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Title: Evolution and Adaptation
Author: Morgan, Thomas Hunt
Language: English
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EVOLUTION AND ADAPTATION
------------------------------------------------------------------------
[Illustration: Publisher's logo (The Macmillan Company)]
------------------------------------------------------------------------
EVOLUTION AND ADAPTATION
by
THOMAS HUNT MORGAN, Ph.D.
New York
The Macmillan Company
London: Macmillan & Co., Ltd.
1908
All rights reserved
Copyright, 1903,
by The Macmillan Company.
Set up and electrotyped. Published October, 1903. Reprinted January,
1908.
Norwood Press
J. S. Cushing Co.—Berwick & Smith Co.
Norwood, Mass., U.S.A.
------------------------------------------------------------------------
TO
Professor William Keith Brooks
AS A TOKEN OF SINCERE ADMIRATION AND RESPECT
------------------------------------------------------------------------
PREFACE
The adaptation of animals and plants to the conditions under which they
live has always excited the interest, and also the imagination, of
philosophers and scientists; for this relation between the organism and
its environment is one of the most characteristic features of living
things. The question at once suggests itself: How has such a relation
been brought about? Is it due to something inherent in the living matter
itself, or is it something that has been, as it were, superimposed upon
it? An example may make my meaning clearer. No one will suppose that
there is anything inherent in iron and other metals that would cause
them to produce an engine if left to themselves. The particular
arrangement of the pieces has been superimposed upon the metals, so that
they now fulfil a purpose, or use. Have the materials of which organisms
are composed been given a definite arrangement, so that they fulfil the
purpose of maintaining the existence of the organism; and if so, how has
this been accomplished? It is the object of the following pages to
discuss this question in all its bearings, and to give, as far as
possible, an idea of the present state of biological thought concerning
the problem. I trust that the reader will not be disappointed if he
finds in the sequel that many of the most fundamental questions in
regard to adaptation are still unsettled.
In attempting to state the problem as clearly as possible, I fear that
it may appear that at times I have “taken sides,” when I should only
have been justified in stating the different aspects of the question.
But this will do little harm provided the issue has been sharply drawn.
Indeed, it seems to me that the only scientific value, that a discussion
of what the French call “les grands problèmes de la Biologie” has, is to
get a clearer understanding of the relation of what is known to what is
unknown or only surmised.
In some quarters speculation concerning the origin of the adaptation of
living things is frowned upon, but I have failed to observe that the
critics themselves refrain entirely from theorizing. They shut one door
only to open another, which also leads out into the dark. To deny the
right to speculative thought would be to deny the right to use one of
the best tools of research.
Yet it must be admitted that all speculation is not equally valuable.
The advance of science in the last hundred years has shown that the kind
of speculation that has real worth is that which leads the way to
further research and possible discovery. Speculation that leads to this
end must be recognized as legitimate. It becomes useless when it deals
with problems that cannot be put to the actual test of observation or
experiment. It is in this spirit that I have approached the topics
discussed in the following pages.
The unsophisticated man believes that all other animals exist to
minister to his welfare; and from this point of view their adaptations
are thought of solely in their relation to himself. A step in advance
was taken when the idea was conceived that adaptations are for the good
of the organisms themselves. It seemed a further advance when the
conclusion was reached that the _origin_ of adaptations could be
accounted for, as the result of the benefit that they conferred on their
possessor. This view was the outcome of the acceptation of the theory of
evolution, combined with Darwin’s theory of natural selection. It is the
view held by most biologists at the present time; but I venture to
prophesy that if any one will undertake to question modern zoologists
and botanists concerning their relation to the Darwinian theory, he will
find that, while professing _in a general way_ to hold this theory, most
biologists have many reservations and doubts, which they either keep to
themselves or, at any rate, do not allow to interfere either with their
teaching of the Darwinian doctrine or with the applications that they
may make of it in their writings. The claim of the opponents of the
theory that Darwinism has become a dogma contains more truth than the
nominal followers of this school find pleasant to hear; but let us not,
therefore, too hastily conclude that Darwin’s theory is without value in
relation to one side of the problem of adaptation; for, while we can
profitably reject, as I believe, much of the theory of natural
selection, and more especially the idea that adaptations have arisen
because of their usefulness, yet the fact that living things must be
adapted more or less well to their environment in order to remain in
existence may, after all, account for the widespread _occurrence_ of
adaptation in animals and plants. It is this point of view that will be
developed in the following pages.
