arteries primarily and secondarily implicated,
cannot go to any extent without growth in the minor blood-vessels on which
their nutrition depends; while their greater contractile power involves
enlargement of the nerves which excite them, and some modification of that
part of the spinal cord whence these nerves proceed. Thus, without tracing
the like remote alterations implied by extra growth of the veins,
lymphatics, glandular organs, and other agencies, it is manifest that a
large amount of rebuilding must be done throughout the organism, before any
organ of importance can be permanently increased in size and power to a
great extent. Hence, though such extra growth in any part as does not
necessitate considerable changes throughout the rest of the organism, may
rapidly take place; a further growth in this part, requiring a re-modelling
of numerous parts remotely and slightly affected, must take place but
slowly.
We have before found our conceptions of vital processes made clearer by
studying analogous social processes. In societies there is a mutual
dependence of functions, essentially like that which exists in organisms;
and there is also an essentially like reaction of functions on structures.
From the laws of adaptive modification in societies, we may therefore hope
to get a clue to the laws of adaptive modification in organisms. Let us
suppose, then, that a society has arrived at a state of equilibrium
analogous to that of a mature animal--a state not like our own, in which
growth and structural development are rapidly going on, but a state of
settled balance among the functional powers of the various classes and
industrial bodies, and a consequent fixity in the relative sizes of such
classes and bodies. Further, let us suppose that in a society thus balanced
there occurs something which throws an unusual demand on one industry--say
an unusual demand for ships (which we will assume to be built of iron) in
consequence of a competing mercantile nation having been prostrated by
famine or pestilence. The immediate result of this additional demand for
iron ships is the employment of more workmen, and the purchase of more
iron, by the ship-builders; and when, presently, the demand continuing, the
ship-builders find their premises and machinery insufficient, they enlarge
them. If the extra requirement persists, the high interest and high wages
bring such extra capital and labour into the business as are needed for new
ship-building establishments. But such extra capital and labour do not come
quickly; since, in a balanced community, not increasing in population and
wealth, labour and capital have to be drawn from other industries, where
they are already yielding the ordinary returns. Let us now go a step
further. Suppose that this iron-ship-building industry, having enlarged as
much as the available capital and labour permit, is still unequal to the
demand; what limits its immediate further growth? The lack of iron. By the
hypothesis, the iron-producing industry, like all the other industries
throughout the community, yields only as much iron as is habitually
required for all the purposes to which iron is applied: ship-building being
only one. If, then, extra iron is required for ship-building, the first
effect is to withdraw part of the iron habitually consumed for other
purposes, and to raise the price of iron. Presently, the iron-makers feel
this change and their stocks dwindle. As, however, the quantity of iron
required for ship-building forms but a small part of the total quantity
required for all purposes, the extra demand on the iron-makers can be
nothing like so great in proportion as is the extra demand on the
ship-builders. Whence it follows that there will be much less tendency to
an immediate enlargement of the iron-producing industry; since the extra
quantity will for some time be obtained by working extra hours.
Nevertheless if, as fast as more iron can be thus supplied, the
ship-building industry goes on growing--if, consequently, the iron-makers
experience a permanently-increased demand, and out of their greater profits
get higher interest on capital, as well as pay higher wages; there will
eventually be an abstraction of capital and labour from other industries to
enlarge the iron-producing industry: new blast-furnaces, new rolling-mills,
new cottages for workmen, will be erected. But obviously, the inertia of
capital and labour to be overcome before the iron-producing industry can
grow by a decrease of certain other industries, will prevent its growth
from taking place until long after the increased ship-building industry has
demanded it; and meanwhile, the growth of the ship-building industry must
be limited by the deficiency of iron. A remoter restraint of the same
nature meets us if we go a step further--a restraint which can be overcome
only in a still longer time. For the manufacture of iron depends on the
supply of coal. The production of coal being previously in equilibrium with
the consumption; and the consumption of coal for the manufacture of iron
being but a small part of the total consumption; it follows that a
considerable extension of the iron manufacture, when it at length takes
place, will cause but a comparatively small additional demand on the
coal-owners and coal-miners--a demand which will not, for a long period,
suffice to cause enlargement of the coal-trade, by drawing capital and
labour from other investments and occupations. And until the permanent
extra demand for coal has become great enough to draw from other
investments and occupations sufficient capital and labour to sink new
mines, the increasing production of iron must be restricted by the scarcity
of coal, and the multiplication of ship-yards and ship-builders must be
checked by the want of iron. Thus, in a community which has reached a state
of moving equilibrium, though any one industry directly affected by an
additional demand may rapidly undergo a small extra growth, yet a growth
beyond this, requiring as it does the building-up of subservient
industries, less directly and strongly affected, as well as the partial
unbuilding of other industries, can take place only with comparative
slowness. And a still further growth, requiring structural modifications of
industries still more distantly affected, must take place still more
slowly.
On returning from this analogy, we see more clearly the truth that any
considerable member of an animal organism, cannot be greatly enlarged
without some general reorganization. Besides a building up of the primary,
secondary, and tertiary groups of the subservient parts, there must be an
unbuilding of sundry non-subservient parts; or, at any rate, there must be
permanently established a lower nutrition of such non-subservient parts.
For it must be remembered that in a mature animal, or one which has reached
a balance between assimilation and expenditure, there cannot (supposing
general conditions to remain constant) be an increase in the nutrition of
some organs without a decrease in the nutrition of others; and an organic
establishment of the increase implies an organic establishment of the
decrease--implies more or less change in the processes and structures
throughout the entire system. And here, indeed, is disclosed one reason why
growing animals undergo adaptations so much more readily than adult ones.
For while there is surplus nutrition, it is possible for
specially-exercised parts to be specially enlarged without any positive
deduction from other parts. There is required only that negative deduction
implied in the diminished growth of other parts.
§ 70. Pursuing the argument further, we reach an explanation of the third
general truth; namely that organisms, and species of organisms, which,
under new conditions, have undergone adaptive modifications, soon return to
something like their original structures when restored to their original
conditions. Seeing, as we have done, how excess of action and excess of
nutrition in any part of an organism, must affect action and nutrition in
subservient parts, and these again in other parts, until the re-action has
divided and subdivided itself throughout the organism, affecting in
decreasing degrees the more and more numerous parts more and more remotely
implicated; we see that the consequent changes in the parts remotely
implicated, constituting the great mass of the organism, must be extremely
slow. Hence, if the need for the adaptive modification ceases before the
great mass of the organism has been much altered in its structure by these
ramified but minute reactions, we shall have a condition in which the
specially-modified part is not in equilibrium with the rest. All the
remotely-affected organs, as yet but little changed, will, in the absence
of the perturbing cause, resume very nearly their previous actions. The
parts that depend on them will consequently by and by do the same. Until at
length, by a reversal of the adaptive process, the organ at first affected
will be brought back almost to its original state. Reconsidering the
above-drawn analogy between an organism and a society, will enable us
better to recognize this necessity. If, in the case supposed, the extra
demand for iron ships, after causing the erection of some additional
ship-yards and the drawing of iron from other manufactures, were to cease;
the old dimensions of the ship-building trade would be quickly returned to:
discharged workmen would seek fresh occupations, and the new yards would be
devoted to other uses. But if the increased need for ships lasted long
enough, and became great enough, to cause a flow of capital and labour from
other industries into the iron-manufacture, a falling off in the demand for
ships, would much less rapidly entail a dwindling of the ship-building
industry. For iron being now produced in greater quantity, a diminished
consumption of it for ships would cause a fall in its price, and a
consequent fall in the cost of ships: thus enabling the ship-builders to
meet the competition which we may suppose led to a decrease in the orders
they received. And since, when new blast-furnaces and rolling-mills, &c.,
had been built with capital drawn from other industries, its transference
back into other industries would involve great loss; the owners, rather
than transfer it, would accept unusually low interest, and an excess of
iron would continue to be produced; resulting in an undue cheapness of
ships, and a maintenance of the ship-building industry at a size beyond the
need. Eventually, however, if the number of ships required still
diminished, the production of iron in excess would become very
unremunerative: some of the blast-furnaces would be blown out; and as much
of the capital and labour as remained available would be re-distributed
among other occupations. Without repeating the steps of the argument, it
will be clear that were the enlargement of the ship-building industry great
enough, and did it last long enough to cause an increase in the number of
coal-mines, the ship-building industry would be still better able to
maintain itself under adverse circumstances; but that it would, though at a
more distant period, end by sinking down to the needful dimensions. Thus
our conclusions are:--First, that if the extra growth caused by extra
activity in a particular industry has lasted long enough only to remodel
the proximately-affected industries; it will dwindle away again after a
moderate period, if the need for it disappears. Second, that a long period
must be required before the re-actions produced by an enlarged industry can
cause a re-construction of the whole society, and before the countless
re-distributions of capital and labour can again reach a state of
equilibrium. And third, that only when such a new state of equilibrium is
eventually reached, can the adaptive modification become a permanent one.
How, in animal organisms the like argument holds, need not be pointed out.
The reader will readily follow the parallel.
That organic types should be comparatively stable, might be anticipated on
the hypothesis of Evolution. The structure of any organism being a product
of the almost infinite series of actions and reactions to which ancestral
organisms have been exposed; any unusual actions and reactions brought to
bear on an individual, can have but an infinitesimal effect in permanently
changing the structure of the organism as a whole. The new set of forces,
compounded with all the antecedent sets of forces, can but inappreciably
modify that moving equilibrium of functions which all these antecedent sets
of forces have established. Though there may result a considerable
perturbation of certain functions--a considerable divergence from their
ordinary rhythms--yet the general centre of equilibrium cannot be sensibly
changed. On the removal of the perturbing cause the previous balance will
be quickly restored: the effect of the new forces being almost obliterated
by the enormous aggregate of forces which the previous balance expresses.
§ 71. As thus understood, the phenomena of adaptation fall into harmony
with first principles. The inference that organic types are fixed, because
the deviations from them which can be produced within assignable periods
are relatively small, and because, when a force producing deviation ceases,
there is a return to something like the original state; proves to be an
invalid inference. Without assuming fixity of species, we find good reasons
for anticipating that kind and degree of stability which is observed. We
find grounds for concluding, _a priori_, that an adaptive change of
structure will soon reach a point beyond which further adaptation will be
slow; for concluding that when the modifying cause has been but a short
time in action, the modification generated will be evanescent; for
concluding that a modifying cause acting even for many generations, will do
but little towards permanently altering the organic equilibrium of a race;
and for concluding that on the cessations of such cause, its effects will
become unapparent in the course of a few generations.
CHAPTER VI.
INDIVIDUALITY.
§ 72. What is an individual? is a question which many readers will think it
easy to answer. Yet it is a question that has led to much controversy among
Zoologists and Botanists, and no quite satisfactory reply to it seems
possible. As applied to a man, or to any one of the higher animals, which
are all sharply-defined and independent, the word individual has a clear
meaning: though even here, when we turn from average cases to exceptional
cases--as a calf with two heads and two pairs of fore-limbs--we find
ourselves in doubt whether to predicate one individuality or two. But when
we extend our range of observation to the organic world at large, we find
that difficulties allied to this exceptional one meets us everywhere under
every variety of form.
Each uniaxial plant may perhaps fairly be regarded as a distinct
individual; though there are botanists who do not make even this admission.
What, however, are we to say of a multiaxial plant? It is, indeed, usual to
speak of a tree with its many branches and shoots as singular; but strong
reasons may be urged for considering it as plural. Every one of its axes
has a more or less independent life, and when cut off and planted may grow
into the likeness of its parent; or, by grafting and budding, parts of this
tree may be developed upon another tree, and there manifest their specific
peculiarities. Shall we regard all the growing axes thus resulting from
slips and grafts and buds, as parts of one individual or as distinct
individuals? If a strawberry-plant sends out runners carrying buds at their
ends, which strike root and grow into independent plants that separate from
the original one by decay of the runners, must we not say that they possess
separate individualities; and yet if we do this, are we not at a loss to
say when their separate individualities were established, unless we admit
that each bud was from the beginning an individual? Commenting on such
perplexities Schleiden says--"Much has been written and disputed concerning
the conception of the individual, without, however, elucidating the
subject, principally owing to the misconception that still exists as to the
origin of the conception. Now the individual is no conception, but the mere
subjective comprehension of an actual object, presented to us under some
given specific conception, and on this latter it alone depends whether the
object is or is not an individual. Under the specific conception of the
solar system, ours is an individual: in relation to the specific conception
of a planetary body, it is an aggregate of many individuals." ... "I think,
however, that looking at the indubitable facts already mentioned, and the
relations treated of in the course of these considerations, it will appear
most advantageous and most useful, in a scientific point of view, to
consider the vegetable cell as the general type of the plant (simple plant
of the first order). Under this conception, _Protococcus_ and other plants
consisting of only one cell, and the spore and pollen-granule, will appear
as individuals. Such individuals may, however, again, with a partial
renunciation of their individual independence, combine under definite laws
into definite forms (somewhat as the individual animals do in the globe of
the _Volvox globator_[25]). These again appear empirically as individual
beings, under a conception of a species (simple plants of the second order)
derived from the form of the normal connexion of the elementary
individuals. But we cannot stop here, since Nature herself combines these
individuals, under a definite form, into larger associations, whence we
draw the third conception of the plant, from a connexion, as it were, of
the second power (compound plants--plants of the third order). The simple
plant proceeding from the combination of the elementary individuals is then
termed a bud (_gemma_), in the composition of plants of the third order."
The animal kingdom presents still greater difficulties. When, from sundry
points on the body of a common polype, there bud out young polypes which,
after acquiring mouths and tentacles and closing up the communications
between their stomachs and the stomach of the parent, finally separate from
the parent; we may with propriety regard them as distinct individuals. But
when in the allied compound _Hydrozoa_, we find that these young polypes
continue permanently connected with the parent; and when by this continuous
budding-out there is presently produced a tree-like aggregation, having a
common alimentary canal into which the digestive cavity of each polype
opens; it is no longer so clear that these little sacs, furnished with
mouths and tentacles, are severally to be regarded as distinct individuals.
We cannot deny a certain individuality to the polypedom. And on discovering
that some of the buds, instead of unfolding in the same manner as the rest,
are transformed into capsules in which eggs are developed--on discovering
that certain of the incipient polypes thus become wholly dependent on the
aggregate for their nutrition, and discharge functions which have nothing
to do with their own maintenance, we have still clearer proof that the
individualities of the members are partially merged in the individuality of
the group. Other organisms belonging to the same order, display still more
decidedly this transition from simple individualities to a complex
individuality. In the _Diphyes_ there is a special modification of one or
more members of the polypedom into a swimming apparatus which, by its
rhythmical contractions, propels itself through the water, drawing the
polypedom after it. And in the more differentiated _Physalia_ various
organs result from the metamorphosis of parts which are the homologues of
individual polypes. In this last instance, the individuality of the
aggregate is so predominant that the individualities of its members are
practically lost. This combination of individualities in such way as to
produce a composite individual, meets us in other forms among the
ascidians. While in some of these, as in the _Clavelina_ and in the
_Botryllidæ_, the animals associated are but little subordinated to the
community they form, in others they are so combined as to form a compound
individual. The pelagic ascidian _Doliolum_ is an example. "Here we find a
large individual which swims by contractions of circular muscular bands,
carries a train of smaller individuals attached to a long dorsal process of
the test. These are arranged in three rows: those constituting the lateral
row have wide mouths and no sexual organs or organs of locomotion--they
subserve the nutrition of the colony, a truth which is illustrated by the
fact that as soon as they are properly developed the large individual (the
mother) loses her alimentary canal;" while from the median row are
eventually derived the sexual zoids.
On the hypothesis of Evolution, perplexities of this nature are just such
as we might anticipate. If Life in general commenced with minute and simple
forms, like those out of which all organisms, however complex, now
originate; and if the transitions from these primordial units to organisms
made up of groups of such units, and to higher organisms made up of groups
of such groups took place by degrees; it is clear that individualities of
the first and simplest order would merge gradually in those of a larger and
more complex order, and these again in others of an order having still
greater bulk and organization. Hence it would be impossible to say where
the lower individualities ceased and the higher individualities commenced.
§ 73. To meet these difficulties, it has been proposed that the whole
product of a single fertilized germ shall be regarded as a single
individual; whether such whole product be organized into one mass, or
whether it be organized into many masses that are partially or completely
separate. It is urged that whether the development of the fertilized germ
be continuous or discontinuous (§ 50) is a matter of secondary importance;
that the totality of living tissue to which the fertilized germ gives rise
in any one case, is the equivalent of the totality to which it gives rise
in any other case; and that we must recognize this equivalence, whether
such totality of living tissue takes a concrete or a discrete arrangement.
In pursuance of this view, a zoological individual is constituted either by
any such single animal as a mammal or bird, which may properly claim the
title of a _zoon_, or by any such group of animals as the numerous _Medusæ_
that have been developed from the same egg, which are to be severally
distinguished as _zooids_.
Admitting it to be very desirable that there should be words for expressing
these relations and this equivalence, it may be objected that to apply the
word individual to a number of separate living bodies, is inconvenient:
conflicting so much, as it does, with the ordinary conception which this
word suggests. It seems a questionable use of language to say that the
countless masses of _Anacharis Alsinastrum_ (now _Eloidea canadensis_)
which, within these few years, have grown up in our rivers, canals, and
ponds, are all parts of one individual: and yet as this plant does not seed
in England, these countless masses, having arisen by discontinuous
development, must be so regarded if we accept the above definition.
It may be contended, too, that while it does violence to our established
way of thinking, this mode of interpreting the facts is not without its
difficulties. Something seems to be gained by restricting the application
of the title individual, to organisms which, being in all respects fully
developed, possess the power of producing their kind after the ordinary
sexual method, and denying this title to those incomplete organisms which
have not this power. But the definition does not really establish this
distinction for us. On the one hand, we have cases in which, as in the
working bee, the whole of the germ-product is aggregated into a single
organism; and yet, though an individual according to the definition, this
organism has no power of reproducing its kind. On the other hand, we have
cases like that of the perfect _Aphis_, where the organism is but an
infinitesimal part of the germ product, and yet has that completeness
required for sexual reproduction. Further, it might be urged with some show
of reason, that if the conception of individuality involves the conception
of completeness, then, an organism which possesses an independent power of
reproducing itself, being more complete than an organism in which this
power is dependent on the aid of another organism, is more individual.
§ 74. There is, indeed, as already implied, no definition of individuality
that is unobjectionable. All we can do is to make the best practicable
compromise.
As applied either to an animate or an inanimate object, the word individual
ordinarily connotes union among the parts of the object and separateness
from other objects. This fundamental element in the conception of
individuality, we cannot with propriety ignore in the biological
application of the word. That which we call an individual plant or animal
must, therefore, be some concrete whole and not a discrete whole. If,
however, we say that each concrete living whole is to be regarded as an
individual, we are still met by the question--What constitutes a concrete
living whole? A young organism arising by internal or external gemmation
from a parent organism, passes gradually from a state in which it is an
indistinguishable part of the parent organism to a state in which it is a
separate organism of like structure with the parent. At what stage does it
become an individual? And if its individuality be conceded only when it
completely separates from the parent, must we deny individuality to all
organisms thus produced which permanently retain their connexions with
their parents? Or again, what must we say of the _Hectocotylus_, which is
an arm of the Cuttle-fish that undergoes a special development and then,
detaching itself, lives independently for a considerable period? And what
must we say of the larval nemertine worm the pilidium of which with its
nervous system is left to move about awhile after the developing worm has
dropped out of it?
To answer such questions we must revert to the definition of life. The
distinction between individual in its biological sense, and individual in
its more general sense, must consist in the manifestation of Life, properly
so called. Life we have seen to be, "the definite combination of
heterogeneous change, both simultaneous and successive, in correspondence
with external co-existences and sequences." Hence, a biological individual
is any concrete whole having a structure which enables it, when placed in
appropriate conditions, to continuously adjust its internal relations to
external relations, so as to maintain the equilibrium of its functions. In
pursuance of this conception, we must consider as individuals all those
wholly or partially independent organized masses which arise by
multicentral and multiaxial development that is either continuous or
discontinuous (§ 50). We must accord the title to each separate aphis, each
polype of a polypedom, each bud or shoot of a lowering plant, whether it
detaches itself as a bulbil or remains attached as a branch.
By thus interpreting the facts we do not, indeed, avoid all anomalies.
While, among flowering plants, the power of independent growth and
development is usually possessed only by shoots or axes; yet, in some
cases, as in that of the Begonia-leaf awhile since mentioned, the appendage
of an axis, or even a small fragment of such appendage, is capable of
initiating and carrying on the functions of life; and in other cases, as
shown by M. Naudin in the _Drosera intermedia_, young plants are
occasionally developed from the surfaces of leaves. Nor among forms like
the compound _Hydrozoa_, does the definition enable us to decide where the
line is to be drawn between the individuality of the group and the
individualities of the members: merging into each other, as these do, in
different degrees. But, as before said, such difficulties must necessarily
present themselves if organic forms have arisen by insensible gradations.
We must be content with a course which commits us to the smallest number of
incongruities; and this course is, to consider as an individual any
organized mass which is capable of independently carrying on that
continuous adjustment of inner to outer relations which constitutes Life.
CHAPTER VI^A.
CELL-LIFE AND CELL-MULTIPLICATION.
§ 74a. The progress of science is simultaneously towards simplification and
towards complication. Analysis simplifies its conceptions by resolving
phenomena into their factors, and by then showing how each simple mode of
action may be traced under multitudinous forms; while, at the same time,
synthesis shows how each factor, by cooperation with various other factors
in countless modes and degrees, produces different results innumerable in
their amounts and varieties. Of course this truth holds alike of processes
and of products. Observation and the grouping into classes make it clear
that through multitudinous things superficially unlike there run the same
cardinal traits of structure; while, along with these major unities,
examination discloses innumerable minor diversities.
A concomitant truth, or the same truth under another aspect, is that Nature
everywhere presents us with complexities within complexities, which go on
revealing themselves as we investigate smaller and smaller objects. In a
preceding chapter (§§ 54a, 54b) it was pointed out that each primitive
organism, in common with each of the units out of which the higher and
larger organisms are built, was found a generation ago to consist of
nucleus, protoplasm, and cell-wall. This general conception of a cell
remained for a time the outcome of inquiry; but with the advance of
microscopy it became manifest that within these minute structures processes
and products of an astonishing nature are to be seen. These we have now to
contemplate.
In the passages just referred to it was said that the external layer or
cell-wall is a non-essential, inanimate part produced by the animate
contents. Itself a product of protoplasmic action, it takes no part in
protoplasmic changes, and may therefore here be ignored.
§ 74b. One of the complexities within complexities was disclosed when it
was found that the protoplasm itself has a complicated structure. Different
observers have described it as constituted by a network or reticulum, a
sponge-work, a foam-work. Of these the first may be rejected; since it
implies a structure lying in one plane. If we accept the second we have to
conceive the threads of protoplasm, corresponding to the fibres of the
sponge, as leaving interstices filled either with liquid or solid. They
cannot be filled with a continuous solid, since all motion of the
protoplasm would be negatived; and that their content is not liquid seems
shown by the fact that its parts move about under the form of granules or
microsomes. But the conception of moving granules implies the conception of
immersion in a liquid or semi-liquid substance in which they move--not a
sponge-work of threads but a foam-work, consisting everywhere of septa
interposed among the granules. This is the hypothesis which sundry
microscopists espouse, and which seems mechanically the most feasible: the
only one which consists with the "streaming" of protoplasm. Ordinarily the
name protoplasm is applied to the aggregate mass--the semi-liquid, hyaline
substance and the granules or microsomes it contains.
What these granules or microsomes are--whether, as some have contended,
they are the essential living elements of the protoplasm, or whether, as is
otherwise held, they are nutritive particles, is at present undecided. But
the fact, alleged by sundry observers, that the microsomes often form rows,
held together by intervening substance, seems to imply that these minute
bodies are not inert. Leaving aside unsettled questions, however, one fact
of significance is manifest--an immense multiplication of surfaces over
which inter-action may take place. Anyone who drops into dilute sulphuric
acid a small nail and then drops a pinch of iron filings, will be shown, by
the rapid disappearance of the last and the long continuance of the first,
how greatly the increasing of surfaces by multiplication of fragments
facilitates change. The effect of subdivision in producing a large area in
a small space, is shown in the lungs, where the air-cells on the sides of
which the blood-vessels ramify, are less than 1/100th of an inch in
diameter, while they number 700,000,000. In the composition of every tissue
we see the same principle. The living part, or protoplasm, is divided into
innumerable protoplasts, among which are distributed the materials and
agencies producing changes. And now we find this principle carried still
deeper in the structure of the protoplasm itself. Each microscopic portion
of it is minutely divided in such ways that its threads or septa have
multitudinous contacts with those included portions of matter which take
part in its activities.
Concerning the protoplasm contained in each cell, named by some cytoplasm,
it remains to say that it always includes a small body called the
centrosome, which appears to have a directive function. Usually the
centrosome lies outside the nucleus, but is alleged to be sometimes within
it. During what is called the "resting stage," or what might more properly
be called the growing stage (for clearly the occasional divisions imply
that in the intervals between them there has been increase) the centrosome
remains quiescent, save in the respect that it exercises some coercive
influence on the protoplasm around. This results in the radially-arranged
lines constituting an "aster." What is the nature of the coercion exercised
by the centrosome--a body hardly distinguishable in size from the
microsomes or granules of protoplasm around--is not known. It can scarcely
be a repelling force; since, in a substance of liquid or semi-liquid kind,
this could not produce approximately straight lines. That it is an
attractive force seems more probable; and the nature of the attraction
would be comprehensible did the centrosome augment in bulk with rapidity.
For if integration were in progress, the drawing in of materials might well
produce converging lines. But this seems scarcely a tenable interpretation;
since, during the so-called "resting stage," this star-like structure
exists--exists, that is, while no active growth of the centrosome is going
on.
Respecting this small body we have further to note that, like the cell as a
whole, it multiplies by fission, and that the bisection of it terminates
the resting or growing stage and initiates those complicated processes by
which two cells are produced out of one: the first step following the
fission being the movement of the halves, with their respective completed
asters, to the opposite sides of the nucleus.
§ 74c. With the hypothesis, now general, that the nucleus or kernel of a
cell is its essential part, there has not unnaturally grown up the dogma
that it is always present; but there is reason to think that the evidence
is somewhat strained to justify this dogma.
In the first place, beyond the cases in which the nucleus, though
ordinarily invisible, is said to have been rendered visible by a re-agent,
there are cases, as in the already-named _Archerina_, where no re-agent
makes one visible. In the second place, there is the admitted fact that
some nuclei are diffused; as in _Trachelocerca_ and some other Infusoria.
In them the numerous scattered granules are supposed to constitute a
nucleus: an interpretation obviously biassed by the desire to save the
generalization. In the third place, the nucleus is frequently multiple in
cells of low types; as in some families of Algæ and predominantly among
Fungi. Once more, the so-called nucleus is occasionally a branching
structure scarcely to be called a "kernel."
The facts as thus grouped suggest that the nucleus has arisen in conformity
with the law of evolution--that the primitive protoplast, though not
homogeneous in the full sense, was homogeneous in the sense of being a
uniformly granular protoplasm; and that the protoplasts with diffused
nuclei, together with those which are multi-nucleate, and those which have
nuclei of a branching form, represent stages in that process by which the
relatively homogeneous protoplast passed into the relatively heterogeneous
one now almost universal.
Concerning the structure and composition of the developed nucleus, the
primary fact to be named is that, like the surrounding granular cytoplasm,
it is formed of two distinct elements. It has a groundwork or matrix not
differing much from that of the cytoplasm, and at some periods continuous
with it; and immersed in this it has a special matter named chromatin,
distinguished from its matrix by becoming dyed more or less deeply when
exposed to fit re-agents. During the "resting stage," or period of growth
and activity which comes between periods of division, the chromatin is
dispersed throughout the ground-substance, either in discrete portions or
in such way as to form an irregular network or sponge-work, various in
appearance. When the time for fission is approaching this dispersed
chromatin begins to gather itself together: reaching its eventual
concentration through several stages. By its concentration are produced the
chromosomes, constant in number in each species of plant or animal. It is
alleged that the substance of the chromosomes is not continuous, but
consists of separate elements or granules, which have been named
chromomeres; and it is also alleged that, whether in the dispersed or
integrated form, each chromosome retains its individuality--that the
chromomeres composing it, now spreading out into a network and now uniting
into a worm-like body, form a group which never loses its identity. Be this
as it may, however, the essential fact is that during the growth-period the
chromatin substance is widely distributed, and concentration of it is one
of the chief steps towards a division of the nucleus and presently of the
cell.
During this process of mitosis or karyokinesis, the dispersed chromatin
having passed through the coil-stage, reaches presently the star-stage, in
which the chromosomes are arranged symmetrically about the equatorial plane
of the nucleus. Meanwhile in each of them there has been a preparation for
splitting longitudinally in such way that the halves when separated contain
(or are assumed to contain) equal numbers of the granules or chromomeres,
which some think are the ultimate morphological units of the chromosomes. A
simultaneous change has occurred: there has been in course of formation a
structure known as the _amphiaster_. The two centrosomes which, as before
said, place themselves on opposite sides of the nucleus, become the
terminal poles of a spindle-shaped arrangement of fibres, arising mainly
from the groundwork of the nucleus, now continuous with the groundwork of
the cytoplasm. A conception of this structure may be formed by supposing
that the radiating fibres of the respective asters, meeting one another and
uniting in the intermediate space, thereafter exercise a tractive force;
since it is clear that, while the central fibres of the bundle will form
straight lines, the outer ones, pulling against one another not in straight
lines, will form curved lines, becoming more pronounced in their curvatures
as the distance from the axis increases. That a tractive force is at work
seems inferable from the results. For the separated halves of the split
chromosomes, which now form clusters on the two sides of the equatorial
plane, gradually part company, and are apparently drawn as clusters towards
the opposing centrosomes. As this change progresses the original nucleus
loses its individuality. The new chromosomes, halves of the previous
chromosomes, concentrate to found two new nuclei; and, by something like a
reversal of the stages above described, the chromatin becomes dispersed
throughout the substance of each new nucleus. While this is going on the
cell itself, undergoing constriction round its equator, divides into two.
Many parts of this complex process are still imperfectly understood, and
various opinions concerning them are current. But the essential facts are
that this peculiar substance, the chromatin, at other times existing
dispersed, is, when division is approaching, gathered together and dealt
with in such manner as apparently to insure equal quantities being
bequeathed by the mother-cell to the two daughter-cells.
§ 74d. What is the physiological interpretation of these structures and
changes? What function does the nucleus discharge; and, more especially,
what is the function discharged by the chromatin? There have been to these
questions sundry speculative answers.
The theory espoused by some, that the nucleus is the regulative organ of
the cell, is met by difficulties. One of them is that, as pointed out in
the chapter on "Structure," the nucleus, though morphologically central, is
not central geometrically considered; and that its position, often near to
some parts of the periphery and remote from others, almost of itself
negatives the conclusion that its function is directive in the ordinary
sense of the word. It could not well control the cytoplasm in the same ways
in all directions and at different distances. A further difficulty is that
the cytoplasm when deprived of its nucleus can perform for some time
various of its actions, though it eventually dies without reproducing
itself.
For the hypothesis that the nucleus is a vehicle for transmitting
hereditary characters, the evidence seems strong. When it was shown that
the head of a spermatozoon is simply a detached nucleus, and that its
fusion with the nucleus of an ovum is the essential process initiating the
development of a new organism, the legitimate inference appeared to be that
these two nuclei convey respectively the paternal and maternal traits which
are mingled in the offspring. And when there came to be discerned the
karyokinesis by which the chromatin is, during cell-fission, exactly halved
between the nuclei of the daughter-cells, the conclusion was drawn that the
chromatin is more especially the agent of inheritance. But though, taken by
themselves, the phenomena of fertilization seem to warrant this inference,
the inference does not seem congruous with the phenomena of ordinary
cell-multiplication--phenomena which have nothing to do with fertilization
and the transmission of hereditary characters. No explanation is yielded of
the fact that ordinary cell-multiplication exhibits an elaborate process
for exact halving of the chromatin. Why should this substance be so
carefully portioned out among the cells of tissues which are not even
remotely concerned with propagation of the species? If it be said that the
end achieved is the conveyance of paternal and maternal qualities in equal
degrees to every tissue; then the reply is that they do not seem to be
conveyed in equal degrees. In the offspring there is not a uniform
diffusion of the two sets of traits throughout all parts, but an irregular
mixture of traits of the one with traits of the other.
In presence of these two suggested hypotheses and these respective
difficulties, may we not suspect that the action of the chromatin is one
which in a way fulfils both functions? Let us consider what action may do
this.
§ 74e. The chemical composition of chromatin is highly complex, and its
complexity, apart from other traits, implies relative instability. This is
further implied by the special natures of its components. Various analyses
have shown that it consists of an organic acid (which has been called
nucleic acid) rich in phosphorus, combined with an albuminous substance:
probably a combination of various proteids. And the evidence, as summarised
by Wilson, seems to show that where the proportion of phosphorized acid is
high the activity of the substance is great, as in the heads of
spermatozoa; while, conversely, where the quantity of phosphorus is
relatively small, the substance approximates in character to the cytoplasm.
Now (like sulphur, present in the albuminoid base), phosphorus is an
element which, besides having several allotropic forms, has a great
affinity for oxygen; and an organic compound into which it enters, beyond
the instability otherwise caused, has a special instability caused by its
presence. The tendency to undergo change will therefore be great when the
proportion of the phosphorized component is great. Hence the statement that
"the chemical differences between chromatin and cytoplasm, striking and
constant as they are, are differences of degree only;" and the conclusion
that the activity of the chromatin is specially associated with the
phosphorus.[26]
What, now, are the implications? Molecular agitation results from
decomposition of each phosphorized molecule: shocks are continually
propagated around. From the chromatin, units of which are thus ever falling
into stabler states, there are ever being diffused waves of molecular
motion, setting up molecular changes in the cytoplasm. The chromatin stands
towards the other contents of the cell in the same relation that a
nerve-element stands to any element of an organism which it excites: an
interpretation congruous with the fact that the chromatin is as near to as,
and indeed nearer than, a nerve-ending to any minute structure stimulated
by it.
Several confirmatory facts may be named. During the intervals between
cell-fissions, when growth and the usual cell-activities are being carried
on, the chromatin is dispersed throughout the nucleus into an irregular
network: thus greatly increasing the surface of contact between its
substance and the substances in which it is imbedded. As has been remarked,
this wide distribution furthers metabolism--a metabolism which in this case
has, as we infer, the function of generating, not special matters but
special motions. Moreover, just as the wave of disturbance a nerve carries
produces an effect which is determined, not by anything which is peculiar
in itself, but by the peculiar nature of the organ to which it is
carried--muscular, glandular or other; so here, the waves diffused from the
chromatin do not determine the kinds of changes in the cytoplasm, but
simply excite it: its particular activities, whether of movement,
absorption, or structural excretion, being determined by its constitution.
And then, further, we observe a parallelism between the metabolic changes
in the two cases; for, on the one hand, "diminished staining capacity of
the chromatin [implying a decreased amount of phosphorus, which gives the
staining capacity] occurs during a period of intense constructive activity
in the cytoplasm;" and, on the other hand, in high organisms having nervous
systems, the intensity of nervous action is measured by the excretion of
phosphates--by the using up of the phosphorus contained in nerve-cells.
For thus interpreting the respective functions of chromatin and cytoplasm,
yet a further reason may be given. One of the earliest general steps in the
evolution of the _Metazoa_, is the differentiation of parts which act from
parts which make them act. The _Hydrozoa_ show us this. In the hydroid
stage there are no specialized contractile organs: these are but incipient:
individual ectoderm cells have muscular processes. Nor is there any
"special aggregation of nerve-cells." If any stimulating units exist they
are scattered. But in the _Medusa_-stage nerve-matter is collected into a
ring round the edge of the umbrella. That is to say, in the undeveloped
form such motor action as occurs is not effected by a specialized part
which excites another part; but in the developed form a differentiation of
the two has taken place. All higher types exhibit this differentiation. Be
it muscle or gland or other operating organ, the cause of its activity lies
not in itself but in a nervous agent, local or central, with which it is
connected. Hence, then, there is congruity between the above interpretation
and certain general truths displayed by animal organization at large. We
may infer that in a way parallel to that just indicated, cell-evolution
was, under one of its aspects, a change from a stage in which the exciting
substance and the substance excited were mingled with approximate
uniformity, to a stage in which the exciting substance was gathered
together into the nucleus and finally into the chromosomes: leaving behind
the substance excited, now distinguished as cytoplasm.
§ 74f. Some further general aspects of the phenomena appear to be in
harmony with this interpretation. Let us glance at them.
In Chapters III and IIIA of the First Part, reasons were given for
concluding that in the animal organism nitrogenous substances play the part
of decomposing agents to the carbo-hydrates--that the molecular disturbance
set up by the collapse of a proteid molecule destroys the equilibrium of
sundry adjacent carbo-hydrate molecules, and causes that evolution of
energy which accompanies their fall into molecules of simpler compounds.
Here, if the foregoing argument is valid, we may conclude that this highly
complex phosphorized compound which chromatin contains, plays the same part
to the adjacent nitrogenous compounds as these play to the carbo-hydrates.
If so, we see arising a stage earlier that "general physiological method"
illustrated in § 23f. It was there pointed out that in animal organisms the
various structures are so arranged that evolution of a small amount of
energy in one, sets up evolution of a larger amount of energy in another;
and often this multiplied energy undergoes a second multiplication of like
kind. If this view is tenable, we may now suspect that this method
displayed in the structures of the _Metazoa_ was initiated in the
structures of the _Protozoa_, and consequently characterizes those
homologues of them which compose the _Metazoa_.
When contemplated from the suggested point of view, karyokinesis appears to
be not wholly incomprehensible. For if the chromatin yields the energy
which initiates changes throughout the rest of the cell, we may see why
there eventually arises a process for exact halving of the chromatin in a
mother-cell between two daughter-cells. To make clear the reason, let us
suppose the portioning out of the chromatin leaves one of the two with a
sensibly smaller amount than the other. What must result? Its source of
activity being relatively less, its rate of growth and its energy of action
will be less. If a protozoon, the weaker progeny arising by division of it
will originate an inferior stirp, unable to compete successfully with that
arising from the sister-cell endowed with a larger portion of chromatin. By
continual elimination of the varieties which produce unequal halving,
necessarily at a disadvantage if a moiety of their members tend continually
to disappear, there will be established a variety in which the halving is
exact: the character of this variety being such that all its members aid
the permanent multiplication of the species. If, again, the case is that of
a metazoon, there will be the same eventual result. An animal or plant in
which the chromatin is unequally divided among the cells, must have tissues
of uncertain formation. Assume that an organ has, by survival of the
fittest, been adjusted in the proportions and qualities of its parts to a
given function. If the multiplying protoplasts, instead of taking equal
portions of chromatin, have some of them smaller portions, the parts of the
organ formed of these, developing less rapidly and having inferior
energies, will throw the organ out of adjustment, and the individual will
suffer in the struggle for life. That is to say, irregular division of the
chromatin will introduce a deranging factor and natural selection will weed
out individuals in which it occurs. Of course no interpretation is thus
yielded of the special process known as karyokinesis. Probably other modes
of equal division might have arisen. Here the argument implies merely that
the tendency of evolution is to establish _some_ mode. In verification of
the view that equal division arises from the cause named, it is pointed out
to me that amitosis, which is a negation of mitosis or karyokinesis, occurs
in transitory tissues or diseased tissues or where degeneracy is going on.
But how does all this consist with the conclusion that the chromatin
conveys hereditary traits--that it is the vehicle in which the
constitutional structure, primarily of the species and secondarily of
recent ancestors and parents, is represented? To this question there seems
to be no definite answer. We may say only that this second function is not
necessarily in conflict with the first. While the unstable units of
chromatin, ever undergoing changes, diffuse energy around, they may also be
units which, under the conditions furnished by fertilization, gravitate
towards the organization of the species. Possibly it may be that the
complex combination of proteids, common to chromatin and cytoplasm, is that
part in which the constitutional characters inhere; while the phosphorized
component, falling from its unstable union and decomposing, evolves the
energy which, ordinarily the cause of changes, now excites the more active
changes following fertilization. This suggestion harmonizes with the fact
that the fertilizing substance which in animals constitutes the head of the
spermatozoon, and in plants that of the spermatozoid or antherozoid, is
distinguished from the other agents concerned by having the highest
proportion of the phosphorized element; and it also harmonizes with the
fact that the extremely active changes set up by fertilization are
accompanied by decrease of this phosphorized element. Speculation aside,
however, we may say that the two functions of the chromatin do not exclude
one another, but that the general activity which originates from it may be
but a lower phase of that special activity caused by fertilization.[27]
§ 74g. Here we come unawares to the remaining topic embraced under the
title Cell-Life and Cell-Multiplication. We pass naturally from asexual
multiplication of cells to sexual multiplication--from cell-reproduction to
cell-generation. The phenomena are so numerous and so varied that a large
part of them must be passed over. Conjugation among the _Protophyta_ and
_Protozoa_, beginning with cases in which there is a mingling of the
contents of two cells in no visible respect different from one another, and
developing into a great variety of processes in which they differ, must be
left aside, and attention limited to the terminal process of fertilization
as displayed in higher types of organisms.
Before fertilization there occurs in the ovum an incidental process of a
strange kind--"strange" because it is a collateral change taking no part in
subsequent changes. I refer to the production and extrusion of the "polar
bodies." It is recognized that the formation of each is analogous to
cell-formation in general; though process and product are both dwarfed.
Apart from any ascribed meaning, the fact itself is clear. There is an
abortive cell-formation. Abortiveness is seen firstly in the diminutive
size of the separated body or cell, and secondly in the deficient number of
its chromosomes: a corresponding deficiency being displayed in the group of
chromosomes remaining in the egg--remaining, that is (on the hypothesis
here to be suggested), in the sister-cell, supposing the polar body to be
an aborted cell. It is currently assumed that the end to be achieved by
thus extruding part of the chromosomes, is to reduce the remainder to half
the number characterizing the species; so that when, to this group in the
germ-cell, the sperm-cell brings a similarly-reduced group, union of the
two shall bring the chromosomes to the normal number. I venture to suggest
another interpretation. In doing this, however, I must forestall a
conclusion contained in the next chapter; namely, the conclusion that
gamogenesis begins when agamogenesis is being arrested by unfavourable
conditions, and that the failing agamogenesis initiates the gamogenesis. Of
numerous illustrations to be presently given I will, to make clear the
conception, name only one--the formation of fructifying organs in plants at
times when, and in places where, shoots are falling off in vigour and
leaves in size. Here the successive foliar organs, decreasingly fitted
alike in quality and dimensions for carrying on their normal lives, show us
an approaching cessation of asexual multiplication, ending in the aborted
individuals we call stamens; and the fact that sudden increase of nutrition
while gamogenesis is being thus initiated, causes resumption of
agamogenesis, shows that the gamogenesis is consequent upon the failing
agamogenesis. See then the parallel. On going back from multicellular
organisms to unicellular organisms (or those homologues of them which form
the reproductive agents in multicellular organisms), we find the same law
hold. The polar bodies are aborted cells, indicating that asexual
multiplication can no longer go on, and that the conditions leading to
sexual multiplication have arisen. If this be so, decrease in the chromatin
becomes an initial cause of the change instead of an accompanying incident;
and we need no longer assume that a quantity of precious matter is lost,
not by passive incapacity, but by active expulsion. Another anomaly
disappears. If from the germ-cell there takes place this extrusion of
superfluous chromatin, the implication would seem to be that a parallel
extrusion takes place from the sperm-cell. But this is not true. In the
sperm-cell there occurs just that failure in the production of chromatin
which, according to the hypothesis above sketched out, is to be expected;
for, in the process of cell-multiplication, the cells which become
spermatozoa are _left_ with half the number of chromosomes possessed by
preceding cells: there is actually that impoverishment and declining vigour
here suggested as the antecedent of fertilization. It needs only to imagine
the ovum and the polar body to be alike in size, to see the parallelism;
and to see that obscuration of it arises from the accumulation of cytoplasm
in the ovum.
A test fact remains. Sometimes the first polar body extruded undergoes
fission while the second is being formed. This can have nothing to do with
reducing the number of chromosomes in the ovum. Unquestionably, however,
this change is included with the preceding changes in one transaction,
effected by one influence. If, then, it is irrelevant to the decrease of
chromosomes, so must the preceding changes be irrelevant: the hypothesis
lapses. Contrariwise this fact supports the view suggested above. That
extrusion of a polar body is a process of cell-fission is congruous with
the fact that another fission occurs after extrusion. And that this occurs
irregularly shows that the vital activities, seen in cell-growth and
cell-multiplication, now succeed in producing further fission of the
dwarfed cell and now fail: the energies causing asexual multiplication are
exhausted and there arises the state which initiates sexual multiplication.
Maturation of the ovum having been completed, entrance of the spermatozoon,
sometimes through the limiting membrane and sometimes through a micropyle
or opening in it, takes place. This instantly initiates a series of
complicated changes: not many seconds passing before there begins the
formation of an aster around one end of the spermatozoon-head. The growth
of this aster, apparently by linear rangings of the granules composing the
reticulum of the germ-cell, progresses rapidly; while the whole structure
hence arising moves inward. Soon there takes place the fusion of this
sperm-nucleus with the germ-nucleus to form the cleavage-nucleus, which,
after a pause, begins to divide and subdivide in the same manner as cells
at large: so presently forming a cluster of cells out of which arise the
layers originating the embyro. The details of this process do not concern
us. It suffices to indicate thus briefly its general nature.
And now ending thus the account of genesis under its histological aspect,
we pass to the account of genesis under its wider and more significant
aspects.
CHAPTER VII.
GENESIS.
§ 75. Having, in the last chapter but one, concluded what constitutes an
individual, and having, in the last chapter, contemplated the histological
process which initiates a new individual, we are in a position to deal with
the multiplication of individuals. For this, the title Genesis is here
chosen as being the most comprehensive title--the least specialized in its
meaning. By some biologists Generation has been used to signify one method
of multiplication, and Reproduction to signify another method; and each of
these words has been thus rendered in some degree unfit to signify
multiplication in general.
Here the reader is indirectly introduced to the fact that the production of
new organisms is carried on in fundamentally unlike ways. Up to quite
recent times it was believed, even by naturalists, that all the various
processes of multiplication observable in different kinds of organisms,
have one essential character in common: it was supposed that in every
species the successive generations are alike. It has now been proved,
however, that in many plants and in numerous animals, the successive
generations are not alike; that from one generation there proceeds another
whose members differ more or less in structure from their parents; that
these produce others like themselves, or like their parents, or like
neither; but that eventually, the original form re-appears. Instead of
there being, as in the cases most familiar to us, a constant recurrence of
the same form, there is a cyclical recurrence of the same form. These two
distinct processes of multiplication, may be aptly termed _homogenesis_ and
_heterogenesis_.[28] Under these heads let us consider them.
There are two kinds of homogenesis, the simplest of them, probably once
universal but now exceptional, being that in which there is no other form
of multiplication than one resulting from perpetual spontaneous fission.
The rise of distinct sexes was doubtless a step in evolution, and before it
took place the formation of new individuals could have arisen only by
division of the old, either into two or into many. At present this process
survives, so far as appears, among _Bacteria_, certain _Algæ_, and sundry
_Protozoa_; though it is possible that a rarely-occurring conjugation has
in these cases not yet been observed. It is a probable conclusion, however,
that in the _Bacteria_ at any rate, the once universal mode of
multiplication still survives as an exceptional mode. But now passing over
these cases, we have to note that the kind of genesis (once supposed to be
the sole kind), in which the successive generations are alike, is sexual
genesis, or, as it has been otherwise called--_gamogenesis_. In every
species which multiplies by this kind of homogenesis, each generation
consists of males and females; and from the fertilized germs they produce
the next generation of similar males and females arises: the only needful
qualification of this statement being that in many _Protophyta_ and
_Protozoa_ the conjugating cells or protoplasts are not distinguishable in
character. This mode of propagation has the further trait, that each
fertilized germ usually gives rise to but one individual--the product of
development is organized round one axis and not round several axes,
Homogenesis in contrast with heterogenesis as exhibited in species which
display distinct sexuality, has also the characteristic that each new
individual begins as an egg detached from the maternal tissues, instead of
being a portion of protoplasm continuous with them, and that its
development proceeds independently. This development may be carried on
either internally or externally; whence results the division into the
oviparous and the viviparous. The oviparous kind is that in which the
fertilized germ is extruded from the parent before it has undergone any
considerable development. The viviparous kind is that in which development
is considerably advanced, or almost completed, before extrusion takes
place. This distinction is, however, not a sharply-defined one: there are
transitions between the oviparous and the viviparous processes. In
ovo-viviparous genesis there is an internal incubation; and though the
young are in this case finally extruded from the parent in the shape of
eggs, they do not leave the parent's body until after they have assumed
something like the parental form. Looking around, we find that homogenesis
is universal among the _Vertebrata_. Every vertebrate animal arises from a
fertilized germ, and unites into its single individuality the whole product
of this fertilized germ. In the mammals or highest _Vertebrata_, this
homogenesis is in every case viviparous; in birds it is uniformly
oviparous; and in reptiles and fishes it is always essentially oviparous,
though there are cases of the kind above referred to, in which viviparity
is simulated. Passing to the _Invertebrata_, we find oviparous homogenesis
universal among the _Arachnida_ (except the Scorpions, which are
ovo-viviparous); universal among the higher _Crustacea_, but not among the
lower; extremely general, though not universal, among Insects; and
universal among the higher _Mollusca_ though not among the lower. Along
with extreme inferiority among animals, we find homogenesis to be the
exception rather than the rule; and in the vegetal kingdom there appear to
be no cases, except among the _Algæ_ and a few aberrant parasites like the
_Rafflesiaceæ_, in which the centre or axis which arises from a fertilized
germ becomes the immediate producer of fertilized germs.
In propagation characterized by unlikeness of the successive generations,
there is asexual genesis with occasionally-recurring sexual genesis; in
other words--_agamogenesis_ interrupted more or less frequently by
_gamogenesis_. If we set out with a generation of perfect males and
females, then, from their ova arise individuals which are neither males nor
females, but which produce the next generation from buds. By this method of
multiplication many individuals originate from a single fertilized germ.
The product of development is organized round more than one centre or axis.
The simplest form of heterogenesis is that seen in most uniaxial plants.
If, as we find ourselves obliged to do, we regard each separate shoot or
axis of growth as a distinct individual, homogenesis is seen in those which
have absolutely terminal flowers; but in all other uniaxial plants, the
successive individuals are not represented by the series A, A, A, A, &c.,
but they are represented by the series A, B, A, B, A, B, &c. For in the
majority of plants which were classed as uniaxial (§ 50), and which may be
conveniently so distinguished from other plants, the axis which shoots up
from the seed, and substantially constitutes the plant, does not itself
flower but gives lateral origin to flowering axes. Though in ordinary
uniaxial plants the fructifying apparatus _appears_ to be at the end of the
primary, vertical axis; yet dissection shows that, morphologically
considered, each fructifying axis is an offspring from the primary axis.
There arises from the seed a sexless individual, from which spring by
gemmation individuals having reproductive organs; and from these there
result fertilized germs or seeds that give rise to sexless individuals.
That is to say, gamogenesis and agamogenesis alternate: the peculiarity
being that the sexual individuals arise from the sexless ones by continuous
development. The _Salpæ_ show us an allied form of heterogenesis in the
animal kingdom. Individuals developed from fertilized ova, instead of
themselves producing fertilized ova, produce, by gemmation, strings of
individuals from which fertilized ova again originate. In multiaxial
plants, we have a succession of generations represented by the series A, B,
B, B, &c., A, B, B, B, &c. Supposing A to be a flowering axis or sexual
individual, then, from any fertilized germ it casts off, there grows up a
sexless individual, B; from this there bud-out other sexless individuals,
B, and so on for generations more or less numerous, until at length, from
some of these sexless individuals, there bud-out seed-bearing individuals
of the original form A. Branched herbs, shrubs, and trees, exhibit this
form of heterogenesis: the successive generations of sexless individuals
thus produced being, in most cases, continuously developed, or aggregated
into a compound individual, but being in some cases discontinuously
developed. Among animals a kind of heterogenesis represented by the same
succession of letters, occurs in such compound polypes as the _Sertularia_,
and in those of the _Hydrozoa_ which assume alternately the polypoid form
and the form of the _Medusa_. The chief differences presented by these
groups arise from the fact that the successive generations of sexless
individuals produced by budding, are in some cases continuously developed,
and in others discontinuously developed; and from the fact that, in some
cases, the sexual individuals give off their fertilized germs while still
growing on the parent-polypedom, but in other cases not until after leaving
the parent-polypedom and undergoing further development. Where, as in all
the foregoing kinds of agamogenesis, the new individuals bud out, not from
any specialized reproductive organs but from unspecialized parts of the
parent, the process has been named, by Prof. Owen, _metagenesis_. In most
instances the individuals thus produced grow from the outsides of the
parents--the metagenesis is external. But there is also a kind of
metagenesis which we may distinguish as internal. Certain _entozoa_ of the
genus _Distoma_ exhibit it. From the egg of a _Distoma_ there results a
rudely-formed creature known as a sporocyst and from this a redia.
Gradually, as this divides and buds, the greater part of the inner
substance is transformed into young animals called _Cercariæ_ (which are
the larvæ of _Distomata_); until at length it becomes little more than a
living sac full of living offspring. In the _Distoma pacifica_, the brood
of young animals thus arising by internal gemmation are not _Cercariæ_, but
are like their parent: themselves becoming the producers of _Cercariæ_,
after the same manner, at a subsequent period. So that now the succession
of forms is represented by the series A, B, A, B, &c., now by the series A,
B, B, A, B, B, &c., and now by A, B, B, C, A. Both cases, however,
exemplify internal metagenesis in contrast with the several kinds of
external metagenesis described above. That agamogenesis which is carried on
in a reproductive organ--either an ovarium or the homologue of one--has
been called, by Prof. Owen, _parthenogenesis_. It is the process familiarly
exemplified in the _Aphides_. Here, from the fertilized eggs laid by
perfect females there grow up imperfect females, in the ovaria of which are
developed ova that though unfertilized, rapidly assume the organization of
other imperfect females, and are born viviparously. From this second
generation of imperfect females, there by-and-by arises, in the same
manner, a third generation of the same kind; and so on for many
generations: the series being thus symbolized by the letters A, B, B, B, B,
B, &c., A. Respecting this kind of heterogenesis it should be added that,
in animals as in plants, the number of generations of sexless individuals
produced before the re-appearance of sexual ones, is indefinite; both in
the sense that in the same species it may go on to a greater or less extent
according to circumstances, and in the sense that among the generations of
individuals proceeding from the same fertilized germ, a recurrence of
sexual individuals takes place earlier in some of the diverging lines of
multiplication than in others. In trees we see that on some branches
flower-bearing axes arise while other branches are still producing only
leaf-bearing axes; and in the successive generations of _Aphides_ a
parallel fact has been observed. Lastly has to be set down that kind of
heterogenesis in which, along with gamogenesis, there occurs a form of
agamogenesis exactly like it, save in the absence of fecundation. This is
called true parthenogenesis--reproduction carried on by virgin mothers
which are in all respects like other mothers. Among silk-worm-moths this
parthenogenesis is exceptional rather than ordinary. Usually the eggs of
these insects are fertilized; but if they are not they are still laid, and
some of them produce larvæ. In certain _Lepidoptera_, however, of the
groups _Psychidæ_ and _Tineidæ_, parthenogenesis appears to be a normal
process--indeed, so far as is known, the only process; for of some species
the males have never been found.
A general conception of the relations among the different modes of Genesis,
thus briefly described, will be best given by the following tabular
statement.
GENESIS is
{ Oviparous
{ or
Homogenesis, which is usually Gamogenesis { Ovo-viviparous
{ or
{ Viviparous
or
{ Gamogenesis
{ alternating
Heterogenesis, which is { with { Parthenogenesis
{ Agamogenesis { or { Internal
{ Metagenesis { or
{ External
This, like all other classifications of such phenomena, presents anomalies.
It may be justly objected that the processes here grouped under the head
agamogenesis, are the same as those before grouped under the head of
discontinuous development (§ 50): thus making development and genesis
partially coincident. Doubtless it seems awkward that what are from one
point of view considered as structural changes are from another point of
view considered as modes of multiplication.[29] There is, however, nothing
for us but a choice of imperfections. We cannot by any logical dichotomies
accurately express relations which, in Nature, graduate into one another
insensibly. Neither the above, nor any other scheme, can do more than give
an approximate idea of the truth.
§ 76. Genesis under every form is a process of negative or positive
disintegration; and is thus essentially opposed to that process of
integration which is the primary process in individual evolution. Negative
disintegration occurs in those cases where, as among the compound
_Hydrozoa_, there is a continuous development of new individuals by budding
from the bodies of older individuals; and where the older individuals are
thus prevented from growing to a greater size, or reaching a higher degree
of integration. Positive disintegration occurs in those forms of
agamogenesis where the production of new individuals is discontinuous, as
well as in all cases of gamogenesis. The degrees of disintegration are
various. At the one extreme the parent organism is completely broken up, or
dissolved into new individuals; and at the other extreme each new
individual forms but a small deduction from the parent organism. _Protozoa_
and _Protophyta_ show us that form of disintegration called spontaneous
fission: two or more individuals being produced by the splitting-up of the
original one. The _Volvox_ and the _Hydrodictyon_ are plants which, having
developed broods within themselves, give them exit by bursting; and among
animals the one lately referred to which arises from the _Distoma_ egg,
entirely loses its individuality in the individualities of the numerous
_Distoma_-larvæ with which it becomes filled. Speaking generally, the
degree of disintegration becomes less marked as we approach the higher
organic forms. Plants of superior types throw off from themselves, whether
by gamogenesis or agamogenesis, parts that are relatively small; and among
superior animals there is no case in which the parent individuality is
habitually lost in the production of new individuals. To the last, however,
there is of necessity a greater or less disintegration. The seeds and
pollen-grains of a flowering plant are disintegrated portions of tissue; as
are also the ova and spermatozoa of animals. And whether the fertilized
germs carry away from their parents small or large quantities of nutriment,
these quantities in all cases involve further negative or positive
disintegrations of the parents.
Except in spore-producing plants, new individuals which result from
agamogenesis usually do not separate from the parent-individuals until they
have undergone considerable development, if not complete development. The
agamogenetic offspring of those lowest organisms which develop centrally,
do not, of course, pass beyond central structure; but the agamogenetic
offspring of organisms which develop axially, commonly assume an axial
structure before they become independent. The vegetal kingdom shows us this
in the advanced organization of detached bulbils, and of buds that root
themselves before separating. Of animals, the _Hydrozoa_, the _Trematoda_,
and the _Salpæ_, present us with different kinds of agamogenesis, in all of
which the new individuals are organized to a considerable extent before
being cast off. This rule is not without exceptions, however. The
statoblasts of the _Plumatella_ (which play the part of winter eggs),
developed in an unspecialized part of the body, furnish a case of
metagenesis in which centres of development, instead of axes, are detached;
and in the above-described parthenogenesis of moths and bees, such centres
are detached from an ovarium.
When produced by gamogenesis, the new individuals become (in a
morphological sense) independent of the parents while still in the shape of
centres of development, rather than axes of development; and this even
where the reverse is apparently the case. The fertilized germs of those
inferior plants which are central, or multicentral, in their development,
are of course thrown off as centres; and the same is usually the case even
in those which are uniaxial or multiaxial. In the higher plants, of the two
elements that go to the formation of the fertilized germ, the pollen-cell
is absolutely separated from the parent-plant under the shape of a centre,
and the egg-cell, though not absolutely separated from the parent, is still
no longer subordinate to the organizing forces of the parent. So that when,
after the egg-cell has been fertilized by matter from the pollen-tube, the
development commences, it proceeds without parental control: the new
individual, though remaining physically united with the old individual,
becomes structurally and functionally separate: the old individual doing no
more than supply materials. Throughout the animal kingdom, the new
individuals produced by gamogenesis are obviously separated in the shape of
centres of development wherever the reproduction is oviparous: the only
conspicuous variation being in the quantity of nutritive matter bequeathed
by the parent at the time of separation. And though, where the reproduction
is viviparous, the process appears to be different, and in one sense is so,
yet, intrinsically, it is the same. For in these cases the new individual
really detaches itself from the parent while still only a centre of
development; but instead of being finally cast off in this state it is
re-attached, and supplied with nutriment until it assumes a more or less
complete axial structure.
§ 77. As we have lately seen, the essential act in gamogenesis is the union
of two cell-nuclei, produced in the great majority of cases by different
parent organisms. Nearly always the containing cells, often called
_gametes_, are unlike: the sperm-cell being the male product, and the
germ-cell the female. But among some _Protozoa_ and many of the lower
_Algæ_ and _Fungi_, the uniting cells show no differentiation. Sexuality is
only nascent.
There are very many modes and modifications of modes in which these cells
are produced; very many modes and modifications of modes by which they are
brought into contact; and very many modes and modifications of modes by
which the resulting fertilized germs have secured to them the fit
conditions for their development. But passing over these divergent and
re-divergent kinds of sexual multiplication, which it would take too much
space here to specify, the one universal trait is this coalescence of a
detached portion of one organism with a more or less detached portion of
another.
Such simple _Algæ_ as the _Desmidieæ_, which are sometimes called
unicellular plants, show us a coalescence, not of detached portions of two
organisms, but of two entire organisms: the entire contents of the
individuals uniting to form the germ-mass. Where, as among the
_Confervoideæ_, we have aggregated cells whose individualities are scarcely
at all subordinate to that of the aggregate, the gamogenetic act is often
effected by the union "of separate motile protoplasmic masses produced by
the division of the contents of any cell of the aggregate. These
free-swimming masses of protoplasm, which are quite similar to (but
generally smaller than) the agamogenetic 'zoospores' of the same plants,
and to the free-swimming individuals of many _Protophyta_, are apparently
the primitive type of gametes (conjugating cells); but it is noteworthy
that such a gamete nearly always unites with one derived from another cell
or from another individual. The same fact holds with regard to the gametes
of the Protophytes themselves, which are formed in the same way from the
single cell of the mother individual. In the higher types of
_Confervoideæ_, and in _Vaucheria_, we find these equivalent,
free-swimming, gametes replaced by sexually differentiated sperm- and
germ-cells, in some cases arising in different organs set apart for their
production, and essentially representing those found in the higher plants.
Transitional forms, intermediate between these and the cases where
equivalent gametes are formed from any cell of the plant are also known."
Recent investigations concerning the conjugation of _Protozoa_ have shown
that there is not, as was at one time thought, a fusion of two
individualities, but a fusion of parts of their nuclei. The macro-nucleus
having disappeared, and the micro-nucleus having broken up into portions,
each individual receives from the other one of these portions, which
becomes fused with its own nuclear matter. So that even in these humble
forms, where there is no differentiation of sexes, the union is not between
elements that have arisen in the same individual but between those which
have arisen in different individuals: the parts being in this case alike.
The marvellous phenomena initiated by the meeting of sperm-cell and
germ-cell, or rather of their nuclei, naturally suggest the conception of
some quite special and peculiar properties possessed by these cells. It
seems obvious that this mysterious power which they display of originating
a new and complex organism, distinguishes them in the broadest way from
portions of organic substance in general. Nevertheless, the more we study
the evidence the more are we led towards the conclusion that these cells
are not fundamentally different from other cells. The first fact which
points to this conclusion is the fact recently dwelt upon (§ 63), that in
many plants and inferior animals, a small fragment of tissue which is but
little differentiated, is capable of developing into an organism like that
from which it was taken. This implies that the component units of tissues
have inherent powers of arranging themselves into the forms of the
organisms which originated them. And if in these component units, which we
distinguished as physiological, such powers exist,--if, under fit
conditions, and when not much specialized, they manifest such powers in a
way as marked as that in which the contents of sperm-cells and germ-cells
manifest them; then, it becomes clear that the properties of sperm-cells
and germ-cells are not so peculiar as we are apt to assume. Again, the
organs emitting sperm-cells and germ-cells have none of the specialities of
structure which might be looked for, did sperm-cells and germ-cells need
endowing with properties unlike those of all other organic agents. On the
contrary, these reproductive centres proceed from tissues characterized by
their low organization. In plants, for example, it is not appendages that
have acquired considerable structure which produce the fructifying
particles: these arise at the extremities of the axes where the degree of
structure is the least. The cells out of which come the egg and the
pollen-grains, are formed from undifferentiated tissue in the interior of
the ovule and of the stamen. Among many inferior animals devoid of special
reproductive organs, such as the _Hydra_, the ova and spermatozoa originate
from the interstitial cells of the ectoderm, which lie among the bases of
the functional cells--have not been differentiated for function; and in the
_Medusæ_, according to Weismann, they arise in the homologous layer, save
where the medusoid form remains attached, and then they arise in the
endoderm and migrate to the ectoderm: lack of specialization being in all
cases implied. Then in the higher animals these same generative agents
appear to be merely modified epithelium-cells--cells not remarkable for
their complexity of structure but rather for their simplicity. If, by way
of demurrer to this view, it be asked why other epithelium-cells do not
exhibit like properties; there are two replies. The first is that other
epithelium-cells are usually so far changed to fit them to their special
functions that they are unfitted for assuming the reproductive function.
The second is that in some cases, where they are but little specialized,
they _do_ exhibit the like properties: not, indeed, by uniting with other
cells to produce new germs but by producing new germs without such union. I
learn from Dr. Hooker that the _Begonia phyllomaniaca_ habitually develops
young plants from the scales of its stem and leaves--nay, that many young
plants are developed by a single scale. The epidermal cells composing one
of these scales swell, here and there, into large globular cells; form
chlorophyll in their interiors; shoot out rudimentary axes; and then, by
spontaneous constrictions, cut themselves off; drop to the ground; and grow
into Begonias. Moreover, in a succulent English plant, the _Malaxis
paludosa_, a like process occurs: the self-detached cells being, in this
case, produced by the surfaces of the leaves.[30] Thus, there is no
warrant for the assumption that sperm-cells and germ-cells possess powers
fundamentally unlike those of other cells. The inference to which the facts
point, is, that they differ from the rest mainly in not having undergone
functional adaptations. They are cells which have departed but little from
the original and most general type: such specializations as some of them
exhibit in the shape of locomotive appliances, being interpretable as
extrinsic modifications which have reference to nothing beyond certain
mechanical requirements. Sundry facts tend likewise to show that there does
not exist the profound distinction we are apt to assume between the male
and female reproductive elements. In the common polype sperm-cells and
germ-cells are developed in the same layer of indifferent tissue; and in
_Tethya_, one of the sponges, Prof. Huxley has observed that they occur
mingled together in the general parenchyma. The pollen-grains and
embryo-cells of plants arise in adjacent parts of the meristematic tissue
of the flower-bud; and from the description of a monstrosity in the
Passion-flower, recently given by Mr. Salter to the Linnæan Society, it
appears both that ovules may, in their general structure, graduate into
anthers, and that they may produce pollen in their interiors. Moreover,
among the lower _Algæ_, which show the beginning of sexual differentiation,
the smaller gametes, which we must regard as incipient sperm-cells, are
sometimes able to fuse _inter se_, and give rise to a zygote which will
produce a new plant. All which evidence is in perfect harmony with the
foregoing conclusion; since, if sperm-cells and germ-cells have natures not
essentially unlike those of unspecialized cells in general, their natures
cannot be essentially unlike each other.
The next general fact to be noted is that these cells whose union
constitutes the essential act of gamogenesis, are cells in which the
developmental changes have come to a close--cells which are incapable of
further evolution. Though they are not, as many cells are, unfitted for
growth and metamorphosis by being highly specialized, yet they have lost
the power of growth and metamorphosis. They have severally reached a state
of equilibrium. And while the internal balance of forces prevents a
continuance of constructive changes, it is readily overthrown by external
destructive forces. For it almost uniformly happens that sperm-cells and
germ-cells which are not brought in contact disappear. In a plant, the
egg-cell, if not fertilized, is absorbed or dissipated, while the ovule
aborts; and the unimpregnated ovum eventually decomposes: save, indeed, in
those types in which parthenogenesis is a part of the normal cycle.
Such being the characters of these cells, and such being their fates if
kept apart, we have now to observe what happens when they are united. In
plants the extremity of the elongated pollen-cell applies itself to the
surface of the embryo-sac, and one of its nuclei having, with some
protoplasm, passed into the egg-cell, there becomes fused with the nucleus
of the egg-cell. Similarly in animals, the spermatozoon passes through the
limiting membrane of the ovum, and a mixture takes place between the
substance of its nucleus and the substance of the nucleus of the ovum. But
the important fact which it chiefly concerns us to notice, is that on the
union of these reproductive elements there begins, either at once or on the
return of favourable conditions, a new series of developmental changes. The
state of equilibrium at which each had arrived is destroyed by their mutual
influence, and the constructive changes, which had come to a close,
recommence. A process of cell-multiplication is set up; and the resulting
cells presently begin to aggregate into the rudiment of a new organism.
Thus, passing over the variable concomitants of gamogenesis, and confining
our attention to what is constant in it, we see:--that there is habitually,
if not universally, a fusion of two portions of organic substance which are
either themselves distinct individuals, or are thrown off by distinct
individuals; that these portions of organic substance, which are severally
distinguished by their low degree of specialization, have arrived at states
of structural quiescence or equilibrium; that if they are not united this
equilibrium ends in dissolution; but that by the mixture of them this
equilibrium is destroyed and a new evolution initiated.
§ 78. What are the conditions under which Genesis takes place? How does it
happen that some organisms multiply by homogenesis and others by
heterogenesis? Why is it that where agamogenesis prevails it is usually
from time to time interrupted by gamogenesis? A survey of the facts
discloses certain correlations which, if not universal, are too general to
be without significance.
Where multiplication is carried on by heterogenesis we find, in numerous
cases, that agamogenesis continues as long as the forces which result in
growth are greatly in excess of the antagonist forces. Conversely, we find
that the recurrence of gamogenesis takes place when the conditions are no
longer so favourable to growth. In like manner where there is homogenetic
multiplication, new individuals are usually not formed while the preceding
individuals are still rapidly growing--that is, while the forces producing
growth exceed the opposing forces to a great extent; but the formation of
new individuals begins when nutrition is nearly equalled by expenditure. A
few out of the many facts which seem to warrant these inductions must
suffice.
The relation in plants between fructification and innutrition (or rather,
between fructification and such diminished nutrition as makes growth
relatively slow) was long ago asserted by a German biologist--Wolff, I am
told. Since meeting with this assertion I have examined into the facts for
myself. The result has been a conviction, strengthened by every inquiry,
that some such relation exists. Uniaxial plants begin to produce their
lateral, flowering axes, only after the main axis has developed the great
mass of its leaves, and is showing its diminished nutrition by smaller
leaves, or shorter internodes, or both. In multiaxial plants two, three, or
more generations of leaf-bearing axes, or sexless individuals, are produced
before any seed-bearing individuals show themselves. When, after this first
stage of rapid growth and agamogenetic multiplication, some gamogenetic
individuals arise, they do so where the nutrition is least;--not on the
main axis, or on secondary axes, or even on tertiary axes, but on axes that
are the most removed from the channels which supply nutriment. Again, a
flowering axis is commonly less bulky than the others: either much shorter
or, if long, much thinner. And further, it is an axis of which the terminal
internodes are undeveloped: the foliar organs, which instead of becoming
leaves become sepals, and petals, and stamens, follow each other in close
succession, instead of being separated by portions of the still-growing
axis. Another group of evidences meets us when we observe the variations of
fruit-bearing which accompany variations of nutrition in the plant regarded
as a whole. Besides finding, as above, that gamogenesis commences only when
growth has been checked by extension of the remoter parts to some distance
from the roots, we find that gamogenesis is induced at an earlier stage
than usual by checking the nutrition. Trees are made to fruit while still
quite small by cutting their roots or putting them into pots; and luxuriant
branches which have had the flow of sap into them diminished, by what
gardeners call "ringing," begin to produce flower-shoots instead of
leaf-shoots. Moreover, it is to be remarked that trees which, by flowering
early in the year, seem to show a direct relation between gamogenesis and
increasing nutrition, really do the reverse; for in such trees the
flower-buds are formed in the autumn. That structure which determines these
buds into sexual individuals is given when the nutrition is declining.
Conversely, very high nutrition in plants prevents, or arrests,
gamogenesis. It is notorious that unusual richness of soil, or too large a
quantity of manure, results in a continuous production of leaf-bearing or
sexless shoots; and a like result happens when the cutting down of a tree,
or of a large part of it, is followed by the sending out of new shoots:
these, supplied with excess of sap, are luxuriant and sexless. Besides
being prevented from producing sexual individuals by excessive nutrition,
plants are, by excessive nutrition, made to change the sexual individuals
they were about to produce, into sexless ones. This arrest of gamogenesis
may be seen in various stages. The familiar instance of flowers made barren
by the transformation of their stamens into petals, shows us the lowest
degree of this reversed metamorphosis. Where the petals and stamens are
partially changed into green leaves, the return towards the agamogenetic
structure is more marked; and it is still more marked when, as occasionally
happens in luxuriantly-growing plants, new flowering axes, and even
leaf-bearing axes, grow out of the centres of flowers.[31] The anatomical
structure of the sexual axis affords corroborative evidence: giving the
impression, as it does, of an aborted sexless axis. Besides lacking those
internodes which the leaf-bearing axis commonly possesses, the flowering
axis differs by the absence of rudimentary lateral axes. In a leaf-bearing
shoot the axil of every leaf usually contains a small bud, which may or may
not develop into a lateral shoot; but though the petals of a flower are
homologous with leaves, they do not bear homologous buds at their bases.
Ordinarily, too, the foliar appendages of sexual axes are much smaller than
those of sexless ones--the stamens and pistils especially, which are the
last formed, being extremely dwarfed; and it may be that the absence of
chlorophyll from the parts of fructification is a fact of like meaning.
Moreover, the formation of the seed-vessel appears to be a direct
consequence of arrested nutrition. If a gloved-finger be taken to represent
a growing shoot, (the finger standing for the pith of the shoot and the
glove for the peripheral layers of meristem and young tissue, in which the
process of growth takes place); and if it be supposed that there is a
diminished supply of material for growth; then, it seems a fair inference
that growth will first cease at the apex of the axis, represented by the
end of the glove-finger; and supposing growth to continue in those parts of
the peripheral layers of young tissue that are nearer to the supply of
nutriment, their further longitudinal extension will lead to the formation
of a cavity at the extremity of the shoot, like that which results in a
glove-finger when the finger is partially withdrawn and the glove sticks to
its end. Whence it seems, both that this introversion of the apical
meristem may be considered as due to failing nutrition, and that the ovules
growing from its introverted surface (which would have been its outer
surface but for the defective nutrition) are extremely aborted homologues
of external appendages: both they and the pollen-grains being either
morphologically or literally quite terminal, and the last showing by their
dehiscence the exhaustion of the organizing power.[32]
Those kinds of animals which multiply by heterogenesis, present us with a
parallel relation between the recurrence of gamogenesis and the recurrence
of conditions checking rapid growth: at least, this is shown where
experiments have thrown light on the connexion of cause and effect; namely,
among the _Aphides_. These creatures, hatched from eggs in the spring,
multiply by agamogenesis, which in this case is parthenogenesis, throughout
the summer. When the weather becomes cold and plants no longer afford
abundant sap, perfect males and females are produced; and from gamogenesis
result fertilized ova. But beyond this evidence we have much more
conclusive evidence. For it has been shown, both that the rapidity of the
agamogenesis is proportionate to the warmth and nutrition, and that if the
temperature and supply of food be artificially maintained, the agamogenesis
continues through the winter. Nay more--it not only, under these
conditions, continues through one winter, but it has been known to continue
for four successive years: some forty or fifty sexless generations being
thus produced. And those who have investigated the matter see no reason to
doubt the indefinite continuance of this agamogenetic multiplication, so
long as the external requirements are duly met. Evidence of another kind,
complicated by special influences, is furnished by the heterogenesis of the
_Daphnia_--a small crustacean commonly known as the Water-flea, which
inhabits ponds and ditches. From the nature of its habitat this little
creature is exposed to very variable conditions. Besides being frozen in
winter, the small bodies of water in which it lives are often unduly heated
by the summer Sun, or dried up by continued drought. The circumstances
favourable to the _Daphnia's_ life and growth, being thus liable to
interruptions which, in our climate, have a regular irregularity of
recurrence; we may, in conformity with the hypothesis, expect to find both
that the gamogenesis recurs along with declining physical prosperity and
that its recurrence is very variable. I use the expression "declining
physical prosperity" advisedly; since "declining nutrition," as measured by
supply of food, does not cover all the conditions. This is shown by the
experiments of Weismann (abstracted for me by Mr. Cunningham) who found
that in various _Daphnideæ_ which bring forth resting eggs, sexual and
asexual reproduction go on simultaneously, as well as separately, in the
spring and summer: these variable results being adapted to variable
conditions. For not only are these creatures liable to die from lack of
food, from the winter's cold, and from the drying up of their ditches, &c.,
as well as from the over-heating of them, but during this period of
over-heating they are liable to die from that deoxygenation of the water
which heat causes. Manifestly the favourable and unfavourable conditions
recurring in combinations that are rarely twice alike, cannot be met by any
regularly recurring form of heterogenesis; and it is interesting to see how
survival of the fittest has established a mixed form. In the spring, as
well as in the autumn, there is in some cases a formation of resting or
winter eggs; and evidently these provide against the killing off of the
whole population by summer drought. Meanwhile, by ordinary males and
females there is a production of summer eggs adapted to meet the incident
of drying up by drought and subsequent re-supply of water. And all along
successive generations of parthenogenetic females effect a rapid
multiplication as long as conditions permit. Since life and growth are
impeded or arrested not by lack of food only, but by other unfavourable
conditions, we may understand how change in one or more of these may set up
one or other form of genesis, and how the mixture of them may cause a mixed
mode of multiplication which, originally initiated by external causes,
becomes by inheritance and selection a trait of the species.[33] And then
in proof that external causes initiate these peculiarities, we have the
fact that in certain _Daphnideæ_ "which live in places where existence and
parthenogenesis are possible throughout the year, the sexual period has
disappeared:" there are no males.
Passing now to animals which multiply by homogenesis--animals in which the
whole product of a fertilized germ aggregates round a single centre or axis
instead of round many centres or axes--we see, as before, that so long as
the conditions allow rapid increase in the mass of this germ-product, the
formation of new individuals by gamogenesis does not take place. Only when
growth is declining in relative rate, do perfect sperm-cells and germ-cells
begin to appear; and the fullest activity of the reproductive function
arises as growth ceases: speaking generally, at least; for though this
relation is tolerably definite in the highest orders of animals which
multiply by gamogenesis, it is less definite in the lower orders. This
admission does not militate against the hypothesis, as it seems to do; for
the indefiniteness of the relation occurs where the limit of growth is
comparatively indefinite. We saw (§ 46) that among active, hot-blooded
creatures, such as mammals and birds, the inevitable balancing of
assimilation by expenditure establishes, for each species, an almost
uniform adult size; and among creatures of these kinds (birds especially,
in which this restrictive effect of expenditure is most conspicuous), the
connexion between cessation of growth and commencement of reproduction is
distinct. But we also saw (§ 46) that where, as in the Crocodile and the
Pike, the conditions and habits of life are such that expenditure does not
overtake assimilation as size increases, there is no precise limit of
growth; and in creatures thus circumstanced we may naturally look for a
comparatively indeterminate relation between declining growth and
commencing reproduction.[34] There is, indeed, among fishes, at least one
case which appears very anomalous. The male parr, or young of the male
salmon, a fish of four or five inches in length, is said to produce milt.
Having, at this early stage of its growth, not one-hundredth of the weight
of a full-grown salmon, how does its production of milt consist with the
alleged general law? The answer must be in great measure hypothetical. If
the salmon is (as it appears to be in its young state) a species of
fresh-water trout that has contracted the habit of annually migrating to
the sea, where it finds a food on which it thrives--if the original size of
this species was not much greater than that of the parr (which is nearly as
large as some varieties of trout)--and if the limit of growth in the trout
tribe is very indefinite, as we know it to be; then we may reasonably infer
that the parr has nearly the adult form and size which this species of
trout had before it acquired its migratory habit; and that this production
of milt is, in such case, a concomitant of the incipient decline of growth
naturally arising in the species when living under the conditions of the
ancestral species. Should this be so, the immense subsequent growth of the
parr into the salmon, consequent on a suddenly-increased facility in
obtaining food, removes to a great distance the limit at which assimilation
is balanced by expenditure; and has the effect, analogous to that produced
in plants, of arresting the incipient reproductive process. A confirmation
of this view may be drawn from the fact that when the parr, after its first
migration to the sea, returns to fresh water, having increased in a few
months from a couple of ounces to five or six pounds, it no longer shows
any fitness for propagation: the grilse, or immature salmon, does not
produce milt or spawn.
We conclude, then, that the products of a fertilized germ go on
accumulating by simple growth, so long as the forces whence growth results
are greatly in excess of the antagonist forces; but that when diminution of
the one set of forces or increase of the other, causes a considerable
decline in this excess and an approach towards equilibrium, fertilized
germs are again produced. Whether the germ-product be organized round one
axis or round the many axes that arise by agamogenesis, matters not.
Whether, as in the higher animals, this approach to equilibrium results
from that disproportionate increase of expenditure entailed by increase of
size; or whether, as in most plants and many inferior animals, it results
from absolute or relative decline of nutrition; matters not. In any case
the recurrence of gamogenesis is associated with a decrease in the excess
of tissue-producing power. We cannot say, indeed, that this decrease always
results in gamogenesis: some organisms multiply for an indefinite period by
agamogenesis only. The Weeping Willow, which has been propagated throughout
Europe, does not seed in Europe; and yet, as the Weeping Willow, by its
large size and the multiplication of generation upon generation of lateral
axes, presents the same causes of local innutrition as other trees, we
cannot ascribe the absence of sexual axes to the continued predominance of
nutrition. Among animals, too, the anomalous case of the _Tineidæ_, a group
of moths in which parthenogenetic multiplication goes on for generation
after generation, seems to imply that gamogenesis does not necessarily
result from an approximate balance of assimilation by expenditure. What we
must say is that an approach towards equilibrium between the forces which
cause growth and the forces which oppose growth, is the chief condition to
the recurrence of gamogenesis; but that there appear to be other
conditions, in the absence of which approach to equilibrium is not followed
by gamogenesis.
§ 79. The above induction is an approximate answer to the question--_When_
does gamogenesis recur? but not to the question which was propounded--_Why_
does gamogenesis recur?--_Why_ cannot multiplication be carried on in all
cases, as it is in many cases, by agamogenesis? As already said, biologic
science is not yet advanced enough to reply. Meanwhile, the evidence above
brought together suggests a certain hypothetical answer.
Seeing, on the one hand, that gamogenesis recurs only in individuals which
are approaching a state of organic equilibrium; and seeing, on the other
hand, that the sperm-cells and germ-cells thrown off by such individuals
are cells in which developmental changes have ended in quiescence, but in
which, after their union, there arises a process of active cell-formation;
we may suspect that the approach towards a state of general equilibrium in
such gamogenetic individuals, is accompanied by an approach towards
molecular equilibrium in them; and that the need for this union of
sperm-cell and germ-cell is the need for overthrowing this equilibrium, and
re-establishing active molecular change in the detached germ--a result
probably effected by mixing the slightly different physiological units of
slightly different individuals. The several arguments which support this
view, cannot be satisfactorily set forth until after the topics of Heredity
and Variation have been dealt with. Leaving it for the present, I propose
hereafter to re-consider it in connexion with sundry others raised by the
phenomena of Genesis.
But before ending the chapter, it may be well to note the relations between
these different modes of multiplication, and the conditions of existence
under which they are respectively habitual. While the explanation of the
teleologist is untrue, it is often an obverse to the truth; for though, on
the hypothesis of Evolution, it is clear that things are not arranged thus
or thus for the securing of special ends, it is also clear that
arrangements which _do_ secure these special ends tend to establish
themselves--are established by their fulfilment of these ends. Besides
insuring a structural fitness between each kind of organism and its
circumstances, the working of "natural selection" also insures a fitness
between the mode and rate of multiplication of each kind of organism and
its circumstances. We may, therefore, without any teleological implication,
consider the fitness of homogenesis and heterogenesis to the needs of the
different classes of organisms which exhibit them.
Heterogenesis prevails among organisms of which the food, though abundant
compared with their expenditure, is dispersed in such a way that it cannot
be appropriated in a wholesale manner. _Protophyta_, subsisting on diffused
gases and decaying organic matter in a state of minute subdivision, and
_Protozoa_, to which food comes in the shape of extremely small floating
particles, are enabled, by their rapid agamogenetic multiplication, to
obtain materials for growth better than they would do did they not thus
continually divide and disperse in pursuit of it. The higher plants, having
for nutriment the carbonic acid of the air and certain mineral components
of the soil, show us modes of multiplication adapted to the fullest
utilization of these substances. A herb with but little power of forming
the woody fibre requisite to make a stem that can support wide-spreading
branches, after producing a few sexless axes produces sexual ones; and
maintains its race better, by the consequent early dispersion of seeds,
than by a further production of sexless axes. But a tree, able to lift its
successive generations of sexless axes high into the air, where each gets
carbonic acid and light almost as freely as if it grew by itself, may with
advantage go on budding-out sexless axes year after year; since it thereby
increases its subsequent power of budding-out sexual axes. Meanwhile it may
advantageously transform into seed-bearers those axes which, in consequence
of their less direct access to materials absorbed by the roots, are failing
in their nutrition; for it thus throws off from a point at which sustenance
is deficient, a migrating group of germs that may find sustenance
elsewhere. The heterogenesis displayed by animals of the Coelenterate type
has evidently a like utility. A polype, feeding on minute annelids and
crustaceans which, flitting through the water, come in contact with its
tentacles, and limited to that quantity of prey which chance brings within
its grasp, buds out young polypes which, either as a colony or as dispersed
individuals, spread their tentacles through a larger space of water than
the parent alone can; and by producing them, the parent better insures the
continuance of its species than it would do if it went on slowly growing
until its nutrition was nearly balanced by its waste, and then multiplied
by gamogenesis. Similarly with the _Aphis_. Living on sap sucked from
tender shoots and leaves, and able thus to take in but a very small
quantity in a given time, this creature's race is more likely to be
preserved by a rapid asexual propagation of small individuals, which
disperse themselves over a wide area of nutrition, than it would be did the
individual growth continue so as to produce large individuals multiplying
sexually. And then when autumnal cold and diminishing supply of sap put a
check to growth, the recurrence of gamogenesis, or production of fertilized
ova which remain dormant through the winter, is more favourable to the
preservation of the race than would be a further continuance of
agamogenesis. On the other hand, among the higher animals living on food
which, though dispersed, is more or less aggregated into large masses, this
alternation of gamic and agamic reproduction ceases to be useful. The
development of the germ-product into a single organism of considerable
bulk, is in many cases a condition without which these large masses of
nutriment could not be appropriated; and here the formation of many
individuals instead of one would be fatal. But we still see the beneficial
results of the general law--the postponement of gamogenesis until the rate
of growth begins to decline. For so long as the rate of growth continues
rapid, there is proof that the organism gets food with facility--that
expenditure does not seriously check accumulation; and that the size
reached is as yet not disadvantageous: or rather, indeed, that it is
advantageous. But when the rate of growth is much decreased by the increase
of expenditure--when the excess of assimilative power is diminishing so
fast as to indicate its approaching disappearance--it becomes needful, for
the maintenance of the species, that this excess shall be turned to the
production of new individuals; since, did growth continue until there was a
complete balancing of assimilation and expenditure, the production of new
individuals would be either impossible or fatal to the parent. And it is
clear that "natural selection" will continually tend to determine the
period at which gamogenesis commences, in such a way as most favours the
maintenance of the race.
Here, too, may fitly be pointed out the fact that, by "natural selection,"
there will in every case be produced the most advantageous proportion of
males and females. If the conditions of life render numerical inequality of
the sexes beneficial to the species, in respect either of the number of the
offspring or the character of the offspring; then, those varieties of the
species which approach more than other varieties towards this beneficial
degree of inequality, will be apt to supplant other varieties. And
conversely, where equality in the number of males and females is
beneficial, the equilibrium will be maintained by the dying out of such
varieties as produce offspring among which the sexes are not balanced.
NOTE.--Such alterations of statement in this chapter as have been made
necessary by the advance of biological knowledge since 1864 have not, I
think, tended to invalidate its main theses, but have tended to verify
them. Some explanations to be here added may remove remaining difficulties.
Certain types, which are transitional between _Protozoa_ and _Metazoa_,
exhibit under its simplest form the relation between self-maintenance and
race-maintenance--the integration primarily effecting the one and the
disintegration primarily effecting the other. Among the _Mycetozoa_ a
number of amoeba-like individuals aggregate into what is called a
plasmodium; and while, in some orders, they become fused into a mass of
protoplasm through which their nuclei are dispersed, in other orders
(_Sorophora_) they retain their individualities and simply form a coherent
aggregate. These last, presumably the earliest in order of evolution,
remain united so long as the plasmodium, having a small power of
locomotion, furthers the general nutrition; but when this is impeded by
drought or cold, there arise spores. Each spore contains an amoeboid
individual; and this, escaping when favourable conditions return,
establishes by fission and by union with others like itself a new colony or
plasmodium. Reduced to its lowest terms, we here see the antagonism between
that growth of the coherent mass of units which accompanies its physical
prosperity, and that incoherence and dispersion of the units which follows
unfavourable conditions and arrest of growth, and which presently initiates
new plasmodia.
This antagonism, seen in these incipient _Metazoa_ which show us none of
that organization characterizing the _Metazoa_ in general, is everywhere in
more or less disguised forms exhibited by them--must necessarily be so if
growth of the individual is a process of integration while formation of new
individuals is a process of disintegration. And, primarily, it is an
implication that whatever furthers the one impedes the other.
But now while recognizing the truth that nutrition and innutrition (using
these words to cover not supply of nutriment only but the presence of other
influences favourable or unfavourable to the vital processes) primarily
determine the alternations of these; we have also to recognize the truth
that from the beginning survival of the fittest has been shaping the forms
and effects of their antagonism. By inheritance a physiological habit which
modifies the form of the antagonism in a way favourable to the species,
will become established. Especially will this be the case where the lives
of the individuals have become relatively definite and where special organs
have been evolved for casting off reproductive centres. The resulting
physiological rhythm may in such cases become so pronounced as greatly to
obscure the primitive relation. Among plants we see this in the fact that
those which have been transferred from one habitat to another having widely
different seasons, long continue their original time of flowering, though
it is inappropriate to the new circumstances--the reproductive periodicity
has become organic. Similarly in each species of higher animal, development
of the reproductive organs and maturation of reproductive cells take place
at a settled age, whether the conditions have been favourable or
unfavourable to physical prosperity. The established constitutional
tendency, adapted to the needs of the species, over-rides the
constitutional needs of the individual.
Even here, however, the primitive antagonism, though greatly obscured,
occasionally shows itself. Instance the fact that in plants where
gamogenesis is commencing a sudden access of nutrition will cause
resumption of agamogenesis; and I suspect that an illustration may be found
among human beings in the earlier establishment of the reproductive
function among the ill-fed poor than among the well-fed rich.
One other qualification has to be added. In plants and animals which have
become so definitely constituted that at an approximately fixed stage, the
proclivity towards the production of new individuals becomes pronounced, it
naturally happens that good nutrition aids it. Surplus nutriment being
turned into the reproductive channel, the reproduction is efficient in
proportion as the surplus is great. Hence the fact that in fruit trees
which have reached the flowering stage, manuring has the effect that though
it does not increase the quantity of blossoms it increases the quantity of
fruit; and hence the fact that well-fed and easy-living races of men are
prolific.
CHAPTER VIII.
HEREDITY.
§ 80. Already, in the last two chapters, the law of hereditary transmission
has been tacitly assumed; as, indeed, it unavoidably is in all such
discussions. Understood in its entirety, the law is that each plant or
animal, if it reproduces, gives origin to others like itself: the likeness
consisting, not so much in the repetition of individual traits as in the
assumption of the same general structure. This truth has been rendered so
familiar by daily illustration as almost to have lost its significance.
That wheat produces wheat--that existing oxen have descended from ancestral
oxen--that every unfolding organism eventually takes the form of the class,
order, genus, and species from which it sprang; is a fact which, by force
of repetition, has acquired in our minds almost the aspect of a necessity.
It is in this, however, that Heredity is principally displayed: the
manifestations of it commonly referred to being quite subordinate. And, as
thus understood, Heredity is universal. The various instances of
heterogenesis lately contemplated seem, indeed, to be at variance with this
assertion. But they are not really so. Though the recurrence of like forms
is, in these instances, not direct but cyclical, still, the like forms do
recur; and, when taken together, the group of forms produced during one of
the cycles is as much like the groups produced in preceding cycles, as the
single individual arising by homogenesis is like ancestral individuals.
While, however, the general truth that organisms of a given type uniformly
descend from organisms of the same type, is so well established by infinite
illustrations as to have assumed the character of an axiom; it is not
universally admitted that non-typical peculiarities are inherited. Many
entertain a vague belief that the law of Heredity applies only to main
characters of structure and not to details; or, at any rate, that though it
applies to such details as constitute differences of species, it does not
apply to smaller details. The circumstance that the tendency to repetition
is in a slight degree qualified by the tendency to variation (which, as we
shall hereafter see, is but an indirect result of the tendency to
repetition), leads some to doubt whether Heredity is unlimited. A careful
weighing of the evidence, however, and a due allowance for the influences
by which the minuter manifestations of Heredity are obscured, may remove
this scepticism.
First in order of importance comes the fact that not only are there
uniformly transmitted from an organism to its offspring, those traits of
structure which distinguish the class, order, genus, and species; but also
those which distinguish the variety. We have numerous cases, among both
plants and animals, where, by natural or artificial conditions, there have
been produced divergent modifications of the same species; and abundant
proof exists that the members of any one sub-species habitually transmit
their distinctive peculiarities to their descendants. Agriculturists and
gardeners can furnish unquestionable illustrations. Several varieties of
wheat are known, of which each reproduces itself. Since the potato was
introduced into England there have been formed from it a number of
sub-species; some of them differing greatly in their forms, sizes,
qualities, and periods of ripening. Of peas, also, the like may be said.
And the case of the cabbage-tribe is often cited as showing the permanent
establishment of races which have diverged widely from a common stock.
Among fruits and flowers the multiplication of kinds, and the continuance
of each kind with certainty by agamogenesis, and to some extent by
gamogenesis, might be exemplified without end. From all sides evidence may
be gathered showing a like persistence of varieties among animals. We have
our distinct breeds of sheep, our distinct breeds of cattle, our distinct
breeds of horses: each breed maintaining its characteristics. The many
sorts of dogs which, if we accept the physiological test, we must consider
as all of one species, show us in a marked manner the hereditary
transmission of small differences--each sort, when kept pure, reproducing
itself not only in size, form, colour, and quality of hair, but also in
disposition and speciality of intelligence. Poultry, too, have their
permanently-established races. And the Isle of Man sends us a tail-less
kind of cat. Even in the absence of other evidence, that which ethnology
furnishes would suffice. Grant them to be derived from one stock, and the
varieties of man yield proof upon proof that non-specific traits of
structure are bequeathed from generation to generation. Or grant only their
derivation from several stocks, and we still have, between races descended
from a common stock, distinctions which prove the inheritance of minor
peculiarities. Besides seeing the Negroes continue to produce Negroes,
copper-coloured men to produce men of a copper colour, and the fair-skinned
races to perpetuate their fair skins--besides seeing that the broad-faced
and flat-nosed Calmuck begets children with broad faces and flat noses,
while the Jew bequeaths to his offspring the features which have so long
characterized Jews; we see that those small unlikenesses which distinguish
more nearly-allied varieties of men, are maintained from generation to
generation. In Germany, the ordinary shape of skull is appreciably
different from that common in Britain: near akin though the Germans are to
the British. The average Italian face continues to be unlike the faces of
northern nations. The French character is now, as it was centuries ago,
contrasted in sundry respects with the characters of neighbouring peoples.
Nay, even between races so closely allied as the Scotch Celts, the Welsh
Celts, and the Irish Celts, appreciable differences of form and nature have
become established.
The fact that sub-species and sub-sub-species thus exemplify the general
law of inheritance which shows itself in the perpetuation of ordinal,
generic, and species peculiarities, is strong reason for the belief that
this general lay is unlimited in its application. This has the support of
still more special evidences. They are divisible into two classes. In the
one come cases where congenital peculiarities, not traceable to any obvious
causes, are bequeathed to descendants. In the other come cases where the
peculiarities thus bequeathed are not congenital, but have resulted from
changes of functions during the lives of the individuals bequeathing them.
We will consider first the cases that come in the first class.
§ 81. Note at the outset the character of the chief testimony. Excluding
those inductions that have been so fully verified as to rank with exact
science, there are no inductions so trustworthy as those which have
undergone the mercantile test. When we have thousands of men whose profit
or loss depends on the truth of their inferences from perpetually-repeated
observations; and when we find that their inferences, handed down from
generation to generation, have generated an unshakable conviction; we may
accept it without hesitation. In breeders of animals we have such a class,
led by such experiences, and entertaining such a conviction--the conviction
that minor peculiarities of organization are inherited as well as major
peculiarities. Hence the immense prices given for successful racers, bulls
of superior forms, sheep that have certain desired peculiarities. Hence the
careful record of pedigrees of high-bred horses and sporting dogs. Hence
the care taken to avoid intermixture with inferior stocks. As quoted by Mr.
Darwin, Youatt says the principle of selection "enables the agriculturist
not only to modify the character of his flock but to change it altogether."
Lord Somerville, speaking of what breeders have done for sheep, says:--"It
would seem that they have chalked upon a wall a form perfect in itself and
then given it existence." That most skilful breeder, Sir John Sebright,
used to say, with respect to pigeons, that "he would produce any given
feather in three years, but it would take him six years to obtain head and
beak." In all which statements the tacit assertion is, that individual
traits are bequeathed from generation to generation, and may be so
perpetuated and increased as to become permanent distinctions.
Of special instances there are many besides that of the often-cited
Otto-breed of sheep, descended from a single short-legged lamb, and that of
the six-fingered Gratio Kelleia, who transmitted his peculiarity, in
different degrees, to several of his children and to some of his
grandchildren. In a paper contributed to the _Edinburgh New Philosophical
Journal_ for July, 1863, Dr. (now Sir John) Struthers gives cases of
hereditary digital variations. Esther P----, who had six fingers on one
hand, bequeathed this malformation along some lines of her descendants for
two, three, and four generations. A---- S---- inherited an extra digit on
each hand and each foot from his father; and C---- G----, who also had six
fingers and six toes, had an aunt and a grandmother similarly formed. A
collection of evidence published by Mr. Sedgwick in the _Medico-Chirurgical
Review_ for April and for July, 1863, in two articles on "The Influence of
Sex in limiting Hereditary Transmission," includes the following
cases:--Augustin Duforet, a pastry-cook of Douai, who had but two instead
of three phalanges to all his fingers and toes, inherited this malformation
from his grandfather and father, and had it in common with an uncle and
numerous cousins. An account has been given by Dr. Lepine, of a man with
only three fingers on each hand and four toes on each foot, and whose
grandfather and son exhibited the like anomaly. Béchet describes Victoire
Barré as a woman who, like her father and sister, had but one developed
finger on each hand and but two toes on each foot, and whose monstrosity
re-appeared in two daughters. And there is a case where the absence of two
distal phalanges on the hands was traced for two generations. The various
recorded instances in which there has been transmission from one generation
to another, of webbed-fingers, of webbed-toes, of hare-lip, of congenital
luxation of the thigh, of absent patellæ, of club-foot, &c., would occupy
more space than can here be spared. Defects in the organs of sense are also
not unfrequently inherited. Four sisters, their mother, and grandmother,
are described by Duval as similarly affected by cataract. Prosper Lucas
details an example of amaurosis affecting the females of a family for three
generations. Duval, Graffe, Dufon, and others testify to like cases coming
under their observation.[35] Deafness, too, is occasionally transmitted
from parent to child. There are deaf-mutes whose imperfections have been
derived from ancestors; and malformations of the external ears have also
been perpetuated in offspring. Of transmitted peculiarities of the skin and
its appendages, many cases have been noted. One is that of a family
remarkable for enormous black eyebrows; another that of a family in which
every member had a lock of hair of a lighter colour than the rest on the
top of the head; and there are also instances of congenital baldness being
hereditary. From one of our leading sculptors I learn that his wife has a
flat mole under the foot near the little toe, and one of her sons has the
same. Entire absence of teeth, absence of particular teeth, and anomalous
arrangements of teeth, are recorded as traits that have descended to
children. And we have evidence that soundness and unsoundness of teeth are
transmissible.