I am fully aware of the danger in attempting to cover so wide a field as
that of “Evolution and Adaptation,” and I cannot hope to escape the
criticism that is certain to be directed against a specialist who
ventures nowadays beyond the immediate field of his own researches; yet,
in my own defence, I may state that the whole point of view underlying
the position here taken is the immediate outcome of my work on
regeneration. One of the general questions that I have always kept
before me in my study of regenerative phenomena is how such a useful
acquirement as the power to replace lost parts has arisen, and whether
the Darwinian hypothesis is adequate to explain the result. The
conclusion that I have reached is that the theory is entirely inadequate
to account for the _origin_ of the power to regenerate; and it seemed to
me, therefore, desirable to reëxamine the whole question of adaptation,
for might it not prove true here, also, that the theory of natural
selection was inapplicable? This was my starting-point. The results of
my examination are given in the following pages.
I am deeply indebted to Professor G. H. Parker and to Professor E. G.
Conklin for advice and friendly criticism; and in connection with the
revision of the proof I am under many obligations to Professor Joseph W.
Warren and to Professor E. A. Andrews. Without their generous help I
should scarcely have ventured into a field so full of pitfalls.
Bryn Mawr, Penn., June 10, 1903.
------------------------------------------------------------------------
CONTENTS
CHAPTER I
PAGE
The Problem of Adaptation 1
– Structural Adaptations 1
– Adaptations for the Good of the 19
Species
– Organs of Little Use to the 22
Individual
– Changes in the Organism that are 25
of No Use to the Individual or to
the Race
– Comparison with Inorganic 26
Phenomena
CHAPTER II
The Theory of Evolution 30
– Evidence in Favor of the 32
Transmutation Theory
– – Evidence from Classification 32
and from Comparative Anatomy
– – The Geological Evidence 39
– – Evidence from Direct 43
Observation and Experiment
– – Modern Criticism of the Theory 44
of Evolution
CHAPTER III
The Theory of Evolution (continued) 58
– The Evidence from Embryology 58
– – The Recapitulation Theory 58
– Conclusions 84
CHAPTER IV
Darwin’s Theories of Artificial and 91
of Natural Selection
– The Principle of Selection 91
– Variation and Competition in 104
Nature
– The Theory of Natural Selection 116
CHAPTER V
The Theory of Natural Selection 129
(continued)
– Objections to the Theory of 129
Natural Selection
– Sterility between Species 147
– Weismann’s Germinal Selection 154
CHAPTER VI
Darwin’s Theory of Sexual Selection 167
– Sexual Selection 167
– General Criticism of the Theory 213
of Sexual Selection
CHAPTER VII
The Inheritance of Acquired 222
Characters
– Lamarck’s Theory 222
– Darwin’s Hypothesis of Pangenesis 233
– The Neo-Lamarckian School 240
CHAPTER VIII
Continuous and Discontinuous 261
Variation and Heredity
– Continuous Variation 261
– Heredity and Continuous Variation 270
– Discontinuous Variation 272
– Mendel’s Law 278
– The Mutation Theory of De Vries 287
– Conclusions 297
CHAPTER IX
Evolution as the Result of External 300
and Internal Factors
– The Effect of External Influences 300
– Responsive Changes in the 319
Organism that adapt it to the New
Environment
– Nägeli’s Perfecting Principle 325
CHAPTER X
The Origin of the Different Kinds 340
of Adaptations
– Form and Symmetry 340
– Mutual Adaptation of Colonial 350
Forms
– Degeneration 352
– Protective Coloration 357
– Sexual Dimorphism and Trimorphism 360
– Length of Life as an Adaptation 370
– Organs of Extreme Perfection 371
– Secondary Sexual Organs as 372
Adaptations
– Individual Adjustments as 375
Adaptations
– Color Changes as Individual 375
Adaptations
– Increase of Organs through Use 376
and Decrease through Disuse
– Reactions of the Organism to 377
Poisons, etc.
– Regeneration 379
CHAPTER XI
Tropisms and Instincts as 382
Adaptations
CHAPTER XII
Sex as an Adaptation 414
– The Different Kinds of Sexual 414
Individuals
– The Determination of Sex 422
– Sex as a Phenomenon of Adaptation 439
CHAPTER XIII
Summary and General Conclusions 452
INDEX 465
------------------------------------------------------------------------
EVOLUTION AND ADAPTATION
CHAPTER I
THE PROBLEM OF ADAPTATION
Between an organism and its environment there takes place a constant
interchange of energy and of material. This is, in general, also true
for all bodies whether living or lifeless; but in the living organism
this relation is a peculiar one; first, because the plant or the animal
is so constructed that it is suited to a particular set of physical
conditions, and, second, because it may so respond to a change in the
outer world that it further adjusts itself to changing conditions,
_i.e._ the response may be of such a kind that it better insures the
existence of the individual, or of the race. The two ideas contained in
the foregoing statement cover, in a general way, what we mean by the
adaptation of living things. The following examples will serve to
illustrate some of the very diverse phenomena that are generally
included under this head.
Structural Adaptations
The most striking cases of adaptations are those in which a special, in
the sense of an unusual, relation exists between the individual and its
surroundings. For example, the foreleg of the mole is admirably suited
for digging underground. A similar modification is found in an entirely
different group of the animal kingdom, namely, in the mole-cricket, in
which the first legs are also well suited for digging. By their use the
mole-cricket makes a burrow near the surface of the ground, similar to,
but of course much smaller than, that made by the mole. In both of these
cases the adaptation is the more obvious, because, while the leg of the
mole is formed on the same general plan as that of other vertebrates,
and the leg of the mole-cricket has the same fundamental structure as
that of other insects, yet in both cases the details of structure and
the general proportions have been so altered, that the leg is fitted for
entirely different purposes from that to which the legs of other
vertebrates and of other insects are put. The wing of the bat is another
excellent case of a special adaptation. It is a modified fore-limb
having a strong membrane stretched between the fingers, which are
greatly elongated. Here we find a structure, which in other mammals is
used as an organ for supporting the body, and for progression on the
ground, changed into one for flying in the air.
The tails of mammals show a number of different adaptations. The tail is
prehensile in some of the monkeys; and not only can the monkey direct
its tail toward a branch in order to grasp it, but the tail can be
wrapped around the branch and hold on so firmly that the monkey can
swing freely, hanging by its tail alone. The animal has thus a sort of
fifth hand, one as it were in the middle line of the body, which can be
used as a hold-fast, while the fingered hands are put to other uses. In
the squirrels the bushy tail serves as a protection during the winter
for those parts of the body not so thickly covered by hair. The tail of
the horse is used to brush away the flies that settle on the hind parts
of the body. In other mammals, the dog, the cat, and the rat, for
example, the tail is of less obvious use, although the suggestion has
been made that it may serve as a sort of rudder when the animal is
running rapidly. In several other cases, as in the rabbit and in the
higher apes, the tail is very short, and is of no apparent use; and in
man it has completely disappeared.