The inheritance of tendencies to such diseases as gout, consumption, and
insanity is universally admitted. Among the less-common diseases of which
the descent has been observed, are ichthyosis, leprosy, pityriasis,
sebaceous tumours, plica polonica, dipsomania, somnambulism, catalepsy,
epilepsy, asthma, apoplexy, elephantiasis. General nervousness displayed by
parents almost always re-appears in their children. Even a bias towards
suicide appears to be sometimes hereditary.
§ 82. To prove the transmission of those structural peculiarities which
have resulted from functional peculiarities, is, for several reasons,
comparatively difficult. Changes produced in the sizes of parts by changes
in their amounts of action, are mostly unobtrusive. A muscle which has
increased in bulk is usually so obscured by natural or artificial clothing,
that unless the alteration is extreme it passes without remark. Such
nervous developments as are possible in the course of a single life, cannot
be seen externally. Visceral modifications of a normal kind are observable
but obscurely, or not at all. And if the changes of structure worked in
individuals by changes in their habits are thus difficult to trace, still
more difficult to trace must be the transmission of them: further hidden,
as this is, by the influences of other individuals who are often otherwise
modified by other habits. Moreover, such specialities of structure as are
due to specialities of function, are usually entangled with specialities of
structure which are, or may be, due to selection, natural or artificial. In
most cases it is impossible to say that a structural peculiarity which
seems to have arisen in offspring from a functional peculiarity in a
parent, is wholly independent of some congenital peculiarity of structure
in the parent, whence this functional peculiarity arose. We are restricted
to cases with which natural or artificial selection can have had nothing to
do, and such cases are difficult to find. Some, however, may be noted.
A species of plant that has been transferred from one soil or climate to
another, frequently undergoes what botanists call "change of habit"--a
change which, without affecting its specific characters, is yet
conspicuous. In its new locality the species is distinguished by leaves
that are much larger or much smaller, or differently shaped, or more
fleshy; or instead of being as before comparatively smooth, it becomes
hairy; or its stem becomes woody instead of being herbaceous; or its
branches, no longer growing upwards, assume a drooping character. Now these
"changes of habit" are clearly determined by functional changes. Occurring,
as they do, in many individuals which have undergone the same
transportation, they cannot be classed as "spontaneous variations." They
are modifications of structure consequent on modifications of function that
have been produced by modifications in the actions of external forces. And
as these modifications re-appear in succeeding generations, we have, in
them, examples of functionally-established variations that are hereditarily
transmitted.
Evidence of analogous changes in animals is difficult to disentangle. Only
among domesticated kinds have we any opportunity of tracing the results of
altered habits; and here, in nearly all cases, artificial selection has
obscured them. Still, there are some facts which seem to the point. Mr.
Darwin, while ascribing almost wholly to "natural selection" the production
of those modifications which eventuate in differences of species,
nevertheless admits the effects of use and disuse. He says--"I find in the
domestic duck that the bones of the wing weigh less and the bones of the
leg more, in proportion to the whole skeleton, than do the same bones in
the wild duck; and I presume that this change may be safely attributed to
the domestic duck flying much less, and walking more, than its wild parent.
The great and inherited development of the udders in cows and goats in
countries where they are habitually milked, in comparison with the state of
these organs in other countries, is another instance of the effect of use.
Not a single domestic animal can be named which has not in some country
drooping ears; and the view suggested by some authors, that the drooping is
due to the disuse of the muscles of the ear, from the animals not being
much alarmed by danger, seems probable." Again--"The eyes of moles and of
some burrowing rodents are rudimentary in size, and in some cases are quite
covered up by skin and fur. This state of the eyes is probably due to
gradual reduction from disuse, but aided perhaps by natural selection." ...
"It is well known that several animals belonging to the most different
classes, which inhabit the caves of Styria and of Kentucky, are blind. In
some of the crabs the footstalk of the eye remains, though the eye is gone;
the stand for the telescope is there, though the telescope with its glasses
has been lost. As it is difficult to imagine that eyes, though useless,
could be in any way injurious to animals living in darkness, I attribute
their loss wholly to disuse."[36] The direct inheritance of an acquired
peculiarity is sometimes observable. Mr. Lewes gives a case. He "had a
puppy taken from its mother at six weeks old, who, although never taught
'to beg' (an accomplishment his mother had been taught), spontaneously took
to begging for everything he wanted when about seven or eight months old:
he would beg for food, beg to be let out of the room, and one day was found
opposite a rabbit hutch begging for rabbits." Instances are on record, too,
of sporting dogs which spontaneously adopted in the field, certain modes of
behaviour which their parents had learnt.
But the best examples of inherited modifications produced by modifications
of function, occur in mankind. To no other cause can be ascribed the rapid
metamorphoses undergone by the British races when placed in new conditions.
In the United States the descendants of the immigrant Irish lose their
Celtic aspect, and become Americanized. This cannot be ascribed to mixture,
since the feeling with which Irish are regarded by Americans prevents any
considerable amount of intermarriage. Equally marked is the case of the
immigrant Germans who, though they keep very much apart, rapidly assume the
prevailing type. To say that "spontaneous variation" increased by natural
selection, can have produced this effect, is going too far. Peoples so
numerous cannot have been supplanted in the course of two or three
generations by varieties springing from them. Hence the implication is that
physical and social conditions have wrought modifications of function and
structure, which offspring have inherited and increased. Similarly with
special cases. In the _Cyclopædia of Practical Medicine_, Vol. II., p. 419,
Dr. Brown states that he "has in many instances observed in the case of
individuals whose complexion and general appearance has been modified by
residence in hot climates, that children born to them subsequently to such
residence, have resembled them rather in their acquired than primary mien."
Some visible modifications of organs caused by changes in their functions,
may be noted. That large hands are inherited by those whose ancestors led
laborious lives, and that those descended from ancestors unused to manual
labour commonly have small hands, are established opinions. It seems very
unlikely that in the absence of any such connexion, the size of the hand
should have come to be generally regarded as some index of extraction. That
there exists a like relation between habitual use of the feet and largeness
of the feet, we have strong evidence in the customs of the Chinese. The
torturing practice of artificially arresting the growth of the feet, could
never have become established among the ladies of China, had they not seen
that a small foot was significant of superior rank--that is of a luxurious
life--that is of a life without bodily labour. There is evidence, too, that
modifications of the eyes, caused by particular uses of the eyes, are
inherited. Short sight appears to be uncommon among peasants; but it is
frequent among classes who use their eyes much for reading and writing, and
is often congenital. Still more marked is this relation in Germany. There,
the educated are notoriously studious, and judging from the numbers of
young Germans who wear spectacles, there is reason to think that congenital
myopia is very frequent among them.
Some of the best illustrations of functional heredity, are furnished by
mental characteristics. Certain powers which mankind have gained in the
course of civilization cannot, I think, be accounted for without admitting
the inheritance of acquired modifications. The musical faculty is one of
these. To say that "natural selection" has developed it by preserving the
most musically endowed, seems an inadequate explanation. Even now that the
development and prevalence of the faculty have made music an occupation by
which the most musical can get sustenance and bring up families; it is very
questionable whether, taking the musical career as a whole, it has any
advantage over other careers in the struggle for existence and
multiplication. Still more if we look back to those early stages through
which the faculty must have passed before definite perception of melody was
arrived at, we fail to see how those possessing the rudimentary faculty in
a somewhat greater degree than the rest, would thereby be enabled the
better to maintain themselves and their children. There is no explanation
but that the habitual association of certain cadences of speech with
certain emotions, has slowly established in the race an organized and
inherited connection between such cadences and such emotions; that the
combination of such cadences, more or less idealized, which constitutes
melody, has all along had a meaning in the average mind, only because of
the meaning which cadences had acquired in the average mind; and that by
the continual hearing and practice of melody there has been gained and
transmitted an increasing musical sensibility. Confirmation of this view
may be drawn from individual cases. Grant that among a people endowed with
musical faculty to a certain degree, spontaneous variation will
occasionally produce men possessing it in a higher degree; it cannot be
granted that spontaneous variation accounts for the frequent production, by
such highly-endowed men, of men still more highly endowed. On the average,
the children of marriages with others not similarly endowed, will be less
distinguished rather than more distinguished. The most that can be expected
is that this unusual amount of faculty shall re-appear in the next
generation undiminished. How then shall we explain cases like those of
Bach, Mozart, and Beethoven, all of them sons of men having unusual musical
powers who were constantly exercising those powers, and who greatly
excelled their fathers in their musical powers? What shall we say to the
facts that Haydn was the son of an organist, that Hummel was born to a
music master, and that Weber's father was a distinguished violinist? The
occurrence of so many cases in one nation within a short period of time,
cannot rationally be ascribed to the coincidence of "spontaneous
variations." It can be ascribed to nothing but inherited developments of
structure caused by augmentations of function.
But the clearest proof that structural alterations caused by alterations of
function are inherited, occurs when the alterations are morbid. I had
originally named in this place the results of M. Brown-Sequard's
experiments on guinea-pigs, showing that those which had been artificially
made epileptic had offspring which were epileptic; and I name them again
though his inference is by many rejected. For, as exemplified a few pages
back, strong evidence is often disregarded for trivial reasons by those who
dislike the conclusion drawn. Just naming this evidence and its possible
invalidity, let me pass to some results of experiences recently set forth
by Dr. Savage, President of the Neurological Society. In an essay on
"Heredity and Neurosis" published in _Brain_, Parts LXXVII, LXXVIII, 1897,
he says:--"We recognise the transmission of a tendency to develop gout, and
we recognise that the disease produced by the individual himself differs
little from that which may have been inherited." [That is, acquired gout
may be transmitted as constitutional gout.] "I have seen several patients
whose history I have been able to examine carefully, in whom mental tricks
have been transmitted from one generation to another." In the "musical
prodigies" descending from musical parents, "there seemed to be a
transmission of a greatly increased aptitude or tendency which is all one
is contending for." "Though there is, in my opinion, power to transmit
acquired peculiarities, yet the tendency is to transmit a predisposition."
(pp. 19-21.) And an authority on nervous diseases who is second to
none--Dr. Hughlings Jackson--takes the same view. The liability to
consumption shown by children of consumptive parents, which no one doubts,
shows us the same thing. It is admitted that consumption may be produced by
conditions very unfavourable to life; and unless it is held that the
disease so produced differs from the disease when inherited, the conclusion
must be that here, too, there is a transmission of functionally-produced
organic changes. This holds true whether the production of tubercle is due
to innate defect or whether it is due to the invasion of a bacillus. For in
this last case the consumptive diathesis must be regarded as a state of
body more than usually liable to invasion by the bacillus, and this is the
same when acquired as when transmitted.
§ 83. Two modified manifestations of Heredity remain to be noticed. The one
is the re-appearance in offspring of traits not borne by the parents, but
borne by the grandparents or by remoter ancestors. The other is the
limitation of Heredity by sex--the restriction of transmitted peculiarities
to offspring of the same sex as the parent possessing them.
Atavism, which is the name given to the recurrence of ancestral traits, is
proved by many and varied facts. In the picture-galleries of old families,
and on the monumental brasses in the adjacent churches, are often seen
types of feature which are still, from time to time, repeated in members of
these families. It is a matter of common remark that some constitutional
diseases, such as gout and insanity, after missing a generation, will show
themselves in the next. Dr. Struthers, in his above-quoted paper "On
Variation in the Number of Fingers and Toes, and in the Phalanges in Man,"
gives cases of malformations common to grandparent and grandchild, but of
which the parent had no trace. M. Girou (as quoted by Mr. Sedgwick)
says--"One is often surprised to see lambs black, or spotted with black,
born of ewes and rams with white wool, but if one takes the trouble to go
back to the origin of this phenomena, it is found in the ancestors."
Instances still more remarkable, in which the remoteness of the ancestors
copied is very great, are given by Mr. Darwin. He points out that in
crosses between varieties of the pigeon, there will sometimes re-appear the
plumage of the original rock-pigeon, from which these varieties descended;
and he thinks the faint zebra-like markings occasionally traceable in
horses have probably a like meaning.
The other modified manifestation of heredity above referred to is the
limitation of heredity by sex. In Mr. Sedgwick's essays, already named,
will be found evidence implying that there exists some such tendency to
limitation, which does or does not show itself distinctly according to the
nature of the organic modification to be conveyed. On joining to the
evidence he gives certain bodies of allied evidence we shall, I think, find
the inconsistences comprehensible.
Beyond the familiar facts that in ourselves, along with the essential
organs of sex there go minor structures and traits distinctive of sex, such
as the beard and the voice in man, we have numerous cases in which, along
with different sex-organs there go general differences, sometimes immense
and often conspicuous. We have those in which (as in sundry parasites) the
male is extremely small compared with the female; we have those in which
the male is winged and the female wingless; we have those, as among birds,
in which the plumage of males contrasts strongly with that of females; and
among butterflies we have kindred instances in which the wings of the two
sexes are wholly unlike--some, indeed, in which there is not simply
dimorphism but polymorphism: two kinds of females both differing from the
male. How shall we range these facts with the ordinary facts of
inheritance? Without difficulty if heredity results from the proclivity
which the component units contained in a germ-cell or a sperm-cell have to
arrange themselves into a structure like that of the structure from which
they were derived. For the obvious corollary is that where there is
gamogenesis there will result partly concurring and partly conflicting
proclivities. In the fertilized germ we have two groups of physiological
units, slightly different in their structures. These slightly-different
units severally multiply at the expense of the nutriment supplied to the
unfolding germ--each kind moulding this nutriment into units of its own
type. Throughout the process of development the two kinds of units, mainly
agreeing in their proclivities and in the form which they tend to build
themselves into, but having minor differences, work in unison to produce an
organism of the species from which they were derived, but work in
antagonism to produce copies of their respective parent-organisms. And
hence ultimately results an organism in which traits of the one are mixed
with traits of the other; and in which, according to the predominance of
one or other group of units, one or other sex with all its concomitants is
produced.
If so, it becomes comprehensible that with the predominance of either
group, and the production of the same sex as that of the parent whence it
was derived, there will go the repetition not only of the minor sex-traits
of that parent but also of any peculiarities he or she possessed, such as
monstrosities. Since the two groups are nearly balanced, and since
inheritance is never an average of the two parents but a mixture of traits
of the one with traits of the other, it is not difficult to see why there
should be some irregularity in the transmission of these monstrosities and
constitutional tendencies, though they are most frequently transmitted only
to those of the same sex.[37]
§ 84. Unawares in the last paragraph there has been taken for granted the
truth of that suggestion concerning Heredity ventured in § 66. Anything
like a positive explanation is not to be expected in the present stage of
Biology, if at all. We can look for nothing beyond a simplification of the
problem; and a reduction of it to the same category with certain other
problems which also admit of hypothetical solutions only. If an hypothesis
which sundry widespread phenomena have already thrust upon us, can be shown
to render the phenomena of Heredity more intelligible than they at present
seem, we shall have reason to entertain it. The applicability of any method
of interpretation to two different but allied classes of facts, is evidence
of its truth.
The power which many animals display of reproducing lost parts, we saw to
be inexplicable except on the assumption that the units of which any
organism is built have a tendency to arrange themselves into the shape of
that organism (§ 65). This power is sufficiently remarkable in cases where
a lost limb or tail is replaced, but it is still more remarkable in cases
where, as among some annelids, the pieces into which an individual is cut
severally complete themselves by developing heads and tails, or in cases
like that of the _Holothuria_, which having, when alarmed, ejected its
viscera, reproduces them. Such facts compel us to admit that the components
of an organism have a proclivity towards a special structure--that the
adult organism when mutilated exhibits that same proclivity which is
exhibited by the young organism in the course of its normal development. As
before said, we may, for want of a better name, figuratively call this
power organic polarity: meaning by this phrase nothing more than the
observed tendency towards a special arrangement. And such facts as those
presented by the fragments of a _Hydra_, and by fragments of leaves from
which complete plants are produced, oblige us to recognize this proclivity
as existing throughout the tissues in general--nay, in the case of the
_Begonia phyllomaniaca_, obliges us to recognize this proclivity as
existing in the physiological units contained in each undifferentiated
cell. Quite in harmony with this conclusion, are certain implications since
noticed, respecting the characters of sperm-cells and germ-cells. We saw
sundry reasons for rejecting the supposition that these are
highly-specialized cells and for accepting the opposite supposition, that
they are cells differing from others rather in being unspecialized. And
here the assumption to which we seem driven by the _ensemble_ of the
evidence, is, that sperm-cells and germ-cells are essentially nothing more
than vehicles in which are contained small groups of the physiological
units in a fit state for obeying their proclivity towards the structural
arrangement of the species they belong to.
If the likeness of offspring to parents is thus determined, it becomes
manifest, _à priori_, that besides the transmission of generic and specific
peculiarities, there will be a transmission of those individual
peculiarities which, arising without assignable causes, are classed as
"spontaneous." For if the assumption of a special arrangement of parts by
an organism, is due to the proclivity of its physiological units towards
that arrangement; then the assumption of an arrangement of parts slightly
different from that of the species, implies physiological units slightly
unlike those of the species; and these slightly-unlike physiological units,
communicated through the medium of sperm-cell or germ-cell, will tend, in
the offspring, to build themselves into a structure similarly diverging
from the average of the species.
But it is not equally manifest that, on this hypothesis, alterations of
structure caused by alterations of function must be transmitted to
offspring. It is not obvious that change in the form of a part, caused by
changed action, involves such change in the physiological units throughout
the organism that these, when groups of them are thrown off in the shape of
reproductive centres, will unfold into organisms that have this part
similarly changed in form. Indeed, when treating of Adaptation (§ 69), we
saw that an organ modified by increase or decrease of function, can but
slowly re-act on the system at large, so as to bring about those
correlative changes required to produce a new equilibrium; and yet only
when such new equilibrium has been established, can we expect it to be
_fully_ expressed in the modified physiological units of which the organism
is built--only then can we count on a complete transfer of the modification
to descendants. Nevertheless, that changes of structure caused by changes
of action must also be transmitted, however obscurely, appears to be a
deduction from first principles--or if not a specific deduction, still, a
general implication. For if an organism A, has, by any peculiar habit or
condition of life, been modified into the form A', it follows that all the
functions of A', reproductive function included, must be in some degree
different from the functions of A. An organism being a combination of
rhythmically-acting parts in moving equilibrium, the action and structure
of any one part cannot be altered without causing alterations of action and
structure in all the rest; just as no member of the Solar System could be
modified in motion or mass, without producing rearrangements throughout the
whole Solar System. And if the organism A, when changed to A', must be
changed in all its functions; then the offspring of A' cannot be the same
as they would have been had it retained the form A. That the change in the
offspring must, other things equal, be in the same direction as the change
in the parent, appears implied by the fact that the change propagated
throughout the parental system is a change towards a new state of
equilibrium--a change tending to bring the actions of all organs,
reproductive included, into harmony with these new actions. Or, bringing
the question to its ultimate and simplest form, we may say that as, on the
one hand, physiological units will, because of their special polarities,
build themselves into an organism of a special structure; so, on the other
hand, if the structure of this organism is modified by modified function,
it will impress some corresponding modification on the structures and
polarities of its units. The units and the aggregate must act and re-act on
each other. If nothing prevents, the units will mould the aggregate into a
form in equilibrium with their pre-existing polarities. If, contrariwise,
the aggregate is made by incident actions to take a new form, its forces
must tend to re-mould the units into harmony with this new form. And to say
that the physiological units are in any degree so re-moulded as to bring
their polar forces towards equilibrium with the forces of the modified
aggregate, is to say that when separated in the shape of reproductive
centres, these units will tend to build themselves up into an aggregate
modified in the same direction.
NOTE.--A large amount of additional evidence supporting the belief that
functionally produced modifications are inherited, will be found in
Appendix B.
CHAPTER IX.
VARIATION.
§ 85. Equally conspicuous with the truth that every organism bears a
general likeness to its parents, is the truth that no organism is exactly
like either parent. Though similar to both in generic and specific traits,
and usually, too, in those traits which distinguish the variety, it
diverges in numerous traits of minor importance. No two plants are
indistinguishable; and no two animals are without differences. Variation is
co-extensive with Heredity.
The degrees of variation have a wide range. There are deviations so small
as to be not easily detected; and there are deviations great enough to be
called monstrosities. In plants we may pass from cases of slight alteration
in the shape of a leaf, to cases where, instead of a flower with its calyx
above the seed-vessel, there is produced a flower with its calyx below the
seed-vessel; and while in one animal there arises a scarcely noticeable
unlikeness in the length or colour of the hair, in another an organ is
absent or a supernumerary organ appears. Though small variations are by far
the most general, yet variations of considerable magnitude are not
uncommon; and even those variations constituted by additions or
suppressions of parts, are not so rare as to be excluded from the list of
causes by which organic forms are changed. Cattle without horns are
frequent. Of sheep there are horned breeds and breeds that have lost their
horns. At one time there existed in Scotland a race of pigs with solid feet
instead of cleft feet. In pigeons, according to Mr. Darwin, "the number of
the caudal and sacral vertebræ vary; as does the number of the ribs,
together with their relative breadth and the presence of processes."
That variations, both small and large, which arise without any specific
assignable cause, tend to become hereditary, was shown in the last chapter.
Indeed the evidence which proves Heredity in its smaller manifestations is
the same evidence which proves Variation; since it is only when there occur
variations that the inheritance of anything beyond the structural
peculiarities of the species can be proved. It remains here, however, to be
observed that the transmission of variations is itself variable; and that
it varies both in the direction of decrease and in the direction of
increase. An individual trait of one parent may be so counteracted by the
influence of the other parent, that it may not appear in the offspring; or,
not being so counteracted, the offspring may possess it, perhaps in an
equal degree or perhaps in a less degree; or the offspring may exhibit the
trait in even a still higher degree. Among illustrations of this, one must
suffice. I quote it from the essay by Sir J. Struthers referred to in the
last chapter.
"The great-great-grandmother, Esther P---- (who married A---- L----), had a
sixth little finger on one hand. Of their eighteen children (twelve
daughters and six sons), only one (Charles) is known to have had digital
variety. We have the history of the descendants of three of the sons,
Andrew, Charles, and James.
"(1.) Andrew L---- had two sons, Thomas and Andrew; and Thomas had two sons
all without digital variety. Here we have three successive generations
without the variety possessed by the great-grandmother showing itself.
"(2.) James L----, who was normal, had two sons and seven daughters, also
normal. One of the daughters became Mrs. J---- (one of the informants), and
had three daughters and five sons, all normal except one of the sons, James
J----, now æt. 17, who had six fingers on each hand....
"In this branch of the descendants of Esther, we see it passing over two
generations and reappearing in one member of the third generation, and now
on both hands.
"(3.) Charles L----, the only child of Esther who had digital variety, had
six fingers on each hand. He had three sons, James, Thomas, and John, all
of whom were born with six fingers on each hand, while John has also a
sixth toe on one foot. He had also five other sons and four daughters, all
of whom were normal.
"(a.) Of the normal children of this, the third generation, the five sons
had twelve sons and twelve daughters, and the four daughters have had four
sons and four daughters, being the fourth generation, all of whom were
normal. A fifth generation in this sub-group consists as yet of only two
boys and two girls who are also normal.
"In this sub-branch, we see the variety of the first generation present in
the second, passing over the third and fourth, and also the fifth as far as
it has yet gone.
"(b.) James had three sons and two daughters, who are normal.
"(c.) Thomas had four sons and five daughters, who are normal; and has two
grandsons, also normal.
"In this sub-branch of the descent, we see the variety of the first
generation, showing itself in the second and third, and passing over the
fourth, and (as far as it yet exists) the fifth generation.
"(d.) John L---- (one of the informants) had six fingers, the additional
finger being attached on the outer side, as in the case of his brothers
James and Thomas. All of them had the additional digits removed. John has
also a sixth toe on one foot, situated on the outer side. The fifth and
sixth toes have a common proximal phalange, and a common integument invests
the middle and distal phalanges, each having a separate nail.
"John L---- has a son who is normal, and a daughter, Jane, who was born
with six fingers on each hand and six toes on each foot. The sixth fingers
were removed. The sixth toes are not wrapped with the fifth as in her
father's case, but are distinct from them. The son has a son and daughter,
who, like himself, are normal.
"In this, the most interesting sub-branch of the descent, we see digital
increase, which appeared in the first generation on one limb, appearing in
the second on two limbs, the hands; in the third on three limbs, the hands
and one foot; in the fourth on all the four limbs. There is as yet no fifth
generation in uninterrupted transmission of the variety. The variety does
not yet occur in any member of the fifth generation of Esther's
descendants, which consists, as yet, only of three boys and one girl, whose
parents were normal, and of two boys and two girls, whose grandparents were
normal. It is not known whether in the case of the great-great-grandmother,
Esther P----, the variety was original or inherited."[38]
§ 86. Where there is great uniformity among the members of a species, the
divergences of offspring from the average type are usually small; but
where, among the members of a species, considerable unlikenesses have once
been established, unlikenesses among the offspring are frequent and great.
Wild plants growing in their natural habitats are uniform over large areas,
and maintain from generation to generation like structures; but when
cultivation has caused appreciable differences among the members of any
species of plant, extensive and numerous deviations are apt to arise.
Similarly, between wild and domesticated animals of the same species, we
see the contrast that though the homogeneous wild race maintains its type
with great persistence, the comparatively heterogeneous domestic race
frequently produces individuals more unlike the average type than the
parents are.
Though unlikeness among progenitors is one antecedent of variation, it is
by no means the sole antecedent. Were it so, the young ones successively
born to the same parents would be alike. If any peculiarity in a new
organism were a direct resultant of the structural differences between the
two organisms which produced it; then all subsequent new organisms produced
by these two would show the same peculiarity. But we know that the
successive offspring have different peculiarities: no two of them are ever
exactly alike.
One cause of such structural variation in progeny, is functional variation
in parents. Proof of this is given by the fact that, among progeny of the
same parents, there is more difference between those begotten under
different constitutional states than between those begotten under the same
constitutional state. It is notorious that twins are more nearly alike than
children borne in succession. The functional conditions of the parents
being the same for twins, but not the same for their brothers and sisters
(all other antecedents being constant), we have no choice but to admit that
variations in the functional conditions of the parents, are the antecedents
of those greater unlikenesses which their brothers and sisters exhibit.
Some other antecedent remains, however. The parents being the same, and
their constitutional states the same, variation, more or less marked, still
manifests itself. Plants grown from seeds out of one pod, or animals
produced at one birth, are not alike. Sometimes they differ considerably.
In a litter of pigs or of kittens, we rarely see uniformity of markings;
and occasionally there are important structural contrasts. I have myself
recently been shown a litter of Newfoundland puppies, some of which had
four digits to their feet, while in others there was present, on each
hind-foot, what is called the "dew-claw"--a rudimentary fifth digit.
Thus, induction points to three causes of variation, all in action
together. We have heterogeneity among progenitors, which, did it act
uniformly and alone in generating, by composition of forces, new
deviations, would impress such new deviations to the same extent on all
offspring of the same parents; which it does not. We have functional
variation in the parents, which, acting either alone or in combination with
the preceding cause, would entail the same structural variations on all
young ones simultaneously produced; which it does not. Consequently there
is some third cause of variation, yet to be found, which acts along with
the structural and functional variations of ancestors and parents.
§ 87. Already, in the last section, there has been implied some relation
between variation and the action of external conditions. The above-cited
contrast between the uniformity of a wild species and the multiformity of
the same species when cultivated or domesticated, thrusts this truth upon
us. Respecting the variations of plants, Mr. Darwin remarks that "'sports'
are extremely rare under nature, but far from rare under cultivation."
Others who have studied the matter assert that if a species of plant which,
up to a certain time, has maintained great uniformity, once has its
constitution thoroughly disturbed, it will go on varying indefinitely.
Though, in consequence of the remoteness of the periods at which they were
domesticated, there is a lack of positive proof that our extremely variable
domestic animals have become variable under the changed conditions implied
by domestication, having been previously constant; yet competent judges do
not doubt that this has been the case.
Now the constitutional disturbance which precedes variation, can be nothing
else than an overthrowing of the pre-established equilibrium of functions.
Transferring a plant from forest lands to a ploughed field or a manured
garden, is altering the balance of forces to which it has been hitherto
subject, by supplying it with different proportions of the assimilable
matters it requires, and taking away some of the positive impediments to
its growth which competing wild plants before offered. An animal taken from
woods or plains, where it lived on wild food of its own procuring, and
placed under restraint while artificially supplied with food not quite like
what it had before, is an animal subject to new outer actions to which its
inner actions must be adjusted. From the general law of equilibration we
found it to follow that "the maintenance of such a moving equilibrium" as
an organism displays, "requires the habitual genesis of internal forces
corresponding in number, directions, and amounts, to the external incident
forces--as many inner functions, single or combined, as there are single or
combined outer actions to be met" (_First Principles_, § 173); and more
recently (§ 27), we have seen that Life itself is "the definite combination
of heterogeneous changes, both simultaneous and successive, in
correspondence with external co-existences and sequences." Necessarily,
therefore, an organism exposed to a permanent change in the arrangement of
outer forces must undergo a permanent change in the arrangement of inner
forces. The old equilibrium has been destroyed; and a new equilibrium must
be established. There must be functional perturbations, ending in a
re-adjusted balance of functions.
If, then, change of conditions is the only known cause by which the
original homogeneity of a species is destroyed; and if change of conditions
can affect an organism only by altering its functions; it follows that
alteration of functions is the only known internal cause to which the
commencement of variation can be ascribed. That such minor functional
changes as parents undergo from year to year are influential on the
offspring, we have seen is proved by the greater unlikeness that exists
between children born to the same parents at different times, than exists
between twins. And here we seem forced to conclude that the larger
functional variations produced by greater external changes, are the
initiators of those structural variations which, when once commenced in a
species, lead by their combinations and antagonisms to multiform results.
Whether they are or are not the direct initiators, they must still be the
indirect initiators.
§ 87a. In the foregoing sentence those pronounced structural variations
from which may presently arise new varieties and eventually species, are
ascribed to "the larger functional variations produced by greater external
changes"; and this limitation is a needful one, since there is a constant
cause of minor variations of a wholly different kind.
There are the variations arising from differences in the conditions to
which the germ is subject, both before detachment from the parent and
after. At first sight it seems that plants grown from seeds out of the same
seed-vessel and animals belonging to the same litter, ought, in the absence
of any differences of ancestral antecedents, to be entirely alike. But this
is not so. Inevitably they are subject from the very outset to slightly
different sets of agencies. The seeds in a seed-vessel do not stand in
exactly the same relations to the sources of nutriment: some are nearer
than others. They are somewhat differently exposed to the heat and light
penetrating their envelope; and some are more impeded in their growth by
neighbours than others are. Similarly with young animals belonging to the
same litter. Their uterine lives are made to some extent unlike by unlike
connexions with the blood-supply, by mutual interferences not all the same,
and even by different relations to the disturbances caused by the mother's
movements. So, too, is it after separation from the parent plant or animal.
Even the biblical parable reminds us that seeds fall into places here
favourable and there unfavourable in various degrees. In respect of soil,
in respect of space for growth, in respect of shares of light, none of them
are circumstanced in quite the same ways. With animals the like holds. In a
litter of pigs some, weaker than others, do not succeed as often in getting
possession of teats. And then in both cases the differences thus initiated
become increasingly pronounced. Among young plants the smaller, outgrown by
their better-placed neighbours, are continually more shaded and more left
behind; and among the litter the weakly ones, continually thrust aside by
the stronger, become relatively more weakly from deficient nutrition.
Differentiations thus arising, both before and after separation from
parents, though primarily differences of growth, entail structural
differences; for it is a general law of nutrition that when there is
deficiency of food the non-essential organs suffer more than the essential
ones, and the unlikenesses of proportion hence arising constitute
unlikenesses of structure. It may be concluded, however, that variations
generated in this manner usually have no permanent results. In the first
place, the individuals which, primarily in growth and secondarily in
smaller developments of less-important organs, are by implication inferior,
are likely to be eliminated from the species. In the second place,
differences of structure produced in the way shown do not express
differences of constitution--are not the effects of somewhat divergent
physiological units; and consequently are not likely to be repeated in
posterity.
§ 88. We have still, therefore, to explain those variations which have no
manifest causes of the kinds thus far considered. These are the variations
termed "spontaneous." Not that those who apply to them this word, or some
equivalent, mean to imply that they are uncaused. Mr. Darwin expressly
guards himself against such an interpretation. He says:--"I have hitherto
sometimes spoken as if the variations--so common and multiform in organic
beings under domestication, and in a lesser degree in those in a state of
nature--had been due to chance. This, of course, is a wholly incorrect
expression, but it serves to acknowledge plainly our ignorance of the cause
of each particular variation." Not only, however, do I hold, in common with
Mr. Darwin, that there must be some cause for these apparently-spontaneous
variations, but it seems to me that a definite cause is assignable. I think
it may be shown that unlikenesses must necessarily arise even between the
new individuals simultaneously produced by the same parents. Instead of the
occurrence of such variations being inexplicable, the absence of them would
be inexplicable.
In any series of dependent changes a small initial difference often works a
marked difference in the results. The mode in which a particular breaker
bursts on the beach, may determine whether the seed of some foreign plant
which it bears is or is not stranded--may cause the presence or absence of
this plant from the Flora of the land; and may so affect, for millions of
years, in countless ways, the living creatures throughout the land. A
single touch, by introducing into the body some morbid matter, may set up
an immensely involved set of functional disturbances and structural
alterations. The whole tenor of a life may be changed by a word of advice;
or a glance may determine an action which alters thoughts, feelings, and
deeds throughout a long series of years. In those still more involved
combinations of changes which societies exhibit, this truth is still more
conspicuous. A hair's-breadth difference in the direction of some soldier's
musket at the battle of Arcola, by killing Napoleon, might have changed
events throughout Europe; and though the type of social organization in
each European country would have been now very much what it is, yet in
countless details it would have been different.
Illustrations like these, with which pages might be filled, prepare us for
the conclusion that organisms produced by the same parents at the same
time, must be more or less differentiated, both by insensible initial
differences and by slight differences in the conditions to which they are
subject during their evolution. We need not, however, rest with assuming
such initial differences: the necessity of them is demonstrable. The
individual germ-cells which, in succession or simultaneously, are separated
from the same parent, can never be exactly alike; nor can the sperm-cells
which fertilize them. When treating of the instability of the homogeneous
(_First Principles_, § 149), we saw that no two parts of any aggregate can
be similarly conditioned with respect to incident forces; and that being
subject to forces that are more or less unlike, they must become more or
less unlike. Hence, no two ova in an ovarium or ovules in a seed-vessel--no
two spermatozoa or pollen-cells, can be identical. Whether or not there
arise other contrasts, there are certain to arise quantitative contrasts;
since the process of nutrition cannot be absolutely alike for all. The
reproductive centres must begin to differentiate from the very outset. Such
being the necessities of the case, what will happen on any successive or
simultaneous fertilizations? Inevitably unlikenesses between the respective
parental influences must result. Quantitative differences among the
sperm-cells and among the germ-cells, will insure this. Grant that the
number of physiological units contained in any one reproductive cell, can
rarely if ever be exactly equal to the number contained in any other,
ripened at the same time or at a different time; and it follows that among
the fertilized germs produced by the same parents, the physiological units
derived from them respectively will bear a different numerical ratio to
each other in every case. If the parents are constitutionally quite alike,
the variation in the ratio between the units they severally bequeath,
cannot cause unlikenesses among the offspring. But if otherwise, no two of
the offspring can be alike. In every case the small initial difference in
the proportions of the slightly-unlike units, will lead, during evolution,
to a continual multiplication of differences. The insensible divergence at
the outset will generate sensible divergences at the conclusion. Possibly
some may hence infer that though, in such case, the offspring must differ
somewhat from each other and from both parents, yet that in every one of
them there must result a homogeneous mixture of the traits of the two
parents. A little consideration shows that the reverse is inferable. If,
throughout the process of development, the physiological units derived from
each parent preserved the same ratio in all parts of the growing organism,
each organ would show as much as every other, the influence of either
parent. But no such uniform distribution is possible. It has been shown
(_First Principles_, § 163), that in any aggregate of mixed units
segregation must inevitably go on. Incident forces will tend ever to cause
separation of the two orders of units from each other--will tend to
integrate groups of the one order in one place and groups of the other
order in another place. Hence there must arise not a homogeneous mean
between the two parents, but a mixture of organs, some of which mainly
follow the one and some the other. And this is the kind of mixture which
observation shows us.
Still it may be fairly objected that however the attributes of the two
parents are variously mingled in their offspring, they must in all of them
fall between the extremes displayed in the parents. In no characteristic
could one of the young exceed both parents, were there no cause of
"spontaneous variation" but the one alleged. Evidently, then, there is a
cause yet unfound.
§ 89. Thus far we have contemplated the process under its simplest aspect.
While we have assumed the two parents to be somewhat unlike, we have
assumed that each parent has a homogeneous constitution--is built up of
physiological units which are exactly alike. But in no case can such a
homogeneity exist. Each parent had parents who were more or less
contrasted--each parent inherited at least two orders of physiological
units not quite identical. Here then we have a further cause of variation.
The sperm-cells or germ-cells which any organism produces, will differ from
each other not quantitatively only but qualitatively. Of the
slightly-unlike physiological units bequeathed to it, the reproductive
cells it casts off cannot habitually contain the same proportions; and we
may expect the proportions to vary not slightly but greatly. Just as,
during the evolution of an organism, the physiological units derived from
the two parents tend to segregate, and produce likeness to the male parent
in this part and to the female parent in that; so, during the formation of
reproductive cells, there will arise in one a predominance of the
physiological units derived from the father, and in another a predominance
of the physiological units derived from the mother. Thus, then, every
fertilized germ, besides containing different _amounts_ of the two parental
influences, will contain different _kinds_ of influences--this having
received a marked impress from one grandparent, and that from another.
Without further exposition the reader will see how this cause of
complication, running back through each line of ancestry, must produce in
every germ numerous minute differences among the units.
Here, then, we have a clue to the multiplied variations, and sometimes
extreme variations, that arise in races which have once begun to vary. Amid
countless different combinations of units derived from parents, and through
them from ancestors, immediate and remote--amid the various conflicts in
their slightly-different organic polarities, opposing and conspiring with
one another in all ways and degrees; there will from time to time arise
special proportions causing special deviations. From the general law of
probabilities it may be concluded that while these involved influences,
derived from many progenitors, must, on the average of cases, obscure and
partially neutralize one another; there must occasionally result such
combinations of them as will produce considerable divergences from average
structures; and, at rare intervals, such combinations as will produce very
marked divergences. There is thus a correspondence between the inferable
results and the results as habitually witnessed.
§ 90. Still there remains a difficulty. It may be said that admitting
functional change to be the initiator of variation--granting that the
physiological units of an organism long subject to new conditions, will
tend to become modified in such way as to cause change of structure in
offspring; yet there will still be no cause of the supposed heterogeneity
among the physiological units of different individuals. There seems
validity in the objection, that as all the members of a species whose
circumstances have been altered will be affected in the same manner, the
results, when they begin to show themselves in descendants, will show
themselves in the same manner: not multiform variations will arise, but
deviations all in one direction.
The reply is simple. The members of a species thus circumstanced will _not_
be similarly affected. In the absence of absolute uniformity among them,
the functional changes caused in them will be more or less dissimilar. Just
as men of slightly-unlike dispositions behave in quite opposite ways under
the same circumstances; or just as men of slightly-unlike constitutions get
diverse disorders from the same cause, and are diversely acted on by the
same medicine; so, the insensibly-differentiated members of a species whose
conditions have been changed, may at once begin to undergo various kinds of
functional changes. As we have already seen, small initial contrasts may
lead to large terminal contrasts. The intenser cold of the climate into
which a species has migrated, may cause in one individual increased
consumption of food to balance the greater loss of heat; while in another
individual the requirement may be met by a thicker growth of fur. Or, when
meeting with the new foods which a new region furnishes, accident may
determine one member of the species to begin with one kind and another
member with another kind; and hence may arise established habits in these
respective members and their descendants. Now when the functional
divergences thus set up in sundry families of a species have lasted long
enough to affect their constitutions, and to modify somewhat the
physiological units thrown off in their reproductive cells, the divergences
produced by these in offspring will be of divers kinds. And the original
homogeneity of constitution having been thus destroyed, variation may go on
with increasing facility. There will result a heterogeneous mixture of
modifications of structure caused by modifications of function; and of
still more numerous correlated modifications, indirectly so caused. By
natural selection of the most divergent forms, the unlikenesses of parents
will be rendered more marked, and the limits of variation wider. Until at
length the divergences of constitutions and modes of life, become great
enough to lead to segregation of the varieties.
§ 91. That variations must occur, and that they must ever tend, both
directly and indirectly, towards adaptive modifications, are conclusions
deducible from first principles; apart from any detailed interpretations
like the above. That the state of homogeneity is an unstable state we have
found to be a universal truth. Each species must pass from the uniform into
the more or less multiform, unless the incidence of external forces is
exactly the same for all its members, which it never can be. Through the
process of differentiation and integration, which of necessity brings
together, or keeps together, like individuals, and separates unlike ones
from them, there must nevertheless be maintained a tolerably uniform
species, so long as there continues a tolerably uniform set of conditions
in which it may exist. But if the conditions change, either absolutely by
some disturbance of the habitat or relatively by spread of the species into
other habitats, then the divergent individuals that result must be
segregated by the divergent sets of conditions into distinct varieties
(_First Principles_, § 166). When, instead of contemplating a species in
the aggregate, we confine our attention to a single member and its
descendants, we see it to be a corollary from the general law of
equilibration that the moving equilibrium constituted by the vital actions
in each member of this family, must remain constant so long as the external
actions to which they correspond remain constant; and that if the external
actions are changed, the disturbed balance of internal changes, if not
overthrown, cannot cease undergoing modification until the internal changes
are again in equilibrium with the external actions: corresponding
structural alterations having arisen.
On passing from these derivative laws to the ultimate law, we see that
Variation is necessitated by the persistence of force. The members of a
species inhabiting any area cannot be subject to like sets of forces over
the whole of that area. And if, in different parts of the area, different
kinds or amounts or combinations of forces act on them, they cannot but
become different in themselves and in their progeny. To say otherwise, is
to say that differences in the forces will not produce differences in the
effects; which is to deny the persistence of force.
CHAPTER X.
GENESIS, HEREDITY, AND VARIATION.
§ 92. A question raised, and hypothetically answered, in §§ 78 and 79, was
there postponed until we had dealt with the topics of Heredity and
Variation. Let us now resume the consideration of this question, in
connexion with sundry others which the facts suggest.
After contemplating the several methods by which the multiplication of
organisms is carried on--after ranging them under the two heads of
Homogenesis, in which the successive generations are similarly produced,
and Heterogenesis, in which they are dissimilarly produced--after observing
that Homogenesis is nearly always sexual genesis, while Heterogenesis is
asexual genesis with occasionally-recurring sexual genesis; we came to the
questions--why is it that some organisms multiply in the one way and some
in the other? and why is it that where agamogenesis prevails it is usually,
from time to time, interrupted by gamogenesis? In seeking answers to these
questions, we inquired whether there are common to both Homogenesis and
Heterogenesis, any conditions under which alone sperm-cells and germ-cells
arise and are united for the production of new organisms; and we reached
the conclusion that, in all cases, they arise only when there is an
approach to equilibrium between the forces which produce growth and the
forces which oppose growth. This answer to the question--_when_ does
gamogenesis recur? still left unanswered the question--_why_ does
gamogenesis recur? And to this the reply suggested was, that the approach
towards general equilibrium in organisms, "is accompanied by an approach
towards molecular equilibrium in them; and that the need for this union of
sperm-cell with germ-cell is the need for overthrowing this equilibrium,
and re-establishing active molecular change in the detached germ--a result
probably effected by mixing the slightly-different physiological units of
slightly-different individuals." This is the hypothesis which we have now
to consider. Let us first look at the evidences which certain inorganic
phenomena furnish.
The molecules of any aggregate which have not a balanced arrangement,
inevitably tend towards a balanced arrangement. As before mentioned (_First
Principles_, § 100), amorphous wrought iron, when subject to continuous
jar, begins to arrange itself into crystals--its atoms assume a condition
of polar equilibrium. The particles of unannealed glass, which are so
unstably arranged that slight disturbing forces make them separate into
small groups, take advantage of that greater freedom of movement given by a
raised temperature, to adjust themselves into a state of relative rest.
During any such re-arrangement the aggregate exercises a coercive force
over its units. Just as in a growing crystal the atoms successively
assimilated from the solution, are made by the already crystallized atoms
to take a certain form, and even to re-complete that form when it is
broken; so in any mass of unstably-arranged atoms which passes into a
stable arrangement, each atom conforms to the forces exercised on it by all
the other atoms. This is a corollary from the general law of equilibration.
We saw (_First Principles_, § 170) that every change is towards
equilibrium; and that change can never cease until equilibrium is reached.
Organisms, above all other aggregates, conspicuously display this
progressive equilibration; because their units are of such kinds, and so
conditioned, as to admit of easy re-arrangement. Those extremely active
changes which go on during the early stages of evolution, imply an immense
excess of the molecular forces over those antagonist forces which the
aggregate exercises on the molecules. While this excess continues, it is
expended in growth, development, and function: expenditure for any of these
purposes being proof that part of the force constituting molecular tensions
remains unbalanced. Eventually, however, this excess diminishes. Either, as
in organisms which do not expend much energy, decrease of assimilation
leads to its decline; or, as in organisms which expend much energy, it is
counterbalanced by the rapidly-increasing reactions of the aggregate
(§ 46). The cessation of growth when followed, as in some organisms, by
death, implies the arrival at an equilibrium between the molecular forces
and those forces which the aggregate opposes to them. When, as in other
organisms, growth ends in the establishment of a moving equilibrium, there
is implied such a decreased preponderance of the molecular forces, as
leaves no surplus beyond that which is used up in functions. The declining
functional activity characteristic of advancing life, expresses a further
decline in this surplus. And when all vital movements come to an end, the
implication is that the actions of the units on the aggregate and the
reactions of the aggregate on the units are completely balanced. Hence,
while a state of rapid growth indicates such a play of forces among the
units of an aggregate as will produce active re-distribution, the
diminution and arrest of growth shows that the units have fallen into such
relative positions that re-distribution is no longer so facile. When,
therefore, we see that gamogenesis recurs only when growth is decreasing,
or has come to an end, we must say that it recurs only when the organic
units are approximating to equilibrium--only when their mutual restraints
prevent them from readily changing their arrangements in obedience to
incident forces.
That units of like forms can be built up into a more stable aggregate than
units of slightly unlike forms, is tolerably manifest _à priori_. And we
have facts which prove that mixing allied but somewhat different units,
_does_ lead to comparative instability. Most metallic alloys exemplify this
truth. Common solder, which is a mixture of lead and tin, melts at a much
lower temperature than either lead or tin. The compound of lead, tin, and
bismuth, called "fusible metal," becomes fluid at the temperature of
boiling water; while the temperatures at which lead, tin, and bismuth
become fluid are, respectively, 612°, 442°, and 497° F. Still more
remarkable is the illustration furnished by potassium and sodium. These
metals are very near akin in all respects--in their specific gravities,
their atomic weights, their chemical affinities, and the properties of
their compounds. That is to say, all the evidences unite to show that their
units, though not identical, have a close resemblance. What now happens
when they are mixed? Potassium alone melts at 136°, sodium alone melts at
190°, but the alloy of potassium and sodium is liquid at the ordinary
temperature of the air. Observe the meaning of these facts, expressed in
general terms. The maintenance of a solid form by any group of units
implies among them an arrangement so stable that it is not overthrown by
the incident forces. Whereas the assumption of a liquid form implies that
the incident forces suffice to destroy the arrangement of the units. In the
one case the thermal undulations fail to dislocate the parts; while in the
other case the parts are so dislocated by the thermal undulations that they
fall into total disorder--a disorder admitting of easy re-arrangement into
any other order. For the liquid state is a state in which the units become
so far free from mutual restraints, that incident forces can change their
relative positions very readily. Thus we have reason to conclude that an
aggregate of units which, though in the main similar to one another, have
minor differences, must be more unstable than an aggregate of homogeneous
units. The one will yield to disturbing forces which the other successfully
resists.
Now though the colloidal molecules of which organisms are mainly built, are
themselves highly composite; and though the physiological units compounded
out of these colloidal molecules must have structures far more involved;
yet it must happen with such units, as with simple units, that those which
have exactly like forms will admit of arrangement into a more stable
aggregate than those which have slightly-unlike forms. Among units of this
order, as among units of a simpler order, imperfect similarity must entail
imperfect balance in anything formed of them, and consequent diminished
ability to withstand disturbing forces. Hence, given two organisms which,
by diminished nutrition or increased expenditure, are being arrested in
their growths--given in each an approaching equilibrium between the forces
of the units and the forces of the aggregate--given, that is, such a
comparatively balanced state among the units that re-arrangement of them by
incident forces is no longer so easy; and it will follow that by uniting a
group of units from the one organism with a group of slightly-different
units from the other, the tendency towards equilibrium will be diminished,
and the mixed units will be rendered more modifiable in their arrangements
by the forces acting on them: they will be so far freed as to become again
capable of that re-distribution which constitutes evolution.
And now let us test this hypothesis by seeing what power it gives us of
interpreting established inductions.
§ 93. The majority of plants being hermaphrodites, it has, until quite
recently, been supposed that the ovules of each flower are fertilized by
pollen from the anthers of the same flower. Mr. Darwin, however, has shown
that the arrangements are generally such as to prevent this. Either the
ovules and the pollen are not ripe simultaneously, or obstacles prevent
access of the one to the other. At the same time he has shown that there
exist arrangements, often of a remarkable kind, which facilitate the
transfer of pollen by insects from the stamens of one flower to the pistil
of another. Similarly, it has been found that among the lower animals,
hermaphrodism does not usually involve the production of fertile ova by the
union of sperm-cells and germ-cells developed in the same individual; but
that the reproductive centres of one individual are united with those of
another to produce fertile ova. Either, as in _Pyrosoma_, _Perophora_, and
in many higher molluscs, the ova and spermatozoa are matured at different
times; or, as in annelids, they are prevented by their relative positions
from coming in contact.
Remembering the fact that among the higher classes of organisms,
fertilization is always effected by combining the sperm-cell of one
individual with the germ-cell of another; and joining with it the above
fact that among hermaphrodite organisms, the germ-cells developed in any
individual are usually not fertilized by sperm-cells developed in the same
individual; we see reason for thinking that the essential thing in
fertilization, is the union of specially-fitted portions of _different_
organisms. If fertilization depended on the peculiar properties of
sperm-cell and germ-cell, as such; then, in hermaphrodite organisms, it
would be a matter of indifference whether the united sperm-cells and
germ-cells were those of the same individual or those of different
individuals. But the circumstance that there exist in such organisms
elaborate appliances for mutual fertilization, shows that unlikeness of
derivation in the united reproductive centres, is the desideratum. Now this
is just what the foregoing hypothesis implies. If, as was concluded,
fertilization has for its object the disturbance of that approaching
equilibrium existing among the physiological units separated from an adult
organism; and if, as we saw reason to think, this object is effected by
mixture with the slightly-different physiological units of another
organism; then, we at the same time see that this object will not be
effected by mixture with physiological units belonging to the same
organism. Thus, the hypothesis leads us to expect such provisions as we
find.
§ 94. But here a difficulty presents itself. These propositions seem to
involve the conclusion that self-fertilization is impossible. It apparently
follows from them, that a group of physiological units from one part of an
organism ought to have no power of altering the state of approaching
balance in a group from another part of it. Yet self-fertilization does
occur. Though the ovules of one plant are generally fertilized by pollen
from another plant of the same kind, yet they may be, some of them,
fertilized by pollen of the same plant; and, indeed, there are plants in
which self-fertilization is the rule: even provision being in some cases
made to prevent fertilization by pollen from other individuals. And though,
among hermaphrodite animals, self-fertilization is usually negatived by
structural or functional arrangements, yet in certain _Entozoa_ there
appear to be special provisions by which the sperm-cells and the germ-cells
of the same individual may be united, when not previously united with those
of another individual. Nay, it has even been shown that in certain
Ascidians the contents of oviduct and spermiduct of the same individual
produce, when united, fertile ova whence evolve perfect individuals.
Certainly, at first sight, these facts do not consist with the above
supposition. Nevertheless there is something like a solution.
In the last chapter, when considering the variations caused in offspring
from uniting elements representing unlike parental constitutions, it was
pointed out that in an unfolding organism, composed of slightly-different
physiological units derived from slightly-different parents, there cannot
be maintained an even distribution of the two orders of units. We saw that
the instability of the homogeneous negatives the uniform blending of them;
and that, by the process of differentiation and integration, they must be
more or less separated; so that in one part of the body the influence of
one parent will predominate, and in another part of the body the influence
of the other parent: an inference which harmonizes with daily observation.
We also saw that the sperm-cells or germ-cells produced by such an organism
must, in virtue of these same laws, be more or less unlike one another. It
was shown that through segregation, some of the sperm-cells or germ-cells
will get an excess of the physiological units derived from one side, and
some of them an excess of those derived from the other side: a cause which
accounts for the unlikenesses among offspring simultaneously produced. Now
from this segregation of the different orders of physiological units,
inherited from different parents and lines of ancestry, there arises the
possibility of self-fertilization in hermaphrodite organisms. If the
physiological units contained in the sperm-cells and germ-cells of the same
flower, are not quite homogeneous--if in some of the ovules the
physiological units derived from the one parent greatly predominate, and in
some of the ovules those derived from the other parent; and if the like is
true of the pollen-cells; then, some of the ovules may be nearly as much
contrasted with some of the pollen-cells in the characters of their
contained units, as were the ovules and pollen-cells of the parents from
which the plant proceeded. Between part of the sperm-cells and part of the
germ-cells, the community of nature will be such that fertilization will
not result from their union; but between some of them, the differences of
constitution will be such that their union will produce the requisite
molecular instability. The facts, so far as they are known, seem in harmony
with this deduction. Self-fertilization in flowers, when it takes place, is
not so efficient as mutual fertilization. Though some of the ovules produce
seeds, yet more of them than usual are abortive. From which, indeed,
results the establishment of varieties that have structures favourable to
mutual fertilization; since, being more prolific, these have, other things
equal, greater chances in the "struggle for existence."
Further evidence is at hand supporting this interpretation. There is reason
to believe that self-fertilization, which at the best is comparatively
inefficient, loses all efficiency in course of time. After giving an
account of the provisions for an occasional, or a frequent, or a constant
crossing between flowers; and after quoting Prof. Huxley to the effect that
among hermaphrodite animals, there is no case in which "the occasional
influence of a distinct individual can be shown to be physically
impossible;" Mr. Darwin writes--"from these several considerations and from
the many special facts which I have collected, but which I am not here able
to give, I am strongly inclined to suspect that, both in the vegetable and
animal kingdoms, an occasional intercross with a distinct individual is a
law of nature ... in none, as I suspect, can self-fertilization go on for
perpetuity." This conclusion, based wholly on observed facts, is just the
conclusion to which the foregoing argument points. That necessary action
and the re-action between the parts of an organism and the organism as a
whole--that power of an aggregate to re-mould the units, which is the
correlative of the power of the units to build up into such an aggregate;
implies that any differences existing among the units inherited by an
organism, must gradually diminish. Being subject in common to the total
forces of the organism, they will in common be modified towards congruity
with these forces, and therefore towards likeness with one another. If,
then, in a self-fertilizing organism and its self-fertilizing descendants,
such contrasts as originally existed among the physiological units are
progressively obliterated--if, consequently, there can no longer be a
segregation of different physiological units in different sperm-cells and
germ-cells; self-fertilization will become impossible. Step by step the
fertility will diminish, and the series will finally die out.
And now observe, in confirmation of this view, that self-fertilization is
limited to organisms in which an approximate equilibrium among the organic
forces is not long maintained. While growth is actively going on, and the
physiological units are subject to a continually-changing distribution of
forces, no decided assimilation of the units can be expected: like forces
acting on the unlike units will tend to segregate them, so long as
continuance of evolution permits further segregation; and only when further
segregation cannot go on, will the like forces tend to assimilate the
units. Hence, where there is no prolonged maintenance of an approximate
organic balance, self-fertilization may be possible for some generations;
but it will be impossible in organisms distinguished by a sustained moving
equilibrium.
§ 95. The interpretation which it affords of sundry phenomena familiar to
breeders of animals, adds probability to the hypothesis. Mr. Darwin has
collected a large "body of facts, showing, in accordance with the almost
universal belief of breeders, that with animals and plants a cross between
different varieties, or between individuals of the same variety but of
another strain, gives vigour and fertility to the offspring; and on the
other hand, that _close_ interbreeding diminishes vigour and fertility,"--a
conclusion harmonizing with the current belief respecting
family-intermarriages in the human race. Have we not here a solution of
these facts? Relations must, on the average of cases, be individuals whose
physiological units are more nearly alike than usual. Animals of different
varieties must be those whose physiological units are more unlike than
usual. In the one case, the unlikeness of the units may frequently be
insufficient to produce fertilization; or, if sufficient to produce
fertilization, not sufficient to produce that active molecular change
required for vigorous development. In the other case, both fertilization
and vigorous development will be made probable.
Nor are we without a cause for the irregular manifestations of these
general tendencies. The mixed physiological units composing any organism
being, as we have seen, more or less segregated in the reproductive centres
it throws off; there may arise various results according to the degrees of
difference among the units, and the degrees in which the units are
segregated. Of two cousins who have married, the common grandparents may
have had either similar or dissimilar constitutions; and if their
constitutions were dissimilar, the probability that their married
grandchildren will have offspring will be greater than if their
constitutions were similar. Or the brothers and sisters from whom these
cousins descended, instead of severally inheriting the constitutions of
their parents in tolerably equal degrees, may have severally inherited them
in very different degrees: in which last case, intermarriages among the
cousins will be less likely to prove infertile. Or the brothers and sisters
from whom these cousins descended, may severally have married persons very
like, or very unlike, themselves; and from this cause there may have
resulted, either an undue likeness, or a due unlikeness, between the
married cousins.[39] These several causes, conspiring and conflicting in
endless ways and degrees, will work multiform effects. Moreover,
differences of segregation will make the reproductive centres produced by
the same nearly-related organisms, vary considerably in their amounts of
unlikeness; and therefore, supposing their amounts of unlikeness great
enough to cause fertilization, this fertilization will be effective in
various degrees. Hence it may happen that among offspring of nearly-related
parents, there may be some in which the want of vigour is not marked, and
others in which there is decided want of vigour. So that we are alike shown
why in-and-in breeding tends to diminish both fertility and vigour: and why
the effect cannot be a uniform effect, but only an average effect.
§ 96. While, if the foregoing arguments are valid, gamogenesis has for its
main result the initiation of a new development by the overthrow of that
approximate equilibrium arrived at among the molecules of the
parent-organisms, a further result appears to be subserved by it. Those
inferior organisms which habitually multiply by agamogenesis, have
conditions of life that are simple and uniform; while those organisms which
have highly-complex and variable conditions of life, habitually multiply by
gamogenesis. Now if a species has complex and variable conditions of life,
its members must be severally exposed to sets of conditions that are
slightly different: the aggregates of incident forces cannot be alike for
all the scattered individuals. Hence, as functional deviation must ever be
inducing structural deviation, each individual throughout the area occupied
tends to become fitted for the particular habits which its particular
conditions necessitate; and in so far, _un_fitted for the average habits
proper to the species. But these undue specializations are continually
checked by gamogenesis. As Mr. Darwin remarks, "intercrossing plays a very
important part in nature in keeping the individuals of the same species, or
of the variety, true and uniform in character:" the idiosyncratic
divergences obliterate one another. Gamogenesis, then, is a means of
turning to positive advantage the individual differentiations which, in its
absence, would result in positive disadvantage. Were it not that
individuals are ever being made unlike one another by their unlike
conditions, there would not arise in them those contrasts of molecular
constitution, which we have seen to be needful for producing the fertilized
germs of new individuals. And were not these individual differentiations
ever being mutually cancelled, they would end in a fatal narrowness of
adaptation.
This truth will be most clearly seen if we reduce it to its purely abstract
form, thus:--Suppose a quite homogeneous species, placed in quite
homogeneous conditions; and suppose the constitutions of all its members in
complete concord with their absolutely-uniform and constant conditions;
what must happen? The species, individually and collectively, is in a state
of perfect moving equilibrium. All disturbing forces have been eliminated.
There remains no force which can, in any way, change the state of this
moving equilibrium; either in the species as a whole or in its members. But
we have seen (_First Principles_, § 173) that a moving equilibrium is but a
transition towards complete equilibration, or death. The absence of
differential or un-equilibrated forces among the members of a species, is
the absence of all forces which can cause changes in the conditions of its
members--is the absence of all forces which can initiate new organisms. To
say, as above, that complete molecular homogeneity existing among the
members of a species, must render impossible that mutual molecular
disturbance which constitutes fertilization, is but another way of saying
that the actions and re-actions of each organism, being in perfect balance
with the actions and re-actions of the environment upon it, there remains
in each organism no force by which it differs from any other--no force
which any other does not meet with an equal force--no force which can set
up a new evolution among the units of any other.
And so we reach the remarkable conclusion that the life of a species, like
the life of an individual, is maintained by the unequal and ever-varying
actions of incident forces on its different parts.[40] An individual
homogeneous throughout, and having its substance everywhere continuously
subject to like actions, could undergo none of those changes which life
consists of; and similarly, an absolutely-uniform species, having all its
members exposed to identical influences, would be deprived of that
initiator of change which maintains its existence as a species. Just as, in
each organism, incident forces constantly produce divergences from the mean
state in various directions, which are constantly balanced by opposite
divergences indirectly produced by other incident forces; and just as the
combination of rhythmical functions thus maintained, constitutes the life
of the organism; so, in a species, there is, through gamogenesis, a
perpetual neutralization of those contrary deviations from the mean state
which are caused in its different parts by different sets of incident
forces; and it is similarly by the rhythmical production and compensation
of these contrary deviations, that the species continues to live. The
moving equilibrium in a species, like the moving equilibrium in an
individual, would rapidly end in complete equilibration, or death, were not
its continually-dissipated forces continually re-supplied from without.
Besides owing to the external world those energies which, from moment to
moment, keep up the lives of its individual members, every species owes to
certain more indirect actions of the external world, those energies which
enable it to perpetuate itself in successive generations.
§ 97. What evidence still remains may be conveniently woven up along with a
recapitulation of the argument pursued through the last three chapters. Let
us contemplate the facts in their synthetic order.
That compounding and re-compounding through which we pass from the simplest
inorganic substances to the most complex organic substances, has several
concomitants. Each successive stage of composition presents us with
molecules that are severally larger or more integrated, that are severally
more heterogeneous, that are severally more unstable, and that are more
numerous in their kinds (_First Principles_, § 151). And when we come to
the substances of which living bodies are formed, we find ourselves among
innumerable divergent groups and sub-groups of compounds, the units of
which are large, heterogeneous, and unstable, in high degrees. There is no
reason to assume that this process ends with the formation of those complex
colloids which constitute organic matter. A more probable assumption is
that out of the complex colloidal molecules there are evolved, by a still
further integration, molecules which are still more heterogeneous, and of
kinds which are still more multitudinous. What must be their properties?
Already the colloidal molecules are extremely unstable--capable of being
variously modified in their characters by very slight incident forces; and
already the complexity of their polarities prevents them from readily
falling into such positions of equilibrium as results in crystallization.
Now the organic molecules composed of these colloidal molecules, must be
similarly characterized in far higher degrees. Far more numerous must be
the minute changes that can be wrought in them by minute external forces;
far more free must they remain for a long time to obey forces tending to
re-distribute them; and far greater must be the number of their kinds.
Setting out with these physiological units, the existence of which various
organic phenomena compel us to recognize, and the production of which the
general law of Evolution thus leads us to anticipate; we get an insight
into the phenomena of Genesis, Heredity, and Variation. If each organism is
built of certain of these highly-plastic units peculiar to its
species--units which slowly work towards an equilibrium of their complex
proclivities, in producing an aggregate of the specific structure, and
which are at the same time slowly modifiable by the re-actions of this
aggregate--we see why the multiplication of organisms proceeds in the
several ways, and with the various results, which naturalists have
observed.
Heredity, as shown not only in the repetition of the specific structure but
in the repetition of ancestral deviations from it, becomes a matter of
course; and it falls into unison with the fact that, in various inferior
organisms, lost parts can be replaced, and that, in still lower organisms,
a fragment can develop into a whole.
While an aggregate of physiological units continues to grow by the
assimilation of matter which it moulds into other units of like type; and
while it continues to undergo changes of structure; no equilibrium can be
arrived at between the whole and its parts. Under these conditions, then,
an un-differentiated portion of the aggregate--a group of physiological
units not bound up into a specialized tissue--will be able to arrange
itself into the structure peculiar to the species; and will so arrange
itself, if freed from controlling forces and placed in fit conditions of
nutrition and temperature. Hence the continuance of agamogenesis in
little-differentiated organisms, so long as assimilation continues to be
greatly in excess of expenditure.
But let growth be checked and development approach its completion--let the
units of the aggregate be severally exposed to an almost constant
distribution of forces; and they must begin to equilibrate themselves.
Arranged, as they will gradually be, into comparatively stable attitudes in
relation to one another, their mobility will diminish; and groups of them,
partially or wholly detached, will no longer readily re-arrange themselves
into the specific form. Agamogenesis will be no longer possible; or, if
possible, will be no longer easy.
When we remember that the force which keeps the Earth in its orbit is the
gravitation of each particle in the Earth towards every one of the group of
particles existing 92,000,000 of miles off; we cannot reasonably doubt that
each unit in an organism acts on all the other units, and is reacted on by
them: not by gravitation only but chiefly by other energies. When, too, we
learn that glass has its molecular constitution changed by light, and that
substances so rigid and stable as metals have their atoms re-arranged by
forces radiated in the dark from adjacent objects;[41] we are obliged to
conclude that the excessively-unstable units of which organisms are built,
must be sensitive in a transcendant degree to all the forces pervading the
organisms composed of them--must be tending ever to re-adjust, not only
their relative attitudes but their molecular structures, into equilibrium
with these forces. Hence, if aggregates of the same species are differently
conditioned, and re-act differently on their component units, their
component units will be rendered somewhat different; and they will become
the more different the more widely the re-actions of the aggregates upon
them differ, and the greater the number of generations through which these
different re-actions of the aggregates upon them are continued.
If, then, unlikenesses of function among individuals of the same species,
produce unlikenesses between the physiological units of one individual and
those of another, it becomes comprehensible that when groups of units
derived from two individuals are united, the group formed will be more
unstable than either of the groups was before their union. The mixed units
will be less able to resist those re-distributing forces which cause
evolution; and may thus have restored to them the capacity for development
which they had lost.
This view harmonizes with the conclusion, which we saw reason to draw, that
fertilization does not depend on any intrinsic peculiarities of sperm-cells
and germ-cells, but depends on their derivation from different individuals.
It explains the facts that nearly-related individuals are less likely to
have offspring than others, and that their offspring, when they have them,
are frequently feeble. And it gives us a key to the converse fact that the
crossing of varieties results in unusual vigour.
Bearing in mind that the slightly-different orders of physiological units
which an organism inherits from its parents, are subject to the same set of
forces, and that when the organism is fully developed this set of forces,
becoming constant, tends slowly to re-mould the two orders of units into
the same form; we see how it happens that self-fertilization becomes
impossible in the higher organisms, while it remains possible in the lower
organisms. In long-lived creatures which have tolerably-definite limits of
growth, this assimilation of the somewhat-unlike physiological units is
liable to go on to an appreciable extent; whereas in organisms which do not
continuously subject their component units to constant forces, there will
be much less of this assimilation. And where the assimilation is not
considerable, the segregation of mixed units may cause the sperm-cells and
germ-cells developed in the same individual, to be sufficiently different
to produce, by their union, fertile germs; and several generations of
self-fertilizing descendants may succeed one another, before the two orders
of units have had their unlikenesses so far diminished that they will no
longer do this. The same principles explain for us the variable results of
union between nearly-related organisms. According to the contrasts among
the physiological units they inherit from parents and ancestors; according
to the unlike proportions of the contrasted units which they severally
inherit; and according to the degrees of segregation of such units in
different sperm-cells and germ-cells; it may happen that two kindred
individuals will produce the ordinary number of offspring or will produce
none; or will at one time be fertile and at another not; or will at one
time have offspring of tolerable strength and at another time feeble
offspring.
To the like causes are also ascribable the phenomena of Variation. These
are unobtrusive while the tolerably-uniform conditions of a species
maintain tolerable uniformity among the physiological units of its members;
but they become obtrusive when differences of conditions, entailing
considerable functional differences, have entailed decided differences
among the physiological units, and when the different physiological units,
differently mingled in every individual, come to be variously segregated
and variously combined.
Did space permit, it might be shown that this hypothesis is a key to many
further facts--to the fact that mixed races are comparatively plastic under
new conditions; to the fact that pure races show predominant influences in
the offspring when crossed with mixed races; to the fact that while mixed
breeds are often of larger growth, pure breeds are the more hardy--have
functions less-easily thrown out of balance. But without further argument
it will, I think, be admitted that the power of this hypothesis to explain
so many phenomena, and to bring under a common bond phenomena which seem so
little allied, is strong evidence of its truth. And such evidence gains
greatly in strength on observing that this hypothesis brings the facts of
Genesis, Heredity, and Variation into harmony with first principles. We see
that these plastic physiological units, which we find ourselves obliged to
assume, are just such more integrated, more heterogeneous, more unstable,
and more multiform molecules, as would result from continuance of the steps
through which organic matter is reached. We see that the differentiations
of them assumed to occur in differently-conditioned aggregates, and the
equilibrations of them assumed to occur in aggregates which maintain
constant conditions, are but corollaries from those universal principles
implied by the persistence of force. We see that the maintenance of life in
the successive generations of a species, becomes a consequence of the
continual incidence of new forces on the species, to replace the forces
that are ever being rhythmically equilibrated in the propagation of the
species. And we thus see that these apparently-exceptional phenomena
displayed in the multiplication of organic beings, fall into their places
as results of the general laws of Evolution. We have, therefore, weighty
reasons for entertaining the hypothesis which affords us this
interpretation.
CHAPTER X^A.
GENESIS, HEREDITY, AND VARIATION
_CONCLUDED_.
§ 97a. Since the foregoing four chapters were written, thirty-four years
ago, the topics with which they deal have been widely discussed and many
views propounded. Ancient hypotheses have been abandoned, and other
hypotheses, referring tacitly or avowedly to the cell-doctrine, have been
set forth. Before proceeding it will be well to describe the chief among
these.
Most if not all of them proceed on the assumption, shown in § 66 to be
needful, that the structural characters of organisms are determined by the
special natures of units which are intermediate between the chemical units
and the morphological units--between the invisible molecules of
proteid-substances and the visible tissue-components called cells.
Four years after the first edition of this volume was published, appeared
Mr. Darwin's work, _The Variation of Animals and Plants under
Domestication_; and in this he set forth his doctrine of Pangenesis.
Referring to the doctrine of physiological units which the preceding
chapters work out, he at first expressed a doubt whether his own was or was
not the same, but finally concluded that it was different. He was right in
so concluding. Throughout my argument the implication everywhere is that
the physiological units are all of one kind; whereas Mr. Darwin regards his
component units, or "gemmules," as being of innumerable unlike kinds. He
supposes that every cell of every tissue gives off gemmules special to
itself, and capable of developing into similar cells. We may here, in
passing, note that this view implies a fundamental distinction between
unicellular organisms and the component cells of multicellular organisms,
which are otherwise homologous with them. For while in their essential
structures, their essential internal changes, and their essential processes
of division, the _Protozoa_ and the component units of the _Metazoa_ are
alike, the doctrine of Pangenesis implies that though the units when
separate do not give off invisible gemmules the grouped units do.
Much more recently have been enunciated the hypotheses of Prof. Weismann,
differing from the foregoing hypotheses in two respects. In the first place
it is assumed that the fragment of matter out of which each organism arises
consists of two portions--one of them, the germ-plasm, reserved within the
generative organ of the incipient individual, representing in its
components the structure of the species, and gives origin to the germs of
future individuals; and the other of them, similarly representative of the
specific structure, giving origin to the rest of the body, or soma, but
contains in its components none of those latent powers possessed by those
of the germ-plasm. In the second place the germ-plasm, in common with the
soma-plasm, consists of multitudinous kinds of units portioned out to
originate the various organs. Of these there are groups, sub-groups, and
sub-sub-groups. The largest of them, called "idants," are supposed each to
contain a number of "ids"; within each id there are numerous
"determinants"; and each determinant is made up of many "biophors"--the
smallest elements possessing vitality. Passing over details, the essential
assumption is that there exists a separate determinant for each part of the
organism capable of independent variation; and Prof. Weismann infers that
while there may be but one for the blood and but one for a considerable
area of skin (as a stripe of the zebra) there must be a determinant for
each scale on a butterfly's wing: the number on the four wings being over
two hundred thousand. And then each cluster of biophors composing a
determinant has to find its way to the place where there is to be formed
the part it represents.
Here it is needless to specify the modifications of these hypotheses
espoused by various biologists--all of them assuming that the structural
traits of each species are expressed in certain units intermediate between
morphological units and chemical units.
§ 97b. A true theory of heredity must be one which recognizes the relevant
phenomena displayed by all classes of organism. We cannot assume two kinds
of heredity, one for plants and another for animals. Hence a theory of
heredity may be first tested by observing whether it is equally applicable
to both kingdoms of living things. Genesis, heredity, and variation, as
seen in plants, are simpler and more accessible than as seen in animals.
Let us then note what these imply.
Already in § 77 I have illustrated the power which some plants possess of
developing new individuals from mere fragments of leaves and even from
detached scales. Striking as are the facts there instanced, they are
scarcely more significant than some which are familiar. The formation of
cauline buds, presently growing into shoots, shows us a kind of inheritance
which a true theory must explain. As described by Kerner, such buds arise
in Pimpernel, Toad-flax, etc., below the seed-leaves, even while yet there
are no axils in which buds usually grow; and in many plants they arise from
intermediate places on the stem: that is, without definite relations to
pre-existing structures. How fortuitous is their origin is shown when a
branch is induced to bud by keeping it wrapped round with a wet cloth. Even
still better proved is the absence of any relation between cauline buds and
normal germs by the frequent growth of them out of "callus"--the tissue
which spreads over wounds and the cut ends of branches. It is not easy to
reconcile these facts with Mr. Darwin's hypothesis of gemmules. We have to
assume that where a cauline bud emerges there are present in due
proportions gemmules of all the parts which will presently arise from
it--leaves, stipules, bracts, petals, stamens, anthers, etc. We have to
assume this though, at the time the bud originates, sundry of these organs,
as the parts of flowers, do not exist on the plant or tree. And we have to
assume that the gemmules of such parts are duly provided in a portion of
adventitious callus, far away from the normal places of fructification.
Moreover, the resulting shoot may or may not produce all the parts which
the gemmules represent; and when, perhaps after years, flowers are produced
on its side shoots, there must exist at each point the needful proportion
of the required gemmules; though there have been no cells continually
giving them off.
Still less does the hypothesis of Prof. Weismann harmonize with the
evidence as plants display it. Plant-embryogeny yields no sign of
separation between germ-plasm and soma-plasm; and, indeed, the absence of
such separation is admitted. After instancing cases among certain of the
lower animals, in which no differentiation of the two arises in the first
generation resulting from a fertilized ovum, Prof. Weismann continues:--
"The same is true as regards the higher plants, in which the first shoot
arising from the seed never contains germ-cells, or even cells which
subsequently become differentiated into germ cells. In all these
last-mentioned cases the germ-cells are not present in the first person
arising by embryogeny as special cells, but are only formed in much later
cell-generations from the offspring of certain cells of which this first
person was composed." (_Germ-Plasm_, p. 185.)
How this admission consists with the general theory it is difficult to
understand. The units of the soma-plasm are here recognized as having the
same generative powers as the units of the germ-plasm. In so far as one
organic kingdom and a considerable part of the other are concerned the
doctrine is relinquished. Relinquishment is, indeed, necessitated even by
the ordinary facts, and still more by the facts just instanced. Defence of
it involves the assertion that where buds arise, normal or cauline, there
exist in due proportion the various ids with their contained
determinants--that these are diffused throughout the growing part of the
soma; and this implies that the somatic tissue does not differ in
generative power from the germ-plasm.
The hypothesis of physiological units, then, remains outstanding. For
cauline buds imply that throughout the plant-tissue, where not unduly
differentiated, the local physiological units have a power of arranging
themselves into the structure of the species.
But this hypothesis, too, as it now stands, is inadequate. Under the form
thus far given to it, it fails to explain some accompanying facts. For if
the branch just instanced as producing a cauline bud be cut off and its end
stuck in the ground, or if it be bent down and a portion of it covered with
earth, there will grow from it rootlets and presently roots. The same
portion of tissue which otherwise would have produced a shoot with all its
appendages, constituting an individual, now produces only a special part of
an individual.
§ 97c. Certain kindred facts of animal development may now be considered.
Similar insufficiencies are disclosed.
The often-cited reproduction of a crab's lost claw or a lizard's tail, Mr.
Darwin thought explicable by his hypothesis of diffused gemmules,
representing all organs or their component cells. But though, after simple
amputation, regrowth of the proximate part of the tail is conceivable as
hence resulting, it is not easy to understand how the remoter part, the
components of which are now absent from the organism, can arise afresh from
gemmules no longer originated in due proportion. Prof. Weismann's
hypothesis, again, implies that there must exist at the place of
separation, a ready-provided supply of determinants, previously latent,
able to reproduce the missing tail in all its details--nay, even to do this
several times over: a strong supposition! The hypothesis of physiological
units, as set forth in preceding chapters, appears less incompetent:
reproduction of the lost part would seem to be a normal result of the
proclivity towards the form of the entire organism. But now what are we to
say when, instead of being cut off transversely, the tail is divided
longitudinally and each half becomes a complete tail? What are we to say
when, if these two tails are similarly dealt with, the halves again
complete themselves; and so until as many as sixteen tails have been
formed? Here the hypothesis of physiological units appears to fail utterly;
for the tendency it implies is to complete the specific form, by
reproducing a single tail only.
Various annulose animals display anomalies of development difficult to
explain on any hypothesis. We have creatures like the fresh-water _Nais_
which, though it has advanced structures, including a vascular system,
branchiæ, and a nervous cord ending with cephalic ganglia, nevertheless
shows us an ability like that of the _Hydra_ to reproduce the whole from a
small part: nearly forty pieces into which a _Nais_ was cut having
severally grown into complete animals. Again we have, in the order
_Polychætæ_, types like _Myrianida_, in which by longitudinal budding a
string of individuals, sometimes numbering even thirty, severally develop
certain segments into heads, while increasing their segments in number. In
yet other types there occurs not longitudinal gemmation only, but lateral
gemmation: a segment will send out sideways a bud which presently becomes a
complete worm. Once more, _Syllis ramosa_ is a species in which the
individual worms growing from lateral buds, while remaining attached to the
parent, themselves give origin to buds; and so produce a branched aggregate
of worms. How shall we explain the reparative and reproductive powers thus
exemplified? It seems undeniable that each portion has an ability to
produce, according to circumstances, the whole creature or a missing part
of the creature. When we read of Sir J. Dalyell that he "cut a _Dasychone_
into three pieces; the hindermost produced a head, the anterior piece
developed an anus, and the middle portion formed both a head and a tail" we
are not furnished with an explanation by the hypothesis of gemmules or by
the hypothesis of determinants; for we cannot arbitrarily assume that
wherever a missing organ has to be produced there exists the needful supply
of gemmules or of determinants representing that organ. The hypothesis that
physiological units have everywhere a proclivity towards the organic form
of the species, appears more congruous with the facts; but even this does
not cover the cases in which a new worm grows from a lateral bud. The
tendency to complete the individual structure might be expected rather to
restrain this breaking of the lines of complete structure.
Still less explicable in any way thus far proposed are certain remedial
actions seen in animals. An example of them was furnished in § 67, where
"false joints" were described--joints formed at places where the ends of a
broken bone, failing to unite, remain moveable one upon the other.
According to the character of the habitual motions there results a rudely
formed hinge-joint or a ball-and-socket joint, either having the various
constituent parts--periosteum, fibrous tissue, capsule, ligaments. Now Mr.
Darwin's hypothesis, contemplating only normal structures, fails to account
for this formation of an abnormal structure. Neither can we ascribe this
local development to determinants: there were no appropriate ones in the
germ-plasm, since no such structure was provided for. Nor does the
hypothesis of physiological units, as presented in preceding chapters,
yield an interpretation. These could have no other tendency than to restore
the normal form of the limb, and might be expected to oppose the genesis of
these new parts.
Thus we have to seek, if not another hypothesis, then some such
qualification of an existing hypothesis as will harmonize it with various
exceptional phenomena.
§ 97d. In Part II of the _Principles of Sociology_, published in 1876, will
be found elaborated in detail that analogy between individual organization
and social organization which was briefly sketched out in an essay on "The
Social Organism" published in 1860. In §§ 241-3 a parallel is drawn between
the developments of the sustaining systems of the two; and it is pointed
out how, in the one case as in the other, the components--here organic
units and there citizens--have their activities and arrangements mainly
settled by local conditions. One leading example is that the parts
constituting the alimentary canal, while jointly fitted to the nature of
the food, are severally adapted to the successive stages at which the food
arrives in its progress; and that in an analogous way the industries
carried on by peoples forming different parts of a society, are primarily
determined by the natures of things around--agriculture, pastoral and
arable, special manufactures and minings, ship-building and fishing: the
respective groups falling into fit combinations and becoming partially
modified to suit their work. The implication is that while the organization
of a society as a whole depends on the characters of its units, in such way
that by some types of men despotisms are always evolved while by other
types there are evolved forms of government partially free--forms which
repeat themselves in colonies--there is, on the other hand, in every case a
local power of developing appropriate structures. And it might have been
pointed out that similarly in types of creatures not showing much
consolidation, as the _Annelida_, many of the component divisions, largely
independent in their vitalities, are but little affected in their
structures by the entire aggregate.
My purpose at that time being the elucidation of sociological truths, it
did not concern me to carry further the biological half of this comparison.
Otherwise there might have been named the case in which a supernumerary
finger, beginning to bud out, completes itself as a local organ with bones,
muscles, skin, nail, etc., in defiance of central control: even repeating
itself when cut off. There might also have been instanced the above-named
formation of a false joint with its appurtenances. For the implication in
both cases is that a local group of units, determined by circumstances
towards a certain structure, coerces its individual units into that
structure.
Now let us contemplate the essential fact in the analogy. The men in an
Australian mining-camp, as M. Pierre Leroy Beaulieu points out, fall into
Anglo-Saxon usages different from those which would characterize a French
mining-camp. Emigrants to a far West settlement in America quickly
establish post-office, bank, hotel, newspaper, and other urban
institutions. We are thus shown that along with certain traits leading to a
general type of social organization, there go traits which independently
produce fit local organizations. Individuals are led into occupations and
official posts, often quite new to them, by the wants of those around--are
now influenced and now coerced into social arrangements which, as shown
perhaps by gambling saloons, by shootings at sight, and by lynchings, are
scarcely at all affected by the central government. Now the physiological
units in each species appear to have a similar combination of capacities.
Besides their general proclivity towards the specific organization, they
show us abilities to organize themselves locally; and these abilities are
in some cases displayed in defiance of the general control, as in the
supernumerary finger or the false joint. Apparently each physiological
unit, while having in a manner the whole organism as the structure which,
along with the rest, it tends to form, has also an aptitude to take part in
forming any local structure, and to assume its place in that structure
under the influence of adjacent physiological units.
A familiar fact supports this conclusion. Everyone has at hand, not
figuratively but literally, an illustration. Let him compare the veins on
the backs of his two hands, either with one another or with the veins on
another person's hands, and he will see that the branchings and
inosculations do not correspond: there is no fixed pattern. But on
progressing inwards from the extremities, the distribution of the veins
becomes settled--there is a pattern-arrangement common to all persons.
These facts imply a predominating control by adjacent parts where control
by the aggregate is less easy. A constant combination of forces which,
towards the centre, produces a typical structure, fails to do this at the
periphery where, during development, the play of forces is less settled.
This peripheral vascular structure, not having become fixed because one
arrangement is as good as another, is in each determined by the immediately
surrounding influences.
§ 97e. And now let us contemplate the verifications which recent
experiments have furnished--experiments made by Prof. G. Born of Breslau,
confirming results earlier reached by Vulpian and adding more striking
results of kindred nature. They leave no longer doubtful the large share
taken by local organizing power as distinguished from central organizing
power.
The independent vitality shown by separated portions of ventral skin from
frog-larvæ may be named as the first illustration. With their attached
yolk-cells these lived for days, and underwent such transformations as
proved some structural proclivity, though of course the product was
amorphous. Detached portions of tails of larvæ went on developing their
component parts in much the same ways as they would have done if remaining
attached. More striking still was the evidence furnished by experiments in
grafting. These proved that the undifferentiated rudiment of an organ will,
when cut off and joined to a non-homologous place in another individual,
develop itself as it would have done if left in its original place. In
brief, then, we may say that each part is in chief measure autogenous.
These strange facts presented by small aggregates of organic matter, which
are the seats of extremely complex forces, will seem less incomprehensible
if we observe what has taken place in a vast aggregate of inorganic matter
which is the seat of very simple forces--the Solar System. Transcendently
different as this is in all other respects, it is analogous in the respect
that, as factors of local structures, local influences predominate over the
influences of the aggregate. For while the members of the Solar System,
considered as a whole, are subordinate to the totality of its forces, the
arrangements in each part of it are produced almost wholly by the play of
forces in that part. Though the Sun affects the motions of the Moon, and
though during the evolution of the Earth-and-Moon system the Sun exercised
an influence, yet the relations of our world and its satellite in respect
of masses and motions were in the main locally determined. Still more
clearly was it thus with Jupiter and his satellites or Saturn with his
rings and satellites. Remembering that the ultimate units of matter of
which the Solar System is composed are of the same kinds, and that they act
on one another in conformity with the same laws, we see that, remote as the
case is from the one we are considering in all other respects, it is
similar in the respect that during organization the energies in each
locality work effects which are almost independent of the effects worked by
the general energies. In this vast aggregate, as in the minute aggregates
now in question, the parts are practically autogenous.
Having thus seen that in a way we have not hitherto recognized the same
general principles pervade inorganic and organic evolution, let us revert
to the case of super-organic evolution from which a parallel was drawn
above. As analogous to the germinal mass of units out of which a new
organism is to evolve, let us take an assemblage of colonists not yet
socially organized but placed in a fertile region--men derived from a
society (or rather a succession of societies) of long-established type, who
have in their adapted natures the proclivity towards that type. In passing
from its wholly unorganized state to an organized state, what will be the
first step? Clearly this assemblage, though it may have within the
constitutions of its units the potentialities of a specific structure, will
not develop forthwith the details of that structure. The inherited natures
of its units will first show themselves by separating into large groups
devoted to strongly-distinguished occupations. The great mass, dispersing
over promising lands, will make preparations for farming. Another
considerable portion, prompted by the general needs, will begin to form a
cluster of habitations and a trading centre. Yet a third group, recognizing
the demand for wood, alike for agricultural and building purposes, will
betake themselves to the adjacent forests. But in no case will the primary
assemblage, before these separations, settle the arrangements and actions
of each group: it will leave each group to settle them for itself. So, too,
after these divisions have arisen. The agricultural division will not as a
whole prescribe the doings of its members. Spontaneous segregation will
occur: some going to a pastoral region and some to a tract which promises
good crops. Nor within each of these bodies will the organization be
dictated by the whole. The pastoral group will separate itself into
clusters who tend sheep on the hills and clusters who feed cattle on the
plains. Meanwhile those who have gravitated towards urban occupations will
some of them make bricks or quarry stone, while others fall into classes
who build walls, classes who prepare fittings, classes who supply
furniture. Then along with completion of the houses will go occupation of
them by men who bake bread, who make clothing, who sell liquors, and so on.
Thus each great group will go on organizing itself irrespective of the
rest; the sub-groups within each will do the same; and so will the
sub-sub-groups. Quite independently of the people on the hills and the
plains and in the town, those in the forest will divide spontaneously into
parties who cut down trees, parties who trim and saw them, parties who
carry away the timbers; while every party forms for itself an organization
of "butty" or "boss," and those who work under him. Similarly with the
ultimate divisions--the separate families: the arrangements and
apportionments of duties in each are internally determined. Mark the fact
which here chiefly concerns us. This formation of a heterogeneous aggregate
with its variously adapted parts, which while influenced by the whole are
mainly self-formed, goes on among units of essentially the same natures,
inherited from units who belonged to similar societies. And now, carrying
this conception with us, we may dimly perceive how, in a developing embryo,
there may take place the formation, first of the great divisions--the
primary layers--then of the outlines of systems, then of component organs,
and so on continually with the minor structures contained in major
structures; and how each of these progressively smaller divisions develops
its own organization, irrespective of the changes going on throughout the
rest of the embryo. So that though all parts are composed of physiological
units of the same nature, yet everywhere, in virtue of local conditions and
the influence of its neighbours, each unit joins in forming the particular
structure appropriate to the place. Thus conceiving the matter, we may in a
vague way understand the strange facts of autogenous development disclosed
by the above named experiments.
§ 97f. "But how immeasurably complex must be the physiological units which
can behave thus!" will be remarked by the reader. "To be able to play all
parts, alike as members of the whole and as members of this or that organ,
they must have an unimaginable variety of potentialities in their natures.
Each must, indeed, be almost a microcosm within a microcosm."
Doubtless this is true. Still we have a _consensus_ of proofs that the
component units of organisms have constitutions of extremely involved
kinds. Contemplate the facts and their implications. (1) Here is some large
division of the animal kingdom--say the _Vertebrata_. The component units
of all its members have certain fundamental traits in common: all of them
have proclivities towards formation of a vertebral column. Leaving behind
the great assemblage of Fishes with its multitudinous types, each having
special units of composition, we pass to the _Amphibia_, in the units of
which there exist certain traits superposed upon the traits they have in
common with those of Fishes. Through unknown links we ascend to incipient
Mammalian types and then to developed Mammalian types, the units of which
must have further superposed traits. Additional traits distinguish the
units of each Mammalian order; and, again, those of every genus included in
it; while others severally characterize the units of each species.
Similarly with the varieties in each species, and the stirps in each
variety. Now the ability of any component unit to carry within itself the
traits of the sub-kingdom, class, order, genus, species, variety, and at
the same time to bear the traits of immediate ancestors, can exist only in
a something having multitudinous proximate elements arranged in innumerable
ways. (2) Again, these units must be at once in some respects fixed and in
other respects plastic. While their fundamental traits, expressing the
structure of the type, must be unchangeable, their superficial traits must
admit of modification without much difficulty; and the modified traits,
expressing variations in the parents and immediate ancestors, though
unstable, must be considered as capable of becoming stable in course of
time. (3) Once more we have to think of these physiological units (or
constitutional units as I would now re-name them) as having such natures
that while a minute modification, representing some small change of local
structure, is inoperative on the proclivities of the units throughout the
rest of the system, it becomes operative in the units which fall into the
locality where that change occurs.
But unimaginable as all this is, the facts may nevertheless in some way
answer to it. As before remarked, progressing science reveals complexity
within complexity--tissues made up of cells, cells containing nuclei and
cytoplasm, cytoplasm formed of a protoplasmic matrix containing granules;
and if now we conclude that the unit of protoplasm is itself an
inconceivably elaborate structure, we do but recognize the complexity as
going still deeper. Further, if we must assume that these component units
are in every part of the body acting on one another by extremely
complicated sets of forces (ethereal undulations emanating from each of the
constituent molecules) determining their relative positions and actions, we
are warranted by the discoveries which every day disclose more of the
marvellous properties of matter. When to such examples as were given in
§ 36e we add the example yielded by recent experiments, showing that even a
piece of bread, after subjection to pressure, exhibits diamagnetic
properties unlike those it previously exhibited, we cannot doubt that these
complex units composing living bodies are all of them seats of energies
diffused around, enabling them to act and re-act so as to modify one
another's states and positions. We are shown, too, that whatever be the
natures of the complex forces emanating from each, it will, as a matter of
course, happen that the power of each will be relatively great in its own
neighbourhood and become gradually smaller in parts increasingly remote:
making more comprehensible the autogenous character of each local
structure.
Whatever be their supposed natures we are compelled to ascribe extreme
complexity to these unknown somethings which have the power of organizing
themselves into a structure of this or that species. If gemmules be
alleged, then the ability of every organ and part of an organ to vary,
implies that the gemmules it gives off are severally capable of receiving
minute modifications of their ordinary structures: they must have many
parts admitting of innumerable relations. Supposing determinants be
assumed, then in addition to the complexity which each must have to express
in itself the structure of the part evolved from it, it must have the
further complexity implied by every superposed modification which causes a
variation of that part. And, as we have just seen, the hypothesis of
physiological units does not relieve us from the need for kindred
suppositions.
One more assumption seems necessary if we are to imagine how changes of
structure caused by changes of function can be transmitted. Reverting to
§ 54d, where an unceasing circulation of protoplasm throughout an organism
was inferred, we must conceive that the complex forces of which each
constitutional unit is the centre, and by which it acts on other units
while it is acted on by them, tend continually to remould each unit into
congruity with the structures around: superposing on it modifications
answering to the modifications which have arisen in those structures.
Whence is to be drawn the corollary that in course of time all the
circulating units,--physiological, or constitutional if we prefer so to
call them--visiting all parts of the organism, are severally made bearers
of traits expressing local modifications; and that those units which are
eventually gathered into sperm-cells and germ-cells also bear these
superposed traits.
If against all this it be urged that such a combination of structures and
forces and processes is inconceivably involved, then the reply is that so
astonishing a transformation as that which an unfolding organism displays
cannot possibly be effected by simple agencies.
§ 97g. But now let it be confessed that none of these hypotheses serves to
render the phenomena really intelligible; and that probably no hypothesis
which can be framed will do this. Many problems beyond those which
embryology presents have to be solved; and no solution is furnished.
What are we to say of the familiar fact that certain small organs which,
with the approach to maturity, become active, entail changes of structure
in remote parts--that after the testes have undergone certain final
developments, the hairs on the chin grow and the voice deepens? It has been
contended that certain concomitant modifications in the fluids throughout
the body may produce correlated sexual traits; and there is proof that in
many of the lower animals the period of sexual activity is accompanied by a
special bodily state--sometimes such that the flesh becomes unwholesome and
even poisonous. But a change of this kind can hardly account for a
structural change in the vocal organs in Man. No hypothesis of gemmules or
determinants or physiological units enables us to understand how removal of
the testes prevents those developments of the larynx and vocal cords which
take place if they remain.
The inadequacy of our explanations we at once see in presence of a
structure like a peacock's tail-feather. Mr. Darwin's hypothesis is that
all parts of every organ are continually giving off gemmules, which are
consequently everywhere present in their due proportions. But a completed
feather is an inanimate product and, once formed, can add to the
circulating fluids no gemmules representing all its parts. If we follow
Prof. Weismann we are led into an astounding supposition. He admits that
every variable part must have a special determinant, and that this results
in the assumption of over two hundred thousand for the four wings of a
butterfly. Let us ask what must happen in the case of a peacock's feather.
On looking at the eye near its end, we see that the minute processes on the
edge of each lateral thread must have been in some way exactly adjusted, in
colour and position, so as to fall into line with the processes on adjacent
threads: otherwise the symmetrical arrangement of coloured rings would be
impossible. Each of these processes, then, being an independent variable,
must have had its particular determinant. Now there are about 300 threads
on the shaft of a large feather, and each of them bears on the average
1,600 processes, making for the whole feather 480,000 of these processes.
For one feather alone there must have been 480,000 determinants, and for
the whole tail many millions. And these, along with the determinants for
the detailed parts of all the other feathers, and for the variable
components of all organs forming the body at large, must have been
contained in the microscopic head of a spermatozoon! Hardly a credible
supposition. Nor is it easy to see how we are helped by the hypothesis of
constitutional units. Take the feather in its budding state and ask how the
group of such units, alike in structure and perpetually multiplying while
the unfolding goes on, can be supposed by their mutual actions so to affect
one another as eventually to produce the symmetrically-adjusted processes
which constitute the terminal eye. Imagination, whatever licence may be
given, utterly fails us.
At last then we are obliged to admit that the actual organizing process
transcends conception. It is not enough to say that we cannot know it; we
must say that we cannot even conceive it. And this is just the conclusion
which might have been drawn before contemplating the facts. For if, as we
saw in the chapter on "The Dynamic Element in Life," it is impossible for
us to understand the nature of this element--if even the ordinary
manifestations of it which a living body yields from moment to moment are
at bottom incomprehensible; then, still more incomprehensible must be that
astonishing manifestation of it which we have in the initiation and
unfolding of a new organism.
Thus all we can do is to find some way of symbolizing the process so as to
enable us most conveniently to generalize its phenomena; and the only
reason for adopting the hypothesis of physiological units or constitutional
units is that it best serves this purpose.
CHAPTER XI.
CLASSIFICATION.
§ 98. That orderly arrangement of objects called Classification has two
purposes, which, though not absolutely distinct, are distinct in great
part. It may be employed to facilitate identification, or it may be
employed to organize our knowledge. If a librarian places his books in the
alphabetical succession of the author's names, he places them in such way
that any particular book may easily be found, but not in such way that
books of a given nature stand together. When, otherwise, he makes a
distribution of books according to their subjects, he neglects various
superficial similarities and distinctions, and groups them according to
certain primary and secondary and tertiary attributes, which severally
imply many other attributes--groups them so that any one volume being
inspected, the general characters of all the neighbouring volumes may be
inferred. He puts together in one great division all works on History; in
another all Biographical works; in another all works that treat of Science;
in another Voyages and Travels; and so on. Each of his great groups he
separates into sub-groups; as when he puts different kinds of Literature
under the heads of Fiction, Poetry, and the Drama. In some cases he makes
sub-sub-groups; as when, having divided his Scientific treatises into
abstract and concrete, putting in the one Logic and Mathematics and in the
other Physics, Astronomy, Geology, Chemistry, Physiology, &c.; he goes on
to sub-divide his books on Physics, into those which treat of Mechanical
Motion, those which treat of Heat, those which treat of Light, of
Electricity, of Magnetism.
Between these two modes of classification note the essential distinctions.
Arrangement according to any single conspicuous attribute is comparatively
easy, and is the first that suggests itself: a child may place books in the
order of their sizes, or according to the styles of their bindings. But
arrangement according to combinations of attributes which, though
fundamental, are not conspicuous, requires analysis; and does not suggest
itself till analysis has made some progress. Even when aided by the
information which the author gives on his title page, it requires
considerable knowledge to classify rightly an essay on Polarization; and in
the absence of a title page it requires much more knowledge. Again,
classification by a single attribute, which the objects possess in
different degrees, may be more or less serial, or linear. Books may be put
in the order of their dates, in single file; or if they are grouped as
works in one volume, works in two volumes, works in three volumes, &c., the
groups may be placed in an ascending succession. But groups severally
formed of things distinguished by some common attribute which implies many
other attributes, do not admit of serial arrangement. You cannot rationally
say either that Historical Works should come before Biographical Works, or
Biographical Works before Historical Works; nor of the sub-divisions of
creative Literature, into Fiction, Poetry, and the Drama, can you give a
good reason why any one should take precedence of the others.
Hence this grouping of the like and separation of the unlike which
constitutes Classification, can reach its complete form only by slow steps.