A peculiar case of adaptation is the so-called basket on the third pair
of legs of the worker honey-bee. A depression of the outer surface of
the tibia is arched over by stiff hairs. The pollen collected from the
stamens of flowers is stowed away in this receptacle by means of the
other pairs of legs. The structure is unique, and is not found in any
other insects except the bees. It is, moreover, present only in the
worker bees, and is absent in the queen and the males.
The preceding cases, in which the adapted parts are used for the
ordinary purposes of life of the individual, are not essentially
different from the cases in which the organ is used to protect the
animal from its enemies. The bad taste of certain insects is supposed to
protect them from being eaten by birds. Cases like this of passive
protection grade off in turn into those in which, by some reflex or
voluntary act, the animal protects itself. The bad-smelling horns of the
caterpillar of the black swallow-tailed butterfly (_Papilio polyxenes_)
are thrust out when the animal is touched, and it is believed that they
serve to protect the caterpillar from attack. The fœtid secretion of the
glands of the skunk is believed to serve as a protection to the animal,
although the presence of the nauseous odor may lead finally to the
extermination of the skunk by man. The sting of bees and of wasps serves
to protect the individual from attack. The sting was originally an
ovipositor, and used in laying the eggs. It has, secondarily, been
changed into an organ of offence.
The special instincts and reflex acts furnish a striking group of
adaptations. The building of the spider’s web is one of the most
remarkable cases of this kind. The construction of the web cannot be the
result of imitation, since, in many instances, the young are born in the
spring of the year following the death of the parents. Each species of
spider has its own type of web, and each web has as characteristic a
form as has the spider itself. It is also important to find that a
certain type of web may be characteristic of an entire family of
spiders. Since, in many cases, the web is the means of securing the
insects used for food, it fulfils a purpose necessary for the welfare of
the spider.
The making of the nests by birds appears to be also in large part an
instinctive act; although some writers are inclined to think that memory
of the nest in which the young birds lived plays a part in their
actions, and imitation of the old birds at the time of nest-building
may, perhaps, also enter into the result. It has been stated that the
first nest built by young birds is less perfect than that built by older
birds, but this may be due to the bird’s learning something themselves
in building their nests, _i.e._ to the perfecting of the instinct in the
individual that makes use of it. In any case much remains that must be
purely instinctive. The construction of the comb by bees appears to be
largely, perhaps entirely, an instinctive act. That this is the case was
shown by isolating young workers as soon as they emerged from the cell,
and before they could have had any experience in seeing comb built. When
given some wax they set to work to make a comb, and made the
characteristic six-sided structures like those made by the bees in a
hive. The formation of so remarkable a structure as the comb is worthy
of admiration, for, with the greatest economy of material, a most
perfect storeroom for the preservation of the honey is secured. This
adaptation appears almost in the nature of foresight, for the store of
honey is used not only to feed the young, but may be drawn on by the
bees themselves in time of need. It is true that a comparison with other
kinds of bees makes it probable that the comb was first made for the
eggs and larvæ, and only later became used as a storehouse, but so far
as its form is concerned there is the same economy of constructive
materials in either case.
The behavior of young birds, more especially those that take care of
themselves from the moment they leave the egg, furnishes a number of
cases of instincts that are protective. If, for example, a flock of
young pheasants is suddenly disturbed, the birds at once squat down on
the ground, and remain perfectly quiet until the danger is past. Their
resemblance to the ground is so perfect that they are almost invisible
so long as they remain quiet. If, instead of remaining still, they were
to attempt to run away when disturbed, they would be much more easily
seen.