I have shown (_Essays_, Vol. II., pp. 145-7) that, other things equal, the
relations among phenomena are recognized in the order of their
conspicuousness; and that, other things equal, they are recognized in the
order of their simplicity. The first classifications are sure, therefore,
to be groupings of objects which resemble one another in external or
easily-perceived attributes, and attributes that are not of complex
characters. Those likenesses among things which are due to their possession
in common of simple obvious properties, may or may not coexist with further
likenesses among them. When geometrical figures are classed as curvilinear
and rectilinear, or when the rectilinear are divided into trilateral,
quadrilateral, &c., the distinctions made connote various other
distinctions with which they are necessarily bound up; but if liquids be
classed according to their visible characters--if water, alcohol, sulphuret
of carbon, &c., be grouped as colourless and transparent, we have things
placed together which are unlike in their essential natures. Thus, where
the objects classed have numerous attributes, the probabilities are that
the early classifications, based on simple and manifest attributes, unite
under the same head many objects that have no resemblance in the majority
of their attributes. As the knowledge of objects increases, it becomes
possible to make groups of which the members have more numerous properties
in common; and to ascertain what property, or combination of properties, is
most characteristic of each group. And the classification eventually
arrived at is of such kind that the objects in each group have more
attributes in common with one another than they have in common with any
excluded objects; one in which the groups of such groups are integrated on
the same principle; and one in which the degrees of differentiation and
integration are proportioned to the degrees of intrinsic unlikeness and
likeness. And this ultimate classification, while it serves to identify the
things completely, serves also to express the greatest amount of knowledge
concerning the things--enables us to predicate the greatest number of facts
about each thing; and by so doing implies the most precise correspondence
between our conceptions and the realities.
§ 99. Biological classifications illustrate well these phases through which
classifications in general pass. In early attempts to arrange organisms in
some systematic manner, we see at first a guidance by conspicuous and
simple characters, and a tendency towards arrangement in linear order. In
successively later attempts, we see more regard paid to combinations of
characters which are essential but often inconspicuous, and an abandonment
of a linear arrangement for an arrangement in divergent groups and
re-divergent sub-groups.
In the popular mind, plants are still classed under the heads of Trees,
Shrubs, and Herbs; and this serial classing according to the single
attribute of magnitude, swayed the earliest observers. They would have
thought it absurd to call a bamboo, thirty feet high, a kind of grass; and
would have been incredulous if told that the Hart's-tongue should be placed
in the same great division with the Tree-ferns. The zoological
classifications current before Natural History became a science, had
divisions similarly superficial and simple. Beasts, Birds, Fishes, and
Creeping-things are names of groups marked off from one another by
conspicuous differences of appearance and modes of life--creatures that
walk and run, creatures that fly, creatures that live in the water,
creatures that crawl. And these groups were thought of in the order of
their importance.
The first arrangements made by naturalists were based either on single
characters or on very simple combinations of characters; as that of
Clusius, and afterwards the more scientific system of Cesalpino,
recognizing the importance of inconspicuous structures. Describing
plant-classifications, Lindley says:--"Rivinus invented, in 1690, a system
depending upon the formation of the corolla; Kamel, in 1693, upon the fruit
alone; Magnol, in 1720, on the calyx and corolla; and finally, Linnæus, in
1731, on variations in the stamens and pistil." In this last system, which
has been for so long current as a means of identification (regarded by its
author as transitional), simple external attributes are still depended on;
and an arrangement, in great measure serial, is based on the degrees in
which these attributes are possessed. In 1703, some thirty years before the
time of Linnæus, our countryman Ray had sketched the outlines of a more
advanced system. He said that--
Plants are either
Flowerless, or
Flowering; and these are
Dicotyledones, or
Monocotyledones.
Among the minor groups which he placed under these general heads, "were
Fungi, Mosses, Ferns, Composites, Cichoraceæ, Umbellifers, Papilionaceous
plants, Conifers, Labiates, &c., under other names, but with limits not
very different from those now assigned to them." Being much in advance of
his age, Ray's ideas remained dormant until the time of Jussieu; by whom
they were developed into what has become known as the Natural System: a
system subsequently improved by De Candolle. Passing through various
modifications in the hands of successive botanists, the Natural System is
now represented by the following form, which is based upon the table of
contents prefixed to Vol. II. of Prof. Oliver's translation of the _Natural
History of Plants_, by Prof. Kerner. His first division, Myxothallophyta (=
Myxomycetes), I have ventured to omit. The territory it occupies is in
dispute between zoologists and botanists, and as I have included the group
in the zoological classification, agreeing that its traits are more animal
than vegetal, I cannot also include it in the botanical classification.
Here, linear arrangement has disappeared: there is a breaking up into
groups and sub-groups and sub-sub-groups, which do not admit of being
placed in serial order, but only in divergent and re-divergent order. Were
there space to exhibit the way in which the Alliances are subdivided into
Orders, and these into Genera, and these into Species, the same principle
of co-ordination would be still further manifested.
PHYLA. CLASSES. ALLIANCES.
SUB-PHYLA. SUB-CLASSES.
THALLOPHYTA
I. Schizophyta
2. Cyanophyceæ. Blue-green Algæ.
3. Schizomycetes.
II. Dinoflagellata
Peridineæ
4.
III. Bacillariales
5.
IV. Gamophyceæ
I. Chlorophyceæ
6. Protococcoideæ.
7. Siphoneæ.
8. Confervoideæ.
9. Conjugatæ.
10. Charales.
11. Phæophyceæ.
12. Dictyotales.
13. Florideæ, Red Seaweeds.
V. Fungi
I. Phycomycetes
14. Oomycetes.
15. Zygomycetes.
II. Mesomycetes
16.
17.
III. Mycomycetes
18.
19.
Additional group of Fungi, Lichenes.
ARCHEGONIATÆ
I. Bryophyta
20. Hepaticæ, Liverworts.
21. Musci, Mosses.
II. Pteridophyta
Vas. Cryptogams
22. Filices, Ferns.
23. Hydropterides, Rhizocarps.
24. Equisetales, Horse-tails.
25. Lycopodiales, Club-mosses.
PHANEROGAMIA (Flowering Plants.)
GYMNOSPERMÆ
I. Cycadales, Cycads
26.
II. Coniferæ
27.
III. Gnetales
28.
ANGIOSPERMÆ
I. Monocotyledons
29. Liliifloræ.
30. Scitamineæ.
31. Gynandræ.
32. Fluviales.
33. Spadicifloræ.
34. Glumifloræ.
II. Dicotyledons
I. Monochlamydæ
35. Centrospermæ.
36. Protiales.
37. Daphnales.
38. Santalales.
39. Rafflesiales.
40. Asarales.
41. Euphorbiales.
42. Podostemales.
43. Viridifloræ.
44. Amentales.
45. Balanophorales.
II. Monopetalæ
46. Caprifoliales.
47. Asterales.
48. Campanales.
49. Ericales.
50. Vaccinales.
51. Primulales.
52. Tubifloræ.
III. Polypetalæ
53. Ranales.
54. Parietales.
55. Malvales.
56. Discifloræ.
57. Crateranthæ.
58. Myrtales.
59. Melastomales.
60. Lythrales.
61. Hygrobiæ.
62. Passifloræ.
63. Pepones.
64. Cactales.
65. Ficoidales.
66. Umbellales.
On studying the definitions of these primary, secondary, and tertiary
classes, it will be found that the largest are marked off from one another
by some attribute which connotes sundry other attributes; that each of the
smaller classes comprehended in one of these largest classes, is marked off
in a similar way from the other smaller classes bound up with it; and that
so, each successively smaller class has an increased number of co-existing
attributes.
§ 100. Zoological classification has had a parallel history. The first
attempt which we need notice, to arrange animals in such a way as to
display their affinities, is that of Linnæus. He grouped them thus:[42]--
CL. 1. MAMMALIA. _Ord._ Primates, Bruta, Feræ, Glires, Pecora, Belluæ,
Cete.
CL. 2. AVES. _Ord._ Accipitres, Picæ, Anseres, Grallæ, Gallinæ, Passeres.
CL. 3. AMPHIBIA. _Ord._ Reptiles, Serpentes, Nantes.
CL. 4. PISCES. _Ord._ Apodes, Jugulares, Thoracici, Abdominales.
CL. 5. INSECTA. _Ord._ Coleoptera, Hemiptera, Lepidoptera, Neuroptera,
Diptera, Aptera.
CL. 6. VERMES. _Ord._ Intestina, Mollusca, Testacea, Lithophyta,
Zoophyta.
This arrangement of classes is obviously based on apparent gradations of
rank; and the placing of the orders similarly betrays an endeavour to make
successions, beginning with the most superior forms and ending with the
most inferior forms. While the general and vague idea of perfection
determines the leading character of the classification, its detailed
groupings are determined by the most conspicuous external attributes. Not
only Linnæus but his opponents, who proposed other systems, were "under the
impression that animals were to be arranged together into classes, orders,
genera, and species, according to their more or less close external
resemblance." This conception survived until the time of Cuvier.
"Naturalists," says Agassiz, "were bent upon establishing one continual
uniform series to embrace all animals, between the links of which it was
supposed there were no unequal intervals. The watchword of their school
was: _Natura non facit saltum_. They called their system _la chaine des
êtres_."
The classification of Cuvier, based on internal organization instead of
external appearance, was a great advance. He asserted that there are four
principal forms, or four general plans, on which animals are constructed;
and, in pursuance of this assertion, he drew out the following scheme.
First Branch. ANIMALIA VERTEBRATA.
Cl. 1. Mammalia.
Cl. 2. Birds.
Cl. 3. Reptilia.
Cl. 4. Fishes.
Second Branch. ANIMALIA MOLLUSCA.
Cl. 1. Cephalapoda.
Cl. 2. Pteropoda.
Cl. 3. Gasteropoda.
Cl. 4. Acephala.
Cl. 5. Brachiopoda.
Cl. 6. Cirrhopoda.
Third Branch. ANIMALIA ARTICULATA.
Cl. 1. Annelides.
Cl. 2. Crustacea.
Cl. 3. Arachnides.
Cl. 4. Insects.
Fourth Branch. ANIMALIA RADIATA.
Cl. 1. Echinoderms.
Cl. 2. Intestinal Worms.
Cl. 3. Acalephæ.
Cl. 4. Polypi.
Cl. 5. Infusoria.
But though Cuvier emancipated himself from the conception of a serial
progression throughout the Animal Kingdom, sundry of his contemporaries and
successors remained fettered by the old error. Less regardful of the
differently-combined sets of attributes distinguishing the different
sub-kingdoms, and swayed by the belief in a progressive development which
was erroneously supposed to imply a linear arrangement of animals, they
persisted in thrusting organic forms into a quite unnatural order. The
following classification of Lamarck illustrates this.
INVERTEBRATA.
I. APATHETIC ANIMALS. }
}
Cl. 1. Infusoria. } Do not feel, and move only by their
Cl. 2. Polypi. } excited irritability. No brain, no
Cl. 3. Radiaria. } elongated medullary mass; no senses;
Cl. 4. Tunicata. } forms varied; rarely articulations.
Cl. 5. Vermes. }
II. SENSITIVE ANIMALS. } Feel, but obtain from their sensations
} only perceptions of objects, a
Cl. 6. Insects. } sort of simple ideas, which they are
Cl. 7. Arachnids. } unable to combine to obtain complex
Cl. 8. Crustacea. } ones. No vertebral column; a brain
Cl. 9. Annelids. } and mostly an elongated medullary
Cl. 10. Cirripeds. } mass; some distinct senses; muscles
Cl. 11. Conchifera. } attached under the skin; form symmetrical,
Cl. 12. Mollusks. } the parts being in pairs.
VERTEBRATA.
{ Feel; acquire preservable ideas;
III. INTELLIGENT ANIMALS. { perform with them operations by which
{ they obtain others; are intelligent in
Cl. 13. Fishes. { different degrees. A vertebral column;
Cl. 14. Reptiles. { a brain and a spinal marrow; distinct
Cl. 15. Birds. { senses; the muscles attached to the
Cl. 16. Mammalia. { internal skeleton; form symmetrical,
{ the parts being in pairs.
Passing over sundry classifications in which the serial arrangement
dictated by the notion of ascending complexity, is variously modified by
the recognition of conspicuous anatomical facts, we come to classifications
which recognize another order of facts--those of development. The
embryological inquiries of Von Baer led him to arrange animals as
follows:--
I. Peripheric Type. (RADIATA.) _Evolutio radiata._ The development
proceeds from a centre, producing identical parts in a radiating order.
II. Massive Type. (MOLLUSCA.) _Evolutio contorta._ The development
produces identical parts curved around a conical or other space.
III. Longitudinal Type. (ARTICULATA.) _Evolutio gemina._ The development
produces identical parts arising on both sides of an axis, and closing up
along a line opposite the axis.
IV. Doubly Symmetrical Type. (VERTEBRATA.) _Evolutio bigemina._ The
development produces identical parts arising on both sides of an axis,
growing upwards and downwards, and shutting up along two lines, so that
the inner layer of the germ is inclosed below, and the upper layer above.
The embryos of these animals have a dorsal cord, dorsal plates, and
ventral plates, a nervous tube and branchial fissures.
Recognizing these fundamental differences in the modes of development, as
answering to fundamental divisions in the animal kingdom, Von Baer shows
(among the _Vertebrata_ at least) how the minor differences which arise at
successively later embryonic stages, correspond with the minor divisions.
Like the modern classification of plants, the modern classification of
animals shows us the assumed linear order completely broken up. In his
lectures at the Royal Institution, in 1857, Prof. Huxley expressed the
relations existing among the several great groups of the animal kingdom, by
placing them at the ends of four or five radii, diverging from a centre.
The diagram I cannot obtain; but in the published reports of his lectures
at the School of Mines the groups were arranged as on the following page.
What remnant there may seem to be of linear succession in some of the
sub-groups contained in it, is merely an accident of typographical
convenience. Each of them is to be regarded simply as a cluster. And if
Prof. Huxley had further developed the arrangement, by dispersing the
sub-groups and sub-sub-groups on the same principle, there would result an
arrangement perhaps not much unlike that shown on the page succeeding this.
VERTEBRATA
(_Abranchiata_)
Mammalia
Aves
Reptilia
(_Branchiata_)
Amphibia
Pisces.
MOLLUSCA ANNULOSA
Cephalopoda Heteropoda } _Articulata._
Gasteropoda-dioecia } Insecta Arachnida
} Myriapoda Crustacea
{ Pulmonata Gasteropoda-monoecia
{ Pteropoda _Annuloida._
Lamellibranchiata Annellata Scoleidæ
Echinodermata Trematoda
Rotifera Tæniadæ
Turbellaria
Nematoidea
COELENTERATA
Hydrozoa Actinozoa.
PROTOZOA
Infusoria Spongiadæ Gregarinidæ
_Noctilucidæ_ Foraminifera _Thallassicollidæ_
In the woodcut, the dots represent orders, the names of which it is
impracticable to insert. If it be supposed that when magnified, each of
these dots resolves itself into a cluster of clusters, representing genera
and species, an approximate idea will be formed of the relations among the
successively-subordinate groups constituting the animal kingdom. Besides
the subordination of groups and their general distribution, some other
facts are indicated. By the distances of the great divisions from the
general centre, are rudely symbolized their respective degrees of
divergence from the form of simple, undifferentiated organic matter; which
we may regard as their common source. Within each group, the remoteness
from the local centre represents, in a rough way, the degree of departure
from the general plan of the group. And the distribution of the sub-groups
within each group, is in most cases such that those which come nearest to
neighbouring groups, are those which show the nearest resemblances to
them--in their analogies though not in their homologies. No such scheme,
however, can give a correct conception. Even supposing the above diagram
expressed the relations of animals to one another as truly as they can be
expressed on a plane surface (which of course it does not), it would still
be inadequate. Such relations cannot be represented in space of two
dimensions, but only in space of three dimensions.
_Mammalia_
_Aves_
_Reptilia_
VERTEBRATA
\
_Amphibia_\ _Pisces_
\
\ _Arachnida_
\ _Insecta_
\ _Crustacea_
\
\ Articulata
\ |
\ | _Myriapoda_
\ |
\ ANNULOSA
\ |
\ |_Annelida_
\ _Scolecida_
\ |
\ Annuloida
\ | /
\ _Echinodermata_
\ | /
_Pteropoda_ _Cephalopoda_\ | |
_Gasteropoda dioecia_ | |
_Gasteropoda monoecia_ _Pulmonata_ | |
\ | |
MOLLUSCA------------- \ | |
\ \ | |
_Lamellibranchiata_ \ \ | |
\ \ | |
_Brachiopoda_ \ \ | |
\ \ _Gregarinida_
Molluscoida------------
_Rhizopoda_ \
_Ascidioida_ _Polyzoa_ / \
/ PROTOZOA
/ _Spongida_ _Infusoria_
_Hydrozoa_ /
/
COELENTERATA
_Actinozoa_
§ 100a. Two motives have prompted me to include in its original form the
foregoing sketch: the one being that in conformity with the course
previously pursued, of giving the successive forms of classifications, it
seems desirable to give this form which was approved thirty-odd years ago;
and the other being that the explanatory comments remain now as applicable
as they were then. Replacement of the diagram by one expressing the
relations of classes as they are now conceived, is by no means an easy
task; for the conceptions formed of them are unsettled. Concerning the
present attitude of zoologists, Prof. MacBride writes:--
"They all recognize a certain number of phyla. Each phylum includes a
group of animals about whose relation to each other no one entertains a
doubt. Each zoologist, however, has his own idea as to the relationship
which the various phyla bear to each other.
"The phyla recognized at present are:--
(1) Protozoa.
(2) Porifera (Sponges).
(3) Coelenterata.
(4) Echinodermata.
{ Cestodes.
(5) Platyhelminthes { Trematodes.
{ Turbellaria.
(6) Nemertea.
(7) Nematoda.
(8) Acanthocephala (Echinorhyncus).
(9) Chætognatha (Sagitta).
(10) Rotifera.
(11) Annelida (Includes Leeches and Gephyrea, Chætifera).
(12) Gephyrea, Achæta.
{ Tracheata (Peripatus, Myriapods, Insects).
(13) Arthropods { Arachnids.
{ Crustacea.
{ Pycnogonida.
(14) Mollusca.
(15) Polyzoa (Including Phoronis).
(16) Brachiopoda.
(17) Chordata (Includes Balanoglossus and Tunicates. Some
continental zoologists do not admit Balanoglossus)."
[This last phylum of course includes the _Vertebrata_.]
Though under present conditions, as above implied, it would be absurd to
attempt a definite scheme of relationships, yet it has seemed to me that
the adumbration of a scheme, presenting in a vague way such relationships
as are generally agreed upon and leaving others indeterminate, may be
ventured; and that a general impression hence resulting may be useful. On
the adjacent page I have tried to make a tentative arrangement of this
kind.
At the bottom of the table I have placed together, under the name "Compound
_Protozoa_," those kinds of aggregated _Protozoa_ which show no
differentiations among the members of groups, and are thus distinguished
from _Metazoa_; and I have further marked the distinction by their
position, which implies that from them no evolution of higher types has
taken place. Respecting the naming of the sub-kingdoms, phyla, classes,
orders, &c., I have not maintained entire consistency. The relative values
of groups cannot be typographically expressed in a small space with a
limited variety of letters. The sizes of the letters mark the
classificatory ranks, and by the thickness I have rudely indicated their
zoological importance. In fixing the order of subordination of groups I
have been aided by the table of contents prefixed to Mr. Adam Sedgwick's
_Student's Text Book of Zoology_ and have also made use of Prof. Ray
Lankester's classifications of several sub-kingdoms.
_Placental_-----+
| _Aves_
_Mammalia_----+ | _Arachnida_
| | |
_Implacental_---+ | _Insecta_ | _Crustacea_
| | | | |
VERTEBRATA | | | | |
| | | | |
_Reptilia_ _Chilopoda_
| |
_Amphibia_ ARTHROPODA |
| |
| _Diplopoda_
_Pisces_ |
| _Chætopoda_
_Cephalochorda_ | |
CHORDATA ANNELIDA
_Urochorda_ | | _Echiuroidea_
_(Tunicata)_ | | _Hirudinea_
+---------+ _Archiannelida_
| |
BRACHIOPODA | | ROTIFERA
_Dibranchiata_ | | | |
_Cephalopoda_ | | +----+
_Tetrabranchiata_ | | | _Crinoidea_
MOLLUSCA ---------------+ | | | +------ ECHINODERMATA
_Scaphopoda_ | | | | | _Asteroidea_
_Solenogastres_ | | | | | _Echinoidea_
_Gasteropoda_ | | | | | _Holothuroidea_
_Lammellibranchiata_----+ | | | | _Enteropneusta_
| | | | |
| | | | | _Acanthocephala_
POLYZOA --------------------+ | | | | | +--- NEMATHELMINTHES
| | | | | | | _Nematomorpha_
| | | | | | | _Nematoda_
| | | | | | |
_Zoantharia_ Ctenophora
_Rugosa_ _Acalephos_
_Alcyonaria_ COELENTERATA _Hydromedusae_
Actinozoa Hydrozoa |
| | |-------- NEMERTEA
| | |
| +----------+ _Turbellaria_
----------------|
|
PROTOZOA PLATYHELMINTHES
_Corticata_ _Trematoda_
_Gymnomyxa_ _Cestoda_
| |
| |
Myxomycetes | _Triaronia_
| _Vobrocina_ _Calcarea_
_Foraminifera_ | PORIFERA
Compound Protozoa _Demospongiae_
_Radiolaria_
Let me again emphasize the fact that the relationships of these diverging
and re-diverging groups cannot be expressed on a flat surface. If we
imagine a laurel-bush to be squashed flat by a horizontal plane descending
upon it, we shall see that sundry of the upper branches and twigs which
were previously close together will become remote, and that the relative
positions of parts can remain partially true only with the minor branches.
The reader must therefore expect to find some of the zoological divisions
which in the order of nature are near one another, shown in the table as
quite distant.
§ 101. While the classifications of botanists and zoologists have become
more and more natural in their arrangements, there has grown up a certain
artificiality in their abstract nomenclature. When aggregating the smallest
groups into larger groups and these into groups still larger, they have
adopted certain general terms expressive of the successively more
comprehensive divisions; and the habitual use of these terms, needful for
purposes of convenience, has led to the tacit assumption that they answer
to actualities in Nature. It has been taken for granted that species,
genera, orders, and classes, are assemblages of definite values--that every
genus is the equivalent of every other genus in respect of its degree of
distinctness; and that orders are separated by lines of demarcation which
are as broad in one place as another. Though this conviction is not a
formulated one, the disputes continually occurring among naturalists on the
questions, whether such and such organisms are specifically or generically
distinct, and whether this or that peculiarity is or is not of ordinal
importance, imply that the conviction is entertained even where not avowed.
Yet that differences of opinion like these arise and remain unsettled,
except when they end in the establishment of sub-species, sub-genera,
sub-orders, and sub-classes, sufficiently shows that the conviction is
ill-based. And this is equally shown by the impossibility of obtaining any
definition of the degree of difference which warrants each further
elevation in the hierarchy of classes.
It is, indeed, a wholly gratuitous assumption that organisms admit of being
placed in groups of equivalent values; and that these may be united into
larger groups which are also of equivalent values; and so on. There is no
_à priori_ reason for expecting this; and there is no _à posteriori_
evidence implying it, save that which begs the question--that which asserts
one distinction to be generic and another to be ordinal, because it is
assumed that such distinctions must be either generic or ordinal. The
endeavour to thrust plants and animals into these definite partitions is of
the same nature as the endeavour to thrust them into linear series. Not
that it does violence to the facts in anything like the same degree; but
still, it does violence to the facts. Doubtless the making of divisions and
sub-divisions, is extremely useful; or rather, it is necessary. Doubtless,
too, in reducing the facts to something like order they must be partially
distorted. So long as the distorted form is not mistaken for the actual
form, no harm results. But it is needful for us to remember that while our
successively subordinate groups have a certain general correspondence with
the realities, they tacitly ascribe to the realities a regularity which
does not exist.
§ 102. A general truth of much significance is exhibited in these
classifications. On observing the natures of the attributes which are
common to the members of any group of the first, second, third, or fourth
rank, we see that groups of the widest generality are based on characters
of the greatest importance, physiologically considered; and that the
characters of the successively-subordinate groups, are characters of
successively-subordinate importance. The structural peculiarity in which
all members of one sub-kingdom differ from all members of another
sub-kingdom, is a peculiarity that affects the vital actions more
profoundly than does the structural peculiarity which distinguishes all
members of one class from all members of another class. Let us look at a
few cases.
We saw (§ 56), that the broadest division among the functions is the
division into "the _accumulation of energy_ (latent in food); the
_expenditure of energy_ (latent in the tissues and certain matters absorbed
by them); and the _transfer of energy_ (latent in the prepared nutriment or
blood) from the parts which accumulate to the parts which expend." Now in
the lowest animals, united under the general name _Protozoa_, there is
either no separation of the parts performing these functions or very
indistinct separation: in the _Rhizopoda_, all parts are alike accumulators
of energy, expenders of energy and transferers of energy; and though in the
higher members of the group, the _Infusoria_, there are some
specializations corresponding to these functions, yet there are no distinct
tissues appropriated to them. Similarly when we pass from simple types to
compound types--from _Protozoa_ to _Metazoa_. The animals known as
_Coelenterata_ are characterized in common by the possession of a part
which accumulates energy more or less marked off from the part which does
not accumulate energy, but only expends it; and the _Hydrozoa_ and
_Actinozoa_, which are sub-divisions of the _Coelenterata_, are contrasted
in this, that in the second these parts are much more differentiated from
one another, as well as more complicated. Besides a completer
differentiation of the organs respectively devoted to the accumulation of
energy and the expenditure of energy, animals next above the _Coelenterata_
possess rude appliances for the transfer of energy: the peri-visceral sac,
or closed cavity between the intestine and the walls of the body, serves as
a reservoir of absorbed nutriment, from which the surrounding tissues take
up the materials they need. And then out of this sac originates a more
efficient appliance for the transfer of energy: the more highly-organized
animals, belonging to whichever sub-kingdom, all of them possess
definitely-constructed channels for distributing the matters containing
energy. In all of them, too, the function of expenditure is divided between
a directive apparatus and an executive apparatus--a nervous system and a
muscular system. But these higher sub-kingdoms are clearly separated from
one another by differences in the relative positions of their component
sets of organs. The habitual attitudes of annulose and molluscous
creatures, is such that the neural centres are below the alimentary canal
and the hæmal centres above. And while by these traits the annulose and
molluscous types are separated from the vertebrate, they are separated from
each other by this, that in the one the body is "composed of successive
segments, usually provided with limbs," but in the other, the body is not
segmented, "and no true articulated limbs are ever developed."
The sub-kingdoms being thus distinguished from one another, by the presence
or absence of specialized parts devoted to fundamental functions, or else
by differences in the distributions of such parts, we find, on descending
to the classes, that these are distinguished from one another, either by
modifications in the structures of fundamental parts, or by the presence or
absence of subsidiary parts, or by both. Fishes and _Amphibia_ are unlike
higher vertebrates in possessing branchiæ, either throughout life or early
in life. And every higher vertebrate, besides having lungs, is
characterized by having, during development, an amnion and an allantois.
Mammals, again, are marked off from Birds and Reptiles by the presence of
mammæ, as well as by the form of the occipital condyles. Among Mammals, the
next division is based on the presence or absence of a placenta. And
divisions of the _Placentalia_ are mainly determined by the characters of
the organs of external action.
Thus, without multiplying illustrations and without descending to genera
and species, we see that, speaking generally, the successively smaller
groups are distinguished from one another by traits of successively less
importance, physiologically considered. The attributes possessed in common
by the largest assemblages of organisms, are few in number but
all-essential in kind. Each secondary assemblage, included in one of the
primary assemblages, is characterized by further common attributes that
influence the functions less profoundly. And so on with each lower grade.
§ 103. What interpretation is to be put on these truths of classification?
We find that organic forms admit of an arrangement everywhere indicating
the fact, that along with certain attributes, certain other attributes,
which are not directly connected with them, always exist. How are we to
account for this fact? And how are we to account for the fact that the
attributes possessed in common by the largest assemblages of forms, are the
most vitally-important attributes?
No one can believe that combinations of this kind have arisen fortuitously.
Even supposing fortuitous combinations of attributes might produce
organisms that would work, we should still be without a clue to this
special mode of combination. The chances would be infinity to one against
organisms which possessed in common certain fundamental attributes, having
also in common numerous non-essential attributes.
Nor, again, can any one allege that such combinations are necessary, in the
sense that all other combinations are impracticable. There is not, in the
nature of things, a reason why creatures covered with feathers should
always have beaks: jaws carrying teeth would, in many cases, have served
them equally well or better. The most general characteristic of an entire
sub-kingdom, equal in extent to the _Vertebrata_, might have been the
possession of nictitating membranes; while the internal organizations
throughout this sub-kingdom might have been on many different plans.
If, as an alternative, this peculiar subordination of traits which organic
forms display be ascribed to design, other difficulties suggest themselves.
To suppose that a certain plan of organization was fixed on by a Creator
for each vast and varied group, the members of which were to have many
different modes of life, and that he bound himself to adhere rigidly to
this plan, even in the most aberrant forms of the group where some other
plan would have been more appropriate, is to ascribe a very strange motive.
When we discover that the possession of seven cervical vertebræ is a
general characteristic of mammals, whether the neck be immensely long as in
the giraffe, or quite rudimentary as in the whale, shall we say that
though, for the whale's neck, one vertebra would have been equally good,
and though, for the giraffe's neck, a dozen would probably have been better
than seven, yet seven was the number adhered to in both cases, because
seven was fixed upon for the mammalian type? And then, when it turns out
that this possession of seven cervical vertebræ is not an
absolutely-universal characteristic of mammals (there is one which has
eight), shall we conclude that while, in a host of cases, there was a
needless adherence to a plan for the sake of consistency, there was yet, in
some cases, an inconsistent abandonment of the plan? I think we may
properly refuse to draw any such conclusion.
What, then, is the meaning of these peculiar relations of organic forms?
The answer to this question must be postponed. Having here contemplated the
problem as presented in these wide inductions which naturalists have
reached; and having seen what proposed solutions of it are inadmissible; we
shall see, in the next division of this work, what is the only possible
solution.
CHAPTER XII.
DISTRIBUTION.
§ 104. There is a distribution of organisms in Space, and there is a
distribution of organisms in Time. Looking first at their distribution in
Space, we observe in it two different classes of facts. On the one hand,
the plants and animals of each species have their habitats limited by
external conditions: they are necessarily restricted to spaces in which
their vital actions can be performed. On the other hand, the existence of
certain conditions does not determine the presence of organisms that are
fit for them. There are many spaces perfectly adapted for life of a high
order in which only life of a much lower order is found.
While, in the inevitable restriction of organisms to environments with
which their natures correspond we find a _negative_ cause of distribution,
there remains to be found that _positive_ cause whence results the presence
of organisms in some places appropriate to them and their absence from
other places equally appropriate or more appropriate. Let us consider the
phenomena as thus classed.
§ 105. Facts which illustrate the limiting influence of surrounding
conditions are abundant, and familiar to all readers. It will be needful,
however, here to cite a few typical ones of each order.
The confinement of different kinds of plants and different kinds of
animals, to the media for which they are severally adapted, is the broadest
fact of distribution. We have extensive groups of plants that are
respectively sub-aerial and sub-aqueous; and of the sub-aqueous some are
exclusively marine, while others exist only in rivers and lakes. Among
animals we similarly find some classes confined to the air and others to
the water; and of the water-breathers some are restricted to salt water and
others to fresh water. Less conspicuous is the fact that within each of
these contrasted media there are further widespread limitations. In the
sea, certain organisms exist only between certain depths, and others only
between other depths--the limpet and the mussel within the littoral zone,
and numerous kinds at the bottom of the ocean; and on the land, there are
Floras and Faunas peculiar to low regions and others peculiar to high
regions. Next we have the familiar geographical limitations made by
climate. There are temperatures which restrict each kind of organism
between certain isothermal lines, and hygrometric states which prevent the
spread of each kind of organism beyond areas having a certain humidity or a
certain dryness. Besides such general limitations we find much more special
limitations. Some minute vegetal forms occur only in snow. Hot springs have
their peculiar _Infusoria_. The habitats of certain Fungi are mines or
other dark places. And there are creatures unknown beyond the water
contained in particular caves. After these limits to distribution imposed
by physical conditions, come limits imposed by the presence or absence of
other organisms. Obviously, graminivorous animals are confined within
tracts which produce plants fit for them to feed on. The great carnivores
cannot exist out of regions where there are creatures large enough and
numerous enough to serve for prey. The needs of the sloth limit it to
certain forest-covered spaces; and there can be no insectivorous bats where
there are no night-flying insects. To these dependences of the
relatively-superior organisms on the relatively-inferior organisms which
they consume, must be added certain reciprocal dependences of the inferior
on the superior. Mr. Darwin's inquiries have shown how generally the
fertilization of plants is due to the agency of insects, and how certain
plants, being fertilizable only by insects of certain structures, are
limited to regions inhabited by insects of such structures. Conversely, the
spread of organisms is often bounded by the presence of particular
organisms beyond the bounds--either competing organisms or organisms
directly inimical. A plant fit for some territory adjacent to its own,
fails to overrun it because the territory is pre-occupied by some plant
which is its superior, either in fertility or power of resisting
destructive agencies; or else fails because there lives in the territory
some mammal which browses on its foliage or bird which devours nearly all
its seeds. Similarly, an area in which animals of a particular species
might thrive, is not colonized by them because they are not fleet enough to
escape some beast of prey inhabiting this area, or because the area is
infested by some insect which destroys them, as the tsetse destroys the
cattle in parts of Africa. Yet another more special series of limitations
accompanies parasitism. There are parasitic plants that flourish only on
trees of some few species, and others that have particular animals for
their habitats--as the fungus which is fatal to the silk-worm, or that
which so strangely grows out of a New Zealand caterpillar. Of
animal-parasites various kinds lead lives involving specialities of
distribution. We have kinds which use other creatures for purposes of
locomotion, as the _Chelonobia_ uses the turtle, and as a certain _Actinia_
uses the shell inhabited by a hermit-crab. We have the parasitism in which
one creature habitually accompanies another to share its prey, like the
annelid which takes up its abode in a hermit-crab's shell, and snatches
from the hermit-crab the morsels of food it is eating. We have again the
commoner parasitism of the _Epizoa_--animals which attach themselves to the
surfaces of other animals, and feed on their juices or on their secretions.
And once more, we have the equally common parasitism of the
_Entozoa_--creatures which live within other creatures. Besides being
restricted to the bodies of the organisms it infests, each species has
usually still narrower limits of distribution; in some cases the infested
organisms furnish fit habitats for the parasites only in certain regions,
and in other cases only when in certain constitutional states. There are
more indirect modes in which the distributions of organisms affect one
another. Plants of some kinds are eaten by animals only in the absence of
kinds that are preferred to them; and hence the prosperity of such plants
partly depends on the presence of the preferred plants. Mr. Bates has shown
that some South American butterflies thrive in regions where insectivorous
birds would destroy them, did they not closely resemble butterflies of
another genus which are disliked by those birds. And Mr. Darwin gives cases
of dependence still more remote and involved.
Such are the chief negative causes of distribution--the inorganic and
organic agencies that set bounds to the spaces which organisms of each
species inhabit. Fully to understand their actions we must contemplate them
as working not separately but in concert. We have to regard the physical
influences, varying from year to year, as now producing an extension or
restriction of the habitat in this direction and now in that, and as
producing secondary extensions and restrictions by their effects on other
kinds of organisms. We have to regard the distribution of each species as
affected not only by causes which favour multiplication of prey or of
enemies within its own area, but also by causes which produce such results
in neighbouring areas. We have to conceive the forces by which the limit is
maintained, as including all meteorologic influences, united with the
influences, direct or remote, of numerous co-existing species.
One general truth, indicated by sundry of the above illustrations, calls
for special notice--the truth that all kinds of organisms intrude on one
another's spheres of existence. Of the ways in which they do this the
commonest is invasion of territory. That tendency which we see in the human
races, to overrun and occupy one another's lands, as well as the lands
inhabited by inferior creatures, is a tendency exhibited by all classes of
organisms in various ways. Among them, as among mankind, there are
permanent conquests, temporary occupations, and occasional raids. Every
spring an inroad is made into the area which our own birds occupy, by birds
from the South; and every winter the fieldfares of the North come to share
the hips and haws of our hedges, and thus entail on our native birds some
mortality. Besides these regularly-recurring incursions there are irregular
ones; as of locusts into countries not usually visited by them, or of
certain rodents which from time to time swarm into areas adjacent to their
own. Every now and then an incursion ends in permanent settlement--perhaps
in conquest over indigenous species. Within these few years an American
water-weed has taken possession of our ponds and rivers, and to some extent
supplanted native water-weeds. Of animals may be named a small kind of red
ant, having habits allied to those of tropical ants, which has of late
overrun many houses in London. The rat, which must have taken to infesting
ships within these few centuries, furnishes a good illustration of the
readiness of animals to occupy new places that are available. And the way
in which vessels visiting India are cleared of the European cockroach by
the kindred _Blatta orientalis_, shows us how these successful invasions
last only until there come more powerful invaders. Animals encroach on one
another's spheres of existence in further ways than by trespassing on one
another's areas: they adopt one another's modes of life. There are cases in
which this usurpation of habits is slight and temporary; and there are
cases where it is marked and permanent. Grey crows often join gulls in
picking up food between tide-marks; and gulls may occasionally be seen many
miles inland, feeding in ploughed fields and on moors. Mr. Darwin has
watched a fly-catcher catching fish. He says that the greater titmouse
sometimes adopts the practices of the shrike, and sometimes of the
nuthatch, and that some South American woodpeckers are frugivorous while
others chase insects on the wing. Of habitual intrusions on the occupations
of other creatures, one case is furnished by the sea-eagle, which, besides
hunting the surface of the land for prey, like the rest of the hawk-tribe,
often swoops down upon fish. And Mr. Darwin names a species of petrel that
has taken to diving, and has a considerably modified organization. The
last cases introduce a still more remarkable class of facts of kindred
meaning. This intrusion of organisms on one another's modes of life goes to
the extent of intruding on one another's media. The great mass of flowering
plants are terrestrial, and (aside from other needs) are required to be so
by their process of fructification. But there are some which live in the
water, and protrude their flowers above the surface. Nay, there is a still
more striking instance. At the sea-side may be found an alga a hundred
yards inland, and a phænogam rooted in salt water. Among animals these
interchanges of media are numerous. Nearly all coleopterous insects are
terrestrial; but the water-beetle, which like the rest of its order is an
air-breather, has aquatic habits. Water appears to be an extremely unfit
medium for a fly; and yet Mr. [now Sir John] Lubbock has discovered more
than one species of fly living beneath the surface of the water and coming
up occasionally for air. Birds, as a class, are specially fitted for an
aerial existence; but certain tribes of them have taken to an aquatic
existence--swimming on the surface of the water and making continual
incursions beneath it, and some kinds have wholly lost the power of flight.
Among mammals, too, which have limbs and lungs implying an organization for
terrestrial life, may be named kinds living more or less in the water and
are more or less adapted to it. We have water-rats and otters which unite
the two kinds of life, and show but little modification; hippopotami
passing the greater part of their time in the water, and somewhat more
fitted to it; seals living almost exclusively in the sea, and having the
mammalian form greatly obscured; whales wholly confined to the sea, and
having so little the aspect of mammals as to be mistaken for fish.
Conversely, sundry inhabitants of the water make excursions on the land.
Eels migrate at night from one pool to another. There are fish with
specially-modified gills and fin-rays serving as stilts, which, when the
rivers they inhabit are partially dried-up, travel in search of better
quarters. And while some kinds of crabs do not make land-excursions beyond
high-water mark, other kinds pursue lives almost wholly terrestrial.
Guided by these two classes of facts, we must regard the bounds to each
species' sphere of existence as determined by the balancing of two
antagonist sets of forces. The tendency which every species has to intrude
on other areas, other modes of life, and other media, is restrained by the
direct and indirect resistance of conditions, organic and inorganic. And
these expansive and repressive energies, varying continually in their
respective intensities, rhythmically equilibrate each other--maintain a
limit that perpetually oscillates from side to side of a certain mean.
§ 106. As implied at the outset, the character of a region, when
unfavourable to any species, sufficiently accounts for the absence of this
species; and thus its absence is not inconsistent with the hypothesis that
each species was originally placed in the regions most favourable to it.
But the absence of a species from regions that _are_ favourable to it
cannot be thus accounted for. Were plants and animals localized wholly with
reference to the fitness of their constitutions to surrounding conditions,
we might expect Floras to be similar, and Faunas to be similar, where the
conditions are similar; and we might expect dissimilarities among Floras
and among Faunas, proportionate to the dissimilarities of their conditions.
But we do not find such anticipations verified.
Mr. Darwin says that "in the Southern hemisphere, if we compare large
tracts of land in Australia, South Africa, and western South America,
between latitudes 25° and 35°, we shall find parts extremely similar in all
their conditions, yet it would not be possible to point out three faunas
and floras more utterly dissimilar. Or again we may compare the productions
of South America south of lat. 35° with those north of 25°, which
consequently inhabit a considerably different climate, and they will be
found incomparably more closely related to each other, than they are to the
productions of Australia or Africa under nearly the same climate." Still
more striking are the contrasts which Mr. Darwin points out between
adjacent areas that are totally cut off from each other. "No two marine
faunas are more distinct, with hardly a fish, shell, or crab in common,
than those of the eastern and western shores of South and Central America;
yet these great faunas are separated only by the narrow, but impassable,
isthmus of Panama." On opposite sides of high mountain-chains, also, there
are marked differences in the organic forms--differences not so marked as
where the barriers are absolutely impassable, but much more marked than are
necessitated by unlikenesses of physical conditions.
Not less suggestive is the converse fact that wide geographical areas which
offer decided geologic and meteorologic contrasts, are peopled by
nearly-allied groups of organisms, if there are no barriers to migration.
"The naturalist in travelling, for instance, from north to south never
fails to be struck by the manner in which successive groups of beings,
specifically distinct, yet clearly related, replace each other. He hears
from closely allied, yet distinct kinds of birds, notes nearly similar, and
sees their nests similarly constructed, but not quite alike, with eggs
coloured in nearly the same manner. The plains near the Straits of Magellan
are inhabited by one species of Rhea (American Ostrich), and northward the
plains of La Plata by another species of the same genus; and not by a true
ostrich or emu, like those found in Africa and Australia under the same
latitude. On these same plains of La Plata, we see the agouti and bizcacha,
animals having nearly the same habits as our hares and rabbits and
belonging to the same order of Rodents, but they plainly display an
American type of structure. We ascend the lofty peaks of the Cordillera and
we find an alpine species of bizcacha; we look to the waters, and we do not
find the beaver or musk-rat, but the coypu and capybara, rodents of the
American type. Innumerable other instances could be given. If we look to
the islands off the American shore, however much they may differ in
geological structure, the inhabitants, though they may be all peculiar
species, are essentially American."
What is the generalization implied by these two groups of facts? On the one
hand, we have similarly-conditioned, and sometimes nearly-adjacent, areas,
occupied by quite different Faunas. On the one hand, we have areas remote
from one another in latitude, and contrasted in soil as well as climate,
occupied by closely-allied Faunas. Clearly then, as like organisms are not
universally, or even generally, found in like habitats, nor very unlike
organisms in very unlike habitats, there is no manifest pre-determined
adaptation of the organisms to the habitats. The organisms do no occur in
such and such places solely because they are either specially fit for those
places, or more fit for them than all other organisms.
The induction under which these facts come, and which unites them with
various other facts, is a totally-different one. When we see that the
similar areas peopled by dissimilar forms, are those between which there
are impassable barriers; while the dissimilar areas peopled by similar
forms, are those between which there are no such barriers; we are at once
reminded of the general truth exemplified in the last section--the truth
that each species of organism tends ever to expand its sphere of
existence--to intrude on other areas, other modes of life, other media. And
we are shown that through these perpetually-recurring attempts to thrust
itself into every accessible habitat, each species spreads until it reaches
limits which are for the time insurmountable.
§ 107. We pass now to the distribution of organic forms in Time. Geological
inquiry has established the truth that during a Past of immeasurable
duration, plants and animals have existed on the Earth. In all countries
their buried remains are found in greater or less abundance. From
comparatively small areas multitudinous different types have been exhumed.
Every exploration of new areas, and every closer inspection of areas
already explored, brings more types to light. And beyond question, an
exhaustive examination of all exposed strata, and of all strata now covered
by the sea, would disclose types immensely out-numbering those at present
known. Further, geologists agree that even had we before us every kind of
fossil which exists, we should still have nothing like a complete index to
the past inhabitants of our globe. Many sedimentary deposits have been so
altered by the heat of adjacent molten matter, as greatly to obscure the
organic remains contained in them. The extensive formations once called
"transition," and now re-named "metamorphic," are acknowledged to be
formations of sedimentary origin, from which all traces of such fossils as
they probably included have been obliterated by igneous action. And the
accepted conclusion is that igneous rock has everywhere resulted from the
melting-up of beds of detritus originally deposited by water. How long the
reactions of the Earth's molten nucleus on its cooling crust, have been
thus destroying the records of Life, it is impossible to say; but there are
strong reasons for believing that the records which remain bear but a small
ratio to the records which have been destroyed. Thus we have but extremely
imperfect data for conclusions respecting the distribution of organic forms
in Time. Some few generalizations, however, may be regarded as established.
One is that the plants and animals now existing mostly differ from the
plants and animals which have existed. Though there are species common to
our present Fauna and to past Faunas, yet the _facies_ of our present Fauna
differs, more or less, from the _facies_ of each past Fauna. On carrying
out the comparison, we find that past Faunas differ from one another, and
that the differences between them are proportionate to their degrees of
remoteness from one another in Time, as measured by their relative
positions in the sedimentary series. So that if we take the assemblage of
organic forms living now, and compare it with the successive assemblages of
organic forms which have lived in successive geologic epochs, we find that
the farther we go back into the past, the greater does the unlikeness
become. The number of species and genera common to the compared
assemblages, becomes smaller and smaller; and the assemblages differ more
and more in their general characters. Though a species of brachiopod now
extant is almost identical with a species found in Silurian strata, though
between the Silurian Fauna and our own there are sundry common genera of
molluscs, yet it is undeniable that there is a proportion between lapse of
time and divergence of organic forms.
This divergence is comparatively slow and continuous where there is
continuity in the geological formations, but is sudden, and comparatively
wide, wherever there occurs a great break in the succession of strata. The
contrasts which thus arise, gradually or all at once, in formations that
are continuous or discontinuous, are of two kinds. Faunas of different eras
are distinguished partly by the absence from the one of type's present in
the other, and partly by the unlikenesses between the types common to both.
Such contrasts between Faunas as are due to the appearance or disappearance
of types, are of secondary significance: they possibly, or probably, do not
imply anything more than migrations or extinctions. The most significant
contrasts are those between successive groups of organisms of the same
type. And among such, as above said, the differences are, speaking
generally, small and continuous where a series of conformable strata gives
proof of continued existence of the type in the locality; while they are
comparatively large and abrupt where the adjacent formations are shown to
have been separated by long intervals.
Another general fact, referred to by Mr. Darwin as one which palæontology
has made tolerably certain, is that forms and groups of forms which have
once disappeared from the Earth, do not reappear. Passing over the few
species which have continued throughout the whole period geologically
recorded, it may be said that each species after arising, spreading for an
era, and continuing abundant for an era, eventually declines and becomes
extinct; and that similarly, each genus during a longer period increases in
the number of its species, and during a longer period dwindles and at last
dies out. After making its exit neither species nor genus ever re-enters.
The like is true even of those larger groups called orders. Four types of
reptiles which were once abundant have not been found in modern formations,
and do not at present exist. Though nothing less than an exhaustive
examination of all strata, can prove conclusively that a type of
organization when once lost is never reproduced, yet so many facts point to
this inference that its truth can scarcely be doubted.
To frame a conception of the total amount and general direction of the
change in organic forms during the time measured by our sedimentary series,
is at present impossible--the data are insufficient. The immense contrast
between the few and low forms of the earliest-known Fauna, and the many and
high forms of our existing Fauna, has been commonly supposed to prove, not
only great change but great progress. Nevertheless, this appearance of
progress may be, and probably is, mainly illusive. Wider knowledge has
shown that remains of comparatively well-organized creatures really existed
in strata long supposed to be devoid of them, and that where they are
absent, the nature of the strata often explains their absence, without
assuming that they did not exist when these strata were formed. It is a
tenable hypothesis that the successively-higher types fossilized in our
successively-later deposits, indicate nothing more than successive
migrations from pre-existing continents to continents that were step by
step emerging from the ocean--migrations which necessarily began with the
inferior orders of organisms, and included the successively-superior orders
as the new lands became more accessible to them and better fitted for
them.[43]
While the evidence usually supposed to prove progression is thus
untrustworthy, there is trustworthy evidence that there has been, in many
cases, little or no progression. Though the orders which have existed from
palæozoic and mesozoic times down to the present day, are almost
universally changed, yet a comparison of ancient and modern members of
these orders shows that the total amount of change is not relatively great,
and that it is not manifestly towards a higher organization. Though nearly
all the living forms which have prototypes in early formations differ from
these prototypes specially, and in most cases generically, yet ordinal
peculiarities are, in numerous cases, maintained from the earliest times
geologically recorded, down to our own time; and we have no visible
evidence of superiority in the existing genera of these orders. In his
lecture "On the Persistent Types of Animal Life," Prof. Huxley enumerated
many cases. On the authority of Dr. Hooker he stated "that there are
Carboniferous plants which appear to be generically identical with some now
living: that the cone of the Oolitic _Araucaria_ is hardly distinguishable
from that of an existing species; that a true _Pinus_ appears in the
Purbecks and a _Juglans_ in the chalk." Among animals he named palæozoic
and mesozoic corals which are very like certain extant corals; genera of
Silurian molluscs that answer to existing genera; insects and arachnids in
the coal-formations that are not more than generically distinct from some
of our own insects and arachnids. He instanced "the Devonian and
Carboniferous _Pleuracanthus_, which differs no more from existing sharks
than these do from one another;" early mesozoic reptiles "identical in the
essential characters of their organization with those now living;" and
Triassic mammals which did not differ "nearly so much from some of those
which now live, as these differ from one another." Continuing the argument
in his "Anniversary Address to the Geological Society" in 1862, Prof.
Huxley gave many cases in which the changes that have taken place, are not
changes towards a more specialized or higher organization--asking "in what
sense are the Liassic Chelonia inferior to those which now exist? How are
the Cretaceous Ichthyosauria, Plesiosauria, or Pterosauria less embryonic
or more differentiated species than those of the Lias?" While, however,
contending that in most instances "positive evidence fails to demonstrate
any sort of progressive modification towards a less embryonic or less
generalized type in a great many groups of animals of long-continued
geological existence," Prof. Huxley added that there are other groups,
"co-existing with them under the same conditions, in which more or less
distinct indications of such a process seem to be traceable." And in
illustration of this, he named that better development of the vertebræ
which characterizes some of the more modern fishes and reptiles, when
compared with ancient fishes and reptiles of the same orders; and the
"regularity and evenness of the dentition of the _Anoplotherium_ as
contrasting with that of existing Artiodactyles."[44]
The facts thus summed up do not show that higher forms have not arisen in
the course of geologic time, any more than the facts commonly cited prove
that higher forms have arisen; nor are they regarded by Professor Huxley as
showing this. Were those which have survived from palæozoic and mesozoic
days down to our own day, the only types; and did the modifications, rarely
of more than generic value, which these types have undergone, give no
better evidences of increased complexity than are actually given by them;
then it would be inferable that there has been no appreciable advance. But
there now exist, and have existed during the more recent geologic epochs,
various types which are not known to have existed in earlier epochs--some
of them widely unlike these persistent types and some of them nearly allied
to these persistent types. As yet, we know nothing about the origins of
these new types. But it is possible that causes like those which have
produced generic differences in the persistent types, have, in some or many
cases, produced modifications great enough to constitute ordinal
differences. If structural contrasts not exceeding certain moderate limits
are held to mark only generic distinctions; and if organisms displaying
larger contrasts are regarded as ordinally or typically distinct; it is
obvious that the persistence of a given type through a long geologic period
without apparently undergoing deviations of more than generic value, by no
means disproves the occurrence of far greater deviations in other cases;
since the forms resulting from such far greater deviations, being regarded
as typically distinct forms, will not be taken as evidence of great change
in an original type. That which Prof. Huxley's argument proves, and that
only which he considers it to prove, is that organisms have no innate
tendencies to assume higher forms; and that "any admissible hypothesis of
progressive modification, must be compatible with persistence without
progression through indefinite periods."
One very significant fact must be added concerning the relation between
distribution in Time and distribution in Space. I quote it from Mr.
Darwin:--"Mr. Clift many years ago showed that the fossil mammals from the
Australian caves were closely allied to the living marsupials of that
continent. In South America a similar relationship is manifest, even to an
uneducated eye, in the gigantic pieces of armour like those of the
armadillo, found in several parts of La Plata; and Professor Owen has shown
in the most striking manner that most of the fossil mammals, buried there
in such numbers, are related to the South American types. This relationship
is even more clearly seen in the wonderland collection of fossil bones made
by MM. Lund and Clausen in the caves of Brazil. I was so much impressed
with these facts that I strongly insisted, in 1839 and 1845, on this 'law
of the succession of types,'--on 'this wonderful relationship in the same
continent between the dead and the living.' Professor Owen has subsequently
extended the same generalization to the Mammals of the Old World. We see
the same law in this author's restorations of the extinct and gigantic
birds of New Zealand. We see it also in the birds of the caves of Brazil.
Mr. Woodward has shown that the same law holds good with sea-shells, but
from the wide distribution of most genera of molluscs, it is not well
displayed by them. Other cases could be added, as the relation between the
extinct and living landshells of Madeira, and between the extinct and
living brackish-water shells of the Aralo-Caspian Sea."
The general results, then, are these. Our knowledge of distribution in
Time, being derived wholly from the evidence afforded by fossils, is
limited to that geologic time of which some records remain--cannot extend
to those remoter times the records of which have been obliterated. From
these remaining records, which probably form but a small fraction of the
whole, the general facts deducible are these:--That such organic types as
have lived through successive epochs, have almost universally undergone
modifications of specific and generic values--modifications which have
commonly been great in proportion as the period has been long. That besides
the types which have persisted from ancient eras down to our own era, other
types have from time to time made their appearance in the ascending series
of strata--types of which some are lower and some higher than the types
previously recorded; but whence these new types came, and whether any of
them arose by divergence from the previously-recorded types, the evidence
does not yet enable us to say. That in the course of long geologic epochs
nearly all species, most genera, and a few orders, have become extinct; and
that a species, genus, or order, which has once disappeared from the Earth
never reappears. And, lastly, that the Fauna now occupying each separate
area of the Earth's surface is very nearly allied to the Fauna which
existed on that area during recent geologic times.
§ 108. Omitting sundry minor generalizations, the exposition of which would
involve too much detail, what is to be said of these major generalizations?
The distribution in Space cannot be said to imply that organisms have been
designed for their particular habitats and placed in them; since, besides
the habitat in which each kind of organism is found there are commonly
other habitats, as good or better for it, from which it is absent--habitats
to which it is so much better fitted than organisms now occupying them,
that it extrudes these organisms when allowed the opportunity. Neither can
we suppose that the purpose has been to establish varieties of Floras and
Faunas; since, if so, why are the Floras and Faunas but little divergent in
widely-sundered areas between which migration is possible, while they are
markedly divergent in adjacent areas between which migration is impossible?
Passing to distributions in Time, there arise the questions--why during
nearly the whole of that vast period geologically recorded have there
existed none of those highest organic forms which have now overrun the
Earth?--how is it that we find no traces of a creature endowed with large
capacities for knowledge and happiness? The answer that the Earth was not,
in remote times, a fit habitation for such a creature, besides being
unwarranted by the evidence, suggests the equally awkward question--why
during untold millions of years did the Earth remain fit only for inferior
creatures? What, again, is the meaning of extinction of types? To conclude
that the saurian type was replaced by other types at the beginning of the
tertiary period, because it was not adapted to the conditions which then
arose, is to conclude that it could not be modified into fitness for the
conditions; and this conclusion is at variance with the hypothesis that
creative skill is shown in the multiform adaptations of one type to many
ends.
What interpretations may rationally be put on these and other general facts
of distribution in Space and Time, will be seen in the next division of
this work.
PART III.
THE EVOLUTION OF LIFE.
CHAPTER I.
PRELIMINARY.
§ 109. In the foregoing Part, we have contemplated the most important of
the generalizations to which biologists have been led by observation of
organisms; as well as some others which contemplation of the facts has
suggested to me. These Inductions of Biology have also been severally
glanced at on their deductive sides; for the purpose of noting the harmony
existing between them and those primordial truths set forth in _First
Principles_. Having thus studied the leading phenomena of life separately,
we are prepared for studying them as an aggregate, with the view of
arriving at the most general interpretation of them.
There is an _ensemble_ of vital phenomena presented by each organism in the
course of its growth, development, and decay; and there is an _ensemble_ of
vital phenomena presented by the organic world as a whole. Neither of these
can be properly dealt with apart from the other. But the last of them may
be separately treated more conveniently than the first. What interpretation
we put on the facts of structure and function in each living body, depends
entirely on our conception of the mode in which living bodies in general
have originated. To form some conclusion respecting this mode--a
provisional if not a permanent conclusion--must therefore be our first
step.
We have to choose between two hypotheses--the hypothesis of Special
Creation and the hypothesis of Evolution. Either the multitudinous kinds of
organisms which now exist, and the far more multitudinous kinds which have
existed during past geologic eras, have been from time to time separately
made; or they have arisen by insensible steps, through actions such as we
see habitually going on. Both hypotheses imply a Cause. The last, certainly
as much as the first, recognizes this Cause as inscrutable. The point at
issue is, how this inscrutable Cause has worked in the production of living
forms. This point, if it is to be decided at all, is to be decided only by
examination of evidence. Let us inquire which of these antagonist
hypotheses is most congruous with established facts.
CHAPTER II.
GENERAL ASPECTS OF THE SPECIAL-CREATION-HYPOTHESIS.[45]
§ 110. Early ideas are not usually true ideas. Undeveloped intellect, be it
that of an individual or that of the race, forms conclusions which require
to be revised and re-revised, before they reach a tolerable correspondence
with realities. Were it otherwise there would be no discovery, no increase
of intelligence. What we call the progress of knowledge, is the bringing of
Thoughts into harmony with Things; and it implies that the first Thoughts
are either wholly out of harmony with Things, or in very incomplete harmony
with them.
If illustrations be needed the history of every science furnishes them. The
primitive notions of mankind as to the structure of the heavens were wrong;
and the notions which replaced them were successively less wrong. The
original belief respecting the form of the Earth was wrong; and this wrong
belief survived through the first civilizations. The earliest ideas that
have come down to us concerning the natures of the elements were wrong; and
only in quite recent times has the composition of matter in its various
forms been better understood. The interpretations of mechanical facts, of
meteorological facts, of physiological facts, were at first wrong. In all
these cases men set out with beliefs which, if not absolutely false,
contained but small amounts of truth disguised by immense amounts of error.
Hence the hypothesis that living beings resulted from special creations,
being a primitive hypothesis, is probably an untrue hypothesis. It would be
strange if, while early men failed to reach the truth in so many cases
where it is comparatively conspicuous, they reached it in a case where it
is comparatively hidden.
§ 111. Besides the improbability given to the belief in special creations,
by its association with mistaken beliefs in general, a further
improbability is given to it by its association with a special class of
mistaken beliefs. It belongs to a family of beliefs which have one after
another been destroyed by advancing knowledge; and is, indeed, almost the
only member of the family surviving among educated people.
We all know that the savage thinks of each striking phenomenon, or group of
phenomena, as caused by some separate personal agent; that out of this
conception there grows up a polytheistic conception, in which these minor
personalities are variously generalized into deities presiding over
different divisions of nature; and that these are eventually further
generalized. This progressive consolidation of causal agencies may be
traced in the creeds of all races, and is far from complete in the creed of
the most advanced races. The unlettered rustics who till our fields, do not
let the consciousness of a supreme power wholly absorb the aboriginal
conceptions of good and evil spirits, and of charms or secret potencies
dwelling in particular objects. The earliest mode of thinking changes only
as fast as the constant relations among phenomena are established. Scarcely
less familiar is the truth, that while accumulating knowledge makes these
conceptions of personal causal agents gradually more vague, as it merges
them into general causes, it also destroys the habit of thinking of them as
working after the methods of personal agents. We do not now, like Kepler,
assume guiding spirits to keep the planets in their orbits. It is no longer
the universal belief that the sea was once for all mechanically parted from
the dry land; or that the mountains were placed where we see them by a
sudden creative act. All but a narrow class have ceased to suppose sunshine
and storm to be sent in some arbitrary succession. The majority of educated
people have given up thinking of epidemics of punishments inflicted by an
angry deity. Nor do even the common people regard a madman as one possessed
by a demon. That is to say, we everywhere see fading away the
anthropomorphic conception of Cause. In one case after another, is
abandoned the ascription of phenomena to a will analogous to the human
will, working by methods analogous to human methods.
If, then, of this once-numerous family of beliefs the immense majority have
become extinct, we may not unreasonably expect that the few remaining
members of the family will become extinct. One of these is the belief we
are here considering--the belief that each species of organism was
specially created. Many who in all else have abandoned the aboriginal
theory of things, still hold this remnant of the aboriginal theory. Ask any
well-informed man whether he accepts the cosmogony of the Indians, or the
Greeks, or the Hebrews, and he will regard the question as next to an
insult. Yet one element common to these cosmogonies he very likely retains:
not bearing in mind its origin. For whence did he get the doctrine of
special creations? Catechise him, and he is forced to confess that it was
put into his mind in childhood, as one portion of a story which, as a
whole, he has long since rejected. Why this fragment is likely to be right
while all the rest is wrong, he is unable to say. May we not then expect
that the relinquishment of all other parts of this story, will by and by be
followed by the relinquishment of this remaining part of it?
§ 112. The belief which we find thus questionable, both as being a
primitive belief and as being a belief belonging to an almost-extinct
family, is a belief not countenanced by a single fact. No one ever saw a
special creation; no one ever found proof of an indirect kind that a
special creation had taken place. It is significant, as Dr. Hooker remarks,
that naturalists who suppose new species to be miraculously originated,
habitually suppose the origination to occur in some region remote from
human observation. Wherever the order of organic nature is exposed to the
view of zoologists and botanists, it expels this conception; and the
conception survives only in connexion with imagined places, where the order
of organic nature is unknown.
Besides being absolutely without evidence to give it external support, this
hypothesis of special creations cannot support itself internally--cannot be
framed into a coherent thought. It is one of those illegitimate symbolic
conceptions which are mistaken for legitimate symbolic conceptions (_First
Principles_, § 9), because they remain untested. Immediately an attempt is
made to elaborate the idea into anything like a definite shape, it proves
to be a pseud-idea, admitting of no definite shape. Is it supposed that a
new organism, when specially created, is created out of nothing? If so,
there is a supposed creation of matter; and the creation of matter is
inconceivable--implies the establishment of a relation in thought between
nothing and something--a relation of which one term is absent--an
impossible relation. Is it supposed that the matter of which the new
organism consists is not created for the occasion, but is taken out of its
pre-existing forms and arranged into a new form? If so, we are met by the
question--how is the re-arrangement effected? Of the myriad atoms going to
the composition of the new organism, all of them previously dispersed
through the neighbouring air and earth, does each, suddenly disengaging
itself from its combinations, rush to meet the rest, unite with them into
the appropriate chemical compounds, and then fall with certain others into
its appointed place in the aggregate of complex tissues and organs? Surely
thus to assume a myriad supernatural impulses, differing in their
directions and amounts, given to as many different atoms, is a
multiplication of mysteries rather than the solution of a mystery. For
every one of these impulses, not being the result of a force locally
existing in some other form, implies the creation of force; and the
creation of force is just as inconceivable as the creation of matter. It is
thus with all attempted ways of representing the process. The old Hebrew
idea that God takes clay and moulds a new creature, as a potter moulds a
vessel, is probably too grossly anthropomorphic to be accepted by any
modern defender of the special-creation doctrine. But having abandoned this
crude belief, what belief is he prepared to substitute? If a new organism
is not thus produced, then in what way is one produced? or rather--in what
way does he conceive a new organism to be produced? We will not ask for the
ascertained mode, but will be content with a mode which can be consistently
imagined. No such mode, however, is assignable. Those who entertain the
proposition that each kind of organism results from a divine interposition,
do so because they refrain from translating words into thoughts. They do
not really believe, but rather _believe they believe_. For belief, properly
so called, implies a mental representation of the thing believed, and no
such mental representation is here possible.
§ 113. If we imagine mankind to be contemplated by some being as
short-lived as an ephemeron, but possessing intelligence like our own--if
we imagine such a being studying men and women, during his few hours of
life, and speculating as to the mode in which they came into existence; it
is manifest that, reasoning in the usual way, he would suppose each man and
woman to have been separately created. No appreciable changes of structure
occurring in any of them during the time over which his observations
extended, this being would probably infer that no changes of structure were
taking place, or had taken place; and that from the outset each man and
woman had possessed all the characters then visible--had been originally
formed with them. The application is obvious. A human life is ephemeral
compared with the life of a species; and even the period over which the
records of all human lives extend, is ephemeral compared with the life of a
species. There is thus a parallel contrast between the immensely-long
series of changes which have occurred during the life of a species, and
that small portion of the series open to our view. And there is no reason
to suppose that the first conclusion drawn by mankind from this small part
of the series visible to them, is any nearer the truth than would be the
conclusion of the supposed ephemeral being respecting men and women.
This analogy, suggesting as it does how the hypothesis of special creations
is merely a formula for our ignorance, raises the question--What reason
have we to assume special creations of species but not of individuals;
unless it be that in the case of individuals we directly know the process
to be otherwise, but in the case of species do not directly know it to be
otherwise? Have we any ground for concluding that species were specially
created, except the ground that we have no immediate knowledge of their
origin? And does our ignorance of the manner in which they arose warrant us
in asserting that they arose by special creation?
Another question is suggested by this analogy. Those who, in the absence of
immediate evidence of the way in which species arose, assert that they
arose not in a natural way allied to that in which individuals arise, but
in a supernatural way, think that by this supposition they honour the
Unknown Cause of things; and they oppose any antagonist doctrine as
amounting to an exclusion of divine power from the world. But if divine
power is demonstrated by the separate creation of each species, would it
not have been still better demonstrated by the separate creation of each
individual? Why should there exist this process of natural genesis? Why
should not omnipotence have been proved by the supernatural production of
plants and animals everywhere throughout the world from hour to hour? Is it
replied that the Creator was able to make individuals arise from one
another in a natural succession, but not to make species thus arise? This
is to assign a limit to power instead of magnifying it. Either it was
possible or not possible to create species and individuals after the same
general method. To say that it was not possible is suicidal in those who
use this argument; and if it was possible, it is required to say what end
is served by the special creation of species which would not have been
better served by the special creation of individuals. Again, what is to be
thought of the fact that the immense majority of these supposed special
creations took place before mankind existed? Those who think that divine
power is demonstrated by special creations, have to answer the question--to
whom demonstrated? Tacitly or avowedly, they regard the demonstrations as
being for the benefit of mankind. But if so, to what purpose were the
millions of these demonstrations which took place on the Earth when there
were no intelligent beings to contemplate them? Did the Unknowable thus
demonstrate his power to himself? Few will have the hardihood to say that
any such demonstration was needful. There is no choice but to regard them,
either as superfluous exercises of power, which is a derogatory
supposition, or as exercises of power that were necessary because species
could not be otherwise produced, which is also a derogatory supposition.
§ 113a. Other implications concerning the divine character must be
recognized by those who contend that each species arose by divine fiat. It
is hardly supposable that Infinite Power is exercised in trivial actions
effecting trivial changes. Yet the organic world in its hundreds of
thousands of species shows in each sub-division multitudinous forms which,
though unlike enough to be classed as specifically distinct, diverge from
one another only in small details which have no significance in relation to
the life led. Sometimes the number of specific distinctions is so great
that did they result from human agency we should call them whimsical.
For example, in Lake Baikal are found 115 species of an amphipod,
_Gammarus_; and the multiplicity becomes startling on learning that this
number exceeds the number of all other species of the genus: various as are
the conditions to which, throughout the rest of the world, the genus is
subject. Still stranger seems the superfluous exercise of power on
examining the carpet of living forms at the bottom of the ocean. Not
dwelling on the immense variety of creatures unlike in type which live
miles below the surface in absolute darkness, it will suffice to instance
the _Polyzoa_ alone: low types of animals so small that a thousand of them
would not cover a square inch, and on which, nevertheless, there has been,
according to the view we are considering, an exercise of creative skill
such that by small variations of structure more than 350 species have been
produced!
Kindred illustrations are furnished by the fauna of caverns. Are we to
suppose that numerous blind creatures--crustaceans, myriapods, spiders,
insects, fishes--were specially made sightless to fit them for the Mammoth
Cave? Or what shall we say of the _Proteus_, a low amphibian with
rudimentary eyes, which inhabits certain caves in Carniola, Carinthia and
Dalmatia and is not found elsewhere. Must we conclude that God went out of
his way to devise an animal for these places?
More puzzling still is a problem presented to the special-creationist by a
batrachian inhabiting Central Australia. In a region once peopled by
numerous animals but now made unfit by continuous droughts, there exists a
frog which, when the pools are drying up, fills itself with water and
burrowing in the mud hibernates until the next rains; which may come in a
year or may be delayed for two years. What is to be thought of this
creature? Were its structure and the accompanying instinct divinely planned
to fit it to this particular habitat?
Many such questions might be asked which, if answered as the current theory
necessitates, imply a divine nature hardly like that otherwise assumed.
§ 114. Those who espouse the aboriginal hypothesis entangle themselves in
yet other theological difficulties. This assumption that each kind of
organism was specially designed, carries with it the implication that the
designer intended everything which results from the design. There is no
escape from the admission that if organisms were severally constructed with
a view to their respective ends, then the character of the constructor is
indicated both by the ends themselves, and the perfection or imperfection
with which the organisms are fitted to them. Observe the consequences.
Without dwelling on the question recently raised, why during untold
millions of years there existed on the Earth no beings endowed with
capacities for wide thought and high feeling, we may content ourselves with
asking why, at present, the Earth is largely peopled by creatures which
inflict on one another so much suffering? Omitting the human race, whose
defects and miseries the current theology professes to account for, and
limiting ourselves to the lower creation, what must we think of the
countless different pain-inflicting appliances and instincts with which
animals are endowed? Not only now, and not only ever since men have lived,
has the Earth been a scene of warfare among all sentient creatures; but
palæontology shows us that from the earliest eras geologically recorded,
there has been going on this universal carnage. Fossil structures, in
common with the structures of existing animals, show us elaborate weapons
for destroying other animals. We have unmistakable proof that throughout
all past time, there has been a ceaseless devouring of the weak by the
strong. How is this to be explained? How happens it that animals were so
designed as to render this bloodshed necessary? How happens it that in
almost every species the number of individuals annually born is such that
the majority die by starvation or by violence before arriving at maturity?
Whoever contends that each kind of animal was specially designed, must
assert either that there was a deliberate intention on the part of the
Creator to produce these results, or that there was an inability to prevent
them. Which alternative does he prefer?--to cast an imputation on the
divine character or to assert a limitation of the divine power? It is
useless for him to plead that the destruction of the less powerful by the
more powerful, is a means of preventing the miseries of decrepitude and
incapacity, and therefore works beneficently. For even were the chief
mortality among the aged instead of among the young, there would still
arise the unanswerable question--why were not animals constructed in such
ways as to avoid these evils? why were not their rates of multiplication,
their degrees of intelligence, and their propensities, so adjusted that
these sufferings might be escaped? And if decline of vigour was a necessary
accompaniment of age, why was it not provided that the organic actions
should end in sudden death, whenever they fell below the level required for
pleasurable existence? Will any one who contends that organisms were
specially designed, assert that they could not have been so designed as to
prevent suffering? And if he admits that they could have been made so as to
prevent suffering, will he assert that the Creator preferred making them in
such ways as to inflict suffering?
Even as thus presented the difficulty is sufficiently great; but it appears
immensely greater when we examine the facts more closely. So long as we
contemplate only the preying of the superior on the inferior, some good
appears to be extracted from the evil--a certain amount of life of a higher
order, is supported by sacrificing a great deal of life of a lower order.
So long, too, as we leave out all mortality but that which, by carrying off
the least perfect members of each species, leaves the most perfect members
to survive and multiply; we see some compensating benefit reached through
the suffering inflicted. But what shall we say on finding innumerable cases
in which the suffering inflicted brings no compensating benefit? What shall
we say when we see the inferior destroying the superior? What shall we say
on finding elaborate appliances for furthering the multiplication of
organisms incapable of feeling, at the expense of misery to organisms
capable of happiness?
Of the animal kingdom as a whole, more than half the species are parasites.
"The number of these parasites," says Prof. Owen, "may be conceived when it
is stated that almost every known animal has its peculiar species, and
generally more than one, sometimes as many as, or even more kinds than,
infest the human body." This parasitism begins among the most minute
creatures and pervades the entire animal kingdom from the lowest to the
highest. Even _Protozoa_, made visible to us only by the microscope, are
infested, as is _Paramoecium_ by broods of _Sphærophrya_; while in large
and complex animals parasites are everywhere present in great variety. More
than this is true. There are parasites upon parasites--an arrangement such
that those which are torturing the creatures they inhabit are themselves
tortured by indwelling creatures still smaller: looking like an ingenious
accumulation of pains upon pains.
But passing over the evils thus inflicted on animals of inferior dignity,
let us limit ourselves to the case of Man. The _Bothriocephalus latus_ and
the _Tænia solium_, are two kinds of tape-worm, which flourish in the human
intestines; producing great constitutional disturbances, sometimes ending
in insanity; and from the germs of the _Tænia_, when carried into other
parts of the body, arise certain partially-developed forms known as
_Cysticerci_, _Echinococci_, and _Coenuri_, which cause disorganization
more or less extensive in the brain, the lungs, the liver, the heart, the
eye, &c., often ending fatally after long-continued suffering. Five other
parasites, belonging to a different class, are found in the viscera of
man--the _Trichocephalus_, the _Oxyuris_, the _Strongylus_ (two species),
the _Ancylostomum_ and the _Ascaris_; which, beyond that defect of
nutrition which they necessarily cause, sometimes induce certain
irritations that lead to complete demoralization. Of another class of
_entozoa_, belonging to the subdivision _Trematoda_, there are five kinds
found in different organs of the human body--the liver and gall-duct, the
portal vein, the intestine, the bladder, the eye. Then we have the
_Trichina spiralis_, which passes through one phase of its existence
imbedded in the muscles and through another phase of its existence in the
intestine; and which, by the induced disease _Trichinosis_, has lately
committed such ravages in Germany as to cause a panic. To these we must add
the Guinea-worm, which in some part of Africa and India makes men miserable
by burrowing in their legs; and the more terrible African parasite the
_Bilharzia_, which affects 30 per cent. of the natives on the east coast
with bleeding of the bladder. From _entozoa_, let us pass to _epizoa_.
There are two kinds of _Acari_, one of them inhabiting the follicles of the
skin and the other producing the itch. There are creatures that bury
themselves beneath the skin and lay their eggs there; and there are three
species of lice which infest the surface of the body. Nor is this all.
Besides animal parasites there are sundry vegetal parasites, which grow and
multiply at our cost. The _Sarcina ventriculi_ inhabits the stomach, and
produces gastric disturbance. The _Leptothrix buccalis_ is extremely
general in the mouth, and may have something to do with the decay of teeth.
And besides these there are microscopic fungi which produce ringworm,
porrigo, pityriasis, thrush, &c. Thus the human body is the habitat of
parasites, internal and external, animal and vegetal, numbering, if all are
set down, between two and three dozen species; sundry of which are peculiar
to Man, and many of which produce great suffering and not unfrequently
death. What interpretation is to be put on these facts by those who espouse
the hypothesis of special creations? According to this hypothesis, all
these parasites were designed for their respective modes of life. They were
endowed with constitutions fitting them to live by absorbing nutriment from
the human body; they were furnished with appliances, often of a formidable
kind, enabling them to root themselves in and upon the human body; and they
were made prolific in an almost incredible degree, that their germs might
have a sufficient number of chances of finding their way into the human
body. In short, elaborate contrivances were combined to insure the
continuance of their respective races; and to make it impossible for the
successive generations of men to avoid being preyed on by them. What shall
we say to this arrangement? Shall we say that "the head and crown of
things," was provided as a habitat for these parasites? Shall we say that
these degraded creatures, incapable of thought or enjoyment, were created
that they might cause human misery? One or other of these alternatives must
be chosen by those who contend that every kind of organism was separately
devised by the Creator. Which do they prefer? With the conception of two
antagonist powers, which severally work good and evil in the world, the
facts are congruous enough. But with the conception of a supreme
beneficence, this gratuitous infliction of pain is absolutely incompatible.
§ 115. See then the results of our examination. The belief in special
creations of organisms arose among men during the era of profoundest
darkness; and it belongs to a family of beliefs which have nearly all died
out as enlightenment has increased. It is without a solitary established
fact on which to stand; and when the attempt is made to put it into
definite shape in the mind, it turns out to be only a pseud-idea. This
mere verbal hypothesis, which men idly accept as a real or thinkable
hypothesis, is of the same nature as would be one, based on a day's
observation of human life, that each man and woman was specially
created--an hypothesis not suggested by evidence but by lack of
evidence--an hypothesis which formulates ignorance into a semblance of
knowledge. Further, we see that this hypothesis, failing to satisfy men's
intellectual need of an interpretation, fails also to satisfy their moral
sentiment. It is quite inconsistent with those conceptions of the divine
nature which they profess to entertain. If infinite power was to be
demonstrated, then, either by the special creation of every individual, or
by the production of species by some method of natural genesis, it would be
better demonstrated than by the use of two methods, as assumed by the
hypothesis. And if infinite goodness was to be demonstrated, then, not only
do the provisions of organic structure, if they are specially devised, fail
to demonstrate it, but there is an enormous mass of them which imply
malevolence rather than benevolence.
Thus the hypothesis of special creations turns out to be worthless by its
derivation; worthless in its intrinsic incoherence; worthless as absolutely
without evidence; worthless as not supplying an intellectual need;
worthless as not satisfying a moral want. We must therefore consider it as
counting for nothing, in opposition to any other hypothesis respecting the
origin of organic beings.
CHAPTER III.
GENERAL ASPECTS OF THE EVOLUTION-HYPOTHESIS.
§ 116. Just as the supposition that races of organisms have been specially
created, is discredited by its origin; so, conversely, the supposition that
races of organisms have been evolved, is credited by its origin. Instead of
being a conception suggested and accepted when mankind were profoundly
ignorant, it is a conception born in times of comparative enlightenment.
Moreover, the belief that plants and animals have arisen in pursuance of
uniform laws, instead of through breaches of uniform laws, is a belief
which has come into existence in the most-instructed class, living in these
better-instructed times. Not among those who have disregarded the order of
Nature, has this idea made its appearance; but among those who have
familiarized themselves with the order of Nature. Thus the derivation of
this modern hypothesis is as favourable as that of the ancient hypothesis
is unfavourable.
§ 117. A kindred antithesis exists between the two families of beliefs, to
which the beliefs we are comparing severally belong. While the one family
has been dying out the other family has been multiplying. As fast as men
have ceased to regard different classes of phenomena as caused by special
personal agents, acting irregularly; so fast have they come to regard these
different classes of phenomena as caused by a general agency acting
uniformly--the two changes being correlatives. And as, on the one hand, the
hypothesis that each species resulted from a supernatural act, having lost
nearly all its kindred hypotheses, may be expected soon to die; so, on the
other hand, the hypothesis that each species resulted from the action of
natural causes, being one of an increasing family of hypotheses, may be
expected to survive.
Still greater will the probability of its survival and establishment
appear, when we observe that it is one of a particular genus of hypotheses
which has been rapidly extending. The interpretation of phenomena as
results of Evolution, has been independently showing itself in various
fields of inquiry, quite remote from one another. The supposition that the
Solar System has been evolved out of diffused matter, is a supposition
wholly astronomical in its origin and application. Geologists, without
being led thereto by astronomical considerations, have been step by step
advancing towards the conviction that the Earth has reached its present
varied structure by modification upon modification. The inquiries of
biologists have proved the falsity of the once general belief, that the
germ of each organism is a minute repetition of the mature organism,
differing from it only in bulk; and they have shown, contrariwise, that
every organism advances from simplicity to complexity through insensible
changes. Among philosophical politicians, there has been spreading the
perception that the progress of society is an evolution: the truth that
"constitutions are not made but grow," is seen to be a part of the more
general truth that societies are not made but grow. It is now universally
admitted by philologists that languages, instead of being artificially or
supernaturally formed, have been developed. And the histories of religion,
of science, of the fine arts, of the industrial arts, show that these have
passed through stages as unobtrusive as those through which the mind of a
child passes on its way to maturity. If, then, the recognition of evolution
as the law of many diverse orders of phenomena, has been spreading; may we
not say that there thence arises the probability that evolution will
presently be recognized as the law of the phenomena we are considering?
Each further advance of knowledge confirms the belief in the unity of
Nature; and the discovery that evolution has gone on, or is going on, in so
many departments of Nature, becomes a reason for believing that there is no
department of Nature in which it does not go on.
§ 118. The hypotheses of Special Creation and Evolution, are no less
contrasted in respect of their legitimacy as hypotheses. While, as we have
seen, the one belongs to that order of symbolic conceptions which are
proved to be illusive by the impossibility of realizing them in thought;
the other is one of those symbolic conceptions which are more or less fully
realizable in thought. The production of all organic forms by the
accumulation of modifications and of divergences by the continual addition
of differences to differences, is mentally representable in outline, if not
in detail. Various orders of our experiences enable us to conceive the
process. Let us look at one of the simplest.
There is no apparent similarity between a straight line and a circle. The
one is a curve; the other is defined as without curvature. The one encloses
a space; the other will not enclose a space though produced for ever. The
one is finite; the other may be infinite. Yet, opposite as the two are in
their characters, they may be connected together by a series of lines no
one of which differs from the adjacent ones in any appreciable degree.
Thus, if a cone be cut by a plane at right angles to its axis we get a
circle. If, instead of being perfectly at right angles, the plane subtends
with the axis an angle of 89° 59', we have an ellipse which no human eye,
even when aided by an accurate pair of compasses, can distinguish from a
circle. Decreasing the angle minute by minute, this closed curve becomes
perceptibly eccentric, then manifestly so, and by and by acquires so
immensely elongated a form so as to bear no recognizable resemblance to a
circle. By continuing this process the ellipse changes insensibly into a
parabola. On still further diminishing the angle, the parabola becomes an
hyperbola. And finally, if the cone be made gradually more obtuse, the
hyperbola passes into a straight line as the angle of the cone approaches
180°. Here then we have five different species of line--circle, ellipse,
parabola, hyperbola, and straight line--each having its peculiar properties
and its separate equation, and the first and last of which are quite
opposite in nature, connected together as members of one series, all
producible by a single process of insensible modification.
But the experiences which most clearly illustrate the process of general
evolution, are our experiences of special evolution, repeated in every
plant and animal. Each organism exhibits, within a short time, a series of
changes which, when supposed to occupy a period indefinitely great, and to
go on in various ways instead of one way, give us a tolerably clear
conception of organic evolution at large. In an individual development, we
see brought into a comparatively infinitesimal time, a series of
metamorphoses equally great with each of those which the hypothesis of
evolution assumes to have taken place during immeasurable geologic epochs.
A tree differs from a seed in every respect--in bulk, in structure, in
colour, in form, in chemical composition. Yet is the one changed in the
course of a few years into the other: changed so gradually, that at no
moment can it be said--Now the seed ceases to be and the tree exists. What
can be more widely contrasted than a newly-born child and the small,
semi-transparent, gelatinous spherule constituting the human ovum? The
infant is so complex in structure that a cyclopædia is needed to describe
its constituent parts. The germinal vesicle is so simple that it may be
defined in a line. Nevertheless, nine months suffice to develop the one out
of the other; and that, too, by a series of modifications so small, that
were the embryo examined at successive minutes, even a microscope would not
disclose any sensible changes. Aided by such facts, the conception of
general evolution may be rendered as definite a conception as any of our
complex conceptions can be rendered. If, instead of the successive minutes
of a child's foetal life, we take the lives of successive generations of
creatures--if we regard the successive generations as differing from one
another no more than the foetus differs in successive minutes; our
imaginations must indeed be feeble if we fail to realize in thought, the
evolution of the most complex organism out of the simplest. If a single
cell, under appropriate conditions, becomes a man in the space of a few
years; there can surely be no difficulty in understanding how, under
appropriate conditions, a cell may, in the course of untold millions of
years, give origin to the human race.
Doubtless many minds are so unfurnished with those experiences of Nature
out of which this conception is built, that they find difficulty in forming
it. Looking at things rather in their statical than in their dynamical
aspects, they never realize the fact that, by small increments of
modification, any amount of modification may in time be generated. The
surprise they feel on finding one whom they last saw as a boy, grown into a
man, becomes incredulity when the degree of change is greater. To such, the
hypothesis that by any series of changes a protozoon can give origin to a
mammal, seems grotesque--as grotesque as Galileo's assertion of the Earth's
movement seemed to his persecutors; or as grotesque as the assertion of the
Earth's sphericity seems now to the New Zealanders. But those who accept a
literally-unthinkable proposition as quite satisfactory, may not
unnaturally be expected to make a converse mistake.
§ 119. The hypothesis of evolution is contrasted with the hypothesis of
special creations, in a further respect. It is not simply legitimate
instead of illegitimate, because representable in thought instead of
unrepresentable; but it has the support of some evidence, instead of being
absolutely unsupported by evidence. Though the facts at present assignable
in _direct_ proof that by progressive modifications, races of organisms
which are apparently distinct from antecedent races have descended from
them, are not sufficient; yet there are numerous facts of the order
required. Beyond all question unlikenesses of structure gradually arise
among the members of successive generations. We find that there is going on
a modifying process of the kind alleged as the source of specific
differences: a process which, though slow, does, in time, produce
conspicuous changes--a process which, to all appearance, would produce in
millions of years, any amount of change.
In the chapters on "Heredity" and "Variation," contained in the preceding
Part, many such facts were given, and more might be added. Although little
attention has been paid to the matter until recent times, the evidence
already collected shows that there take place in successive generations,
alterations of structure quite as marked as those which, in successive
short intervals, arise in a developing embryo--nay, often much more marked;
since, besides differences due to changes in the relative sizes or parts,
there sometimes arise differences due to additions and suppressions of
parts. The structural modification proved to have taken place since
organisms have been observed, is not less than the hypothesis
demands--bears as great a ratio to this brief period, as the total amount
of structural change seen in the evolution of a complex organism out of a
simple germ, bears to that vast period during which living forms have
existed on the Earth.
We have, indeed, much the same kind and quantity of direct evidence that
all organic beings have arisen through the actions of natural causes, which
we have that all the structural complexities of the Earth's crust have
arisen through the actions of natural causes. Between the known
modifications undergone by organisms, and the totality of modifications
displayed in their structures, there is no greater disproportion than
between the observed geological changes, and the totality of geological
changes supposed to have been similarly caused. Here and there are
sedimentary deposits now slowly taking place. At this place a shore has
been greatly encroached on by the sea during recorded times; and at another
place an estuary has become shallower within some generations. In one
region an upheaval is going on at the rate of a few feet in a century;
while in another region occasional earthquakes cause slight variations of
level. Appreciable amounts of denudation by water are visible in some
localities; and in other localities glaciers are detected in the act of
grinding down the rocky surfaces over which they glide. But these changes
are infinitesimal compared with the aggregate of changes to which the
Earth's crust testifies, even in its still extant systems of strata. If,
then, the small changes now being wrought on the Earth's crust by natural
agencies, yield warrant for concluding that by such agencies acting through
vast epochs, all the structural complexities of the Earth's crust have been
produced; do not the small known modifications produced in races of
organisms by natural agencies, yield warrant for concluding that by natural
agencies have been produced all those structural complexities which we see
in them?
The hypothesis of Evolution then, has direct support from facts which,
though small in amount, are of the kind required; and the ratio which these
facts bear to the generalization based on them, seems as great as is the
ratio between facts and generalization which, in another case, produces
conviction.
§ 120. Let us put ourselves for a moment in the position of those who, from
their experiences of human modes of action, draw differences respecting the
mode of action of that Ultimate Power manifested to us through phenomena.
We shall find the supposition that each kind of organism was separately
designed and put together, to be much less consistent with their professed
conception of this Ultimate Power, than is the supposition that all kinds
of organisms have resulted from one unbroken process. Irregularity of
method is a mark of weakness. Uniformity of method is a mark of strength.
Continual interposition to alter a pre-arranged set of actions, implies
defective arrangement in those actions. The maintenance of those actions,
and the working out by them of the highest results, implies completeness of
arrangement. If human workmen, whose machines as at first constructed
require perpetual adjustment, show their increasing skill by making their
machines self-adjusting; then, those who figure to themselves the
production of the world and its inhabitants by a "Great Artificer," must
admit that the achievement of this end by a persistent process, adapted to
all contingencies, implies greater skill than its achievement by the
process of meeting the contingencies as they severally arise.
So, too, it is with the contrast under its moral aspect. We saw that to the
hypothesis of special creations, a difficulty is presented by the absence
of high forms of life during immeasurable epochs of the Earth's existence.
But to the hypothesis of evolution, absence of them is no such obstacle.
Suppose evolution, and this question is necessarily excluded. Suppose
special creations, and this question can have no satisfactory answer. Still
more marked is the contrast between the two hypotheses, in presence of that
vast amount of suffering entailed on all orders of sentient beings by their
imperfect adaptations to their conditions of life, and the further vast
amount of suffering entailed on them by enemies and by parasites. We saw
that if organisms were severally designed for their respective places in
Nature, the inevitable conclusion is that these innumerable kinds of
inferior organisms which prey on superior organisms, were intended to
inflict all the pain and mortality which results. But the hypothesis of
evolution involves us in no such dilemma. Slowly, but surely, evolution
brings about an increasing amount of happiness. In all forms of
organization there is a progressive adaptation, and a survival of the most
adapted. If, in the uniform working out of the process, there are evolved
organisms of low types which prey on those of higher types, the evils
inflicted form but a deduction from the average benefits. The universal
multiplication of the most adapted must cause the spread of those superior
organisms which, in one way or other, escape the invasions of the inferior;
and so tends to produce a type less liable to the invasions of the
inferior. Thus the evils accompanying evolution are ever being
self-eliminated. Though there may arise the question--Why could they not
have been avoided? there does not arise the question--Why were they
deliberately inflicted? Whatever may be thought of them, it is clear that
they do not imply gratuitous malevolence.
§ 121. In all respects, then, the hypothesis of evolution contrasts
favourably with the hypothesis of special creation. It has arisen in
comparatively-instructed times and in the most cultivated class. It is one
of those beliefs in the uniform concurrence of phenomena, which are
gradually supplanting beliefs in their irregular and arbitrary concurrence;
and it belongs to a genus of these beliefs which has of late been rapidly
spreading. It is a definitely-conceivable hypothesis; being simply an
extension to the organic world at large, of a conception framed from our
experiences of individual organisms; just as the hypothesis of universal
gravitation was an extension of the conception which our experiences of
terrestrial gravitation had produced. This definitely-conceivable
hypothesis, besides the support of numerous analogies, has the support of
direct evidence. We have proof that there is going on a process of the kind
alleged; and though the results of this process, as actually witnessed, are
minute in comparison with the totality of results ascribed to it, yet they
bear to such totality a ratio as great as that by which an analogous
hypothesis is justified. Lastly, that sentiment which the doctrine of
special creations is thought necessary to satisfy, is much better satisfied
by the doctrine of evolution; since this doctrine raises no contradictory
implications respecting the Unknown Cause, such as are raised by the
antagonist doctrine.
And now, having observed how, under its most general aspects, the
hypothesis of organic evolution commends itself to us by its derivation, by
its coherence, by its analogies, by its direct evidence, by its
implications; let us go on to consider the several orders of facts which
yield indirect support to it. We will begin by noting the harmonies between
it and sundry of the inductions set forth in Part II.
CHAPTER IV.
THE ARGUMENTS FROM CLASSIFICATION.
§ 122. In § 103, we saw that the relations which exist among the species,
genera, orders, and classes of organisms, are not interpretable as results
of any such causes as have usually been assigned. We will here consider
whether they are interpretable as the results of evolution. Let us first
contemplate some familiar facts.
The Norwegians, Swedes, Danes, Germans, Dutch, and Anglo-Saxons, form
together a group of Scandinavian races, which are but slightly divergent in
their characters. Welsh, Irish, and Highlanders, though they have
differences, have not such differences as hide a decided community of
nature: they are classed together as Celts. Between the Scandinavian race
as a whole and the Celtic race as a whole, there is a distinction greater
than that between the sub-divisions which make up the one or the other.
Similarly, the several peoples inhabiting Southern Europe are more nearly
allied to one another, than the aggregate they form is allied to the
aggregates of Northern peoples. If, again, we compare these European
varieties of Man, taken as a group, with that group of Eastern varieties
which had a common origin with it, we see a stronger contrast than between
the groups of European varieties themselves. And once more, ethnologists
find differences of still higher importance between the Aryan stock as a
whole and the Mongolian stock as a whole, or the Negro stock as a whole.
Though these contrasts are partially obscured by intermixtures, they are
not so much obscured as to hide the truths that the most-nearly-allied
varieties of Man are those which diverged from one another at
comparatively-recent periods; that each group of nearly-allied varieties is
more strongly contrasted with other such groups that had a common origin
with it at a remoter period; and so on until we come to the largest groups,
which are the most strongly contrasted, and of whose divergence no trace is
extant.
The relations existing among the classes and sub-classes of languages, have
been briefly referred to by Mr. Darwin in illustration of his argument. We
know that languages have arisen by evolution. Let us then see what grouping
of them evolution has produced. On comparing the dialects of adjacent
counties in England, we find that their differences are so small as
scarcely to distinguish them. Between the dialects of the Northern counties
taken together, and those of the Southern counties taken together, the
contrast is stronger. These clusters of dialects, together with those of
Scotland and Ireland, are nevertheless so similar that we regard them as
one language. The several languages of Scandinavian Europe, including
English, are much more unlike one another than are the several dialects
which each of them includes; in correspondence with the fact that they
diverged from one another at earlier periods than did their respective
dialects. The Scandinavian languages have nevertheless a certain community
of character, distinguishing them as a group from the languages of Southern
Europe; between which there are general and special affinities that
similarly unite them into a group formed of sub-groups containing
sub-sub-groups. And this wider divergence between the order of languages
spoken in Northern Europe and the order of languages spoken in Southern
Europe, answers to the longer time that has elapsed since their
differentiation commenced. Further, these two orders of modern European
languages, as well as Latin and Greek and certain extinct and spoken
languages of the East, are shown to have traits in common which unite them
into one great class known as Aryan languages; radically distinguished from
the classes of languages spoken by the other main divisions of the human
race.
§ 123. Now this kind of subordination of groups which we see arises in the
course of continuous descent, multiplication, and divergence, is just the
kind of subordination of groups which plants and animals exhibit: it is
just the kind of subordination which has thrust itself on the attention of
naturalists in spite of pre-conceptions.
The original idea was that of arrangement in linear order. We saw that even
after a considerable acquaintance with the structures of organisms had been
acquired, naturalists continued their efforts to reconcile the facts with
the notion of a uni-serial succession. The accumulation of evidence
necessitated the breaking up of the imagined chain into groups and
sub-groups. Gradually there arose the conviction that these groups do not
admit of being placed in a line. And the conception finally arrived at, is
that of certain great sub-kingdoms, very widely divergent, each made up of
classes much less divergent, severally containing orders still less
divergent; and so on with genera and species.
Hence this "grand fact in natural history of the subordination of group
under group, which from its familiarity does not always sufficiently strike
us," is perfectly in harmony with the hypothesis of evolution. The extreme
significance of this kind of relation among organic forms is dwelt on by
Mr. Darwin, who shows how an ordinary genealogical tree represents, on a
small scale, a system of grouping analogous to that which exists among
organisms in general, and which is explained on the supposition of a
genealogical tree by which all organisms are affiliated. If, wherever we
can trace direct descent, multiplication, and divergence, this formation of
groups within groups takes place; there results a strong presumption that
the groups within groups which constitute the animal and vegetal kingdoms,
have arisen by direct descent, multiplication, and divergence--that is, by
evolution.
§ 124. Strong confirmation of this inference is yielded by the fact, that
the more marked differences which divide groups are, in both cases,
distinguished from the less marked differences which divide sub-groups, by
this, that they are not simply greater in _degree_, but they are more
radical in _kind_. Objects, as the stars, may present themselves in small
clusters, which are again more or less aggravated into clusters of
clusters, in such manner that the individuals of each simple cluster are
much closer together than are the simple clusters gathered into a compound
cluster: in which case, the trait that unites groups of groups differs from
the trait that unites groups, not in _nature_ but only in _amount_. But
this is not so either with the groups and sub-groups which we know have
resulted from evolution, or with those which we here infer have resulted
from evolution. In both cases the highest or most general classes, are
marked off from one another by fundamental differences that have no common
measure with the differences that mark off small classes. Observe the
parallelism.
We saw that each sub-kingdom of animals is distinguished from other
sub-kingdoms, by some unlikeness in its main plan of organization; such as
the presence or absence of a peri-visceral cavity. Contrariwise, the
members of the smallest groups are united together, and separated from the
members of other small groups, by modifications which do not affect the
relations of essential parts. That this is just the kind of arrangement
which results from evolution, the case of languages will show.
On comparing the dialects spoken in different parts of England, we find
scarcely any difference but those of pronunciation: the structures of the
sentences are almost uniform. Between English and the allied modern
languages there are divergences of structure: there are some unlikenesses
of idiom; some unlikenesses in the ways of modifying the meanings of verbs;
and considerable unlikenesses in the uses of genders. But these
unlikenesses are not sufficient to hide a general community of
organization. A greater contrast of structure exists between these modern
languages of Western Europe, and the classic languages. Differentiation
into abstract and concrete elements, which is shown by the substitution of
auxiliary words for inflections, has produced a higher specialization,
distinguishing these languages as a group from the older languages.
Nevertheless, both the ancient and modern languages of Europe, together
with some Eastern languages derived from the same original, have, under all
their differences of organization, a fundamental likeness; since in all of
them words are formed by such a coalescence and integration of roots as
destroys the independent meanings of the roots. These Aryan languages, and
others which have the _amalgamate_ character, are united by it into a class
distinguished from the _aptotic_ and _agglutinate_ languages; in which the
roots are either not united at all, or so incompletely united that one of
them still retains its independent meaning. And philologists find that
these radical traits which severally determine the grammatical forms, or
modes of combining ideas, characterize the primary divisions among
languages.
So that among languages, where we know that evolution has been going on,
the greatest groups are marked off from one another by the strongest
structural contrasts; and as the like holds among groups of organisms,
there results a further reason for inferring that these have been evolved.
§ 125. There is yet another parallelism of like meaning. We saw (§ 101)
that the successively-subordinate groups--classes, orders, genera, and
species--into which zoologists and botanists segregate animals and plants,
have not, in reality, those definite values conventionally given to them.
There are well-marked species, and species so imperfectly marked that some
systematists regard them as varieties. Between genera strong contrasts
exist in many cases, and in other cases contrasts so much less decided as
to leave it doubtful whether they imply generic distinctions. So, too, is
it with orders and classes: in some of which there have been introduced
sub-divisions, having no equivalents in others. Even of the sub-kingdoms
the same truth holds. The contrast between the _Coelenterata_ and the
_Mollusca_, is far less than that between the _Coelenterata_ and the
_Vertebrata_.
Now just this same indefiniteness of value, or incompleteness of
equivalence, is observable in those simple and compound and re-compound
groups which we see arising by evolution. In every case the endeavour to
arrange the divergent products of evolution, is met by a difficulty like
that which would meet the endeavour to classify the branches of a tree,
into branches of the first, second, third, fourth, &c., orders--the
difficulty, namely, that branches of intermediate degrees of composition
exist. The illustration furnished by languages will serve us once more.
Some dialects of English are but little contrasted; others are strongly
contrasted. The alliances of the several Scandinavian tongues with one
another are different in degree. Dutch is much less distinct from German
than Swedish is; while between Danish and Swedish there is so close a
kinship that they might almost be regarded as widely-divergent dialects.
Similarly on comparing the larger divisions, we see that the various
languages of the Aryan stock have deviated from their original to very
unlike distances. The general conclusion is manifest. While the kinds of
human speech fall into groups, and sub-groups, and sub-sub-groups; yet the
groups are not equal to one another in value, nor have the sub-groups equal
values, nor the sub-sub-groups.
If, then, when classified, organisms fall into assemblages such that those
of the same grade are but indefinitely equivalent; and if, where evolution
is known to have taken place, there have arisen assemblages between which
the equivalence is similarly indefinite; there is additional reason for
inferring that organisms are products of evolution.
§ 126. A fact of much significance remains. If groups of organic forms have
arisen by divergence and re-divergence; and if, while the groups have been
developing from simple groups into compound groups, each group and
sub-group has been giving origin to more complex forms of its own type;
then it is inferable that there once existed greater structural likenesses
between the members of allied groups than exists now. This, speaking
generally, proves to be so.
Between the sub-kingdoms the gaps are extremely wide; but such distant
kinships as may be discerned, bear out anticipation. Thus in the formation
of the germinal layers there is a general agreement among them; and there
is a further agreement among sundry of them in the formation of a gastrula.
This simplest and earliest likeness, significant of primitive kinship, is
in most cases soon obscured by divergent modes of development; but sundry
sub-kingdoms continue to show relationships by the likenesses of their
larval forms; as we see in the trochophores of the _Polyzoa_, _Annelida_,
and _Mollusca_--sub-kingdoms the members of which by their later structural
changes are rendered widely unlike.
More decided approximations exist between the lower members of classes. In
tracing down the _Crustacea_ and the _Arachnida_ from their more complex to
their simpler forms, zoologists meet with difficulties: respecting some of
these simpler forms, it becomes a question which class they belong to. The
_Lepidosiren_, about which there have been disputes whether it is a fish or
an amphibian, is inferior, in the organization of its skeleton, to the
great majority of both fishes and amphibia. Widely as they differ from
them, the lower mammals have some characters in common with birds, which
the higher mammals do not possess.
Now since this kind of relationship of groups is not accounted for by any
other hypothesis, while the hypothesis of evolution gives us a clue to it;
we must include it among the supports of this hypothesis which the facts of
classification furnish.
§ 127. What shall we say of these leading truths when taken together? That
naturalists have been gradually compelled to arrange organisms in groups
within groups, and that this is the arrangement which we see arises by
descent, alike in individual families and among races of men, is a striking
circumstance. That while the smallest groups are the most nearly related,
there exist between the great sub-kingdoms, structural contrasts of the
profoundest kind, cannot but impress us as remarkable, when we see that
where it is known to take place evolution actually produces these
feebly-distinguished small groups, and these strongly-distinguished great
groups. The impression made by these two parallelisms, which add meaning to
each other, is deepened by the third parallelism, which enforces the
meaning of both--the parallelism, namely, that as, between the species,
genera, orders, classes, &c., which naturalists have formed, there are
transitional types; so between the groups, sub-groups, and sub-sub-groups,
which we know to have been evolved, types of intermediate values exist. And
these three correspondences between the known results of evolution and the
results here ascribed to evolution, have further weight given to them by
the fact, that the kinship of groups through their lowest members is just
the kinship which the hypothesis of evolution implies.
Even in the absence of these specific agreements, the broad fact of unity
amid multiformity, which organisms so strikingly display, is strongly
suggestive of evolution. Freeing ourselves from pre-conceptions, we shall
see good reason to think with Mr. Darwin, "that propinquity of descent--the
only known cause of the similarity of organic beings--is the bond, hidden
as it is by various degrees of modification, which is partly revealed to us
by our classifications." When we consider that this only known cause of
similarity, joined with the only known cause of divergence (the influence
of conditions), gives us a key to these likenesses obscured by
unlikenesses; we shall see that were there none of those remarkable
harmonies above pointed out, the truths of classification would still yield
strong support to our conclusion.
CHAPTER V.
THE ARGUMENTS FROM EMBRYOLOGY.
§ 127a. Already I have emphasized the truth that Nature is always more
complex than we suppose (§ 74a)--that there are complexities within
complexities. Here we find illustrated this truth under another aspect.
When seeking to formulate the arguments from Embryology, we are shown that
the facts as presented in Nature are not to be expressed in the simple
generalizations we at first make.
While we recognize this truth we must also recognize the truth that only by
enunciation and acceptance of imperfect generalizations can we progress to
perfect ones. The order of Evolution is conformed to by ideas as by other
things. The advance is, and must be, from the indefinite to the definite.
It is impossible to express the totality of any natural phenomenon in a
single proposition. To the primary statement expressing that which is most
dominant have to be added secondary statements qualifying it. We see this
even in so simple a case as the flight of a projectile. The young artillery
officer is first taught that a cannon-shot describes a curve treated as a
parabola, though literally part of an extremely eccentric ellipse not
distinguishable from a parabola. Presently he learns that atmospheric
resistance, causing a continual decrease of velocity, entails a deviation
from that theoretical path which is calculated on the supposition that the
velocity is uniform; and this incorrectness he has to allow for. Then,
further, there comes the lateral deviation due to wind, which may be
appreciable if the wind is strong and the range great. To introduce him all
at once to the correct conception thus finally reached would be impossible:
it has to be reached through successive qualifications. And that which
holds even in this simple case necessarily holds more conspicuously in
complex cases.
The title of the chapter suggests a metaphor, which is, indeed, something
more than a metaphor. There is an embryology of conceptions. That this
statement is not wholly a figure of speech, we shall see on considering
that cerebral organization is a part of organization at large; and that the
evolving nervous plexus which is the correlative of an evolving conception,
must conform to the general law of change conformed to in the evolution of
the whole nervous structure as well as in the evolution of the whole bodily
structure. As the body has at first a rude form, very remotely suggesting
that which is presently developed by the superposing of modifications on
modifications; so the brain as a whole and its contained ideas together
make up an inner world answering with extreme indefiniteness to that outer
world to which it is brought by successive approximations into tolerable
correspondence; and so any nervous plexus and its associated hypothesis,
which refer to some external group of phenomena under investigation, have
to reach their final developments by successive corrections.
This being the course of discovery must also be the course of exposition.
In pursuance of this course we may therefore fitly contemplate that early
_formula_ of embryological development which we owe to von Baer.
§ 128. Already in § 52, where the generalization of von Baer respecting the
relations of embryos was set forth, there was given the warning, above
repeated with greater distinctness, that it is only an adumbration.
In the words of his translator, he "found that in its earliest stage, every
organism has the greatest number of characters in common with all other
organisms in their earliest stages; that at a stage somewhat later, its
structure is like the structures displayed at corresponding phases by a
less extensive multitude of organisms; that at each subsequent stage,
traits are acquired which successively distinguished the developing embryo
from groups of embryos that it previously resembled--thus step by step
diminishing the class of embryos which it still resembles; and that thus
the class of similar forms is finally narrowed to the species of which it
is a member."
Assuming for a moment that this generalization is true as it stands, or
rather, assuming that the qualifications needed are not such as destroy its
correspondence with the average facts, we shall see that it has profound
significance. For if we follow out in thought the implications--if we
conceive the germs of all kinds of organisms simultaneously developing, and
imagine that after taking their first step together, at the second step one
half of the vast multitude diverges from the other half; if, at the next
step, we mentally watch the parts of each great assemblage beginning to
take two or more routes of development; if we represent to ourselves such
bifurcations going on, stage after stage, in all the branches; we shall see
that there must result an aggregate analogous, in its arrangement of parts,
to a tree. If this vast genealogical tree be contemplated as a whole, made
up of trunk, main branches, secondary branches, and so on as far as the
terminal twigs; it will be perceived that all the various kinds of
organisms represented by these terminal twigs, forming the periphery of the
tree, will stand related to one another in small groups, which are united
into groups of groups, and so on. The embryological tree, expressing the
developmental relations of organisms, will be similar to the tree which
symbolizes their classificatory relations. That subordination of classes,
orders, genera, and species, to which naturalists have been gradually led,
is just that subordination which results from the divergence and
re-divergence of embryos, as they all unfold. On the hypothesis of
evolution this parallelism has a meaning--indicates that primordial kinship
of all organisms, and that progressive differentiation of them, which the
hypothesis alleges. But on any other hypothesis the parallelism is
meaningless; or rather, it raises a difficulty; since it implies either an
effect without a cause or a design without a purpose.
§ 129. This conception of a tree, symbolizing the relationships of types
and a species derived from the same root, has a concomitant conception. The
implication is that each organism, setting out from the simple nucleated
cell, must in the course of its development follow the line of the trunk,
some main branch, some sub-branch, some sub-sub-branch, &c., of this
embryological tree; and so on till it reaches that ultimate twig
representing the species of which it is a member. It must in a general way
go through the particular line of forms which preceded it in all past
times: there must be what has been aptly called a "recapitulation" of the
successive ancestral structures. This, at least, is the conclusion
necessitated by the generalization we are considering under its original
crude form.
Von Baer lived in the days when the Development Hypothesis was mentioned
only to be ridiculed, and he joined in the ridicule. What he conceived to
be the meaning of these groupings of organisms and these relations among
their embryological histories, is not obvious. The only alternative to the
hypothesis of Evolution is the hypothesis of Special Creation; and as he
did not accept the one it is inferable that he accepted the other. But if
he did this he must in the first place have found no answer to the inquiry
why organisms specially created should have the embryological kinships he
described. And in the second place, after discovering that his alleged law
was traversed by many and various nonconformities, he would have been
without any explanation of these. Observe the positions which were open to
him and the reasons which show them to be untenable.
If it be said that the conditions of the case necessitated the derivation
of all organisms from simple germs, and therefore necessitated a
morphological unity in their primitive states; there arises the obvious
answer, that the morphological unity thus implied, is not the only
morphological unity to be accounted for. Were this the only unity, the
various kinds of organisms, setting out from a common primordial form,
should all begin from the first to diverge individually, as so many radii
from a centre; which they do not. If, otherwise, it be said that organisms
were framed upon certain types, and that those of the same type continue
developing together in the same direction, until it is time for them to
begin putting on their specialities of structure; then the answer is, that
when they do finally diverge they ought severally to develop in direct
lines towards their final forms. No reason can be assigned why, having
parted company, some should progress towards their final forms by irregular
or circuitous routes. On the hypothesis of design such deviations are
inexplicable.
The hypothesis of evolution, however, while it pre-supposes those kinships
among embryos in their early forms which are found to exist, also leads us
to expect nonconformities in their courses of development. If, as any
rational theory of evolution implies, the progressive differentiations of
types from one another during past times, have resulted from the direct and
indirect effects of external conditions--if races of organisms have become
different, either by immediate adaptations to unlike habits of life, or by
the mediate adaptations resulting from preservation of the individuals most
fitted for such habits of life, or by both; and if most embryonic changes
are significant of changes that were undergone by ancestral races; then
these irregularities must be anticipated. For the successive changes in
modes of life pursued by successive ancestral races, can have had no
regularity of sequence. In some cases they must have been more numerous
than in others; in some cases they must have been greater in degree than in
others; in some cases they must have been to simpler modes, in some cases
to more complex modes, and in some cases to modes neither higher nor lower.
Of two cognate races which diverged in the remote past, the one may have
had descendants that have remained tolerably constant in their habits,
while the other may have had descendants that have passed through
widely-aberrant modes of life; and yet some of these last may have
eventually taken to modes of life like those of the other races derived
from the same stock. And if the metamorphoses of embryos indicate, in a
general way, the changes of structure undergone by ancestors; then, the
later embryologic changes of such two allied races will be somewhat
different, though they may end in very similar forms. An illustration will
make this clear. Mr. Darwin says: "Petrels are the most aërial and oceanic
of birds, but in the quiet sounds of Tierra del Fuego, the _Puffinuria
berardi_, in its general habits, in its astonishing power of diving, its
manner of swimming, and of flying when unwillingly it takes flight, would
be mistaken by any one for an auk or grebe; nevertheless, it is essentially
a petrel, but with many parts of its organization profoundly modified." Now
if we suppose these grebe-like habits to be continued through a long epoch,
the petrel-form to be still more obscured, and the approximation to the
grebe-form still closer; it is manifest that while the chicks of the grebe
and the _Puffinuria_ will, during their early stages of development,
display that likeness involved by their common derivation from some early
type of bird, the chick of the _Puffinuria_ will eventually begin to show
deviations, representative of the ancestral petrel-structure, and will
afterwards begin to lose these distinctions and assume the grebe-structure.
Hence, remembering the perpetual intrusions of organisms on one another's
modes of life, often widely different; and remembering that these
intrusions have been going on from the beginning; we shall be prepared to
find that the general law of embryonic parallelism is qualified by
irregularities which are mostly small, in many cases considerable, and
occasionally great. The hypothesis of evolution accounts for these: it does
more--it implies the necessity of them.
§ 130. The substitutions of organs and the suppressions of organs, are
among those secondary embryological phenomena which harmonize with the
belief in evolution but cannot be reconciled with any other belief. Some
embryos, during early stages of development, possess organs that afterwards
dwindle away, as there arise other organs to discharge the same functions.
And in other embryos organs make their appearance, grow to certain points,
have no functions to discharge, and disappear by absorption.
We have a remarkable instance of substitution in the temporary appliances
for respiration, which some embryos exhibit. During the first phase of its
development, the mammalian embryo possesses a system of blood-vessels
distributed over what is called the _area vasculosa_--a system of vessels
homologous with one which, among fishes, serves for aërating the blood
until the permanent respiratory organs come into play. Now since this
system of blood-vessels, not being in proximity to an oxygenated medium,
cannot be serviceable to the mammalian embryo during development of the
lungs, as it is serviceable in the embryo-fish during development of the
gills, this needless formation of it is unaccountable as a result of
design. But it is quite congruous with the supposition that the mammalian
type arose out of lower vertebrate types. For in such case the mammalian
embryo, passing through states representing in a general way those which
its remote ancestors had in common with the lower _Vertebrata_, develops
this system of vessels in like manner with them. An instance more
significant still is furnished by certain _Amphibia_. One of the facts
early made familiar to the natural-history student is that the tadpole
breathes by external branchiæ, and that these, needful during its aquatic
life, dwindle away as fast as it develops the lungs fitting it for
terrestrial life. But in one of the higher _Amphibia_, the viviparous
Salamander, these transformations ordinarily undergone during the free life
of the larva, are undergone by the embryo in the egg. The branchiæ are
developed though there is no use for them: lungs being substituted as
breathing appliances before the creature is born.
Even more striking than the substitutions of organs are the suppressions of
organs. Mr. Darwin names some cases as "extremely curious; for instance,
the presence of teeth in foetal whales, which when grown up have not a
tooth in their heads;... It has even been stated on good authority that
rudiments of teeth can be detected in the beaks of certain embryonic
birds." Irreconcilable with any teleological theory, these facts do not
even harmonize with the theory of fixed types which are maintained by the
development of all the typical parts, even where not wanted; seeing that
the disappearance of these incipient organs during foetal life spoils the
typical resemblance. But while to other hypotheses these facts are
stumbling-blocks, they yield strong support to the hypothesis of evolution.
Allied to these cases, are the cases of what has been called retrograde
development. Many parasitic creatures and creatures which, after leading
active lives for a time, become fixed, lose, in their adult states, the
limbs and senses they had when young. It may be alleged, however, that
these creatures could not secure the habitats needful for them, without
possessing, during their larval stages, eyes and swimming appendages which
eventually become useless; that though, by losing these, their organization
retrogresses in one direction, it progresses in another direction; and
that, therefore, they do not exhibit the needless development of a higher
type on the way to a lower type. Nevertheless there are instances of a
descent in organization, following an apparently-superfluous ascent. Mr.
Darwin says that in some genera of cirripedes, "the larvæ become developed
either into hermaphrodites having the ordinary structure, or into what I
have called complemental males, and in the latter, the development has
assuredly been retrograde; for the male is a mere sack, which lives for a
short time, and is destitute of mouth, stomach, or other organ of
importance, excepting for reproduction."
§ 130a. But now let us contemplate more closely the energies at work in the
unfolding embryo, or rather the energies which the facts appear to imply.
Whatever natures we ascribe to the hypothetical units proper to each kind
of organism, we must conclude that from the beginning of embryonic
development, they have a proclivity towards the structure of that organism.
Because of their phylogenetic origin, they must tend towards the form of
the primitive type; but the superposed modifications, conflicting with
their initial tendency, must cause a swerving towards each successively
higher type. To take an illustration:--If in the germ-plasm out of which
will come a vertebrate animal there is a proclivity towards the primitive
piscine form, there must, if the germ-plasm is derived from a mammal, be
also from the outset a proclivity towards the mammalian form. While the
initial type tends continually to establish itself the terminal type tends
also to establish itself. The intermediate structures must be influenced by
their conflict, as well as by the conflict of each with the proclivities
towards the amphibian and reptilian types. This complication of tendencies
is increased by the intervention of several other factors.
There is the factor of economy. An embryo in which the transformations have
absorbed the smallest amount of energy and wasted the smallest amount of
matter, will have an advantage over embryos the transformations of which
have cost more in energy and matter: the young animal will set out with a
greater surplus of vitality, and will be more likely than others to live
and propagate. Again, in the embryos of its descendants, inheriting the
tendency to economical transformation, those which evolve at the least cost
will thrive more than the rest and be more likely to have posterity. Thus
will result a continual shortening of the processes. We can see alike that
this must take place and that it does take place. If the whole series of
phylogenetic changes had to be repeated--if the embryo mammal had to become
a complete fish, and then a complete amphibian, and then a complete
reptile, there would be an immense amount of superfluous building up and
pulling down, entailing great waste of time and of materials. Evidently
these abridgments which economy entails, necessitate that unfolding embryos
bear but rude resemblances to lower types ancestrally passed
through--vaguely represent their dominant traits only.
From this principle of economy arise several derivative principles, which
may be best dealt with separately.
§ 130b. In some cases the substitution of an abridged for an unabridged
course of evolution causes the entire disappearance of certain intermediate
forms. Structural arrangements once passed through during the unfolding are
dropped out of the series.
In the evolution of these embryos with which there is not laid up a large
amount of food-yolk there occurs at the outset a striking omission of this
kind. When, by successive fissions, the fertilized cell has given rise to a
cluster of cells constituting a hollow sphere, known as a _blastula_, the
next change under its original form is the introversion of one side, so as
to produce two layers in place of one. An idea of the change may be
obtained by taking an india-rubber ball (having a hole through which the
air may escape) and thrusting in one side until its anterior surface
touches the interior surface of the other side. If the cup-shaped structure
resulting be supposed to have its wide opening gradually narrowed, until it
becomes the mouth of an internal chamber, it will represent what is known
as a _gastrula_--a double layer of cells, of which the outer is called
epiblast and the inner hypoblast (answering to ectoderm and endoderm)
inclosing a cavity known as the _archenteron_, or primitive digestive sac.
But now in place of this original mode of forming the _gastrula_, there
occurs a mode known as delamination. Throughout its whole extent the single
layer splits so as to become a double layer--one sphere of cells inclosing
the other; and after this direct formation of the double layer there is a
direct formation of an opening through it into the internal cavity. There
is thus a shortening of the primitive process: a number of changes are left
out.
Often a kindred passing over of stages at later periods of development may
be observed. In certain of the _Mollusca_, as the _Patella chiton_, the egg
gives origin to a free-swimming larva known as a trochosphere, from which
presently comes the ordinary molluscous organization. In the highest
division of the Molluscs, however, the Cephalopods, no trochosphere is
formed. The nutritive matter laid up in the egg is used in building up the
young animal without any indication of an ancestral larva.
§ 130c. Among principles derived from the principle of economy is the
principle of pre-adaptation--a name which we may appropriately coin to
indicate an adaptation made in advance of the time at which it could have
arisen in the course of phylogenetic history.
How pre-adaptation may result from economy will be shown by an illustration
which human methods of construction furnish. Let us assume that building
houses of a certain type has become an established habit, and that, as a
part of each house, there is a staircase of given size. And suppose that in
consequence of changed conditions--say the walling in of the town, limiting
the internal space and increasing ground-rents--it becomes the policy to
build houses of many stories, let out in flats to different tenants. For
the increased passing up and down, a staircase wider at its lower part will
be required. If now the builder, when putting up the ground floor, follows
the old dimensions, then after all the stories are built, the lower part of
the staircase, if it is to yield equal facilities for passage, must be
reconstructed. Instead of a staircase adapted to those few stories which
the original type of house had, economy will dictate a pre-adaptation of
the staircase to the additional stories.
On carrying this idea with us, we shall see that if from some type of
organism there is evolved a type in which enlargement of a certain part is
needed to meet increased functions, the greater size of this part will
begin to show itself during early stages of unfolding. That unbuilding and
rebuilding which would be needful were it laid down of its original size,
will be made needless if from the beginning it is laid down of a larger
size. Hence, in successive generations, the greater prosperity and
multiplication of individuals in which this part is at the outset somewhat
larger than usual, must eventually establish a marked excess in its
development at an early stage. The facts agree with this inference.
Referring to the contrasts between embryos, Mr. Adam Sedgwick says that "a
species is distinct and distinguishable from its allies from the very
earliest stages." Whereas, according to the law of von Baer, "animals so
closely allied as the fowl and duck would be indistinguishable in the early
stages of development," "yet I can distinguish a fowl and a duck embryo on
the second day by the inspection of a single transverse section through the
trunk." This experience harmonizes with the statement of the late Prof.
Agassiz, that in some cases traits characterizing the species appear at an
earlier period than traits characterizing the genus.
Similar in their implications are the facts recently published by Dr. E.
Mehnert, concerning the feet of pentadactyle vertebrates. A leading example
is furnished by the foot in the struthious birds. Out of the original five
digits the two which eventually become large while the others disappear,
soon give sign of their future predominance: their early sizes being in
excess of those required for the usual functional requirements in birds,
and preparing the way for their special requirements in the struthious
birds. Dr. Mehnert shows that a like lesson is given by the relative
developments of legs and wings in these birds. Ordinarily in vertebrates
the fore limbs grow more rapidly than the hind limbs; but in the ostrich,
in which the hind limbs or legs have to become so large while the wings are
but little wanted, the leg development goes in advance of the
wing-development in early embryonic stages: there is a pre-adaptation.
Much more striking are examples furnished by creatures whose modes of
existence require that they shall have enormous fertility--require that the
generative system shall be very large. Ordinarily the organs devoted to
maintenance of the race develop later than the organs devoted to
maintenance of the individual. But this order is inverted in certain
_Entozoa_. To these creatures, imbedded in nutritive matters,
self-maintenance cost nothing, and the structures devoted to it are
relatively of less importance than the structures devoted to
race-maintenance, which, to make up for the small chance any one germ has
of getting into a fit habitat, have to produce immense numbers of germs.
Here the rudiments of the generative systems are the first to become
visible--here, in virtue of the principle of pre-adaptation, a structure
belonging to the terminal form asserts itself so early in the developmental
process as almost to obliterate the structure of the initial form.
It may be that in some cases where the growth of certain organs goes in
advance of the normal order, the element of time comes into play--the
greater time required for construction. To elucidate this let us revert to
our simile. Suppose that the staircase above instanced, or at any rate its
lower part, is required to be of marble with balusters finely carved. If
this piece of work is not promptly commenced and pushed on fast, it will
not be completed when the rest of the house is ready: workmen and tools
will still block it up at a time when it should be available. Similarly
among the parts of an unfolding embryo, those in which there is a great
deal of constructive work must early take such shape as will allow of this.
Now of all the tissues the nervous tissue is that which takes longest to
repair when injured; and it seems a not improbable inference that it is a
tissue which is slower in its histological development than others. If this
be so, we may see why, in the embryos of the higher vertebrates, the
central nervous system quickly grows large in comparison to the other
systems--why by pre-adaptation the brain of a chick develops in advance of
other organs so much more than the brain of a fish.
§ 130d. Yet another complication has to be noted. From the principle of
economy, it seems inferable that decrease and disappearance of organs which
were useful in ancestral types but have ceased to be useful, should take
place uniformly; but they do not. In the words of Mr. Adam Sedgwick, "some
ancestral organs persist in the embryo in a functionless rudimentary
(vestigial) condition and at the same time without any reference to adult
structures, while other ancestral organs have disappeared without leaving a
trace."[46] This anomaly is rendered more striking when joined with the
fact that some of the structures which remain conspicuous are relatively
ancient, while some which have been obliterated are relatively modern--_e.
g._, "gill slits [which date back to the fish-ancestor], have been retained
in embryology, whereas other organs which have much more recently
disappeared, _e. g._ teeth of birds, fore-limbs of snakes [dating back to
the reptile ancestor], have been entirely lost."[47] Mr. Sedgwick ascribes
these anomalies to the difference between larval development and embryonic
development, and expresses his general belief thus:--
"The conclusion here reached is that, whereas larval development must
retain traces (it may be very faint) of ancestral stages of structure
because they are built out of ancestral stages, embryonic development
need not necessarily do so, and very often does not; that embryonic
development in so far as it is a record at all, is a record of structural
features of previous larval stages. Characters which disappear during
free life disappear also in the embryo, but characters which though lost
by the adult are retained in the larva may ultimately be absorbed into
the embryonic phase and leave their traces in embryonic development."[48]
To set forth the evidence justifying this view would encumber too much the
general argument. Towards elucidation of such irregularities let me name
two factors which should I think be taken into account.
Abridgment of embryonic stages cannot go on uniformly with all disused
organs. Where an organ is of such size that progressive diminution of it
will appreciably profit the young animal, by leaving it a larger surplus of
unused material, we may expect progressive diminution to occur.
Contrariwise, if the organ is relatively so small that each decrease will
not, by sensibly increasing the reserve of nutriment, give the young animal
an advantage over others, decrease must not be looked for: there may be a
survival of it even though of very ancient origin.
Again, the reduction of a superfluous part can take place only on condition
that the economy resulting from each descending variation of it, is of
greater importance than are the effects of variations simultaneously
occurring in other parts. If by increase or decrease of any other parts of
the embryo, survival of the animal is furthered in a greater degree than by
decrease of this superfluous part, then such decrease is unlikely; since it
is illegitimate to count upon the repeated concurrence of favourable
variations in two or more parts which are independent. So that if changes
of an advantageous kind are going on elsewhere in the embryo a useless part
may remain long undiminished.
Yet another cause operates, and perhaps cooperates. Embryonic survival of
an organ which has become functionless, may readily happen if, during
subsequent stages of development, parts of it are utilized as parts of
other organs. In the words of Mr. J. T. Cunningham:--
"It seems to be a general fact that a structure which in metamorphosis
disappears completely may easily be omitted altogether in embryonic
development, while one which is modified into something else continues to
pass more or less through its original larval condition." (_Science
Progress_, July, 1897, p. 488.)
One more factor of considerable importance should be taken into account. A
disused organ which entails evil because construction of it involves
needless cost, may entail further evil by being in the way. This, it seems
to me, is the reason why the fore-limbs of snakes have disappeared from
their embryos. When the long-bodied lizard out of which the ophidian type
evolved, crept through stiff herbage, and moved its head from side to side
to find openings, there resulted alternate bends of its body, which were
the beginnings of lateral undulations; and we may easily see that in
proportion as it thus progressed by insinuating itself through interstices,
the fore-limbs, less and less used for walking, would be more and more in
the way; and the lengthening of the body, increasing the undulatory motion
and decreasing the use of the fore-limbs, would eventually make them
absolute impediments. Hence besides the benefit in economy of construction
gained by embryos in which the fore-limbs were in early stages a little
less developed than usual, they would gain an advantage by having, when
mature, smaller fore-limbs than usual, leading to greater facility of
locomotion. There would be a double set of influences causing, through
selection, a comparatively rapid decrease of these appendages. And we may I
think see also, on contemplating the kind of movement, that the fore-limbs
would be more in the way than the hind limbs, which would consequently
dwindle with such smaller rapidity as to make continuance of the rudiments
of them comprehensible.
§ 131-132. So that while the embryonic law enunciated by von Baer is in
harmony with the hypothesis of evolution, and is, indeed, a law which this
hypothesis implies, the nonconformities to the law are also interpretable
by this hypothesis.
Parallelism between the courses of development in species allied by remote
ancestry, is liable to be variously modified in correspondence with the
later ancestral forms passed through after divergence of such species. The
substitution of a direct for an indirect process of formation, which we
have reason to believe will show itself, must obscure the embryonic
history. And the principle of economy which leads to this substitution
produces effects that are very irregular and uncertain in consequence of
the endlessly varied conditions. Thus several causes conspire to produce
deviations from the general law.
Let it be remarked, finally, that the ability to trace out embryologic
kinships and the inability to do this, occur just where, according to the
hypothesis of Evolution, they should occur. We saw in § 100a that
zoologists are agreed in grouping animals into some 17 phyla--_Mollusca_,
_Arthropoda_, _Echinodermata_, &c.--each of which includes a number of
classes severally sub-divided into orders, genera, species. All the members
of each phylum are so related embryologically, that the existence of a
common ancestor of them in the remote past is considered certain. But when
it comes to the relations among the archaic ancestors, opinion is
unsettled. Whether, for instance, the primitive _Chordata_, out of which
the _Vertebrata_ emerged, have molluscan affinities or annelidan
affinities, is still a matter in dispute. With regard to the origins of
various other types no settled conclusions are held. Now it is clear that
on tracing down each branch of the great genealogical tree, kinships would
be much more manifest among the recently-differentiated forms than among
those forms which diverged from one another in the earliest stages of
organic life, and had separated widely before any of the types we now know
had come into existence.
CHAPTER VI.
THE ARGUMENTS FROM MORPHOLOGY.
§ 133. Leaving out of consideration those parallelisms among their modes of
development which characterize organisms belonging to each group, that
community of plan which exists among them when mature is extremely
remarkable and extremely suggestive. As before shown (§ 103), neither the
supposition that these combinations of attributes which unite classes are
fortuitous, nor the supposition that no other combinations were
practicable, nor the supposition of adherence to pre-determined typical
plans, suffices to explain the facts. An instance will best prepare the
reader for seeing the true meaning of these fundamental likenesses.
Under the immensely-varied forms of insects, greatly elongated like the
dragon-fly or contracted in shape like the lady-bird, winged like the
butterfly or wingless like the flea, we find this character in
common--there are primarily seventeen segments.[49] These segments may be
distinctly marked or they may be so fused as to make it difficult to find
the divisions between them, but they always exist. What now can be the
meaning of this community of structure throughout the hundred thousand
kinds of insects filling the air, burrowing in the earth, swimming in the
water? Why under the down-covered body of a moth and under the hard
wing-cases of a beetle, should there be discovered the same number of
divisions? Why should there be no more somites in the Stick-insect, or
other Phasmid a foot long, than there are in a small creature like the
louse? Why should the inert _Aphis_ and the swift-flying Emperor-butterfly
be constructed on the same fundamental plan? It cannot be by chance that
there exist equal numbers of segments in all these multitudes of species.
There is no reason to think it was _necessary_, in the sense that no other
number would have made a possible organism. And to say that it is the
result of _design_--to say that the Creator followed this pattern
throughout, merely for the purpose of maintaining the pattern--is to assign
an absurd motive. No rational interpretation of these and countless like
morphological facts, can be given except by the hypothesis of evolution;
and from the hypothesis of evolution they are corollaries. If organic forms
have arisen from common stocks by perpetual divergences and
re-divergences--if they have continued to inherit, more or less clearly,
the characters of ancestral races; then there will naturally result these
communities of fundamental structure among creatures which have severally
become modified in multitudinous ways and degrees, in adaptation to their
respective modes of life. To this let it be added that while the belief in
an intentional adhesion to a pre-determined pattern throughout a whole
group, is negatived by the occurrence of occasional deviations from the
pattern; such deviations are reconcilable with the belief in evolution. As
pointed out in the last chapter, ancestral traits will be obscured more or
less according as the superposed modifications of structure, have or have
not been furthered by the conditions of life and development to which the
type has been subjected.
§ 134. Besides these wide-embracing and often deeply-hidden homologies,
which hold together different animals, there are the scarcely-less
significant homologies between different organs of the same animal. These,
like the others, are obstacles to the supernatural interpretations and
supports of the natural interpretation.
One of the most familiar and instructive examples is furnished by the
vertebral column. Snakes, which move sinuously through and over plants and
stones, obviously need a segmentation of the bony axis from end to end; and
inasmuch as flexibility is required throughout the whole length of the
body, there is advantage in the comparative uniformity of this
segmentation. The movements would be impeded if, instead of a chain of
vertebræ varying but little in their lengths, there existed in the middle
of the series some long bony mass that would not bend. But in the higher
_Vertebrata_, the mechanical actions and reactions demand that while some
parts of the vertebral column shall be flexible, other parts shall be
inflexible. Inflexibility is specially requisite in that part of it called
the sacrum; which, in mammals and birds, forms a fulcrum exposed to the
greatest strains the skeleton has to bear. Now in both mammals and birds,
this rigid portion of the vertebral column is not made of one long segment
or vertebra, but of several segments fused together. In man there are five
of these confluent sacral vertebræ; and in the ostrich tribe they number
from seventeen to twenty. Why is this? Why, if the skeleton of each species
was separately contrived, was this bony mass made by soldering together a
number of vertebræ like those forming the rest of the column, instead of
being made out of one single piece? And why, if typical uniformity was to
be maintained, does the number of sacral vertebræ vary within the same
order of birds? Why, too, should the development of the sacrum be by the
round-about process of first forming its separate constituent vertebræ, and
then destroying their separateness? In the embryo of a mammal or bird, the
central element of the vertebral column is, at the outset, continuous. The
segments that are to become vertebræ, arise gradually in the adjacent
mesoderm, and enwrap this originally-homogeneous axis or notochord. Equally
in those parts of the spine which are to remain flexible, and in those
parts which are to grow rigid, these segments are formed; and that part of
the spine which is to compose the sacrum, having acquired this segmental
structure, loses it again by coalescence of the segments. To what end is
this construction and re-construction? If, originally, the spine in
vertebrate animals consisted from head to tail of separate moveable
segments, as it does still in fishes and some reptiles--if, in the
evolution of the higher _Vertebrata_, certain of these moveable segments
were rendered less moveable with respect to one another, by the mechanical
conditions they were exposed to, and at length became relatively immovable;
it is comprehensible why the sacrum formed out of them, should continue
ever after to show its originally-segmented structure. But on any other
hypothesis this segmented structure is inexplicable. "We see the same law
in comparing the wonderfully complex jaws and legs in crustaceans," says
Mr. Darwin: referring to the fact that those numerous lateral appendages
which, in the lower crustaceans, most of them serve as legs, and have like
shapes, are, in the higher crustaceans, some of them represented by
enormously-developed claws, and others by variously-modified foot-jaws. "It
is familiar to almost every one," he continues, "that in a flower the
relative position of the sepals, petals, stamens, and pistils, as well as
their intimate structure, are intelligible on the view that they consist of
metamorphosed leaves arranged in a spire. In monstrous plants we often get
direct evidence of the possibility of one organ being transformed into
another; and we can actually see in embryonic crustaceans and in many other
animals, and in flowers, that organs, which when mature become extremely
different, are at an early stage of growth exactly alike." ... "Why should
one crustacean, which has an extremely complex mouth formed of many parts
consequently always have fewer legs; or conversely, those with many legs
have simpler mouths? Why should the sepals, petals, stamens, and pistils in
any individual flower, though fitted for such widely-different purposes, be
all constructed on the same pattern?"
To these and countless similar questions, the theory of evolution furnishes
the only rational answer. In the course of that change from homogeneity to
heterogeneity of structure displayed in evolution under every form, it will
necessarily happen that from organisms made up of numerous like parts,
there will arise organisms made up of parts more and more unlike: which
unlike parts will nevertheless continue to bear traces of their primitive
likeness.
§ 135. One more striking morphological fact, near akin to some of the facts
dwelt on in the last chapter, must be here set down--the frequent
occurrence, in adult animals and plants, of rudimentary and useless organs,
which are homologous with organs that are developed and useful in allied
animals and plants. In the last chapter we saw that during the development
of embryos, there often arise organs which disappear on being replaced by
other organs discharging the same functions in better ways; and that in
some cases, organs develop to certain points and are then re-absorbed
without performing any functions. Very generally, however, the
partially-developed organs are retained throughout life.
The osteology of the higher _Vertebrata_ supplies abundant examples.
Vertebral processes which, in one tribe, are fully formed and ossified from
independent centres, are, in other tribes, mere tubercles not having
independent centres of ossification. While in the tail of this animal the
vertebræ are severally composed of centrum and appendages, in the tail of
that animal they are simple osseous masses without any appendages; and in
another animal they have lost their individualities by coalescence with
neighbouring vertebræ into a rudimentary tail. From the structures of the
limbs analogous facts are cited by comparative anatomists. The undeveloped
state of certain metacarpal bones, characterizes whole groups of mammals.
In one case we find the normal number of digits; and, in another case, a
smaller number with an atrophied digit to make out the complement. Here is
a digit with its full number of phalanges; and there a digit of which one
phalange has been arrested in its growth. Still more remarkable are the
instances of entire limbs being rudimentary; as in certain snakes, which
have hind legs hidden beneath the integument. So, too, is it with dermal
appendages. Some of the smooth-skinned amphibia have scales buried in the
skin. The seal, which is a mammal considerably modified in adaptation to an
aquatic life, and which uses its feet mainly as paddles, has toes that
still bear external nails; but the manatee, which is a much more
transformed mammal, has nailless paddles which, when the skin is removed,
are said, by Humboldt, to display rudimentary nails at the ends of the
imbedded digits. Nearly all birds are covered with developed feathers,
severally composed of a shaft bearing fibres, each of which, again, bears a
fringe of down. But in some birds, as in the ostrich, various stages of
arrested development of the feathers may be traced: between the
unusually-elaborated feathers of the tail, and those about the beak which
are reduced to simple hairs, there are transitions. Nor is this the extreme
case. In the _Apteryx_ we see the whole of the feathers reduced to a
hair-like form. Again, the hair which commonly covers the body in mammals
is, over the greater part of the human body almost rudimentary, and is in
some parts reduced to mere down--down which nevertheless proves itself to
be homologous with the hair of mammals in general, by occasionally
developing into the original form. Numerous cases of aborted organs are
given by Mr. Darwin, of which a few may be here added. "Nothing can be
plainer," he remarks, "than that wings are formed for flight, yet in how
many insects do we see wings so reduced in size as to be utterly incapable
of flight, and not rarely lying under wing-cases, firmly soldered
together?" ... "In plants with separated sexes, the male flowers often have
a rudiment of a pistil; and Kölreuter found that by crossing such male
plants with an hermaphrodite species, the rudiment of the pistil in the
hybrid offspring was much increased in size; and this shows that the
rudiment and the perfect pistil are essentially alike in nature." And then,
to complete the proof that these undeveloped parts are marks of descent
from races in which they were developed, there are not a few direct
experiences of this relation. "We have plenty of cases of rudimentary
organs in our domestic productions--as the stump of a tail in tailless
breeds--the vestige of an ear in earless breeds--the re-appearance of
minute dangling horns in hornless breeds of cattle." (_Origin of Species_,
1859, pp. 451, 454.)
Here, as before, the teleological doctrine fails utterly; for these
rudimentary organs are useless, and occasionally even detrimental; as is
the _appendix vermiformis_, in Man--a part of the cæcum which is of no
value for the purpose of absorption but which, by detaining small foreign
bodies, often causes severe inflammation and death. The doctrine of typical
plans is equally out of court; for while, in some members of a group,
rudimentary organs completing the general type are traceable, in other
members of the same group such organs are unrepresented. There remains only
the doctrine of evolution; and to this, these rudimentary organs offer no
difficulties. On the contrary, they are among its most striking evidences.
§ 136. The general truths of morphology thus coincide in their
implications. Unity of type, maintained under extreme dissimilarities of
form and mode of life, is explicable as resulting from descent with
modification; but is otherwise inexplicable. The likenesses disguised by
unlikenesses, which the comparative anatomist discovers between various
organs in the same organism, are worse than meaningless if it be supposed
that organisms were severally framed as we now see them; but they fit in
quite harmoniously with the belief that each kind of organism is a product
of accumulated modifications upon modifications. And the presence, in all
kinds of animals and plants, of functionally-useless parts corresponding to
parts that are functionally-useful in allied animals and plants, while it
is totally incongruous with the belief in a construction of each organism
by miraculous interposition, is just what we are led to expect by the
belief that organisms have arisen by progression.
CHAPTER VII.
THE ARGUMENTS FROM DISTRIBUTION.
§ 137. In §§ 105 and 106, we contemplated the phenomena of distribution in
Space. The general conclusions reached, in great part based on the evidence
brought together by Mr. Darwin, were that, "on the one hand, we have
similarly-conditioned, and sometimes nearly-adjacent, areas, occupied by
quite different Faunas. On the other hand, we have areas remote from each
other in latitude, and contrasted in soil as well as climate, which are
occupied by closely-allied Faunas." Whence it was inferred that "as like
organisms are not universally, or even generally, found in like habitats;
nor very unlike organisms, in very unlike habitats; there is no manifest
pre-determined adaptation of the organisms to the habitats." In other
words, the facts of distribution in Space do not conform to the hypothesis
of design. At the same time we saw that "the similar areas peopled by
dissimilar forms, are those between which there are impassable barriers;
while the dissimilar areas peopled by similar forms, are those between
which there are no such barriers;" and these generalizations appeared to
harmonize with the abundantly-illustrated truth, "that each species of
organism tends ever to expand its sphere of existence--to intrude on other
areas, other modes of life, other media."
By way of showing still more clearly the effects of competition among races
of organisms, let me here add some recently-published instances of the
usurpations of areas, and changes of distribution hence resulting. In the
_Natural History Review_ for January, 1864, Dr. Hooker quotes as follows
from some New Zealand naturalists:--"You would be surprised at the rapid
spread of European and other foreign plants in this country. All along the
sides of the main lines of road through the plains, a _Polygonum_
(_aviculare_), called 'Cow Grass,' grows most luxuriantly, the roots
sometimes two feet in depth, and the plants spreading over an area from
four to five feet in diameter. The dock (_Rumex obtusifolius_ or _R.
crispus_) is to be found in every river bed, extending into the valleys of
the mountain rivers, until these become mere torrents. The sow-thistle is
spread all over the country, growing luxuriantly nearly up to 6000 feet.
The water-cress increases in our still rivers to such an extent, as to
threaten to choke them altogether ... I have measured stems twelve feet
long and three-quarters of an inch in diameter. In some of the mountain
districts, where the soil is loose, the white clover is completely
displacing the native grasses, forming a close sward.... In fact, the young
native vegetation appears to shrink from competition with these more
vigorous intruders." "The native (Maori) saying is 'as the white man's rat
has driven away the native rat, so the European fly drives away our own,
and the clover kills our fern, so will the Maoris disappear before the
white man himself.'"
Given this universal tendency of the superior to overrun the habitats of
the inferior,[50] let us consider what, on the hypothesis of evolution,
will be the effects on the geographical relationships of species.
§ 138. A race of organisms cannot expand its sphere of existence without
subjecting itself to new external conditions. Those of its members which
spread over adjacent areas, inevitably come in contact with circumstances
partially different from their previous circumstances; and such of them as
adopt the habits of other organisms, necessarily experience re-actions more
or less contrasted with the re-actions before experienced. Now if changes
of organic structure are caused, directly or indirectly, by changes in the
incidence of forces; there must result unlikenesses of structure between
the divisions of a race which colonizes new habitats. Hence, in the absence
of obstacles to migration, we may anticipate manifest kinships between the
animals and plants of one area, and those of areas adjoining it. This
inference corresponds with an induction before set down (§ 106). In
addition to illustrations of it already quoted from Mr. Darwin, his pages
furnish others. One is that species which inhabit islands are allied to
species which inhabit neighbouring main lands; and another is that the
faunas of clustered islands show marked similarities. "Thus the several
islands of the Galapagos Archipelago are tenanted," says Mr. Darwin, "in a
quite marvellous manner, by very closely related species; so that the
inhabitants of each separate island, though mostly distinct, are related in
an incomparably closer degree to each other than to the inhabitants of any
other part of the world." Mr. Wallace has traced "variation as specially
influenced by locality" among the _Papilionidæ_ inhabiting the East Indian
Archipelago: showing how "the species and varieties of Celebes possess a
striking character in the form of the anterior wings, different from that
of the allied species and varieties of all the surrounding islands;" and
how "tailed species in India and the western islands lose their tails as
they spread eastward through the archipelago." During his travels on the
Upper Amazons, Mr. Bates found that "the greater part of the species of
_Ithomiæ_ changed from one locality to another, not further removed than
100 to 200 miles;" that "many of these local species have the appearance of
being geographical varieties;" and that in some species "most of the local
varieties are connected with their parent form by individuals exhibiting
all the shades of variation."
Further general relationships are to be inferred. If races of organisms,
ever being thrust by pressure of population into new habitats, undergo
modifications of structure as they diverge more and more widely in Space,
it follows that, speaking generally, the widest divergences in Space will
indicate the longest periods during which the descendants from a common
stock have been subject to modifying conditions; and hence that, among
organisms of the same group, the smaller contrasts of structure will be
limited to the smaller areas. This we find: "varieties being," as Dr.
Hooker says in his _Flora of Tasmania_, "more restricted in locality than
species, and these again than genera." Again, if races of organisms spread,
and as they spread are altered by changing incident forces; it follows that
where the incident forces vary greatly within given areas, the alterations
will be more numerous than in equal areas which are less-variously
conditioned. This, too, proves to be the fact. Dr. Hooker points out that
the relatively uniform regions have the fewest species; while in the most
multiform regions the species are the most numerous.
§ 139. Let us consider next, how the hypothesis of evolution corresponds
with the facts of distribution, not over different areas but through
different media. If all forms of organisms have descended from some
primordial form, it follows that since this primordial form must have
inhabited some one medium out of the several media now inhabited, the
peopling of other media by its descendants implies migration from one
medium to others--implies adaptations to media quite unlike the original
medium. To speak specifically--water being the medium in which the lowest
living forms exist, the implication is that the earth and the air have been
colonized from the water. Great difficulties appear to stand in the way of
this assumption. Ridiculing those who alleged the uniserial development of
organic forms, who, indeed, laid themselves open to ridicule by their many
untenable propositions, Von Baer writes--"A fish, swimming towards the
shore desires to take a walk, but finds his fins useless. They diminish in
breadth for want of use, and at the same time elongate. This goes on with
children and grandchildren for a few millions of years, and at last who can
be astonished that the fins become feet? It is still more natural that the
fish in the meadow, finding no water, should gape after air, thereby, in a
like period of time developing lungs; the only difficulty being that in the
meanwhile, a few generations must manage without breathing at all."
Though, as thus presented, the belief in a transition looks laughable; and
though such derivation of terrestrial vertebrates by direct modification of
piscine vertebrates, is untenable; yet we must not conclude that no
migrations of the kind alleged can have taken place. The adage that "truth
is stranger than fiction," applies quite as much to Nature in general as to
human life. Besides the fact that certain fish actually do "take a walk"
without any obvious reason; and besides the fact that sundry kinds of fish
ramble about on land when prompted by the drying-up of the waters they
inhabit; there is the still more astounding fact that one kind of fish
climbs trees. Few things seem more manifestly impossible, than that a
water-breathing creature without efficient limbs, should ascend eight or
ten feet up the trunk of a palm; and yet the _Anabas scandens_ does as
much. To previous testimonies on this point Capt. Mitchell has recently
added others. Such remarkable cases of temporary changes of media, will
prepare us for conceiving how, under special conditions, permanent changes
of media may have taken place; and for considering how the doctrine of
evolution is elucidated by them.
Inhabitants of the sea, of rivers, and of lakes, are many of them left from
time to time partially or completely without water; and those which show
the power to change their media temporarily or permanently, are in very
many cases of the kinds most liable to be thus deserted by their medium.
Let us consider what the sea-shore shows us. Twice a day the rise and the
fall of the tide covers and uncovers plants and animals, fixed and moving;
and through the alternation of spring and neap tides, it results that the
exposure of the organisms living low down on the beach, varies both in
frequency and duration: while some of them are left dry only once a
fortnight for a very short time, others, a little higher up, are left dry
during two or three hours at several ebb tides every fortnight. Then by
small gradations we come to such as, living at the top of the beach, are
bathed by salt-water only at long intervals; and still higher to some which
are but occasionally splashed in stormy weather. What, now, do we find
among the organisms thus subject to various regular and irregular
alterations of media? Besides many plants and many fixed animals, we find
moving animals of numerous kinds; some of which are confined to the lower
zones of this littoral region, but others of which wander over the whole of
it. Omitting the humbler types, it will suffice to observe that each of the
two great sub-kingdoms, _Mollusca_ and _Arthropoda_, supplies examples of
creatures having a wide excursiveness within this region. We have
gasteropods which, when the tide is down, habitually creep snail-like over
sand and sea-weed, even up as far as high-water mark. We have several kinds
of crustaceans, of which the crab is the most conspicuous, running about on
the wet beach, and sometimes rambling beyond the reach of the water. And
then note the striking fact that each of the forms thus habituated to
changes of media, is allied to forms which are mainly or wholly
terrestrial. On the West Coast of Ireland marine gasteropods are found on
the rocks three hundred feet above the sea, where they are only at long
intervals wetted by the spray; and though between gasteropods of this class
and land-gasteropods the differences are considerable, yet the
land-gasteropods are more closely allied to them than to any other
_Mollusca_. Similarly, the two highest orders of crustaceans have their
species which live occasionally, or almost entirely, out of the water:
there is a kind of lobster in the Mauritius which climbs trees; and there
is the land-crab of the West Indies, which deserts the sea when it reaches
maturity and re-visits it only to spawn. Seeing, thus, how there are many
kinds of marine creatures whose habitats expose them to frequent changes of
media; how some of the higher kinds so circumstanced, show a considerable
adaptation to both media; and how these amphibious kinds are allied to
kinds that are mainly or wholly terrestrial; we shall see that the
migrations from one medium to another, which evolution pre-supposes, are by
no means impracticable. With such evidence before us, the assumption that
the distribution of the _Vertebrata_ through media so different as air and
water, may have been gradually effected in some analogous manner, would not
be altogether unwarranted even had we no clue to the process. We shall
find, however, a tolerably distinct clue. Though rivers, and lakes, and
pools, have no sensible tidal variations, they have their rises and falls,
regular and irregular, moderate and extreme. Especially in tropical
climates, we see them annually full for a certain number of months, and
then dwindling away and drying up. The drying up may reach various degrees
and last for various periods. It may go to the extent only of producing a
liquid mud, or it may reduce the mud to a hardened, fissured solid. It may
last for a few days or for months. That is to say, aquatic forms which are
in one place annually subject to a slight want of water for a short time,
are elsewhere subject to greater wants for longer times: we have gradations
of transition, analogous to those which the tides furnish. Now it is well
known that creatures inhabiting such waters have, in various degrees,
powers of meeting these contingencies. The contained fish either bury
themselves in the mud when the dry season comes, or ramble in search of
other waters. This is proved by evidence from India, Guiana, Siam, Ceylon;
and some of these fish, as the _Anabas scandens_, are known to survive for
days out of the water. But the facts of greatest significance are furnished
by an allied class of _Vertebrata_, almost peculiar to habitats of this
kind. The _Amphibia_ are not, like fish, usually found in waters that are
never partially or wholly dried up; but they nearly all inhabit waters
which, at certain seasons, evaporate, in great measure or
completely--waters in which most kinds of fish cannot exist. And what are
the leading structural traits of these _Amphibia_? They have two
respiratory systems--pulmonic and branchial--variously developed in
different orders; and they have two or four limbs, also variously
developed. Further, the class _Amphibia_ consists of two groups, in one of
which this duality of the respiratory system is permanent, and the
development of the limbs always incomplete; and in the other of which the
branchiæ disappear as the lungs and limbs become fully developed. The
lowest group, the _Perennibranchiata_, have internal organs for aerating
the blood which approach in various degrees to lungs, until "in the
_Siren_, the pulmonic respiration is more extensive and important than the
branchial;" and to these creatures, having a habitat partially aërial and
partially aquatic, there are at the same time supplied, in the shallow
water covering soft mud, the mechanical conditions which render swimming
difficult and rudimentary limbs useful. In the higher group, the
_Caducibranchiata_, we find still more suggestive transformations. Having
at first a structure resembling that which is permanent in the
perennibranchiate amphibian, the larva of the caducibranchiate amphibian
pursues for a time a similar life; but, eventually, while the branchial
appendages dwindle the lungs grow: the respiration of air, originally
supplementary to the respiration of water, predominates over it more and
more, till it replaces it entirely; and an additional pair of legs is
produced. This having been done, the creature either becomes, like the
_Triton_, one which quits the water only occasionally; or, like the Frog,
one which pursues a life mainly terrestrial, and returns to the water now
and then. Finally, if we ask under what conditions this metamorphosis of a
water-breather into an air-breather completes itself, the answer is--it
completes itself at the time when the shallow pools inhabited by the larvæ
are being dried up, or in danger of being dried up, by the summer's
sun.[51]
See, then, how significant are the facts when thus brought together. There
are particular habitats in which animals are subject to changes of media.
In such habitats exist animals having, in various degrees, the power to
live in both media, consequent on various phases of transitional
organization. Near akin to these animals there are some that, after passing
their early lives in the water, acquire more completely the structures
fitting them to live on land, to which they then migrate. Lastly, we have
closely-allied creatures, like the Surinam toad and the terrestrial
salamander, which, though they belong by their structures to the class
_Amphibia_, are not amphibious in their habits--creatures the larvæ of
which do not pass their early lives in the water, and yet go through these
same metamorphoses! Must we then think, like Von Baer, that the
distribution of kindred organisms through different media presents an
insurmountable difficulty? On the contrary, with facts like these before
us, the evolution-hypothesis supplies possible interpretations of many
phenomena that are else unaccountable. After seeing the ways in which such
changes of media are in some cases gradually imposed by physical
conditions, and in other cases voluntarily commenced and slowly increased
in the search after food; we shall begin to understand how, in the course
of evolution, there have arisen strange obscurations of one type by the
externals of another type. When we see land-birds occasionally feeding by
the water-side, and then learn that one of them, the water-ouzel, an
"anomalous member of the strictly terrestrial thrush family, wholly
subsists by diving--grasping the stones with its feet and using its wings
under water"--we are enabled to comprehend how, under pressure of
population, aquatic habits may be acquired by creatures organized for
aërial life; and how there may eventually arise an ornithic type in which
the traits of the bird are very much disguised. On finding among mammals
some that, in search of prey or shelter, have taken to the water in various
degrees, we shall cease to be perplexed on discovering the mammalian
structure hidden under a fish-like form, as it is in the _Cetacea_ and the
_Sirenia_: especially on finding that in the sea-lion and the seals there
are transitional forms. Grant that there has ever been going on that
re-distribution of organisms which we see still resulting from their
intrusions on one another's areas, media, and modes of life; and we have an
explanation of those multitudinous cases in which homologies of structure
are complicated with analogies. And while it accounts for the occurrence in
one medium of organic types fundamentally organized for another medium, the
doctrine of evolution accounts also for the accompanying unfitnesses.
Either the seal has descended from some mammal which little by little
became aquatic in its habits, in which case the structure of its hind limbs
has a meaning; or else it was specially framed for its present habitat, in
which case the structure of its hind limbs is incomprehensible.
§ 140. The facts respecting distribution in Time, which have more than any
others been cited both in proof and in disproof of evolution, are too
fragmentary to be conclusive either way. Were the geological record
complete, or did it, as both Uniformitarians and Progressionists have
commonly assumed, give us traces of the earliest organic forms; the
evidence hence derived, for or against, would have had more weight than any
other evidence. As it is, all we can do is to see whether such fragmentary
evidence as remains, is congruous with the hypothesis.
Palæontology has shown that there is a "general relation between lapse of
time and divergence of organic forms" (§ 107); and that "this divergence is
comparatively slow and continuous where there is continuity in the
geological formations, but is sudden and comparatively wide wherever there
occurs a great break in the succession of strata." Now this is obviously
what we should expect. The hypothesis implies structural changes that are
not sudden but gradual. Hence, where conformable strata indicate a
continuous record, we may anticipate successions of forms only slightly
different from one another; while we may rationally look for marked
contrasts between the groups of forms fossilized in adjacent strata, where
there is evidence of a great blank in the record.
The permanent disappearances of species, of genera, and of orders, which we
saw to be a fact tolerably-well established, is also a fact for which the
belief in evolution prepares us. If later organic forms have in all cases
descended from earlier organic forms, and have diverged during their
descent, both from their prototypes and from one another; then it follows
that such of them as become extinct at any epoch, will never re-appear at a
subsequent epoch; since there can never again arise a concurrence and
succession of conditions such as those under which each type was evolved.
Though comparisons of ancient and modern organic forms, prove that many
types have persisted through enormous periods of time, without undergoing
great changes; it was shown that such comparisons do not disprove the
occurrence in other organic forms, of changes great enough to produce what
are called different types. The result of inductive inquiry we saw to be,
that while a few modern higher types yield signs of having been developed
from ancient lower types; and that while there are many modern types which
_may_ have been thus developed, though we are without evidence that they
have been so; yet that "any admissible hypothesis of progressive
modification must be compatible with persistence without progression
through indefinite periods." Now these results are quite congruous with the
hypothesis of evolution. As rationally interpreted, evolution must in all
cases be understood to result, directly or indirectly, from the incidence
of forces. If there are no changes of conditions entailing organic changes,
organic changes are not to be expected. Only in organisms which fall under
conditions leading to additional modifications answering to additional
needs, will there be that increased heterogeneity which characterizes
higher forms. Hence, though the facts of palæontology cannot be held
conclusive proof of evolution, yet they are congruous with it; and some of
them yield it strong support.
§ 141. One general truth respecting distribution in Time, is profoundly
significant. If, instead of contemplating the relations among past forms of
life taken by themselves, we contemplate the relations between them and the
forms now existing, we find a connexion which is in harmony with the belief
in evolution but irreconcilable with any other belief.
Note, first, how full of meaning is the close kinship existing between the
aggregate of organisms now living, and the aggregate of organisms which
lived in the most recent geologic times. In the last-formed strata, nearly
all the imbedded remains are those of species which still flourish. Strata
a little older contain a few fossils of species now extinct, though,
usually, species greatly resembling extant ones. Of the remains found in
strata of still earlier date, the extinct species form a larger percentage;
and the differences between them and the allied species now living are more
marked. That is to say, the gradual change of organic types in Time, which
we before saw is indicated by the geological record, is equally indicated
by the relation between existing organic types and organic types of the
epochs preceding our own. The evidence completely accords with the belief
in a descent of present life from past life. Doubtless such a kinship is
not incongruous with the doctrine of special creations. It may be argued
that the introduction, from time to time, of new species better fitted to
the somewhat changed conditions of the Earth's surface, would result in an
apparent alliance between our living Flora and Fauna, and the Floras and
Faunas that lately lived. No one can deny it. But on passing from the most
general aspect of the alliance to its more special aspects, we shall find
this interpretation completely negatived.
For besides a close kinship between the aggregate of surviving forms and
the aggregate of forms which have died out in recent geologic times; there
is a peculiar connexion of like nature between present and past forms in
each great geographical region. The instructive fact, before cited from Mr.
Darwin, is the "wonderful relationship in the same continent between the
dead and the living." This relationship is not explained by the supposition
that new species have been at intervals supernaturally placed in each
habitat, as the habitat became modified; since, as we saw, species are by
no means uniformly found in the habitats to which they are best adapted. It
cannot be said that the marsupials imbedded in recent Australian strata,
having become extinct because of unfitness to some new external condition,
the existing marsupials were then specially created to fit the modified
environment; since sundry animals found elsewhere are so much more in
harmony with these new Australian conditions that, when taken to Australia,
they rapidly extrude the marsupials. While, therefore, the similarity
between the existing Australian Fauna and the Fauna which immediately
preceded it over the same area, is just that which the belief in evolution
leads us to expect; it is a similarity which cannot be otherwise accounted
for. And so is it with parallel relations in New England, in South America,
and in Europe.
§ 142. Given, then, that pressure which species exercise on one another, in
consequence of the universal overfilling of their respective
habitats--given the resulting tendency to thrust themselves into one
another's areas, and media, and modes of life, along such lines of least
resistance as from time to time are found--given besides the changes in
modes of life, hence arising, those other changes which physical
alterations of habitats necessitate--given the structural modifications
directly or indirectly produced in organisms by modified conditions; and
the facts of distribution in Space and Time are accounted for. That
divergence and re-divergence of organic forms, which we saw to be shadowed
forth by the truths of classification and the truths of embryology, we see
to be also shadowed forth by the truths of distribution. If that aptitude
to multiply, to spread, to separate, and to differentiate, which the human
races have in all times shown, be a tendency common to races in general, as
we have ample reason to assume; then there will result those kinds of
spacial relations and chronological relations among the species, and
genera, and orders, peopling the Earth's surface, which we find exist. The
remarkable identities of type discovered between organisms inhabiting one
medium, and strangely modified organisms inhabiting another medium, are at
the same time rendered comprehensible. And the appearances and
disappearances of species which the geological record shows us, as well as
the connexions between successive groups of species from early eras down to
our own, cease to be inexplicable.
CHAPTER VIII.
HOW IS ORGANIC EVOLUTION CAUSED?
§ 143. Already it has been necessary to speak of the causes of organic
evolution in general terms; and now we are prepared for considering them
specifically. The task before us is to affiliate the leading facts of
organic evolution, on those same first principles conformed to by evolution
at large.
Before attempting this, however, it will be instructive to glance at the
causes of organic evolution which have been from time to time alleged.
§ 144. The theory that plants and animals of all kinds were gradually
evolved, seems to have been at first accompanied only by the vaguest
conception of cause--or rather, by no conception of cause properly so
called, but only by the blank form of a conception. One of the earliest who
in modern times (1735) contended that organisms are indefinitely
modifiable, and that through their modifications they have become adapted
to various modes of existence, was De Maillet. But though De Maillet
supposed all living beings to have arisen by a natural, continuous process,
he does not appear to have had any definite idea of that which determines
this process. In 1794, in his _Zoonomia_, Dr. Erasmus Darwin gave reasons
(sundry of them valid ones) for believing that organized beings of every
kind, have descended from one, or a few, primordial germs; and along with
some observable causes of modification, which he points out as aiding the
developmental process, he apparently ascribes it, in part, to a tendency
given to such germ or germs when created. He suggests the possibility "that
all warm-blooded animals have arisen from one living filament, which THE
GREAT FIRST CAUSE endued with animality, with the power of acquiring new
parts, attended with new propensities, directed by irritations, sensations,
volitions, and associations; and thus possessing the faculty of continuing
to improve by its own inherent activity." In this passage we see the idea
to be, that evolution is pre-determined by some intrinsic proclivity. "It
is curious," says Mr. Charles Darwin, "how largely my grandfather, Dr.
Erasmus Darwin, anticipated the erroneous grounds of opinion, and the views
of Lamarck." One of the anticipations was this ascription of development to
some inherent tendency. To the "plan général de la nature, et sa marche
uniforme dans ses opérations," Lamarck attributes "la progression évidente
qui existe dans la composition de l'organisation des animaux;" and "la
_gradation_ régulière qu'ils devroient offrir dans la composition de leur
organisation," he thinks is rendered irregular by secondary causes.
Essentially the same in kind, though somewhat different in form, is the
conception put forth in the _Vestiges of Creation_; the author of which
contends "that the several series of animated beings, from the simplest and
oldest up to the highest and most recent, are, under the providence of God,
the results, _first_, of an impulse which has been imparted to the forms of
life, advancing them, in definite times, by generation, through grades of
organization terminating in the highest dicotyledons and vertebrata;" and
that the progression resulting from these impulses, is modified by certain
other causes. The broad contrasts between lower and higher forms of life,
are regarded by him as implying an innate aptitude to give birth to forms
of more perfect structures. The last to re-enunciate this doctrine has been
Prof. Owen; who asserts "the axiom of the continuous operation of creative
power, or of the ordained becoming of living things." Though these words do
not suggest a very definite idea, yet they indicate the belief that organic
progress is a result of some in-dwelling tendency to develop,
supernaturally impressed on living matter at the outset--some ever-acting
constructive force which, independently of other forces, moulds organisms
into higher and higher forms.
In whatever way it is formulated, or by whatever language it is obscured,
this ascription of organic evolution to some aptitude naturally possessed
by organisms, or miraculously imposed on them, is unphilosophical. It is
one of those explanations which explain nothing--a shaping of ignorance
into the semblance of knowledge. The cause assigned is not a true
cause--not a cause assimilable to known causes--not a cause that can be
anywhere shown to produce analogous effects. It is a cause unrepresentable
in thought: one of those illegitimate symbolic conceptions which cannot by
any mental process be elaborated into a real conception. In brief, this
assumption of a persistent formative power inherent in organisms, and
making them unfold into higher types, is an assumption no more tenable than
the assumption of special creations: of which, indeed, it is but a
modification; differing only by the fusion of separate unknown processes
into a continuous unknown process.
§ 145. Besides this intrinsic tendency to progress which Dr. Darwin
ascribes to animals, he says they have a capacity for being modified by
processes which their own desires initiate. He speaks of powers as "excited
into action by the necessities of the creatures which possess them, and on
which their existence depends;" and more specifically he says that "from
their first rudiment or primordium, to the termination of their lives, all
animals undergo perpetual transformations; which are in part produced by
their own exertions, in consequence of their desires and aversions, of
their pleasures and their pains, or of irritations, or of associations; and
many of these acquired forms or properties are transmitted to their
posterity." While it embodies a belief for which much may be said, this
passage involves the assumption that desires and aversions, existing before
experiences of the actions to which they are related, were the originators
of the actions, and therefore of the structural modifications caused by
them. In his _Philosophie Zoologique_, Lamarck much more specifically
asserts "le _sentiment intérieur_," to be in all creatures that have
developed nervous systems, an independent cause of those changes of form
which are due to the exercise of organs: distinguishing it from that simple
_irritability_ possessed by inferior animals, which cannot produce what we
call a desire or emotion; and holding that these last, along with all "qui
manquent de système nerveux, ne vivent qu'à l'aide des excitations qu'ils
reçoivent de l'extérieur." Afterwards he says--"je reconnus que la nature,
obligée d'abord d'emprunter des milieux environnants la _puissance
excitatrice_ des mouvements vitaux et des actions des animaux imparfaits,
sut, en composant de plus en plus l'organisation animale, transporter cette
puissance dans l'intérieur même de ces êtres, et qu'à la fin, elle parvint
à mettre cette même puissance à la disposition de l'individu." And still
more definitely he contends that if one considers "la _progression_ qui se
montre dans la composition de l'organisation," ... "alors on eût pu
apercevoir comment les _besoins_, d'abord réduits à nullité, et dont le
nombre ensuite s'est accru graduellement, ont amené le penchant aux actions
propres à y satisfaire: comment les actions devenues habituelles et
énergiques, ont occasionné le développement des organes qui les exécutent."
Now though this conception of Lamarck is more precisely stated, and worked
out with much greater elaboration and wider knowledge of the facts, it is
essentially the same as that of Dr. Darwin; and along with the truth it
contains, contains also the same error more distinctly pronounced. Merely
noting that desires or wants, acting directly only on the nervo-muscular
system, can have no immediate influence on very many organs, as the
viscera, or such external appendages as hair and feathers; and observing,
further, that even some parts which belong to the apparatus of external
action, such as the bones of the skull, cannot be made to grow by increase
of function called forth by desire; it will suffice to point out that the
difficulty is not solved, but simply slurred over, when needs or wants are
introduced as independent causes of evolution. True though it is, as Dr.
Darwin and Lamarck contend, that desires, by leading to increased actions
of motor organs, may induce further developments of such organs; and true,
as it probably is, that the modifications hence arising are transmissible
to offspring; yet there remains the unanswered question--Whence do these
desires originate? The transference of the exciting power from the exterior
to the interior, as described by Lamarck, begs the question. How comes
there a wish to perform an action not before performed? Until some
beneficial result has been felt from going through certain movements, what
can suggest the execution of such movements? Every desire consists
primarily of a mental representation of that which is desired, and
secondarily excites a mental representation of the actions by which it is
attained; and any such mental representations of the end and the means,
imply antecedent experience of the end and antecedent use of the means. To
assume that in the course of evolution there from time to time arise new
kinds of actions dictated by new desires, is simply to remove the
difficulty a step back.
§ 146. Changes of external conditions are named, by Dr. Darwin, as causes
of modifications in organisms. Assigning as evidence of original kinship,
that marked similarity of type which exists among animals, he regards their
deviations from one another, as caused by differences in their modes of
life: such deviations being directly adaptive. After enumerating various
appliances for procuring food, he says they all "seem to have been
gradually produced during many generations by the perpetual endeavour of
the creatures to supply the want of food, and to have been delivered to
their posterity with constant improvement of them for the purposes
required." And the creatures possessing these various appliances are
considered as having been rendered unlike by seeking for food in unlike
ways. As illustrating the alterations wrought by changed circumstances, he
names the acquired characters of domestic animals. Lamarck has elaborated
the same view in detail: using for the purpose, with great ingenuity, his
extensive knowledge of the animal kingdom. From a passage in the
_Avertissement_ it would at first sight seem that he looks upon direct
adaptation to new conditions as the chief cause of evolution. He says--"Je
regardai comme certain que le _mouvement des fluides_ dans l'intérieur des
animaux, mouvement qui c'est progressivement accéléré avec la composition
plus grande de l'organisation; et que _l'influence des circonstances_
nouvelles, à mesure que les animaux s'y exposèrent en se répandant dans
tous les lieux habitables, furent les deux causes générales qui ont amené
les différents animaux à l'état où nous les voyons actuellement." But
elsewhere the view he expresses appears decidedly different from this. He
asserts that "dans sa marche, la nature a commencé, et recommence encore
tous les jours, par former les corps organisés les plus simples;" and that
"les premières ébauches de l'animal et du végétal étant formées dans les
lieux et les circonstances convenables, les facultés d'une vie commençante
et d'un mouvement organique établi, ont nécessairement développé peu à peu
les organes, et qu'avec le temps elles les ont diversifies ainsi que les
parties." And then, further on, he puts in italics this proposition:--"_La
progression dans la composition de l'organisation subit, çà et là, dans la
série générale des animaux, des anomalies opérées par l'influence des
circonstances d'habitation, et par celle des habitudes contractées._"
These, and sundry other passages, joined with his general scheme of
classification, make it clear that Lamarck conceived adaptive modification
to be, not the cause of progression, but the cause of irregularities in
progression. The inherent tendency which organisms have to develop into
more perfect forms, would, according to him, result in a uniform series of
forms; but varieties in their conditions work divergences of structure,
which break up the series into groups: groups which he nevertheless places
in uni-serial order, and regards as still substantially composing an
ascending succession.
§ 147. These speculations, crude as they may be considered, show much
sagacity in their respective authors, and have done good service. Without
embodying the truth in definite shapes, they contain adumbrations of it.
Not directly, but by successive approximations, do mankind reach correct
conclusions; and those who first think in the right direction, loose as may
be their reasonings, and wide of the mark as their inferences may be, yield
indispensable aid by framing provisional conceptions and giving a bent to
inquiry.
Contrasted with the dogmas of his age, the idea of De Maillet was a great
advance. Before it can be ascertained how organized beings have been
gradually evolved, there must be reached the conviction that they _have_
been gradually evolved; and this conviction he reached. His wild notions
about the way in which natural causes acted in the production of plants and
animals, must not make us forget the merit of his intuition that animals
and plants _were_ produced by natural causes. In Dr. Darwin's brief
exposition, the belief in a progressive genesis of organisms is joined with
an interpretation having considerable definiteness and coherence. In the
space of ten pages he not only indicates several of the leading classes of
facts which support the hypothesis of development, but he does something
towards suggesting the process of development. His reasonings show an
unconscious mingling of the belief in a supernaturally-impressed tendency
to develop, with the belief in a development arising from the changing
incidence of conditions. Probably had he pursued the inquiry further, this
last belief would have grown at the expense of the first. Lamarck, in
elaborating this general conception, has given greater precision both to
its truth and to its error. Asserting the same imaginary factors and the
same real factors, he has traced out their supposed actions in detail; and
has, in consequence, committed himself to a greater number of untenable
positions. But while, in trying to reconcile the facts with a theory which
is only an adumbration of the truth, he laid himself open to the criticisms
of his contemporaries; he proved himself profounder than his contemporaries
by seeing that natural genesis, however caused, has been going on. If they
were wise in not indorsing a theory which fails to account for a great part
of the facts; they were unwise in ignoring that degree of congruity with
the facts, which shows the theory to contain some fundamental verity.
Leaving out, however, the imaginary factors of evolution which these
speculations allege, and looking only at the one actual factor which Dr.
Darwin and Lamarck assign as accounting for some of the phenomena; it is
manifest, from our present stand-point, that this, so far as it is a cause
of evolution, is a proximate cause and not an ultimate cause. To say that
functionally-produced adaptation to conditions originates either evolution
in general, or the irregularities of evolution, is to raise the further
question--why is there a functionally-produced adaptation to
conditions?--why do use and disuse generate appropriate changes of
structure? Neither this nor any other interpretation of biologic evolution
which rests simply on the basis of biologic induction, is an ultimate
interpretation. The biologic induction must itself be interpreted. Only
when the process of evolution of organisms is affiliated on the process of
evolution in general, can it be truly said to be explained. The thing
required is to show that its various results are corollaries from first
principles. We have to reconcile the facts with the universal laws of the
re-distribution of matter and motion.
CHAPTER IX.
EXTERNAL FACTORS.
§ 148. When illustrating the rhythm of motion (_First Principles_, § 83) it
was pointed out that besides the daily and annual alternations in the
quantities of light and heat which any portion of the Earth's surface
receives from the Sun, there are alternations which require
immensely-greater periods to complete. Reference was made to the fact that
"every planet, during a certain long period, presents more of its northern
than of its southern hemisphere to the Sun at the time of its nearest
approach to him; and then again, during a like period, presents more of its
southern hemisphere than of its northern--a recurring coincidence which,
though it causes in some planets no sensible alterations of climate,
involves, in the case of the Earth, an epoch of 21,000 years during which
each hemisphere goes through a cycle of temperate seasons, and seasons that
are extreme in their heat and cold." Further, we saw that there is a
variation of this variation. The slow rhythm of temperate and intemperate
climates, which takes 21,000 years to complete itself, undergoes
exaggeration and mitigation during epochs that are far longer. The Earth's
orbit slowly alters in form: now approximating to a circle, and now
becoming more eccentric. During the period in which the Earth's orbit has
least eccentricity, the temperate and intemperate climates which repeat
their cycle in 21,000 years, are severally less temperate and less
intemperate, than when, some one or two millions of years later, the
Earth's orbit has reached its extreme of eccentricity.
Thus, besides those daily variations in the quantities of light and heat
received by organisms, and responded to by variations in their functions;
and besides the annual variations in the quantities of light and heat which
organisms receive, and similarly respond to by variations in their
functions; there are variations that severally complete themselves in
21,000 years and in some millions of years--variations to which there must
also be responses in the changed functions of organisms. The whole vegetal
and animal kingdoms, are subject to quadruply-compounded rhythms in the
incidence of the forces on which life primarily depends--rhythms so
involved in their slow working round that at no time during one of these
vast epochs, can the incidence of these various forces be exactly the same
as at any other time. To the direct effects so produced on organisms, have
to be added much more important indirect effects. Changes of distribution
must result. Certain redistributions are occasioned even by the annual
variations in the quantities of the solar rays received by each part of the
Earth's surface. The migrations of birds thus caused are familiar. So, too,
are the migrations of certain fishes: in some cases from one part of the
sea to another; in some cases from salt water to fresh water; and in some
cases from fresh water to salt water. Now just as the yearly changes in the
amounts of light and heat falling on each locality, yearly extend and
restrict the habitats of many organisms which are able to move about with
some rapidity; so must the alterations of temperate and intemperate
climates produce extensions and restrictions of habitats. These, though
slow, must be universal--must affect the habitats of stationary organisms
as well as those of locomotive ones. For if, during an astronomic era,
there is going on at any limit to a plant's habitat, a diminution of the
winter's cold or summer's heat, which had before stopped its spread at that
limit; then, though the individual plants are fixed, yet the species will
move: the seeds of plants living at the limit, will produce individuals
which survive beyond the limit. The gradual spread so effected, having gone
on for some ten thousand years, the opposite change of climate will begin
to cause retreat. The tide of each species will, during one half of a long
epoch, slowly flow into new regions, and then will slowly ebb away from
them. Further, this rise and fall in the tide of each species will, during
far longer intervals, undergo increasing rises and falls and then
decreasing rises and falls. There will be an alteration of spring tides and
neap tides, answering to the changing eccentricity of the Earth's orbit.
These astronomical rhythms, therefore, entail on organisms unceasing
changes in the incidence of forces in two ways. They directly subject them
to variations of solar influences, in such a manner that each generation is
somewhat differently affected in its functions; and they indirectly bring
about complicated alterations in the environing agencies, by carrying each
species into the presence of new physical conditions, new soil and surface.
§ 149. The power of geological actions to modify everywhere the
circumstances in which plants and animals are placed, is conspicuous. In
each locality denudation slowly uncovers different deposits, and slowly
changes the exposed areas of deposits already uncovered. Simultaneously,
the alluvial beds in course of formation, are qualitatively affected by
these progressive changes in the natures and proportions of the strata
denuded. The inclinations of surfaces and their directions with respect to
the Sun, are at the same time modified; and the organisms existing on them
are thus having their thermal conditions continually altered, as well as
their drainage. Igneous action, too, complicates these gradual
modifications. A flat region cannot be step by step thrust up into a
protuberance without unlike climatic changes being produced in its several
parts, by their exposures to different aspects. Extrusions of trap,
wherever they take place, revolutionize the localities; both over the areas
covered and over the areas on to which their detritus is carried. And where
volcanoes are formed, the ashes they occasionally send out modify the
character of the soil throughout large surrounding tracts.
In like manner alterations in the Earth's crust cause the ocean to be ever
subjecting the organisms it contains to new combinations of conditions.
Here the water is being deepened by subsidence, and there shallowed by
upheaval. While the falling upon it of sediment brought down by
neighbouring large rivers, is raising the sea-bottom in one place, in
another the habitual rush of the tide is carrying away the sediment
deposited in past times. The mineral character of the submerged surface on
which sea-weeds grow and molluscs crawl, is everywhere occasionally
changed; now by the bringing away from an adjacent shore some previously
untouched strata; and now by the accumulation of organic remains, such as
the shells of pteropods or of foraminifera. A further series of alterations
in the circumstances of marine organisms, is entailed by changes in the
movements of the water. Each modification in the outlines of neighbouring
shores makes the tidal streams vary their directions or velocities or both.
And the local temperature is from time to time raised or lowered, because
some far-distant change of form in the Earth's crust has wrought a
divergence in those circulating currents of warm and cold water which
pervade the ocean.
These geologically-caused changes in the physical characters of each
environment, occur in ever-new combinations, and with ever-increasing
complexity. As already shown (_First Principles_, § 158), it follows from
the law of the multiplication of effects, that during long periods each
tract of the Earth's surface increases in heterogeneity of both form and
substance. So that plants and animals of all kinds are, in the course of
generations, subjected by alterations in the crust of the Earth, to sets of
incident forces differing from previous sets, both by changes in the
proportions of the factors and, occasionally, by the addition of new
factors.
§ 150. Variations in the astronomical conditions joined with variations in
the geological conditions, bring about variations in the meteorological
conditions. Those slow alternations of elevation and subsidence which take
place over immense areas, here producing a continent where once there was a
fathomless ocean, and there causing wide seas to spread where in a long
past epoch there stood snow-capped mountains, gradually work great
atmospheric changes. While the highest parts of an emerging surface of the
Earth's crust exist as a cluster of islands, the plants and animals which
in course of time migrate to them have climates that are peculiar to small
tracts of land surrounded by large tracts of water. As, by successive
upheavals, greater areas are exposed, there begin to arise sensible
contrasts between the states of their peripheral parts and their central
parts. The breezes which daily moderate the extremes of temperature near
the shores, cease to affect the interiors; and the interiors, less
qualified too in their heat and cold by such ocean-currents as approach the
coast, acquire more decidedly the characters due to their latitudes. Along
with the further elevations which unite the members of the archipelago into
a continent, there come new meteorologic changes, as well as exacerbations
of the old. The winds, which were comparatively uniform in their directions
and periods when only islands existed, grow involved in their distribution,
and widely-different in different parts of the continent. The quantities of
rain which they discharge and of moisture which they absorb, vary
everywhere according to the proximity to the sea and to surfaces of land
having special characters.
Other complications result from variations of height above the sea:
elevation producing a decrease of heat and consequently an increase in the
precipitation of water--a precipitation which takes the shape of snow where
the elevation is very great, and of rain where it is not so great. The
gatherings of clouds and descents of showers around mountain tops, are
familiar to every tourist. Inquiries in the neighbouring valleys prove that
within distances of a mile or two the recurring storms differ in their
frequency and violence. Nay, even a few yards off, the meteorological
conditions vary in such regions: as witness the way in which the condensing
vapour keeps eddying round on one side of some high crag, while the other
side is clear; or the way in which the snowline runs irregularly to
different heights, in all the hollows and ravines of each mountain side.
As climatic variations thus geologically produced, are compounded with
those which result from slow astronomical changes; and as no correspondence
exists between the geologic and the astronomic rhythms; it results that the
same plexus of actions never recurs. Hence the incident forces to which the
organisms of every locality are exposed by atmospheric agencies, are ever
passing into unparalleled combinations; and these are on the average ever
becoming more complex.
§ 151. Besides changes in the incidence of inorganic forces, there are
equally continuous, and still more involved, changes in the incidence of
forces which organisms exercise on one another. As before pointed out
(§ 105), the plants and animals inhabiting each locality are held together
in so entangled a web of relations, that any considerable modification
which one species undergoes, acts indirectly on many other species, and
eventually changes, in some degree, the circumstances of nearly all the
rest. If an increase of heat, or modification of soil, or decrease of
humidity, causes a particular kind of plant either to thrive or to dwindle,
an unfavourable or favourable effect is wrought on all such competing kinds
of plants as are not immediately influenced in the same way. The animals
which eat the seeds or browse on the leaves, either of the plant primarily
affected or those of its competitors, are severally altered in their states
of nutrition and in their numbers; and this change presently tells on
various predatory animals and parasites. And since each of these secondary
and tertiary changes becomes itself a centre of others, the increase or
decrease of each species produces waves of influence which spread and
reverberate and re-reverberate throughout the whole Flora and Fauna of the
locality.
More marked and multiplied still, are the ultimate effects of those causes
which make possible the colonization of neighbouring areas. Each intruding
plant or animal, besides the new inorganic conditions to which it is
subject, is subject to organic conditions different from those to which it
has been accustomed. It has to compete with some organisms unlike those of
its preceding habitat. It must preserve itself from enemies not before
encountered. Or it may meet with a species over which it has some advantage
greater than any it had over the species it was previously in contact with.
Even where migration does not bring it face to face with new competitors or
new enemies or new prey, it inevitably experiences new proportions among
these. Further, an expanding species is almost certain to invade more than
one adjacent region. Spreading both north and south, or east and west, it
will come among the plants and animals, here of a level district and there
of a hilly one--here of an inland tract and there of a tract bordered by
the sea. And while different groups of its members will thus expose
themselves to the actions and reactions of different Floras and Faunas,
these different Floras and Faunas will simultaneously have their organic
conditions changed by the intruders.
This process becomes gradually more active and more complicated. Though, in
particular cases, a plant or animal may fall into simpler relations with
the living things around than those it was before placed in, yet it is
manifest that, on the average, the organic environments of organisms have
been advancing in heterogeneity. As the number of species with which each
species is directly or indirectly implicated, multiplies, each species is
oftener subject to changes in the organic actions which influence it. These
more frequent changes severally grow more involved. And the corresponding
reactions affect larger Floras and Faunas, in ways increasingly complex and
varied.
§ 152. When the astronomic, geologic, meteorologic, and organic agencies
which are at work on each species of plant and animal are contemplated as
becoming severally more complicated in themselves, and as co-operating in
ways that are always partially new; it will be seen that throughout all
time there has been an exposure of organisms to endless successions of
modifying causes which gradually acquire an intricacy scarcely conceivable.
Every kind of plant and animal may be regarded as for ever passing into a
new environment--as perpetually having its relations to external
circumstances altered, either by their changes with respect to it when it
remains stationary, or by its changes with respect to them when it
migrates, or by both.
Yet a further cause of progressive alteration and complication in the
incident forces, exists. All other things continuing the same, every
additional faculty by which an organism is brought into relation with
external objects, as well as every improvement in such faculty, becomes a
means of subjecting the organism to a greater number and variety of
external stimuli, and to new combinations of external stimuli. So that each
advance in complexity of organization, itself becomes an added source of
complexity in the incidence of external forces.
Once more, every increase in the locomotive powers of animals, increases
both the multiplicity and the multiformity of the actions of things upon
them, and of their reactions upon things. Doubling a creature's activity
quadruples the area that comes within the range of its excursions; thus
augmenting in number and heterogeneity, the external agencies which act on
it during any given interval.
By compounding the actions of these several orders of factors, there is
produced a geometric progression of changes, increasing with immense
rapidity. And there goes on an equally rapid increase in the frequency with
which the combinations of the actions are altered, and the intricacies of
their co-operations enhanced.
CHAPTER X.
INTERNAL FACTORS.
§ 153. We saw at the outset (§§ 10-16), that organic matter is built up of
molecules so unstable, that the slightest variation in their conditions
destroys their equilibrium, and causes them either to assume altered
structures or to decompose. But a substance which is beyond all others
changeable by the actions and reactions of the forces liberated from
instant to instant within its own mass, must be a substance which is beyond
all others changeable by the forces acting on it from without. If their
composition fits organic aggregates for undergoing with special facility
and rapidity those re-distributions of matter and motion whence result
individual organization and life; then their composition must make them
similarly apt to undergo those permanent re-distributions of matter and
motion which are expressed by changes of structure, in correspondence with
permanent re-distributions of matter and motion in their environments.
In _First Principles_, when considering the phenomena of Evolution at
large, the leading characters and causes of those changes which constitute
organic evolution were briefly traced. Under each of the derivative laws of
force to which the passage from an incoherent, indefinite homogeneity to a
coherent, definite heterogeneity, conforms, were given illustrations drawn
from the metamorphoses of living bodies. Here it will be needful to
contemplate the several resulting processes as going on at once, in both
individuals and species.
§ 154. Our postulate being that organic evolution in general commenced with
homogeneous organic matter, we have first to remember that the state of
homogeneity is an unstable state (_First Principles_, § 149). In any
aggregate "the relations of outside and inside, and of comparative nearness
to neighbouring sources of influence, imply the reception of influences
that are unlike in quantity, or quality, or both; and it follows that
unlike changes will be produced in the parts thus dissimilarly acted upon."
Further, "if any given whole, instead of being absolutely uniform
throughout, consists of parts distinguishable from one another--if each of
these parts, while somewhat unlike other parts, is uniform within itself;
then, each of them being in unstable equilibrium, it follows that while the
changes set up within it must render it multiform, they must at the same
time render the whole more multiform than before;" and hence, "whether that
state with which we commence be or be not one of perfect homogeneity, the
process must equally be towards a relative heterogeneity." This loss of
homogeneity which the special instability of organic aggregates fits them
to display more promptly and variously than any other aggregates, must be
shown in more numerous ways in proportion as the incident forces are more
numerous. Every differentiation of structure being a result of some
difference in the relations of the parts to the agencies acting on them, it
follows that the more multiplied and more unlike the agencies, the more
varied must be the differentiations wrought. Hence the change from a state
of homogeneity to a state of heterogeneity, will be marked in proportion as
the environing actions to which the organism is supposes it is only are
complex. This transition from a uniform to a multiform state, must continue
through successive individuals. Given a series of organisms, each of which
is developed from a portion of a preceding organism, and the question is
whether, after exposure of the series for a million years to changed
incident forces, one of its members will be the same as though the incident
forces had only just changed. To say that it will, is implicitly to deny
the persistence of force. In relation to any cause of divergence, the whole
series of such organisms may be considered as fused together into a
continuously-existing organism; and when so considered, it becomes manifest
that a continuously-acting cause will go on working a
continuously-increasing effect, until some counteracting cause prevents any
further effect.
But now if any primordial organic aggregate must, in itself and through its
descendants, gravitate from uniformity to multiformity, in obedience to the
more or less multiform forces acting on it; what must happen if these
multiform forces are themselves undergoing slow variations and
complications? Clearly the process, ever-advancing towards a temporary
limit but ever having its limit removed, must go on unceasingly. On those
structural changes wrought in the once homogeneous aggregate by an original
set of incident forces, will be superposed further changes wrought by a
modified set of incident forces; and so on throughout all time. Omitting
for the present those circumstances which check and qualify its
consequences, the instability of the homogeneous must be recognized as an
ever-acting cause of organic evolution, as of all other evolution.
While it follows that every organism, considered as an individual and as
one of a series, tends thus to pass into a more heterogeneous state; it
also follows that every species, considered as an aggregate of individuals,
tends to do the like. Throughout the area it inhabits, the conditions can
never be absolutely uniform: its members must, in different parts of the
area, be exposed to different sets of incident forces. Still more decided
must this difference of exposure be when its members spread into other
habitats. Those expansive and repressive energies which set to each species
a limit that perpetually oscillates from side to side of a certain mean,
are, as we lately saw, frequently changed by new combinations of the
external factors--astronomic, geologic, meteorologic, and organic. Hence
there from time to time arise lines of diminished resistance, along which
the species flows into new localities. Such portions of the species as thus
migrate, are subject to circumstances unlike its previous average
circumstances. And from multiformity of the circumstances, must come
multiformity of the species.
Thus the law of the instability of the homogeneous has here a three-fold
corollary. As interpreted in connexion with the ever-progressing,
ever-complicating changes in external factors, it involves the conclusion
that there is a prevailing tendency towards greater heterogeneity in all
kinds of organisms, considered both individually and in successive
generations; as well as in each assemblage of organisms constituting a
species; and, by consequence, in each genus, order, and class.
§ 155. When considering the causes of evolution in general, we further saw
(_First Principles_, § 156), that the multiplication of effects aids
continually to increase that heterogeneity into which homogeneity
inevitably lapses. It was pointed out that since "the several parts of an
aggregate are differently modified by any incident force;" and since "by
the reactions of the differently modified parts the incident force itself
must be divided into differently modified parts;" it follows that "each
differentiated division of the aggregate thus becomes a centre from which a
differentiated division of the original force is again diffused. And since
unlike forces must produce unlike results, each of these differentiated
forces must produce, throughout the aggregate, a further series of
differentiations." To this it was added that, in proportion as the
heterogeneity increases, the complications arising from this multiplication
of effects grow more marked; because the more strongly contrasted the parts
of an aggregate become, the more different must be their reactions on
incident forces, and the more unlike must be the secondary effects which
these initiate; and because every increase in the number of unlike parts
adds to the number of such differentiated incident forces, and such
secondary effects.
How this multiplication of effects conspires, with the instability of the
homogeneous, to work an increasing multiformity of structure in an
organism, was shown at the time; and the foregoing pages contain further
incidental illustrations. In § 69 it was pointed out that a change in one
function must produce ever-complicating perturbations in other functions;
and that, eventually, all parts of the organism must be modified in their
states. Suppose that the head of a bison becomes much heavier, what must be
the indirect results? The muscles of the neck are put to greater exertions;
and its vertebræ have to bear additional tensions and pressures, caused
both by the increased weight of the head, and by the stronger contractions
of the muscles that support and move it. These muscles also affect their
special attachments: several of the dorsal spines suffer augmented strains;
and the vertebræ to which they are fixed are more severely taxed. Further,
this heavier head and the more massive neck it necessitates, require a
stronger fulcrum: the whole thoracic arch, and the fore-limbs which support
it, are subject to greater continuous stress and more violent occasional
shocks. And the required strengthening of the fore-quarters cannot take
place without the centre of gravity being changed, and the hind limbs being
differently reacted upon during locomotion. Any one who compares the
outline of the bison with that of its congener, the ox, will see how
profoundly a heavier head affects the entire osseous and muscular systems.
Besides this multiplication of mechanical effects, there is a
multiplication of physiological effects. The vascular apparatus is modified
throughout its whole structure by each considerable modification in the
proportions of the body. Increase in the size of any organ implies a
quantitative, and often a qualitative, reaction on the blood; and thus
alters the nutrition of all other organs. Such physiological correlations
are exemplified in the many differences which accompany difference of sex.
That the minor sexual peculiarities are brought about by the physiological
actions and reactions, is shown both by the fact that they are commonly but
faintly marked until the fundamentally distinctive organs are developed,
and that when the development of these is prevented, the minor sexual
peculiarities do not arise. No further proof is, I think, needed, that in
any individual organism or its descendants, a new external action must,
besides the primary internal change which it works, work many secondary
changes, as well as tertiary changes still more multiplied. That tendency
towards greater heterogeneity which is given to an organism by disturbing
its environment, is helped by the tendency which every modification has to
produce other modifications--modifications which must become more numerous
in proportion as the organism becomes more complex. Lastly, among the
indirect and involved manifestations of this tendency, we must not omit the
innumerable small irregularities of structure which result from the
crossing of dissimilarly-modified individuals. It was shown (§§ 89, 90)
that what are called "spontaneous variations," are interpretable as results
of miscellaneously compounding the changes wrought in different lines of
ancestors by different conditions of life. These still more complex and
multitudinous effects so produced, are further illustrations of the
multiplication of effects.
Equally in the aggregate of individuals constituting a species, does
multiplication of effects become the continual cause of increasing
multiformity. The lapse of a species into divergent varieties, initiates
fresh combinations of forces tending to work further divergences. The new
varieties compete with the parent species in new ways; and so add new
elements to its circumstances. They modify somewhat the conditions of other
species existing in their habitat, or in the habitat they have invaded; and
the modifications wrought in such other species become additional sources
of influence. The Flora and Fauna of every region are united by their
entangled relations into a whole, of which no part can be affected without
affecting the rest. Hence, each differentiation in a local assemblage of
species, becomes the cause of further differentiations.
§ 156. One of the universal principles to which we saw that the
re-distribution of matter and motion conforms, is that in any aggregate
made up of mixed units, incident forces produce segregation--separate
unlike units and bring together like units; and it was shown that the
increasing integration and definiteness which characterizes each part of an
evolving organic aggregate, as of every other aggregate, results from this
(_First Principles_, § 166). It remains here to say that while the actions
and reactions between organisms and their changing environments, add to the
heterogeneity of organic structures, they also give to the heterogeneity
this growing distinctness. At first sight the reverse might be inferred. It
might be argued that any new set of effects wrought in an organism by some
new set of external forces, must tend more or less to obliterate the
effects previously wrought--must produce confusion or indefiniteness. A
little consideration, however, will dissipate this impression.
Doubtless the condition under which alone increasing definiteness of
structure can be acquired by any part of an organism, either in an
individual or in successive generations, is that such part shall be exposed
to some set of tolerably-constant forces; and doubtless, continual change
of circumstances interferes with this. But the interference can never be
considerable. For the pre-existing structure of an organism prevents it
from living under any new conditions except such as are congruous with the
fundamental characters of its organization--such as subject its essential
organs to actions substantially the same as before. Great changes must kill
it. Hence, it can continuously expose itself and its descendants, only to
those moderate changes which do not destroy the general harmony between the
aggregate of incident forces and the aggregate of its functions. That is,
it must remain under influences calculated to make greater the definiteness
of the chief differentiations already produced. If, for example, we set out
with an animal in which a rudimentary vertebral column with its attached
muscular system has been established; it is clear that the mechanical
arrangements have become thereby so far determined, that subsequent
modifications are extremely likely, if not certain, to be consistent with
the production of movement by the actions of muscles on a flexible central
axis. Hence, there will continue a general similarity in the play of forces
to which the flexible central axis is subject; and so, notwithstanding the
metamorphoses which the vertebrate type undergoes, there will be a
maintenance of conditions favourable to increasing definiteness and
integration of the vertebral column. Moreover, this maintenance of such
conditions becomes secure in proportion as organization advances. Each
further complexity of structure, implying some further complexity in the
relations between an organism and its environment, must tend to specialize
the actions and reactions between it and its environment--must tend to
increase the stringency with which it is restrained within such
environments as admit of those special actions and reactions for which its
structure fits it; that is, must further guarantee the continuance of those
actions and reactions to which its essential organs respond, and therefore
the continuance of the segregating process.
How in each species, considered as an aggregate of individuals, there must
arise stronger and stronger contrasts among those divergent varieties which
result from the instability of the homogeneous and the multiplication of
effects, need only be briefly indicated. It has already been shown (_First
Principles_, § 166), that in conformity to the universal law that mixed
units are segregated by like incident forces, there are produced
increasingly-definite distinctions among varieties, wherever there occur
definitely-distinguished sets of conditions to which the varieties are
respectively subject.
§ 157. Probably in the minds of some, the reading of this chapter has been
accompanied by a running commentary, to the effect that the argument proves
too much. The apparent implication is, that the passage from an indefinite,
incoherent homogeneity to a definite, coherent heterogeneity in organic
aggregates, must have been going on universally; whereas we find that in
many cases there has been persistence without progression. This apparent
implication, however, is not a real one.
For though every environment on the Earth's surface undergoes changes; and
though usually the organisms which each environment contains, cannot escape
certain resulting new influences; yet occasionally such new influences are
escaped, by the survival of species in the unchanged parts of their
habitats, or by their spread into neighbouring habitats which the change
has rendered like their original habitats, or by both. Any alteration in
the temperature of a climate or its degree of humidity, is unlikely to
affect simultaneously the whole area occupied by a species; and further, it
can scarcely fail to happen that the addition or subtraction of heat or
moisture, will give to a part of some adjacent area, a climate like that to
which the species has been habituated. If, again, the circumstances of a
species are modified by the intrusion of some foreign kind of plant or
animal, it follows that since the intruders will probably not spread
throughout its whole habitat, the species will, in one or more localities,
remain unaffected by them. Especially among marine creatures, must there
frequently occur cases in which modifying causes are continually eluded.
Comparatively uniform as are the physical conditions to which the sea
exposes its inhabitants, it becomes possible for such of them as live on
widely-diffused food, to be widely distributed; and wide distribution
generally prevents the members of a species from being all subject to the
same cause. Our commonest cirriped, for instance, subsisting on minute
creatures everywhere dispersed through the water; needing only to have some
firm surface on which to build up its shell; and in scarcely any danger
from surrounding animals; is able to exist on shores so widely remote from
one another, that nearly every change in the incident forces must fall
within narrower areas than that which the species occupies. Nearly always,
therefore, a portion of the species will survive unmodified. Its
easily-transported germs will take possession of such new habitats as have
been rendered fitter by the change that has unfitted some parts of its
original habitat. Hence, on successive occasions, while some parts of the
species are slightly transformed, another part may continually escape
transformation by migrating hither and thither, where the simple conditions
needed for its existence recur in nearly the same combinations as before.
And it will so become possible for it to survive, with insignificant
structural changes, throughout long geologic periods.
§ 158. The results to which we find ourselves led, are these.
In subordination to the different amounts and kinds of forces to which its
different parts are exposed, every individual organic aggregate, like all
other aggregates, tends to pass from its original indistinct simplicity
towards a more distinct complexity. Unless we deny the persistence of
force, we must admit that the lapse of an organism's structure from an
indefinitely homogeneous to a definitely heterogeneous state, must be
cumulative in successive generations, if the forces causing it continue to
act. And for the like reasons, the increasing assemblage of individuals
arising from a common stock, is also liable to lose its original
uniformity; and, in successive generations, to grow more pronounced in its
multiformity.
These changes, which would go to but a comparatively small extent were
organisms exposed to constant external conditions, are kept up by the
continual changes in external conditions, produced by astronomic, geologic,
meteorologic, and organic agencies: the average result being, that on
previous complications wrought by previous incident forces, new
complications are continually superposed by new incident forces. And hence
simultaneously arises increasing heterogeneity in the structures of
individuals, in the structures of species, and in the structures of the
Earth's Flora and Fauna.