Certain solitary wasps (_Ammophila_) have the habit of stinging
caterpillars and spiders, and dragging them to their nests, where they
are stored away for the future use of the young that hatch from the eggs
laid by the wasp on the body of the prey. As a result of the sting which
the wasp administers to the caterpillar, the latter is paralyzed, and
cannot escape from the hole in which it is stored, where it serves as
food for the young wasp that emerges from the egg. It was originally
claimed by Forel that the wasp stings the caterpillar in such a way that
the central nervous system is always pierced, and many subsequent
naturalists have marvelled at the perfection of such a wonderful
instinct. But the recent results of the Peckhams have made it clear that
the act of the wasp is not carried out with the precision previously
supposed, although it is true that the wasp pierces the caterpillar on
the lower surface where the ventral chain of ganglia lies. The habit of
this wasp is not very dissimilar from that shown by many other kinds of
wasps that sting their captive in order to quiet it. We need not imagine
in this case that the act carries with it the consciousness that the
caterpillar, quieted in this way, will be unable to escape before the
young wasps have hatched.
The resemblance in color of many animals to their natural backgrounds
has in recent years excited the interest and imagination of many
naturalists. The name of protective coloration has been given to this
group of phenomena. The following cases which have less the appearance
of purely imaginative writing may serve by way of illustration. A
striking example is that of the ptarmigan which has a pure white coat in
winter, and a brown coat in summer. The white winter plumage renders the
animal less conspicuous against the background of snow, while in summer
the plumage is said to closely resemble the lichen-covered ground on
which the bird rests. The snowy owl is a northern bird, whose color is
supposed to make it less conspicuous, and may serve either as a
protection against enemies, or may allow the owl to approach its prey
unseen. It should not pass unnoticed, however, that there are white
birds in other parts of the world, where their white color cannot be of
any use to them as a protection. The white cockatoos, for example, are
tropical birds, living amongst green foliage, where their color must
make them conspicuous, rather than the reverse.
The polar bear is the only member of the family that is white, and while
this can scarcely be said to protect it from enemies, because it is
improbable that it has anything to fear from the other animals of the
ice-fields, yet it may be claimed that the color is an adaptation to
allow the animal to approach unseen its prey.
In the desert many animals are sand-colored, as seen for instance in the
tawny color of the lion, the giraffe, the antelopes, and of many birds
that live on or near the ground.
It has been pointed out that in the tropics and temperate zones there
are many greenish and yellowish birds whose colors harmonize with the
green and yellow of the trees amongst which they live; but on the other
hand we must not forget that in all climes there are numbers of birds
brilliantly colored, and many of these do not appear to be protected in
any special way. The tanagers, humming-birds, parrots, Chinese
pheasants, birds of paradise, etc., are extremely conspicuous, and so
far as we can see they must be much exposed on account of the color of
their plumage. Whether, therefore, we are justified in picking out
certain cases as examples of adaptation, because of an agreement in
color between the organism and its surroundings, and in neglecting all
others, is, as has been already said, a point to be further examined.
Not only among mammals and birds have many cases of protective
coloration been described by writers dealing with this subject, but in
nearly every group of the animal kingdom similar cases have been
recognized. The green and brown color of lizards may protect them, the
green color of many frogs is supposed to conceal them as they sit
amongst the plants on the edge of a stream or pond. The gray-brown color
of the toad has been described as a resemblance to the dry ground, while
the brilliant green of several tree-frogs conceals them very effectively
amongst the leaves. Many fishes are brilliantly colored, and it has even
been suggested that those living amongst corals and sea-anemonies have
acquired their colors as a protection, but Darwin states that they
appeared to him very conspicuous even in their highly colored
environment.
Amongst insects innumerable cases of adaptive coloration have been
described. In fact this is the favorite group for illustrating the
marvels of protective coloration. A few examples will here serve our
purpose. The oft-cited case of the butterfly _Kallima_ is, apparently, a
striking instance of protective resemblance. When at rest the wings are
held together over the back, as in nearly all butterflies, so that only
the under surface is exposed. This surface has an unquestionably close
resemblance to a brown leaf. It is said on no less authority than that
of Wallace that when this butterfly alights on a bush it is almost
impossible to distinguish between it and a dead leaf. The special point
in the resemblance to which attention is most often called is the
distinct line running obliquely across the wings which looks like the
midrib of a leaf. Whether the need of such a close resemblance to a leaf
is requisite for the life of this butterfly, we do not know, of course,
and so long as we do not have this information there is danger that the
case may prove too much, for, if it should turn out that this remarkable
case is accidental the view in regard to the resemblance may be
endangered.
Amongst caterpillars there are many cases of remarkable resemblances in
color between the animal and its surroundings. The green color of many
of those forms that remain on the leaves of the food-plant during the
day will give, even to the most casual observer, the impression that the
color is for the purpose of concealment; and that it does serve to
conceal the animal there can be no doubt. But even from the point of
view of those who maintain that this color has been acquired because of
its protective value it must be admitted that the color is insufficient,
because some of these same green caterpillars are marvellously armed
with an array of spines which are also supposed to be a protection
against enemies. Equally well protected are the brown and mottled
geometrid caterpillars. These have, moreover, the striking and unusual
habit of fixing themselves by the posterior pairs of false legs, and
standing still and rigid in an oblique position on the twigs to which
they are affixed. So close is their resemblance to a short twig, that
even when their exact position is known it is very difficult to
distinguish them.
Grasshoppers that alight on the ground are, in many cases, so similar to
the surface of the ground that unless their exact location is known they
easily escape attention, while the green color of the katydid, a member
of the same group of orthoptera, protects it from view in the green
foliage of the trees where it lives. The veinlike wings certainly
suggest a resemblance to a leaf, but whether there is any necessity for
so close an imitation may be questioned.
There can be little doubt in some of these cases that the color of the
animal may be a protection to it, but as has been hinted already, it is
another question whether it acquired these colors because of their
usefulness. Nevertheless, if the color is useful to its possessor, it is
an adaptation in our sense of the word, without regard to the way in
which it has been acquired. Even, for instance, if the resemblance were
purely the outcome of chance in the sense that the color appeared
without relation to the surroundings, it would still be an adaptation if
it were of use to the animal under the ordinary conditions of life.
In the lower groups numerous cases in which animals resemble their
surroundings could be given. Such cases are known in crustacea, worms,
mollusks, hydroids, etc., and the possible value of these resemblances
may be admitted in many instances.
It is rather curious that so few cases of adaptive coloration have been
described for plants. No one supposes that the slate color of the lichen
is connected with the color of the rocks on which it grows, in the sense
that the resemblance is of any use to the lichen. Nor does the color of
the marine red algæ serve in any way to protect the plants so far as is
known. The green color of nearly all the higher plants is obviously
connected with the substance, chlorophyl, that is essential for the
processes of assimilation, and has no relation to external objects. But
when we come to the colors of flowers we meet with curious cases of
adaptation, at least according to the generally accepted point of view.
For it is believed by many naturalists that the color of the corolla of
flowering plants is connected with the visits of insects to the flowers,
and these visits are in many cases essential for the cross-fertilization
of the flowers. This adaptation is one useful to the species, rather
than the individual, and belongs to another category.
The leaf of the Venus’s fly-trap, which suddenly closes together from
the sides when a fly or other light body comes to rest on it, is
certainly a remarkable adaptation. A copious secretion of a digestive
fluid is poured out on the surface of the leaf, and the products of
digestion are absorbed. There can be no question that this contrivance
is of some use to the plant. In other insectivorous plants, the pitcher
plants, the leaves are transformed into pitchers. In Nepenthes a
digestive fluid is secreted from the walls. A line of glands secreting a
sweet fluid serves to attract insects to the top of the pitcher, whence
they may wander or fall into the fluid inside, and there being drowned,
they are digested. A lidlike cover projecting over the opening of the
pitcher is supposed to be of use to keep out the rain.
In _Utricularia_, a submerged water-plant, the tips of the leaves are
changed into small bladders, each having a small entrance closed by an
elastic valve opening inwards. Small snails and crustaceans can pass
into this opening, to which they are guided by small outgrowths; but
once in the cup they cannot get out again, and, in fact, small animals
are generally found in the bladders where they die and their substance
is absorbed by forked hairs projecting into the interior of the bladder.