But while, in very many or in most cases, the ever-changing incidence of
forces is ever adding to the complexity of organisms, and to the complexity
of the organic world as a whole; it does this only where its action cannot
be eluded. And since, by migration, it is possible for a species to keep
itself under conditions that are tolerably constant, there must be a
proportion of cases in which greater heterogeneity of structure is not to
be expected.
To show, however, that there must arise a certain average tendency to the
production of greater heterogeneity is not sufficient. Aggregates might be
rendered more heterogeneous by changing incident forces, without having
given to them that kind of heterogeneity required for carrying on life.
Hence it remains now to inquire how the production and maintenance of this
kind of heterogeneity is insured.
CHAPTER XI.
DIRECT EQUILIBRATION.
§ 159. Every change is towards a balance of forces; and of necessity can
never cease until a balance of forces is reached. When treating of
equilibration under its general aspects (_First Principles_, Part II.,
Chap. xxii.), we saw that every aggregate having compound movements tends
continually towards a moving equilibrium; since any unequilibrated force to
which such an aggregate is subject, if not of a kind to overthrow it
altogether, must continue modifying its state until an equilibrium is
brought about. And we saw that the structure simultaneously reached must be
"one presenting an arrangement of forces that counterbalance all the forces
to which the aggregate is subject;" since, "so long as there remains a
residual force in any direction--be it excess of a force exercised by an
aggregate on its environment, or of a force exercised by its environment on
the aggregate, equilibrium does not exist; and therefore the
re-distribution of matter must continue."
It is essential that this truth should here be fully comprehended; and to
the end of insuring clear comprehension of it, some re-illustration is
desirable. The case of the Solar System will best serve our purpose. An
assemblage of bodies, each of which has its simple and compound motions
that severally alternate between two extremes, and the whole of which has
its involved perturbations, that now increase and now decrease, is here
presented to us. Suppose a new factor were brought to bear on this moving
equilibrium--say by the arrival of some wandering mass, or by an additional
momentum given to one of the existing masses--what would be the result? If
the strange body or the extra energy were very large, it might so derange
the entire system as to cause its collapse. But what if the incident
energy, falling on the system from without, proved insufficient to
overthrow it? There would then arise a set of perturbations which would, in
the course of an enormous period, slowly work round into a modified moving
equilibrium. The effects primarily impressed on the adjacent masses, and in
a smaller degree on the remoter masses, would presently become complicated
with the secondary effects impressed by the disturbed masses on one
another; and these again with tertiary effects. Waves of perturbation would
continue to be propagated throughout the entire system; until, around a new
centre of gravity, there had been established a set of planetary motions
different from the preceding ones. The new energy must gradually be used up
in overcoming the energies resisting the divergence it generates; which
antagonizing energies, when no longer opposed, set up a counter-action,
ending in a compensating divergence in the opposite direction, followed by
a re-compensating divergence, and so on. Now though instead of being, like
the Solar System, in a state of _independent_ moving equilibrium, an
organism is in a state of _dependent_ moving equilibrium (_First
Principles_, § 170); yet this does not prevent the manifestation of the
same law. Every animal daily obtains from without, a supply of energy to
replace the energy it expends; but this continual giving to its parts a new
momentum, to make up for the momentum continually lost, does not interfere
with the carrying on of actions and reactions like those just described.
Here, as before, we have a definitely-arranged aggregate of parts, called
organs, having their definitely-established actions and reactions, called
functions. These rhythmical actions or functions, and the various compound
rhythms resulting from their combinations, are so adjusted as to balance
the actions to which the organism is subject: there is a constant or
periodic genesis of energies which, in their kinds, amounts, and
directions, suffice to antagonize the energies the organism has constantly
or periodically to bear. If, then, there exists this moving equilibrium
among a set of internal actions, exposed to a set of external actions, what
must result if any of the external actions are changed? Of course there is
no longer an equilibrium. Some energy which the organism habitually
generates, is too great or too small to balance some incident energy; and
there arises a residual energy exerted by the environment on the organism,
or by the organism on the environment. This residual or unbalanced energy,
of necessity expends itself in producing some change of state in the
organism. Acting directly on some organ and modifying its function, it
indirectly modifies dependent functions and remotely influences all the
functions. As we have already seen (§§ 68, 69), if this new energy is
permanent, its effects must be gradually diffused throughout the entire
system; until it has come to be equilibrated in producing those structural
rearrangements whence result a counter-balancing energy.
The bearing of this general truth on the question we are now dealing with
is obvious. Those modifications upon modifications, which the unceasing
mutations of their environments have been all along generating in
organisms, have been in each case modifications involved by the
establishment of a new balance with the new combination of actions. In
every species throughout all geologic time, there has been perpetually
going on a rectification of the equilibrium, which has been perpetually
disturbed by the alteration of its circumstances; and every further
heterogeneity has been the addition of a structural change entailed by a
new equilibration, to the structural changes entailed by previous
equilibrations. There can be no other ultimate interpretation of the
matter, since change can have no other goal.
This equilibration between the functions of an organism and the actions in
its environment, may be either direct or indirect. The new incident force
may either immediately call forth some counteracting force, and its
concomitant structural change; or it may be eventually balanced by some
otherwise-produced change of function and structure. These two processes of
equilibration are quite distinct, and must be separately dealt with. We
will devote this chapter to the first of them.
§ 160. Direct equilibration is that process currently known as
_adaptation_. We have already seen (Part II., Chap, v.), that individual
organisms become modified when placed in new conditions of life--so
modified as to re-adjust the powers to the requirements; and though there
is great difficulty in disentangling the evidence, we found reason for
thinking (§ 82) that structural changes thus caused by functional changes
are inherited. In the last chapter, it was argued that if, instead of the
succession of individuals constituting a species, there were a
continuously-existing individual, any functional and structural divergence
produced by a new incident action, would increase until the new incident
action was counterpoised; and that the replacing of a continuously-existing
individual by a succession of individuals, each formed out of the modified
substance of its predecessor, will not prevent the like effect from being
produced. Here we further find that this limit towards which any such
organic change advances, in the species as in the individual, is a new
moving equilibrium adjusted to the new arrangement of external forces.
But now what are the conditions under which alone, direct equilibration can
occur? Are all the modifications that serve to re-fit organisms to their
environments, directly adaptive modifications? And if otherwise, which are
the directly adaptive and which are not? How are we to distinguish between
them?
There can be no direct equilibration with an external agency which, if it
acts at all, acts fatally; since the organism to be adapted disappears.
Conversely, some inaccessible benefit which a small modification in the
organism would make accessible, cannot by its action tend to produce this
modification: the modification and the benefit do not stand in dynamic
relation. The only new incident forces which can work the changes of
function and structure required to bring any animal or plant into
equilibrium with them, are such incident forces as operate on this animal
or plant, either continuously or frequently. They must be capable of
appreciably changing that set of complex rhythmical actions and reactions
constituting the life of the organism; and yet must not usually produce
perturbations that are fatal. Let us see what are the limits to direct
equilibration hence arising.
§ 161. In plants, organs engaged in nutrition, and exposed to variations in
the amounts and proportions of matters and forces utilized in nutrition,
may be expected to undergo corresponding variations. We find evidence that
they do this. The "changes of habit" which are common in plants, when taken
to places unlike in climate or soil to those before inhabited by them, are
changes of parts in which the modified external actions directly produce
modified internal actions. The characters of the stem and shoots as woody
or succulent, erect or procumbent; of the leaves in respect of their sizes,
thicknesses, and textures; of the roots in their degrees of development and
modes of growth; are obviously in immediate relation to the characters of
the environment. A permanent difference in the quantity of light or heat
affects, day after day, the processes going on in the leaves. Habitual rain
or drought alters all the assimilative actions, and appreciably influences
the organs that carry them on. Some particular substance, by its presence
in the soil, gives new qualities to some of the tissues; causing greater
rigidity or flexibility, and so affecting the general aspect. Here then we
have changes towards modified sets of functions and structures, in
equilibrium with modified sets of external forces.
But now let us turn to other classes of organs possessed by plants--organs
which are not at once affected in their actions by variations of incident
forces. Take first the organs of defence. Many plants are shielded against
animals that would else devour them, by formidable thorns; and others, like
the nettle, by stinging hairs. These must be counted among the appliances
by which equilibrium is maintained between the actions in the organism and
the actions in its environment; seeing that were these defences absent, the
destruction by herbivorous animals would be so much increased, that the
number of young plants annually produced would not suffice, as it now does,
to balance the mortality, and the species would disappear. But these
defensive appliances, though they aid in maintaining the balance between
inner and outer actions, cannot have been directly called forth by the
outer actions which they serve to neutralize; for these outer actions do
not continuously affect the functions of the plant even in a general way,
still less in the special way required. Suppose a species of nettle bare of
poison-hairs, to be habitually eaten by some mammal intruding on its
habitat. The actions of this mammal would have no direct tendency to
develop poison-hairs in the plant; since the individuals devoured could not
bequeath changes of structure, even were the actions of a kind to produce
fit ones; and since the individuals which perpetuated themselves would be
those on which the new incident force had not fallen. Organs of another
class, similarly circumstanced, are those of reproduction. Like the organs
of defence these are not, during the life of the individual plant, variably
exercised by variable external actions; and therefore do not fulfil those
conditions under which structural changes may be directly caused by changes
in the environment. The generative apparatus contained in every flower acts
only once during its existence; and even then, the parts subserve their
ends in a passive rather than an active way. Functionally-produced
modifications are therefore out of the question. If a plant's anthers are
so placed that the insect which most commonly frequents its flowers, must
come in contact with the pollen, and fertilize with it other flowers of the
same species; and if this insect, dwindling away or disappearing from the
locality, leaves behind no insects having such shapes and habits as cause
them to do the same thing efficiently, but only some which do it
inefficiently; it is clear that this change of its conditions has no
immediate tendency to work in the plant any such structural change as shall
bring about a new balance with its conditions. For the anthers, which, even
when they discharge their functions, do it simply by standing in the way of
the insect, are, under the supposed circumstances, left untouched by the
insect; and this remaining untouched cannot have the effect of so modifying
the stamens as to bring the anthers into a position to be touched by some
other insect. Only those individuals whose parts of fructification so far
differed from the average form that some other insect could serve them as
pollen-carrier, would have good chances of perpetuating themselves. And on
their progeny, inheriting the deviation, there would act no external force
directly tending to make the deviation greater; since the new circumstances
to which re-adaptation is required, are such as do not in the least alter
the equilibrium of functions constituting the life of the individual plant.
§ 162. Among animals, adaptation by direct equilibration is similarly
traceable wherever, during the life of the individual, an external change
generates some constant or repeated change of function. This is
conspicuously the case with such parts of an animal as are immediately
exposed to diffused influences, like those of climate, and with such parts
of an animal as are occupied in its mechanical actions on the environment.
Of the one class of cases, the darkening of the skin which follows exposure
to one or other extreme of temperature, may be taken as an instance; and
with the other class of cases we are made familiar by the increase and
decrease which use and disuse cause in the organs of motion. It is needless
here to exemplify these: they were treated of in the Second Part of this
work.
But in animals, as in plants, there are many indispensable offices
fulfilled by parts between which and the external conditions they respond
to, there is no such action and reaction as can directly produce an
equilibrium. This is especially manifest with dermal appendages. Some
ground exists for the conclusion that the greater or less development of
hairs, is in part immediately due to increase or decrease of demand on the
passive function, as forming a non-conducting coat; but be this as it may,
it is impossible that there can exist any such cause for those immense
developments of hairs which we see in the quills of the porcupine, or those
complex developments of them known as feathers. Such an enamelled armour as
is worn by _Lepidosteus_, is inexplicable as a direct result of any
functionally-worked change. For purposes of defence, such an armour is as
needful, or more needful, for hosts of other fishes; and did it result from
any direct reaction of the organism against any offensive actions it was
subject to, there seems no reason why other fishes should not have
developed similar protective coverings. Of sundry reproductive appliances
the like may be said. The secretion of an egg-shell round the substance of
an egg, in the oviduct of a bird, is quite inexplicable as a consequence of
some functionally-wrought modification of structure, immediately caused by
some modification of external conditions. The end fulfilled by the
egg-shell, is that of protecting the contained mass against certain slight
pressures and collisions, to which it is liable during incubation. How, by
any process of direct equilibration, could it come to have the required
thickness? or, indeed, how could it come to exist at all? Suppose this
protective envelope to be too weak, so that some of the eggs a bird lays
are broken or cracked. In the first place, the breakages or crackings are
actions which cannot react on the maternal organism in such ways as to
cause the secretion of thicker shells for the future: to suppose that they
can, is to suppose that the bird understands the cause of the evil, and
that the secretion of thicker shells can be effected by its will. In the
second place, such developing chicks as are contained in the shells which
crack or break, are almost certain to die; and cannot, therefore, acquire
appropriately-modified constitutions: even supposing any relation could
exist between the impression received and the change required. Meanwhile,
such eggs as escape breakage are not influenced at all by the requirement;
and hence, on the birds developed from them, there cannot have acted any
force tending to work the needful adjustment of functions. In no way,
therefore, can a direct equilibration between constitution and conditions
be here produced. Even in organs that can be modified by certain incident
actions into correspondence with such incident actions, there are some
re-adjustments which cannot be effected by direct balancing. It is thus
with the bones. The majority of the bones have to resist muscular strains;
and variations in the muscular strains call forth, by reaction, variations
in the strengths of the bones. Here there is direct equilibration. But
though the greater massiveness acquired by bones subject to greater
strains, may be ascribed to counter-acting forces evoked by forces brought
into action; it is impossible that the acquirement of greater lengths by
bones can be thus accounted for. It has been supposed that the elongation
of the metatarsals in wading birds, has resulted from direct adaptation to
conditions of life. To justify this supposition, however, it must be shown
that the mechanical actions and reactions in the legs of a wading bird,
differ from those in the legs of other birds; and that the differential
actions are equilibrated by the extra lengths. There is not the slightest
evidence of this. The metatarsals of a bird have to bear no appreciable
strains but those due to the superincumbent weight. Standing in the water
does not appreciably alter such strains; and even if it did, an increase in
the lengths of these bones would not fit them any better to meet the
altered strains.
§ 163. The conclusion at which we arrive is, then, that there go on in all
organisms, certain changes of function and structure that are directly
consequent on changes in the incident forces--inner changes by which the
outer changes are balanced, and the equilibrium restored. Such
re-equilibrations, which are often conspicuously exhibited in individuals,
we have reason to believe continue in successive generations; until they
are completed by the arrival at structures fitted to the modified
conditions. But, at the same time, we see that the modified conditions to
which organisms may be adapted by direct equilibration, are conditions of
certain classes only. That a new external action may be met by a new
internal action, it is needful that it shall either continuously or
frequently be borne by the individuals of the species, without killing or
seriously injuring them; and shall act in such way as to affect their
functions. And we find that many of the environing agencies--evil or
good--to which organisms have to be adjusted, are not of these kinds: being
agencies which either do not immediately affect the functions at all, or
else affect them in ways that prove fatal.
Hence there must be at work some other process which equilibrates the
actions of organisms with the actions they are exposed to. Plants and
animals that continue to exist, are necessarily plants and animals whose
powers balance the powers acting on them; and as their environments change,
the changes which plants and animals undergo must necessarily be changes
towards re-establishment of the balance. Besides direct equilibration,
there must therefore be an indirect equilibration. How this goes on we have
now to inquire.
CHAPTER XII.
INDIRECT EQUILIBRATION.
§ 164. Besides those perturbations produced in any organism by special
disturbing forces, there are ever going on many others--the reverberating
effects of disturbing forces previously experienced by the individual, or
by ancestors; and the multiplied deviations of function so caused imply
multiplied deviations of structure. In § 155 there was re-illustrated the
truth, set forth at length when treating of Adaptation (§ 69), that an
organism in a state of moving equilibrium, cannot have extra function
thrown on any organ, and extra growth produced in such organ, without
correlative changes being entailed throughout all other functions, and
eventually throughout all other organs. And when treating of Variation
(§ 90), we saw that individuals which have been made, by their different
circumstances, to deviate functionally and structurally from the average
type in different directions, will bequeath to their joint offspring,
compound perturbations of function and compound deviations of structure,
endlessly varied in their kinds and amounts.
Now if the individuals of a species are thus necessarily made unlike in
countless ways and degrees--if in one individual the amount of energy in a
particular direction is greater than in any other individual, or if here a
peculiar combination gives a resulting action which is not found elsewhere;
then, among all the individuals, some will be less liable than others to
have their equilibria overthrown by a particular incident force previously
unexperienced. Unless the change in the environment is so violent as to be
universally fatal to the species, it must affect more or less differently
the slightly-different moving equilibria which the members of the species
present. Inevitably some will be more stable than others when exposed to
this new or altered factor. That is to say, those individuals whose
functions are most out of equilibrium with the modified aggregate of
external forces, will be those to die; and those will survive whose
functions happen to be most nearly in equilibrium with the modified
aggregate of external forces.
But this survival of the fittest[52] implies multiplication of the fittest.
Out of the fittest thus multiplied there will, as before, be an
overthrowing of the moving equilibrium wherever it presents the least
opposing force to the new incident force. And by the continual destruction
of the individuals least capable of maintaining their equilibria in
presence of this new incident force, there must eventually be reached an
altered type completely in equilibrium with the altered conditions.
§ 165. This survival of the fittest, which I have here sought to express in
mechanical terms, is that which Mr. Darwin has called "natural selection,
or the preservation of favoured races in the struggle for life." That there
goes on a process of this kind throughout the organic world, Mr. Darwin's
great work on the _Origin of Species_ has shown to the satisfaction of
nearly all naturalists. Indeed, when once enunciated, the truth of his
hypothesis is so obvious as scarcely to need proof. Though evidence may be
required to show that natural selection accounts for everything ascribed to
it, yet no evidence is required to show that natural selection has always
been going on, is going on now, and must ever continue to go on.
Recognizing this as an _à priori_ certainty, let us contemplate it under
its two distinct aspects.
That organisms which live, thereby prove themselves fit for living, in so
far as they have been tried, while organisms which die, thereby prove
themselves in some respects unfitted for living, are facts no less manifest
than is the fact that this self-purification of a species must tend ever to
insure adaptation between it and its environment. This adaptation may be
either so _maintained_ or so _produced_. Doubtless many who have looked at
Nature with philosophic eyes, have observed that death of the worst and
multiplication of the best, tends towards maintenance of a constitution in
harmony with surrounding circumstances. That the average vigour of any race
would be diminished did the diseased and feeble habitually survive and
propagate; and that the destruction of such, through failure to fulfil some
of the conditions to life, leaves behind those which are able to fulfil the
conditions to life, and thus keeps up the average fitness to the conditions
of life; are almost self-evident truths. But to recognize "Natural
Selection" as a means of preserving an already-established balance between
the powers of a species and the forces to which it is subject, is to
recognize only its simplest and most general mode of action. It is the more
special mode of action with which we are here concerned. This more special
mode of action, Mr. Darwin has been the first to recognize as an
all-important factor, though, besides his co-discoverer Mr. A. R. Wallace,
some others have perceived that such a factor is at work. To him we owe due
appreciation of the fact that natural selection is capable of _producing_
fitness between organisms and their circumstances. He has worked up an
enormous mass of evidence showing that this "preservation of favoured races
in the struggle for life," is an ever-acting cause of divergence among
organic forms. He has traced out the involved results of the process with
marvellous subtlety. He has shown how hosts of otherwise inexplicable
facts, are accounted for by it. In brief, he has proved that the cause he
alleges is a true cause; that it is a cause which we see habitually in
action; and that the results to be inferred from it are in harmony with the
phenomena which the Organic Creation presents, both as a whole and in its
details. Let us glance at a few of the more important interpretations which
the hypothesis furnishes.
A soil possessing some ingredient in unusual quantity, may supply to a
plant an excess of the matter required for certain of its tissues; and may
cause all the parts formed of such tissues to be abnormally developed.
Suppose that among these are the hairs clothing its surfaces, including
those which grow on its seeds. Thus furnished with somewhat longer fibres,
its seeds, when shed, are carried a little further by the wind before they
fall to the ground. The plants growing from them, being rather more widely
dispersed than those produced by other individuals of the same species,
will be less liable to smother one another; and a greater number may
therefore reach maturity and fructify. Supposing the next generation
subject to the same peculiarity of nutrition, some of the seeds borne by
its members will not simply inherit this increased development of hairs,
but will carry it further; and these, still more advantaged in the same way
as before, will, on the average, have still more numerous chances of
continuing the race. Thus, by the survival, generation after generation, of
those possessing these longer hairs, and the inheritance of successive
increments of growth in the hairs, there may result a seed deviating
greatly from the original. Other individuals of the same species, subject
to the different physical conditions of other localities, may develop
somewhat thicker or harder coatings to their seeds: so rendering their
seeds less digestible by the birds which devour them. Such thicker-coated
seeds, by escaping undigested more frequently than thinner-coated ones,
will have additional chances of growing and leaving offspring; and this
process, acting in a cumulative manner season after season, will produce a
seed diverging in another direction from the ancestral type. Again,
elsewhere, some modification in the physiologic actions of the plant may
lead to an unusual secretion of an essential oil in the seeds; rendering
them unpalatable to creatures which would otherwise feed on them: so giving
an advantage to the variety in its rate of multiplication. This incidental
peculiarity, proving a preservative, will, as before, be increased by
natural selection until it constitutes another divergence. Now in such
cases, we see that plants may become better adapted, or re-adapted, to the
aggregate of surrounding agencies, not through any _direct_ action of such
agencies on them, but through their _indirect_ action--through the
destruction by them of the individuals least congruous with them, and the
survival of those most congruous with them. All these slight variations of
function and structure, arising among the members of a species, serve as so
many experiments; the great majority of which fail, but a few of which
succeed. Just as each plant bears a multitude of seeds, out of which some
two or three happen to fulfil all the conditions required for reaching
maturity and continuing the race; so each species is ever producing
numerous slightly-modified forms, deviating in all directions from the
average, out of which most fit the surrounding conditions no better than
their parents, or not so well, but some few of which fit the conditions
better; and, doing so, are enabled the better to preserve themselves, and
to produce offspring similarly capable of preserving themselves. Among
animals the like process results in the like development of various
structures which cannot have been affected by the performance of
functions--their functions being purely passive. The thick shell of a
mollusk cannot have arisen from direct reactions of the organism against
the external actions to which it is exposed; but it is quite explicable as
an effect of the survival, generation after generation, of individuals
whose thicker coverings protected them against enemies. Similarly with such
dermal structure as that of the tortoise. Though we have evidence that the
skin, where it is continually exposed to pressure and friction, may
thicken, and so re-establish the equilibrium by opposing a greater inner
force to a greater outer force; yet we have no evidence that a coat of
armour like that of the tortoise can be so produced. Nor, indeed, are the
conditions under which alone its production in such a manner could be
accounted for, fulfilled; since the surface of the tortoise is not exposed
to greater pressure and friction than the surfaces of other creatures. This
massive carapace, and the strangely-adapted osseous frame-work which
supports it, are inexplicable as results of evolution, unless through the
process of natural selection. So, too, is it with the formation of
odoriferous glands in some mammals, or the growth of such excrescences as
those of the camel. Thus, in short, is it with all those organs of animals
which do not play active parts.
Besides giving us explanations of structural characters that are otherwise
unaccountable, Mr. Darwin shows how natural selection explains peculiar
relations between individuals in certain species. Such facts as the
dimorphism of the primrose and other flowers, he proves to be in harmony
with his hypothesis, though stumbling-blocks to all other hypotheses. The
various differences which accompany difference of sex, sometimes slight,
sometimes very great, are similarly accounted for. As before suggested
(§ 79), natural selection appears capable of producing and maintaining the
right proportion of the sexes in each species; and it requires but to
contemplate the bearings of the argument, to see that the formation of
different sexes may itself have been determined in the same way.
To convey here an adequate idea of Mr. Darwin's doctrine, throughout the
immense range of its applications, is of course impossible. The few
illustrations just given, are intended simply to remind the reader what Mr.
Darwin's hypothesis is, and what are the else insoluble problems which it
solves for us.
§ 166. But now, though it seems to me that we are thus supplied with a key
to phenomena which are multitudinous and varied beyond all conception; it
also seems to me that there is a moiety of the phenomena which this key
will not unlock. Mr. Darwin himself recognizes use and disuse of parts, as
causes of modifications in organisms; and does this, indeed, to a greater
extent than do some who accept his general conclusion. But I conceive that
he does not recognize them to a sufficient extent. While he shows that the
inheritance of changes of structure caused by changes of function, is
utterly insufficient to explain a great mass--probably the greater mass--of
morphological phenomena; I think he leaves unconsidered a mass of
morphological phenomena which are explicable as results of
functionally-produced modifications, and are not explicable as results of
natural selection.
By induction, as well as by inference from the hypothesis of natural
selection, we know that there exists a balance among the powers of organs
which habitually act together--such proportions among them that no one has
any considerable excess of efficiency. We see, for example, that throughout
the vascular system there is maintained an equilibrium of the component
parts: in some cases, under continued excess of exertion, the heart gives
way, and we have enlargement; in other cases the large arteries give way,
and we have aneurisms; in other cases the minute blood-vessels give
way--now bursting, now becoming chronically congested. That is to say, in
the average constitution, no superfluous strength is possessed by any of
the appliances for circulating the blood. Take, again, a set of motor
organs. Great strain here causes the fibres of a muscle to tear. There the
muscle does not yield but the tendon snaps. Elsewhere neither muscle nor
tendon is damaged, but the bone breaks. Joining with these instances the
general fact that, under the same adverse conditions, different individuals
show their slight differences of constitution by going wrong some in one
way and some in another; and that even in the same individual, similar
adverse conditions will now affect one viscus and now another; it becomes
manifest that though there cannot be maintained an accurate balance among
the powers of the organs composing an organism, yet their excesses and
deficiencies of power are extremely slight. That they must be extremely
slight, is, as before said, a deduction from the hypothesis of natural
selection. Mr. Darwin himself argues "that natural selection is continually
trying to economise in every part of the organization. If under changed
conditions of life a structure before useful becomes less useful, any
diminution, however slight, in its development, will be seized on by
natural selection, for it will profit the individual not to have its
nutriment wasted in building up an useless structure." In other words, if
any muscle has more fibres than are required, or if a bone is stronger than
needful, no advantage results but rather a disadvantage--a disadvantage
which will decrease the chances of survival. Hence it follows that among
any organs which habitually act in concert, an increase of one can be of no
service unless there is a concomitant increase of the rest. The
co-operative parts must vary together; otherwise variation will be
detrimental. A stronger muscle must have a stronger bone to resist its
contractions; must have stronger correlated muscles and ligaments to secure
the neighbouring articulations; must have larger blood-vessels to bring it
supplies; must have a more massive nerve to transmit stimulus, and some
extra development of a nervous centre to supply the extra stimulus. The
question arises, then,--do variations of the appropriate kinds occur
simultaneously in all these co-operative parts? Have we any reason to think
that the parts spontaneously increase or decrease together? The assumption
that they do seems to me untenable; and its untenability will, I think,
become conspicuous if we take a case, and observe how extremely numerous
and involved are the variations which must be supposed to occur together.
In illustration of another point, we have already considered the
modifications required to accompany increased weight of the head (§ 155).
Instead of the bison, the moose deer, or the extinct Irish elk, will here
best serve our purpose. In this last species the male has
enormously-developed horns, used for purposes of offence and defence. These
horns, weighing upwards of a hundred-weight, are carried at great
mechanical disadvantage: supported as they are, along with the massive
skull which bears them, at the extremity of the outstretched neck. Further,
that these heavy horns may be of use in fighting, the supporting bones and
muscles must be strong enough, not simply to carry them, but to put them in
motion with the rapidity needed for giving blows. Let us, then, ask how, by
natural selection, this complex apparatus of bones and muscles can have
been developed, _pari passu_ with the horns? If we suppose the horns to
have been originally of like size with those borne by other kinds of deer;
and if we suppose that in some individual they became larger by spontaneous
variation; what would be the concomitant changes required to render their
greater size useful? Other things equal, the blow given by a larger horn
would be a blow given by a heavier mass moving at a smaller velocity: the
momentum would be the same as before; and the area of contact with the body
struck being somewhat increased, while the velocity was decreased, the
injury done would be less. That horns may become better weapons, the whole
apparatus concerned in moving them must be so strengthened as to impress
more force on them, and to bear the more violent reactions of the blows
given. The bones of the skull on which the horns are seated must be
thickened; otherwise they will break. The vertebræ of the neck must be
further developed; and unless the ligaments which hold together these
vertebræ, and the muscles which move them, are also enlarged, nothing will
be gained. Again the upper dorsal vertebræ and their spines must be
strengthened, that they may withstand the stronger contractions of the
neck-muscles; and like changes must be made on the scapular arch. Still
more must there be required a simultaneous development of the bones and
muscles of the fore-legs; since these extra growths in the horns, in the
skull, in the neck, in the shoulders, add to the burden they have to bear;
and without they are strengthened the creature will not only suffer from
loss of speed but will fail in fight. Hence, to make larger horns of use,
additional sizes must be acquired by numerous bones, muscles, and
ligaments, as well as by the blood-vessels and nerves on which their
actions depend. On calling to mind how the spraining of a single small
muscle in the foot incapacitates for walking, or how permanent weakness in
a knee-ligament will diminish the power of the leg, it will be seen that
unless all these many changes are simultaneously made, they may as well be
none of them made--or rather, they would better be none of them made; since
the enlargements of some parts, by putting greater strains on connected
parts, would render them relatively weaker if they remained unenlarged. Can
we with any propriety assume that these many enlargements duly proportioned
will be simultaneously effected by spontaneous variations? I think not. It
would be a strong supposition that the vertebræ and muscles of the neck
suddenly became bigger at the same time as the horns. It would be a still
stronger supposition that the upper dorsal vertebræ not only at the same
time became more massive, but appropriately altered their proportions, by
the development of their immense neural spines. And it would be an
assumption still more straining our powers of belief, that along with
heavier horns there should spontaneously take place the required
strengthenings in the bones, muscles, arteries, and nerves of the scapular
and the fore-legs.
Besides the multiplicity of directly-coöperative organs, the multiplicity
of organs which do not coöperate, save in the degree implied by their
combination in the same organism, seems to me a further hindrance to the
development of special structures by natural selection alone. Where the
life is simple, or where circumstances render some one function supremely
important, survival of the fittest may readily bring about the appropriate
structural change, without aid from the transmission of functionally-caused
modifications. But in proportion as the life grows complex--in proportion
as a healthy existence cannot be secured by a large endowment of some one
power, but demands many powers; in the same proportion do there arise
obstacles to the increase of any particular power by "the preservation of
favoured races in the struggle for life." As fast as the faculties are
multiplied, so fast does it become possible for the several members of a
species to have various kinds of superiorities over one another. While one
saves its life by higher speed, another does the like by clearer vision,
another by keener scent, another by quicker hearing, another by greater
strength, another by unusual power of enduring cold or hunger, another by
special sagacity, another by special timidity, another by special courage;
and others by other bodily and mental attributes. Conditions being alike,
each of these life-saving attributes is likely to be transmitted to
posterity. But we may not assume that it will be increased in subsequent
generations by natural selection. Increase of it can result only if
individuals possessing average endowments of it are more frequently killed
off than individuals highly endowed with it; and this can happen only when
the attribute is one of greater importance, for the time being, than most
of the other attributes. If those members of the species which have but
ordinary shares of it, nevertheless survive by virtue of other
superiorities which they severally possess; then it is not easy to see how
this particular attribute can be developed by natural selection in
subsequent generations. The probability seems rather to be that, by
gamogenesis, this extra endowment will, on the average, be diminished in
posterity--just serving in the long run to make up for the deficient
endowments of those whose special powers lie in other directions; and so to
keep up the normal structure of the species. As fast as the number of
bodily and mental faculties increases, and as fast as maintenance of life
comes to depend less on the amount of any one and more on the combined
actions of all; so fast does the production of specialities of character by
natural selection alone, become difficult. Particularly does this seem to
be so with a species so multitudinous in its powers as mankind; and above
all does it seem to be so with such of the human powers as have but minor
shares in aiding the struggle for life--the æsthetic faculties, for
example.
It by no means follows, however, that in cases of this kind, and cases of
the preceding kind, natural selection plays no part. Wherever it is not the
chief agent in working organic changes, it is still, very generally, a
secondary agent. The survival of the fittest must nearly always further the
production of modifications which produce fitness, whether they be
incidental modifications, or modifications caused by direct adaptation.
Evidently, those individuals whose constitutions have facilitated the
production in them of any structural change consequent on any functional
change demanded by some new external condition, will be the individuals
most likely to live and to leave descendants. There must be a natural
selection of functionally-acquired peculiarities, as well as of
spontaneously-acquired peculiarities; and hence such structural changes in
a species as result from changes of habit necessitated by changed
circumstances, natural selection will render more rapid than they would
otherwise be.
There are, however, some modifications in the sizes and forms of parts,
which cannot have been aided by natural selection; but which must have
resulted wholly from the inheritance of functionally-caused alterations.
The dwindling of organs of which the undue sizes entail no appreciable
evils, furnishes the best evidence of this. Take, for an example, that
diminution of the jaws and teeth which characterizes the civilized races,
as contrasted with the savage races.[53] How can the civilized races have
been benefited in the struggle for life, by the slight decrease in these
comparatively-small bones? No functional superiority possessed by a small
jaw over a large jaw in civilized life, can be named as having caused the
more frequent survival of small-jawed individuals. The only advantage
accompanying smallness of jaw, is the advantage of economized nutrition;
and this cannot be great enough to further the preservation of those
distinguished by it. The decrease of weight in the jaw and co-operative
parts, which has arisen in the course of thousands of years, does not
amount to more than a few ounces. This decrease has to be divided among the
many generations which have lived and died in the interval. Let us admit
that the weight of these parts diminished to the extent of an ounce in a
single generation (which is a large admission); it still cannot be
contended that the having to carry an ounce less in weight, and to keep in
repair an ounce less of tissue, could sensibly affect any man's fate. And
if it never did this--nay, if it did not cause a _frequent_ survival of
small-jawed individuals where large-jawed individuals died; natural
selection could neither cause nor aid diminution of the jaw and its
appendages. Here, therefore, the decreased action which has accompanied the
growth of civilized habits (the use of tools and the disuse of coarse
food), must have been the sole cause at work. Through direct equilibration,
diminished external stress on these parts has resulted in diminution of the
internal forces by which this stress is met. From generation to generation,
this lessening of the parts consequent on functional decline has been
inherited. And since the survival of individuals must always have been
determined by more important structural traits, this trait can have neither
been facilitated nor retarded by natural selection.
§ 167. Returning from these extensive classes of facts for which Mr.
Darwin's hypothesis does not account, to the still more extensive classes
of facts for which it does account, and which are unaccountable on any
other hypothesis; let us consider in what way this hypothesis is
expressible in terms of the general doctrine of evolution. Already it has
been pointed out that the evolving of modified types by "natural selection
or the preservation of favoured races in the struggle for life," must be a
process of equilibration; since it results in the production of organisms
which are in equilibrium with their environments. At the outset of this
chapter, something was done towards showing how this continual survival of
the fittest may be understood as the progressive establishment of a balance
between inner and outer forces. Here, however, we must consider the matter
more closely.
On previous occasions we have contemplated the assemblage of individuals
composing a species, as an aggregate in a state of moving equilibrium. We
have seen that its powers of multiplication give it an expansive energy
which is antagonized by other energies; and that through the rhythmical
variations in these two sets of energies there is maintained an oscillating
limit to its habitat, and an oscillating limit to its numbers. On another
occasion (§ 96) it was shown that the aggregate of individuals constituting
a species, has a kind of general life which, "like the life of an
individual, is maintained by the unequal and ever-varying actions of
incident forces on its different parts." We saw that "just as, in each
organism, incident forces constantly produce divergences from the mean
state in various directions, which are constantly balanced by opposite
divergences indirectly produced by other incident forces; and just as the
combination of rhythmical functions thus maintained, constitutes the life
of the organism; so, in a species there is, through gamogenesis, a
perpetual neutralization of those contrary deviations from the mean state,
which are caused in its different parts by different sets of incident
forces; and it is similarly by the rhythmical production and compensation
of these contrary deviations that the species continues to live." Hence, to
understand how a species is affected by causes which destroy some of its
units and favour the multiplication of others, we must consider it as a
whole whose parts are held together by complex forces that are ever
re-balancing themselves--a whole whose moving equilibrium is continually
disturbed and continually rectified. Thus much premised, let us next call
to mind how moving equilibria in general are changed. In the first place, a
new incident force falling on any part of an aggregate with balanced
motions, produces a new motion in the direction of least resistance. In the
second place, the new incident force is gradually used up in overcoming the
opposing forces, and when it is all expended the opposing forces produce a
recoil--a reverse deviation which counter-balances the original deviation.
Consequently, to consider whether the moving equilibrium of a species is
modified in the same way as moving equilibria in general, is to consider
whether, when exposed to a new force, a species yields in the direction of
least resistance; and whether, by its thus yielding, there is generated in
the species a compensating change in the opposite direction. We shall find
that it does both these things.
For what, expressed in mechanical terms, is the effect wrought on a species
by some previously-unknown enemy, that kills such of its members as fail in
defending themselves? The disappearance of those individuals which meet the
destroying forces by the smallest preserving forces, is tantamount to the
yielding of the species as a whole at the places where the resistances are
the least. Or if by some general influence, such as alteration of climate,
the members of a species are subject to increase of external actions which
are ever tending to overthrow their equilibria, and which they are ever
counter-balancing by certain physiological actions, which are the first to
die? Those least able to generate the internal energies which antagonize
these external energies. If the change be an increase of the winter's cold,
then such members of the species as have unusual powers of getting food or
of digesting food, or such as are by their constitutional aptitude for
making fat, furnished with reserve stores of force, available in times of
scarcity, or such as have the thickest coats and so lose least heat by
radiation, survive; and their survival implies that in each of them the
moving equilibrium of functions presents such an adjustment of internal
forces, as prevents overthrow by the modified aggregate of external forces.
Conversely, the members which die are, other things equal, those deficient
in the power of meeting the new action by an equivalent counter-action.
Thus, in all cases, a species considered as an aggregate in a state of
moving equilibrium, has its state changed by the yielding of its
fluctuating mass wherever this mass is weakest in relation to the special
forces acting on it. The conclusion is, indeed, a truism. But now what must
follow from the destruction of the least-resisting individuals and survival
of the most-resisting individuals? On the moving equilibrium of the species
as a whole, existing from generation to generation, the effect of this
deviation from the mean state is to produce a compensating deviation. For
if all such as are deficient of power in a certain direction are destroyed,
what must be the effect on posterity? Had they lived and left offspring,
the next generation would have had the same average powers as preceding
generations: there would have been a like proportion of individuals less
endowed with the needful power, and individuals more endowed with it. But
the more-endowed individuals being alone left to continue the race, there
must result a new generation characterized by a larger average endowment of
this power. That is to say, on the moving equilibrium of a species, an
action producing change in a given direction is followed, in the next
generation, by a reaction producing an opposite change. Observe, too, that
these effects correspond in their degrees of violence. If the alteration
of some external factor is so great that it leaves alive only the few
individuals possessing extreme endowments of the power required to
antagonize it; then, in succeeding generations, there is a rapid
multiplication of individuals similarly possessing extreme endowments of
this power--the force impressed calls out an equivalent conflicting force.
Moreover, the change is temporary where the cause is temporary, and
permanent where the cause is permanent. All that are deficient in the
needful attribute having been killed off, and the survivors having the
needful attribute in a comparatively high degree, there will descend from
them, not only some possessing equal amounts of this attribute with
themselves, but also some possessing less amounts of it. If the destructive
agency has not continued in action, such less-endowed individuals will
multiply; and the species, after sundry oscillations, will return to its
previous mean state. But if this agency be a persistent one, such less
endowed individuals will be continually killed off, and eventually none but
highly-endowed individuals will be produced--a new moving equilibrium,
adapted to the new environing conditions, will result.
It may be objected that this mode of expressing the facts does not include
the cases in which a species becomes modified in relation to surrounding
agencies of a passive kind--cases like that of a plant which acquires
hooked seed-vessels, by which it lays hold of the skins of passing animals,
and makes them the distributors of its seeds--cases in which the outer
agency has no direct tendency at first to affect the species, but in which
the species so alters itself as to take advantage of the outer agency. To
cases of this kind, however, the same mode of interpretation applies on
simply changing the terms. While, in the aggregate of influences amid which
a species exists, there are some which tend to overthrow the moving
equilibria of its members, there are others which facilitate the
maintenance of their moving equilibria, and some which are capable of
giving their moving equilibria increased stability: instance the spread
into their habitat of some new kind of prey, which is abundant at seasons
when other prey is scarce. Now what is the process by which the moving
equilibrium in any species becomes adapted to some additional external
factor furthering its maintenance? Instead of an increased resistance to be
met and counterbalanced, there is here a diminished resistance; and the
diminished resistance is equilibrated in the same way as the increased
resistance. As, in the one case, there is a more frequent survival of
individuals whose peculiarities enable them to resist the new adverse
factor; so, in the other case, there is a more frequent survival of
individuals whose peculiarities enable them to take advantage of the new
favourable factor. In each member of the species, the balance of functions
and correlated arrangement of structures, differ slightly from those
existing in other members. To say that among all its members, one is better
fitted than the rest to benefit by some before-unused agency in the
environment, is to say that its moving equilibrium is, in so far, more
stably adjusted to the sum of surrounding influences. And if, consequently,
this individual maintains its moving equilibrium when others fail, and has
offspring which do the like--that is, if individuals thus characterized
multiply and supplant the rest; there is, as before, a process which
effects equilibration between the organism and its environment, not
immediately but mediately, through the continuous intercourse between the
species as a whole and the environment.
§ 168. Thus we see that indirect equilibration does whatever direct
equilibration cannot do. All these processes by which organisms are
re-fitted to their ever-changing environments, must be equilibrations of
one kind or other. As authority for this conclusion, we have not simply the
universal truth that change of every order is towards equilibrium; but we
have also the truth that life itself is a moving equilibrium between inner
and outer actions--a continuous adjustment of internal relations to
external relations; or the maintenance of a balance between the forces to
which an organism is subject and the forces which it evolves. Hence all
changes which enable a species to live under altered conditions, are
changes towards equilibrium with the altered conditions; and therefore
those which do not come within the class of direct equilibrations, must
come within the class of indirect equilibrations.
And now we reach an interpretation of Natural Selection regarded as a part
of Evolution at large. As understood in _First Principles_, Evolution is a
continuous redistribution of matter and motion; and a process of evolution
which is not expressible in terms of matter and motion has not been reduced
to its ultimate form. The conception of Natural Selection is manifestly one
not known to physical science: its terms are not of a kind physical science
can take cognisance of. But here we have found in what manner it may be
brought within the realm of physical science. Rejecting metaphor we see
that the process called Natural Selection is literally a survival of the
fittest; and the outcome of the above argument is that survival of the
fittest is a maintenance of the moving equilibrium of the functions in
presence of outer actions: implying the possession of an equilibrium which
is relatively stable in contrast with the unstable equilibria of those
which do not survive.
CHAPTER XIII.
THE CO-OPERATION OF THE FACTORS.
§ 169. Thus the phenomena of Organic Evolution may be interpreted in the
same way as the phenomena of all other Evolution. Fully to see this, it
will be needful for us to contemplate in their _ensemble_, the several
processes separately described in the four preceding chapters.
If the forces acting on any aggregate remain the same, the changes produced
by them will presently reach a limit, at which the outer forces are
balanced by the inner forces; and thereafter no further metamorphosis will
take place. Hence, that there may be continuous changes of structure in
organisms, there must be continuous changes in the incident forces. This
condition to the evolution of animal and vegetal forms, we find to be fully
satisfied. The astronomic, geologic, and meteorologic changes that have
been slowly but incessantly going on, and have been increasing in the
complexity of their combinations, have been perpetually altering the
circumstances of organisms; and organisms, becoming more numerous in their
kinds and higher in their kinds, have been perpetually altering one
another's circumstances. Thus, for those progressive modifications upon
modifications which organic evolution implies, we find a sufficient cause.
The increasing inner changes for which we thus find a cause in the
perpetual outer changes, conform, so far as we can trace them, to the
universal law of the instability of the homogeneous. In organisms, as in
all other things, the exposure of different parts to different kinds and
amounts of incident forces, has necessitated their differentiation; and,
for the like reason, aggregates of individuals have been lapsing into
varieties, and species, and genera, and orders. Further, in each type of
organism, as in the aggregate of types, the multiplication of effects has
continually aided this transition from a more homogeneous to a more
heterogeneous state. And yet again, that increasing segregation and
concomitant increasing definiteness, associated with the growing
heterogeneity of organisms, has been aided by the continual destruction of
those which expose themselves to aggregates of external actions markedly
incongruous with the aggregates of their internal actions, and the survival
of those subject only to comparatively small incongruities. Finally, we
have found that each change of structure, superposed on preceding changes,
has been a re-equilibration necessitated by the disturbance of a preceding
equilibrium. The maintenance of life being the maintenance of a balanced
combination of functions, it follows that individuals and species that have
continued to live, are individuals and species in which the balance of
functions has not been overthrown. Hence survival through successive
changes of conditions, implies successive adjustments of the balance to the
new conditions.
The actions that are here specified in succession, are in reality
simultaneous; and they must be so conceived before organic evolution can be
rightly understood. Some aid towards so conceiving them will be given by
the annexed table, representing the co-operation of the factors.
§ 170. Respecting this co-operation, it remains only to point out the
respective shares of the factors in producing the total result; and the way
in which the proportions of their respective shares vary as evolution
progresses.
Astronomic }
changes }
} alter the }
Geologic } incidence }
changes } of inorganic }
} forces. }
Meteorologic } }
changes } }
}
}
} on each species: affecting
} |
} |
} |
} |
} |
Enemies } } |
Competitors } } |
} varying in } } |
Co-operators } number } } |
Prey } } alter the } |
} incidence } |
Enemies } } of organic } |
Competitors } } forces. } |
} varying in } |
Co-operators } kind } |
Prey } |
|
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|
| { which, partially in the first
| { generation, and completely in
| { the course of generations, are
| { directly equilibrated with the
| { changed agencies.
| { immediately {
| { through their { which have their direct
| { functions; { equilibration with the changed
| { { agencies, aided by indirect
| { { equilibration, through the more
| { { frequent survival of those in
| { { which the direct equilibration
| { { is most rapid.
| {
| { its individuals, { { positively--by aiding the
| { { { multiplication of those whose
| { { { moving equilibria happen to be
| { { { most congruous with the
| { { mediately { changed agencies: thus, in the
| { { through the { course of generations, indirectly
| { { aggregate of { equilibrating certain individuals
| { { individuals; { with them.
--{ {
{ { negatively--by killing those
{ { whose moving equilibria are
{ { most incongruous with the
{ { changed agencies: thus, in
{ { the course of generations,
{ { indirectly equilibrating each
{ { of its surviving individuals
{ { with them.
{
{ { by acting on it in some parts of the habitat
{ { more than in others; and thus differentiating
{ its aggregate { the species into local varieties.
{ of individuals, {
{ { and thus causing
{ { differentiations of
{ { the species into
{ by acting differently { varieties, irrespective
{ on slightly-unlike { of locality.
{ individuals in the {
{ same locality; { and thus causing
{ modification of the
{ species as a whole,
{ by abstracting a
{ certain class of
{ its units.
At first, changes in the amounts and combinations of inorganic forces,
astronomic, geologic, and meteorologic, were the only causes of the
successive modifications; and these changes have continued to be causes.
But as, through the diffusion of organisms and consequent differential
actions of inorganic forces, there arose unlikenesses among them, producing
varieties, species, genera, orders, classes, the actions of organisms on
one another became new sources of organic modifications. And as fast as
types have multiplied and become more complex, so fast have the mutual
actions of organisms come to be more influential factors in their
respective evolutions: eventually becoming the chief factors.
Passing from the external causes of change to the internal processes of
change entailed by them, we see that these, too, have varied in their
proportions: that which was originally the most important and almost the
sole process, becoming gradually less important, if not at last the least
important. Always there must have been, and always there must continue to
be, a survival of the fittest; natural selection must have been in
operation at the outset, and can never cease to operate. While yet
organisms had small abilities to coordinate their actions, and adjust them
to environing actions, natural selection worked almost alone in moulding
and remoulding organisms into fitness for their changing environments; and
natural selection has remained almost the sole agency by which plants and
inferior orders of animals have been modified and developed. The
equilibration of organisms that are almost passive, is necessarily effected
indirectly, by the action of incident forces on the species as a whole. But
along with the evolution of organisms having some activity, there grows up
a kind of equilibration which is in part direct. In proportion as the
activity increases direct equilibration plays a more important part. Until,
when the nervo-muscular apparatus becomes greatly developed, and the power
of varying the actions to fit the varying requirements becomes
considerable, the share taken by direct equilibration rises into
co-ordinate importance or greater importance. As fast as essential
faculties multiply, and as fast as the number of organs which co-operate in
any given function increases, indirect equilibration through natural
selection becomes less and less capable of producing specific adaptations;
and remains capable only of maintaining the general fitness of constitution
to conditions. The production of adaptations by direct equilibration then
takes the first place: indirect equilibration serving to facilitate it.
Until at length, among the civilized human races, the equilibration becomes
mainly direct: the action of natural selection being limited to the
destruction of those who are constitutionally too feeble to live, even with
external aid. As the preservation of incapables is secured by our social
arrangements; and as very few save incarcerated criminals are prevented by
their inferiorities from leaving the average number of offspring; it
results that survival of the fittest can scarcely at all act in such way as
to produce specialities of nature, either bodily or mental. Here the
specialities of nature, chiefly mental, which we see produced, and which
are so rapidly produced that a few centuries show a considerable change,
must be ascribed almost wholly to direct equilibration.[54]
CHAPTER XIV.
THE CONVERGENCE OF THE EVIDENCES.
§ 171. Of the three classes of evidences that have been assigned in proof
of Evolution, the _à priori_, which we took first, were partly negative,
partly positive.
On considering the "General Aspects of the Special-creation hypothesis," we
discovered it to be worthless. Discredited by its origin, and wholly
without any basis of observed fact, we found that it was not even a
thinkable hypothesis; and, while thus intellectually illusive, it turned
out to have moral implications irreconcilable with the professed beliefs of
those who hold it.
Contrariwise, the "General Aspects of the Evolution-hypothesis" begot the
stronger faith in it the more nearly they were considered. By its lineage
and its kindred, it was found to be as closely allied with the proved
truths of modern science, as is the antagonist hypothesis with the proved
errors of ancient ignorance. We saw that instead of being a mere
pseud-idea, it admits of elaboration into a definite conception: so showing
its legitimacy as an hypothesis. Instead of positing a purely fictitious
process, the process which it alleges proves to be one actually going on
around us. To which add that, morally considered, this hypothesis presents
no radical incongruities.
Thus, even were we without further means of judging, there could be no
rational hesitation which of the two views should be entertained.
§ 172. Further means of judging, however, we found to be afforded by
bringing the two hypotheses face to face with the general truths
established by naturalists. These inductive evidences were dealt with in
four chapters.
"The Arguments from Classification" were these. Organisms fall into groups
within groups; and this is the arrangement which we see results from
evolution, where it is known to take place. Of these groups within groups,
the great or primary ones are the most unlike, the sub-groups are less
unlike, the sub-sub-groups still less unlike, and so on; and this, too, is
a characteristic of groups demonstrably produced by evolution. Moreover,
indefiniteness of equivalence among the groups is common to those which we
know have been evolved, and those here supposed to have been evolved. And
then there is the further significant fact, that divergent groups are
allied through their lowest rather than their highest members.
Of "the Arguments from Embryology," the first is that when developing
embryos are traced from their common starting point, and their divergences
and re-divergences symbolized by a genealogical tree, there is manifest a
general parallelism between the arrangement of its primary, secondary, and
tertiary branches, and the arrangement of the divisions and sub-divisions
of our classifications. Nor do the minor deviations from this general
parallelism, which look like difficulties, fail, on closer observation, to
furnish additional evidence; since those traits of a common ancestry which
embryology reveals, are, if modifications have resulted from changed
conditions, liable to be disguised in different ways and degrees in
different lines of descendants.
We next considered "the Arguments from Morphology." Apart from those
kinships among organisms disclosed by their developmental changes, the
kinships which their adult forms show are profoundly significant. The
unities of type found under such different externals, are inexplicable
except as results of community of descent with non-community of
modification. Again, each organism analyzed apart, shows, in the likenesses
obscured by unlikenesses of its component parts, a peculiarity which can be
ascribed only to the formation of a more heterogeneous organism out of a
more homogeneous one. And once more, the existence of rudimentary organs,
homologous with organs that are developed in allied animals or plants,
while it admits of no other rational interpretation, is satisfactorily
interpreted by the hypothesis of evolution.
Last of the inductive evidences, came "the Arguments from Distribution."
While the facts of distribution in Space are unaccountable as results of
designed adaptation of organisms to their habitats, they are accountable as
results of the competition of species, and the spread of the more fit into
the habitats of the less fit, followed by the changes which new conditions
induce. Though the facts of distribution in Time are so fragmentary that no
positive conclusion can be drawn, yet all of them are reconcilable with the
hypothesis of evolution, and some of them yield it strong support:
especially the near relationship existing between the living and extinct
types in each great geographical area.
Thus of these four groups, each furnished several arguments which point to
the same conclusion; and the conclusion pointed to by the arguments of any
one group, is that pointed to by the arguments of every other group. This
coincidence of coincidences would give to the induction a very high degree
of probability, even were it not enforced by deduction. But the conclusion
deductively reached, is in harmony with the inductive conclusion.
§ 173. Passing from the evidence that evolution has taken place, to the
question--How has it taken place? we find in known agencies and known
processes, adequate causes of its phenomena.
In astronomic, geologic, and meteorologic changes, ever in progress, ever
combining in new and more involved ways, we have a set of inorganic factors
to which all organisms are exposed; and in the varying and complicating
actions of organisms on one another, we have a set of organic factors that
alter with increasing rapidity. Thus, speaking generally, all members of
the Earth's Flora and Fauna experience perpetual re-arrangements of
external forces.
Each organic aggregate, whether considered individually or as a
continuously-existing species, is modified afresh by each fresh
distribution of external forces. To its pre-existing differentiations new
differentiations are added; and thus that lapse to a more heterogeneous
state, which would have a fixed limit were the circumstances fixed, has its
limit perpetually removed by the perpetual change of the circumstances.
These modifications upon modifications which result in evolution
structurally considered, are the accompaniments of those functional
alterations continually required to re-equilibrate inner with outer
actions. That moving equilibrium of inner actions corresponding with outer
actions, which constitutes the life of an organism, must either be
overthrown by a change in the outer actions, or must undergo perturbations
that cannot end until there is a re-adjusted balance of functions and
correlative adaptation of structures.
But where the external changes are either such as are fatal when
experienced by the individuals, or such as act on the individuals in ways
that do not affect the equilibrium of their functions; then the
re-adjustment results through the effects produced on the species as a
whole--there is indirect equilibration. By the preservation in successive
generations of those whose moving equilibria are least at variance with the
requirements, there is produced a changed equilibrium completely in harmony
with the requirements.
§ 174. Even were this the whole of the evidence assignable for the belief
that organisms have been gradually evolved, it would have a warrant higher
than that of many beliefs which are regarded as established. But the
evidence is far from exhausted.
At the outset it was remarked that the phenomena presented by the organic
world as a whole, cannot be properly dealt with apart from the phenomena
presented by each organism, in the course of its growth, development, and
decay. The interpretation of either implies interpretation of the other;
since the two are in reality parts of one process. Hence, the validity of
any hypothesis respecting the one class of phenomena, may be tested by its
congruity with phenomena of the other class. We are now about to pass to
the more special phenomena of development, as displayed in the structures
and functions of individual organisms. If the hypothesis that plants and
animals have been progressively evolved be true, it must furnish us with
keys to these phenomena. We shall find that it does this; and by doing it
gives numberless additional vouchers for its truth.
CHAPTER XIV^A.
RECENT CRITICISMS AND HYPOTHESES.
§ 174a. Since the first edition of this work was published, and more
especially since the death of Mr. Darwin, an active discussion of the
Evolution hypothesis has led to some significant results.
That organic evolution has been going on from the dawn of life down to the
present time, is now a belief almost universally accepted by zoologists and
botanists--"almost universally," I say, because the surviving influence of
Cuvier prevents acceptance of it by some of them in France. Omitting the
ideas of these, all biological interpretations, speculations, and
investigations, tacitly assume that organisms of every kind in every era
and in every region have come into existence by the process of descent with
modification.
But while concerning the fact of evolution there is agreement, concerning
its causes there is disagreement. The ideas of naturalists have, in this
respect, undergone a differentiation increasingly pronounced; which has
ended in the production of two diametrically opposed beliefs. The cause
which Mr. Darwin first made conspicuous has come to be regarded by some as
the sole cause; while, on the part of others there has been a growing
recognition of the cause which he at first disregarded but afterwards
admitted. Prof. Weismann and his supporters contend that natural selection
suffices to explain everything. Contrariwise, among many who recognize the
inheritance of functionally-produced changes, there are a few, like the
Rev. Prof. Henslow, who regard it as the sole factor.
The foregoing chapters imply that the beliefs of neither extreme are here
adopted. Agreeing with Mr. Darwin that both factors have been operative, I
hold that the inheritance of functionally-caused alterations has played a
larger part than he admitted even at the close of his life; and that,
coming more to the front as evolution has advanced, it has played the chief
part in producing the highest types. I am not now about to discuss afresh
these questions, but to deal with certain further questions.
For while there has been taking place in the biological world the major
differentiation above indicated, there have been taking place certain minor
differentiations--there have been arising special views respecting the
process of organic evolution. Concerning each of these it is needful to say
something.
§ 174b. Among the implied controversies the most conspicuous one has
concerned the alleged process called by Prof. Weismann _Panmixia_--a
process which Dr. Romanes had foreshadowed under the name of "the Cessation
of Selection." Dr. Romanes says:--"At that time it appeared to me, as it
now appears to Weismann, entirely to supersede the necessity of supposing
that the effect of disuse is ever inherited in any degree at all."[55] The
alleged mode of action is exemplified by Prof. Weismann as follows:--
"A goose or a duck must possess strong powers of flight in the natural
state, but such powers are no longer necessary for obtaining food when it
is brought into the poultry-yard, so that a rigid selection of
individuals with well-developed wings, at once ceases among its
descendants. Hence in the course of generations, a deterioration of the
organs of flight must necessarily ensue, and the other members and organs
of the bird will be similarly affected."[56]
Here, and throughout the arguments of those who accept the hypothesis of
Panmixia, there is an unwarranted assumption--nay, an assumption at
variance with the doctrine in support of which it is made. It is contended
that in such cases as the one given there will, apart from any effects of
disuse, be decrease in the disused organs because, not being kept by
Natural Selection up to the level of strength previously needed, they will
vary in the direction of decrease; and that variations in the direction of
decrease, occurring in some individuals, will, by interbreeding, produce an
average decrease throughout the species. But why will the disused organs
vary in the direction of decrease more than in the direction of increase?
The hypothesis of Natural Selection postulates indeterminate
variations--deviations no more in one direction than in the opposite
direction: implying that increases and decreases of size will occur to
equal extents and with equal frequencies. With any other assumption the
hypothesis lapses; for if the variations in one direction exceed those in
another the question arises--What makes them do this? And whatever makes
them do this becomes the essential cause of the modification: the selection
of favourable variations is tacitly admitted to be an insufficient
explanation. But if the hypothesis of Natural Selection itself implies the
occurrence of equal variations on all sides of the mean, how can Panmixia
produce decrease? _Plus_ deviations will cancel _minus_ deviations, and the
organ will remain where it was.[57]
"But you have forgotten the tendency to economy of growth," will be the
reply--"you have forgotten that in Mr. Darwin's words 'natural selection is
continually trying to economize in every part of the organization;' and
that this is a constant cause favouring _minus_ variations." I have not
forgotten it; but have remembered it as showing how, to support the
hypothesis of Panmixia, there is invoked the aid of that very hypothesis
which it is to replace. For this principle of economy is but another aspect
of the principle of functionally-produced modifications. Nearly forty years
ago I contended that "the different parts of ... an individual organism
compete for nutriment; and severally obtain more or less of it according as
they are discharging more or less duty:"[58] the implication being that as
all other organs are demanding blood, decrease of duty in any one,
entailing decreased supply of blood, brings about decreased size. In other
words, the alleged economy is nothing else than the abstraction, by active
parts, of nutriment from an inactive part; and is merely another name for
functionally-produced decrease. So that if the variations are supposed to
take place predominantly in the direction of decrease, it can only be by
silently assuming the cause which is overtly denied.
But now we come to the strange fact that the particular case in which
panmixia is assigned in disproof of alleged inheritance of
functionally-produced modifications, is a case in which it would be
inapplicable even were its assumption legitimate--the case of disused
organs in domestic animals. For since nutrition is here abundant, the
principle of economy under the form alleged does not come into play.
Contrariwise, there even occurs a partial re-development of rudimentary
organs: instances named by Mr. Darwin being the supplementary mammæ in
cows, fifth toes on the hind feet of dogs, spurs and comb in hens, and
canine teeth in mares. Now clearly, if organs disused for innumerable
generations may thus vary in the direction of increase, it must, _a
fortiori_, be so with recently disused organs, and there disappears all
plea (even the illegitimate plea) for assuming that in the wing of a wild
duck which has become domesticated, the _minus_ variations will exceed the
_plus_ variations: the hypothesis of panmixia loses its postulate.
If it be said that Mr. Darwin's argument is based on the changed ratio
between the weights of leg-bones and wing-bones, and that this changed
ratio may result not from decrease of the wing-bones but from increase of
the leg-bones, then there comes a fatal reply. Such, increase cannot be
ascribed to selection of varieties, since there is no selective breeding to
obtain larger legs, and as it is not pretended that panmixia accounts for
increase the case is lost: there remains no cause for such increase save
increase of function.
§ 174c. The doctrine of determinate evolution or definitely-directed
evolution, which appears to be in one form or other entertained by sundry
naturalists, has been set forth by the late Prof. Eimer under the title
"Orthogenesis." A distinct statement of his conception is not easily made
for the reason that, as I think, the conception itself is indistinct. Here
are some extracts from a translation of his paper published at Chicago. Out
of these the reader may form a notion of the theory:
"Orthogenesis shows that organisms develop in definite directions without
the least regard for utility through purely physiological causes as the
result of _organic growth_, as I term the process."
"I am concerned in this paper with definitely directed evolution as the
cause of transmutation, and not with the effects of the use and activity
of organs which with Lamarck I adopted as the second main explanatory
cause thereof."
"The causes of definitely directed evolution are contained, according to
my view, in the effects produced by outward circumstances and influences
such as climate and nutrition upon the constitution of a given organism."
"At variance with all the facts of definitely directed evolution ... is
also the contention of my opponent [Weismann] ... that the variations
demonstrably oscillate to and fro in the most diverse directions about a
given zero-point. There is no oscillation in the direction of
development, but simply an advance forwards in a straight line with
occasional lateral divergences whereby the forkings of the ancestral tree
are produced."[59]
These sentences contain one of those explanations which explain nothing;
for we are not enabled to see how the "outward circumstances and
influences" produce the effects ascribed to them. We are not shown in what
way they cause organic evolution in general, still less in what way they
cause the infinitely-varied forms in which organic evolution results. The
assertion that evolution takes definitely-directed lines is accompanied by
no indication of the reasons why particular lines are followed rather than
others. In short, we are simply taken a step back, and for further
interpretation referred to a cause said to be adequate, but the operations
of which we are to imagine as best we may.
This is a re-introduction of supernaturalism under a disguise. It may pair
off with the conception made popular by the _Vestiges of the Natural
History of Creation_, in which it was contended that there exists a
persistent tendency towards the birth of a higher form of creature; or it
may be bracketed with the idea entertained by the late Prof. Owen, who
alleged an "ordained becoming" of living things.
§ 174d. An objection to the Darwinian doctrine which has risen into
prominence, is that Natural Selection does not explain that which it
professes to explain. In the words of Mr. J. T. Cunningham:--
"Everybody knows that the theory of natural selection was put forward by
Darwin as a theory of the origin of species, and yet it is only a theory
of the origin of adaptations. The question is: Are the differences
between species differences of adaptation? If so, then the origin of
species and the origin of adaptations are equivalent terms. But there is
scarcely a single instance in which a specific character has been shown
to be useful, to be adaptive."[60]
To illustrate this last statement Mr. Cunningham names the plaice,
flounder, and dab as three flat fishes in which, along with the adaptive
characters related to the mode of life common to them all, each has
specific characters which are not adaptive. No evidence is forthcoming that
these in any way conduce to the welfare of the species. Two propositions
are here involved which should be separately dealt with.
The first is that the adaptive modifications which survival of the fittest
is able to produce, do not become specific traits: they are traits separate
in kind from those which mark off groups proved to be specifically distinct
by their inability to breed together. Such evidence as we at present have
seems to warrant this statement. Out of the many varieties of dogs most, if
not all, have been rendered distinct by adaptive modifications, mostly
produced by selection. But, notwithstanding the immense divergences of
structure so produced, the varieties inter-breed. To this, however, it may
be replied that sufficient time has not elapsed--that the process by which
a structural adaptation so reacts on the constitution as to make it a
distinct one, possibly, or probably, takes many thousands of years. Let us
accept for the moment Lord Kelvin's low estimate of the geologic time
during which life has existed--one hundred million years. Suppose we divide
that time into as many parts as there are hours occupied in the development
of a human foetus. And suppose that during these hundred million years
there has been going on with some uniformity the evolution of the various
organic types now existing. Then the amount of change undergone by the
foetus in an hour, will be equivalent to the amount of change undergone by
an evolving organic form in fifteen thousand years. That is to say, during
general evolution it may have taken fifteen thousand years to establish, as
distinct, two species differing from one another no more than the foetus
differs from itself after the lapse of an hour. Hence, though we lack proof
that adaptive modifications become specific traits, it is quite possible
that they are in course of becoming specific traits.
The converse proposition, that the traits by which species are ordinarily
distinguished are non-adaptive traits is well sustained; and the statement
that, if not themselves useful they are correlated with those which are
useful, is, to say the least, unproved. For the instances given by Mr.
Darwin of correlated traits are not those between adaptive traits and the
traits regarded as specific, but between traits none of which are specific;
as between skull and limbs in swine, tusks and bristles in swine, horns and
wool in sheep, beak and feet in pigeons.
If we seek a clue in those processes by which correlations are brought
about--the physiological actions and reactions--we may at once see that any
organic modification, be it adaptive or not, must entail secondary
modifications throughout the rest of the organism, most of them insensible
but some of them sensible. The competition for blood among organs, referred
to above, necessitates that, other things remaining the same, the extra
growth of any one tells on all others, in variable degrees according to
conditions, and may cause appreciable diminutions of some. This is not all.
While the quantity of blood supplied to other organs is affected, its
quality also is in some cases affected. Each organ, or at any rate each
class of organs, has special nutrition--abstracts from the blood a
proportion of ingredients different from that abstracted by other organs or
classes of organs. Hence may result a deficiency or a surplus of some
element: instance the change in the blood which must be caused by growth of
a stag's antlers. Now if such effects are always produced, and if, further,
a change of general nutrition caused by a new food or by a difference of
ability to utilize certain components of food, similarly operates (instance
the above named correlation between horns and wool), then every
modification must entail throughout the organism multitudinous alterations
of structure. Such alterations will ordinarily be neither in themselves
useful nor necessarily correlated with those which are useful; since they
must arise as concomitants of any change, whether adaptive or not. There
will consequently arise the innumerable minute differences presented by
allied species in addition to the differences called specific.
On joining with recognition of this general process a recognition of the
tendency towards localization of deposit, one possible origin of specific
marks is suggested. When in an organism the circulating fluids contain
useless matter, normal or abnormal, the excretion of it, once determined
towards a certain place, continues at that place. Trees furnish examples in
the casting out of gums and resins. Animal life yields evidence in gouty
concretions and such morbid products as tubercle. A place of enfeebled
nutrition is commonly chosen--not unfrequently a place where a local injury
has occurred. Now if we extend this principle, well recognized in
pathological processes, to physiological processes, we may infer that where
an adaptive modification has so reacted on the blood as to leave some
matter to be got rid of, the deposit of this, initiated at some place of
least resistance, may produce a local structure which eventually becomes a
species-mark. A relevant inquiry suggests itself--What proportion of
species-marks are formed out of inanimate tissue or tissue of low
vitality--tissue which, like hair, feathers, horns, teeth, is composed of
by-products unfit for carrying on vital actions.
§ 174e. In the days when, not having been better instructed by Mr. Darwin,
I believed that all changes of structure in organisms result from changes
of function, I held that the cause of such changes of function is
migration. Assuming as the antecedent of migration a great geologic change,
such as upheaval of the East Indian Archipelago step by step into a
continent, it was argued, in an essay I then wrote, that, subjected
primarily to new influences in its original habitat, each kind of plant and
animal would secondarily be subjected to the altered conditions consequent
on spreading over the upheaved regions.
"Each species being distributed over an area of some extent, and tending
continually to colonize the new area exposed, its different members would
be subject to different sets of changes. Plants and animals spreading
towards the equator would not be affected in the same way with others
spreading from it. Those spreading towards the new shores would undergo
changes unlike the changes undergone by those spreading into the
mountains. Thus, each original race of organisms would become the root
from which diverged several races differing more or less from it and from
one another."
It was further argued that, beyond modifications caused by change of
physical conditions and food, others would be caused by contact of the
Flora and Fauna of each island with the Floras and Faunas of other islands:
bringing experience of animals and plants before unknown.[61]
While this conception was wrong in so far as it ascribed the production of
new species entirely to inheritance of functionally-wrought alterations
(thus failing to recognize Natural Selection, which was not yet
enunciated), it was right in so far as it ascribed organic changes to
changes of conditions. And it was, I think, also right in so far as it
implied that isolation is a condition precedent to such changes. Apparently
it did not occur to me as needful to specify this isolation as making
possible the differentiation of species; since it goes without saying that
members of a species spreading east, west, north, south, and forming groups
hundreds of miles apart, must, while breeding with those of the same group
be prevented from breeding with those of other groups--prevented from
having their locally-caused modifications mutually cancelled.
The importance of isolation has of late been emphasized by Dr. Romanes and
others, who, to that isolation consequent on geographical diffusion, have
added that isolation which results from difference of station in the same
habitat, and also that due to differences in the breeding periods arising
in members of the same species. Doubtless in whatever way effected, the
isolation of a group subject to new conditions and in course of being
changed, is requisite as a means to permanent differentiation. Doubtless
also, as contended by Mr. Gulick and Dr. Romanes, there is a difference
between the case in which an entire species being subject to the same
conditions is throughout modified in character, thus illustrating what Mr.
Gulick calls "monotypic evolution," and the case in which different parts
of the species, leading different lives, will, if they are by any means
prevented from inter-breeding with other parts, form divergent varieties:
thus illustrating "polytypic evolution."
§ 174f. Beyond geographical and topographical isolation, there is an
isolation of another kind regarded by some as having had an important share
in organic evolution. Foreshadowed by Mr. Belt, subsequently enunciated by
Mr. Catchpool, fully thought out by Mr. Gulick, and more recently
elaborated by Dr. Romanes, "Physiological Selection" is held to account for
the genesis of marked varieties side by side with their parents. It is
contended that without the kind of isolation implied by it, variations will
be swamped by inter-crossing, and divergence prevented; but that by the aid
of this kind of isolation, a uniform species may be differentiated into two
or more species, though its members continue to live in the same area.
Facts are assigned to show that slightly unlike varieties may become unable
to inter-breed either with the parent-species or with one another. This
mutual inferiority is not of the kind we might expect. We might reasonably
suppose that when varieties had diverged widely, crossing would be
impracticable, because their constitutions had become so far unlike as to
form an unworkable mixture. But there seems evidence that the infertility
arises long before such a cause could operate, and that instead of failure
to produce a workable constitution, there is failure to produce any
constitution at all--failure to fertilize. Some change in the sexual system
is suggested as accounting for this. That a minute difference in the
reproductive elements may suffice, plants prove by the fact that when two
members of slightly-divergent varieties are fertilized by each other's
pollen, the fertility is less than if each were fertilized by the pollen of
its own variety; and where the two kinds of pollen are both used, that
derived from members of the same variety is prepotent in its effect over
that derived from members of the other variety.
The writers above named contend that variations must occur in the
reproductive organs as well as in other organs; that such variations may
produce relative infertility in particular directions; and that such
relative infertility may be the first step towards prevention of crossing
and establishment of isolation: so making possible the accumulation of such
differences as mark off new species. Without doubt we have here a
legitimate supposition and a legitimate inference. Necessarily there must
happen variations of the kind alleged, and considering how sensitive the
reproductive system is to occult influences (witness among ourselves the
frequent infertility of healthy people while feeble unhealthy ones are
fertile), it is reasonable to infer that minute and obscure alterations of
this kind may make slightly-different varieties unable to inter-breed.
Granting that there goes on this "physiological selection," we must
recognize it as one among the causes by which isolation is produced, and
the differentiating influence of natural selection in the same locality
made possible.
§ 174g. The foregoing criticisms and hypotheses do not, however, affect in
any essential way the pre-existing conceptions. If, as in the foregoing
chapters, we interpret the facts in terms of that redistribution of matter
and motion constituting Evolution at large, we shall see that the general
theory, as previously held, remains outstanding.
It is indisputable that to maintain its life an organism must maintain the
moving equilibrium of its functions in presence of environing actions. This
is a truism: overthrow of the equilibrium is death. It is a corollary that
when the environment is changed, the equilibrium of functions is disturbed,
and there must follow one of two results--either the equilibrium is
overthrown or it is re-adjusted: there is a re-equilibration. Only two
possible ways of effecting the re-adjustment exist--the direct and the
indirect. In the one case the changed outer action so alters the moving
equilibrium as to call forth an equivalent reaction which balances it. If
re-equilibration is not thus effected in the individual it is effected in
the succession of individuals. Either the species altogether disappears, or
else there disappear, generation after generation, those members of it the
equilibria of whose functions are least congruous with the changed actions
in the environment; and this is the survival of the fittest or natural
selection.
If now we persist in thus contemplating the problem as a statico-dynamical
one, we shall see that much of the discussion commonly carried on is beside
the question. The centre around which the collision of arguments has taken
place, is the question of the formation of species. But here we see that
this question is a secondary and, in a sense, irrelevant one. We are
concerned with the production of evolving and diverging organic forms; and
whether these are or are not marked off by so-called specific traits, and
whether they will or will not breed together, matters little to the general
argument. If two divisions of a species, falling into unlike conditions and
becoming re-equilibrated with them, eventually acquire the differences of
nature called specific, this is but a collateral result. The _essential_
result is the formation of divergent organic forms. The biologic
atmosphere, so to speak, has been vitiated by the conceptions of past
naturalists, with whom the identification and classification of species was
the be-all and end-all of their science, and who regarded the traits which
enabled them to mark off their specimens from one another, as the traits of
cardinal importance in Nature. But after ignoring these technical ideas it
becomes manifest that the distinctions, morphological or physiological,
taken as tests of species, are merely incidental phenomena.
Moreover, on continuing thus to look at the facts, we shall better
understand the relation between adaptive and specific characters, and
between specific characters and those many small differences which always
accompany them. For during re-equilibration there must, beyond those
changes of structure required to balance outer actions by inner actions, be
numerous minor changes. In any complex moving equilibrium alterations of
larger elements inevitably cause alterations of elements immediately
dependent on them, and these again of others: the effects reverberate and
re-reverberate throughout the entire aggregate of actions down to the most
minute. Of resulting structural changes a few will be conspicuous, more
will be less conspicuous, and so on continuously multiplying in number and
decreasing in amount.
Here seems a fit place for remarking that there are certain processes which
do not enter into these re-equilibrations but in a sense interfere with
them. One example must suffice. Among dogs may be observed the trick of
rolling on some mass having a strong animal smell: commonly a decaying
carcase. This trick has probably been derived from the trick of rolling on
the body of an animal caught and killed, and so gaining a tempting odour. A
male dog which first did this, and left a trail apt to be mistaken for that
of prey, would be more easily found by a female, and would be more likely
than others to leave posterity. Now such a trick could have no relation to
better maintenance of the moving equilibrium, and might very well arise in
a dog having no superiority. If it arose in one of the worst it would be
eliminated from the species, but if it arose in one of medium constitution,
fairly capable of self-preservation, it would tend to produce survival of
certain of the less fit rather than the fittest. Probably there are many
such minor traits which are in a sense accidental, and are neither adaptive
nor specific in the ordinary sense.
§ 174h. But now let it be confessed that though all phenomena of organic
evolution must fall within the lines above indicated, there remain many
unsolved problems.
Take as an instance the descent of the testes in the _Mammalia_. Neither
direct nor indirect equilibration accounts for this. We cannot consider it
an adaptive change, since there seems no way in which the production of
sperm-cells, internally carried on in a bird, is made external by
adjustment to the changed requirements of mammalian life. Nor can we
ascribe it to survival of the fittest; for it is incredible that any mammal
was ever advantaged in the struggle for life by this changed position of
these organs. Contrariwise, the removal of them from a place of safety to a
place of danger, would seem to be negatived by natural selection. Nor can
we regard the transposition as a concomitant of re-equilibration; since it
can hardly be due to some change in the general physiological balance.
An example of another order is furnished by the mason-wasp. Several
instincts, capacities, peculiarities, which are in a sense independent
though they cooperate to the same end, are here displayed. There is the
instinct to build a cell of grains of sand, and the ability to do this,
which though in a sense separate may be regarded as an accompaniment; and
there is the secretion of a cement--a physiological process not directly
connected with the psychological process. After oviposition there comes
into play the instinct to seek, carry home, and pack into the cell, the
small caterpillars, spiders, &c., which are to serve as food for the larva;
and then there is the instinct to sting each of them at a spot where the
injected hypnotic poison keeps the creature insensible though alive till it
is wanted. These cannot be regarded as parts of a whole developed in
simultaneous coordination. There is no direct connexion between the
building instinct and the hypnotizing instinct; still less between these
instincts and the associated appliances. What were the early stages they
passed through imagination fails to suggest. Their usefulness depends on
their combination; and this combination would seem to have been useless
until they had all reached something like their present completeness. Nor
can we in this case ascribe anything to the influence of teaching by
imitation, supposed to explain the doings of social insects; for the
mason-wasp is solitary.
Thus the process of organic evolution is far from being fully understood.
We can only suppose that as there are devised by human beings many puzzles
apparently unanswerable till the answer is given, and many necromantic
tricks which seem impossible till the mode of performance is shown; so
there are apparently incomprehensible results which are really achieved by
natural processes. Or, otherwise, we must conclude that since Life itself
proves to be in its ultimate nature inconceivable, there is probably an
inconceivable element in its ultimate workings.
END OF VOL. I.
APPENDICES.
APPENDIX A.
THE GENERAL LAW OF ANIMAL FERTILITY.
[_In the_ Westminster Review _for April, 1852, I published an essay under
the title "A Theory of Population deduced from the General Law of Animal
Fertility." That essay was the germ of Part VI of this work, "The Laws of
Multiplication," in which its essential theses are fully developed. When
developing them, I omitted some portions of the original essay--one which
was not directly relevant, and another which contained a speculation open
to criticism. As indicated in § 74f, I find that this speculation has an
unexpected congruity with recent results of inquiry. I therefore decide to
reproduce it here along with the definition of Life propounded in that
essay, which, though subsequently replaced by the definition elaborated in
Part I, contains an element of truth._]
* * * * *
Some clear idea of the nature of Life itself, must, indeed, form a needful
preliminary. We may be sure that a search for the influences determining
the maintenance and multiplication of living organisms, cannot be
successfully carried out unless we understand what is the peculiar property
of a living organism--what is the widest generalization of the phenomena
that indicate life. By way of preparation, therefore, for the Theory of
Population presently to be developed, we propose devoting a brief space to
this prior question.
* * * * *
Employing the term, then, in its usual sense, as applicable only to
organisms, Life may be defined as--_the co-ordination of actions_. The
growth of a crystal, which is the highest inorganic process we are
acquainted with, involves but one action--that of accretion. The growth of
a cell, which is the lowest organic process, involves two
actions--accretion and disintegration--repair and waste--assimilation and
oxidation. Wholly deprive a cell of oxygen, and it becomes inert--ceases to
manifest vital phenomena; or, as we say, dies. Give it no matter to
assimilate, and it wastes away and disappears, from continual oxidation.
Evidently, then, it is in the balance of these two actions that the life
consists. It is not in the assimilation alone; for the crystal assimilates:
neither is it in the oxidation alone; for oxidation is common to inorganic
matter: but it is in the joint maintenance of these--the _co-ordination_ of
them. So long as the two go on together, life continues: suspend either of
them, and the result is--death.
The attribute which thus distinguishes the lowest organic from the highest
inorganic bodies, similarly distinguishes the higher organisms from the
lower ones. It is in the greater complexity of the co-ordination--that is,
in the greater number and variety of the co-ordinated actions--that every
advance in the scale of being essentially consists. And whether we regard
the numerous vital processes carried on in a creature of complex structure
as so many additional processes, or whether, more philosophically, we
regard them as subdivisions of the two fundamental ones--oxidation and
accretion--the co-ordination of them is still the life. Thus turning to
what is physiologically classified as the _vegetative system_, we see that
stomach, lungs, heart, liver, skin, and the rest, must work in concert. If
one of them does too much or too little--that is, if the co-ordination be
imperfect--the life is disturbed; and if one of them ceases to act--that
is, if the co-ordination be destroyed--the life is destroyed. So likewise
is it with the _animal system_, which indirectly assists in co-ordinating
the actions of the viscera by supplying food and oxygen. Its component
parts, the limbs, senses, and instruments of attack or defence must perform
their several offices in proper sequence; and further, must conjointly
minister to the periodic demands of the viscera, that these may in turn
supply blood. How completely the several attributes of animal life come
within the definition, we shall best see on going through them _seriatim_.
Thus _Strength_ results from the co-ordination of actions; for it is
produced by the simultaneous contraction of many muscles and many fibres of
each muscle; and the strength is great in proportion to the number of these
acting together--that is, in proportion to the co-ordination. _Swiftness_
also, depending partly on strength, but requiring also the rapid
alternation of movements, equally comes under the expression; seeing that,
other things equal, the more quickly sequent actions can be made to follow
each other, the more completely are they co-ordinated. So, too, is it with
_Agility_; the power of a chamois to spring with safety from crag to crag
implies accurate co-ordination in the movements of many different muscles,
and a due subordination of them all to the perceptions. The definition
similarly includes _Instinct_, which consists in the uniform succession of
certain actions or series of actions after certain sensations or groups of
sensations; and that which surprises us in instinct is the accuracy with
which these compound actions respond to these compound sensations; that
is--the completeness of their co-ordination. Thus, likewise, is it with
_Intelligence_, even in its highest manifestations. That which we call
rationality is the power to combine, or co-ordinate a great number and a
great variety of complex actions for the achievement of a desired result.
The husbandman has in the course of years, by drainage and manuring, to
bring his ground into a fertile state; in the autumn he must plough,
harrow, and sow, for his next year's crop; must subsequently hoe and weed,
keep out cattle, and scare away birds; when harvest comes, must adapt the
mode and time of getting in his produce to the weather and the labour
market; he must afterwards decide when, and where, and how to sell to the
best advantage; and must do all this that he may get food and clothing for
his family. By properly coordinating these various processes (each of which
involves many others)--by choosing right modes, right times, right
quantities, right qualities, and performing his acts in right order, he
attains his end. But if he have done too little of this, or too much of
that; or have done one thing when he should have done another--if his
proceedings have been badly co-ordinated--that is, if he have lacked
intelligence--he fails.
We find, then, that _the co-ordination of actions_ is a definition of Life,
which includes alike its highest and its lowest manifestations; and not
only so, but expresses likewise the degree of Life, seeing that the Life is
high in proportion as the co-ordination is great. Proceeding upwards, from
the simplest organic cell in which there are but two interdependent
actions, on through the group in which many such cells are acting in
concert, on through the higher group in which some of these cells assume
mainly the respiratory and others the assimilative function--proceeding
still higher to organisms in which these two functions are subdivided into
many others, and in which some cells begin to act together as contractile
fibres; next to organisms in which the visceral division of labour is
carried yet further, and in which many contractile fibres act together as
muscles--ascending again to creatures that combine the movements of several
limbs and many bones and muscles in one action; and further, to creatures
in which complex impressions are followed by the complex acts we term
instinctive--and arriving finally at man, in whom not only are the separate
acts complex, but who achieves his ends by combining together an immense
number and variety of acts often extending through years--we see that the
progress is uniformly towards greater co-ordination of actions. Moreover,
this co-ordination of actions unconsciously constitutes the essence of our
common notion of life; for we shall find, on inquiry, that when we infer
the death of an animal, which does not move on being touched, we infer it
because we miss the usual co-ordination of a sensation and a motion: and we
shall also find, that the test by which we habitually rank creatures high
or low in the scale of vitality is the degree of co-ordination their
actions exhibit.
* * * * *
There remains but to notice the objection which possibly may be raised,
that the co-ordination of actions is not life, but the ability to maintain
life. Lack of space forbids going into this at length. It must suffice to
say, that life and the ability to maintain life will be found the same. We
perpetually expend the vitality we have that we may continue our vitality.
Our power to breathe a minute hence depends upon our breathing now. We must
digest during this week that we may have strength to digest next. That we
may get more food, we must use the force which the food we have eaten gives
us. Everywhere vigorous life is the strength, activity, and sagacity
whereby life is maintained; and equally in descending the scale of being,
or in watching the decline of an invalid, we see that the ebbing away of
life is the ebbing away of the ability to preserve life.[62]
[Only on now coming to re-read the definition of Life enunciated at the
commencement of this essay with the arguments used in justification of it,
does it occur to me that its essential thought ought to have been
incorporated in the definition of Life given in Part I. The idea of
co-ordination is there implied in the idea of correspondence, but the idea
of co-ordination is so cardinal a one that it should be expressed not by
implication but overtly. It is too late to make the required amendment in
the proper place, for the first part of this work is already stereotyped
and printed. Being unable to do better I make the amendment here. The
formula as completed will run:--The definite combination of heterogeneous
changes, both simultaneous and successful, _co-ordinated into_
correspondence with external co-existences and sequences.]
* * * * *
Ending here this preliminary dissertation, let us now proceed to our
special subject.
§ 1. On contemplating its general circumstances, we perceive that any race
of organisms is subject to two sets of conflicting influences. On the one
hand by natural death, by enemies, by lack of food, by atmospheric changes,
&c., it is constantly being destroyed. On the other hand, partly by the
strength, swiftness and sagacity of its members, and partly by their
fertility, it is constantly being maintained. These conflicting sets of
influences may be conveniently generalized as--the forces destructive of
race, and the forces preservative of race.
§ 2. Whilst any race continues to exist, the forces destructive of it and
the forces preservative of it must perpetually tend towards equilibrium. If
the forces destructive of it decrease, the race must gradually become more
numerous, until, either from lack of food or from increase of enemies, the
destroying forces again balance the preserving forces. If, reversely, the
forces destructive of it increase, then the race must diminish, until,
either from its food becoming relatively more abundant, or from its enemies
dying of hunger, the destroying forces sink to the level of the preserving
forces. Should the destroying forces be of a kind that cannot be thus met
(as great change of climate), the race, by becoming extinct, is removed out
of the category. Hence this is necessarily the _law of maintenance_ of all
races; seeing that when they cease to conform to it they cease to be.
Now the forces preservative of race are two--ability in each member of the
race to preserve itself, and ability to produce other members--power to
maintain individual life, and power to propagate the species. These must
vary inversely. When, from lowness of organization, the ability to contend
with external dangers is small, there must be great fertility to compensate
for the consequent mortality; otherwise the race must die out. When, on the
contrary, high endowments give much capacity of self-preservation, there
needs a correspondingly low degree of fertility. Given the dangers to be
met as a constant quantity; then, as the ability of any species to meet
them must be a constant quantity too, and as this is made up of the two
factors--power to maintain individual life and power to multiply--these
cannot do other than vary inversely.
§ 3. To show that observed phenomena harmonise with this _à priori_
principle seems scarcely needful But, though axiomatic in its character,
and therefore incapable of being rendered more certain, yet illustrations
of the conformity to it which nature everywhere exhibits, will facilitate
the general apprehension of it.
In the vegetable kingdom we find that the species consisting of simple
cells, exhibit the highest reproductive power. The yeast fungus, which in a
few hours propagates itself throughout a large mass of wort, offers a
familiar example of the extreme rapidity with which these lowly organisms
multiply. In the _Protococcus nivalis_, a microscopic plant which in the
course of a night reddens many square miles of snow, we have a like
example; as also in the minute _Algæ_, which colour the waters of stagnant
pools. The sudden appearance of green films on damp decaying surfaces, the
spread of mould over stale food, and the rapid destruction of crops by
mildew, afford further instances. If we ascend a step to plants of
appreciable size, we still find that in proportion as the organization is
low the fertility is great. Thus of the common puff-ball, which is little
more than a mere aggregation of cells, Fries says, "in a single individual
of _Reticularia maxima_, I have counted (calculated?) 10,000,000 sporules."
From this point upwards, increase of bulk and greater complexity of
structure are still accompanied by diminished reproductive power; instance
the _Macrocystis pyrifera_, a gigantic sea-weed, which sometimes attains a
length of 1500 feet, of which Carpenter remarks, "This development of the
nutritive surface takes place at the expense of the fructifying apparatus,
which is here quite subordinate."[63] And when we arrive at the
highly-organized exogenous trees, we find that not only are they many years
before beginning to bear with any abundance, but that even then they
produce, at the outside, but a few thousand seeds in a twelvemonth. During
its centuries of existence, an oak does not develop as many acorns as a
fungus does spores in a single night.
Still more clearly is this truth illustrated throughout the animal kingdom.
Though not so great as the fertility of the Protophyta, which, as Prof.
Henslow says, in some cases passes comprehension, the fertility of the
Protozoa is yet almost beyond belief. In the polygastric animalcules
spontaneous fission takes place so rapidly that "it has been calculated by
Prof. Ehrenberg that no fewer than 268 millions might be produced in a
month from a single _Paramecium_;"[64] and even this astonishing rate of
increase is far exceeded in another species, one individual of which, "only
to be perceived by means of a high magnifying power, is calculated to
generate 170 billions in four days."[65] Amongst the larger organisms
exhibiting this lowest mode of reproduction under a modified form--that of
gemmation--we see that, though not nearly so rapid as in the Infusoria, the
rate of multiplication is still extremely high. This fact is well
illustrated by the polypes; and in the apparent suddenness with which whole
districts are blighted by the Aphis (multiplying by internal gemmation), we
have a familiar instance of the startling results which the parthenogenetic
process can achieve. Where reproduction becomes occasional instead of
continuous, as it does amongst higher creatures, the fertility equally
bears an inverse ratio to the development. "The queen ant of the African
_Termites_ lays 80,000 eggs in twenty-four hours; and the common hairworm
(_Gordius_) as many as 8,000,000 in less than one day."[66] Amongst the
_Vertebrata_ the lowest are still the most prolific. "It has been
calculated," says Carpenter, "that above a million of eggs are produced at
once by a single codfish."[67] In the strong and sagacious shark
comparatively few are found. Still less fertile are the higher reptiles.
And amongst the Mammalia, beginning with small Rodents, which quickly reach
maturity, produce large litters, and several litters in the year; advancing
step by step to the higher mammals, some of which are long in attaining the
reproductive age, others of which produce but one litter in a year, others
but one young one at a time, others who unite these peculiarities; and
ending with the elephant and man, the least prolific of all, we find that
throughout this class, as throughout the rest, ability to multiply
decreases as ability to maintain individual life increases.
§ 4. The _à priori_ principle thus exemplified has an obverse of a like
axiomatic character. We have seen that for the continuance of any race of
organisms it is needful that the power of self-preservation and the power
of reproduction should vary inversely.
We shall now see that, quite irrespective of such an end to be subserved,
these powers could not do otherwise than vary inversely. In the nature of
things species can subsist only by conforming to this law; and equally in
the nature of things they cannot help conforming to it.
Reproduction, under all its forms, may be described as the separation of
portions of a parent plant or animal for the purpose of forming other
plants or animals. Whether it be by spontaneous fission, by gemmation, or
by gemmules; whether the detached products be bulbels, spores or seeds,
ovisacs, ova or spermatozoa; or however the process of multiplication be
modified, it essentially consists in the throwing off of parts of adult
organisms for the purpose of making new organisms. On the other hand, self
preservation is fundamentally a maintenance of the organism in undiminished
bulk. Amongst the lowest forms of life, aggregation of tissue is the only
mode in which the self-preserving power is shown. Even in the highest,
sustaining the body in its integrity is that in which self-preservation
most truly consists--is the end which the widest intelligence is indirectly
made to subserve. Whilst, on the one side, it cannot be denied that the
increase of tissue constituting growth is self-preservation both in essence
and in result; neither can it, on the other side, be denied that a
diminution of tissue, either from injury, disease, or old age, is in both
essence and result the reverse.
Hence the maintenance of the individual and the propagation of the race
being respectively aggregative and separative, _necessarily_ vary
inversely. Every generative product is a deduction from the parental life;
and, as already pointed out, to diminish life is to diminish the ability to
preserve life. The portion thrown off is organised matter; vital force has
been expended in the organisation of it, and in the assimilation of its
component elements; which vital force, had no such portion been made and
thrown off, _would have been available for the preservation of the parent_.
Neither of these forces, therefore, can increase, save at the expense of
the other. The one draws in and incorporates new material; the other throws
off material previously incorporated. The one adds to; the other takes
from. Using a convenient expression for describing the facts (though one
that must not be construed into an hypothesis), we may say that the force
which builds up and repairs the individual is an attractive force, whilst
that which throws off germs is a repulsive force. But whatever may turn out
to be the true nature of the two processes, it is clear that they are
mutually destructive; or, stating the proposition in its briefest
form--Individuation and Reproduction are antagonistic.
Again, illustrating the abstract by reference to the concrete, let us now
trace throughout the organic world the various phases of this antagonism.
§ 5. All the lowest animal and vegetable forms--_Protozoa_ and
_Protophyta_--consist essentially of a single cell containing fluid, and
having usually a solid nucleus. This is true of the Infusoria, the simplest
Entozoa, and the microscopic Algæ and Fungi. The organisms so constituted
uniformly multiply by spontaneous fission. The nucleus, originally
spherical, becomes elongated, then constricted across its smallest
diameter, and ultimately separates, when "its divisions," says Prof. Owen,
describing the process in the Infusoria, "seem to repel each other to
positions equidistant from each other, and from the pole or end of the body
to which they are nearest. The influence of these distinct centres of
assimilation is to divert the flow of the plasmatic fluid from a common
course through the body of the polygastrian to two special courses about
those centres. So much of the primary developmental process is renewed, as
leads to the insulation of the sphere of the influence of each assimilative
centre from that of the other by the progressive formation of a double
party wall of integument, attended by progressive separation of one party
wall from the other, and by concomitant constriction of the body of the
polygastrian, until the vibratile action of the superficial cilia of each
separating moiety severs the narrowed neck of union, and they become two
distinct individuals."[68] Similar in its general view is Dr. Carpenter's
description of the multiplication of vegetable cells, which he says divide,
"in virtue, it may be surmised, of a sort of mutual repulsion between the
two halves of the endochrome (coloured cell-contents) which leads to their
spontaneous separation."[69] Under a modified form of this process, the
cell-contents, instead of undergoing bisection, divide into numerous parts,
each of which ultimately becomes a separate individual. In some of the Algæ
"a whole brood of young cells may thus be at once generated in the cavity
of the parent-cell, which subsequently bursts and sets them free."[70] The
_Achlya prolifera_ multiplies after this fashion. Amongst the Fungi, too,
the same mode of increase is exemplified by the _Protococcus nivalis_. And
"it would appear that certain Infusoria, especially the _Kolpodinæ_,
propagate by the breaking-up of their own mass into reproductive
particles."[71]
Now in this fissiparous mode of multiplication, which "is amazingly
productive, and indeed surpasses in fertility any other with which we are
acquainted,"[72] we see most clearly the antagonism between individuation
and reproduction. We see that the reproductive process involves destruction
of the individual; for in becoming two, the parent fungus or polygastrian
must be held to lose its own proper existence; and when it breaks up into a
numerous progeny, does so still more completely. Moreover, this rapid mode
of multiplication not only destroys the individuals in whom it takes place,
but also involves that their individualities, whilst they continue, shall
be of the lowest kind. For assume a protozoon to be growing by imbibition
at a given rate, and it follows that the oftener it divides the smaller
must be the size it attains to; that is, the smaller the development of its
individuality. And a further manifestation of the same truth is seen in the
fact that the more frequent the spontaneous fission the shorter the
existence of each individual. So that alike by preventing anything beyond a
microscopic bulk being attained, by preventing the continuance of this in
its integrity beyond a few hours, and by being fatal when it occurs, this
most active mode of reproduction shows the strongest antagonism to
individual life.
§ 6. Whether or not we regard reproduction as resulting from a repulsive
force (and, as seen above, both Owen and Carpenter lean to some such view),
and whether or not we consider the formation of the individual as due to
the reverse of this--an attractive force--we cannot, on studying the
phenomena, help admitting that two opposite activities thus generalized are
at work; we cannot help admitting that the aggregative and separative
tendencies do in each case determine the respective developments of the
individual and the race. On ascending one degree in the scale of organic
life, we shall find this truth clearly exemplified.
For if these single-celled organisms which multiply so rapidly be supposed
to lose some of their separative tendency, what must be the result? They
now not only divide frequently, but the divided portions fly apart. How,
then, will a diminution of this separative tendency first show itself? May
we not expect that it will show itself in the divided portions _not_ flying
apart, but remaining near each other, and forming a group? This we find in
nature to be the first step in advance. The lowest compound organisms are
"_simple aggregations of vesicles without any definite arrangement,
sometimes united, but capable of existing separately_."[73] In these cases,
"every component cell of the aggregate mass that springs from a single
germ, being capable of existing independently of the rest, may be regarded
as a distinct individual."[74] The several stages of this aggregation are
very clearly seen in both the animal and vegetable kingdoms. In the
_Hæmatococcus binalis_, the plant producing the reddish slime seen on damp
surfaces, not only does each of the cells retain its original sphericity,
but each is separated from its neighbour by a wide interval filled with
mucus; so that it is only as being diffused through a mass of mucus common
to them all, that these cells can be held to constitute one individual. We
find, too, that "the component cells, even in the highest Algæ, are
generally separated from each other by a large quantity of mucilaginous
intercellular substance."[75] And, again, the tissue of the simpler
Lichens, "in consequence of the very slight adhesion of its component
cells, is said to be pulverulent."[76] Similarly the Protozoa, by their
feeble union, constitute the organisms next above them. Amongst the
Polygastrica there are many cases "in which the individuals produced by
fission or gemmation do not become completely detached from each
other."[77] The _Ophrydium_, for instance, "exists under the form of a
motionless jelly-like mass ... made up of millions of distinct and similar
individuals imbedded in a gelatinous connecting substance;"[78] and again,
the _Uvella_, or "grape monad," consists of a cluster "which strongly
resembles a transparent mulberry rolling itself across the field of view by
the ciliary action of its component individuals."[79] The parenchyma of the
Sponge, too, is made up of cells "each of which has the character of a
distinct animalcule, having a certain power of spontaneous motion,
obtaining and assimilating its own food, and altogether living _by_ and
_for_ itself;" and so small is the cohesion of these individual cells, that
the tissue they constitute "drains away when the mass is removed from the
water, like white of egg."[80]
Of course in proportion as the aggregate tendency leading to the formation
of these groups of monads is strong, we may expect that, other things
equal, the groups will be large. Proceeding upwards from the yeast fungus,
whose cells hold together in groups of four, five, and six,[81] there must
be found in each species of these composite organisms a size of group
determined by the strength of the aggregative tendency in that species.