The cactus is a plant that is well suited to a dry climate. Its leaves
have completely disappeared, and the stem has become swollen into a
water-reservoir. “It has been estimated that the amount of water
evaporated by a melon cactus is reduced to one six-hundredth of that
given off by any equally heavy climbing-plant.”
[Illustration:
Fig. 1.—The fertilization of _Aristolochia Clematitis_.
A, portion of stem with flowers in axil of leaf in different stages.
B and C, longitudinal sections of two flowers, before and after
fertilization. (After Sachs.)]
Sachs gives the following account of the fertilization process in
_Aristolochia Clematitis_, which he refers to as a conspicuous and
peculiar adaptation. In Figure 1 A a group of flowers is shown, and in
Figure 1 B and C a single flower is split open to show the interior. In
B a small fly has entered, and has brought in upon its back some pollen
that has stuck to it in another flower. The fly has entered through the
long neck which is beset with hairs which are turned inwards so that the
fly can enter but cannot get out. In roaming about, the pollen that is
sticking to its back will be rubbed against the stigmatic surface. “As
soon as this has taken place the anthers, which have been closed
hitherto, dehisc and become freely accessible,” as a result in the
change in the stigma and of the collapse of the hairs at the base of the
enlargement which has widened. The fly can now crawl under the anthers,
and, if it does so, new pollen may stick to its back. At this time the
hairs in the throat dry up, and the fly can leave its prison house,
Figure 1 C. If the fly now enters another flower this is fertilized by
repeating the process. The unfertilized flowers stand erect with widely
open mouths. As soon as they have been fertilized they bend down, as
seen in Figure 1 A, and at the same time the terminal flap bends over
the open mouth of the throat, “stopping the entrance to the flies, which
have now nothing more to do here.”
Adjustments of the Individual to Changes in the Environment
The most familiar cases of adjustments of the individual to the
environment are those that we recognize in our own bodies. After violent
exercise we breathe more rapidly, and take deeper inspirations. Since
during exercise our blood loses more oxygen and takes in more carbon
dioxide from the muscles, it is clear that one result of more rapid
breathing is to get more oxygen into the blood and more carbon dioxide
out of it. The process of sweating, that also follows exercise, may be
also looked upon as an adaptive process, since by evaporation the skin
is kept cooler, and, in consequence, the blood, which at this time flows
in larger quantities to the skin, is cooled also.
More permanent adaptive changes than these also take place as the result
of prolonged use of certain parts. If the muscles work against powerful
resistance, they become larger after several days or weeks, and are
capable of doing more work than at first. Conversely, when any group of
muscles is not used, it becomes smaller than the normal and capable of
doing less work. It would be a nice point to decide whether this latter
change is also an adaptation. If so it is one in a somewhat different
sense from that usually employed. The result is of no direct advantage
to the animal, except possibly in saving a certain amount of food, but
since the same change will take place when an abundance of food is
consumed, the result is, under these conditions, of no use.
The thickening of the skin on those parts of the body where continued
pressure is brought to bear on it is a change in a useful direction. The
thickening on the soles of the feet and on the palms of the hands is a
case in point. Not only is the skin thicker at birth in these parts, but
it becomes thicker through use. In other parts of the body also, the
skin hardens and becomes thicker if pressure is brought to bear on it.
We may regard this as a general property of the skin, which is present
even in those parts where, under ordinary circumstances, it can rarely
or never be brought into use.
Even as complicated and as much used an organ as the eye can become
adaptively improved. It is said that the lateral region of the field of
vision can be trained to perceive more accurately; and every one who has
used a microscope is familiar with the fact that if one eye is
habitually used it becomes capable of seeing more distinctly and better
than the other eye. This seems to be due, in part at least, to the
greater contraction of the iris.