Hence we may expect that, when this limit is passed, the group no longer
remains united, but divides. Such we find to be the fact. These groups of
cells undergo the same process that the cells themselves do. They increase
up to a certain point, and then multiply either by simple spontaneous
fission or by that modification of it called gemmation. The _Volvox
globator_, which is made up of a number of monads associated together in
the form of a hollow sphere, develops within itself a number of smaller
spheres similarly constituted; and after these, swimming freely in its
interior, have reached a certain size, the parent group of animalcules
bursts and sets the interior groups free. And here we may observe how this
compound individuality of the Volvox is destroyed in the act of
reproduction as the simple individuality of the monad is. Again, in the
higher forms of grouped cells, where something like organisation begins to
show itself, the aggregations are not only larger, but the separative
process, now carried on by the method of gemmation, no longer wholly
destroys the individual. And in fact, this gemmation may be regarded as the
form which spontaneous fission must assume in ceasing to be fatal; seeing
that gemmation essentially consists in the separation, not into halves, but
into a larger part and a smaller part; the larger part continuing to
represent the original individual. Thus in the common _Hydra_ or
fresh-water polype, "little bud-like processes are developed from the
external surface, which are soon observed to resemble the parent in
character, possessing a digestive sac, mouth, and tentacula; for a long
time, however, their cavity is connected with that of the parent; but at
last the communication is cut off, and the young polype quits its
attachment, and goes in quest of its own maintenance."[82]
§ 7. Progress from these forms of organisation to still higher forms is
similarly characterized by increase of the aggregative tendency or
diminution of the separative, and similarly exhibits the necessary
antagonism between the development of the individual and the increase of
the race. That process of grouping which constitutes the first step towards
the production of complex organisms, we shall now find repeated in the
formation of series of groups. Just as a diminution of the separative
tendency is shown in the aggregation of divided monads, so is a further
diminution of it shown in the aggregation of the divided groups of monads.
The first instance that occurs is afforded by the compound polypes. "Some
of the simpler forms of the composite _Hydroida_," says Carpenter, "may be
likened to a _Hydra_, whose gemmæ, instead of becoming detached, remain
permanently connected with the parent; and as these in their turn may
develop gemmæ from their own bodies, a structure of more or less
arborescent character may be produced."[83] A similar species of
combination is observable amongst the _Bryozoa_, and the compound
_Tunicata_. Every degree of union may be found amongst these associated
organisms; from the one extreme in which the individuals can exist as well
apart as together, to the other extreme in which the individuals are lost
in the general mass. Whilst each _Bryozoon_ is tolerably independent of its
neighbour, "in the compound _Hydroida_, the lives of the polypes are
subordinate to that of the polypdom."[84] Of the _Salpidæ_ and
_Pyrosomidæ_, Carpenter says:--"Although closely attached to one another,
these associated animals are capable of being separated by a smart shock
applied to the sides of the vessel in which they are swimming.... In other
species, however, the separate animals are imbedded in a gelatinous mass,"
and in one kind "there is an absolute union between the vascular systems of
the different individuals."[85]
In the same manner that with a given aggregative tendency there is a limit
to the size of groups, so is there a similarly-determined limit to the size
of series of groups; and that spontaneous fission which we have seen in
cells and groups of cells we here find repeated. In the lower _Annelida_,
for example, "after the number of segments in the body has been greatly
multiplied by gemmation, a separation of those of the posterior portion
begins to take place; a constriction forms itself about the beginning of
the posterior third of the body, in front of which the alimentary canal
undergoes a dilatation, whilst on the segment behind it a proboscis and
eyes are developed, so as to form the head of the young animal which is to
be budded off; and in due time, by the narrowing of the constriction, a
complete separation is effected."[86] Not unfrequently in the _Nais_ this
process is repeated in the young one before it becomes independent of the
parent. The higher _Annelida_ are distinguished by the greater number of
segments held in continuity; an obvious result of comparatively infrequent
fission. In the class _Myriapoda_, which stands next above, "there is no
known instance of multiplication by fission."[87] Yet even here the law may
be traced both in the number and structure of the segments. The length of
the body is still increased after birth "by gemmation from (or partial
fission of) the penultimate segment." The lower members of the class are
distinguished from the higher by the greater extent to which this gemmation
is carried. Moreover, the growing aggregative tendency is seen in the fact,
that each segment of the Julus "is formed by the coalescence of two
original segments,"[88] whilst in the _Scolopendridæ_, which are the
highest of this class, "the head, according to Mr. Newport, is composed of
eight segments, which are often consolidated into one piece;"[89] both of
which phenomena may be understood as arrests of that process of fission,
which, if allowed to go a little further, would have produced distinct
segments; and, if allowed to go further still, would have separated these
segments into groups.
§ 8. Remarking, first, how gradually this mode of multiplication
disappears--how there are some creatures that spontaneously divide or not
according to circumstances; others that divide when in danger (the several
parts being capable of growing into complete individuals); others which,
though not self-dividing, can live on in each half if artificially divided;
and others in which only one of the divided halves can live--how, again, in
the Crustaceans the power is limited to the reproduction of lost limbs; how
there are certain reptiles that can re-supply a lost tail, but only
imperfectly; and how amongst the higher _Vertebrata_ the ability to repair
small injuries is all that remains--remarking thus much, let us now, by way
of preparation for what is to follow, consider the significance of the
foregoing facts taken in connection with the definition of Life awhile
since given.
This spontaneous fission, which we have seen to be, in all cases, more or
less destructive of individual life, is simply a cessation in the
co-ordination of actions. From the single cell, the halves of whose
nucleus, instead of continuing to act together, begin to repel each other,
fly apart, establish distinct centres of assimilation, and finally cause
the cell to divide; up to the Annelidan, whose string of segments
separates, after reaching a certain length; we everywhere see the
phenomenon to be fundamentally this. The tendency to separate is the
tendency not to act together, probably arising from inability to act
together any longer; and the process of separation is the process of
ceasing to act together. How truly non-co-ordination is the essence of the
matter will be seen on observing that fission takes place more or less
rapidly, according as the co-ordinating apparatus is less or more
developed. Thus, "the capability of spontaneous division is one of the most
distinctive attributes of the acrite type of structure;"[90] the acrite
type of structure being that in which the neurine or nervous matter is
supposed to be diffused through the tissues in a molecular state, and in
which, therefore, there exists no distinct nervous or co-ordinating system.
From this point upwards the gradual disappearance of spontaneous fission is
clearly related to the gradual appearance of nerves and ganglia--a fact
well exemplified by the several grades of _Annelida_ and _Myriapoda_. And
when we remember that in the embryotic development of these classes, the
nervous system does not make its appearance until after the rest of the
organism has made great progress, we may even suspect that that coalescence
of segments characteristic of the _Myriapoda_, exhibits the co-ordinating
power of the rapidly-growing nervous system overtaking and arresting the
separative tendency; and doing this most where it (the nervous system) is
most developed, namely, in the head.
And here let us remark, in passing, how, from this point of view, we still
more clearly discern the antagonism of individuation and reproduction. We
before saw that the propagation of the race is at the expense of the
individual: in the above facts we may contemplate the obverse of this--may
see that the formation of the individual is at the expense of the race.
This combination of parts that are tending to separate and become distinct
beings--this union of many incipient minor individualities into one large
individuality--is an arrest of reproduction--a diminution in the number
produced. Either these units may part and lead independent lives, or they
may remain together and have their actions co-ordinated. Either they may,
by their diffusion, form a small, simple, and prolific race, or, by their
aggregation, a large, complex, and infertile one. But manifestly the
aggregation involves the infertility; and the fertility involves the
smallness.
§ 9. The ability to multiply by spontaneous fission, and the ability to
maintain individual life, are opposed in yet another mode. It is not in
respect of size only, but still more in respect of structure, that the
antagonism exists.
Higher organisms are distinguished from lower ones partly by bulk, and
partly by complexity. This complexity essentially consists in the mutual
dependence of numerous different organs, each subserving the lives of the
rest, and each living by the help of the rest. Instead of being made up of
many like parts, performing like functions, as the Crinoid, the Star-fish,
or the Millipede, a vertebrate animal is made up of many unlike parts,
performing unlike functions. From that initial form of a compound organism,
in which a number of minor individuals are simply grouped together, we may,
more or less distinctly, trace not only the increasing closeness of their
union, and the gradual disappearance of their individualities in that of
the mass, but the gradual assumption by them of special duties. And this
"physiological division of labour," as it has been termed, has the same
effect as the division of labour amongst men. As the preservation of a
number of persons is better secured when, uniting into a society, they
severally undertake different kinds of work, than when they are separate
and each performs for himself every kind of work; so the preservation of a
congeries of parts, which, combining into one organism, respectively assume
nutrition, respiration, circulation, locomotion, as separate functions, is
better secured than when those parts are independent, and each fulfils for
itself all these functions.
But the condition under which this increased ability to maintain life
becomes possible is, that the parts shall cease to separate. While they are
perpetually separating, it is clear that they cannot assume mutually
subservient duties. And it is further clear that the more the tendency to
separate diminishes, that is, the larger the groups that remain connected,
_the more minutely and perfectly can that subdivision of functions which we
call organization be carried out_.
Thus we see that in its most active form the ability to multiply is
antagonistic to the ability to maintain individual life, not only as
preventing increase of bulk, but also as preventing organization--not only
as preventing homogeneous co-ordination, but as preventing heterogeneous
co-ordination.
§ 10. To establish the unbroken continuity of this law of fertility, it
will be needful, before tracing its results amongst the higher animals, to
explain in what manner spontaneous fission is now understood, and what the
cessation of it essentially means. Originally, naturalists supposed that
creatures which multiply by self-division, under any of its several forms,
continue so to multiply perpetually. In many cases, however, it has
latterly been shown that they do not do this; and it is now becoming a
received opinion that they do not, and cannot, do this, in any case. A
fertilised germ appears here, as amongst higher organisms, to be the point
of departure; and that constant formation of new tissue implied in the
production of a great number of individuals by fission, seems gradually to
exhaust the germinal capacity in the same way that the constant formation
of new tissue, during the development of a single mammal, exhausts it. The
phenomena classified by Steenstrup as "Alternate Generation," and since
generalised by Professor Owen in his work "On Parthenogenesis," illustrate
this. The egg of a _Medusa_ (jellyfish) develops into a polypoid animal
called the _Strobila_. This _Strobila_ lives as the polype does, and, like
it, multiplies rapidly by gemmation. After a great number of individuals
has been thus produced, and when, as we must suppose, the germinal capacity
is approaching exhaustion, each _Strobila_ begins to exhibit a series of
constrictions, giving it some resemblance to a rouleau of coin or a pile of
saucers. These constrictions deepen; the segments gradually develop
tentacula; the terminal segment finally separates itself, and swims away in
the form of a young _Medusa_; the other segments, in succession, do the
same; and from the eggs which these _Medusæ_ produce, other like series of
polypoid animals, multiplying by gemmation, originate. In the compound
Polypes, in the _Tunicata_, in the _Trematoda_, and in the Aphis, we find
repeated, under various modifications, the same phenomenon.
Understanding then, this lowest and most rapid mode of multiplication to
consist essentially in the production of a great number of individuals from
a single germ--perceiving, further, that diminished activity of this mode
of multiplication consists essentially in the aggregation of the
germ-product into larger masses--and seeing, lastly, that the disappearance
of this mode of multiplication consists essentially in the aggregation of
the germ-product into _one_ mass--we shall be in a position to comprehend,
amongst the higher animals, that new aspect of the law, under which
increased individuation still involves diminished reproduction. Progressing
from those lowest forms of life in which a single ovum originates countless
organisms, through the successive stages in which the number of organisms
so originated becomes smaller and smaller; and finally arriving at a stage
in which one ovum produces but one organism; we have now, in our further
ascent, to observe the modified mode in which this same necessary
antagonism between the ability to multiply, and the ability to preserve
individual life, is exhibited.
§ 11. Throughout both the animal and vegetable kingdoms, generation is
effected "by the union of the contents of a 'sperm-cell' with those of a
'germ-cell;' the latter being that from within which the embryo is evolved,
whilst the former supplies some material or influence necessary to its
evolution."[91] Amongst the lowest vegetable organisms, as in the
_Desmideæ_, the _Diatomaceæ_, and other families of the inferior _Algæ_,
those cells do not appreciably differ; and the application to them of the
terms "sperm-cell" and "germ-cell" is hypothetical. From this point
upwards, however, distinctions become visible. As we advance to higher and
higher types of structure, marked differences arise in the character of
these cells, in the organs evolving them, and in the position of these
organs, which are finally located in separate sexes. Doubtless a separation
in the _functions_ of "sperm-cell" and "germ-cell" has simultaneously
arisen. That change from homogeneity of function to heterogeneity of
function which essentially constitutes progress in organization may be
assumed to take place here also; and, indeed, it is probable that the
distinction gradually established between these cells, in origin and
appearance, is merely significant of, and consequent upon, the distinction
that has arisen between them in constitution and office. Let us now inquire
in what this distinction consists.
If the foundation of every new organism be laid by the combination of two
elements, we may reasonably suspect that these two elements are typical of
some two fundamental divisions of which the new organism is to consist. As
nothing in nature is without meaning and purpose, we may be sure that the
universality of this binary origin, signifies the universality of a binary
structure. The simplest and broadest division of which an organism is
capable must be that signified. What, then, must this division be?
The proposed definition of organic life supplies an answer. If organic life
be the co-ordination of actions, then an organism may be primarily divided
into parts whose actions are co-ordinated, and parts which co-ordinate
them--organs which are made to work in concert, and the apparatus which
makes them so work--or, in other words, the assimilative, vascular,
excretory, and muscular systems on the one hand, and the nervous system on
the other. The justness of this classification will become further
apparent, when it is remembered that by the nervous system alone is the
individuality established. By it all parts are made one in purpose, instead
of separate; by it the organism is rendered a conscious whole--is enabled
to recognise its own extent and limits; and by it are all injuries
notified, repairs directed, and the general conservation secured. The more
the nervous system is developed, the more reciprocally subservient do the
components of the body become--the less can they bear separating. And that
which thus individuates many parts into one whole, must be considered as
more broadly distinguished from the parts individuated, than any of these
parts from each other. Further evidence in support of this position may be
drawn from the fact, that as we ascend in the scale of animal life, that
is, as the co-ordination of actions becomes greater, we find the
co-ordinating or nervous system becoming more and more definitely separated
from the rest; and in the vertebrate or highest type of structure we find
the division above insisted on distinctly marked. The co-ordinating parts
and the parts co-ordinated are placed on opposite sides of the vertebral
column. With the exception of a few ganglia, the whole of the nervous
masses are contained within the neural arches of the vertebræ; whilst all
the viscera and limbs are contained within, or appended to, the hæmal
arches--the terms neural and hæmal having, indeed, been chosen to express
this fundamental division.
If, then, there be truth in the assumption that the two elements, which, by
their union, give origin to a new organism, typify the two essential
constituents of such new organism, we must infer that the sperm-cell and
germ-cell respectively consist of co-ordinating matter and matter to be
co-ordinated--neurine and nutriment. That apparent identity of sperm-cell
and germ-cell seen in the lowest forms of life may thus be understood as
significant to the fact that no extended co-ordination of actions exists in
the generative product--each cell being a separate individual; and the
dissimilarity seen in higher organic types may, conversely, be understood
as expressive of, and consequent upon, the increasing degree of
co-ordination exhibited.[92]
That the sperm-cell and germ-cell are thus contrasted in nature and
function may further be suspected on considering the distinctive
characteristics of the sexes. Of the two elements they respectively
contribute to the formation of a fertile germ, it may be reasonably
supposed that each furnishes that which it possesses in greatest abundance
and can best spare. Well, in the greater size of the nervous centres in the
male, as well as in the fact that during famines men succumb sooner than
women, we see that in the male the co-ordinating system is relatively
predominant. From the same evidence, as well as from the greater abundance
of the cellular and adipose tissues in women, we may infer that the
nutritive system predominates in the female.[93] Here, then, is additional
support for the hypothesis that the sperm-cell, which is supplied by the
male, contains co-ordinating matter, and the germ-cell, which is supplied
by the female, contains matter to be co-ordinated.
The same inference may, again, be drawn from a general view of the maternal
function. For if, as we see, it is the office of the mother to afford milk
to the infant, and during a previous period to afford blood to the foetus,
it becomes probable that during a yet earlier stage it is still the
function to supply nutriment, though in another form. Indeed when,
ascending gradually the scale of animal life, we perceive that this
supplying of milk, and before that of blood, is simply a continuation of
the previous process, we may be sure that, with Nature's usual consistency,
this process is essentially one from the beginning.
Quite in harmony with this hypothesis concerning the respective natures of
the sperm-cell and germ-cell is a remark of Carpenter's on the same
point:--
"Looking," he says, "to the very equal mode in which the characters of
the two parents are mingled in _hybrid_ offspring, and to the certainty
that the _material_ conditions which determine the development of the
germ are almost exclusively female, it would seem probable that the
_dynamical_ conditions are, in great part, furnished by the male."[94]
§ 12. Could nothing but the foregoing indirect evidence be adduced in proof
of the proposition that the spermatozoon is essentially a neural element,
and the ovum essentially a hæmal element, we should scarcely claim for it
anything more than plausibility. On finding, however, that this indirect
evidence is merely introductory to evidence of a quite direct nature, its
significance will become apparent. Adding to their weight taken separately
the force of their mutual confirmation, these two series of proofs will be
seen to give the hypothesis a high degree of probability. The direct
evidence now to be considered is of several kinds.
On referring to the description of the process of multiplication in monads,
quoted some pages back (§ 5), from Professor Owen, the reader will perceive
that it is by the pellucid nucleus that the growth and reproduction of
these single-celled creatures are regulated. The nucleus controls the
circulation of the plasmatic fluid; the fission of the nucleus is the first
step towards the formation of another cell; each half of the divided
nucleus establishes round itself an independent current; and, apparently,
it is by the repulsion of the nuclei that the separation into two
individuals is finally effected. All which facts, when generalised, imply
that the nucleus is the governing or _co-ordinating_ part. Now, Professor
Owen subsequently points out that the matter of the sperm-cell performs in
the fertilised germ-cell just this same function which the nucleus performs
in a single-celled animal. We find the absorption by a germ-cell of the
contents of a sperm-cell "followed by the appearance of a pellucid nucleus
in the centre of the opaque and altered germ-cell; we further see its
successive fissions governed by the preliminary division of the pellucid
centre;" and, led by these and other facts, Professor Owen thinks that "one
cannot reasonably suppose that the nature and properties of the nucleus of
the impregnated germ-cell and that of the monad can be different."[95] And
hence he further infers that "the nucleus of the monad is of a nature
similar to, if not identical with," the matter of the spermatozoon. But we
have seen that in the monad the nucleus is the co-ordinating part; and
hence to say that the sperm-cell is, in nature, identical with it, is to
say that the sperm-cell consists of co-ordinating matter.
Chemical analysis affords further evidence, though, from the imperfect data
at present obtained, less conclusive evidence than could be wished. Partly
from the white and gray nervous substances having been analysed together
instead of separately, and partly from the difficulty of isolating the
efficient contents of the sperm-cells, a satisfactory comparison cannot be
made. Nevertheless, possessing in common, as they do, one element, by which
they are specially characterised, the analysis, as far as it goes, supports
our argument. The following table, which has been made up from data given
in the _Cyclopædia of Anatomy and Physiology, Art._ NERVOUS SYSTEM, gives
the proportion of this element in the brain in different conditions, and
shows how important is its presence.
+-----------------------------+--------+-------+-------+--------+-------+
| | In | In | In | In | In |
| |Infants.| Youth.|Adults.|Old Men.|Idiots.|
| +--------+-------+-------+--------+-------+
| Solid constituents in a | | | | | |
| hundred parts of the brain | 17.21 | 25.74 | 27.49 | 26.15 | 29.07 |
| Of these solid constituents | | | | | |
| the phosphorus amounts to | 0.8 | 1.65 | 1.80 | 1.00 | 0.85 |
| Which gives a percentage of | | | | | |
| phosphorus in the solid | | | | | |
| constituents of | 4.65 | 6.41 | 6.54 | 3.82 | 2.92 |
+-----------------------------+--------+-------+-------+--------+-------+
This connection between the quantity of phosphorus present and the degree
of mental power exhibited, is sufficiently significant; and the fact that
in the same individual the varying degrees of cerebral activity are
indicated by the varying quantities of alkaline phosphates excreted by the
kidneys,[96] still more clearly shows the essentialness of phosphorus as a
constituent of nervous matter. Respecting the constitution of sperm-cells
chemists do not altogether agree. One thing, however, is certain--that they
contain unoxidized phosphorus; and also a fatty acid, that is not
improbably similar to the fatty acid contained in neurine.[97] In fact,
there would seem to be present the constituents of that oleophosphoric acid
which forms so distinctive an element of the brain. That a large quantity
of binoxide of protein is also present, may be ascribed to the fact that a
great part of the sperm-cell consists merely of the protective membrane and
its locomotive appendage; the really efficient portion being but the
central contents.[98]
Evidence of a more conclusive nature--evidence, too, which will show in
what direction our argument tends--is seen in the marked antagonism of the
nervous and generative systems. Thus, the fact that intense mental
application, involving great waste of the nervous tissues, and a
corresponding consumption of nervous matter for their repair, is
accompanied by a cessation in the production of sperm-cells, gives strong
support to the hypothesis that the sperm-cells consist essentially of
neurine. And this becomes yet clearer on finding that the converse fact is
true--that undue production of sperm-cells involves cerebral inactivity.
The first result of a morbid excess in this direction is headache, which
may be taken to indicate that the brain is out of repair; this is followed
by stupidity; should the disorder continue, imbecility supervenes, ending
occasionally in insanity.
That the sperm-cell is co-ordinating matter, and the germ-cell matter to be
co-ordinated, is, therefore, an hypothesis not only having much _à priori_
probability, but one supported by numerous facts.
§ 13. This hypothesis alike explains, and is confirmed by, the truth, that
throughout the vertebrate tribes the degree of fertility varies inversely
as the development of the nervous system.
The necessary antagonism of Individuation and Reproduction does indeed show
itself amongst the higher animals, in some degree in the manner hitherto
traced; namely, as determining the total bulk. Though the parts now thrown
off, being no longer segments or gemmæ, are not obvious diminutions of the
parent, yet they must be really such. Under the form of internal fission,
the separative tendency is as much opposed to the aggregative tendency as
ever; and, _other things equal_, the greater or less development of the
individual depends upon the less or greater production of new individuals
or germs of new individuals. As in groups of cells, and series of groups of
cells, we saw that there was in each species a limit, passing which, the
germ product would not remain united; so in each species of higher animal
there is a limit, passing which, the process of cell-multiplication results
in the throwing off of cells, instead of resulting in the formation of more
tissue. Hence, taking an average view, we see why the smaller animals so
soon arrive at a reproductive age, and produce large and frequent broods;
and why, conversely, increased size is accompanied by retarded and
diminished fertility.
But, as above implied, it is not so much to the bulk of the body as a
whole, as to the bulk of the nervous system, that fertility stands related
amongst the higher animals. Probably, indeed, it stands thus related in all
cases; the difference simply arising from the fact, that whereas in the
lower organisms, where the nervous system is not concentrated, its bulk
varies as the bulk of the body, in the higher organisms it does not do so.
Be this as it may, however, we see clearly that, amongst the vertebrata,
the bodily development is not the determining circumstance. In a fish, a
reptile, a bird, and a mammal of the same weight, there is nothing like
equality of fecundity. Cattle and horses, arriving as they do so soon at a
reproductive age, are much more prolific than the human race, at the same
time that they are much larger. And whilst, again, the difference in size
between the elephant and man is far greater, their respective powers of
multiplication are less unlike. Looking in these cases at the nervous
systems, however, we find no such discrepancy. On learning that the average
ratio of the brain to the body is--in fishes, 1 to 5668; in reptiles, 1 to
1321; in birds, 1 to 212; and in mammals, 1 to 186;[99] their different
degrees of fecundity are accounted for. Though an ox will outweigh
half-a-dozen men, yet its brain and spinal cord are far less than those of
one man; and though in bodily development the elephant so immensely exceeds
the human being, yet the elephant's cerebro-spinal system is only thrice
the size attained by that of civilized men.[100] Unfortunately, it is
impossible to trace throughout the animal kingdom this inverse relationship
between the nervous and reproductive systems with any accuracy. Partly from
the fact that, in each case, the degree of fertility depends on three
variable elements--the age at which reproduction begins, the number
produced at a birth, and the frequency of the births; partly from the fact
that, in respect to most animals, these data are not satisfactorily
attainable, and that, when they are attainable, they are vitiated by the
influence of domesticity; and partly from the fact that no precise
measurement of the respective nervous systems has been made, we are unable
to draw any but general and somewhat vague comparisons. These, however, as
far as they go, are in our favour. Ascending from beings of the acrite
nerveless type, which are the most prolific of all, through the various
invertebrate sub-kingdoms, amongst which spontaneous fission disappears as
the nervous system becomes developed; passing again to the least nervous
and most fertile of the vertebrate series--Fishes, of which, too, the
comparatively large-brained cartilaginous kinds multiply much less rapidly
than the others; progressing through the more highly endowed and less
prolific Reptiles to the Mammalia, amongst which the Rodents, with their
unconvoluted brains, are noted for their fecundity; and ending with man and
the elephant, the least fertile and largest-brained of all--there seems to
be throughout a constant relationship between these attributes.
And indeed, on turning back to our _à priori_ principle, no other
relationship appears possible. We found it to be the necessary law of
maintenance of races, that the ability to maintain individual life and the
ability to multiply vary inversely. But the ability to maintain individual
life _is in all cases measured by the development of the nervous system_.
If it be in good visceral organization that the power of self-preservation
is shown, this implies some corresponding nervous apparatus to secure
sufficient food. If it be in strength, there must be a provision of nerves
and nervous centres answering to the number and size of the muscles. If it
be in swiftness and agility, a proportionate development of the cerebellum
is presupposed. If it be in intelligence, this varies with the size of the
cerebrum. As in all cases co-ordination of actions constitutes the life,
or, what is the same thing, the ability to maintain life; and as throughout
the animal kingdom this co-ordination, under all its forms, is effected by
nervous agents of some kind or other; and as each of these nervous agents
performs but one function; it follows that in proportion to the number of
the actions co-ordinated must be the number of nervous agents. Hence the
nervous system becomes the universal measure of the degree of co-ordination
of actions; that is, of the life, or ability to maintain life. And if the
nervous system varies directly as the ability to maintain life, it _must_
vary inversely as the ability to multiply.[101]
And here, assuming the constitution of the sperm-cell above inferred to be
the true one, we see how the obverse _à priori_ principle is fulfilled.
Where, as amongst the lowest organisms, bulk is expressive of life, the
antagonism of individuation and reproduction was broadly exhibited in the
fact that the making of two or more new individuals was the _un_making of
the original individual. And now, amongst the higher organisms, where bulk
is no longer the measure of life, we see that this antagonism is between
the neural elements thrown off, and that internal neural mass whose bulk
_is_ the measure of life. The production of co-ordinating cells must be at
the expense of the co-ordinating apparatus; and the aggregation of the
co-ordinating apparatus must be at the expense of co-ordinating cells. How
the antagonism affects the female economy is not so clear. Possibly the
provision required to be made for supplying nervous as well as other
nutriment to the embryo, involves an arrest in the development of the
nervous system; and if so, probably this arrest takes place early in
proportion as the number of the coming offspring makes the required
provision great: or rather, to put the facts in their right sequence, an
early arrest renders the production of a numerous offspring possible.
§ 14. The law which we have thus traced throughout the animal kingdom, and
which must alike determine the different fertilities of different species,
and the variations of fertility in the same species, we have now to
consider in its application to mankind.
[_The remainder of the essay, which as implied, deals with the
application of this general principle to the multiplication of the human
race, need not be here reproduced. The subject is treated in full in Part
VI._]
APPENDIX B.
THE INADEQUACY OF NATURAL SELECTION, ETC., ETC.
[_In this Appendix are included four essays originally published in the_
Contemporary Review _and subsequently republished as pamphlets. The first
appeared under the above title in February and March, 1893; the second in
May of that year under the title "Prof. Weismann's Theories;" the third in
December of that year under the title "A Rejoinder to Prof. Weismann;" and
the fourth in October, 1894, under the title "Weismannism Once More." As
these successive essays practically form parts of one whole, I have thought
it needless to keep them separate by repeating their titles, and have
simply marked them off from one another by the numbers I, II, III, IV. Of
course, as they are components of a controversy, some incompleteness arises
from the absence of the essays to which portions of them were replies; but
in each the course of the argument sufficiently indicates the
counter-arguments which were met._]
I.
Students of psychology are familiar with the experiments of Weber on the
sense of touch. He found that different parts of the surface differ widely
in their ability to give information concerning the things touched. Some
parts, which yielded vivid sensations, yielded little or no knowledge of
the sizes or forms of the things exciting them; whereas other parts, from
which there came sensations much less acute, furnished clear impressions
respecting the tangible characters, even of relatively small objects. These
unlikenesses of tactual discriminativeness he ingeniously expressed by
actual measurements. Taking a pair of compasses, he found that if they were
closed so nearly that the points were less than one-twelfth of an inch
apart, the end of the forefinger could not perceive that there were two
points: the two points seemed one. But when the compasses were opened so
that the points were one-twelfth of an inch apart, then the end of the
forefinger distinguished the two points. At the same time, he found that
the compasses must be opened to the extent of two and a half inches, before
the middle of the back could distinguish between two points and one. That
is to say, as thus measured, the end of the forefinger has thirty times the
tactual discriminativeness which the middle of the back has.
Between these extremes he found gradations. The inner surfaces of the
second joints of the fingers can distinguish separateness of positions only
half as well as the tip of the forefinger. The innermost joints are still
less discriminating, but have powers of discrimination equal to that of the
tip of the nose. The end of the great toe, the palm of the hand, and the
cheek, have alike one-fifth of the perceptiveness which the tip of the
forefinger has; and the lower part of the forehead has but one-half that
possessed by the cheek. The back of the hand and the crown of the head are
nearly alike in having but a fourteenth or a fifteenth of the ability to
perceive positions as distinct, which is possessed by the finger-end. The
thigh, near the knee, has rather less, and the breast less still; so that
the compasses must be opened more than an inch and a half before the breast
distinguishes the two points from one another.
What is the meaning of these differences? How, in the course of evolution,
have they been established? If "natural selection," or survival of the
fittest, is the assigned cause, then it is required to show in what way
each of these degrees of endowment has advantaged the possessor to such
extent that not infrequently life has been directly or indirectly preserved
by it. We might reasonably assume that in the absence of some
differentiating process, all parts of the surface would have like powers of
perceiving relative positions. They cannot have become widely unlike in
perceptiveness without some cause. And if the cause alleged is natural
selection, then it is necessary to show that the greater degree of the
power possessed by this part than by that, has not only conduced to the
maintenance of life, but has conduced so much that an individual in whom a
variation has produced better adjustment to needs, thereby maintained life
when some others lost it; and that among the descendants inheriting this
variation, there was a derived advantage such as enabled them to multiply
more than the descendants of individuals not possessing it. Can this, or
anything like this, be shown?
That the superior perceptiveness of the forefinger-tip has thus arisen,
might be contended with some apparent reason. Such perceptiveness is an
important aid to manipulation, and may have sometimes given a life-saving
advantage. In making arrows or fish-hooks, a savage possessing some extra
amount of it may have been thereby enabled to get food where another
failed. In civilized life, too, a sempstress with well-endowed finger-ends
might be expected to gain a better livelihood than one with finger-ends
which were obtuse; though this advantage would not be so great as appears.
I have found that two ladies whose finger-ends were covered with
glove-tips, reducing their sensitiveness from one-twelfth of an inch
between compass-points to one-seventh, lost nothing appreciable of their
quickness and goodness in sewing. An experience of my own here comes in
evidence. Towards the close of my salmon-fishing days I used to observe
what a bungler I had become in putting on and taking off artificial flies.
As the tactual discriminativeness of my finger-ends, recently tested, comes
up to the standard specified by Weber, it is clear that this decrease of
manipulative power, accompanying increase of age, was due to decrease in
the delicacy of muscular co-ordination and sense of pressure--not to
decrease of tactual discriminativeness. But not making much of these
criticisms, let us admit the conclusion that this high perceptive power
possessed by the forefinger-end may have arisen by survival of the fittest;
and let us limit the argument to the other differences.
How about the back of the trunk and its face? Is any advantage derived from
possession of greater tactual discriminativeness by the last than the
first? The tip of the nose has more than three times the power of
distinguishing relative positions which the lower part of the forehead has.
Can this greater power be shown to have any advantage? The back of the hand
has scarcely more discriminative ability than the crown of the head, and
has only one-fourteenth of that which the finger-tip has. Why is this?
Advantage might occasionally be derived if the back of the hand could tell
us more than it does about the shapes of the surfaces touched. Why should
the thigh near the knee be twice as perceptive as the middle of the thigh?
And, last of all, why should the middle of the forearm, middle of the
thigh, middle of the back of the neck, and middle of the back, all stand on
the lowest level, as having but one-thirtieth of the perceptive power which
the tip of the forefinger has? To prove that these differences have arisen
by natural selection, it has to be shown that such small variation in one
of the parts as might occur in a generation--say one-tenth extra
amount--has yielded an appreciably greater power of self-preservation; and
that those inheriting it have continued to be so far advantaged as to
multiply more than those who, in other respects equal, were less endowed
with this trait. Does any one think he can show this?
But if this distribution of tactual perceptiveness cannot be explained by
survival of the fittest, how can it be explained? The reply is that, if
there has been in operation a cause which it is now the fashion among
biologists to ignore or deny, these various differences are at once
accounted for. This cause is the inheritance of acquired characters. As a
preliminary to setting forth the argument showing this, I have made some
experiments.
It is a current belief that the fingers of the blind, more practised in
tactual exploration than the fingers of those who can see, acquire greater
discriminativeness: especially the fingers of those blind who have been
taught to read from raised letters. Not wishing to trust to this current
belief, I recently tested two youths, one of fifteen and the other younger,
at the School for the Blind in Upper Avenue Road, and found the belief to
be correct. I found that instead of being unable to distinguish between
points of the compasses until they were opened to one-twelfth of an inch
apart, both of them could distinguish between points when only
one-fourteenth of an inch apart. They had thick and coarse skins; and
doubtless, had the intervening obstacle, so produced, been less, the
discriminative power would have been greater. It afterwards occurred to me
that a better test would be furnished by those whose finger-ends are
exercised in tactual perceptions, not occasionally, as by the blind in
reading, but all day long in pursuit of their occupations. The facts
answered expectation. Two skilled compositors, on whom I experimented, were
both able to distinguish between points when they were only one-seventeenth
of an inch apart. Thus we have clear proof that constant exercise of the
tactual nervous structure leads to further development.[102]
Now if acquired structural traits are inheritable, the various contrasts
above set down are obvious consequences; for the gradations in tactual
perceptiveness correspond with the gradations in the tactual exercises of
the parts. Save by contact with clothes, which present only broad surfaces
having but slight and indefinite contrast, the trunk has scarcely any
converse with external bodies, and it has but small discriminative power;
but what discriminative power it has is greater on its face than on its
back, corresponding to the fact that the chest and abdomen are much more
frequently explored by the hands: this difference being probably in part
inherited from inferior creatures; for, as we may see in dogs and cats, the
belly is far more accessible to feet and tongue than the back. No less
obtuse than the back are the middle of the back of the neck, the middle of
the forearm, and the middle of the thigh; and these parts have but rare
experiences of irregular foreign bodies. The crown of the head is
occasionally felt by the fingers, as also the back of one hand by the
fingers of the other; but neither of these surfaces, which are only twice
as perceptive as the back, is used with any frequency for touching objects,
much less for examining them. The lower part of the forehead, though more
perceptive than the crown of the head, in correspondence with a somewhat
greater converse with the hands, is less than one-third as perceptive as
the tip of the nose; and manifestly, both in virtue of its relative
prominence, in virtue of its contacts with things smelt at, and in virtue
of its frequent acquaintance with the handkerchief, the tip of the nose has
far greater tactual experience. Passing to the inner surfaces of the hands,
which, taken as wholes, are more constantly occupied in touching than are
the back, breast, thigh, forearm, forehead, or back of the hand, Weber's
scale shows that they are much more perceptive, and that the degrees of
perceptiveness of different parts correspond with their tactual activities.
The palms have but one-fifth the perceptiveness possessed by the
forefinger-ends; the inner surfaces of the finger-joints next the palms
have but one-third; while the inner surfaces of the second joints have but
one-half. These abilities correspond with the facts that whereas the inner
parts of the hand are used only in grasping things, the tips of the fingers
come into play not only when things are grasped, but when such things, as
well as smaller things, are felt at or manipulated. It needs but to observe
the relative actions of these parts in writing, in sewing, in judging
textures, &c., to see that above all other parts the finger-ends, and
especially the forefinger-ends, have the most multiplied experiences. If,
then, it be that the extra perceptiveness acquired from actual tactual
activities, as in a compositor, is inheritable, these gradations of tactual
perceptiveness are explained.
Doubtless some of those who remember Weber's results, have had on the tip
of the tongue the argument derived from the tip of the tongue. This part
exceeds all other parts in power of tactual discrimination: doubling, in
that respect, the power of the forefinger-tip. It can distinguish points
that are only one-twenty-fourth of an inch apart. Why this unparalleled
perceptiveness? If survival of the fittest be the ascribed cause, then it
has to be shown what the advantages achieved have been; and, further, that
those advantages have been sufficiently great to have had effects on the
maintenance of life.
Besides tasting, there are two functions conducive to life, which the
tongue performs. It enables us to move about food during mastication, and
it enables us to make many of the articulations constituting speech. But
how does the extreme discriminativeness of the tongue-tip aid these
functions? The food is moved about, not by the tongue-tip, but by the body
of the tongue; and even were the tip largely employed in this process, it
would still have to be shown that its ability to distinguish between points
one-twenty-fourth of an inch apart, is of service to that end, which cannot
be shown. It may, indeed, be said that the tactual perceptiveness of the
tongue-tip serves for detection of foreign bodies in the food, as
plum-stones or as fish-bones. But such extreme perceptiveness is needless
for the purpose. A perceptiveness equal to that of the finger-ends would
suffice. And further, even were such extreme perceptiveness useful, it
could not have caused survival of individuals who possessed it in slightly
higher degrees than others. It needs but to observe a dog crunching small
bones, and swallowing with impunity the sharp-angled pieces, to see that
but a very small amount of mortality would be prevented.
But what about speech? Well, neither here can there be shown any advantage
derived from this extreme perceptiveness. For making the _s_ and _z_, the
tongue has to be partially applied to a portion of the palate next the
teeth. Not only, however, must the contact be incomplete, but its place is
indefinite--may be half an inch further back. To make the _sh_ and _zh_,
the contact has to be made, not with the tip, but with the upper surface of
the tongue; and must be an incomplete contact. Though, for making the
liquids, the tip of the tongue and the sides of the tongue are used, yet
the requisite is not any exact adjustment of the tip, but an imperfect
contact with the palate. For the _th_, the tip is used along with the edges
of the tongue; but no perfect adjustment is required, either to the edges
of the teeth, or to the junction of the teeth with the palate, where the
sound may equally well be made. Though for the _t_ and _d_ complete contact
of the tip and edges of the tongue with the palate is required, yet the
place of contact is not definite, and the tip takes no more important share
in the action than the sides. Any one who observes the movements of his
tongue in speaking, will find that there occur no cases in which the
adjustments must have an exactness corresponding to the extreme power of
discrimination which the tip possesses: for speech, this endowment is
useless. Even were it useful, it could not be shown that it has been
developed by survival of the fittest; for though perfect articulation is an
aid, yet imperfect articulation has rarely such an effect as to impede a
man in the maintenance of his life. If he is a good workman, a German's
interchanges of _b's_ and _p's_ do not disadvantage him. A Frenchman who,
in place of the sound of _th_, always makes the sound of _z_, succeeds as a
teacher of music or dancing, no less than if he achieved the English
pronunciation. Nay, even such an imperfection of speech as that which
arises from cleft palate, does not prevent a man from getting on if he is
capable. True, it may go against him as a candidate for Parliament, or as
an "orator" of the unemployed (mostly not worth employing). But in the
struggle for life he is not hindered by the effect to the extent of being
less able than others to maintain himself and his offspring. Clearly, then,
even if this unparalleled perceptiveness of the tongue-tip is required for
perfect speech, such use is not sufficiently important to have been
developed by natural selection.
How, then, is this remarkable trait of the tongue-tip to be accounted for?
Without difficulty, if there is inheritance of acquired characters. For the
tongue-tip has, above all other parts of the body, unceasing experiences of
small irregularities of surface. It is in contact with the teeth, and
either consciously or unconsciously is continually exploring them. There is
hardly a moment in which impressions of adjacent but different positions
are not being yielded to it by either the surfaces of the teeth or their
edges; and it is continually being moved about from some of them to others.
No advantage is gained. It is simply that the tongue's position renders
perpetual exploration almost inevitable; and by perpetual exploration is
developed this unique power of discrimination. Thus the law holds
throughout, from this highest degree of perceptiveness of the tongue-tip to
its lowest degree on the back of the trunk; and no other explanation of the
facts seems possible.
"Yes, there is another explanation," I hear some one say: "they may be
explained by _panmixia_." Well, in the first place, as the explanation by
_panmixia_ implies that these gradations of perceptiveness have been
arrived at by the dwindling of nervous structures, there lies at the basis
of the explanation an unproved and improbable assumption; and, in the
second place, even were there no such difficulty, it may with certainty be
denied that _panmixia_ can furnish an explanation. Let us look at its
pretensions.
* * * * *
It was not without good reason that Bentham protested against metaphors.
Figures of speech in general, valuable as they are in poetry and rhetoric,
cannot be used without danger in science and philosophy. The title of Mr.
Darwin's great work furnishes us with an instance of the misleading effects
produced by them. It runs:--_The Origin of Species by means of Natural
Selection, or the Preservation of favoured Races in the Struggle for Life_.
Here are two figures of speech which conspire to produce an impression more
or less erroneous. The expression "natural selection" was chosen as serving
to indicate some parallelism with artificial selection--the selection
exercised by breeders. Now selection connotes volition, and thus gives to
the thoughts of readers a wrong bias. Some increase of this bias is
produced by the words in the second title, "favoured races;" for anything
which is favoured implies the existence of some agent conferring a favour.
I do not mean that Mr. Darwin himself failed to recognize the misleading
connotations of his words, or that he did not avoid being misled by them.
In chapter iv of the _Origin of Species_, he says that, considered
literally, "natural selection is a false term," and that the
personification of Nature is objectionable; but he thinks that readers, and
those who adopt his views, will soon learn to guard themselves against the
wrong implications. Here I venture to think that he was mistaken. For
thinking this, there is the reason that even his disciple, Mr. Wallace--no,
not his disciple, but his co-discoverer, ever to be honoured--has
apparently been influenced by them. When, for example, in combating a view
of mine, he says that "the very thing said to be impossible by variation
and natural selection has been again and again effected, by variation and
artificial selection," he seems clearly to imply that the processes are
analogous, and operate in the same way. Now this is untrue. They are
analogous only within certain narrow limits; and, in the great majority of
cases, natural selection is utterly incapable of doing that which
artificial selection does.
To see this it needs only to de-personalise Nature, and to remember that,
as Mr. Darwin says, Nature is "only the aggregate action and product of
many natural laws [forces]." Observe its relative shortcomings. Artificial
selection can pick out a particular trait, and, regardless of other traits
of the individuals displaying it, can increase it by selective breeding in
successive generations. For, to the breeder or fancier, it matters little
whether such individuals are otherwise well constituted. They may be in
this or that way so unfit for carrying on the struggle for life, that were
they without human care, they would disappear forthwith. On the other hand,
if we regard Nature as that which it is, an assemblage of various forces,
inorganic and organic, some favourable to the maintenance of life and many
at variance with its maintenance--forces which operate blindly--we see that
there is no such selection of this or that trait; but that there is a
selection only of individuals which are, by the aggregate of their traits,
best fitted for living. And here I may note an advantage possessed by the
expression "survival of the fittest;" since this does not tend to raise the
thought of any one character which, more than others, is to be maintained
or increased; but tends rather to raise the thought of a general adaptation
for all purposes. It implies the process which Nature can alone carry
on--the leaving alive of those which are best able to utilize surrounding
aids to life, and best able to combat or avoid surrounding dangers. And
while this phrase covers the great mass of cases in which there are
preserved well-constituted individuals, it also covers those special cases
which are suggested by the phrase "natural selection," in which individuals
succeed beyond others in the struggle for life, by the help of particular
characters which conduce in important ways to prosperity and
multiplication. For now observe the fact which here chiefly concerns us,
that survival of the fittest can increase any serviceable trait, only if
that trait conduces to prosperity of the individual, or of posterity, or of
both, _in an important degree_. There can be no increase of any structure
by natural selection unless, amid all the slightly varying structures
constituting the organism, increase of this particular one is so
advantageous as to cause greater multiplication of the family in which it
arises than of other families. Variations which, though advantageous, fail
to do this, must disappear again. Let us take a case.
Keenness of scent in a deer, by giving early notice of approaching enemies,
subserves life so greatly that, other things equal, an individual having it
in an unusual degree is more likely than others to survive; and, among
descendants, to leave some similarly endowed or more endowed, who again
transmit the variation with, in some cases, increase. Clearly this highly
useful power may be developed by natural selection. So also, for like
reasons, may quickness of vision and delicacy of hearing; though it may be
remarked in passing that since this extra sense-endowment, serving to give
early alarm, profits the herd as a whole, which takes the alarm from one
individual, selection of it is not so easy, unless it occurs in a
conquering stag. But now suppose that one member of the herd--perhaps
because of more efficient teeth, perhaps by greater muscularity of stomach,
perhaps by secretion of more appropriate gastric juices--is enabled to eat
and digest a not uncommon plant which the others refuse. This peculiarity
may, if food is scarce, conduce to better self-maintenance, and better
fostering of young if the individual is a hind. But unless this plant is
abundant, and the advantage consequently great, the advantages which other
members of the herd gain from other slight variations may be equivalent.
This one has unusual agility, and leaps a chasm which others balk at. That
one develops longer hair in winter, and resists the cold better. Another
has a skin less irritated by flies, and can graze without so much
interruption. Here is one which has an unusual power of detecting food
under the snow; and there is one which shows extra sagacity in the choice
of a shelter from wind and rain. That the variation giving ability to eat a
plant before unutilized, may become a trait of the herd, and eventually of
a variety, it is needful that the individual in which it occurs shall have
more descendants, or better descendants, or both, than have the various
other individuals severally having their small superiorities. If these
other individuals severally profit by their small superiorities, and
transmit them to equally large numbers of offspring, no increase of the
variation in question can take place: it must soon be cancelled. Whether in
the _Origin of Species_ Mr. Darwin has recognized this fact, I do not
remember, but he has certainly done it by implication in his _Animals and
Plants under Domestication_. Speaking of variations in domestic animals, he
there says that "any particular variation would generally be lost by
crossing, reversion, and the accidental destruction of the varying
individuals, unless carefully preserved by man." (Vol. II, p. 292.) That
which survival of the fittest does in cases like the one I have instanced,
is to keep all faculties up to the mark, by destroying such individuals as
have faculties in some respect below the mark; and it can produce
development of some one faculty only if that faculty is predominantly
important. It seems to me that many naturalists have practically lost sight
of this, and assume that natural selection will increase _any_ advantageous
trait. Certainly a view now held by some assumes as much.
The consideration of this view, to which the foregoing paragraph is
introductory, may now be entered upon. This view concerns, not direct
selection, but what has been called, in questionable logic, "reversed
selection"--the selection which effects, not increase of an organ, but
decrease of it. For as, under some conditions, it is of advantage to an
individual and its descendants to have some structure of larger size, it
may be, under other conditions--namely, when the organ becomes useless--of
advantage to have it of smaller size; since, even if it is not in the way,
its weight and the cost of its nutrition are injurious taxes on the
organism. But now comes the truth to be emphasized. Just as direct
selection can increase an organ only in certain cases, so can reversed
selection decrease it only in certain cases. Like the increase produced by
a variation, the decrease produced by one must be such as will sensibly
conduce to preservation and multiplication. It is, for instance,
conceivable that were the long and massive tail of the kangaroo to become
useless (say by the forcing of the species into a mountainous and rocky
habitat filled with brushwood), a variation which considerably reduced the
tail might sensibly profit the individual in which it occurred; and, in
seasons when food was scarce, might cause survival when individuals with
large tails died. But the economy of nutrition must be considerable before
any such result could occur. Suppose that in this new habitat the kangaroo
had no enemies; and suppose that, consequently, quickness of hearing not
being called for, large ears gave no greater advantage than small ones.
Would an individual with smaller ears than usual, survive and propagate
better than other individuals, in consequence of the economy of nutrition
achieved? To suppose this is to suppose that the saving of a grain or two
of protein per day would determine the kangaroo's fate.
Long ago I discussed this matter in the _Principles of Biology_ (§ 166),
taking as an instance the decrease of the jaw implied by the crowding of
the teeth, and now proved by measurement to have taken place. Here is the
passage:--
"No functional superiority possessed by a small jaw over a large jaw, in
civilized life, can be named as having caused the more frequent survival
of small-jawed individuals. The only advantage which smallness of jaw
might be supposed to give, is the advantage of economized nutrition; and
this could not be great enough to further the preservation of men
possessing it. The decrease of weight in the jaw and co-operative parts
that has arisen in the course of many thousands of years, does not amount
to more than a few ounces. This decrease has to be divided among the many
generations that have lived and died in the interval. Let us admit that
the weight of these parts diminished to the extent of an ounce in a
single generation (which is a large admission); it still cannot be
contended that the having to carry an ounce less in weight, or having to
keep in repair an ounce less of tissue, could sensibly affect any man's
fate. And if it never did this--nay, if it did not cause a _frequent_
survival of small-jawed individuals where large-jawed individuals died,
natural selection could neither cause nor aid diminution of the jaw and
its appendages."
When writing this passage in 1864, I never dreamt that a quarter of a
century later, the supposable cause of degeneration here examined and
excluded as impossible, would be enunciated as an actual cause and named
"reversed selection."
One of the arguments used to show the adequacy of natural selection under
its direct or indirect form consists of a counter-argument to the effect
that inheritance of functionally-wrought changes, supposing it to be
operative, does not explain certain of the facts. This is alleged by Prof.
Weismann as a part justification for his doctrine of Panmixia. Concerning
the "blind fish and amphibia" found in dark places, which have but
rudimentary eyes "hidden under the skin," he argues that "it is difficult
to reconcile the facts of the case with the ordinary theory that the eyes
of these animals have simply degenerated through disuse." After giving
instances of rapid degeneration of disused organs, he argues that if "the
effects of disuse are so striking in a single life, we should certainly
expect, if such effects can be transmitted, that all traces of an eye would
soon disappear from a species which lives in the dark." Doubtless this is a
reasonable conclusion. To explain the facts on the hypothesis that acquired
characters are inheritable, seems very difficult. One possible explanation
may, indeed, be named. It appears to be a general law of organization that
structures are stable in proportion to their antiquity--that while organs
of relatively modern origin have but a comparatively superficial root in
the constitution, and readily disappear if the conditions do not favour
their maintenance, organs of ancient origin have deep-seated roots in the
constitution, and do not readily disappear. Having been early elements in
the type, and having continued to be reproduced as parts of it during a
period extending throughout many geological epochs, they are comparatively
persistent. Now the eye answers to this description as being a very early
organ. But waiving possible explanations, let us take the particular
instance cited by Prof. Weismann and see what is to be made of it. He
writes:--
"The caverns in Carniola and Carinthia, in which the blind _Proteus_ and
so many other blind animals live, belong geologically to the Jurassic
formation; and although we do not exactly know when for example the
_Proteus_ first entered them, the low organization of this amphibian
certainly indicates that it has been sheltered there for a very long
period of time, and that thousands of generations of this species have
succeeded one another in the caves.
"Hence there is no reason to wonder at the extent to which the
degeneration of the eye has been already carried in the _Proteus_; even
if we assume that it is merely due to the cessation of the conserving
influence of natural selection."[103]
Let me first note a strange oversight on the part of Prof. Weismann. He
points out that the caverns in question belong to the Jurassic formation:
apparently intending to imply that they have an antiquity related to that
of the formation. But there is no such relation, except that the caverns
cannot be older than the formation. They may have originated at any period
since the containing strata were deposited; and they may be therefore
relatively modern. But passing over this, and admitting that the _Proteus_
has inhabited the caverns for an enormous period, what is to be said of the
fact that their eyes have not disappeared entirely, as Prof. Weismann
contends they should have done had the inheritance of the effects of disuse
been all along operative? There is a very sufficient answer--the
rudimentary eyes are not entirely useless. It seems that when the
underground streams it inhabits are unusually swollen, some individuals of
the species are carried out of the caverns into the open (being then
sometimes captured). It is also said that the creatures shun the light;
this trait being, I presume, observed when it is in captivity. Now
obviously, among individuals carried out into the open, those which remain
visible are apt to be carried off by enemies; whereas, those which,
appreciating the difference between light and darkness, shelter themselves
in dark places, survive. Hence the tendency of natural selection is to
prevent the decrease of the eyes beyond that point at which they can
distinguish between light and darkness. Thus the apparent anomaly is
explained.
Let me suggest, as another possible reason for persistence of rudimentary
organs, that the principle of economy of growth will cause diminution of
them only in proportion as their constituents are of value for other uses
in the organism; and that in many cases their constituents are practically
valueless. Hence probably the reason why, in the case of stalk-eyed
crustaceans, the eye is gone but the pedicle remains, or to use Mr.
Darwin's simile, the telescope has disappeared but not its stand.
* * * * *
Along with that inadequacy of natural selection to explain changes of
structure which do not aid life in important ways, alleged in § 166 of _The
Principles of Biology_, a further inadequacy was alleged. It was contended
that the relative powers of co-operative parts cannot be adjusted solely by
survival of the fittest; and especially where the parts are numerous and
the co-operation complex. In illustration it was pointed out that immensely
developed horns, such as those of the extinct Irish elk, weighing over a
hundred-weight, could not, with the massive skull bearing them, be carried
at the extremity of the outstretched neck without many and great
modifications of adjacent bones and muscles of the neck and thorax; and
that without strengthening of the fore-legs, too, there would be failure
alike in fighting and in locomotion. And it was argued that while we cannot
assume spontaneous increase of all these parts proportionate to the
additional strains, we cannot suppose them to increase by variations, one
at once, without supposing the creature to be disadvantaged by the weight
and nutrition of parts that were for the time useless--parts, moreover,
which would revert to their original sizes before the other needful
variations occurred.
When, in reply to me, it was contended that co-operative parts vary
together, I named facts conflicting with this assertion--the fact that the
blind cray-fish of the Kentucky caves have lost their eyes but not the
foot-stalks carrying them; the fact that the normal proportion between
tongue and beak in certain selected varieties of pigeons is lost; the fact
that lack of concomitance in decrease of jaws and teeth in sundry kinds of
pet dogs, has caused great crowding of the teeth ("The Factors of Organic
Evolution," _Essays_, i, 401-402). And I then argued that if co-operative
parts, small in number and so closely associated as these are, do not vary
together, it is unwarrantable to allege that co-operative parts which are
very numerous and remote from one another vary together. After making this
rejoinder I enforced my argument by a further example--that of the giraffe.
Tacitly recognizing the truth that the unusual structure of this creature
must have been, in its most conspicuous traits, the result of survival of
the fittest (since it is absurd to suppose that efforts to reach high
branches could lengthen the legs), I illustrated afresh the obstacles to
co-adaptation. Not dwelling on the objection that increase of any
components of the fore-quarters out of adjustment to the others, would
cause evil rather than good, I went on to argue that the co-adaptation of
parts required to make the giraffe's structure useful, is much greater than
at first appears. This animal has a grotesque gallop, necessitated by the
great difference in length between the fore and the hind limbs. I pointed
out that the mode of action of the hind limbs shows that the bones and
muscles have all been changed in their proportions and adjustments; and I
contended that, difficult as it is to believe that all parts of the
fore-quarters have been co-adapted by the appropriate variations, now of
this part now of that, it becomes impossible to believe that all the parts
in the hind-quarters have been simultaneously co-adapted to one another and
to all the parts of the fore-quarters: adding that want of co-adaptation,
even in a single muscle, would cause fatal results when high speed had to
be maintained while escaping from an enemy.
Since this argument, repeated with this fresh illustration, was published
in 1886, I have met with nothing to be called a reply; and might, I think,
if convictions usually followed proofs, leave the matter as it stands. It
is true that, in his _Darwinism_, Mr. Wallace has adverted to my renewed
objection, and, as already said, contended that changes such as those
instanced can be effected by natural selection, since such changes can be
effected by artificial selection: a contention which, as I have pointed
out, assumes a parallelism that does not exist. But now, instead of
pursuing the argument further along the same line, let me take a somewhat
different line.
If there occurs some change in an organ, say by increase of its size, which
adapts it better to the creature's needs, it is admitted that when, as
commonly happens, the use of the organ demands the co-operation of other
organs, the change in it will generally be of no service unless the
co-operative organs are changed. If, for instance, there takes place such a
modification of a rodent's tail as that which, by successive increases,
produces the trowel-shaped tail of the beaver, no advantage will be derived
unless there also take place certain modifications in the bulks and shapes
of the adjacent vertebræ and their attached muscles, as well as, probably,
in the hind limbs; enabling them to withstand the reactions of the blows
given by the tail. And the question is, by what process these many parts,
changed in different degrees, are co-adapted to the new
requirements--whether variation and natural selection alone can effect the
readjustment. There are three conceivable ways in which the parts may
simultaneously change:--(1) they may all increase or decrease together in
like degree; (2) they may all simultaneously increase or decrease
independently, so as not to maintain their previous proportions, or assume
any other special proportions; (3) they may vary in such ways and degrees
as to make them jointly serviceable for the new end. Let us consider
closely these several conceivabilities.
And first of all, what are we to understand by co-operative parts? In a
general sense, all the organs of the body are co-operative parts, and are
respectively liable to be more or less changed by change in any one. In a
narrower sense, more directly relevant to the argument, we may, if we
choose to multiply difficulties, take the entire framework of bones and
muscles as formed of co-operative parts; for these are so related that any
considerable change in the actions of some entails change in the actions of
most others. It needs only to observe how, when putting out an effort,
there goes, along with a deep breath, an expansion of the chest and a
bracing up of the abdomen, to see that various muscles beyond those
directly concerned are strained along with them. Or, when suffering from
lumbago, an effort to lift a chair will cause an acute consciousness that
not the arms only are brought into action, but also the muscles of the
back. These cases show how the motor organs are so tied together that
altered actions of some implicate others quite remote from them.
But without using the advantage which this interpretation of the words
would give, let us take, as co-operative organs, those which are obviously
such--the organs of locomotion. What, then, shall we say of the fore limbs
and hind limbs of terrestrial mammals, which co-operate closely and
perpetually? Do they vary together? If so, how have there been produced
such contrasted structures as that of the kangaroo, with its large hind
limbs and small fore limbs, and that of the giraffe, in which the hind
limbs are small and the fore limbs large--how does it happen that,
descending from the same primitive mammal, these creatures have diverged in
the proportions of their limbs in opposite directions? Take, again, the
articulate animals. Compare one of the lower types, with its rows of almost
equal-sized limbs, and one of the higher types, as a crab or a lobster,
with limbs some very small and some very large. How came this contrast to
arise in the course of evolution, if there was the equality of variation
supposed?
But now let us narrow the meaning of the phrase still further, giving it a
more favourable interpretation. Instead of considering separate limbs as
co-operative, let us consider the component parts of the same limb as
co-operative, and ask what would result, from varying together. It would in
that case happen that, though the fore and hind limbs of a mammal might
become different in their sizes, they would not become different in their
structures. If so, how have there arisen the unlikenesses between the hind
legs of the kangaroo and those of the elephant? Or if this comparison is
objected to, because the creatures belong to the widely different divisions
of implacental and placental mammals, take the cases of the rabbit and the
elephant, both belonging to the last division. On the hypothesis of
evolution these are both derived from the same original form; but the
proportions of the parts have become so widely unlike that the
corresponding joints are scarcely recognized as such by the unobservant: at
what seem corresponding places the legs bend in opposite ways. Equally
marked, or more marked, is the parallel fact among the _Articulata_. Take
that limb of the lobster which bears the claw and compare it with the
corresponding limb in an inferior articulate animal, or the corresponding
limb of its near ally, the rock lobster, and it becomes obvious that the
component segments of the limb have come to bear to one another in the one
case, proportions immensely different from those they bear in the other
case. Undeniably, then, on contemplating the general facts of organic
structure, we see that the concomitant variations in the parts of limbs,
have not been of a kind to produce equal amounts of change in them, but
quite the opposite--have been everywhere producing inequalities. Moreover,
we are reminded that this production of inequalities among co-operative
parts, is an essential principle of development. Had it not been so, there
could not have been that progress from homogeneity of structure to
heterogeneity of structure which constitutes evolution.
We pass now to the second supposition:--that the variations in co-operative
parts occur irregularly, or in such independent ways that they bear no
definite relations to one another--miscellaneously, let us say. This is the
supposition which best corresponds with the facts. Glances at the faces
around yield conspicuous proofs. Many of the muscles of the face and some
of the bones, are distinctly co-operative; and these respectively vary in
such ways as to produce in each person a different combination. What we see
in the face we have reason to believe holds in the limbs and in all other
parts. Indeed, it needs but to compare people whose arms are of the same
lengths, and observe how stumpy are the fingers of one and how slender
those of another; or it needs but to note the unlikenesses of gait of
passers-by, implying small unlikenesses of structure; to be convinced that
the relations among the variations of co-operative parts are anything but
fixed. And now, confining our attention to limbs, let us consider what must
happen if, by variations taking place miscellaneously, limbs have to be
partially changed from fitness for one function to fitness for another
function--have to be re-adapted. That the reader may fully comprehend the
argument, he must here have patience while a good many anatomical details
are set down.
Let us suppose a species of quadruped of which the members have, for
immense past periods, been accustomed to locomotion over a relatively even
surface, as, for instance, the "prairie-dogs" of North America; and let us
suppose that increase of numbers has driven part of them into a region full
of obstacles to easy locomotion--covered, say, by the decaying stems of
fallen trees, such as one sees in portions of primeval forest. Ability to
leap must then become a useful trait; and, according to the hypothesis we
are considering, this ability will be produced by the selection of
favourable variations. What are the variations required? A leap is effected
chiefly by the bending of the hind limbs so as to make sharp angles at the
joints, and then suddenly straightening them; as any one may see on
watching a cat leap on to the table. The first required change, then, is
increase of the large extensor muscles, by which the hind limbs are
straightened. Their increases must be duly proportioned; for if those which
straightened one joint become much stronger than those which straightened
the other joint, the result must be collapse of the other joint when the
muscles are contracted together. But let us make a large admission, and
suppose these muscles to vary together; what further muscular change is
next required? In a plantigrade mammal the metatarsal bones chiefly bear
the reaction of the leap, though the toes may have a share. In a
digitigrade mammal, however, the toes form almost exclusively the fulcrum,
and if they are to bear the reaction of a higher leap, the flexor muscles
which depress and bend them must be proportionately enlarged: if not, the
leap will fail from want of a firm _point d'appui_. Tendons as well as
muscles must be modified; and, among others, the many tendons which go to
the digits and their phalanges. Stronger muscles and tendons imply greater
strains on the joints; and unless these are strengthened, one or other,
dislocation will be caused by a more vigorous spring. Not only the
articulations themselves must be so modified as to bear greater stress, but
also the numerous ligaments which hold the parts of each in place. Nor can
the bodies of the bones remain unstrengthened; for if they have no more
than the strengths needed for previous movements they will fail to bear
more violent movements. Thus, saying nothing of the required changes in the
pelvis, as well as in the nerves and blood-vessels, there are, counting
bones, muscles, tendons, ligaments, at least fifty different parts in each
hind leg which have to be enlarged. Moreover they have to be enlarged in
unlike degrees. The muscles and tendons of the outer toes, for example,
need not be added to so much as those of the median toes. Now, throughout
their successive stages of growth, all these parts have to be kept fairly
well balanced; as any one may infer on remembering sundry of the accidents
he has known. Among my own friends I could name one who, when playing
lawn-tennis, snapped the Achilles tendon; another who, while swinging his
children, tore some of the muscular fibres in the calf of his leg; another
who, in getting over a fence, tore a ligament of one knee. Such facts,
joined with every one's experience of sprains, show that during the extreme
exertions to which limbs are now and then subject, there is a giving way of
parts not quite up to the required level of strength. How, then, is this
balance to be maintained? Suppose the extensor muscles have all varied
appropriately; their variations are useless unless the other co-operative
parts have also varied appropriately. Worse than this. Saying nothing of
the disadvantage caused by extra weight and cost of nutrition, they will be
causes of mischief--causes of derangement to the rest by contracting with
undue force. And then, how long will it take for the rest to be brought
into adjustment? As Mr. Darwin says concerning domestic animals:--"Any
particular variation would generally be lost by crossing, reversion, &c.
... unless carefully preserved by man." In a state of nature, then,
favourable variations of these muscles would disappear again long before
one or a few of the co-operative parts could be appropriately varied, much
more before all of them could.
With this insurmountable difficulty goes a difficulty still more
insurmountable--if the expression may be allowed. It is not a question of
increased sizes of parts only, but of altered shapes of parts, too. A
glance at the skeletons of mammals shows how unlike are the forms of the
corresponding bones of their limbs; and shows that they have been severally
re-moulded in each species to the different requirements entailed by its
different habits. The change from the structures of hind limbs fitted only
for walking and trotting to hind limbs fitted also for leaping, implies,
therefore, that, along with strengthenings of bones there must go
alterations in their forms. Now the fortuitous alterations of form which
may take place in any bone are countless. How long, then, will it be before
there takes place that particular alteration which will make the bone
fitter for its new action? And what is the probability that the many
required changes of shape, as well as of size, in bones will each of them
be effected before all the others are lost again? If the probabilities
against success are incalculable, when we take account only of changes in
the sizes of parts, what shall we say of their incalculableness when
differences of form also are taken into account?
"Surely this piling up of difficulties has gone far enough"; the reader
will be inclined to say. By no means. There is a difficulty immeasurably
transcending those named. We have thus far omitted the second half of the
leap, and the provisions to be made for it. After ascent of the animal's
body comes descent; and the greater the force with which it is projected
up, the greater is the force with which it comes down. Hence, if the
supposed creature has undergone such changes in the hind limbs as will
enable them to propel it to a greater height, without having undergone any
changes in the fore limbs, the result will be that on its descent the fore
limbs will give way, and it will come down on its nose. The fore limbs,
then, have to be changed simultaneously with the hind. How changed?
Contrast the markedly bent hind limbs of a cat with its almost straight
fore limbs, or contrast the silence of the spring on to the table with the
thud which the fore paws make as it jumps off the table. See how unlike the
actions of the hind and fore limbs are, and how unlike their structures. In
what way, then, is the required co-adaptation to be effected? Even were it
a question of relative sizes only, there would be no answer; for facts
already given show that we may not assume simultaneous increases of size to
take place in the hind and fore limbs; and, indeed, a glance at the various
human races, which differ considerably in the ratios of their legs to their
arms, shows us this. But it is not simply a question of sizes. To bear the
increased shock of descent the fore limbs must be changed throughout in
their structures. Like those in the hind limbs, the changes must be of many
parts in many proportions; and they must be both in sizes and in shapes.
More than this. The scapular arch and its attached muscles must also be
strengthened and re-moulded. See, then, the total requirements. We must
suppose that by natural selection of miscellaneous variations, the parts of
the hind limbs will be co-adapted to one another, in sizes, shapes, and
ratios; that those of the fore limbs will undergo co-adaptation similar in
their complexity, but dissimilar in their kinds; and that the two sets of
co-adaptations will be effected _pari passu_. If, as may be held, the
probabilities are millions to one against the first set of changes being
achieved, then it may be held that the probabilities are billions to one
against the second being simultaneously achieved, in progressive adjustment
to the first.
There remains only to notice the third conceivable mode of adjustment. It
may be imagined that though, by the natural selection of miscellaneous
variations, these adjustments cannot be effected, they may nevertheless be
made to take place appropriately. How made? To suppose them so made is to
suppose that the prescribed end is somewhere recognized; and that the
changes are step by step simultaneously proportioned for achieving it--is
to suppose a designed production of these changes. In such case, then, we
have to fall back in part upon the primitive hypothesis; and if we do this
in part, we may as well do it wholly--may as well avowedly return to the
doctrine of special creations.
What, then, is the only defensible interpretation? If such modifications of
structure produced by modifications of function as we see take place in
each individual, are in any measure transmissible to descendants, then all
these co-adaptations, from the simplest up to the most complex, are
accounted for. In some cases this inheritance of acquired characters
suffices by itself to explain the facts; and in other cases it suffices
when taken in combination with the selection of favourable variations. An
example of the first class is furnished by the change just considered; and
an example of the second class is furnished by the case, before named, of
development in a deer's horns. If, by some extra massiveness spontaneously
arising, or by formation of an additional "point," an advantage is gained
either for attack or defence, then, if the increased muscularity and
strengthened structure of the neck and thorax, which wielding of these
somewhat heavier horns produces, are in a greater or less degree inherited,
and in several successive generations are by this process brought up to the
required extra strength, it becomes possible and advantageous for a further
increase of the horns to take place, and a further increase in the
apparatus for wielding them, and so on continuously. By such processes
only, in which each part gains strength in proportion to function, can
co-operative parts be kept in adjustment, and be re-adjusted to meet new
requirements. Close contemplation of the facts impresses me more strongly
than ever with the two alternatives--either there has been inheritance of
acquired characters, or there has been no evolution.
This very pronounced opinion will be met, on the part of some, by a no less
pronounced demurrer, which involves a denial of possibility. It has been of
late asserted, and by many believed, that inheritance of acquired
characters cannot occur. Weismann, they say, has shown that there is early
established in the evolution of each organism such a distinctness between
those component units which carry on the individual life and those which
are devoted to maintenance of the species, that changes in the one cannot
affect the other. We will look closely into his doctrine.
Basing his argument on the principle of the physiological division of
labour, and assuming that the primary division of labour is that between
such part of an organism as carries on individual life and such part as is
reserved for the production of other lives, Weismann, starting with "the
first multicellular organism," says that--"Hence the single group would
come to be divided into two groups of cells, which may be called somatic
and reproductive--the cells of the body as opposed to those which are
concerned with reproduction." (_Essays upon Heredity_, i, p. 27.)
Though he admits that this differentiation "was not at first absolute, and
indeed is not always so to-day," yet he holds that the differentiation
eventually becomes absolute in the sense that the somatic cells, or those
which compose the body at large, come to have only a limited power of
cell-division, instead of an unlimited power which the reproductive cells
have; and also in the sense that eventually there ceases to be any
communication between the two further than that implied by the supplying of
nutriment to the reproductive cells by the somatic cells. The outcome of
this argument is that, in the absence of communication, changes induced in
the somatic cells, constituting the individual, cannot influence the
natures of the reproductive cells, and cannot therefore be transmitted to
posterity. Such is the theory. Now let us look at a few facts--some
familiar, some unfamiliar.
His investigations led Pasteur to the positive conclusion that the silkworm
diseases are inherited. The transmission from parent to offspring resulted,
not through any contamination of the surface of the egg by the body of the
parent while being deposited, but resulted from infection of the egg
itself--intrusion of the parasitic organism. Generalized observations
concerning the disease called _pébrine_, enabled him to decide, by
inspection of the eggs, which were infected and which were not: certain
modifications of form distinguishing the diseased ones. More than this; the
infection was proved by microscopical examination of the contents of the
egg; in proof of which he quotes as follows from Dr. Carlo Vittadini:--
"Il résulte de mes recherches sur les graines, à l'époque où commence le
développement du germe, que les corpuscules, une fois apparus dans
l'oeuf, augmentent graduellement en nombre, à mesure que l'embryon se
développe; que, dans les derniers jours de l'incubation, l'oeuf en est
plein, au point de faire croire que la majeure partie des granules du
jaune se sont transformés en corpuscules.
"Une autre observation importante est que l'embryon aussi est souillé de
corpuscules, et à un degré tel qu'on peut soupçonner que l'infection du
jaune tire son origine du germe lui-même; en d'autres termes que le germe
est primordialement infecté, et porte en lui-même ces corpuscules tout
comme les vers adultes, frappés du même mal."[104]
Thus, then the substance of the egg and even its innermost vital part, is
permeable by a parasite sufficiently large to be microscopically visible.
It is also of course permeable by the invisible molecules of protein, out
of which its living tissues are formed, and by absorption of which they
subsequently grow. But, according to Weismann, it is _not_ permeable by
those invisible units of protoplasm out of which the vitally active tissues
of the parent are constituted: units composed, as we must assume, of
variously arranged molecules of protein. So that the big thing may pass,
and the little thing may pass, but the intermediate thing may not pass!
A fact of kindred nature, unhappily more familiar, may be next brought in
evidence. It concerns the transmission of a disease not infrequent among
those of unregulated lives. The highest authority concerning this disease,
in its inherited form, is Mr. Jonathan Hutchinson; and the following are
extracts from a letter I have received from him, and which I publish with
his assent:--
"I do not think that there can be any reasonable doubt that a very large
majority of those who suffer from inherited syphilis take the taint from
the male parent.... It is the rule when a man marries who has no
remaining local lesion, but in whom the taint is not eradicated, for his
wife to remain apparently well, whilst her child may suffer. No doubt the
child infects its mother's blood, but this does not usually evoke any
obvious symptoms of syphilis.... I am sure I have seen hundreds of
syphilitic infants whose mothers had not, so far as I could ascertain,
ever displayed a single symptom."
See, then, to what we are committed if we accept Weismann's hypothesis. We
must conclude, that whereas the reproductive cell may be effectually
invaded by an abnormal living element in the parental organism, those
normal living elements which constitute the vital protoplasm of the
parental organism, cannot invade it. Or if it be admitted that both
intrude, then the implication is that, whereas the abnormal element can so
modify the development as to cause changes of structure (as of the teeth),
the normal element can cause no changes of structure![105]
We pass now to evidence not much known to the world at large, but widely
known in the biological world, though known in so incomplete a manner as to
be undervalued in it. Indeed, when I name it, probably many will vent a
mental pooh-pooh. The fact to which I refer is one of which record is
preserved in the museum of the College of Surgeons, in the shape of
paintings of a foal borne by a mare not quite thoroughbred, to a sire which
was thoroughbred--a foal which bears the markings of the quagga. The
history of this remarkable foal is given by the Earl of Morton, F.R.S., in
a letter to the President of the Royal Society (read November 23, 1820). In
it he states that wishing to domesticate the quagga, and having obtained a
male but not a female, he made an experiment.
"I tried to breed from the male quagga and a young chestnut mare of
seven-eighths Arabian blood, and which had never been bred from; the
result was the production of a female hybrid, now five years old, and
bearing, both in her form and in her colour, very decided indications of
her mixed origin. I subsequently parted with the seven-eighths Arabian
mare to Sir Gore Ouseley, who has bred from her by a very fine black
Arabian horse. I yesterday morning examined the produce, namely, a
two-year-old filly and a year-old colt. They have the character of the
Arabian breed as decidedly as can be expected, where fifteen-sixteenths
of the blood are Arabian; and they are fine specimens of that breed; but
both in their colour and in the hair of their manes, they have a striking
resemblance to the quagga. Their colour is bay, marked more or less like
the quagga in a darker tint. Both are distinguished by the dark line
along the ridge of the back, the dark stripes across the forehead, and
the dark bars across the back part of the legs."[106]
Lord Morton then names sundry further correspondences. Dr. Wollaston, at
that time President of the Royal Society, who had seen the animals,
testified to the correctness of his description, and, as shown by his
remarks, entertained no doubt about the alleged facts. But good reason for
doubt may be assigned. There naturally arises the question--How does it
happen that parallel results are not observed in other cases? If in any
progeny certain traits not belonging to the sire, but belonging to a sire
of preceding progeny, are reproduced, how is it that such anomalously
inherited traits are not observed in domestic animals, and indeed in
mankind? How is it that the children of a widow by a second husband do not
bear traceable resemblances to the first husband? To these questions
nothing like satisfactory replies seem forthcoming; and, in the absence of
replies, scepticism, if not disbelief, may be held reasonable.
There is an explanation, however. Forty years ago I made acquaintance with
a fact which impressed me by its significant implications, and has, for
this reason I suppose, remained in my memory. It is set forth in the
_Journal of the Royal Agricultural Society_, Vol. XIV (1853), pp. 214 _et
seq._, and concerns certain results of crossing French and English breeds
of sheep. The writer of the translated paper, M. Malingie-Nouel, Director
of the Agricultural School of La Charmoise, states that when the French
breeds of sheep (in which were included "the _mongrel_ Merinos") were
crossed with an English breed, "the lambs present the following results.
Most of them resemble the mother more than the father; some show no trace
of the father." Joining the admission respecting the mongrels with the
facts subsequently stated, it is tolerably clear that the cases in which
the lambs bore no traces of the father were cases in which the mother was
of pure breed. Speaking of the results of these crossings in the second
generation, "having 75 per cent. of English blood," M. Nouel says:--"The
lambs thrive, wear a beautiful appearance, and complete the joy of the
breeder.... No sooner are the lambs weaned than their strength, their
vigour, and their beauty begin to decay.... At last the constitution gives
way ... he remains stunted for life:" the constitution being thus proved
unstable or unadapted to the requirements. How, then, did M. Nouel succeed
in obtaining a desirable combination of a fine English breed with the
relatively poor French breeds?
He took an animal from "flocks originally sprung from a mixture of the
two distinct races that are established in those two provinces [Berry and
La Sologne]," and these he "united with animals of another mixed breed
... which blended the Tourangelle and native Merino blood of" La Beauce
and Touraine, and obtained a mixture of all four races "without decided
character, without fixity ... but possessing the advantage of being used
to our climate and management."
Putting one of these "mixed blood ewes to a pure New-Kent ram ... one
obtains a lamb containing fifty-hundredths of the purest and most ancient
English blood, with twelve and a half hundredths of four different French
races, which are individually lost in the preponderance of English blood,
and disappear almost entirely, leaving the improving type in the
ascendant.... All the lambs produced strikingly resembled each other, and
even Englishmen took them for animals of their own country."
M. Nouel goes on to remark that when this derived breed was bred with
itself, the marks of the French breeds were lost. "Some slight traces"
could be detected by experts, but these "soon disappeared."
Thus we get proof that relatively pure constitutions predominate in progeny
over much mixed constitutions. The reason is not difficult to see. Every
organism tends to become adapted to its conditions of life; and all the
structures of a species, accustomed through multitudinous generations to
the climate, food, and various influences of its locality, are moulded into
harmonious co-operation favourable to life in that locality: the result
being that in the development of each young individual, the tendencies
conspire to produce the fit organization. It is otherwise when the species
is removed to a habitat of different character, or when it is of mixed
breed. In the one case its organs, partially out of harmony with the
requirements of its new life, become partially out of harmony with one
another; since, while one influence, say of climate, is but little changed,
another influence, say of food, is much changed; and, consequently, the
perturbed relations of the organs interfere with their original stable
equilibrium. Still more in the other case is there a disturbance in
equilibrium. In a mongrel, the constitution derived from each source
repeats itself as far as possible. Hence a conflict of tendencies to evolve
two structures more or less unlike. The tendencies do not harmoniously
conspire, but produce partially incongruous sets of organs. And evidently
where the breed is one in which there are united the traits of various
lines of ancestry, there results an organization so full of small
incongruities of structure and action, that it has a much-diminished power
of maintaining its balance; and while it cannot withstand so well adverse
influences, it cannot so well hold its own in the offspring. Concerning
parents of pure and mixed breeds respectively, severally tending to
reproduce their own structures in progeny, we may therefore say,
figuratively, that the house divided against itself cannot withstand the
house of which the members are in concord.
Now if this is shown to be the case with breeds the purest of which have
been adapted to their habitats and modes of life during some few hundred
years only, what shall we say when the question is of a breed which has had
a constant mode of life in the same locality for ten thousand years or
more, like the quagga? In this the stability of constitution must be such
as no domestic animal can approach. Relatively stable as may have been the
constitutions of Lord Morton's horses, as compared with the constitutions
of ordinary horses, yet, since Arab horses, even in their native country,
have probably in the course of successive conquests and migrations of
tribes become more or less mixed, and since they have been subject to the
conditions of domestic life, differing much from the conditions of their
original wild life, and since the English breed has undergone the
perturbing effects of change from the climate and food of the East to the
climate and food of the West, the organizations of the horse and mare in
question could have had nothing like that perfect balance produced in the
quagga by a hundred centuries of harmonious co-operation. Hence the result.
And hence at the same time the interpretation of the fact that analogous
phenomena are not obvious among most domestic animals, or among ourselves;
since both have relatively mixed, and generally extremely mixed,
constitutions, which, as we see in ourselves, have been made generation
after generation, not by the formation of a mean between two parents, but
by the jumbling of traits of the one with traits of the other; until there
exist no such conspiring tendencies among the parts as cause repetition of
combined details of structure in posterity.
Expectation that scepticism might be felt respecting this alleged anomaly
presented by the quagga-marked foal, had led me to think over the matter;
and I had reached this interpretation before sending to the College of
Surgeons Museum (being unable to go myself) to obtain the particulars and
refer to the records. When there was brought to me a copy of the account as
set forth in the _Philosophical Transactions_, it was joined with the
information that there existed an appended account of pigs, in which a
parallel fact had been observed. To my immediate inquiry--"Was the male a
wild pig?" there came the reply--"I did not observe." Of course I forthwith
obtained the volume, and there found what I expected. It was contained in a
paper communicated by Dr. Wollaston from Daniel Giles, Esq., concerning his
"sow and her produce," which said that--
"she was one of a well-known black and white breed of Mr. Western, the
Member for Essex. About ten years since I put her to a boar of the wild
breed, and of a deep chestnut colour which I had just received from
Hatfield House, and which was soon afterwards drowned by accident. The
pigs produced (which were her first litter) partook in appearance of both
boar and sow, but in some the chestnut colour of the boar strongly
prevailed.
"The sow was afterwards put to a boar of Mr. Western's breed (the wild
boar having been long dead). The produce was a litter of pigs, some of
which, we observed with much surprise, to be stained and clearly marked
with the chestnut colour which had prevailed in the former litter."
Mr. Giles adds that in a second litter of pigs, the father of which was of
Mr. Western's breed, he and his bailiff believe there was a recurrence, in
some, of the chestnut colour, but admits that their "recollection is much
less perfect than I wish it to be." He also adds that, in the course of
many years' experience, he had never known the least appearance of the
chestnut colour in Mr. Western's breed.
What are the probabilities that these two anomalous results should have
arisen, under these exceptional conditions, as a matter of chance?
Evidently the probabilities against such a coincidence are enormous. The
testimony is in both cases so good that, even apart from the coincidence,
it would be unreasonable to reject it; but the coincidence makes acceptance
of it imperative. There is mutual verification, at the same time that there
is a joint interpretation yielded of the strange phenomenon, and of its
non-occurrence under ordinary circumstances.
And now, in presence of these facts, what are we to say? Simply that they
are fatal to Weismann's hypothesis. They show that there is none of the
alleged independence of the reproductive cells; but that the two sets of
cells are in close communion. They prove that while the reproductive cells
multiply and arrange themselves during the evolution of the embryo, some of
their germ-plasm passes into the mass of somatic cells constituting the
parental body, and becomes a permanent component of it. Further, they
necessitate the inference that this introduced germ-plasm, everywhere
diffused, is some of it included in the reproductive cells subsequently
formed. And if we thus get a demonstration that the somewhat different
units of a foreign germ-plasm permeating the organism, permeate also the
subsequently formed reproductive cells, and affect the structures of the
individuals arising from them, the implication is that the like happens
with those native units which have been made somewhat different by modified
functions: there must be a tendency to inheritance of acquired characters.
One more step only has to be taken. It remains to ask what is the flaw in
the assumption with which Weismann's theory sets out. If, as we see, the
conclusions drawn from it do not correspond to the facts, then, either the
reasoning is invalid, or the original postulate is untrue. Leaving aside
all questions concerning the reasoning, it will suffice here to show the
untruth of the postulate. Had his work been written during the early years
of the cell-doctrine, the supposition that the multiplying cells of which
the _Metazoa_ and _Metaphyta_ are composed, become completely separate,
could not have been met by a reasonable scepticism; but now, not only is
scepticism justifiable, but denial is called for. Some dozen years ago it
was discovered that in many cases vegetal cells are connected with one
another by threads of protoplasm--threads which unite the internal
protoplasm of one cell with the internal protoplasms of cells around It is
as though the pseudopodia of imprisoned rhizopods were fused with the
pseudopodia of adjacent imprisoned rhizopods. We cannot reasonably suppose
that the continuous network of protoplasm thus constituted has been
produced after the cells have become adult. These protoplasmic connections
must have survived the process of fission. The implication is that the
cells forming the embryo-plant retained their protoplasmic connections
while they multiplied, and that such connections continued throughout all
subsequent multiplications--an implication which has, I believe, been
established by researches upon germinating palm-seeds. But now we come to a
verifying series of facts which the cell-structures of animals in their
early stages present. In his _Monograph of the Development of Peripatus
Capensis_, Mr. Adam Sedgwick, F.R.S., Reader in Animal Morphology at
Cambridge, writes as follows:--
"All the cells of the ovum, ectodermal as well as endodermal, are
connected together by a fine protoplasmic reticulum." (p. 41)
"The continuity of the various cells of the segmenting ovum is primary,
and not secondary; _i. e._, in the cleavage the segments do not
completely separate from one another. But are we justified in speaking of
cells at all in this case? _The fully segmented ovum is a syncytium, and
there are not and have not been at any stage cell limits._" (p. 41)
"It is becoming more and more clear every day that the cells composing
the tissues of animals are not isolated units, but that they are
connected with one another. I need only refer to the connection known to
exist between connective tissue cells, cartilage cells, epithelial cells,
&c. And not only may the cells of one tissue be continuous with each
other, but they may also be continuous with the cells of other tissues."
(pp. 47-8)
"Finally, if the protoplasm of the body is primitively a syncytium, and
the ovum until maturity a part of that syncytium, the separation of the
generative products does not differ essentially from the internal
gemmation of a Protozoon, and the inheritance by the offspring of
peculiarities first appearing in the parent, though not explained, is
rendered less mysterious; for the protoplasm of the whole body being
continuous, change in the molecular constitution of any part of it would
naturally be expected to spread, in time, through the whole mass." (p.
49)
Mr. Sedgwick's subsequent investigations confirm these conclusions. In a
letter of December 27, 1892, passages which he allows me to publish run as
follows:--
"All the embryological studies that I have made since that to which you
refer confirm me more and more in the view that the connections between
the cells of adults are not secondary connections, but primary, dating
from the time when the embryo was a unicellular structure.... My own
investigations on this subject have been confined to the Arthropoda,
Elasmobranchii, and Aves. I have thoroughly examined the development of
at least one kind of each of these groups, and I have never been able to
detect a stage in which the cells were not continuous with each other;
and I have studied innumerable stages from the beginning of cleavage
onwards."
So that the alleged independence of the reproductive cells does not exist.
The _soma_--to use Weismann's name for the aggregate of cells forming the
body--is, in the words of Mr. Sedgwick, "a continuous mass of vacuolated
protoplasm;" and the reproductive cells are nothing more than portions of
it separated some little time before they are required to perform their
functions.
Thus the theory of Weismann is doubly disproved. Inductively we are shown
that there _does_ take place that communication of characters from the
somatic cells to the reproductive cells, which he says cannot take place;
and deductively we are shown that this communication is a natural sequence
of connections between the two which he ignores; his various conclusions
are deduced from a postulate which is untrue.
* * * * *
From the title of this essay, and from much of its contents, nine readers
out of ten will infer that it is directed against the views of Mr. Darwin.
They will be astonished on being told that, contrariwise, it is directed
against the views of those who, in a considerable measure, dissent from Mr.
Darwin. For the inheritance of acquired characters, which it is now the
fashion in the biological world to deny, was, by Mr. Darwin, fully
recognized and often insisted on. Such of the foregoing arguments as touch
Mr. Darwin's views, simply imply that the cause of evolution which at first
he thought unimportant, but the importance of which he increasingly
perceived as he grew older, is more important than he admitted, even at the
last. The neo-Darwinists, however, do not admit this cause at all.
Let it not be supposed that this explanation implies any disapproval of the
dissentients, considered as such. Seeing how little regard for authority I
have myself usually shown, it would be absurd in me to reflect in any
degree upon those who have rejected certain of Mr. Darwin's teachings, for
reasons which they have held sufficient. But while their independence of
thought is to be applauded rather than blamed, it is, I think, to be
regretted that they have not guarded themselves against a long-standing
bias. It is a common trait of human nature to seek some excuse when found
in the wrong. Invaded self-esteem sets up a defence, and anything is made
to serve. Thus it happened that when geologists and biologists, previously
holding that all kinds of organisms arose by special creations, surrendered
to the battery opened upon them by _The Origin of Species_, they sought to
minimise their irrationality by pointing to irrationality on the other
side. "Well, at any rate, Lamarck was in the wrong." "It is clear that we
were right in rejecting _his_ doctrine." And so, by duly emphasizing the
fact that he overlooked "Natural Selection" as the chief cause, and by
showing how erroneous were some of his interpretations, they succeeded in
mitigating the sense of their own error. It is true their creed was that at
successive periods in the Earth's history, old Floras and Faunas had been
abolished and others introduced; just as though, to use Professor Huxley's
figure, the table had been now and again kicked over and a new pack of
cards brought out. And it is true that Lamarck, while he rejected this
absurd creed, assigned for the facts reasons some of which are absurd. But
in consequence of the feeling described, his defensible belief was
forgotten and only his indefensible ones remembered. This one-sided
estimate has become traditional; so that there is now often shown a subdued
contempt for those who suppose that there can be any truth in the
reasonings of a man whose general conception was partly sense, at a time
when the general conceptions of his contemporaries were wholly nonsense.
Hence results unfair treatment--hence result the different dealings with
the views of Lamarck and of Weismann.
"Where are the facts proving the inheritance of acquired characters?" ask
those who deny it. Well, in the first place, there might be asked the
counter-question--Where are the facts which disprove it? Surely if not only
the general structures of organisms, but also many of the modifications
arising in them, are inheritable, the natural implication is that all
modifications are inheritable; and if any say that the inheritableness is
limited to those arising in a certain way, the _onus_ lies on them of
proving that those otherwise arising are not inheritable.[107] Leaving this
counter-question aside, however, it will suffice if we ask another
counter-question. It is asserted that the dwindling of organs from disuse
is due to the successive survivals in posterity of individuals in which the
organs have varied in the direction of decrease. Where now are the facts
supporting this assertion? Not one has been assigned or can be assigned.
Not a single case can be named in which _panmixia_ is a proved cause of
diminution. Even had the deductive argument for _panmixia_ been as valid as
we have found it to be invalid, there would still have been required, in
pursuance of scientific method, some verifying inductive evidence. Yet,
though not a shred of such evidence has been given, the doctrine is
accepted with acclamation, and adopted as part of current biological
theory. Articles are written and letters published in which it is assumed
that this mere speculation, justified by not a tittle of proof, displaces
large conclusions previously drawn. And then, passing into the outer world,
this unsupported belief affects opinions there too; so that we have
recently had a Right Honourable lecturer who, taking for granted its truth,
represents the inheritance of acquired characters as an exploded
hypothesis, and proceeds to give revised views of human affairs.
Finally, there comes the reply that there _are_ facts proving the
inheritance of acquired characters. All those assigned by Mr. Darwin,
together with others such, remain outstanding when we find that the
interpretation by _panmixia_ is untenable. Indeed, even had that hypothesis
been tenable, it would have been inapplicable to these cases; since in
domestic animals, artificially fed and often overfed, the supposed
advantage from economy cannot be shown to tell; and since, in these cases,
individuals are not naturally selected during the struggle for life, in
which certain traits are advantageous, but are artificially selected by man
without regard to such traits. Should it be urged that the assigned facts
are not numerous, it may be replied that there are no persons whose
occupations and amusements incidentally bring out such facts; and that they
are probably as numerous as those which would have been available for Mr.
Darwin's hypothesis, had there been no breeders and fanciers and gardeners
who, in pursuit of their profits and hobbies, furnished him with evidence.
It may be added that the required facts are not likely to be numerous, if
biologists refuse to seek for them.
See, then, how the case stands. Natural selection, or survival of the
fittest, is almost exclusively operative throughout the vegetal world and
throughout the lower animal world, characterized by relative passivity. But
with the ascent to higher types of animals, its effects are in increasing
degrees involved with those produced by inheritance of acquired characters;
until, in animals of complex structures, inheritance of acquired characters
becomes an important, if not the chief, cause of evolution. We have seen
that natural selection cannot work any changes in organisms save such as
conduce in considerable degrees, directly or indirectly, to the
multiplication of the stirp; whence failure to account for various changes
ascribed to it. And we have seen that it yields no explanation of the
co-adaptation of co-operative parts, even when the co-operation is
relatively simple, and still less when it is complex. On the other hand, we
see that if, along with the transmission of generic and specific
structures, there tend to be transmitted modifications arising in a certain
way, there is a strong _a priori_ probability that there tend to be
transmitted modifications arising in all ways. We have a number of facts
confirming this inference, and showing that acquired characters are
inherited--as large a number as can be expected, considering the difficulty
of observing them and the absence of search. And then to these facts may be
added the facts with which this essay set out, concerning the distribution
of tactual discriminativeness. While we saw that these are inexplicable by
survival of the fittest, we saw that they are clearly explicable as
resulting from the inheritance of acquired characters. And here let it be
added that this conclusion is conspicuously warranted by one of the methods
of inductive logic, known as the method of concomitant variations. For
throughout the whole series of gradations in perceptive power, we saw that
the amount of the effect is proportionate to the amount of the alleged
cause.
II.
Apart from those more special theories of Professor Weismann I lately dealt
with, the wide acceptance of which by the biological world greatly
surprises me, there are certain more general theories of his--fundamental
theories--the acceptance of which surprises me still more. Of the two on
which rests the vast superstructure of his speculations, the first concerns
the distinction between the reproductive elements of each organism and the
non-reproductive elements. He says:--
"Let us now consider how it happened that the multicellular animals and
plants, which arose from unicellular forms of life, came to lose this
power of living for ever.
"The answer to this question is closely bound up with the principle of
division of labour which appeared among multicellular organisms at a very
early stage....
"The first multicellular organism was probably a cluster of similar
cells, but these units soon lost their original homogeneity. As the
result of mere relative position, some of the cells were especially
fitted to provide for the nutrition of the colony, while others undertook
the work of reproduction." (_Essays upon Heredity_, i, p. 27)
Here, then, we have the great principle of the division of labour, which is
the principle of all organization, taken as primarily illustrated in the
division between the reproductive cells and the non-reproductive or somatic
cells--the cells devoted to the continuance of the species, and the cells
which subserve the life of the individual. And the early separation of
reproductive cells from somatic cells, is alleged on the ground that this
primary division of labour is that which arises between elements devoted to
species-life and elements devoted to individual life. Let us not be content
with words but look at the facts.
When Milne-Edwards first used the phrase "physiological division of
labour," he was obviously led to do so by perceiving the analogy between
the division of labour in a society, as described by political economists,
and the division of labour in an organism. Every one who reads has been
familiarized with the first as illustrated in the early stages, when men
were warriors while the cultivation and drudgery were done by slaves and
women; and as illustrated in the later stages, when not only are
agriculture and manufactures carried on by separate classes, but
agriculture is carried on by landlords, farmers, and labourers, while
manufactures, multitudinous in their kinds, severally involve the actions
of capitalists, overseers, workers, &c., and while the great function of
distribution is carried on by wholesale and retail dealers in different
commodities. Meanwhile students of biology, led by Milne-Edwards's phrase,
have come to recognize a parallel arrangement in a living creature; shown,
primarily, in the devoting of the outer parts to the general business of
obtaining food and escaping from enemies, while the inner parts are devoted
to the utilization of food, and supporting themselves and the outer parts;
and shown, secondarily, by the subdivision of these great functions into
those of various limbs and senses in the one case, and in the other case
into those of organs for digestion, respiration, circulation, excretion,
&c. But now let us ask what is the essential nature of this division of
labour. In both cases it is an _exchange of services_--an arrangement under
which, while one part devotes itself to one kind of action and yields
benefits to all the rest, all the rest, jointly and severally performing
their special actions, yield benefits to it in exchange. Otherwise
described, it is a system of _mutual_ dependence: A depends for its welfare
upon B, C, and D; B upon A, C, and D; and so with the rest: all depend upon
each and each upon all. Now let us apply this true conception of the
division of labour, to that which Professor Weismann calls a division of
labour. Where is the _exchange of services_ between somatic cells and
reproductive cells? There is none. The somatic cells render great services
to the reproductive cells, by furnishing them with materials for growth and
multiplication; but the reproductive cells render no services at all to the
somatic cells. If we look for the _mutual_ dependence we look in vain. We
find entire dependence on the one side and none on the other. Between the
parts devoted to individual life and the part devoted to species-life,
there is no division of labour whatever. The individual works for the
species; but the species works not for the individual. Whether at the stage
when the species is represented by reproductive cells, or at the stage when
it is represented by eggs, or at the stage when it is represented by young,
the parent does everything for it, and it does nothing for the parent. The
essential part of the conception is gone: there is no giving and receiving,
no exchange, no mutuality.
But now suppose we pass over this fallacious interpretation, and grant
Professor Weismann his fundamental assumption and his fundamental
corollary. Suppose we grant that because the primary division of labour is
that between somatic cells and reproductive cells, these two groups are the
first to be differentiated. Having granted this corollary, let us compare
it with the facts. As the alleged primary division of labour is universal,
so the alleged primary differentiation should be universal too. Let us see
whether it is so. Already, in the paragraph from which I have quoted above,
a crack in the doctrine is admitted: it is said that "this differentiation
was not at first absolute, and indeed it is not always so to-day." And
then, on turning to page 74, we find that the crack has become a chasm. Of
the reproductive cells it is stated that--"In Vertebrata they do not become
distinct from the other cells of the body until the embryo is completely
formed." That is to say, in this large and most important division of the
animal kingdom, the implied universal law does not hold. Much more than
this is confessed. Lower down the page we read--"There may be in fact cases
in which such separation does not take place until after the animal is
completely formed, and others, as I believe that I have shown, in which it
first arises one or more generations later, viz., in the buds produced by
the parent."
So that in other great divisions of the animal kingdom the alleged law is
broken; as among the _Coelenterata_ by the _Hydrozoa_, as among the
_Mollusca_ by the Ascidians, and as among the _Platyhelminthes_ by the
Trematode worms.
Following this admission concerning the _Vertebrata_, come certain
sentences which I partially italicize:--
"Thus, as their development shows, a marked antithesis exists between the
substance of the undying reproductive cells and that of the perishable
body-cells. We cannot explain this fact except _by the supposition_ that
each reproductive cell potentially contains two kinds of substance, which
at a variable time after the commencement of embryonic development,
separate from one another, and finally produce two sharply contrasted
groups of cells." (p. 74)
And a little lower down the page we meet with the lines:--
"_It is therefore quite conceivable_ that the reproductive cells might
separate from the somatic cells much later than in the examples mentioned
above, without changing the hereditary tendencies of which they are the
bearers."
That is to say, it is "quite conceivable" that after sexless _Cercariæ_
have gone on multiplying by internal gemmation for generations, the "two
kinds of substance" have, notwithstanding innumerable cell-divisions,
preserved their respective natures, and finally separate in such ways as to
produce reproductive cells. Here Professor Weismann does not, as in a case
before noted, assume something which it is "easy to imagine," but he
assumes something which it is difficult to imagine; and apparently thinks
that a scientific conclusion may be thereon safely based.
* * * * *
Associated with the assertion that the primary division of labour is
between the somatic cells and the reproductive cells, and associated with
the corollary that the primary differentiation is that which arises between
them, there goes another corollary. It is alleged that there exists a
fundamental distinction of nature between these two classes of cells. They
are described as respectively mortal and immortal, in the sense that those
of the one class are limited in their powers of multiplication, while those
of the other class are unlimited. And it is contended that this is due to
inherent unlikeness of nature.
Before inquiring into the truth of this proposition, I may fitly remark
upon a preliminary proposition set down by Professor Weismann. Referring to
the hypothesis that death depends "upon causes which lie in the nature of
life itself," he says:--
"I do not however believe in the validity of this explanation: I consider
that death is not a primary necessity, but that it has been secondarily
acquired as an adaptation. I believe that life is endowed with a fixed
duration, not because it is contrary to its nature to be unlimited, but
because the unlimited existence of individuals would be a luxury without
any corresponding advantage." (p. 24)
This last sentence has a teleological sound which would be appropriate did
it come from a theologian, but which seems strange as coming from a man of
science. Assuming, however, that the implication was not intended, I go on
to remark that Professor Weismann has apparently overlooked a universal law
of evolution--not organic only, but inorganic and super-organic--which
implies the necessity of death. The changes of every aggregate, no matter
of what kind, inevitably end in a state of equilibrium. Suns and planets
die, as well as organisms. The process of integration, which constitutes
the fundamental trait of all evolution, continues until it has brought
about a state which negatives further alterations, molar or molecular--a
state of balance among the forces of the aggregate and the forces which
oppose them.[108] In so far, therefore, as Professor Weismann's conclusions
imply the non-necessity of death, they cannot be sustained.
But now let us consider the above-described antithesis between the immortal
_Protozoa_ and the mortal _Metazoa_. An essential part of the theory is
that the _Protozoa_ can go on dividing and subdividing without limit, so
long as the fit external conditions are maintained. But what is the
evidence for this? Even by Professor Weismann's own admission there is no
proof. On p. 285 he says:--
"I could only consent to adopt the hypothesis of rejuvenescence [achieved
by conjugation], if it were rendered absolutely certain that reproduction
by division could never under any circumstances persist indefinitely. But
this cannot be proved with any greater certainty than the converse
proposition, and hence, as far as direct proof is concerned, the facts
are equally uncertain on both sides."
But this is an admission which seems to be entirely ignored when there is
alleged the contrast between the immortal _Protozoa_ and the mortal
_Metazoa_. Following Professor Weismann's method, it would be "easy to
imagine" that occasional conjugation is in all cases essential; and this
easily imagined conclusion might fitly be used to bar out his own. Indeed,
considering how commonly conjugation is observed, it may be held difficult
to imagine that it can in any cases be dispensed with. Apart from
imaginations of either kind, however, here is an acknowledgment that the
immortality of _Protozoa_ is not proved; that the allegation has no better
basis than the failure to observe cessation of fission; and that thus one
term of the above antithesis is not a fact, but is only an assumption.
And now what about the other term of the antithesis--the alleged inherent
mortality of the somatic cells? This we shall, I think, find is no more
defensible than the other. Such plausibility as it possesses disappears
when, instead of contemplating the vast assemblage of familiar cases which
animals present, we contemplate certain less familiar and unfamiliar cases.
By these we are shown that the usual ending of multiplication among somatic
cells is due, not to an intrinsic cause, but to extrinsic causes. Let us,
however, first look at Professor Weismann's own statements:--
"I have endeavoured to explain death as the result of restriction in the
powers of reproduction possessed by the somatic cells, and I have
suggested that such restriction may conceivably follow from a limitation
in the number of cell-generations possible for the cells of each organ
and tissue." (p. 28)
"The above-mentioned considerations show us that the degree of
reproductive activity present in the tissues is regulated by internal
causes while the natural death of an organism is the termination--the
hereditary limitation--of the process of cell-division, which began in
the segmentation of the ovum." (p. 30)
Now, though, in the above extracts there is mention of "internal causes"
determining "the degree of reproductive activity" of tissue cells, and
though, on page 28, the "causes of the loss" of the power of unlimited
cell-production "must be sought outside the organism, that is to say, in
the external conditions of life," yet the doctrine is that somatic cells
have become constitutionally unfitted for continued cell-multiplication.
"The somatic cells have lost this power to a gradually increasing extent,
so that at length they became restricted to a fixed, though perhaps very
large, number of cell-generations." (p. 28)
Examination will soon disclose good reasons for denying this inherent
restriction. We will look at the various causes which affect their
multiplication, and usually put a stop to increase after a certain point is
reached.
There is first the amount of vital capital given by the parent; partly in
the shape of a more or less developed structure, and partly in the shape of
bequeathed nutriment. Where this vital capital is small, and the young
creature, forthwith obliged to carry on physiological business for itself,
has to expend effort in obtaining materials for daily consumption as well
as for growth, a rigid restraint is put on that cell-multiplication
required for a large size. Clearly, the young elephant, starting with a big
and well-organized body, and supplied _gratis_ with milk during early
stages of growth, can begin physiological business on his own account on a
great scale; and by its large transactions his system is enabled to supply
nutriment to its multiplying somatic cells until they have formed a vast
aggregate--an aggregate such as it is impossible for a young mouse to
reach, obliged as it is to begin physiological business in a small way.
Then there is the character of the food in respect of its digestibility and
its nutritiveness. Here, that which the creature takes in requires much
grinding-up, or, when duly prepared, contains but a small amount of
available matter in comparison with the matter that has to be thrown away;
while there, the prey seized is almost pure nutriment, and requires but
little trituration. Hence, in some cases, an unprofitable physiological
business, and in other cases a profitable one; resulting in small or large
supplies to the multiplying somatic cells. Further, there has to be noted
the grade of visceral development, which, if low, yields only crude
nutriment slowly distributed, but which, if high, serves by its good
appliances for solution, depuration, absorption, and circulation, to yield
to the multiplying somatic cells a rich and pure blood. Then we come to an
all-important factor, the cost of obtaining food. Here large expenditure of
energy in locomotion is necessitated, and there but little--here great
efforts for small portions of food, and there small efforts for great
portions: again resulting in physiological poverty or physiological wealth.
Next, beyond the cost of nervo-muscular activities in foraging, there is
the cost of maintaining bodily heat. So much heat implies so much consumed
nutriment, and the loss by radiation or conduction, which has perpetually
to be made good, varies according to many circumstances--climate, medium
(as air or water), covering, size of body (small cooling relatively faster
than large); and in proportion to the cost of maintaining heat is the
abstraction from the supplies for cell-formation. Finally, there are three
all-important co-operative factors, or rather laws of factors, the effects
of which vary with the size of the animal. The first is that, while the
mass of the body varies as the cubes of its dimensions (_proportions_ being
supposed constant), the absorbing surface varies as the squares of its
dimensions; whence it results that, other things equal, increase of size
implies relative decrease of nutrition, and therefore increased obstacles
to cell-multiplication.[109] The second is a further sequence from these
laws--namely, that while the weight of the body increases as the cubes of
the dimensions, the sectional areas of its muscles and bones increase as
their squares; whence follows a decreasing power of resisting strains, and
a relative weakness of structure. This is implied in the ability of a small
animal to leap many times its own length, while a great animal, like the
elephant, cannot leap at all: its bones and muscles being unable to bear
the stress which would be required to propel its body through the air. What
increasing cost of keeping together the bodily fabric is thus entailed, we
cannot say; but that there is an increasing cost, which diminishes the
available, materials for increase of size, is beyond question.[110] And
then, in the third place, we have augmented expense of distribution of
nutriment. The greater the size becomes, the more force must be exerted to
send blood to the periphery; and this once more entails deduction from the
cell-forming matters.
Here, then, we have nine factors, several of them involving subdivisions,
which co-operate in aiding or restraining cell-multiplication. They occur
in endlessly varied proportions and combinations; so that every species
differs more or less from every other in respect of their effects. But in
all of them the co-operation is such as eventually arrests that
multiplication of cells which causes further growth; continues thereafter
to entail slow decrease in cell-multiplication, accompanying decline of
vital activities; and eventually brings cell-multiplication to an end. Now
a recognized principle of reasoning--the Law of Parsimony--forbids the
assumption of more causes than are needful for explanation of phenomena;
and since, in all such living aggregates as those above supposed, the
causes named inevitably bring about arrest of cell-multiplication, it is
illegitimate to ascribe this arrest to some inherent property in the cells.
Inadequacy of the other causes must be shown before an inherent property
can be rightly assumed.
For this conclusion we find ample justification when we contemplate types
of animals which lead lives that do not put such decided restraints on
cell-multiplication. First let us take an instance of the extent to which
(irrespective of natures of cells as reproductive or somatic)
cell-multiplication may go, where the conditions render nutrition easy and
reduce expenditure to a minimum. I refer to the case of the _Aphides_.
Though it is early in the season (March), the hothouses at Kew have
furnished a sufficient number of these to show that twelve of them weigh a
grain--a larger number than would be required were they full-sized. Citing
Professor Owen, who adopts the calculations of Tougard to the effect that
by agamic multiplication "a single impregnated ovum of _Aphis_ may give
rise, without fecundation, to a quintillion of _Aphides_," Professor Huxley
says:--
"I will assume that an Aphis weighs 1/1000 of a grain, which is certainly
vastly under the mark. A quintillion of _Aphides_ will, on this estimate,
weigh a quatrillion of grains. He is a very stout man who weighs two
million grains; consequently the tenth brood alone, if all its members
survive the perils to which they are exposed, contains more substance
than 500,000,000 stout men--to say the least, more than the whole
population of China!"[111]
And had Professor Huxley taken the actual weight, one-twelfth of a grain,
the quintillion of _Aphides_ would evidently far outweigh the whole human
population of the globe: five billions of tons being the weight, as brought
out by my own calculation! Of course I do not cite this in proof of the
extent to which multiplication of somatic cells, descending from a single
ovum, may go; because it will be contended, with some reason, that each of
the sexless _Aphides_, viviparously produced, arose by fission of a cell
which had descended from the original reproductive cell. I cite it merely
to show that when the cell-products of a fertilized ovum are perpetually
divided and subdivided into small groups, distributed over an unlimited
nutritive area, so that they can get materials for growth at no cost, and
expend nothing appreciable in motion or maintenance of temperature,
cell-production may go on without limit. For the agamic multiplication of
_Aphides_ has been shown to continue for four years, and to all appearance
would be ceaseless were the temperature and supply of food continued
without break. But now let us pass to analogous illustrations of cause and
consequence, open to no criticism of the kind just indicated. They are
furnished by various kinds of _Entozoa_, of which take the _Trematoda_,
infesting molluscs and fishes. Of one of them we read:--"_Gyrodactylus_
multiplies agamically by the development of a young Trematode within the
body, as a sort of internal bud. A second generation appears within the
first, and even a third within the second, before the young _Gyrodactylus_
is born."[112] And the drawings of Steenstrup, in his _Alternation of
Generations_, show us, among creatures of this group, a sexless individual
the whole interior of which is transformed into smaller sexless
individuals, which severally, before or after their emergence, undergo
similar transformations--a multiplication of somatic cells without any sign
of reproductive cells. Under what circumstances do such modes of agamic
multiplication, variously modified among parasites, occur? They occur where
there is no expenditure whatever in motion or maintenance of temperature,
and where nutriment surrounds the body on all sides. Other instances are
furnished by groups in which, though the nutriment is not abundant, the
cost of living is almost unappreciable. Among the _Coelenterata_ there are
the Hydroid Polyps, simple and compound; and among the _Mollusca_ we have
various types of Ascidians, fixed and floating, _Botryllidæ_ and _Salpæ_.
But now from these low animals in which sexless reproduction, and continued
multiplication of somatic cells, is common, and one class of which is named
"zoophytes," because its form of life simulates that of plants, let us pass
to plants themselves. In these there is no expenditure in effort, there is
no expenditure in maintaining temperature, and the food, some of it
supplied by the earth, is the rest of it supplied by a medium which
everywhere bathes the outer surface: the utilization of its contained
material being effected _gratis_ by the Sun's rays. Just as was to be
expected, we here find that agamogenesis may go on without end. Numerous
plants and trees are propagated to an unlimited extent by cuttings and
buds; and we have sundry plants which cannot be otherwise propagated. The
most familiar are the double roses of our gardens: these do not seed, and
yet have been distributed everywhere by grafts and buds. Hothouses furnish
many cases, as I learn from an authority second to none. Of "the whole host
of tropical orchids, for instance, not one per cent. has ever seeded, and
some have been a century under cultivation." Again, we have the _Acorus
calamus_, "that has hardly been known to seed anywhere, though it is found
wild all over the north temperate hemisphere." And then there is the
conspicuous and conclusive case of _Eloidea Canadensis_ (alias
_Anacharis_,) introduced no one knows how (probably with timber), and first
observed in 1847, in several places; and which, having since spread over
nearly all England, now everywhere infests ponds, canals, and slow rivers.
The plant is dioecious, and only the female exists here. Beyond all
question, therefore, this vast progeny of the first slip or fragment
introduced, sufficient to cover many square miles were it put together, is
constituted entirely of somatic cells. Hence, as far as we can judge, these
somatic cells are immortal in the sense given to the word by Professor
Weismann; and the evidence that they are so is immeasurably stronger than
the evidence which leads him to assert immortality for the
fissiparously-multiplying _Protozoa_. This endless multiplication of
somatic cells has been going on under the eyes of numerous observers for
forty odd years. What observer has watched for forty years to see whether
the fissiparous multiplication of _Protozoa_ does not cease? What observer
has watched for one year, or one month, or one week?[113]
Even were not Professor Weismann's theory disposed of by this evidence, it
might be disposed of by a critical examination of his own evidence, using
his own tests. Clearly, if we are to measure relative mortalities, we must
assume the conditions to be the same and must use the same measure. Let us
do this with some appropriate animal--say Man, as the most open to
observation. The mortality of the somatic cells constituting the mass of
the human body, is, according to Professor Weismann, shown by the decline
and final cessation of cell-multiplication in its various organs. Suppose
we apply this test to all the organs: not to those only in which there
continually arise bile-cells, epithelium-cells, &c., but to those also in
which there arise reproductive cells. What do we find? That the
multiplication of these last comes to an end long before the multiplication
of the first. In a healthy woman, the cells which constitute the various
active tissues of the body, continue to grow and multiply for many years
after germ-cells have died out. If similarly measured, then, these cells of
the last class prove to be more mortal than those of the first. But
Professor Weismann uses a different measure for the two classes of cells.
Passing over the illegitimacy of this proceeding, let us accept his other
mode of measurement, and see what comes of it. As described by him, absence
of death among the _Protozoa_ is implied by that unceasing division and
subdivision of which they are said to be capable. Fission continued without
end, is the definition of the immortality he speaks of. Apply this
conception to the reproductive cells in a _Metazoon_. That the immense
majority of them do not multiply without end, we have already seen: with
very rare exceptions they die and disappear without result, and they cease
their multiplication while the body as a whole still lives. But what of
those extremely exceptional ones which, as being actually instrumental to
the maintenance of the species, are alone contemplated by Professor
Weismann? Do these continue their fissiparous multiplications without end?
By no means. The condition under which alone they preserve a qualified form
of existence, is that, instead of one becoming two, two become one. A
member of series A and a member of series B, coalesce; and so lose their
individualities. Now, obviously, if the immortality of a series is shown if
its members divide and subdivide perpetually, then the opposite of
immortality is shown when, instead of division, there is union. Each series
ends, and there is initiated a new series, differing more or less from
both. Thus the assertion that the reproductive cells are immortal, can be
defended only by changing the conception of immortality otherwise implied.
Even apart from these last criticisms, however, we have clear disproof of
the alleged inherent difference between the two classes of cells. Among
animals, the multiplication of somatic cells is brought to an end by sundry
restraining conditions; but in various plants, where these restraining
conditions are absent, the multiplication is unlimited. It may, indeed, be
said that the alleged distinction should be reversed; since the fissiparous
multiplication of reproductive cells is necessarily interrupted from time
to time by coalescence, while that of the somatic cells may go on for a
century without being interrupted.
* * * * *
In the essay to which this is a postscript, conclusions were drawn from the
remarkable case of the horse and the quagga, there narrated, along with an
analogous case observed among pigs. These conclusions have since been
confirmed. I am much indebted to a distinguished correspondent who has
drawn my attention to verifying facts furnished by the offspring of whites
and negroes in the United States. Referring to information given him many
years ago, he says:--"It was to the effect that the children of white women
by a white father, had been _repeatedly_ observed to show traces of black
blood, in cases when the woman had previous connection with [_i. e._ a
child by] a negro." At the time I received this information, an American
was visiting me; and, on being appealed to, answered that in the United
States there was an established belief to this effect. Not wishing,
however, to depend upon hearsay, I at once wrote to America to make
inquiries. Professor Cope of Philadelphia has written to friends in the
South, but has not yet sent me the results. Professor Marsh, the
distinguished palæontologist, of Yale, New Haven, who is also collecting
evidence, sends a preliminary letter in which he says:--"I do not myself
know of such a case, but have heard many statements that make their
existence probable. One instance, in Connecticut, is vouched for so
strongly by an acquaintance of mine, that I have good reason to believe it
to be authentic."
That cases of the kind should not be frequently seen in the North,
especially nowadays, is of course to be expected. The first of the above
quotations refers to facts observed in the South during slavery days; and
even then, the implied conditions were naturally very infrequent. Dr. W. J.
Youmans of New York has, on my behalf, interviewed several medical
professors, who, though they have not themselves met with instances, say
that the alleged result, described above, "is generally accepted as a
fact." But he gives me what I think must be regarded as authoritative
testimony. It is a quotation from the standard work of Professor Austin
Flint, and runs as follows:--
"A peculiar and, it seems to me, an inexplicable fact is, that previous
pregnancies have an influence upon offspring. This is well known to
breeders of animals. If pure-blooded mares or bitches have been once
covered by an inferior male, in subsequent fecundations the young are
likely to partake of the character of the first male, even if they be
afterwards bred with males of unimpeachable pedigree. What the mechanism
of the influence of the first conception is, it is impossible to say; but
the fact is incontestable. The same influence is observed in the human
subject. A woman may have, by a second husband, children who resemble a
former husband, and this is particularly well marked in certain instances
by the colour of the hair and eyes. A white woman who has had children by
a negro may subsequently bear children to a white man, these children
presenting some of the unmistakable peculiarities of the negro
race."[114]
Dr. Youmans called on Professor Flint, who remembered "investigating the
subject at the time his larger work was written [the above is from an
abridgment], and said that he had never heard the statement questioned."
Some days before I received this letter and its contained quotation, the
remembrance of a remark I heard many years ago concerning dogs, led to the
inquiry whether they furnished analogous evidence. It occurred to me that a
friend who is frequently appointed judge of animals at agricultural shows,
Mr. Fookes, of Fairfield, Pewsey, Wiltshire, might know something about the
matter. A letter to him brought various confirmatory statements. From one
"who had bred dogs for many years" he learnt that--
"It is a well known and admitted fact that if a bitch has two litters by
two different dogs, the character of the first father is sure to be
perpetuated in any litters she may afterwards have, no matter how
pure-bred a dog may be the begetter."
After citing this testimony, Mr. Fookes goes on to give illustrations known
to himself.
"A friend of mine near this had a very valuable Dachshund bitch, which
most unfortunately had a litter by a stray sheep-dog. The next year her
owner sent her on a visit to a pure Dachshund dog, but the produce took
quite as much of the first father as the second, and the next year he
sent her to another Dachshund with the same result. Another case:--A
friend of mine in Devizes had a litter of puppies, unsought for, by a
setter from a favourite pointer bitch, and after this she never bred any
true pointers, no matter of what the paternity was."
[Since the publication of this article additional evidences have come to
hand. One is from the late Prof. Riley, State Entomologist at Washington,
who says that telegony is an "established principle among well-educated
farmers" in the United States, and who gives me a case in horse-breeding
to which he was himself witness.
Mr. W. P. Smith, writing from Stoughton Grange, Guildford, but giving the
results of his experiences in America, says that "the fact of a previous
conception influencing subsequent offspring was so far recognised among
American cattle-breeders" that it was proposed to raise the rank of any
heifer that had borne a first calf by a thoroughbred bull, and though
this resolution when brought before one of the chief societies was not
carried, yet on all sides it was admitted that previous conceptions had
effects of the kind alleged. Mr. Smith in another letter says:--"When I
had a large mule and horse ranche in America I noticed that the foals of
mares by horse stallions had a mulish appearance in those cases where the
mare had previously given birth to a mule foal. Common heifers who have
had calves by a thoroughbred bull are apt thereafter to have well-bred
calves even from the veriest scrubs."
Yet another very interesting piece of evidence is furnished by Mr. W.
Sedgwick, M.R.C.S., in an article on "The Influence of Heredity in
Disease," published in the _British Medical Journal_ for Feb. 22, 1896,
pp. 460-2. It concerns the transmission of a malformation known among
medical men as hypospadias. Referring to a man belonging to a family in
which this defect prevailed, he writes:--"The widow of the man from whom
these three generations of hypospadians were descended married again,
after an interval of eighteen months; and in this instance the second
husband was not only free from the defect, but there was no history of it
in his family. By this second marriage she had four hypospadiac sons and
four hypospadiac grandsons; whilst there were seven grandsons and three
great-grandsons who were not malformed."]
Coming from remote places, from those who have no theory to support, and
who are some of them astonished by the unexpected phenomena, the agreement
dissipates all doubt. In four kinds of mammals, widely divergent in their
natures--man, horse, dog, and pig--we have this same seemingly-anomalous
kind of heredity, made visible under analogous conditions. We must take it
as a demonstrated fact that, during gestation, traits of constitution
inherited from the father produce effects upon the constitution of the
mother; and that these communicated effects are transmitted by her to
subsequent offspring. We are supplied with an absolute disproof of
Professor Weismann's doctrine that the reproductive cells are independent
of, and uninfluenced by, the somatic cells; and there disappears absolutely
the alleged obstacle to the transmission of acquired characters.
* * * * *
Notwithstanding experiences showing the futility of controversy for the
establishment of truth, I am tempted here to answer opponents at some
length. But even could the editor allow me the needful space, I should be
compelled, both by lack of time and by ill-health, to be brief. I must
content myself with noticing a few points which most nearly concern me.
Referring to my argument respecting tactual discriminativeness, Mr. Wallace
thinks that I--
"afford a glaring example of taking the unessential in place of the
essential, and drawing conclusions from a partial and altogether
insufficient survey of the phenomena. For this 'tactual
discriminativeness,' which is alone dealt with by Mr. Spencer, forms the
least important, and probably only an incidental portion of the great
vital phenomenon of skin-sensitiveness, which is at once the watchman and
the shield of the organism against imminent external dangers."
(_Fortnightly Review_, April, 1893, p. 497)
Here Mr. Wallace assumes it to be self-evident that skin-sensitiveness is
due to natural selection, and assumes that this must be admitted by me. He
supposes it is only the unequal distribution of skin-discriminativeness
which I contend is not thus accounted for. But I deny that either the
general sensitiveness or the special sensitiveness results from natural
selection; and I have years ago justified the first disbelief as I have
recently the second. In "The Factors of Organic Evolution" (_Essays_,
454-8), I have given various reasons for inferring that the genesis of the
nervous system cannot be due to survival of the fittest; but that it is due
to the direct effects of converse between the surface and the environment;
and that thus only is to be explained the strange fact that the nervous
centres are originally superficial, and migrate inwards during development.
These conclusions I have, in the essay Mr. Wallace criticizes, upheld by
the evidence which blind boys and skilled compositors furnish; proving, as
this does, that increased nervous development is peripherally initiated.
Mr. Wallace's belief that skin-sensitiveness arose by natural selection, is
unsupported by a single fact. He assumes that it _must_ have been so
produced because it is all-important to self-preservation. My belief that
it is directly initiated by converse with the environment, is supported by
facts; and I have given proof that the assigned cause is now in operation.
Am I called upon to abandon my own supported belief and accept Mr.
Wallace's unsupported belief? I think not.
Referring to my argument concerning blind cave-animals, Professor
Lankester, in _Nature_ of February 23, 1893, writes:--
"Mr. Spencer shows that the saving of ponderable material in the
suppression of an eye is but a small economy: he loses sight of the fact,
however, that possibly, or even probably, the saving to the organism in
the reduction of an eye to a rudimentary state is not to be measured by
mere bulk, but by the non-expenditure of special materials and special
activities which are concerned in the production of an organ so peculiar
and elaborate as is the vertebrate eye."
It seems to me that a supposition is here made to do duty as a fact; and
that I might with equal propriety say that "possibly, or even probably,"
the vertebrate eye is physiologically cheap: its optical part, constituting
nearly its whole bulk, consisting of a low order of tissue. There is,
indeed, strong reason for considering it physiologically cheap. If any one
remembers how relatively enormous are the eyes of a fish just out of the
egg--a pair of eyes with a body and head attached; and if he then remembers
that every egg contains material for such a pair of eyes; he will see that
eye-material constitutes a very considerable part of the fish's roe; and
that, since the female fish provides this quantity every year, it cannot be
expensive. My argument against Weismann is strengthened rather than
weakened by contemplation of these facts.
Professor Lankester asks my attention to a hypothesis of his own, published
in the _Encyclopædia Britannica_, concerning the production of blind
cave-animals. He thinks it can--
"be fully explained by natural selection acting on congenital fortuitous
variations. Many animals are thus born with distorted or defective eyes
whose parents have not had their eyes submitted to any peculiar
conditions. Supposing a number of some species of Arthropod or Fish to be
swept into a cavern or to be carried from less to greater depths in the
sea, those individuals with perfect eyes would follow the glimmer of
light and eventually escape to the outer air or the shallower depths,
leaving behind those with imperfect eyes to breed in the dark place. A
natural selection would thus be effected" in successive generations.
First of all, I demur to the words "many animals." Under the abnormal
conditions of domestication, congenitally defective eyes may be not very
uncommon; but their occurrence under natural conditions is, I fancy,
extremely rare. Supposing, however, that in a shoal of young fish, there
occur some with eyes seriously defective. What will happen? Vision is
all-important to the young fish, both for obtaining food and for escaping
from enemies. This is implied by the immense development of eyes just
referred to; and the obvious conclusion to be drawn is that the partially
blind would disappear. Considering that out of the enormous number of young
fish hatched with perfect eyes, not one in a hundred reaches maturity, what
chance of surviving would there be for those with imperfect eyes?
Inevitably they would be starved or be snapped up. Hence the chances that a
matured or partially matured semi-blind fish, or rather two such, male and
female, would be swept into a cave and left behind are extremely remote.
Still more remote must the chances be in the case of cray-fish. Sheltering
themselves as these do under stones, in crevices, and in burrows which they
make in the banks, and able quickly to anchor themselves to weeds or sticks
by their claws, it seems scarcely supposable that any of them could be
carried into a cave by a flood. What, then, is the probability that there
will be two nearly blind ones, and that these will be thus carried? Then,
after this first extreme improbability, there comes a second, which we may,
I think, rather call an impossibility. How would it be possible for
creatures subject to so violent a change of habitat to survive? Surely
death would quickly follow the subjection to such utterly unlike conditions
and modes of life. The existence of these blind cave-animals can be
accounted for only by supposing that their remote ancestors began making
excursions into the cave, and, finding it profitable, extended them,
generation after generation, further in: undergoing the required
adaptations little by little.[115]
Between Dr. Romanes and myself the first difference concerns the
interpretation of "Panmixia." Clearer conceptions of these matters would be
reached if, instead of thinking in abstract terms, the physiological
processes concerned were brought into the foreground. Beyond the production
of changes in the sizes of parts by the selection of fortuitously-arising
variations, I can see but one other cause for the production of them--the
competition among the parts for nutriment. This has the effect that active
parts are well-supplied and grow, while inactive parts are ill-supplied and
dwindle.[116] This competition is the cause of "economy of growth"; this is
the cause of decrease from disuse; and this is the only conceivable cause
of that decrease which Dr. Romanes contends follows the cessation of
selection. The three things are aspects of the same thing. And now, before
leaving this question, let me remark on the strange proposition which has
to be defended by those who deny the dwindling of organs from disuse. Their
proposition amounts to this:--that for a hundred generations an inactive
organ may be partially denuded of blood all through life, and yet in the
hundredth generation will be produced of just the same size as in the
first!
There is one other passage in Dr. Romanes' criticism--that concerning the
influence of a previous sire on progeny--which calls for comment. He sets
down what he supposes Weismann will say in response to my argument. "First,
he may question the fact." Well, after the additional evidence given above,
I think he is not likely to do that; unless, indeed, it be that along with
readiness to base conclusions on things "it is easy to imagine" there goes
reluctance to accept testimony which it is difficult to doubt. Second, he
is supposed to reply that "the Germ-plasm of the first sire has in some way
or another become partly commingled with that of the immature ova"; and Dr.
Romanes goes on to describe how there may be millions of spermatozoa and
"thousands of millions" of their contained "ids" around the ovaries, to
which these secondary effects are due. But, on the one hand, he does not
explain why in such cases each subsequent ovum, as it becomes matured, is
not fertilized by the sperm-cells present, or their contained germ-plasm,
rendering all subsequent fecundations needless; and, on the other hand, he
does not explain why, if this does not happen, the potency of this
remaining germ-plasm is nevertheless such as to affect not only the next
succeeding offspring, but all subsequent offspring. The irreconcilability
of these two implications would, I think, sufficiently dispose of the
supposition, even had we not daily multitudinous proofs that the surface of
a mammalian ovarium is not a spermatheca. The third reply Dr. Romanes
urges, is the inconceivability of the process by which the germ-plasm of a
preceding male parent affects the constitution of the female and her
subsequent offspring. In response, I have to ask why he piles up a mountain
of difficulties based on the assumption that Mr. Darwin's explanation of
heredity by "Pangenesis" is the only available explanation preceding that
of Weismann? and why he presents these difficulties to me, more especially;
deliberately ignoring my own hypothesis of physiological units? It cannot
be that he is ignorant of this hypothesis, since the work in which it is
variously set forth (_Principles of Biology_, §§ 66-97) is one with which
he is well acquainted: witness his _Scientific Evidences of Organic
Evolution_; and he has had recent reminders of it in Weismann's
_Germ-plasm_, where it is repeatedly referred to. Why, then, does he assume
that I abandon my own hypothesis and adopt that of Darwin; thereby
entangling myself in difficulties which my own hypothesis avoids? If, as I
have argued, the germ-plasm consists of substantially similar units (having
only those minute differences expressive of individual and ancestral
differences of structure), none of the complicated requirements which Dr.
Romanes emphasizes exist; and the alleged inconceivability disappears.
Here I must end: not intending to say more, unless for some very urgent
reason; and leaving others to carry on the discussion. I have, indeed, been
led to suspend for a short time my proper work, only by consciousness of
the transcendent importance of the question at issue. As I have before
contended, a right answer to the question whether acquired characters are
or are not inherited, underlies right beliefs, not only in Biology and
Psychology, but also in Education, Ethics, and Politics.
III.
As a species of literature, controversy is characterised by a terrible
fertility. Each proposition becomes the parent of half a dozen; so that a
few replies and rejoinders produce an unmanageable population of issues,
old and new, which end in being a nuisance to everybody. Remembering this,
I shall refrain from dealing with all the points of Professor Weismann's
answer. I must limit myself to a part; and that there may be no suspicion
of a selection convenient to myself, I will take those contained in his
first article.
Before dealing with his special arguments, let me say something about the
general mode of argument which Professor Weismann adopts.
The title of his article is "The All-Sufficiency of Natural
Selection."[117] Very soon, however, as on p. 322, we come to the
admission, which he has himself italicised, "that _it is really very
difficult to imagine this process of natural selection in its details_; and
to this day it is impossible to demonstrate it in any one point."
Elsewhere, as on pp. 327 and 336 _à propos_ of other cases, there are like
admissions. But now if the sufficiency of an assigned cause cannot in any
case be demonstrated, and if it is "really very difficult to imagine" in
what way it has produced its alleged effects, what becomes of the
"all-sufficiency" of the cause? How can its all-sufficiency be alleged when
its action can neither be demonstrated nor easily imagined? Evidently to
fit Professor Weismann's argument the title of the article should have been
"The Doubtful Sufficiency of Natural Selection."
Observe, again, how entirely opposite are the ways in which he treats his
own interpretation and the antagonist interpretation. He takes the problem
presented by certain beautifully adapted structures on the anterior legs of
"very many insects," which they use for cleansing their antennæ. These, he
argues, cannot have resulted from the inheritance of acquired characters;
since any supposed changes produced by function would be changes in the
chitinous exo-skeleton, which, being a dead substance, cannot have had its
changes transmitted. He then proceeds, very candidly, to point out the
extreme difficulties which lie in the way of supposing these structures to
have resulted from natural selection: admitting that an opponent might "say
that it was absurd" to assume that the successive small variations implied
were severally life-saving in their effects. Nevertheless, he holds it
unquestionable that natural selection has been the cause. See then the
difference. The supposition that the apparatus has been produced by the
inheritance of acquired characters is rejected _because_ it presents
insuperable difficulties. But the supposition that the apparatus has been
produced by natural selection is accepted, _though_ it presents insuperable
difficulties. If this mode of reasoning is allowable, no fair comparison
between diverse hypotheses can be made.
With these remarks on Professor Weismann's method at large, let me now pass
to the particular arguments he uses, taking them _seriatim_.
* * * * *
The first case he deals with is that of the progressive degradation of the
human little toe. This he considers a good test case; and he proceeds to
discuss an assigned cause--the inherited and accumulated effects of
boot-pressure. Without much difficulty he shows that this interpretation is
inadequate; since fusion of the phalanges, which constitutes in part the
progressive degradation, is found among peoples who go barefoot, and has
been found also in Egyptian mummies. Having thus disposed of Mr. Buckman's
interpretation, Professor Weismann forthwith concludes that the ascription
of this anatomical change to the inheritance of acquired characters is
disposed of, and assumes, as the only other possible interpretation, a
dwindling "through panmixia": "the hereditary degeneration of the little
toe is thus quite simply explained from my standpoint."
It is surprising that Professor Weismann should not have seen that there is
an explanation against which his criticism does not tell. If we go back to
the genesis of the human type from some lower type of _primates_, we see
that while the little toe has ceased to be of any use for climbing
purposes, it has not come into any considerable use for walking and
running. A glance at the feet of the sub-human _primates_ in general, shows
that the inner digits are, as compared with those of men, quite small, have
no such relative length and massiveness as the human great toes. Leaving
out the question of cause, it is manifest that the great toes have been
immensely developed, since there took place the change from arboreal habits
to terrestrial habits. A study of the mechanics of walking shows why this
has happened. Stability requires that the "line of direction" (the vertical
line let fall from the centre of gravity) shall fall within the base, and,
in walking, shall be brought at each step within the area of support, or so
near it that any tendency to fall may be checked at the next step. A
necessary result is that if, at each step, the chief stress of support is
thrown on the outer side of the foot, the body must be swayed so that the
"line of direction" may fall within the outer side of the foot, or close to
it; and when the next step is taken it must be similarly swayed in an
opposite way, so that the outer side of the other foot may bear the weight.
That is to say, the body must oscillate from side to side, or waddle. The
movements of a duck when walking or running show what happens when the
points of support are wide apart. Clearly this kind of movement conflicts
with efficient locomotion. There is a waste of muscular energy in making
these lateral movements, and they are at variance with the forward
movement. We may infer, then, that the developing man profited by throwing
the stress as much as possible on the inner sides of the feet; and was
especially led to do this when going fast, which enabled him to abridge the
oscillations: as indeed we now see in a drunken man. Thus there was thrown
a continually increasing stress upon the inner digits as they progressively
developed from the effects of use; until now that the inner digits, so
large compared with the others, bear the greater part of the weight, and
being relatively near one another, render needless any marked swayings from
side to side. But what has meanwhile happened to the outer digits?
Evidently as fast as the great toes have come more and more into play and
developed, the little toes have gone more and more out of play and have
been dwindling for--how long shall we say?--perhaps a hundred thousand
years.
So far, then, am I from feeling that Professor Weismann has here raised a
difficulty in the way of the doctrine I hold, that I feel indebted to him
for having drawn attention to a very strong evidence in its support. This
modification in the form of the foot, which has occurred since arboreal
habits have given place to terrestrial habits, shows the effects of use and
disuse simultaneously. The inner digits have increased by use while the
outer digits have decreased by disuse.
* * * * *
Saying that he will not "pause to refute other apparent proofs of the
transmission of acquired characters," Professor Weismann proceeds to deal
with the argument which, with various illustrations, I have several times
urged--the argument that the natural selection of fortuitously-arising
variations cannot account for the adjustment of co-operative parts. Very
clearly and very fairly he summarises this argument as used in _The
Principles of Biology_ in 1864. Admitting that in this case there are
"enormous difficulties" in the way of any other interpretation than the
inheritance of acquired characters, Professor Weismann before proceeding to
assault this "last bulwark of the Lamarckian principle," premises that the
inheritance of acquired characters cannot be a cause of change because
inactive as well as active parts degenerate when they cease to be of use:
instancing the "skin and skin-armature of crabs and insects." On this I may
remark in the first place that an argument derived from degeneracy of
passive structures scarcely meets the case of development of active
structures; and I may remark in the second place that I have never dreamt
of denying the efficiency of natural selection as a cause of degeneracy in
passive structures when the degeneracy is such as aids the prosperity of
the stirp.
Making this parenthetical reply to his parenthetical criticism I pass to
his discussion of this particular argument which he undertakes to dispose
of.
His _cheval de bataille_ is furnished him by the social insects--not a
fresh one, however, as might be supposed from the way in which he mounts
it. From time to time it has carried other riders, who have couched their
lances with fatal effects as they supposed. But I hope to show that no one
of them has unhorsed an antagonist, and that Professor Weismann fails to do
this just as completely as his predecessors. I am, indeed, not sorry that
he has afforded me the opportunity of criticising the general discussion
concerning the peculiarities of these interesting creatures, which it has
often seemed to me sets out with illegitimate assumptions. The supposition
always is that the specialities of structures and instincts in the unlike
classes of their communities, have arisen during the period in which the
communities have existed in something like their present forms. This cannot
be. It is doubtless true that association without differentiations of
classes may pre-exist for co-operative purposes, as among wolves, and as
among various insects which swarm under certain circumstances. Hence we may
suppose that there arise in some cases permanent swarms--that survival of
the fittest will establish these constant swarms where they are
advantageous. But admitting this, we have also to admit a gradual rise of
the associated state out of the solitary state. Wasps and bees present us
with gradations. If, then, we are to understand how the organized societies
have arisen, either out of the solitary state or out of undifferentiated
swarms, we must assume that the differences of structure and instinct among
the members of them arose little by little, as the social organization
arose little by little. Fortunately we are able to trace the greater part
of the process in the annually-formed communities of the common wasp; and
we shall recognize in it an all-important factor (ignored by Professor
Weismann) to which the phenomena, or at any rate the greater part of them,
are due.
But before describing the wasp's annual history, let me set down certain
observations made when, as a boy, I was given to angling, and, in July or
August, sometimes used for bait "wasp-grubs," as they were called. After
having had two or three days the combs or "cakes" of these, full of unfed
larvæ in all stages of growth, I often saw some of them devouring the edges
of their cells to satisfy their appetites; and saw others, probably the
most advanced in growth, which were spinning the little covering caps to
their cells, in preparation for assuming the pupa state. It is to be
inferred that if, after a certain stage of growth has been reached, the
food-supply becomes inadequate or is stopped altogether, the larva
undergoes its transformation prematurely; and, as we shall presently see,
this premature transformation has several natural sequences.
Let us return now to the wasp's family history. In the spring, a queen-wasp
or mother-wasp which has survived the winter, begins to make a small nest
containing four or more cells in which she lays eggs, and as fast as she
builds additional cells, she lays an egg in each. Presently, to these
activities, is added the feeding of the larvæ: one result being that the
multiplication of larvæ involves a restriction of the food that can be
given to each. If we suppose that the mother-wasp rears no more larvæ than
she can fully feed, there will result queens or mothers like herself,
relatively few in number. But if we suppose that, laying more numerous eggs
she produces more larvæ than she can fully feed, the result will be that
when these have reached a certain stage of growth, inadequate supply of
food will be followed by premature retirement and transformation into pupæ.
What will be the characters of the developed insects? The first effect of
arrested nutrition will be smaller size. This we find. A second effect will
be defective development of parts that are latest formed and least
important for the survival of the individual. Hence we may look for
arrested development of the reproductive organs--non-essential to
individual life. And this expectation is in accord with what we see in
animal development at large; for (passing over entirely sexless
individuals) we see that though the reproductive organs may be marked out
early in the course of development, they are not made fit for action until
after the structures for carrying on individual life are nearly complete.
The implication is, then, that an inadequately-fed and small larva will
become a sterile imago. Having noted this, let us pass to a remarkable
concomitant. In the course of development, organs are formed not alone in
the order of their original succession, but partly in the order of
importance and the share they have to take in adult activities--a change of
order called by Haeckel "heterochrony." Hence the fact that we often see
the maternal instinct precede the sexual instinct. Every little girl with
her doll shows us that the one may become alive while the other remains
dormant. In the case of wasps, then, premature arrest of development may
result in incompleteness of the sexual traits, along with completeness of
the maternal traits. What happens? Leave out the laying of eggs, and the
energies of the mother-wasp are spent wholly in building cells and feeding
larvæ, and the worker-wasp forthwith begins to spend its life in building
cells and feeding larvæ. Thus interpreting the facts, we have no occasion
to assume any constitutional difference between the eggs of worker-wasps
and the eggs of queens; and that, their eggs are not different we see,
first, in the fact that occasionally the worker-wasp is fertile and lays
drone-producing eggs, and we see secondly that (if in this respect they are
like the bees, of which, however, we have no proof) the larva of a
worker-wasp can be changed into the larva of a queen-wasp by special
feeding. But be this as it may, we have good evidence that the feeding
determines everything. Says Dr. Ormerod, in his _British Social Wasps_:--
"When the swarm is strong and food plentiful ... the well fed larvæ
develop into females, full, large, and overflowing with fat. There are
all gradations of size, from the large fat female to the smallest
worker.... The larger the wasp, the larger and better developed, as the
rule, are the female organs, in all their details. In the largest wasps,
which are to be the queens of another year, the ovaries differ to all
appearances in nothing but their size from those of the larger worker
wasps.... Small feeble swarms produce few or no perfect females; but in
large strong swarms they are found by the score." (pp. 248-9)
To this evidence add the further evidence that queens and workers pass
through certain parallel stages in respect of their maternal activities. At
first the queen, besides laying eggs, builds cells and feeds larvæ, but
after a time ceases to build cells, and feeds larvæ only, and eventually
doing neither one nor the other, only lays eggs, and is supplied with food
by the workers. So it is in part with the workers. While the members of
each successive brood, when in full vigour, build cells and feed larvæ,
by-and-by they cease to build cells, and only feed larvæ: the maternal
activities and instincts undergo analogous changes. In this case, then, we
are not obliged to assume that only by a process of natural selection can
the differences of structure and instinct between queens and workers be
produced. The only way in which natural selection here comes into play is
in the better survival of the families of those queens which made as many
cells, and laid as many eggs, as resulted in the best number of half-fed
larvæ, producing workers; since by a rapid multiplication of workers the
family is advantaged, and the ultimate production of more queens surviving
into the next year insured.
The differentiation of classes does not go far among the wasps, because the
cycle of processes is limited to a year, or rather to the few months of the
summer. It goes further among the hive-bees, which, by storing food,
survive from one year into the next. Unlike the queen-wasp, the queen-bee
neither builds cells nor gathers food, but is fed by the workers: egg
laying has become her sole business. On the other hand the workers,
occupied exclusively in building and nursing, have the reproductive organs
more dwarfed than they are in wasps. Still we see that the worker-bee
occasionally lays drone-producing eggs, and that, by giving extra nutriment
and the required extra space, a worker-larva can be developed into a
queen-larva. In respect to the leading traits, therefore, the same
interpretation holds. Doubtless there are subsidiary instincts which are
apparently not thus interpretable. But before it can be assumed that an
interpretation of another kind is necessary, it must be shown that these
instincts cannot be traced back to those pre-social types and semi-social
types which must have preceded the social types we now see. For
unquestionably existing bees must have brought with them from the
pre-social state an extensive endowment of instincts, and, acquiring other
instincts during the unorganized social state, must have brought these into
the present organized social state. It is clear, for instance, that the
cell-building instinct in all its elaboration was mainly developed in the
pre-social stage; for the transition from species building solitary cells
to those building combs is traceable. We are similarly enabled to account
for swarming as being an inheritance from remote ancestral types. For just
in the same way that, with under-feeding of larvæ, there result individuals
with imperfectly developed reproductive systems, so there will result
individuals with imperfect sexual instincts; and just as the imperfect
reproductive system partially operates upon occasion, so will the imperfect
sexual instinct. Whence it will result that on the event which causes a
queen to undertake a nuptial flight which is effectual, the workers may
take abortive nuptial flights: so causing a swarm.
And here, before going further, let us note an instructive class of facts
related to the class of facts above set forth. Summing up, in a chapter on
"The Determination of Sex," an induction from many cases, Professor Geddes
and Mr. Thompson remark that "such conditions as deficient or abnormal
food," and others causing "preponderance of waste over repair ... tend to
result in production of males;" while "abundant and rich nutrition" and
other conditions which "favour constructive processes ... result in the
production of females."[118] Among such evidences of this as immediately
concern us, are these:--J. H. Fabre found that in the nests of _Osmia
tricornis_, eggs at the bottom, first laid, and accompanied by much food,
produced females, while those at the top, last laid, and accompanied by
one-half or one-third the quantity of food, produced males,[119] Huber's
observations on egg-laying by the honey-bee, show that in the normal course
of things, the queen lays eggs of workers for eleven months, and only then
lays eggs of drones: that is, when declining nutrition or exhaustion has
set in. Further, we have the above-named fact, shown by wasps and bees,
that when workers lay eggs these produce drones only.[120] Special
evidence, harmonizing with general evidence, thus proves that among the
social insects the sex is determined by degree of nutrition while the egg
is being formed. See then how congruous this evidence is with the
conclusion above drawn; for it is proved that after an egg, predetermined
as a female, has been laid, the character of the produced insect as a
perfect female or imperfect female is determined by the nutrition of the
larva. _That is, one set of differences in structures and instincts is
determined by nutrition before the egg is laid, and a further set of
differences in structures and instincts is determined by nutrition after
the egg is laid._
We come now to the extreme case--that of the ants. Is it not probable that
the process of differentiation has been similar? There are sundry reasons
for thinking so. With ants as with wasps and bees--the workers occasionally
lay eggs; and an ant-community can, like a bee-community, when need be,
produce queens out of worker-larvæ: presumably in the same manner by extra
feeding. But here we have to add special evidence of great significance.
For observe that the very facts concerning ants, which Professor Weismann
names as exemplifying the formation of the worker type by selection, serve,
as in the case of wasps, to exemplify its formation by arrested nutrition.
He says that in several species the egg-tubes in the ovaries show
progressive decrease in number; and this, like the different degrees of
arrest in the ovaries of the worker-wasps, indicates arrest of
larva-feeding at different stages. He gives cases showing that, in
different degrees, the eyes of workers are less developed in the number of
their facets than those of the perfect insects; and he also refers to the
wings of workers as not being developed: remarking, however, that the
rudiments of their wings show that the ancestral forms had wings. Are not
these traits also results of arrested nutrition? Generally among insects
the larvæ are either blind or have but rudimentary eyes; that is to say,
visual organs are among the latest organs to arise in the genesis of the
perfect organism. Hence early arrest of nutrition will stop formation of
these, while various more ancient structures have become tolerably
complete. Similarly with wings. Wings are late organs in insect phylogeny,
and therefore will be among those most likely to abort where development is
prematurely arrested. And both these traits will, for the same reason,
naturally go along with arrested development of the reproductive system.
Even more significant, however, is some evidence assigned by Mr. Darwin
respecting the caste-gradations among the driver ants of West Africa. He
says:--
"But the most important fact for us is, that, though the workers can be
grouped into castes of different sizes, yet they graduate insensibly into
each other, as does the widely-different structure of their jaws."[121]
"Graduate insensibly," he says; implying that there are very numerous
intermediate forms. This is exactly what is to be expected if arrest of
nutrition be the cause; for unless the ants have definite measures,
enabling them to stop feeding at just the same stages, it must happen that
the stoppage of feeding will be indefinite; and that, therefore, there will
be all gradations between the extreme forms--"insensible gradations," both
in size and in jaw-structure.
In contrast with this interpretation, consider now that of Professor
Weismann. From whichever of the two possible suppositions he sets out, the
result is equally fatal. If he is consistent, he must say that each of
these intermediate forms of workers must have its special set of
"determinants," causing its special set of modifications of organs; for he
cannot assume that while perfect females and the extreme types of workers
have their different sets of determinants, the intermediate types of
workers have not. Hence we are introduced to the strange conclusion that
besides the markedly-distinguished sets of determinants there must be, to
produce these intermediate forms, many other sets slightly distinguished
from one another--a score or more kinds of germ-plasm in addition to the
four chief kinds. Next comes an introduction to the still stranger
conclusion, that these numerous kinds of germ-plasm, producing these
numerous intermediate forms, are not simply needless but injurious--produce
forms not well fitted for either of the functions discharged by the extreme
forms: the implication being that natural selection has originated these
disadvantageous forms! If to escape from this necessity for suicide,
Professor Weismann accepts the inference that the differences among these
numerous intermediate forms are caused by arrested feeding of the larvæ at
different stages, then he is bound to admit that the differences between
the extreme forms, and between these and perfect females, are similarly
caused. But if he does this, what becomes of his hypothesis that the
several castes are constitutionally distinct, and result from the operation
of natural selection? Observe, too, that his theory does not even allow him
to make this choice; for we have clear proof that unlikenesses among the
forms of the same species cannot be determined this way or that way by
differences of nutrition. English greyhounds and Scotch greyhounds do not
differ from one another so much as do the Amazon-workers from the inferior
workers, or the workers from the queens. But no matter how a pregnant
Scotch greyhound is fed, or her pups after they are born, they cannot be
changed into English greyhounds: the different germ-plasms assert
themselves spite of all treatment. But in these social insects the
different structures of queens and workers _are_ determinable by
differences of feeling. Therefore the production of their various castes
does not result from the natural selection of varying germ-plasm.
Before dealing with Professor Weismann's crucial case--that co-adaptation
of parts, which, in the soldier-ants, has, he thinks, arisen without
inheritance of acquired characters--let me deal with an ancillary case
which he puts forward as explicable by "panmixia alone." This is the
"degeneration, in the warlike Amazon-ants, of the instinct to search for
food."[122] Let us first ask what have been the probable antecedents of
these Amazon-ants; for, as I have above said, it is absurd to speculate
about the structures and instincts the species possesses in its existing
organized social state without asking what structures and instincts it
brought with it from its original solitary state and its unorganized social
state. From the outset these ants were predatory. Some variety of them led
to swarm--probably at the sexual season--did not again disperse so soon as
other varieties. Those which thus kept together derived advantages from
making simultaneous attacks on prey, and prospered accordingly. Of
descendants the varieties which carried on longest the associated state
prospered most; until, at length, the associated state became permanent.
All which social progress took place while there existed only perfect males
and females. What was the next step? Ants utilize other insects, and, among
other ways of doing this, sometimes make their nests where there are useful
insects ready to be utilized. Giving an account of certain New Zealand
species of _Tetramorium_, Mr. W. W. Smith says they seek out underground
places where there are "root-feeding aphides and coccids," which they begin
to treat as domestic animals; and further he says that when, after the
pairing season, new nests are being formed, there are "a few ants of both
sexes ... from two up to eight or ten."[123] Carrying with us this fact as
a key, let us ask what habits will be fallen into by the conquering species
of ants. They, too, will seek places where there are creatures to be
utilized; and, finding it profitable, will invade the habitations not of
defenceless creatures only, but of creatures whose powers of defence are
inadequate--weaker species of their own order. A very small modification
will affiliate their habits on habits of their prototypes. Instead of being
supplied with sweet substance excreted by the aphides they are supplied
with sweet substance by the ants among which they parasitically settle
themselves. How easily the subjugated ants may fall into the habit of
feeding them, we shall see on remembering that already they feed not only
larvæ but adults--individuals bigger than themselves. And that attentions
kindred to these paid to parasitic ants may be established without
difficulty, is shown us by the small birds which continue to feed a young
cuckoo in their nest when it has outgrown them. This advanced form of
parasitism grew up while there were yet only perfect males and females, as
happens in the initial stage with these New Zealand ants. What further
modifications of habits were probably then acquired? From the practice of
settling themselves where there already exist colonies of aphides, which
they carry about to suitable places in the nest, like _Tetramorium_, other
ants pass to the practice of making excursions to get aphides, and putting
them in better feeding places where they become more productive of
saccharine matter. By a parallel step these soldier-ants pass from the
stage of settling themselves among other ants which feed them, to the stage
of fetching the pupæ of such ants to the nest: a transition like that which
occurs among slave-making human beings. Thus by processes analogous to
those we see going on, these communities of slave-making ants may be
formed. And since the transition from an unorganized social state to a
social state characterized by castes, must have been gradual, there must
have been a long interval during which the perfect males and females of
these conquering ants could acquire habits and transmit them to progeny. A
small modification accounts for that seemingly-strange habit which
Professor Weismann signalizes. For if, as is observed, those ants which
keep aphides solicit them to excrete a supply of ant-food by stroking them
with the antennæ, they come very near to doing that which Professor
Weismann says the soldier-ants do towards a worker--"they come to it and
beg for food:" the food being put into their mouths in this last case as
almost or quite in the first. And evidently this habit of passively
receiving food, continued through many generations of perfect males and
females, may result in such disuse of the power of self-feeding that this
is eventually lost. The behaviour of young birds, during, and after, their
nest-life, gives us the clue. For a week or more after they are full-grown
and fly about with their parents, they may be seen begging for food and
making no efforts to recognize and pick up food for themselves. If,
generation after generation, feeding of them in full measure continued,
they would not learn to feed themselves: the perceptions and instincts
implied in self-feeding would be later and later developed, until, with
entire disuse of them, they would disappear altogether by inheritance. Thus
self-feeding may readily have ceased among these soldier-ants before the
caste-organization arose among them.
With this interpretation compare the interpretation of Professor Weismann.
I have before protested against arguing in abstracts without descending to
concretes. Here let us ask what are the particular changes which the
alleged explanation by survival of the fittest involves. Suppose we make
the very liberal supposition that an ant's central ganglion bears to its
body the same ratio as the human brain bears to the human body--say,
one-fortieth of its weight. Assuming this, what shall we assume to be the
weight of those ganglion-cells and fibres in which are localized the
perceptions of food and the suggestion to take it? Shall we say that these
amount to one-tenth of the central ganglion? This is a high estimate
considering all the impressions which this ganglion has to receive, and all
the operations which it has to direct. Still we will say one-tenth. Then it
follows that this portion of nervous substance is one-400th of the weight
of its body. By what series of variations shall we say that it is reduced
from full power to entire incapacity? Shall we say five? This is a small
number to assume. Nevertheless we will assume it. What results? That the
economy of nerve-substance achieved by each of these five variations will
amount to one-2000th of the entire mass. Making these highly favourable
assumptions, what follows:--The queen-ant lays eggs that give origin to
individuals in each of which there is achieved an economy in
nerve-substance of one-2000th of its weight; and the implication of the
hypothesis is that such an economy will so advantage this ant-community
that in the competition with other ant-communities it will conquer. For
here let me recall the truth before insisted upon, that natural selection
can operate only on those variations which appreciably benefit the stirp.
Bearing in mind this requirement, is any one now prepared to say that
survival of the fittest can cause this decline of the self-feeding
faculty?[124]
Not limiting himself to the Darwinian interpretation, however, Professor
Weismann says that this degradation may be accounted for by "panmixia
alone." Here I will not discuss the adequacy of this supposed cause, but
will leave it to be dealt with by implication a few pages in advance, where
the general hypothesis of panmixia will be reconsidered.
And now, at length, we are prepared for dealing with Professor Weismann's
crucial case--with his alleged disproof that co-adaptation of co-operative
parts results from inheritance of acquired characters, because in the case
of the Amazon-ants, it has arisen where the inheritance of acquired
characters is impossible. For after what has been said, it will be manifest
that the whole question is begged when it is assumed that this
co-adaptation has arisen since there existed among these ants an organized
social state. Unquestionably this organized social state pre-supposes a
series of modifications through which it has been reached. It follows,
then, that there can be no rational interpretation without a preceding
inquiry concerning that earlier state in which there were no castes, but
only males and females. What kinds of individuals were the ancestral
ants--at first solitary, and then semi-social? They must have had marked
powers of offence and defence. Of predacious creatures, it is the more
powerful which form societies, not the weaker. Instance human races.
Nations originate from the relatively warlike tribes, not from the
relatively peaceful tribes. Among the several types of individuals forming
the existing ant community, to which, then, did the ancestral ants bear the
greatest resemblance? They could not have been like the queens, for these,
now devoted to egg-laying, are unfitted for conquest. They could not have
been like the inferior class of workers, for these, too, are inadequately
armed and lack strength. Hence they must have been most like these
Amazon-ants or soldier-ants, which now make predatory excursions--which now
do, in fact, what their remote ancestors did. What follows? Their
co-adapted parts have not been produced by the selection of variations
within the ant-community, such as we now see it. They have been inherited
from the pre-social and early social types of ants, in which the
co-adaptation of parts had been effected by inheritance of acquired
characters. It is not that the soldier-ants have gained these traits; it is
that the other castes have lost them. Early arrest of development causes
absence of them in the inferior workers; and from the queens they have
slowly disappeared by inheritance of the effects of disuse. For, in
conformity with ordinary facts of development, we may conclude that in a
larva which is being so fed as that the development of the reproductive
organs is becoming pronounced, there will simultaneously commence arrest in
the development of those organs which are not to be used. There are
abundant proofs that along with rapid growth of some organs others abort.
And if these inferences are true, then Professor Weismann's argument falls
to the ground. Nay, it falls to the ground even if conclusions so definite
as these be not insisted upon; for before he can get a basis for his
argument he must give good reasons for concluding that these traits of the
Amazon-ants have _not_ been inherited from remote ancestors.
One more step remains. Let us grant him his basis, and let us pass from the
above negative criticism to a positive criticism. As before, I decline to
follow the practice of talking in abstracts instead of in concretes, and
contend that, difficult as it may be to see how natural selection has in
all cases operated, we ought, at any rate, to trace out its operation
whenever we can, and see where the hypothesis lands us. According to
Professor Weismann's admission, for production of the Amazon-ant by natural
selection, "_many parts must have varied simultaneously and in harmony with
one another_;"[125] and he names as such, larger jaws, muscles to move
them, larger head, and thicker chitin for it, bigger nerves for the
muscles, bigger motor centres in the brain, and, for the support of the big
head, strengthening of the thorax, limbs, and skeleton generally. As he
admits, all these parts must have varied simultaneously in due proportion
to one another. What must have been the proximate causes of their
variations? They must have been variations in what he calls the
"determinants." He says:--
"We have, however, to deal with the transmission of parts which are
_variable_ and this necessitates the assumption that just as many
independent and variable parts exist in the germ-plasm as are present in
the fully formed organism."[126]
Consequently to produce simultaneously these many variations of parts,
adjusted in their sizes and shapes, there must have simultaneously arisen a
set of corresponding variations in the "determinants" composing the
germ-plasm. What made them simultaneously vary in the requisite ways?
Professor Weismann will not say that there was somewhere a foregone
intention. This would imply supernatural agency. He makes no attempt to
assign a physical cause for these simultaneous appropriate variations in
the determinants: an adequate physical cause being inconceivable. What,
then, remains as the only possible interpretation? Nothing but _a
fortuitous concourse of variations_; reminding us of the old "fortuitous
concourse of atoms." Nay, indeed, it is the very same thing. For each of
the "determinants," made up of "biophors," and these again of
protein-molecules, and these again of simpler chemical molecules, must have
had its molecular constitution changed in the required way; and the
molecular constitutions of all the "determinants," severally modified
differently, but in adjustment to one another, must have been thus modified
by "a fortuitous concourse of atoms." Now if this is an allowable
supposition in respect of the "determinants," and the varying organs
arising from them, why is it not an allowable supposition in respect of the
organism as a whole? Why not assume "a fortuitous concourse of atoms" in
its broad, simple form? Nay, indeed, would not this be much the easier? For
observe, this co-adaptation of numerous co-operative parts is not achieved
by one set of variations, but is achieved gradually by a series of such
sets. That is to say, the "fortuitous concourse of atoms" must have
occurred time after time in appropriate ways. We have not one miracle, but
a series of miracles!
* * * * *
Of the two remaining points in Professor Weismann's first article which
demand notice, one concerns his reply to my argument drawn from the
distribution of tactual discriminativeness. In what way does he treat this
argument? He meets it by an argument derived from hypothetical
evidence--not actual evidence. Taking the case of the tongue-tip, I have
carefully inquired whether its extreme power of tactual discrimination can
give any life-saving advantage in moving about the food during mastication,
in detecting foreign bodies in it, or for purposes of speech; and have, I
think, shown that the ability to distinguish between points one
twenty-fourth of an inch apart is useless for such purposes. Professor
Weismann thinks he disposes of this by observing that among the apes the
tongue is used as an organ of touch. But surely a counter-argument
equivalent in weight to mine should have given a case in which power to
discriminate between points one twenty-fourth of an inch apart instead of
one-twentieth of an inch apart (a variation of one-sixth) had a life-saving
efficacy; or, at any rate, should have suggested such a case. Nothing of
the kind is done or even attempted. But now note that his reply, accepted
even as it stands, is suicidal. For what has the trusted process of
panmixia been doing ever since the human being began to evolve from the
ape? Why during thousands of generations has not the nervous structure
giving this extreme discriminativeness dwindled away? Even supposing it had
been proved of life-saving efficacy to our simian ancestors, it ought,
according to Professor Weismann's own hypothesis, to have disappeared in
us. Either there was none of the assumed special capacity in the ape's
tongue, in which case his reply fails, or panmixia has not operated, in
which case his theory of degeneracy fails.
All this, however, is but preface to the chief answer. The argument drawn
from the case of the tongue-tip, with which alone Professor Weismann deals,
is but a small part of my argument, the remainder of which he does not
attempt to touch--does not even mention. Had I never referred to the
tongue-tip at all, the various contrasts in discriminativeness which I have
named, between the one extreme of the forefinger-tip and the other extreme
of the middle of the back, would have abundantly sufficed to establish my
case--would have sufficed to show the inadequacy of natural selection as a
key and the adequacy of the inheritance of acquired characters.
It seems to me, then, that judgment must go against him by default.
Practically he leaves the matter standing just where it did.[127]
The other remaining point concerns the vexed question of panmixia.
Confirming the statement of Dr. Romanes, Professor Weismann says that I
have misunderstood him. Already (_Contemporary Review_, May, 1893, p. 758,
and Reprint, p. 66) I have quoted passages which appeared to justify my
interpretation, arrived at after much seeking.[128] Already, too, in this
review (July, 1893, p. 54) I have said why I did not hit upon the
interpretation now said to be the true one: I never supposed that any one
would assume, without assigned cause, that (apart from the excluded
influence of disuse) the _minus_ variations of a disused organ are greater
than the _plus_ variations. This was a tacit challenge to produce reasons
for the assumption. Professor Weismann does not accept the challenge, but
simply says:--"In my opinion all organs are maintained at the height of
their development only through uninterrupted selection" (p. 332): in the
absence of which they decline. Now it is doubtless true that as a
naturalist he may claim for his "opinion" a relatively great weight. Still,
in pursuance of the methods of science, it seems to me that something more
than an opinion is required as the basis of a far-reaching theory.[129]
Though the counter-opinion of one who is not a naturalist (as Professor
Weismann points out) may be of relatively small value, yet I must here
again give it, along with a final reason for it. And this reason shall be
exhibited, not in a qualitative form, but in a quantitative form. Let us
quantify the terms of the hypothesis by weights; and let us take as our
test case the rudimentary hind-limbs of the whale. Zoologists are agreed
that the whale has been evolved from a mammal which took to aquatic habits,
and that its disused hind-limbs have gradually disappeared. When they
ceased to be used in swimming, natural selection played a part--probably an
important part--in decreasing them; since, being then impediments to
movement through the water, they diminished the attainable speed. It may
be, too, that for a period after disappearance of the limbs beneath the
skin, survival of the fittest had still some effect. But during the latter
stages of the process it had no effect; since the rudiments caused no
inconvenience and entailed no appreciable cost. Here, therefore, the cause,
if Professor Weismann is right, must have been panmixia. Dr. Struthers,
Professor of Anatomy at Aberdeen, whose various publications show him to be
a high, if not the highest, authority on the anatomy of these great
cetaceans, has kindly taken much trouble in furnishing me with the needful
data, based upon direct weighing and measuring and estimation of specific
gravity. In the Black Whale (_Balænoptera borealis_) there are no rudiments
of hind-limbs whatever: rudiments of the pelvic bones only remain. A sample
of the Greenland Right Whale, estimated to weigh 44,800 lbs., had femurs
weighing together 3½ ozs.; while a sample of the Razor-back Whale
(_Balænoptera musculus_), 50 feet long, and estimated to weigh 56,000 lbs.,
had rudimentary femurs weighing together one ounce; so that these vanishing
remnants of hind-limbs weighed but one-896,000th part of the animal. Now in
considering the alleged degeneration by panmixia, we have first to ask why
these femurs must be supposed to have varied in the direction of decrease
rather than in the direction of increase. During its evolution from the
original land-mammal, the whale has grown enormously, implying habitual
excess of nutrition. Alike in the embryo and in the growing animal, there
must have been a chronic plethora. Why, then, should we suppose these
rudiments to have become smaller? Why should they not have enlarged by
deposit in them of superfluous materials? But let us grant the unwarranted
assumption of predominant _minus_ variations. Let us say that the last
variation was a reduction of one-half--that in some individuals the joint
weight of the femurs was suddenly reduced from two ounces to one ounce--a
reduction of one-900,000th of the creature's weight. By inter-crossing with
those inheriting the variation, the reduction, or a part of the reduction,
was made a trait of the species. Now, in the first place, a necessary
implication is that this _minus_ variation was maintained in posterity. So
far from having reason to suppose this, we have reason to suppose the
contrary. As before quoted, Mr. Darwin says that "unless carefully
preserved by man," "any particular variation would generally be lost by
crossing, reversion, and the accidental destruction of the varying
individuals."[130] And Mr. Galton, in his essay on "Regression towards
Mediocrity,"[131] contends that not only do deviations of the whole
organism from the mean size tend to thus disappear, but that deviations in
its components do so. Hence the chances are against such _minus_ variation
being so preserved as to affect the species by panmixia. In the second
place, supposing it to be preserved, may we reasonably assume that, by
inter-crossing, this decrease, amounting to about a millionth part of the
creature's weight, will gradually affect the constitutions of all
Razor-back Whales distributed over the Arctic seas and the North Atlantic
Ocean, from Greenland to the Equator? Is this a credible conclusion? For
three reasons, then, the hypothesis must be rejected.
Thus, the only reasonable interpretation is the inheritance of acquired
characters. If the effects of use and disuse, which are known causes of
change in each individual, influence succeeding individuals--if
functionally-produced modifications of structure are transmissible, as well
as modifications of structure otherwise arising--then this reduction of the
whale's hind limbs to minute rudiments is accounted for. The cause has been
unceasingly operative on all individuals of the species ever since the
transformation began.
In one case see all. If this cause has thus operated on the limbs of the
whale, it has thus operated in all creatures on all parts having active
functions.
* * * * *
At the outset I intimated that I must limit my replies to those arguments
of Professor Weismann which are contained in his first article. That those
contained in his second might be dealt with no less effectually, did time
and space permit, is manifest to me; but about the probability of this the
reader must form his own judgment. My replies thus far may be summed up as
follows:--
Professor Weismann says he has disproved the conclusion that degeneration
of the little toe has resulted from inheritance of acquired characters. But
his reasoning fails against an interpretation he overlooks. A profound
modification of the hind limbs and their appendages must have taken place
during the transition from arboreal habits to terrestrial habits; and
dwindling of the little toe is an obvious consequence of disuse, at the
same time that enlargement of the great toe is an obvious consequence of
increased use.
The entire argument based on the unlike forms and instincts presented by
castes of social insects is invalidated by an omission. Until probable
conclusions are reached respecting the characters which such insects
brought with them into the organized social state, no valid inferences can
be drawn respecting characters developed during that state.
A further large error of interpretation is involved in the assumption that
the different caste-characters are transmitted to them in the eggs laid by
the mother insect. While we have evidence that the unlike structures of the
sexes are determined by nutrition of the germ before egg-laying, we have
evidence that the unlike structures of classes are caused by unlikenesses
of nutrition of the larvæ. That these varieties of forms do not result from
varieties of germ-plasms, is demonstrated by the fact that where there are
varieties of germ-plasms, as in varieties of the same species of mammal, no
deviations in feeding prevent display of their structural results.
For such caste-modifications as those of the Amazon-ants, which are unable
to feed themselves, there is a feasible explanation other than Professor
Weismann's. The relation of common ants to their domestic animals--aphides
and coccids--which yield them food on solicitation, does not differ widely
from this relation between these Amazon-ants and their domestic
animals--the slave-ants. And the habit of being fed, contracted during the
first stages of their parasitic life, when there were perfect males and
females, may, during that stage, have become established by inheritance.
Meanwhile the opposed interpretation--that this incapacity has resulted
from the selection of those ant-communities the queens of which laid eggs
that had so varied as to entail this incapacity--implies that a scarcely
appreciable economy of nerve-matter advantaged the stirp so greatly as to
cause it to spread more than other stirps: an incredible supposition.
As the outcome of these alternative interpretations we saw that the
argument respecting the co-adaptation of co-operative parts, which
Professor Weismann thinks is furnished to him by the Amazon-ants,
disappears. The ancestral ants were conquering ants. These founded the
communities; and hence those members of the present communities which are
most like them are the Amazon-ants. If so, the co-adaptation of the
co-operative parts was effected by inheritance during the solitary and
semi-social stages. Even were there no such solution, the opposed solution
will be unacceptable. These simultaneous appropriate variations of the
co-operative parts in sizes, shapes, and proportions, are supposed to be
effected by simultaneous variations in the "determinants" of the
germ-plasms; and in the absence of an assigned physical cause, this implies
a fortuitous concourse of appropriate variations, which carries us back to
a "fortuitous concourse of atoms." This may just as well be extended to the
entire organism. The old hypothesis of special creations is more consistent
and comprehensible.
To rebut my inference drawn from the distribution of discriminativeness,
Professor Weismann uses not an argument but the blank form of an argument.
The ability to discriminate one twenty-fourth of an inch by the tongue-tip
_may_ have been useful to the ape: no conceivable use being even suggested.
And then the great body of my argument derived from the distribution of
discriminativeness over the skin, which amply suffices, is wholly ignored.
The tacit challenge I gave to name some facts in support of the hypothesis
of panmixia--or even a solitary fact--is passed by. It remains a pure
speculation having no basis but Professor Weismann's "opinion." When from
the abstract statement of it we pass to a concrete test, in the case of the
whale, we find that it necessitates an unproved and improbable assumption
respecting _plus_ and _minus_ variations; that it ignores the unceasing
tendency to reversion; and that it implies an effect out of all proportion
to the cause.
It is curious what entirely opposite conclusions men may draw from the same
evidence. Professor Weismann thinks he has shown that the "last bulwark of
the Lamarckian principle is untenable." Most readers will hold with me that
he is, to use the mildest word, premature in so thinking. Contrariwise my
impression is that he has not shown either this bulwark or any other
bulwark to be untenable; but rather that while his assault has failed it
has furnished opportunity for strengthening sundry of the bulwarks.
IV.
Among those who follow a controversy to its close, not one in a hundred
turns back to its beginning to see whether its chief theses have been dealt
with. Very often the leading arguments of one disputant, seen by the other
to be unanswerable, are quietly ignored, and attention is concentrated on
subordinate arguments to which replies, actually or seemingly valid, can be
made. The original issue is thus commonly lost sight of.
More than once I have pointed out that, as influencing men's views about
Education, Ethics, Sociology, and Politics, the question whether acquired
characters are inherited is the most important question before the
scientific world. Hence I cannot allow the discussion with Professor
Weismann to end in so futile a way as it will do if no summary of results
is made. Here, therefore, I propose to recapitulate the whole case in
brief. Primarily my purpose is to recall certain leading propositions
which, having been passed by unnoticed, remain outstanding. I will turn, in
the second place, to such propositions as have been dealt with; hoping to
show that the replies given are invalid, and consequently that these
propositions also remain outstanding.
But something beyond a summing-up is intended. A few pages at the close
will be devoted to setting forth new evidence which has come to light since
the controversy commenced--evidence which many will think sufficient in
itself to warrant a positive conclusion.
* * * * *
The fact that the tip of the fore finger has thirty times the power of
discrimination possessed by the middle of the back, and that various
intermediate degrees of discriminative power are possessed by various parts
of the skin, was set down as a datum for my first argument. The causes
which might be assigned for these remarkable contrasts were carefully
examined under all their aspects. I showed in detail that the contrasts
could not in any way be accounted for by natural selection. I further
showed that no interpretation of them is afforded by the alleged process of
panmixia: this has no _locus standi_ in the case. Having proved
experimentally, that ability of the fingers to discriminate is increased by
practice, and having pointed out that gradations of discriminativeness in
different parts correspond with gradations in the activities of the parts
as used for tactual exploration, I argued that these contrasts have arisen
from the organized and inherited effects of tactual converse with
surrounding things, varying in its degrees according to the positions of
the parts--in other words, that they are due to the inheritance of acquired
characters. As a crowning proof I instanced the case of the tongue-tip,
which has twice the discriminativeness of the forefinger-tip: pointing out
that consciously, or semi-consciously, or unconsciously, the tongue-tip is
perpetually exploring the inner surfaces of the teeth.
Singling out this last case, Professor Weismann made, or rather adopted
from Dr. Romanes, what professed to be a reply but was nothing more than
the blank form of a reply. It was said that though this extreme
discriminativeness of the tongue-tip is of no use to mankind, it may have
been of use to certain ancestral _primates_. No evidence of any such use
was given; no imaginable use was assigned. It was simply suggested that
there perhaps was a use.
In my rejoinder, after indicating the illusory nature of this proceeding
(which is much like offering a cheque on a bank where no assets have been
deposited to meet it), I pointed out that had the evidence furnished by the
tongue tip never been mentioned, the evidence otherwise furnished amply
sufficed. I then drew attention to the fact that this evidence had been
passed over, and tacitly inquired why.
No reply.[132]
* * * * *
In his essay on "The All-Sufficiency of Natural Selection," Professor
Weismann set out, not by answering one of the arguments I had used, but by
importing into the discussion an argument used by another writer, which it
was easy to meet. It had been contended that the smallness and deformity of
the little toe are consequent upon the effects of boot-pressure, inherited
from generation to generation. To this Professor Weismann made the
sufficient reply that the fusion of the phalanges and otherwise degraded
structure of the little toe, exist among peoples who go barefoot.
In my "Rejoinder" I said that though the inheritance of acquired characters
does not explain this degradation in the way alleged, it explains it in a
way which Professor Weismann overlooks. The cause is one which has been
operating ever since the earliest anthropoid creatures began to decrease
their life in trees and increase their life on the earth's surface. The
mechanics of walking and running, in so far as they concern the question at
issue, were analyzed; and it was shown that effort is economized and
efficiency increased in proportion as the stress is thrown more and more on
the inner digits of the foot and less and less on the outer digits. So that
thus the foot furnishes us simultaneously with an instance of increase from
use and of decrease from disuse; a further disproof being yielded of the
allegation that co-operative parts vary together, since we have here
co-operative parts of which one grows while the other dwindles.
I ended by pointing out that, so far from strengthening his own case,
Professor Weismann had, by bringing into the controversy this changed
structure of the foot, given occasion for strengthening the opposite case.
No reply.
* * * * *
We come now to Professor Weismann's endeavour to disprove my second
thesis--that it is impossible to explain by natural selection alone the
co-adaptation of co-operative parts. It is thirty years since this was set
forth in _The Principles of Biology_. In § 166 I instanced the enormous
horns of the extinct Irish elk, and contended that in this, and in kindred
cases, where for the efficient use of some one enlarged part many other
parts have to be simultaneously enlarged, it is out of the question to
suppose that they can have all spontaneously varied in the required
proportions. In "The Factors of Organic Evolution," by way of enforcing
this argument, which had, so far as I know, never been met, I dwelt upon
the aberrant structure of the giraffe. And then, in the essay which
initiated this controversy, I brought forward yet a third case--that of an
animal which, previously accustomed only to walking, acquires the power of
leaping.
In the first of his articles in the _Contemporary Review_ (September,
1893), Professor Weismann made no direct reply, but he made an indirect
reply. He did not attempt to show how there could have taken place in the
stag the "harmonious variation of the different parts that co-operate to
produce one physiological result" (p. 311); but he contended that such
harmonious variation _must_ have taken place, because the like has taken
place in "the neuters of state-forming insects"--"animal forms which do not
reproduce themselves, but are always propagated anew by parents which are
unlike them" (p. 313), and which therefore cannot have transmitted acquired
characters. Singling out those soldier-neuters which exist among certain
kinds of ants, he described (p. 318) the many co-ordinated parts required
to make their fighting organs efficient. He then argued that the required
simultaneous changes can "only have arisen by a selection of the
parent-ants dependent on the fact that those parents which produced the
best workers had always the best prospect of the persistence of their
colony. No other explanation is conceivable; _and it is just because no
other explanation is conceivable, that it is necessary for us to accept the
principle of natural selection_" (pp. 318-9).
[This passage initiated a collateral controversy, which, as continually
happens, has greatly obscured the primary controversy. It became a question
whether these forms of neuter insects have arisen as Professor Weismann
assumes, or whether they have arisen from arrested development consequent
upon innutrition. To avoid entanglements I must for the present pass over
this collateral controversy, intending to resume it presently, when the
original issues have been dealt with.]
No one will suspect me of thinking that the inconceivability of the
negation is not a valid criterion, since, in "The Universal Postulate,"
published in the _Westminster Review_ in 1852 and afterwards in _The
Principles of Psychology_, I contended that it is the ultimate test of
truth. But then in every case there has to be determined the question--Is
the negation inconceivable; and in assuming that it is so in the case
named, lies the fallacy of the above-quoted passage. The three separate
ways in which I dealt with this position of Professor Weismann are as
follows:--
If we admit the assumption that the form of the soldier-ant has been
developed since the establishment of the organized ant-community in which
it exists, Professor Weismann's assertion that no other process than that
which he alleges is conceivable, is true. But I pointed out that this
assumption is inadmissible; and that no valid conclusion respecting the
genesis of the soldier-ant can be drawn without postulating either the
ascertained, or the probable, structure of those pre-social, or
semi-social, ants from which the organized social ants have descended. I
went on to contend that the pre-social type must have been a conquering
type, and that therefore in all probability the soldier-ants represent most
nearly the structures of those ancestral ants which existed when the
society had perfect males and females and could transmit acquired
characters, while the other members of the existing communities are
degraded forms of the type.
No reply.
A further argument I used was that where there exist different castes among
the neuter-ants, as those seen in the soldiers and workers of the Driver
ants of West Africa, "they graduate insensibly into each other" alike in
their sizes and in their structures; and that Professor Weismann's
hypothesis implies a special set of "determinants" for each intermediate
form. Or if he should say that the intermediate forms result from mixtures
of the determinants of the two extreme forms, there still remains the
further difficulty that natural selection has maintained, for innumerable
generations, these intermediate forms which are injurious deviations from
the useful extreme forms.
No reply.
One further reason--fatal it seems to me--was urged in bar of his
interpretation. No physical cause has been, or can be, assigned, why in the
germ-plasm of any particular queen-ant, the "determinants" initiating these
various co-operative organs, all simultaneously vary in fitting ways and
degrees, and still less why there occur such co-ordinated variations
generation after generation, until by their accumulated results these
efficient co-operative structures have been evolved. I pointed out that in
the absence of any assigned or assignable physical cause, it is necessary
to assume a fortuitous concurrence of favourable variations, which means "a
fortuitous concourse of atoms;" and that it would be just as rational, and
much more consistent, to assume that the structure of the entire organism
thus resulted.
No reply.
* * * * *
It is reasonable to suspect that Professor Weismann recognized these
difficulties as insuperable, for, in his Romanes Lecture on "The Effect of
External Influences upon Development," instead of his previous indirect
reply, he makes a direct reply. Reverting to the stag and its enlarging
horns, he alleges a process by which, as he thinks, we may understand how,
by variation and selection, all the bones and muscles of the neck, of the
thorax, and of the fore-legs, are step by step adjusted in their sizes to
the increasing sizes of the horns. He ascribes this harmonization to the
internal struggle for nutriment, and that survival of the fittest which
takes place among the parts of an organism: a process which he calls
"_intra-individual_-selection, or more briefly--_intra-selection_" (p. 12).
"Wilhelm Roux has given an explanation of the cause of these wonderfully
fine adaptations by applying the principle of selection to the parts of
the organism. Just as there is a struggle for survival among the
individuals of a species, and the fittest are victorious, so also do even
the smallest living particles contend with one another, and those that
succeed best in securing food and place grow and multiply rapidly, and so
displace those that are less suitably equipped" (p. 12).[133]
That I do not explain as he does the co-adaptation of co-operative parts,
Professor Weismann ascribes to my having overlooked this "principle of
intra-selection"--an unlucky supposition, as we see. But I do not think
that when recognizing it a generation ago, I should have seen its relevancy
to the question at issue, had that issue then been raised, and I certainly
do not see it now. Full reproduction of Professor Weismann's explanation is
impracticable, for it occupies several pages, but here are the essential
sentences from it:--
"The great significance of intra-selection appears to me not to depend on
its producing structures that are directly transmissible,--it cannot do
that,--but rather consists in its causing a development of the
germ-structure, acquired by the selection of individuals, which will be
suitable to varying conditions.... We may therefore say that
intra-selection effects the adaptation of the individual to its chance
developmental conditions,--the suiting of the hereditary primary
constituents to fresh circumstances" (p. 16).... "But as the primary
variations in the phyletic metamorphosis occurred little by little, the
secondary adaptations would probably as a rule be able to keep pace with
them. Time would thus be gained till, in the course of generations, by
constant selection of those germs the primary constituents of which are
best suited to one another, the greatest possible degree of harmony may
be reached, and consequently a definitive metamorphosis of the species
involving all the parts of the individual may occur" (p. 19).
The connecting sentences, along with those which precede and succeed, would
not, if quoted, give to the reader clearer conceptions than these by
themselves give. But when disentangled from Professor Weismann's involved
statements, the essential issues are, I think, clear enough. In the case of
the stag, that daily working together of the numerous nerves, muscles, and
bones concerned, by which they are adjusted to the carrying and using of
somewhat heavier horns, produces on them effects which, as I hold, are
inheritable, but which, as Professor Weismann holds, are not inheritable.
If they are not inheritable, what must happen? A fawn of the next
generation is born with no such adjustment of nerves, muscles and bones as
had been produced by greater exercise in the parent, and with no tendency
to such adjustment. Consequently if, in successive generations, the horns
go on enlarging, all these nerves, muscles, and bones, remaining of the
original sizes, become utterly inadequate. The result is loss of life: the
process of adaptation fails. "No," says Professor Weismann, "we must
conclude that the germ-plasm has varied in the needful manner." How so? The
process of "intra-individual selection," as he calls it, can have had no
effect, since the cells of the soma cannot influence the reproductive
cells. In what way, then, has the germ-plasm gained the characters required
for producing simultaneously all these modified co-operative parts. Well,
Professor Weismann tells us merely that we must suppose that the germ-plasm
acquires a certain sensitiveness such as gives it a proclivity to
development in the requisite ways. How is such proclivity obtainable? Only
by having a multitude of its "determinants" simultaneously changed in fit
modes. Emphasizing the fact that even a small failure in any one of the
co-operative parts may be fatal, as the sprain of an over-taxed muscle
shows us, I alleged that the chances are infinity to one against the
needful variations taking place at the same time. Divested of its
elaboration, its abstract words and technical phrases, the outcome of
Professor Weismann's explanation is that he accepts this, and asserts that
the infinitely improbable thing takes place!
Either his argument is a disguised admission of the inheritableness of
acquired characters (the effects of "intra-selection") or else it is, as
before, the assumption of a fortuitous concourse of favourable variations
in the determinants--"a fortuitous concourse of atoms."
* * * * *
Leaving here this main issue, I return now to that collateral issue named
on a preceding page as being postponed--whether the neuters among social
insects result from specially modified germ-plasms or whether they result
from the treatment received during their larval stages.
For the substantiation of his doctrine Professor Weismann is obliged to
adopt the first of these alternatives; and in his Romanes Lecture he found
it needful to deal with the evidence I brought in support of the second
alternative. He says that "poor feeding is not the _causa efficiens_ of
sterility among bees, but is merely the stimulus which _not only results in
the formation of rudimentary ovaries, but at the same time calls forth all
the other distinctive characters of the workers_" (pp. 29-30); and he says
this although he has in preceding lines admitted that it is "true of all
animals that they reproduce only feebly or not at all when badly and
insufficiently nourished:" a known cause being thus displaced by a supposed
cause. But Professor Weismann proceeds to justify his interpretation by
experimentally-obtained evidence.
He "reared large numbers of the eggs of a female blow-fly"; the larvæ of
some he fed abundantly, but the larvæ of others sparingly; and eventually
he obtained, from the one set flies of full size, and from the other small
flies. Nevertheless the small flies were fertile, as well as the others.
Here, then, was proof that innutrition had not produced infertility; and he
contends that therefore among the neuter social insects, infertility has
not resulted from innutrition. The argument seems strong, and to many will
appear conclusive; but there are two differences which entirely vitiate the
comparison Professor Weismann institutes.
One of them has been pointed out by Mr. Cunningham. In the case of the
blow-fly the food supplied to the larvæ though different in quantity was
the same in quality; in the case of the social insects the food supplied,
whether or not different in quantity, differs in quality. Among bees,
wasps, ants, &c., the larvæ of the reproductive forms are fed upon a more
nitrogenous food than are the larvæ of the workers; whereas the two sets of
larvæ of the blow-fly, as fed by Professor Weismann, were alike supplied
with highly nitrogenous food. Hence there did not exist the same cause for
non-development of the reproductive organs. Here, then, is one vitiation of
the supposed parallel. There is a second.
While the development of an embryo follows in a rude way the phyletic
metamorphoses passed through by its ancestry, the order of development of
organs is often gradually modified by the needs of particular species: the
structures being developed in such order as conduces to self-sustentation
and the welfare of offspring. Among other results there arise differences
in the relative dates of maturity of the reproductive system and of the
other systems. It is clear, _à priori_, that it must be fatal to a species
if offspring are habitually produced before the conditions requisite for
their survival are fulfilled. And hence, if the life is a complex one, and
the care taken of offspring is great, reproduction must be much longer
delayed than where the life is simple and the care of offspring absent or
easy. The contrast between men and oxen sufficiently illustrates this
truth. Now the subordination of the order of development of parts to the
needs of the species, is conspicuously shown in the contrast between these
two kinds of insects which Professor Weismann compares as though their
requirements were similar. What happens with the blow fly? If it is able to
suck up some nutriment, to fly tolerably, and to scent out dead flesh,
various of its minor organs may be more or less imperfect without
appreciable detriment to the species: the eggs can be laid in a fit place,
and that is all that is wanted. Hence it profits the species to have the
reproductive system developed comparatively early--in advance, even, of
various less essential parts. Quite otherwise is it with social insects,
which take such remarkable care of their young; or rather to make the case
parallel--quite otherwise is it with those types from which the social
insects have descended, bringing into the social state their inherited
instincts and constitutions. Consider the doings of the mason-wasp, or
mason-bee, or those of the carpenter-bee. What, in these cases, must the
female do that she may rear members of the next generation? There is a fit
place for building or burrowing to be chosen; there is the collecting
together of grains of sand and cementing them into a strong and water-proof
cell, or there is the burrowing into wood and there building several cells;
there is the collecting of food to place along with the eggs deposited in
these cells, solitary or associated, including that intelligent choice of
small caterpillars which, discovered and carried home, are carefully packed
away and hypnotized by a sting, so that they may live until the growing
larva has need of them. For all these proceedings there have to be provided
the fit external organs--cutting instruments, &c., and the fit internal
organs--complicated nerve-centres in which are located these various
remarkable instincts, and ganglia by which these delicate operations have
to be guided. And these special structures have, some if not all of them,
to be made perfect and brought into efficient action before egg-laying
takes place. Ask what would happen if the reproductive system were active
in advance of these ancillary appliances. The eggs would have to be laid
without protection or food, and the species would forthwith disappear. And
if that full development of the reproductive organs which is marked by
their activity, is not needful until these ancillary organs have come into
play, the implication, in conformity with the general law above indicated,
is that the perfect development of the reproductive organs will take place
later than that of these ancillary organs, and that if innutrition checks
the general development, the reproductive organs will be those which
chiefly suffer. Hence, in the social types which have descended from these
solitary types, this order of evolution of parts will be inherited, and
will entail the results I have inferred.
If only deductively reached, this conclusion would, I think, be fully
justified. But now observe that it is more than deductively reached. It is
established by observation. Professor Riley, Ph.D., late Government
Entomologist of the United States, in his annual address as President of
the Biological Society of Washington,[134] on January 29, 1894, said:--
"Among the more curious facts connected with these Termites, because of
their exceptional nature, is the late development of the internal sexual
organs in the reproductive forms." (p. 34.)
Though what has been shown of the Termites has not been shown of the other
social insects, which belong to a different order, yet, considering the
analogies between their social states and between their constitutional
requirements, it is a fair inference that what holds in the one case holds
partially, if not fully, in the other. Should it be said that the larval
forms do not pass into the pupa state in the one case as they do in the
other, the answer is that this does not affect the principle. The larva
carries into the pupa state a fixed quantity of tissue-forming material for
the production of the imago. If the material is sufficient, then a complete
imago is formed. If it is not sufficient, then, while the earlier formed
organs are not affected by the deficiency, the deficiency is felt when the
latest formed organs come to be developed, and they are consequently
imperfect.
Even if left without reply, Professor Weismann's interpretation commits him
to some insuperable difficulties, which I must now point out.
Unquestionably he has "the courage of his opinions;" and it is shown
throughout this collateral discussion as elsewhere. He is compelled by
accumulated evidence to admit "that there is only _one_ kind of egg from
which queens and workers as well as males arise."[135] But if the
production of one or other form from the same germ does not result from
speciality of feeding, what does it result from? Here is his reply:--
"We must rather suppose that the primary constituents of two distinct
reproductive systems--_e. g._ those of the queen and worker--are
contained in the germ-plasm of the egg."[136]
"The courage of his opinions," which Professor Weismann shows in this
assumption, is, however, quite insufficient. For since he himself has just
admitted that there is only one kind of egg for queens, workers, and males,
he must at any rate assume three sets of "determinants." (I find that on a
subsequent page he does so.) But this is not enough, for there are, in many
cases, two if not more kinds of workers, which implies that four sets of
determinants must co-exist in the same egg. Even now we have not got to the
extent of the assumption required. In the address above referred to on
"Social Insects from Psychical and Evolutional Points of View," Professor
Riley gives us (p. 33) the--
_Forms in a Termes Colony under Normal Conditions._
1. Youngest larvæ.
/ \
/ \
/ \
2. Larvæ [of those] unfit 3. Larvæ [that will be] fit
for reproduction. for reproduction.
/ \ / \
/ \ / \
4. Larvæ of 5. Larvæ of 8. Nymphs of 9. Nymphs of 2nd
workers. soldiers. 1st form. form.
| | |
6. Workers. 7. Soldiers. 10. Winged forms.
|
11. True royal pairs.
Hence as, in this family tree, the royal pair includes male and female, it
results that there are _five_ different adult forms (Grassi says there are
two others) arising from like eggs or larvæ; and Professor Weismann's
hypothesis becomes proportionately complicated. Let us observe what the
complications are.
It often happens in controversy--metaphysical controversy more than any
other--that propositions are accepted without their terms having been
mentally represented. In public proceedings documents are often "taken as
read," sometimes with mischievous results; and in discussions propositions
are often taken as thought when they have not been thought and cannot be
thought. It sufficiently taxes imagination to assume, as Professor Weismann
does, that two sets of "ids" or of "determinants" in the same egg are,
throughout all the cell-divisions which end in the formation of the
_morula_, kept separate, so that they may subsequently energize
independently; or that if they are not thus kept separate, they have the
power of segregating in the required ways. But what are we to say when
three, four, and even five sets of "ids" or bundles of "determinants" are
present? How is dichotomous division to keep these sets distinct; or if
they are not kept distinct, what shall we say to the chaos which must arise
after many fissions, when each set in conflict with the others strives to
produce its particular structure? And how are the conquering determinants
to find they ways out of the _mêlée_ to the places where they are to fulfil
their organizing functions? Even were they all intelligent beings and each
had a map by which to guide his movements, the problem would be
sufficiently puzzling. Can we assume it to be solved by unconscious units?
Thus even had Professor Weismann shown that the special structures of the
different individuals in an insect-community are not due to differences in
the nurtures they receive, which he has failed to do, he would still be met
by this difficulty in the way of his own view, in addition to the three
other insuperable difficulties grouped together in a preceding section.
* * * * *
The collateral issue, which has occupied the largest space in the
controversy, has, as commonly happens, begotten a second generation of
collateral issues. Some of these are embodied in the form of questions put
to me, which I must here answer, lest it should be supposed that they are
unanswerable and my view therefore untenable.
In the notes he appends to his Romanes Lecture, Professor Weismann
writes:--
"One of the questions put to Spencer by Ball is quite sufficient to show
the utter weakness of the position of Lamarckism:--if their
characteristics did not arise among the workers themselves, but were
transmitted from the pre-social time, how does it happen that the queens
and drones of every generation can give anew to the workers the
characteristics which they themselves have long ago lost?" (p. 68).
It is curious to see put forward in so triumphant a manner, by a professed
naturalist, a question so easily disposed of. I answer it by putting
another. How does it happen that among those moths of which the female has
but rudimentary wings, she continues to endow the males of her species with
wings? How does it happen, for example, that among the _Geometridæ_, the
peculiar structures and habits of which show that they have all descended
from a common ancestor, some species have winged females and some wingless
females; and that though they have lost the wings the ancestral females
had, these wingless females convey to the males the normal developments of
wings? Or, still better, how is it that in the _Psychidæ_ there are
apterous worm-like females, which lay eggs that bring forth winged males of
the ordinary imago form? If for males we read workers, the case is parallel
to the cases of those social insects, the queens of which bequeath
characteristics they have themselves lost. The ordinary facts of embryonic
evolution yield us analogies. What is the most common trait in the
development of the sexes? When the sexual organs of either become
pronounced, the incipient ancillary organs belonging to the opposite sex
cease to develop and remain rudiments, while the organs special to the sex,
essential and nonessential, become fully developed. What, then, must happen
with the queen-ant, which, through countless generations, has ceased to use
certain structures and has lost them from disuse? If one of the eggs which
she lays, capable, as Professor Weismann admits, of becoming queen, male,
or worker of one or other kind, does not at a certain stage begin actively
to develop its reproductive system, then those organs of the ancestral or
pre-social type which the queen has lost begin to develop, and a worker
results.
Another difficulty in the way of my view, supposed to be fatal, is that
presented by the Honey-ants--aberrant members of certain ant-colonies which
develop so enormously the pouch into which the food is drawn, that the
abdomen becomes little else than a great bladder out of which the head,
thorax, and legs protrude. This, it is thought, cannot be accounted for
otherwise than as a consequence of specially endowed eggs, which it has
become profitable to the community for the queen to produce. But the
explanation fits in quite easily with the view I have set forth. No one
will deny that the taking in of food is the deepest of vital requirements,
and the correlative instinct a dominant one; nor will any one deny that the
instinct of feeding young is less deeply seated--comes later in order of
time. So, too, every one will admit that the worker-bee or worker-ant
before regurgitating food into the mouth of a larva must first of all take
it in. Hence, alike in order of time and necessity, it is to be assumed
that development of the nervous structures which guide self-nutrition,
precedes development of the nervous structures which guide the feeding of
larvæ. What, then, will in some cases happen, supposing there is an
arrested development consequent on innutrition? It will in some cases
happen that while the nervous centres prompting and regulating deglutition
are fully formed, the formation of those prompting and regulating the
regurgitation of the food into the mouths of larvæ are arrested. What will
be the consequence? The life of the worker is mainly passed in taking in
food and putting it out again. If the putting out is stopped its life will
be mainly passed in taking in food. The receptacle will go on enlarging and
it will eventually assume the monstrous form that we see.[137]
Here, however, to exclude misinterpretations, let me explain. I by no means
deny that variation and selection have produced, in these
insect-communities, certain effects such as Mr. Darwin suggested. Doubtless
ant-queens vary; doubtless there are variations in their eggs; doubtless
differences of structure in the resulting progeny sometimes prove
advantageous to the stirp, and originate slight modifications of the
species. But such changes, legitimately to be assumed, are changes in
single parts--in single organs or portions of organs. Admission of this
does not involve admission that there can take place numerous correlated
variations in different and often remote parts, which must take place
simultaneously or else be useless. Assumption of this is what Professor
Weismann's argument requires, and assumption of this we have seen to be
absurd.
Before leaving the general problem presented by the social insects, let me
remark that the various complexities of action not explained by inheritance
from pre-social or semi-social types, are probably due to accumulated and
transmitted knowledge. I recently read an account of the education of a
butterfly, carried to the extent that it became quite friendly with its
protector and would come to be fed. If a non-social and relatively
unintelligent insect is capable of thus far consciously adjusting its
actions, then it seems a reasonable supposition that in a community of
social insects there has arisen a mass of experience and usage into which
each new individual is initiated; just as happens among ourselves. We have
only to consider the chaos which would result were we suddenly bereft of
language, and if the young were left to grow up without precept and
example, to see that very probably the polity of an insect community is
made possible by the addition of intelligence to instinct, and the
transmission of information through sign-language.
* * * * *
There remains now the question of _panmixia_, which stands exactly where it
did when I published the "Rejoinder to Professor Weismann."
After showing that the interpretation I put upon his view was justified by
certain passages quoted; and after pointing out that one of his adherents
had set forth the view which I combated--if not as his view yet as
supplementary to it; I went on to criticize the view as set forth afresh by
Professor Weismann himself. I showed that as thus set forth the actuality
of the supposed cause of decrease in disused organs, implies that _minus_
variations habitually exceed _plus_ variations--in degree or in number, or
in both. Unless it can be proved that such an excess ordinarily occurs, the
hypothesis of _panmixia_ has no place; and I asked, where is the proof that
it occurs.
No reply.
Not content with this abstract form of the question I put it also in a
concrete form, and granted for the nonce Professor Weismann's assumption:
taking the case of the rudimentary hind limbs of the whale. I said that
though, during those early stages of decrease in which the disused limbs
were external, natural selection probably had a share in decreasing them,
since they were then impediments to locomotion, yet when they became
internal, and especially when they had dwindled to nothing but remnants of
the femurs, it is impossible to suppose that natural selection played any
part: no whale could have survived and initiated a more prosperous stirp in
virtue of the economy achieved by such a decrease. The operation of natural
selection being out of the question, I inquired whether such a decrease,
say of one-half when the femurs weighed a few ounces, occurring in one
individual, could be supposed in the ordinary course of reproduction to
affect the whole of the whale species inhabiting the Arctic Seas and the
North Atlantic Ocean; and so on with successive diminutions until the
rudiments had reached their present minuteness. I asked whether such an
interpretation could be rationally entertained.
No reply.
Now in the absence of replies to these two questions it seems to me that
the verdict must go against Professor Weismann by default. If he has to
surrender the hypothesis of _panmixia_, what results? All that evidence
collected by Mr. Darwin and others, regarded by them as proof of the
inheritance of acquired characters, which was cavalierly set aside on the
strength of this alleged process of panmixia, is reinstated. And this
reinstated evidence, joined with much evidence since furnished, suffices to
establish the repudiated interpretation.
In the printed report of his Romanes Lecture, after fifty pages of
complicated speculations which we are expected to accept as proofs,
Professor Weismann ends by saying, in reference to the case of the neuter
insects:--
"This case is of additional interest, as it may serve to convince those
naturalists who are still inclined to maintain that acquired characters
are inherited, and to support the Lamarckian principle of development,
that their view cannot be the right one. It has not proved tenable in a
single instance" (p. 54).
Most readers of the foregoing pages will think that since Professor
Weismann has left one after another of my chief theses without reply, this
is rather a strong assertion; and they will still further raise their
eyebrows on remembering that, as I have shown, where he has given answers
his answers are invalid.
* * * * *
And now we come to the additions which I indicated at the outset as having
to be made--certain evidences which have come to light since this
controversy commenced.
When, by a remembered observation made in boyhood, joined with the familiar
fact that worker-larvæ can be changed into the larvæ of queens by feeding,
I was led to suggest that probably all the variations of form in the social
insects are consequent on differences of nurture, I was unaware that
observations and experiments were being made which have justified this
suggestion. Professor Grassi has recently published accounts of the
food-habits of two European species of Termites, shewing that the various
forms are due to feeding. He is known to be a most careful observer, and
some of the most curious of his facts are confirmed by the collection of
white ants exhibited by Dr. David Sharp, F.R.S., at the _soirée_ of the
Royal Society in May last. He has favoured me with the following account of
Grassi's results, which I publish with his assent:--
"There is great variety as to the constituents of the community and
economy of the species in White Ants. One of the simplest conditions
known is that studied by Grassi in the case of the European species
Calotermes flavicollis. In this species there is no worker caste; the
adult forms are only of two kinds, viz., soldiers, and the males and
females; the sexes are externally almost indistinguishable, and there are
males and females of soldiers as well as of the winged forms, though the
sexual organs do not undergo their full development in any soldier
whether male or female.
"The soldier is not however a mere instance of simple arrested
development. It is true that there is in it arrested development of the
sexual organs, but this is accompanied by change of form of other
parts--changes so extreme that one would hardly suppose the soldier to
have any connection with either the young or the adult of the winged
forms.
"Now according to Grassi the whole of the individuals when born are
undifferentiated forms (except as to sex), and each one is capable of
going on the natural course of development and thus becoming a winged
insect, or can be deviated from this course and made into a soldier; this
is accomplished by the White Ants by special courses of feeding.
"The evidence given by Grassi is not conclusive as to the young being all
born alike; and it may be that there are some individuals born that could
not be deviated from the natural course and made into soldiers. But there
is one case which seems to show positively that the deviation Grassi
believes to occur is real, and not due to the selection by the ants of an
individual that though appearing to our eyes undifferentiated is not
really so. This is that an individual can be made into a soldier after it
has visibly undergone one half or more of the development into a winged
form. The Termites can in fact operate on an individual that has already
acquired the rudiments of wings and whose head is totally destitute of
any appearance of the shape of the armature peculiar to the soldier, and
can turn it into a soldier; the rudiments of the wings being in such a
case nearly entirely re-absorbed."
Grassi has been for many years engaged in investigating these phenomena,
and there is no reason for rejecting his statement. We can scarcely avoid
accepting it, and if so, Professor Weismann's hypothesis is conclusively
disposed of. Were there different sets of "determinants" for the
soldier-form and for the winged sexual form, those "determinants" which had
gone a long way towards producing the winged sexual form, would inevitably
go on to complete that form, and could not have their proclivity changed by
feeding.
[Yet more evidence to the like effect has since become known. At the
meeting of the Entomological Society, on March 14, 1894 (reported in
_Nature_, March 29):--
"Dr. D. Sharp, F.R.S., exhibited a collection of white ants (_Termites_),
formed by Mr. G. D. Haviland in Singapore, which comprised about twelve
species, of most of which the various forms were obtained. He said that
Prof. Grassi had recently made observations on the European species, and
had brought to light some important particulars; and also that in the
discussion that had recently been carried on between Mr. Herbert Spencer
and Prof. Weismann, the former had stated that in his opinion the
different forms of social insects were produced by nutrition. Prof.
Grassi's observations showed this view to be correct, and the specimens
now exhibited confirmed one of the most important points in his
observations. Dr. Sharp also stated that Mr. Haviland found in one nest
eleven neoteinic queens--that is to say, individuals having the
appearance of the queen in some respects, while in others they are still
immature."
Another similarly conclusive verification I published in _Nature_ for
December 6, 1894, under the title "The Origin of Classes among the
'Parasol' Ants." The letter ran as follows:--
"Mr. J. H. Hart is Superintendent of the Royal Botanic Gardens in
Trinidad. He has sent me a copy of his report presented to the
Legislative Council in March, 1893, and has drawn my attention to certain
facts contained in it concerning the 'Parasol' ants--the leaf-cutting
ants which feed on the fungi developed in masses of the cut leaves
carried to their nests. Both Mr. Bates and Mr. Belt described these ants,
but described, it seems, different, though nearly allied, species, the
habits of which are partially unlike. As they are garden-pests, Mr. Hart
was led to examine into the development and social arrangements of these
ants; establishing, to that end, artificial nests, after the manner
adopted by Sir John Lubbock. Several of the facts set down have an
important bearing on a question now under discussion. The following
extracts, in which they are named, I abridge by omitting passages not
relevant to the issue:--
"'The history of my nests is as follows: Nos. 1 and 2 were both taken
(August 9) on the same day, while destroying nests in the Gardens, and
were portions of separate nests but of the same species. No. 3 was
procured on September 5, and is evidently a different although an allied
species to Nos. 1 and 2.
"'Finding neither of my nests had a queen, I procured one from another
nest about to be destroyed, and placed it with No. 1 nest. It was
received by the workers, and at once attended by a numerous retinue in
royal style. On August 30 I removed the queen from No. 1 and placed it
with No. 2, when it was again received in a most loyal manner....
"'Ants taken from Nos. 1 and 2 and placed with No. 3 were immediately
destroyed by the latter, and even the soldiers of No. 3, as well as
workers or nurses, were destroyed when placed with Nos. 1 and 2.
"'In nest No. 2, from which I removed the queen on August 30, there are
now in the pupa stage several queens and several males. The forms of ant
in nests Nos. 1 and 2 are as follows: (_a_) queen, (_b_) male (both
winged, but the queen loses its wings after marital flight), (_c_) large
workers, (_d_) small workers, and (_e_) nurses. In nest No. 3 I have not
yet seen the queen or male, but it possesses--(_a_) soldier, (_b_) larger
workers, (_c_) smaller workers, and (_d_) nurses; but these are different
in form to those of nests No. 1 and No. 2. Probably we might add a third
form of worker, as there are several sizes in the nest....
"'It is curious that in No. 1 nest, from which the queen was removed on
August 30, new queens and males are now being developed, while in No. 2
nest, where the queen is at present, nothing but workers have been
brought out, and if a queen larva or pupa is placed there it is at once
destroyed, while worker larvæ or pupæ are amicably received. In No. 3 all
the eggs, larvæ, and pupæ collected with the nest have been hatched, and
no eggs have since made their appearance to date. There is no queen with
this nest.... On November 14 I attempted to prove by experiment how small
a number of "parasol" ants it required to form a new colony. I placed two
dozen of ants (one dozen workers and one dozen nurses) in two separate
nests, No. 4 and No. 5. With No. 4 I placed a few larvæ with a few rose
petals for them to manipulate. With No. 5 I gave a small piece of nest
covered with mycelium. On the 16th these nests were destroyed by small
foraging ants, known as the "sugar" or "meat" ant, and I had to remove
them and replace with a new colony. My notes on these are not
sufficiently lengthy to be of much importance. But I noted four eggs laid
on the 16th, or two days after being placed in their new quarters; no
queen being present. The experiment is being continued. I may mention
that in No. 4 nest, in which no fungus was present, the larvæ of all
sizes appeared to change into the pupæ stage at once for want of food [a
fact corresponding with the fact I have named as observed by myself sixty
years ago in the case of wasp larvæ]. The circumstance tends to show that
the development of the insect is influenced entirely by the feeding it
gets in the larva stage.
"'In nest No. 2 before the introduction of a queen there were no eggs or
larvæ. The first worker was hatched on October 27, or fifty-seven days
afterwards, and a continual succession has since been maintained, but as
yet (November 19) no males or queens have made their appearance.'
"In a letter accompanying the report, Mr. Hart says:--
"'Since these were published, my notes go to prove that ants can
practically manufacture at will, male, female, soldier, worker, or nurse.
Some of the workers are capable of laying eggs, and from these can be
produced all the various forms as well as from a queen's egg.
"'There does not, however, appear to be any difference in the character
of the food; as I cannot find that the larger larvæ are fed with anything
different to that given to the smaller.'
"These results were obtained before the recent discussion of the question
commenced, and joined with the other evidence entirely dispose of those
arguments which Prof. Weismann bases on facts furnished by the social
insects."]
The other piece of additional evidence I have referred to, is furnished by
two papers contributed to _The Journal of Anatomy and Physiology_ for
October 1893 and April 1894, by R. Havelock Charles, M. D., &c. &c.,
Professor of Anatomy in the Medical College, Lahore. These papers set forth
the differences between the leg-bones of Europeans and those of the Punjaub
people--differences caused by their respective habits of sitting in chairs
and squatting on the ground. He enumerates more than twenty such
differences, chiefly in the structures of the knee-joint and ankle-joint.
From the _résumé_ of his second paper I quote the following passages, which
sufficiently show the data and the inferences:--
"7. The habits as to sitting postures of Europeans differ from those of
their prehistoric ancestors, the Cave-dwellers, &c., who probably
squatted on the ground.
"8. The sitting postures of Orientals are the same now as ever. They have
retained the habits of their ancestors. The Europeans have not done so.
"9. Want of use would induce changes in form and size, and so, gradually,
small differences would be integrated till there would be total
disappearance of the markings on the European skeleton, as no advantage
would accrue to him from the possession of facets on his bones fitting
them for postures not practised by him.
"10. The facets seen on the bones of the Panjabi infant or foetus have
been transmitted to it by the accumulation of peculiarities gained by
habit in the evolution of its racial type--in which an acquisition having
become a permanent possession, 'profitable to the individual under its
conditions of life,' is transmitted as a useful inheritance.
"11. These markings are due to the influence of certain positions, which
are brought about by the use of groups of muscles, and they are the
definite results produced by actions of these muscles.
"12. The effects of the use of the muscles mentioned in No. 11 are
transmitted to the offspring, for the markings are present in the
_foetus-in-utero_, in the child at birth, and in the infant.
"13. The markings are instances of the transmission of acquired
characters, which heritage in the individual, function subsequently
develops."
No other conclusion appears to me possible. _Panmixia_, even were it not
invalidated by its unwarranted assumption as above shown, would be out of
court: the case is not a case of either increase or decrease of size but of
numerous changes of form. Simultaneous variation of co-operative parts
cannot be alleged, since these co-operative parts have not changed in one
way but in various ways and degrees. And even were it permissible to
suppose that the required different variations had taken place
simultaneously, natural selection cannot be supposed to have operated. The
assumption would imply that in the struggle for existence, individuals of
the European races who were less capable than others of crouching and
squatting, gained by those minute changes of structure which incapacitated
them, such advantages that their stirps prevailed over other stirps--an
absurd supposition.
And now I must once more point out that a grave responsibility rests on
biologists in respect of the general question; since wrong answers lead,
among other effects, to wrong beliefs about social affairs and to
disastrous social actions. In me this conviction has unceasingly
strengthened. Though _The Origin of Species_ proved to me that the
transmission of acquired characters cannot be the sole factor in organic
evolution, as I had assumed in _Social Statics_ and in _The Principles of
Biology_, published in pre-Darwinian days, yet I have never wavered in the
belief that it is a factor and an all-important factor. And I have felt
more and more that since all the higher sciences are dependent on the
science of life, and must have their conclusions vitiated if a fundamental
datum given to them by the teachers of this science is erroneous, it
behoves these teachers not to let an erroneous datum pass current: they are
called on to settle this vexed question one way or other. The times give
proof. The work of Mr. Benjamin Kidd on _Social Evolution_, which has been
so much lauded, takes Weismannism as one of its data; and if Weismannism be
untrue, the conclusions Mr. Kidd draws must be in large measure erroneous
and may prove mischievous.
POSTSCRIPT.--Since the foregoing pages have been put in type there has
appeared in _Natural Science_ for September, an abstract of certain parts
of a pamphlet by Professor Oscar Hertwig, setting forth facts directly
bearing on Professor Weismann's doctrine respecting the distinction between
reproductive cells and somatic cells. In _The Principles of Biology_, § 77,
I contended that reproductive cells differ from other cells composing the
organism, only in being unspecialized. And in support of the hypothesis
that tissue-cells in general have a reproductive potentiality, I instanced
the cases of the _Begonia phyllomaniaca_ and _Malaxis paludosa_. In the
thirty years which have since elapsed, many facts of like significance have
been brought to light, and various of these are given by Professor Hertwig.
Here are some of them:--
"Galls are produced under the stimulus of the insect almost anywhere on
the surface of a plant. Yet in most cases these galls, in a sense grown
at random on the surface of a plant, when placed in damp earth will give
rise to a young plant. In the hydroid _Tubularia mesembryanthemum_, when
the polyp heads are cut off, new heads arise. But if both head and root
be cut off, and the upper end be inserted in the mud, then from the
original upper end not head-polyps but root filaments will arise, while
from the original lower end not root filaments but head-polyps will
grow.... Driesch, by separating the first two and the first four
segmentation spheres of an _Echinus_ ovum, obtained two or four normal
plutei, respectively one half and a quarter of the normal size.... So,
also, in the case of _Amphioxus_, Wilson obtained a normal, but
proportionately diminished embryo with complete nervous system from a
separated sphere of a two- or four- or eight celled stage.... Chabry
obtained normal embryos in cases where some of the segmentation-spheres
had been artificially destroyed."
These evidences, furnished by independent observers, unite in showing,
firstly, that all the multiplying cells of the developing embryo are alike;
and, secondly, that the soma-cells of the adult severally retain, in a
latent form, all the powers of the original embryo-cell. If these facts do
not disprove absolutely Professor Weismann's hypothesis, we may wonderingly
ask what facts would disprove it?
Since Hertwig holds that all the cells forming an organism of any species
primarily consist of the same components, I at first thought that his
hypothesis was identical with my own hypothesis of "physiological units,"
or, as I would now call them, constitutional units. It seems otherwise,
however; for he thinks that each cell contains "only those material
particles which are bearers of cell-properties," and that organs "are the
functions of cell-complexes." To this it may be replied that the ability to
form the appropriate cell-complexes, itself depends upon the constitutional
units contained in the cells.
APPENDIX C.
THE INHERITANCE OF FUNCTIONALLY-WROUGHT MODIFICATIONS: A SUMMARY.
The assertion that changes of structure caused by changes of function are
transmitted to descendants is continually met by the question--Where is the
evidence? When some facts are assigned in proof, they are pooh-poohed as
insufficient. If after a time the question is raised afresh and other facts
are named, there is a like supercilious treatment of them. Successively
rejected in this way, the evidences do not accumulate in the minds of
opponents; and hence produce little or no effect. When they are brought
together, however, it turns out that they are numerous and weighty. We will
group them into negative and positive.
* * * * *
Negative evidence is furnished by those cases in which traits otherwise
inexplicable are explained if the structural effects of use and disuse are
transmitted. In the foregoing chapters and appendices three have been
given.
(1) Co-adaptation of co-operative parts comes first. This has been
exemplified by the case of enlarged horns in a stag, by the case of an
animal led into the habit of leaping, and in the case of the giraffe (cited
in "The Factors of Organic Evolution"); and it has been shown that the
implied co-adaptations of parts cannot possibly have been effected by
natural selection.
(2) The possession of unlike powers of discrimination by different parts of
the human skin, was named as a problem to be solved on the hypothesis of
natural selection or the hypothesis of panmixia; and it was shown that
neither of these can by any twisting yield a solution. But the facts
harmonize with the hypothesis that the effects of use are inherited.
(3) Then come the cases of those rudimentary organs which, like the hind
limbs of the whale, have nearly disappeared. Dwindling by natural selection
is here out of the question; and dwindling by panmixia, even were its
assumptions valid, would be incredible. But as a sequence of disuse the
change is clearly explained.
Failure to solve any _one_ of these three problems would, I think, alone
prove the Neo-Darwinian doctrines untenable; and the fact that we have
_three_ unsolved problems seems to me fatal.
* * * * *
From this negative evidence, turn now to the positive evidence. This falls
into several groups.
There are first the facts collected by Mr. Darwin, implying
functionally-altered structures in domestic animals. The hypothesis of
panmixia is, as we have seen, out of court; and therefore Mr. Darwin's
groups of evidences are reinstated. There is the changed ratio of
wing-bones and leg-bones in the duck; there are the drooping ears of cats
in China, of horses in Russia, of sheep in Italy, of guinea-pigs in
Germany, of goats and cattle in India, of rabbits, pigs, and dogs in all
long-civilized countries. Though artificial selection has come into play
where drooping has become a curious trait (as in rabbits), and has probably
caused the greater size of ears which has in some cases gone along with
diminished muscular power over them; yet it could not have been the
initiator, and has not been operative on animals bred for profit. Again
there are the changes produced by climate; as instance, among plants, the
several varieties of maize established in Germany and transformed in the
course of a few generations.
Facts of another class are yielded by the blind inhabitants of caverns. One
who studies the memoir by Mr. Packard on _The Cave Fauna of North America_,
&c., will be astonished at the variety of types in which degeneration or
loss of the eyes has become a concomitant of life passed in darkness. A
great increase in the force of this evidence will be recognized on learning
that absence or extreme imperfection of visual organs is found also in
creatures living in perpetual night at the bottoms of deep oceans.
Endeavours to account for these facts otherwise than by the effects of
disuse we have seen to be futile.
Kindred evidence is yielded by decrease of the jaws in those races which
have had diminished use of them--mankind and certain domestic animals.
Relative smallness in the jaws of civilized men, manifest enough on
comparison, has been proved by direct measurement. In pet dogs--pugs,
household spaniels--we find associated the same cause with the same effect.
Though there has been artificial selection, yet this did not operate until
the diminution had become manifest. Moreover there has been diminution of
the other structures concerned in biting: there are smaller muscles, feeble
zygomata, and diminished areas for insertion of muscles--traits which
cannot have resulted from selection, since they are invisible in the living
animal.
In abnormal vision produced by abnormal use of the eyes we have evidence of
another kind. That the Germans, among whom congenital short sight is
notoriously prevalent, have been made shortsighted by inheritance of
modifications due to continual reading of print requiring close attention,
is by some disputed. It is strange, however, that if there exists no causal
connexion between them, neither trait occurs without the other elsewhere.
But for the belief that there is a causal connexion we have the verifying
testimony of oculists. From Dr. Lindsay Johnson I have cited cases within
his professional experience of functionally-produced myopia transmitted to
children; and he asserts that other oculists have had like experiences.
Development of the musical faculty in the successive members of families
from which the great composers have come, as well as in the civilized races
at large, is not to be explained by natural selection. Even when it is
great, the musical faculty has not a life-saving efficiency as compared
with the average of faculties; for the most highly gifted have commonly
passed less prosperous lives and left fewer offspring than have those
possessed of ordinary abilities. Still less can it be said that the musical
faculty in mankind at large has been developed by survival of the fittest.
No one will assert that men in general have been enabled to survive and
propagate in proportion as their musical appreciation was great.
The transmission of nervous peculiarities functionally produced is alleged
by the highest authorities--Dr. Savage, president of the Neurological
Society, and Dr. Hughlings Jackson. The evidence they assign confirms, and
is confirmed by, that which the development of the musical faculty above
named supplies.
Here, then, we have sundry groups of facts directly supporting the belief
that functionally-wrought modifications descend from parents to offspring.
* * * * *
Now let us consider the position of those Darwinians who dissent from
Darwin, and who make light of all this evidence. We might naturally suppose
that their own hypothesis is unassailable. Yet, strange to say, they admit
that there is no direct proof that any species has been established by
natural selection. The proof is inferential only.
The certainty of an axiom does not give certainty to the deductions drawn
from it. That natural selection is, and always has been, operative is
incontestable. Obviously I should be the last person to deny that survival
of the fittest is a necessity: its negation is inconceivable. The
Neo-Darwinians, however, judging from their attitude, apparently assume
that firmness of the basis implies firmness of the superstructure. But
however high may be the probability of some of the conclusions drawn, none
of them can have more than probability; while some of them remain, and are
likely to remain, very questionable. Observe the difficulties.
(1) The general argument proceeds upon the analogy between natural
selection and artificial selection. Yet all know that the first cannot do
what the last does. Natural selection can do nothing more than preserve
those of which the _aggregate_ characters are most favourable to life. It
cannot pick out those possessed of one particular favourable character,
unless this is of extreme importance.
(2) In many cases a structure is of no service until it has reached a
certain development; and it remains to account for that increase of it by
natural selection which must be supposed to take place before it reaches
the stage of usefulness.
(3) Advantageous variations, not preserved in nature as they are by the
breeder, are liable to be swamped by crossing or to disappear by atavism.
Now whatever replies are made, their component propositions cannot be
necessary truths. So that the conclusion in each case, however reasonable,
cannot claim certainty: the fabric can have no stability like that of its
foundation.
When to uncertainties in the arguments supporting the hypothesis we add its
inability to explain facts of cardinal significance, as proved above, there
is I think ground for asserting that natural selection is less clearly
shown to be a factor in the origination of species than is the inheritance
of functionally-wrought changes.
* * * * *
If, finally, it is said that the mode in which functionally-wrought
changes, especially in small parts, so affect the reproductive elements as
to repeat themselves in offspring, cannot be imagined--if it be held
inconceivable that those minute changes in the organs of vision which cause
myopia can be transmitted through the appropriately-modified sperm-cells or
germ-cells; then the reply is that the opposed hypothesis presents a
corresponding inconceivability. Grant that the habit of a pointer was
produced by selection of those in which an appropriate variation in the
nervous system had occurred; it is impossible to imagine how a
slightly-different arrangement of a few nerve-cells and fibres could be
conveyed by a spermatozoon. So too it is impossible to imagine how in a
spermatozoon there can be conveyed the 480,000 independent variables
required for the construction of a single peacock's feather, each having a
proclivity towards its proper place. Clearly the ultimate process by which
inheritance is effected in either case passes comprehension; and in this
respect neither hypothesis has an advantage over the other.
APPENDIX D.
ON ALLEGED "SPONTANEOUS GENERATION," AND ON THE HYPOTHESIS OF PHYSIOLOGICAL
UNITS.
[_The following letter, originally written for publication in the_ North
American Review, _but declined by the Editor in pursuance of a general
rule, and eventually otherwise published in the United States, I have
thought well to append to this first volume of the_ Principles of Biology.
_I do this because the questions which it discusses are dealt with in this
volume; and because the further explanations it furnishes seem needful to
prevent misapprehensions._]
_The Editor of the North American Review._
SIR,
It is in most cases unwise to notice adverse criticisms. Either they do not
admit of answers or the answers may be left to the penetration of readers.
When, however, a critic's allegations touch the fundamental propositions of
a book, and especially when they appear in a periodical having the position
of the _North American Review_, the case is altered. For these reasons the
article on "Philosophical Biology," published in your last number, demands
from me an attention which ordinary criticisms do not.
It is the more needful for me to notice it, because its two leading
objections have the one an actual fairness and the other an apparent
fairness; and in the absence of explanations from me, they will be
considered as substantiated even by many, or perhaps most, of those who
have read the work itself--much more by those who have not read it. That to
prevent the spread of misapprehensions I ought to say something, is further
shown by the fact that the same two objections have already been made in
England--the one by Dr. Child, of Oxford, in his _Essays on Physiological
Subjects_, and the other by a writer in the _Westminster Review_ for July,
1865.
* * * * *
In the note to which your reviewer refers, I have, as he says, tacitly
repudiated the belief in "spontaneous generation;" and that I have done
this in such a way as to leave open the door for the interpretation given
by him is true. Indeed the fact that Dr. Child, whose criticism is a
sympathetic one, puts the same construction on this note, proves that your
reviewer has but drawn what seems to be a necessary inference.
Nevertheless, the inference is one which I did not intend to be drawn.
In explanation, let me at the outset remark that I am placed at a
disadvantage in having had to omit that part of the System of Philosophy
which deals with Inorganic Evolution. In the original programme will be
found a parenthetic reference to this omitted part, which should, as there
stated, precede the _Principles of Biology_. Two volumes are missing. The
closing chapter of the second, were it written, would deal with the
evolution of organic matter--the step preceding the evolution of living
forms. Habitually carrying with me in thought the contents of this
unwritten chapter, I have, in some cases, expressed myself as though the
reader had it before him; and have thus rendered some of my statements
liable to misconstructions. Apart from this, however, the explanation of
the apparent inconsistency is very simple, if not very obvious. In the
first place, I do not believe in the "spontaneous generation" commonly
alleged, and referred to in the note; and so little have I associated in
thought this alleged "spontaneous generation" which I disbelieve, with the
generation by evolution which I do believe, that the repudiation of the one
never occurred to me as liable to be taken for repudiation of the other.
That creatures having _quite specific structures_ are evolved in the course
of a few hours, without antecedents calculated to determine their specific
forms, is to me incredible. Not only the established truths of Biology, but
the established truths of science in general, negative the supposition that
organisms having structures definite enough to identify them as belonging
to known genera and species, can be produced in the absence of germs
derived from antecedent organisms of the same genera and species. If there
can suddenly be imposed on simple protoplasm the organization which
constitutes it a _Paramoecium_, I see no reason why animals of greater
complexity, or indeed of any complexity, may not be constituted after the
same manner. In brief, I do not accept these alleged facts as exemplifying
Evolution, because they imply something immensely beyond that which
Evolution, as I understand it, can achieve. In the second place, my
disbelief extends not only to the alleged cases of "spontaneous
generation," but to every case akin to them. The very conception of
spontaneity is wholly incongruous with the conception of Evolution. For
this reason I regard as objectionable Mr. Darwin's phrase "spontaneous
variation" (as indeed he does himself); and I have sought to show that
there are always assignable causes of variation. No form of Evolution,
inorganic or organic, can be spontaneous; but in every instance the
antecedent forces must be adequate in their quantities, kinds, and
distributions, to work the observed effects. Neither the alleged cases of
"spontaneous generation," nor any imaginable cases in the least allied to
them, fulfil this requirement.
If, accepting these alleged cases of "spontaneous generation," I had
assumed, as your reviewer seems to do, that the evolution of organic life
commenced in an analogous way; then, indeed, I should have left myself open
to a fatal criticism. This supposed "spontaneous generation" habitually
occurs in menstrua that contain either organic matter, or matter originally
derived from organisms; and such organic matter, proceeding in all known
cases from organisms of a higher kind, implies the pre-existence of such
higher organisms. By what kind of logic, then, is it inferrible that
organic life was initiated after a manner like that in which _Infusoria_
are said to be now spontaneously generated? Where, before life commenced,
were the superior organisms from which these lowest organisms obtained
their organic matter? Without doubting that there are those who, as the
reviewer says, "can penetrate deeper than Mr. Spencer has done into the
idea of universal evolution," and who, as he contends, prove this by
accepting the doctrine of "spontaneous generation"; I nevertheless think
that I can penetrate deep enough to see that a tenable hypothesis
respecting the origin of organic life must be reached by some other clue
than that furnished by experiments on decoction of hay and extract of beef.
From what I do not believe, let me now pass to what I do believe. Granting
that the formation of organic matter, and the evolution of life in its
lowest forms, may go on under existing cosmical conditions; but believing
it more likely that the formation of such matter and such forms, took place
at a time when the heat of the Earth's surface was falling through those
ranges of temperature at which the higher organic compounds are unstable; I
conceive that the moulding of such organic matter into the simplest types,
must have commenced with portions of protoplasm more minute, more
indefinite, and more inconstant in their characters, than the lowest
Rhizopods--less distinguishable from a mere fragment of albumen than even
the _Protogenes_ of Professor Haeckel. The evolution of specific shapes
must, like all other organic evolution, have resulted from the actions and
reactions between such incipient types and their environments, and the
continued survival of those which happened to have specialities best fitted
to the specialities of their environments. To reach by this process the
comparatively well-specialized forms of ordinary _Infusoria_, must, I
conceive, have taken an enormous period of time.
To prevent, as far as may be, future misapprehension, let me elaborate this
conception so as to meet the particular objections raised. The reviewer
takes for granted that a "first organism" must be assumed by me, as it is
by himself. But the conception of a "first organism," in anything like the
current sense of the words, is wholly at variance with conception of
evolution; and scarcely less at variance with the facts revealed by the
microscope. The lowest living things are not properly speaking organisms at
all; for they have no distinctions of parts--no traces of organization. It
is almost a misuse of language to call them "forms" of life: not only are
their outlines, when distinguishable, too unspecific for description, but
they change from moment to moment and are never twice alike, either in two
individuals or in the same individual. Even the word "type" is applicable
in but a loose way; for there is little constancy in their generic
characters: according as the surrounding conditions determine, they undergo
transformations now of one kind and now of another. And the vagueness, the
inconstancy, the want of appreciable structure, displayed by the simplest
of living things as we now see them, are characters (or absences of
characters) which, on the hypothesis of Evolution, must have been still
more decided when, as at first, no "forms," no "types," no "specific
shapes," had been moulded. That "absolute commencement of organic life on
the globe," which the reviewer says I "cannot evade the admission of," I
distinctly deny. The affirmation of universal evolution is in itself the
negation of an "absolute commencement" of anything. Construed in terms of
evolution, every kind of being is conceived as a product of modifications
wrought by insensible gradations on a pre-existing kind of being; and this
holds as fully of the supposed "commencement of organic life" as of all
subsequent developments of organic life. It is no more needful to suppose
an "absolute commencement of organic life" or a "first organism," than it
is needful to suppose an absolute commencement of social life and a first
social organism. The assumption of such a necessity in this last case, made
by early speculators with their theories of "social contracts" and the
like, is disproved by the facts; and the facts, so far as they are
ascertained, disprove the assumption of such a necessity in the first case.
That organic matter was not produced all at once, but was reached through
steps, we are well warranted in believing by the experiences of chemists.
Organic matters are produced in the laboratory by what we may literally
call _artificial evolution_. Chemists find themselves unable to form these
complex combinations directly from their elements; but they succeed in
forming them indirectly, by successive modifications of simpler
combinations. In some binary compound, one element of which is present in
several equivalents, a change is made by substituting for one of these
equivalents an equivalent of some other element; so producing a ternary
compound. Then another of the equivalents is replaced, and so on. For
instance, beginning with ammonia, N H_{3}, a higher form is obtained by
replacing one of the atoms of hydrogen by an atom of methyl, so producing
methyl-amine, N (C H_{3} H_{2}); and then, under the further action of
methyl, ending in a further substitution, there is reached the still more
compound substance dimethyl-amine, N (C H_{3}) (C H_{3}) H. And in this
manner highly complex substances are eventually built up. Another
characteristic of their method is no less significant. Two complex
compounds are employed to generate, by their action upon one another, a
compound of still greater complexity: different heterogeneous molecules of
one stage, become parents of a molecule a stage higher in heterogeneity.
Thus, having built up acetic acid out of its elements, and having by the
process of substitution described above, changed the acetic acid into
propionic acid, and propionic into butyric, of which the formula is
{C(CH_{3})(CH_{3})H}
{CO(HO) };
this complex compound, by operating on another complex compound, such as
the dimethyl-amine named above, generates one of still greater complexity,
butyrate of dimethyl-amine
{C(CH)(CH_{3})H} N(CH_{3})(CH_{3})H.
{CO(HO) }
See, then, the remarkable parallelism. The progress towards higher types of
organic molecules is effected by modifications upon modifications; as
throughout Evolution in general. Each of these modifications is a change of
the molecule into equilibrium with its environment--an adaptation, as it
were, to new surrounding conditions to which it is subjected; as throughout
Evolution in general. Larger, or more integrated, aggregates (for compound
molecules are such) are successively generated; as throughout Evolution in
general. More complex or heterogeneous aggregates are so made to arise, one
out of another; as throughout Evolution in general. A
geometrically-increasing multitude of these larger and more complex
aggregates so produced, at the same time results; as throughout Evolution
in general. And it is by the action of the successively higher forms on one
another, joined with the action of environing conditions, that the highest
forms are reached; as throughout Evolution in general.
When we thus see the identity of method at the two extremes--when we see
that the general laws of evolution, as they are exemplified in known
organisms, have been unconsciously conformed to by chemists in the
artificial evolution of organic matter; we can scarcely doubt that these
laws were conformed to in the natural evolution of organic matter, and
afterwards in the evolution of the simplest organic forms. In the early
world, as in the modern laboratory, inferior types of organic substances,
by their mutual actions under fit conditions, evolved the superior types of
organic substances, ending in organizable protoplasm. And it can hardly be
doubted that the shaping of organizable protoplasm, which is a substance
modifiable in multitudinous ways with extreme facility, went on after the
same manner. As I learn from one of our first chemists, Prof. Frankland,
_protein_ is capable of existing under probably at least a thousand
isomeric forms; and, as we shall presently see, it is capable of forming,
with itself and other elements, substances yet more intricate in
composition, that are practically infinite in their varieties of kind.
Exposed to those innumerable modifications of conditions which the Earth's
surface afforded, here in amount of light, there in amount of heat, and
elsewhere in the mineral quality of its aqueous medium, this extremely
changeable substance must have undergone now one, now another, of its
countless metamorphoses. And to the mutual influences of its metamorphic
forms under favouring conditions, we may ascribe the production of the
still more composite, still more sensitive, still more variously-changeable
portions of organic matter, which, in masses more minute and simpler than
existing _Protozoa_, displayed actions verging little by little into those
called vital--actions which protein itself exhibits in a certain degree,
and which the lowest known living things exhibit only in a greater degree.
Thus, setting out with inductions from the experiences of organic chemists
at the one extreme, and with inductions from the observations of biologists
at the other extreme, we are enabled deductively to bridge the
interval--are enabled to conceive how organic compounds were evolved, and
how, by a continuance of the process, the nascent life displayed in these
became gradually more pronounced. And this it is which has to be explained,
and which the alleged cases of "spontaneous generation" would not, were
they substantiated, help us in the least to explain.
It is thus manifest, I think, that I have not fallen into the alleged
inconsistency. Nevertheless, I admit that your reviewer was justified in
inferring this inconsistency; and I take blame to myself for not having
seen that the statement, as I have left it, is open to misconstruction.
* * * * *
I pass now to the second allegation--that in ascribing to certain specific
molecules, which I have called "physiological units," the aptitude to build
themselves into the structure of the organism to which they are peculiar, I
have abandoned my own principle, and have assumed something beyond the
re-distribution of Matter and Motion. As put by the reviewer, his case
appears to be well made out; and that he is not altogether unwarranted in
so putting it, may be admitted. Nevertheless, there does not in reality
exist the supposed incongruity.
Before attempting to make clear the adequacy of the conception which I am
said to have tacitly abandoned as insufficient, let me remove that excess
of improbability the reviewer gives to it, by the extremely-restricted
meaning with which he uses the word mechanical. In discussing a proposition
of mine he says:--
"He then cites certain remarks of Mr. Paget on the permanent effects
wrought in the blood by the poison of scarlatina and small-pox, as
justifying the belief that such a 'power' exists, and attributes the
repair of a wasted tissue to 'forces analogous to those by which a
crystal reproduces its lost apex.' (Neither of which phenomena, however,
is explicable by mechanical causes.)"
Were it not for the deliberation with which this last statement is made, I
should take it for a slip of the pen. As it is, however, I have no course
left but to suppose the reviewer unaware of the fact that molecular actions
of all kinds are now not only conceived as mechanical actions, but that
calculations based on this conception of them, bring out the results that
correspond with observation. There is no kind of re-arrangement among
molecules (crystallization being one) which the modern physicist does not
think of. and correctly reason upon, in terms of forces and motions like
those of sensible masses. Polarity is regarded as a resultant of such
forces and motions; and when, as happens in many cases, light changes the
molecular structure of a crystal, and alters its polarity, it does this by
impressing, in conformity with mechanical laws, new motions on the
constituent molecules. That the reviewer should present the mechanical
conception under so extremely limited a form, is the more surprising to me
because, at the outset of the very work he reviews, I have, in various
passages, based inferences on those immense extensions of it which he
ignores; indicating, for example, the interpretation it yields of the
inorganic chemical changes effected by heat, and the organic chemical
changes effected by light (_Principles of Biology_, § 13).
Premising, then, that the ordinary idea of mechanical action must be
greatly expanded, let us enter upon the question at issue--the sufficiency
of the hypothesis that the structure of each organism is determined by the
polarities of the special molecules, or physiological units, peculiar to it
as a species, which necessitate tendencies towards special arrangements. My
proposition and the reviewer's criticism upon it, will be most conveniently
presented if I quote in full a passage of his from which I have already
extracted some expressions. He says:--
"It will be noticed, however, that Mr. Spencer attributes the possession
of these 'tendencies,' or 'proclivities,' to natural inheritance from
ancestral organisms; and it may be argued that he thus saves the
mechanist theory and his own consistency at the same time, inasmuch as he
derives even the 'tendencies' themselves ultimately from the environment.
To this we reply, that Mr. Spencer, who advocates the nebular hypothesis,
cannot evade the admission of an absolute commencement of organic life on
the globe, and that the 'formative tendencies,' without which he cannot
explain the evolution of a single individual, could not have been
inherited by the first organism. Besides, by his virtual denial of
spontaneous generation, he denies that the first organism was evolved out
of the inorganic world, and thus shuts himself off from the argument
(otherwise plausible) that its 'tendencies' were ultimately derived from
the environment."
This assertion is already in great measure disposed of by what has been
said above. Holding that, though not "spontaneously generated," those
minute portions of protoplasm which first displayed in the feeblest degree
that changeability taken to imply life, were evolved, I am _not_ debarred
from the argument that the "tendencies" of the physiological units are
derived from the inherited effects of environing actions. If the conception
of a "first organism" were a necessary one, the reviewer's objection would
be valid. If there were an "absolute commencement" of life, a definite line
parting organic matter from the simplest living forms, I should be placed
in the predicament he describes. But as the doctrine of Evolution itself
tacitly negatives any such distinct separation; and as the negation is the
more confirmed by the facts the more we know of them; I do not feel that I
am entangled in the alleged difficulty. My reply might end here; but as the
hypothesis in question is one not easily conceived, and very apt to be
misunderstood, I will attempt a further elucidation of it.
Much evidence now conspires to show that molecules of the substances we
call elementary are in reality compound; and that, by the combination of
these with one another, and re-combinations of the products, there are
formed systems of systems of molecules, unimaginable in their complexity.
Step by step as the aggregate molecules so resulting, grow larger and
increase in heterogeneity, they become more unstable, more readily
transformable by small forces, more capable of assuming various characters.
Those composing organic matter transcend all others in size and intricacy
of structure; and in them these resulting traits reach their extreme. As
implied by its name _protein_, the essential substance of which organisms
are built, is remarkable alike for the variety of its metamorphoses and the
facility with which it undergoes them: it changes from one to another of
its thousand isomeric forms on the slightest change of conditions. Now
there are facts warranting the belief that though these multitudinous
isomeric forms of protein will not unite directly with one another, yet
they admit of being linked together by other elements with which they
combine. And it is very significant that there are habitually present two
other elements, sulphur and phosphorus, which have quite special powers of
holding together many equivalents--the one being pentatomic and the other
hexatomic. So that it is a legitimate supposition (justified by analogies)
that an atom of sulphur may be a bond of union among half-a-dozen different
isomeric forms of protein; and similarly with phosphorus. A moment's
thought will show that, setting out with the thousand isomeric forms of
protein, this makes possible a number of these combinations almost passing
the power of figures to express. Molecules so produced, perhaps exceeding
in size and complexity those of protein as those of protein exceed those of
inorganic matter, may, I conceive, be the special units belonging to
special kinds of organisms. By their constitution they must have a
plasticity, or sensitiveness to modifying forces, far beyond that of
protein; and bearing in mind not only that their varieties are practically
infinite in number, but that closely allied forms of them, chemically
indifferent to one another as they must be, may coexist in the same
aggregate, we shall see that they are fitted for entering into unlimited
varieties of organic structures.
The existence of such physiological units, peculiar to each species of
organism, is not unaccounted for. They are evolved simultaneously with the
evolution of the organisms they compose--they differentiate as fast as
these organisms differentiate; and are made multitudinous in kind by the
same actions which make the organism they compose multitudinous, in kind.
This conception is clearly representable in terms of the mechanical
hypothesis. Every physicist will endorse the proposition that in each
aggregate there tends to establish itself an equilibrium between the forces
exercised by all the units upon each and by each upon all. Even in masses
of substance so rigid as iron and glass, there goes on a molecular
re-arrangement, slow or rapid according as circumstances facilitate, which
ends only when there is a complete balance between the actions of the parts
on the whole and the actions of the whole on the parts: the implications
being that every change in the form or size of the whole, necessitates some
redistribution of the parts. And though in cases like these, there occurs
only a polar re-arrangement of the molecules, without changes in the
molecules themselves; yet where, as often happens, there is a passage from
the colloid to the crystalloid state, a change of constitution occurs in
the molecules themselves. These truths are not limited to inorganic matter:
they unquestionably hold of organic matter. As certainly as molecules of
alum have a form of equilibrium, the octahedron, into which they fall when
the temperature of their solvent allows them to aggregate, so certainly
must organic molecules of each kind, no matter how complex, have a form of
equilibrium in which, when they aggregate, their complex forces are
balanced--a form far less rigid and definite, for the reason that they have
far less definite polarities, are far more unstable, and have their
tendencies more easily modified by environing conditions. Equally certain
is it that the special molecules having a special organic structure as
their form of equilibrium, must be reacted upon by the total forces of this
organic structure; and that, if environing actions lead to any change in
this organic structure, these special molecules, or physiological units,
subject to a changed distribution of the total forces acting upon them will
undergo modification--modification which their extreme plasticity will
render easy. By this action and reaction I conceive the physiological units
peculiar to each kind of organism, to have been moulded along with the
organism itself. Setting out with the stage in which protein in minute
aggregates, took on those simplest differentiations which fitted it for
differently-conditioned parts of its medium, there must have unceasingly
gone on perpetual re-adjustments of balance between aggregates and their
units--actions and reactions of the two, in which the units tended ever to
establish the typical form produced by actions and reactions in all
antecedent generations, while the aggregate, if changed in form by change
of surrounding conditions, tended ever to impress on the units a
corresponding change of polarity, causing them in the next generation to
reproduce the changed form--their new form of equilibrium.
This is the conception which I have sought to convey, though it seems
unsuccessfully, in the _Principles of Biology_; and which I have there used
to interpret the many involved and mysterious phenomena of Genesis,
Heredity, and Variation. In one respect only am I conscious of having so
inadequately explained myself, as to give occasion for a
misinterpretation--the one made by the _Westminster_ reviewer above
referred to. By him, as by your own critic, it is alleged that in the idea
of "inherent tendencies" I have introduced, under a disguise, the
conception of "the archæus, vital principle, _nisus formativus_, and so
on." This allegation is in part answered by the foregoing explanation. That
which I have here to add, and did not adequately explain in the _Principles
of Biology_, is that the proclivity of units of each order towards the
specific arrangement seen in the organism they form, is not to be
understood as resulting from their own structures and actions only; but as
the product of these and the environing forces to which they are exposed.
Organic evolution takes place only on condition that the masses of
protoplasm formed of the physiological units, and of the assimilable
materials out of which others like themselves are to be multiplied, are
subject to heat of a given degree--are subject, that is, to the unceasing
impacts of undulations of a certain strength and period; and, within
limits, the rapidity with which the physiological units pass from their
indefinite arrangement to the definite arrangement they presently assume,
is proportionate to the strengths of the ethereal undulations falling upon
them. In its complete form, then, the conception is that these specific
molecules, having the immense complexity above described, and having
correspondently complex polarities which cannot be mutually balanced by any
simple form of aggregation, have, for the form of aggregation in which all
their forces are equilibrated, the structure of the adult organism to which
they belong; and that they are compelled to fall into this structure by the
co-operation of the environing forces acting on them, and the forces they
exercise on one another--the environing forces being the source of the
_power_ which effects the re-arrangement, and the polarities of the
molecules determining the _direction_ in which that power is turned. Into
this conception there enters no trace of the hypothesis of an "archæus or
vital principle;" and the principles of molecular physics fully justify it.
It is, however, objected that "the living body in its development presents
a long succession of _differing_ forms; a continued series of changes for
the whole length of which, according to Mr. Spencer's hypothesis, the
physiological units must have an 'inherent tendency.' Could we more truly
say of anything, 'it is unrepresentable in thought?'" I reply that if there
is taken into account an element here overlooked, the process will not be
found "unrepresentable in thought." This is the element of size or mass. To
satisfy or balance the polarities of each order of physiological units, not
only a certain structure of organism, but a certain size of organism is
needed; for the complexities of that adult structure in which the
physiological units are equilibrated, cannot be represented within the
small bulk of the embryo. In many minute organisms, where the whole mass of
physiological units required for the structure is present, the very thing
_does_ take place which it is above implied _ought_ to take place. The mass
builds itself directly into the complete form. This is so with _Acari_, and
among the nematoid _Entozoa_. But among higher animals such direct
transformations cannot happen. The mass of physiological units required to
produce the size as well as the structure that approximately equilibrates
them, is not all present, but has to be formed by successive
additions--additions which in viviparous animals are made by absorbing, and
transforming into these special molecules, the organizable materials
directly supplied by the parent, and which in oviparous animals are made by
doing the like with the organizable materials in the "food-yelk," deposited
by the parent in the same envelope with the germ. Hence it results that,
under such conditions, the physiological units which first aggregate into
the rudiment of the future organism, do not form a structure like that of
the adult organism, which, when of such small dimensions, does not
equilibrate them. They distribute themselves so as partly to satisfy the
chief among their complex polarities. The vaguely-differentiated mass thus
produced cannot, however, be in equilibrium. Each increment of
physiological units formed and integrated by it, changes the distribution
of forces; and this has a double effect. It tends to modify the
differentiations already made, bringing them a step nearer to the
equilibrating structure; and the physiological units next integrated, being
brought under the aggregate of polar forces exercised by the whole mass,
which now approaches a step nearer to that ultimate distribution of polar
forces which exists in the adult organism, are coerced more directly into
the typical structure. Thus there is necessitated a series of compromises.
Each successive form assumed is unstable and transitional: approach to the
typical structure going on hand in hand with approach to the typical bulk.
Possibly I have not succeeded by this explanation, any more than by the
original explanation, in making this process "representable in thought." It
is manifestly untrue, however, that I have, as alleged, re-introduced under
a disguise the conception of a "vital principle." That I interpret
embryonic development in terms of Matter and Motion, cannot, I think, be
questioned. Whether the interpretation is adequate, must be a matter of
opinion; but it is clearly a matter of fact, that I have not fallen into
the inconsistency asserted by your reviewer. At the same time I willingly
admit that, in the absence of certain statements which I have now supplied,
he was not unwarranted in representing my conception in the way that he has
done.
----
NOTES
[1] Gross misrepresentations of this statement, which have been from time
to time made, oblige me, much against my will, to add here an
explanation of it. The last of these perversions, uttered in a
lecture delivered at Belfast by the Rev. Professor Watts, D.D., is
reported in the _Belfast Witness_ of December 18, 1874; just while a
third impression of this work is being printed from the plates. The
report commences as follows:--"Dr. Watts, after showing that on his
own confession Spencer was indebted for his facts to Huxley and
Hooker, who," &c., &c.
Wishing in this, as in other cases, to acknowledge indebtedness when
conscious of it, I introduced the words referred to, in recognition
of the fact that I had repeatedly questioned the distinguished
specialists named, on matters beyond my knowledge, which were not
dealt with in the books at my command. Forgetting the habits of
antagonists, and especially theological antagonists, it never
occurred to me that my expression of thanks to my friends for
"information where my own was deficient," would be turned into the
sweeping statement that I was indebted to them for my facts.
Had Professor Watts looked at the preface to the second volume (the
two having been published separately, as the prefaces imply), he
would have seen a second expression of my indebtedness "for their
valuable criticisms, and for the trouble they have taken in
_checking_ the numerous statements of fact on which the arguments
proceed"--no further indebtedness being named. A moment's comparison
of the two volumes in respect of their accumulations of facts, would
have shown him what kind of warrant there was for his interpretation.
Doubtless the Rev. Professor was prompted to make this assertion by
the desire to discredit the work he was attacking; and having so good
an end in view, thought it needless to be particular about the means.
In the art of dealing with the language of opponents, Dr. Watts might
give lessons to Monsignor Capel and Archbishop Manning.
_December 28th, 1874._
[2] In this passage as originally written (in 1862) they were described
as incondensible; since, though reduced to the density of liquids,
they had not been liquefied.
[3] Here and hereafter the word "atom" signifies a unit of something
classed as an element, because thus far undecomposed by us. The word
must not be supposed to mean that which its derivation implies. In
all probability it is not a simple unit but a compound one.
[4] The name hydro-carbons was here used when these pages were written,
thirty-four years ago. It was the name then current. In this case, as
in multitudinous other cases, the substitution of newer words and
phrases for older ones, is somewhat misleading. Putting the thoughts
of 1862 in the language of 1897 gives an illusive impression of
recency.
[5] It will perhaps seem strange to class oxygen as a crystalloid. But
inasmuch as the crystalloids are distinguished from the colloids by
their atomic simplicity, and inasmuch as sundry gases are reducible
to a crystalline state, we are justified in so classing it.
[6] The remark made by a critic to the effect that in a mammal higher
temperature diminishes the rate of molecular change in the tissues,
leads me to add that the exhalation I have alleged is prevented if
the heat rises above the range of variation normal to the organism;
since, then, unusually rapid pulsations with consequent inefficient
propulsion of the blood, cause a diminished rate of circulation. To
produce the effect referred to in the text, heat must be associated
with dryness; for otherwise evaporation is not aided. General
evidence supporting the statement I have made is furnished by the
fact that the hot and dry air of the eastern deserts is extremely
invigorating; by the fact that all the energetic and conquering races
of men have come from the hot and dry regions marked on the maps as
rainless; and by the fact that travellers in Africa comment on the
contrast between the inhabitants of the hot and dry regions
(relatively elevated) and those of the hot and moist regions: active
and inert respectively.
[7] The increase of respiration found to result from the presence of
light, is probably an _indirect_ effect. It is most likely due to the
reception of more vivid impressions through the eyes, and to the
consequent nervous stimulation. Bright light is associated in our
experience with many of our greatest outdoor pleasures, and its
presence partially arouses the consciousness of them, with the
concomitant raised vital functions.
[8] To exclude confusion it may be well here to say that the word "atom"
is, as before explained, used as the name for a unit of a substance
at present undecomposed; while the word "molecule" is used as the
name for a unit of a substance known to be compound.
[9] On now returning to the subject after many years, I meet with some
evidence recently assigned, in a paper read before the Royal Society
by Mr. J. W. Pickering, D.Sc. (detailing results harmonizing with
those obtained by Prof. Grimaux), showing clearly how important an
agent in vital actions is this production of isomeric changes by
slight changes of conditions. Certain artificially produced
substances, simulating proteids in other of their characters and
reactions, were found to simulate them in coagulability by trifling
disturbances. "In the presence of a _trace of neutral salt_ they
coagulate on heating at temperatures very similar to proteid
solutions." And it is shown that by one of these factitious organic
colloids a like effect is produced in coagulating the blood, to that
"produced by the intravenous injection of a nucleoproteid."
[10] After this long interval during which other subjects have occupied
me, I now find that the current view is similar to the view above set
forth, in so far that a small molecular disturbance is supposed
suddenly to initiate a great one, producing a change compared to an
explosion. But while, of two proposed interpretations, one is that
the fuse is nitrogenous and the charge a carbo-hydrate, the other is
that both are nitrogenous. The relative probabilities of these
alternative views will be considered in a subsequent chapter.
[11] When writing this passage I omitted to observe the verification
yielded of the conclusion contained in § 15 concerning the part
played in the vital processes by the nitrogenous compounds. For these
vegeto-alkalies, minute quantities of which produce such great
effects in exalting the functions (_e. g._, a sixteenth of a grain of
strychnia is a dose), are all nitrogenous bodies, and, by
implication, relatively unstable bodies. The small amounts of
molecular change which take place in these small quantities of the
vegeto-alkalies when diffused through the system, initiate larger
amounts of molecular change in the nitrogenous elements of the
tissues.
But the evidence furnished a generation ago by these vegeto-alkalies
has been greatly reinforced by far more striking evidence furnished
by other nitrogenous compounds--the various explosives. These, at the
same time that they produce by their sudden decompositions violent
effects outside the organism, also produce violent effects inside it:
a hundredth of a grain of nitro-glycerine being a sufficient dose.
Investigations made by Dr. J. B. Bradbury, and described by him in
the Bradshaw Lecture on "Some New Vaso-Dilators" (see _The Lancet_,
Nov. 16, 1895), details the effects of kindred
bodies--methyl-nitrate, glycol-dinitrate, erythrol-tetranitrate. The
first two, in common with nitro-glycerine, are stable only when cool
and in the dark--sunlight or warmth decomposes them, and they explode
by rapid heating or percussion. The fact which concerns us here is
that the least stable--glycol-dinitrate--has the most powerful and
rapid physiological effect, which is proportionately transient. In
one minute the blood-pressure is reduced by one-fourth and in four
minutes by nearly two-thirds: an effect which is dissipated in a
quarter of an hour. So that this excessively unstable compound,
decomposing in the body in a very short time, produces within that
short time a vast amount of molecular change: acting, as it seems,
not through the nervous system, but directly on the blood-vessels.
[12] This interpretation is said to be disproved by the fact that the
carbo-hydrate contained in muscle amounts to only about 1.5 of the
total solids. I do not see how this statement is to be reconciled
with the statement cited three pages back from Professor Michael
Foster, that the deposits of glycogen contained in the liver and in
the muscles may be compared to the deposits in a central bank and
branch banks.
[13] Before leaving the topic let me remark that the doctrine of
metabolism is at present in its inchoate stage, and that the
prevailing conclusions should be held tentatively. As showing this
need an anomalous fact may be named. It was long held that gelatine
is of small value as food, and though it is now recognized as
valuable because serving the same purposes as fats and
carbo-hydrates, it is still held to be valueless for structural
purposes (save for some inactive tissue); and this estimate agrees
with the fact that it is a relatively stable nitrogenous compound,
and therefore unfit for those functions performed by unstable
nitrogenous compounds in the muscular and other tissues. But if this
is true, it seems a necessary implication that such substances as
hair, wool, feathers, and all dermal growths chemically akin to
gelatine, and even more stable, ought to be equally innutritive or
more innutritive. In that case, however, what are we to say of the
larva of the clothes-moth, which subsists exclusively on one or other
of these substances, and out of it forms all those unstable
nitrogenous compounds needful for carrying on its life and developing
its tissues? Or again, how are we to understand the nutrition of the
book-worm, which, in the time-stained leaves through which it
burrows, finds no proteid save that contained in the dried-up size,
which is a form of gelatine; or, once more, in what form is the
requisite amount of nitrogenous substance obtained by the
coleopterous larva which eats holes in wood a century old?
[14] This chapter and the following two chapters originally appeared in
Part III of the original edition of the _Principles of Psychology_
(1855): forming a preliminary which, though indispensable to the
argument there developed, was somewhat parenthetical. Having now to
deal with the general science of Biology before the more special one
of Psychology, it becomes possible to transfer these chapters to
their proper place.
[15] See _Westminster Review_ for April, 1852.--Art. IV. "A Theory of
Population." See Appendix A.
[16] This paragraph replaces a sentence that, in _The Principles of
Psychology_, referred to a preceding chapter on "Method;" in which
the mode of procedure here indicated was set forth as a mode to be
systematically pursued in the choice of hypotheses. This chapter on
Method is now included, along with other matter, in a volume entitled
_Various Fragments_.
[17] Speaking of "the general idea of _life_" M. Comte says:--"Cette idée
suppose, en effet, non-seulement celle d'un être organisé de manière
à comporter l'état vital, mais aussi celle, non moins indispensable,
d'un certain ensemble d'influences extérieures propres à son
accomplissement. Une telle harmonie entre l'être vivant et le
_milieu_ correspondant, caractérise evidemment la condition
fondamentale de la vie." Commenting on de Blainville's definition of
life, which he adopts, he says:--"Cette lumineuse définition ne me
paraît laisser rien d'important à désirer, si ce n'est une indication
plus directe et plus explicite de ces deux conditions fondamentales
co-relatives, nécessairement inséparables de l'état vivant, un
_organisme_ déterminé et un _milieu_ convenable." It is strange that
M. Comte should have thus recognized the necessity of a harmony
between an organism and its environment, as a _condition_ essential
to life, and should not have seen that the continuous maintenance of
such inner actions as will counterbalance outer actions,
_constitutes_ life.
[When the original edition was published Dr. J. H. Bridges wrote to
me saying that in the _Politique Positive_, Comte had developed his
conception further. On p. 413, denying "le prétendu antagonisme des
corps vivants envers leurs milieux inorganiques," he says "au lieu de
ce conflit, on a reconnu bientôt que cette relation nécessaire
constitue une condition fondamentale de la vie réelle, dont la notion
systématique consiste dans une intime conciliation permanente entre
la spontanéité intérieure et la fatalité extérieure." Still, this
"conciliation _permanente_" seems to be a "_condition_" to life; not
that varying adjustment of changes which life consists in
maintaining. In presence of an ambiguity, the interpretation which
agrees with his previous statement must be chosen.]
[18] In further elucidation of this general doctrine, see _First
Principles_, § 25.
[19] In ordinary speech Development is often used as synonymous with
Growth. It hence seems needful to say that Development as here and
hereafter used, means _increase of structure_ and not _increase of
bulk_. It may be added that the word Evolution, comprehending growth
as well as Development, is to be reserved for occasions when both are
implied.
[20] This paragraph originally formed part of a review-article on
"Transcendental Physiology," published in 1857.
[21] When, in 1863, the preceding chapter was written, it had not occurred
to me that there needed an accompanying chapter treating of
Structure. The gap left by that oversight I now fill up. In doing
this there have been included certain statements which are tacitly
presupposed in the last chapter, and there may also be some which
overlap statements in the next chapter. I have not thought it needful
so to alter adjacent chapters as to remove these slight defects: the
duplicated ideas will bear re-emphasizing.
[22] In connexion with this matter I add here a statement made by Prof.
Foster which it is difficult to understand: "Indeed it has been
observed that a dormouse actually gained in weight during a
hybernating period; it discharged during this period neither urine
nor fæces, and the gain in weight was the excess of oxygen taken in
over the carbonic acid given out." (_Text-book of Physiology_, 6th
ed., Part II, page 859.)
[23] In the account of James Mitchell, a boy born blind and deaf, given by
James Wardrop, F.R.S. (Edin. 1813), it is said that he acquired a
"preternatural acuteness of touch and smell." The deaf Dr. Kitto
described himself as having an extremely strong visual memory: he
retained "a clear impression or image of everything at which he ever
looked."
[24] Here, as in sundry places throughout this chapter, the necessities of
the argument have obliged me to forestall myself, by assuming the
conclusion reached in a subsequent chapter, that modifications of
structure produced by modifications of function are transmitted to
offspring.
[25] Whether the _Volvox_ is to be classed as animal or vegetal is a
matter of dispute; but its similarity to the blastula stage of many
animals warrants the claim of the zoologists.
[26] While the proof was in my hands there was published in _Science
Progress_ an essay by Dr. T. G. Brodie on "The Phosphorus-containing
Substances of the Cell." In this essay it is pointed out that
"nucleic acid is particularly characterized by its instability.... In
the process of purification it is extremely liable to decompose, with
the result that it loses a considerable part of its phosphorus. In
the second place it is most easily split up in another manner in
which it loses a considerable part of its nitrogen.... To avoid the
latter source of error he [Miescher] found that it was necessary to
keep the temperature of all solutions down to 0°C., the whole time of
the preparation." These facts tend strongly to verify the hypothesis
that the nucleus is a source of perpetual molecular disturbance--not
a regulating centre but a stimulating centre.
[27] The writing of the above section reminded me of certain allied views
which I ventured to suggest nearly 50 years ago. They are contained
in the _Westminster Review_ for April, 1852, in an article entitled
"A Theory of Population deduced from the General Law of Animal
Fertility." It is there suggested that the "spermatozoon is
essentially a neural element, and the ovum essentially a hæmal
element," or, as otherwise stated, that the "sperm-cell is
co-ordinating matter and the germ-cell matter to be co-ordinated"
(pp. 490-493). And along with this proposition there is given some
chemical evidence tending to support it. Now if, in place of "neural"
and "hæmal," we say--the element that is most highly phosphorized and
the element that is phosphorized in a much smaller degree; or if, in
place of co-ordinating matter and matter to be co-ordinated, we
say--the matter which initiates action and the matter which is made
to act; there is disclosed a kinship between this early view and the
view just set forth. In the last part of this work, "Laws of
Multiplication," which is developed from the essay referred to, I
left out the portion containing the quoted sentences, and the
evidence supporting the conclusion drawn. Partly I omitted them
because the speculation did not form an essential link in the general
argument, and partly because I did not see how the suggested
interpretation could hold of plants as well as of animals. If,
however, the alleged greater staining capacity of the male generative
nucleus in plants implies, as in other cases, that the male cell has
a larger proportion of the phosphorized matter than the other
elements concerned, then the difficulty disappears.
As, along with the idea just named, the dropped portion of the
original essay contains other ideas which seem to me worth
preserving, I have thought it as well to reproduce it, in company
with the chief part of the general argument as at first sketched out.
It will be found in Appendix A to this volume.
[28] Unfortunately the word _heterogenesis_ has been already used as a
synonym for "spontaneous generation." Save by those few who believe
in "spontaneous generation," however, little objection will be felt
to using the word in a sense that seems much more appropriate. The
meaning above given to it covers both Metagenesis and
Parthenogenesis.
[29] Prof. Huxley avoids this difficulty by making every kind of Genesis a
mode of development. His classification, which suggested the one
given above, is as follows:--
{ Growth
{ Continuous {
{ { Metamorphosis
{
Development {
{ { Metagenesis
{ { Agamogenesis {
{ Discontinuous { { Parthenogenesis
{ Gamogenesis
[30] The implication is that an essentially similar process occurs in
those fragments of leaves used for artificial propagation. Besides
the Begonias in general, I learn that various other plants are thus
multiplied--Citron and orange trees, _Hoya carnosa_, _Aucuba
japonica_, _Clianthus puniceus_, etc., etc. _Bryophyllum calicinum_,
_Rochea falcata_, and _Echeveria_. I also learn that the following
plants, among others, produce buds from their foliage
leaves:--_Cardamine pratensis_, _Nasturtium officinale_, _Roripa
palustris_, _Brassica oleracea_, _Arabis pumila_, _Chelidonium
majus_, _Nymphæa guianensis_, _Episcia bicolor_, _Chirita sivensis_,
_Pinguicula Backeri_, _Allium_, _Gagea_, _Tolmia_, _Fritillaria_,
_Ornithogalum_, etc. In _Cardamine_ and several others, a complete
miniature plant is at once produced; in other cases bulbils or
similar detachable buds.
[31] Among various examples I have observed, the most remarkable were
among Foxgloves, growing in great numbers and of large size, in a
wood between Whatstandwell Bridge and Crich, in Derbyshire. In one
case the lowest flower on the stem contained, in place of a pistil, a
shoot or spike of flower-buds, similar in structure to the
embryo-buds of the main spike. I counted seventeen buds on it; of
which the first had three stamens, but was otherwise normal; the
second had three; the third, four; the fourth, four; &c. Another
plant, having more varied monstrosities, evinced excess of nutrition
with equal clearness. The following are the notes I took of its
structure:--1st, or lowest flower on the stem, very large; calyx
containing eight divisions, one partly transformed into a corolla,
and another transformed into a small bud with bract (this bud
consisted of a five-cleft calyx, four sessile anthers, a pistil, and
a rudimentary corolla); the corolla of the main flower, which was
complete, contained six stamens, three of them bearing anthers, two
others being flattened and coloured, and one rudimentary; there was
no pistil but, _in place of it_, a large bud, consisting of a
three-cleft calyx of which two divisions were tinted at the ends, an
imperfect corolla marked internally with the usual purple spots and
hairs, three anthers sessile on this mal-formed corolla, a pistil, a
seed vessel with ovules, and, growing to it, another bud of which the
structure was indistinct. 2nd flower, large; calyx of seven
divisions, one being transformed into a bud with bract, but much
smaller than the other; corolla large but cleft along the top; six
stamens with anthers, pistil, and seed-vessel. 3rd flower, large;
six-cleft calyx, cleft corolla, with six stamens, pistil, and
seed-vessel, with a second pistil half unfolded at its apex. 4th
flower, large; divided along the top, six stamens. 5th flower, large;
corolla divided into three parts, six stamens. 6th flower, large;
corolla cleft, calyx six cleft, the rest of the flower normal. 7th,
and all succeeding flowers, normal.
While this chapter is under revision, another noteworthy illustration
has been furnished to me by a wall-trained pear tree which was
covered in the spring by luxuriant "foreright" shoots. As I learned
from the gardener, it was pruned just as the fruit was setting. A
large excess of sap was thus thrown into other branches, with the
result that in a number of them the young pears were made monstrous
by reversion. In some cases, instead of the dried up sepals at the
top of the pear, there were produced good sized leaves; and in other
cases the seed-bearing core of the pear was transformed into a growth
which protruded through the top of the pear in the shape of a new
shoot.
[32] In partial verification, Mr. Tansley writes:--"Prof. Klebs of Basel
has shown that in _Hydrodictyon_, gametes can only be produced by the
cells of a net when these are above a certain size and age; and then
only under conditions unfavourable to growth, such as a feeble light
or poverty of nutritive inorganic salts or absence of oxygen, or a
low temperature in the water containing the plant. The presence of
organic substances, especially sugar, also acts as a stimulus to the
formation of gametes, and this is also the case in _Vaucheria_. Many
other _Algæ_ produce gametes mainly at the end of the vegetative
season, when food is certainly difficult to obtain in their natural
habitat, and we may well suppose that their assimilative power is
waning. Where, however, as is the case in _Vaucheria_, the plant
depends for propagation mainly on the production of fertilized eggs,
we find the sexual organs often produced in conditions very
favourable to vegetative growth, in opposition to those cases such as
_Hydrodictyon_, where the chief means of propagation is by zoospores.
So that side by side with, and to some extent obscuring, the
principle developed above we have a clear adaptation of the
production of reproductive cells to the special circumstances of the
case."
[33] This establishment by survival of the fittest of reproductive
processes adapted to variable conditions, is indirectly elucidated by
the habits of salmon. As salmon thrive in the sea and fall out of
condition in fresh water (having during their sea-life not exercised
the art of catching fresh-water prey), the implication is that the
species would profit if all individuals ran up the rivers just before
spawning time in November. Why then do most of them run up during
many preceding months? Contemplation of the difficulties which lie in
the way to the spawning grounds, will, I think, suggest an
explanation. There are falls to be leaped and shallow rapids to be
ascended. These obstacles cannot be surmounted when the river is low.
A fish which starts early in the season has more chances of getting
up the falls and the rapids than one which starts later; and, out of
condition as it will be, may spawn, though not well. On the other
hand, one which starts in October, if floods occur appropriately, may
reach the upper waters and then spawn to great advantage; but in the
absence of adequate rains it may fail altogether to reach the
spawning grounds. Hence the species profits by an irregularity of
habits adapted to meet irregular contingencies.
[34] I owe to Mr. (now Sir John) Lubbock an important confirmation of this
view. After stating his belief that between Crustaceans and Insects
there exists a physiological relation analogous to that which exists
between water vertebrata and land-vertebrata, he pointed out to me
that while among Insects there is a definite limit of growth, and an
accompanying definite commencement of reproduction, among
Crustaceans, where growth has no definite limit, there is no definite
relation between the commencement of reproduction and the decrease or
arrest of growth.
[35] While this chapter is passing through the press, I learn from Mr.
White Cooper, that not only are near sight, long sight, dull sight,
and squinting, hereditary; but that a peculiarity of vision confined
to one eye is frequently transmitted: re-appearing in the same eye in
offspring.
[36] An instance here occurs of the way in which those who are averse to a
conclusion will assign the most flimsy reasons for rejecting it.
Rather than admit that the eyes of these creatures living in darkness
have disappeared from lack of use, some contend that such creatures
would be liable to have their eyes injured by collisions with
objects, and that therefore natural selection would favour those
individuals in which the eyes had somewhat diminished and were least
liable to injury: the implication being that the immunity from the
inflammations due to injuries would be so important a factor in life
as to cause survival. And this is argued in presence of the fact that
one of the most conspicuous among these blind cave-animals is a
cray-fish, and that the cray-fish in its natural habitat is in the
habit of burrowing in the banks of rivers holes a foot or more deep,
and has its eyes exposed to all those possible blows and frictions
which the burrowing involves!
[37] In addition to the numerous illustrations given by Mr. Sedgwick, here
is one which Colonel A. T. Fraser published in _Nature_ for Nov. 9,
1893, concerning two Hindoo dwarfs:--"In speech and intelligence the
dwarfs were indistinguishable from ordinary natives of India. From an
interrogation of one of them, it appeared that he belonged to a
family all the male members of which have been dwarfs for several
generations. They marry ordinary native girls, and the female
children grow up like those of other people. The males, however,
though they develop at the normal rate until they reach the age of
six, then cease to grow, and become dwarfs."
[38] This remarkable case appears to militate against the conclusion,
drawn a few pages back, that the increase of a peculiarity by
coincidence of "spontaneous variations" in successive generations, is
very improbable; and that the special superiorities of musical
composers cannot have thus arisen. The reply is that the extreme
frequency of the occurrence among so narrow a class as that of
musical composers, forbids the interpretation thus suggested.
[39] I omitted to name here a cause which may be still more potent in
producing irregularity in the results of cousin-marriages. So far as
I can learn, no attempt has been made to distinguish between such
results as arise when the related parents from whom the cousins
descend are of the same sex and those which arise when they are of
different sexes. In the one case two sisters have children who
intermarry; and in the other case a brother and a sister have
children who intermarry. The marriages of cousins in these two cases
may be quite dissimilar in their results. If there is a tendency to
limitation of heredity by sex--if daughters usually inherit more from
the mother than sons do, while sons inherit more from the father than
from the mother, then two sisters will on the average of cases be
more alike in constitution than a sister and a brother. Consequently
the descendants of two sisters will differ less in their
constitutions than the descendants of a brother and a sister; and
marriage in the first case will be more likely to prove injurious
from absence of dissimilarity in the physiological units than
marriage in the second. My own small circle of friends furnishes
evidence tending to verify this conclusion. In one instance two
cousins who intermarried are children of two sisters, and they have
no offspring. In another the cousins who intermarried are children of
two brothers, and they have no offspring. In the third case the
cousins were descendants of two brothers and only one child resulted.
[40] _A propos_ of this sentence one of my critics writes:--"I cannot find
in this book the statement as first made that the 'life of an
individual is maintained by the unequal and ever-varying actions of
incident forces on its different parts.' Recent physiological work
offers a startling example of the statement."
To the question contained in the first sentence the answer is that I
have not made the statement in the above words, but that it is
implied in the chapter entitled "The Degree of Life varies as the
Degree of Correspondence," and more especially in § 36, which,
towards its close, definitely involves the statement. The verifying
evidence my critic gives me is this:--
"Prof. Sherrington has shown that if the sensory roots of the spinal
nerves are cut one by one there is at first no general effect
produced. That is to say, the remainder of the nervous system
continues to function as before. This condition (lack of general
effect) persists until about six pairs have been cut. With the
severance of the seventh pair, however, the whole central nervous
system ceases to function, so that stimulation of intact sensory
nerves produces no reflex action. After a variable period, but one of
many hours duration, the power of functioning is recovered. That is
to say, if the sensory impulses (from the skin, &c.) reaching the
central nervous system are rapidly reduced in amount, there comes a
point where those remaining do not suffice to keep the structure
'awake.' After a time, however, it adjusts itself to work with the
diminished supply. Similarly Strumpell describes the case of a boy
'whose sensory inlets were all paralyzed except one eye and one ear.'
When these were closed he instantly fell asleep."
[41] Fifty years before the discovery of the Röntgen rays and those
habitually emanating from uranium, it had been observed by Moser that
under certain conditions the surfaces of metals receive permanent
impressions from appropriate objects placed upon them. Such facts
show that the molecules of substances propagate in all directions
special ethereal undulations determined by their special
constitutions.
[42] This classification, and the three which follow it, I quote
(abridging some of them) from Prof. Agassiz's "Essay on
Classification."
[43] For explanations, see "Illogical Geology," _Essays_, Vol. I. How much
we may be misled by assuming that because the remains of creatures of
high types have not been found in early strata, such creatures did
not exist when those strata were formed, has recently (1897) been
shown by the discovery of a fossil Sea-cow in the lower Miocene of
Hesse-Darmstadt. The skeleton of this creature proves that it
differed from such Sirenian mammals as the existing Manatee only in
very small particulars: further dwindling of disused parts being an
evident cause. The same is true as regards, now, we consider that
since the beginning of Miocene days this aberrant type of mammal has
not much increased its divergence from the ordinary mammalian type;
if we then consider how long it must have taken for this large
aquatic mammal (some eight or ten feet long) to be derived by
modification from a land-mammal; and if then we contemplate the
probable length of the period required for the evolution of that
land-mammal out of a pre-mammalian type; we seem carried back in
thought to a time preceding any of our geologic records. We are shown
that the process of organic evolution has most likely been far slower
than is commonly supposed.
[44] Since this passage was written, in 1863, there has come to light much
more striking evidence of change from a more generalized to a less
generalized type during geologic time. In a lecture delivered by him
in 1876, Prof. Huxley gave an account of the successive modifications
of skeletal structure in animals allied to the horse. Beginning with
the _Orohippus_ of the Eocene formation, which had four complete toes
on the front limb and three toes on the hind limb, he pointed out the
successive steps by which in the _Mesohippus_, _Miohippus_,
_Protohippus_, and _Pliohippus_, there was a gradual approach to the
existing horse.
[45] Several of the arguments used in this chapter and in that which
follows it, formed parts of an essay on "The Development Hypothesis,"
originally published in 1852.
[46] _Studies from the Morphological Laboratory in the University of
Cambridge_, vol. vi, p. 84.
[47] _Ibid._, p. 81.
[48] _Studies from the Morphological Laboratory in the University of
Cambridge_, vol. vi, p. 89.
[49] Early in our friendship (about 1855) Prof. Huxley expressed to me his
conviction that all the higher articulate animals have twenty
segments or somites. That he adhered to this view in 1880, when his
work on _The Crayfish_ was published, is shown by his analysis there
given of the twenty segments existing in this fluviatile crustacean;
and adhesion to it had been previously shown in 1877, when his work
on _The Anatomy of Invertebrated Animals_ was published. On p. 398 of
that work he writes:--"In the abdomen there are, at most, eleven
somites, none of which, in the adult, bear ambulatory limbs. Thus,
assuming the existence of six somites in the head, the normal number
of somites in the body of insects will be twenty, as in the higher
_Crustacea_ and _Arachnida_." To this passage, however, he puts the
note:--"It is open to question whether the podical plates represent a
somite; and therefore it must be recollected that the total number of
somites, the existence of which can be actually demonstrated in
insects, is only seventeen, viz., four for the head, three for the
thorax, and ten for the abdomen." I have changed the number twenty,
which in the original edition occurred in the text, to the number
seventeen in deference to suggestions made to me; though I find in
Dr. Sharp's careful and elaborate work on the _Insecta_, that
Viallanes and Cholodkovsky agree with Huxley in believing that there
are six somites in the insect-head. The existence of a doubt on this
point, however, does not essentially affect the argument, since there
is agreement among morphologists respecting the _constancy_ of the
total number of somites in insects.
[50] To avoid circumlocution I let these words stand, though they are not
truly descriptive; for the prosperity of imported species is largely,
if not mainly, caused by the absence of those natural enemies which
kept them down at home.
[51] While these pages are passing through the press (in 1864), Dr. Hooker
has obliged me by pointing out that "plants afford many excellent
examples" of analogous transitions. He says that among true "water
plants," there are found, in the same species, varieties which have
some leaves submerged and some floating; other varieties in which
they are all floating; and other varieties in which they are all
submerged. Further, that many plants characterized by floating
leaves, and which have all their leaves floating when they grow in
deeper water, are found with partly aerial leaves when they grow in
shallower water; and that elsewhere they occur in almost dry soil
with all their leaves aerial.
[52] It will be seen that the argument naturally leads up to this
expression--Survival of the Fittest--which was here used for the
first time. Two years later (July, 1866) Mr. A. R. Wallace wrote to
Mr. Darwin contending that it should be substituted for the
expression "Natural Selection." Mr. Darwin demurred to this proposal.
Among reasons for retaining his own expression he said that I had
myself, in many cases, preferred it--"continually using the words
Natural Selection." (_Life and Letters_, &c., vol. III, pp. 45-6.)
Mr. Darwin was quite right in his statement, but not right in the
motive he ascribed to me. My reason for frequently using the phrase
"Natural Selection," after the date at which the phrase "Survival of
the Fittest" was first used above, was that disuse of Mr. Darwin's
phrase would have seemed like an endeavour to keep out of sight my
own indebtedness to him, and the indebtedness of the world at large.
The implied feeling has led me ever since to use the expressions
Natural Selection and Survival of the Fittest with something like
equal frequency.
[53] I am indebted to Mr. [now Sir W.] Flower for the opportunity of
examining the many skulls in the Museum of the College of Surgeons
for verification of this. Unfortunately the absence, in most cases,
of some or many teeth, prevented me from arriving at that specific
result which would have been given by weighing a number of the under
jaws in each race. Simple inspection, however, disclosed a
sufficiently-conspicuous difference. The under jaws of Australians
and Negroes, when collated with those of Englishmen, were visibly
larger, not only relatively but absolutely. One Australian jaw only
seemed about of the same size as an average English jaw; and this
(probably the jaw of a woman), belonging as it did to a smaller
skull, bore a greater ratio to the whole body of which it formed
part, than did an English jaw of the same actual size. In all the
other cases, the under jaws of these inferior races (containing
larger teeth than our own) were _absolutely_ more massive than our
own--often exceeding them in all dimensions; and _relatively_ to
their smaller skeletons were much more massive. Let me add that the
Australian and Negro jaws are thus strongly contrasted, not with all
British jaws, but only with the jaws of the civilized British. An
ancient British skull in the collection possesses a jaw almost or
quite as massive as those of the Australian skulls. All this is in
harmony with the alleged relation between greater size of jaws and
greater action of jaws, involved by the habits of savages.
[In 1891 Mr. F. Howard Collins carefully investigated this matter:
measuring ten Australian, ten Ancient British, and ten recent English
skulls in the College of Surgeons Museum. The result proved an
absolute difference of the kind above indicated, and a far greater
relative difference. To ascertain this last a common standard of
comparison was established--an equal size of skull in all the cases;
and then when the relative masses or cubic sizes of the jaws were
calculated, the result which came out was this:--Australian jaw,
1948; Ancient British jaw, 1135; Recent English jaw, 1030. "Hence,"
in the words of Mr. Collins, "the mass of the Recent English jaw is,
roughly speaking, half that of the Australian relatively to that of
the skull, and a ninth less than that of the Ancient British." He
adds verifying evidence from witnesses who have no hypothesis to
support--members of the Odontological Society. The Vice-President,
Mr. Mummery, remarks of the Australians that "the jaw-bones are
powerfully developed, and large in proportion to the cranium."]
[54] As bearing on the question of the varieties of Man, let me here refer
to a paper on "The Origin of the Human Races" read before the
Anthropological Society, March 1st, 1864, by Mr. Alfred Wallace. In
this paper, Mr. Wallace shows that along with the attainment of that
intelligence implied by the use of implements, clothing, &c., there
arises a tendency for modifications of brain to take the place of
modifications of body: still, however, regarding the natural
selection of spontaneous variations as the cause of the
modifications. But if the foregoing arguments be valid, natural
selection here plays but the secondary part of furthering the
adaptations otherwise caused. It is true that, as Mr. Wallace argues,
and as I have myself briefly indicated (see _Westminster Review_, for
April, 1852, pp. 496-501), the natural selection of races leads to
the survival of the more cerebrally-developed, while the less
cerebrally-developed disappear. But though natural selection acts
freely in the struggle of one society with another; yet, among the
units of each society, its action is so interfered with that there
remains no adequate cause for the acquirement of mental superiority
by one race over another, except the inheritance of
functionally-produced modifications.
[55] _Darwin and after Darwin_, Part II, p. 99.
[56] _Essays upon Heredity_, vol. i, p. 90.
[57] In a letter published by Dr. Romanes in _Nature_, for April 26, 1894,
he alleges three reasons why "as soon as selection is withdrawn from
an organ the _minus_ variations of that organ outnumber the _plus_
variations." The first is that "the survival-mean must descend to the
birth-mean." The interpretation of this is that if the members of a
species are on the average born with an organ of the required size,
and if they are exposed to natural selection, then those in which the
organ is relatively small will some of them die, and consequently the
mean size of the organ at adult age will be greater than at birth.
Contrariwise, if the organ becomes useless and natural selection does
not operate on it, this difference between the birth-mean and the
survival-mean disappears. Now here, again, the _plus_ variations and
their effects are ignored. Supposing the organ to be useful, it is
tacitly assumed that while _minus_ variations are injurious, _plus_
variations are not injurious. This is untrue. Superfluous size of an
organ implies several evils:--Its original cost is greater than
requisite, and other organs suffer; the continuous cost of its
nutrition is unduly great, involving further injury; it adds
needlessly to the weight carried and so again is detrimental; and
there is in some cases yet a further mischief--it is in the way.
Clearly, then, those in which _plus_ variations of the organ have
occurred are likely to be killed off as well as those in which
_minus_ variations have occurred; and hence there is no proof that
the survival-mean will exceed the birth-mean. Moreover the assumption
has a fatal implication. To say that the survival-mean of an organ is
greater than the birth-mean is to say that the organ is greater _in
proportion to other organs_ than it was at birth. What happens if
instead of one organ we consider all the organs? If the survival-mean
of a particular organ is greater than its birth-mean, the survival
mean of each other organ must also be greater. Thus the proposition
is that every organ has become larger in relation to every other
organ!--a marvellous proposition. I need only add that Dr. Romanes'
inferences with respect to the two other causes--atavism and failing
heredity--are similarly vitiated by ignoring the plus variations and
their effects.
[58] _Westminster Review_, January, 1860. See also _Essays, &c._, vol. i,
p. 290.
[59] "On Orthogenesis and the Impotence of Natural Selection in
Species-Formation," pp. 2, 19, 22, 24.
[60] Address to Plymouth Institution, at opening of Session 1895-6.
[61] _Westminster Review_, April, 1857. "Progress: its Law and Cause." See
also _Essays_, vol. i.
[62] It may be needful to remark, that by the proposed expression it is
intended to define--not Life in its essence; but, Life as manifested
to us--not Life as a _noumenon_: but, Life as a _phenomenon_. The
ultimate mystery is as great as ever: seeing that there remains
unsolved the question--What _determines_ the co-ordination of
actions?
[63] _Prin. of Phys._, 2nd edit., p. 77.
[64] _Ibid._, 3rd edit., p 249.
[65] _Ibid._, p. 124.
[66] Agassiz and Gould, p. 274.
[67] _Prin. of Phys._, 3rd edit., p. 964.
[68] "Parthenogenesis," p. 8.
[69] _Prin. of Phys._, p. 92.
[70] _Ibid._, p. 93.
[71] _Ibid._, p. 917.
[72] "A General Outline of the Animal Kingdom." By Prof. T. R. Jones, F.
G. S., p. 61.
[73] Carpenter.
[74] _Prin. of Phys._, p. 873.
[75] _Ibid._, p. 203.
[76] _Ibid._, p. 209.
[77] _Ibid._, p. 249.
[78] _Ibid._, p. 249.
[79] _Ibid._, p. 250.
[80] _Prin. of Phys._, p. 256.
[81] _Ibid._, p. 212.
[82] _Ibid._, p. 266.
[83] _Prin. of. Phys._, p. 267.
[84] _Ibid._, p. 276.
[85] _Ibid._, 2nd edit., p. 115.
[86] _Prin. of Phys._, p. 954.
[87] _Ibid._, p. 958.
[88] _Ibid._, p. 688.
[89] _Ibid._, p. 958.
[90] "A General Outline of the Animal Kingdom." By Professor T. R. Jones,
p. 61.
[91] _Prin. of Phys._, p. 907.
[92] Should it be objected that in the higher plants the sperm-cell and
germ-cell differ, though no distinct co-ordinating system exists, it
is replied that there _is_ co-ordination of actions, though of a
feeble kind, and that there must be some agency by which this is
carried on.
[93] It is a significant fact that amongst the dioecious invertebrata,
where the nutritive system greatly exceeds the other systems in
development, the female is commonly the largest, and often greatly
so. In some of the Rotifera the male has no nutritive system at all.
See _Prin. of Phys._, p. 954.
[94] _Prin. of Phys._, p. 908.
[95] "Parthenogenesis," pp. 66, 67.
[96] "Lectures on Animal Chemistry." By Dr. Bence Jones. _Medical
Times_, Sept. 13th, 1851. See also _Prin. of Phys._, p. 171.
[97] _Cyclopædia of Anatomy and Physiology_, Vol. IV, p. 506.
[98] From a remark of Drs. Wagner and Leuckart this chemical evidence
seems to have already suggested the idea that the sperm-cell becomes
"metamorphosed into the central parts of the nervous system." But
though they reject this assumption, and though the experiments of Mr.
Newport clearly render it untenable, yet none of the facts latterly
brought to light conflict with the hypothesis that the sperm-cell
contains unorganized co-ordinating matter.
[99] Quain's _Elements of Anatomy_, p. 672.
[100] The maximum weight of the horse's brain is 1 lb. 7 ozs.; the human
brain weighs 3 lbs., and occasionally as much as 4 lbs.; the brain of
a whale, 75 feet long, weighed 5 lbs. 5 ozs.; and the elephant's
brain reaches from 8 lbs. to 10 lbs. Of the whale's fertility we know
nothing; but the elephant's quite agrees with the hypothesis. The
elephant does not attain its full size until it is thirty years old,
from which we may infer that it arrives at a reproductive age later
than man does; its period of gestation is two years, and it produces
one at a birth. Evidently, therefore, it is much less prolific than
man. See Müller's _Physiology_ (Baly's translation), p. 815, and
Quain's _Elements of Anatomy_, p. 671.
[101] That the size of the nervous system is the measure of the ability to
maintain life, is a proposition that must, however, be taken with
some qualifications. The ratio between the amounts of gray and white
matter present in each case is probably a circumstance of moment.
Moreover, the temperature of the blood may have a modifying
influence; seeing that small nervous centres exposed to rapid
oxidation will be equivalent to larger ones more slowly oxidized.
Indeed, we see amongst mankind, that though, in the main, size of
brain determines mental power, yet temperament exercises some
control. There is reason to think, too, that certain kinds of nervous
action involve greater consumption of nervous tissue than others; and
this will somewhat complicate the comparisons. Nevertheless, these
admissions do not affect the generalization as a whole, but merely
prepare us to meet with minor irregularities.
[102] Let me here note in passing a highly significant implication. The
development of nervous structures which in such cases take place,
cannot be limited to the finger-ends. If we figure to ourselves the
separate sensitive areas which severally yield independent feelings,
as constituting a network (not, indeed, a network sharply marked out,
but probably one such that the ultimate fibrils in each area intrude
more or less into adjacent areas, so that the separations are
indefinite), it is manifest that when, with exercise, the structure
has become further elaborated, and the meshes of the network smaller,
there must be a multiplication of fibres communicating with the
central nervous system. If two adjacent areas were supplied by
branches of one fibre, the touching of either would yield to
consciousness the same sensation: there could be no discrimination
between points touching the two. That there may be discrimination,
there must be a distinct connection between each area and the tract
of grey matter which receives the impressions. Nay more, there must
be, in this central recipient-tract, an added number of the separate
elements which, by their excitements, yield separate feelings. So
that this increased power of tactual discrimination implies a
peripheral development, a multiplication of fibres in the
trunk-nerve, and a complication of the nerve-centre. It can scarcely
be doubted that analogous changes occur under analogous conditions
throughout all parts of the nervous system--not in its sensory
appliances only, but in all its higher co-ordinating appliances, up
to the highest.
[103] _Essays upon Heredity_, p. 87.
[104] _Les Maladies des Vers à soie_, par L. Pasteur, Vol. I, p. 39.
[105] Curiously enough, Weismann refers to, and recognizes, syphilitic
infection of the reproductive cells. Dealing with Brown-Séquard's
cases of inherited epilepsy (concerning which, let me say, that I do
not commit myself to any derived conclusions), he says:--"In the case
of epilepsy, at any rate, it is easy to imagine [many of Weismann's
arguments are based on things 'it is easy to imagine'] that the
passage of some specific organism through the reproductive cells may
take place, as in the case of syphilis" (p. 82). Here is a sample of
his reasoning. It is well known that epilepsy is frequently caused by
some peripheral irritation (even by the lodging of a small foreign
body under the skin), and that, among peripheral irritations causing
it, imperfect healing is one. Yet though, in Brown-Séquard's cases, a
peripheral irritation caused in the parent by local injury was the
apparent origin, Weismann chooses gratuitously to assume that the
progeny were infected by "some specific organism," which produced the
epilepsy! And then though the epileptic virus, like the syphilitic
virus, makes itself at home in the egg, the parental protoplasm is
not admitted!
[106] _Philosophical Transactions of the Royal Society for the Year 1821_,
Part I, pp. 20-24.
[107] It will, I suppose, be said that the non-inheritance of mutilations
constitutes evidence of the kind here asked for. The first reply is
that the evidence is conflicting, as it may well be. It is forgotten
that to have valid evidence of non-inheritance of mutilations, it is
requisite that both parents shall have undergone mutilation, and that
this does not often happen. If they have not, then, assuming the
inheritableness of mutilations, there would, leaving out other
causes, be an equal tendency to appearance and non-appearance of the
mutilation in offspring. But there is another cause--the tendency to
reversion, which ever works in the direction of cancelling individual
characters by the return to ancestral characters. So that even were
the inheritance of mutilations to be expected (and for myself I may
say that its occurrence surprises me), it could not be reasonably
looked for as more than exceptional: there are two strong
countervailing tendencies. But now, in the second place, let it be
remarked that the inheritance or non-inheritance of mutilations is
beside the question. The question is whether modifications of parts
produced by modifications of functions are inheritable or not. And
then, by way of disproof of their inheritableness, we are referred to
cases in which the modifications of parts are not produced by
modifications of functions, but are otherwise produced!
[108] See _First Principles_, Part II, Chap. XXII, "Equilibration."
[109] _Principles of Biology_, § 46, (No. 8. April, 1863).
[110] _Ibid._ This must not be understood as implying that while the mass
increases as the cubes, the _quantity of motion_ which can be
generated increases only as the squares; for this would not be true.
The quantity of motion is obviously measured, not by the sectional
areas of the muscles alone, but by these multiplied into their
lengths, and therefore increases as the cubes. But this admission
leaves untouched the conclusion that the ability to _bear stress_
increases only as the squares; and thus limits the ability to
generate motion, by relative incoherence of materials.
[111] _The Transactions of the Linnæan Society of London_, Vol. XXII, p.
215. The estimate of Reaumur, cited by Kirby and Spence, is still
higher--"in five generations one Aphis may be the progenitor of
5,904,900,000 descendants; and that it is supposed that in one year
there may be twenty generations." (_Introduction to Entomology_, Vol.
I, p. 175)
[112] _A Manual of the Anatomy of Invertebrated Animals_, by T. H. Huxley,
p. 206.
[113] Respecting the _Eloidea_ I learn that in 1879--thirty years after it
had become a pest--one solitary male plant was found in a pond near
Edinburgh; but "in an exhaustive inquiry on the plant made by Dr.
Groenland, of Copenhagen, he could find no trace of any male
specimens having been found in Europe other than the Scotch." In
waters from which the _Eloidea_ has disappeared, it seems to have
done so in consequence of the growth of an _Alga_, which has produced
turbid water unfavourable to it. That is to say, the decreased
multiplication of somatic cells in some cases, is not due to any
exhaustion, but is caused by the rise of enemies or adverse
conditions; as happens generally with introduced species of plants
and animals which multiply at first enormously, and then, without any
loss of reproductive power, begin to decrease under the antagonizing
influences which grow up.
[114] _A Text Book of Human Physiology._ By Austin Flint, M.D., LL.D.
Fourth edition. New York: D. Appleton & Co. 1888. Page 797.
[115] This supposition I find verified by Mr. A. S. Packard in his
elaborate monograph on "The Cave Fauna of North America, &c.," as
also in his article published in the _American Naturalist_,
September, 1888; for he there mentions "variations in _Pseudotremia
cavernarum_ and _Tomocerus plumbeus_, found living near the entrance
to caves in partial daylight." The facts, as accumulated by Mr.
Packard, furnished a much more complete answer to Prof. Lankester
than is above given, as, for example, the "blindness of _Neotoma_, or
the Wood-Rat of Mammoth Cave." It seems that there are also "cave
beetles, with or without rudimentary eyes," and "eyeless spiders" and
Myriapods. And there are insects, as some "species of Anophthalmus
and Adelops, whose larvæ are lacking in all traces of eyes and optic
nerves and lobes." These instances cannot be explained as sequences
of an inrush of water carrying with it the remote ancestors, some of
which did not find their way out; nor can others of them be explained
by supposing an inrush of air, which did the like.
[116] See "Social Organism" in _Westminster Review_ for January, 1860; also
_Principles of Sociology_, § 247.
[117] _Contemporary Review_, September, 1893.
[118] _Evolution of Sex_, p. 50.
[119] _Souvenirs Entomologiques_, 3^{me} Série, p. 328.
[120] _Natural History of Bees_, new ed., p. 33.
[121] _Origin of Species_, 6th ed., p. 232.
[122] _Contemporary Review_, September, 1893, p. 333.
[123] _The Entomologist's Monthly Magazine_, March, 1892, p. 61.
[124] Perhaps it will be alleged that nerve-matter is costly, and that this
minute economy might be of importance. Anyone who thinks this will no
longer think it after contemplating a litter of half-a-dozen young
rabbits (in the wild rabbit the number varies from four to eight);
and on remembering that the nerve-matter contained in their brains
and spinal cords, as well as the materials for building up the bones,
muscles, and viscera of their bodies, has been supplied by the doe in
the space of a month; at the same time that she has sustained herself
and carried on her activities: all this being done on relatively poor
food. Nerve-matter cannot be so very costly then.
[125] _Loc. cit._, p. 318.
[126] _The Germ Plasm_, p. 54.
[127] While Professor Weismann has not dealt with my argument derived from
the distribution of discriminativeness on the skin, it has been
criticized by Mr. McKeen Cattell, in the last number of _Mind_
(October, 1893). His general argument, vitiated by extreme
misconceptions, I need not deal with. He says:--"Whether changes
acquired by the individual are hereditary, and if so to what extent,
is a question of great interest for ethics no less than for biology.
But Mr. Spencer's application of this doctrine to account for the
origin of species [!] simply begs the question. He assumes useful
variations [!]--whether of structure or habit is immaterial--without
attempting to explain their origin": two absolute misstatements in
two sentences! The only part of Mr. Cattell's criticism requiring
reply is that which concerns the "sensation-areas" on the skin. He
implies that since Weber, experimental psychologists have practically
set aside the theory of sensation areas: showing, among other things,
that relatively great accuracy of discrimination can be quickly
acquired by "increased interest and attention.... Practice for a few
minutes will double the accuracy of discrimination, and practice on
one side of the body is carried over to the other." To me it seems
manifest that "increased interest and attention" will not enable a
patient to discriminate two points where a few minutes before he
could perceive only one. That which he can really do in this short
time is to learn to discriminate between the _massiveness of a
sensation_ produced by two points and the massiveness of that
produced by one, and to _infer_ one point or two points accordingly.
Respecting the existence of sensation-areas marked off from one
another, I may, in the first place, remark that since the eye
originates as a dermal sac, and since its retina is a highly
developed part of the sensitive surface at large, and since the
discriminative power of the retina depends on the division of it into
numerous rods and cones, each of which gives a separate
sensation-area, it would be strange were the discriminative power of
the skin at large achieved by mechanism fundamentally different. In
the second place I may remark that if Mr. Cattell will refer to
Professor Gustav Retzius's _Biologische Untersuchungen_, New Series,
vol. iv (Stockholm, 1892), he will see elaborate diagrams of
superficial nerve-endings in various animals showing many degrees of
separateness. I guarded myself against being supposed to think that
the sensation-areas are sharply marked off from one another; and
suggested, contrariwise, that probably the branching
nerve-terminations intruded among the branches of adjacent
nerve-terminations. Here let me add that the intrusion may vary
greatly in extent; and that where the intruding fibres run far among
those of adjacent areas, the discriminativeness will be but small,
while it will be great in proportion as each set of branching fibres
is restricted more nearly to its own area. All the facts are
explicable on this supposition.
[128] To save space and exclude needless complication I have omitted these
passages from the preceding divisions of this appendix.
[129] Though Professor Weismann does not take up the challenge, Dr. Romanes
does. He says:--"When selection is withdrawn there will be no
excessive _plus_ variations, because so long as selection was present
the efficiency of the organ was maintained at its highest level: it
was only the _minus_ variations which were then eliminated"
(_Contemporary Review_, p. 611). In the first place, it seems to me
that the phrases used in this sentence beg the question. It says that
"the efficiency of the organ was maintained at its _highest_ level";
which implies that the highest level (tacitly identified with the
greatest size) is the best and that the tendency is to fall below it.
This is the very thing I ask proof of. Suppose I invert the idea and
say that the organ is maintained at its right size by natural
selection, because this prevents increase beyond the size which is
best for the organism. Every organ should be in due proportion, and
the welfare of the creature as a whole is interfered with by excess
as well as by defect. It may be directly interfered with--as for
instance by too big an eyelid; and it may be indirectly interfered
with, where the organ is large, by needless weight and cost of
nutrition. In the second place the question which here concerns us is
not what natural selection will do with variations. We are concerned
with the previous question--What variations will arise? An organ
varies in all ways; and, unless reason to the contrary is shown, the
assumption must be that variations in the direction of increase are
as frequent and as great as those in the direction of decrease. Take
the case of the tongue. Certainly there are tongues inconveniently
large, and probably tongues inconveniently small. What reason have we
for assuming that the inconveniently small tongues occur more
frequently than the inconveniently large ones? None that I can see.
Dr. Romanes has not shown that when natural selection ceases to act
on an organ the _minus_ variations in each new generation will exceed
the _plus_ variations. But if they are equal the alleged process of
panmixia has no place.
[130] _The Variation of Animals and Plants under Domestication_, vol. ii,
p. 292.
[131] _Journal of the Anthropological Institute_ for 1885, p. 253.
[132] In "The All-Sufficiency of Natural Selection" (_Contemporary Review_,
Sept., 1893, p. 311), Professor Weismann writes:--"I have ever
contended that the acceptance of a principle of explanation is
justified, if it can be shown that without it certain facts are
inexplicable." Unless, then, Prof. Weismann can show that the
distribution of discriminativeness is otherwise explicable, he is
bound to accept the explanation I have given, and admit the
inheritance of acquired characters.
[133] Prof. Weismann is unaware that the view here ascribed to Roux,
writing in 1881, is of far earlier date. In the _Westminster Review_
for January, 1860, in an essay on "The Social Organism," I
wrote:--"One more parallelism to be here noted, is that the different
parts of a social organism, like the different parts of an individual
organism, compete for nutriment; and severally obtain more or less of
it according as they are discharging more or less duty." (See also
_Essays_, i, 290.) And then, in 1876, in _The Principles of
Sociology_, vol. i, § 247, I amplified the statement thus:--"All
other organs, therefore, jointly and individually, compete for blood
with each organ ... local tissue-formation (which under normal
conditions measures the waste of tissue in discharging function) is
itself a cause of increased supply of materials ... the resulting
competition, not between units simply, but between organs, causes in
a society, as in a living body, high nutrition and growth of parts
called into greatest activity by the requirements of the rest."
Though I did not use the imposing phrase
"intra-individual-selection," the process described is the same.
[134] _Proceedings of the Biological Society of Washington_, vol. ix.
[135] Romanes Lecture, p. 29.
[136] _Ibid._, p. 35.
[137] This interpretation harmonizes with a fact which I learn from Prof.
Riley, that there are gradations in this development, and that in
some species the ordinary neuters swell their abdomens so greatly
with food that they can hardly get home.
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