Another phenomenon, which, I think, must be looked upon as an
adaptation, is the immunity to certain poisons that can be gradually
brought about by slowly increasing the amount introduced into the body.
Nicotine is a most virulent poison, and yet by slowly increasing the
dose an animal can be brought into a condition in which an amount of
nicotine, fatal to an ordinary individual, can be administered without
any ill effects at all resulting.
The same phenomenon has been observed in the case of other poisons, not
only in case of other alkaloids, such as morphine and cocaine, but also
in the case of caffein, alcohol, and even arsenic. There is a curious
phenomenon in regard to arsenic, which appears to be well established,
viz., that a person who has gradually increased the dose to an amount
great enough to kill ten ordinary men, will die if he suddenly ceases
altogether to take arsenic. He can, however, be gradually brought back
to a condition in which arsenic is not necessary for his existence, if
the dose is gradually decreased. It is a curious case of adaptation that
we meet with here, since the man becomes so thoroughly adjusted to a
poison that if he is suddenly brought back to the normal condition of
the race he will die.
Immunity to the poison of venomous snakes can also be acquired by slowly
increasing the amount given to an animal. It is possible to make a
person so immune to the poison of venomous snakes that he would become,
in a sense, adapted to live amongst them without danger to himself. It
is to be noted, moreover, that this result could be reached only by
quite artificial means, for, under natural conditions it is
inconceivable that the nicely graded series of doses of increasing
strength necessary to bring about the immunity could ever be acquired.
Hence we find here a case of response in an adaptive direction that
could not have been the outcome of experience in the past. It is
important to emphasize this capacity of organisms to adapt themselves to
certain conditions entirely new to them.
These cases lead at once to cases of immunity to certain bacterial
diseases. An animal may become immune to a particular disease in several
ways. First, by having the disease itself, which renders it immune for a
longer or a shorter period afterwards; or, second, by having a mild form
of the disease as in the case of smallpox, where immunity is brought
about by vaccination, _i.e._ by giving the individual a mild form of
smallpox; or, third, by introducing into the blood an antidote, in the
form, for example, of antitoxin, which has been made by another animal
itself immune to the disease. The first two classes of immunity may be
looked upon as adaptations which are of the highest importance to the
organism; the last case can scarcely be looked upon as an adaptive
process, since the injurious effect of the poison may as well be
neutralized outside of the body by mixing it with the antitoxin. We may
suppose, then, that in the body a similar process goes on, so that the
animal itself takes no active part in the result.
When we consider that there are a number of bacterial diseases, in each
of which a different poison is made by the bacteria, we cannot but ask
ourselves if the animal really makes a counter-poison for each disease,
or whether a single substance may not be manufactured that counteracts
all alike? That the latter is not the case is shown by the fact that an
animal made immune to one disease is not immune to others. When we
recall that the animal has also the capacity to react in one way or
another to a large number of organic and inorganic poisons, to which it
or its ancestors can have had little or no previous experience, we may
well marvel at this wonderful regulative power.
The healing of wounds, which takes place in all animals, forms another
class of adaptive processes. The immense usefulness of this power is
obvious when it is remembered how exposed most animals are to injuries.
By repairing the injury the animal can better carry on its normal
functions. Moreover, the presence of the wound would give injurious
bacteria a ready means of entering the body. In fact, an intact skin is
one of the best preventives to the entrance of bacteria.
Not only have most organisms the power of repairing injuries, but many
animals have also the closely related power of regenerating new parts if
the old ones are lost. If a crab loses its leg, a new one is
regenerated. If a fresh-water worm (_Lumbriculus_) is cut into pieces,
each piece makes a new head at its anterior end and a new tail at the
posterior end. In this way as many new worms are produced as there are
pieces. And while in a strict sense it cannot be claimed that this power
of regeneration is of any use to the original worm, since the original
worm, as such, no longer exists, yet since it has not died but has
simply changed over into several new worms, the process is of use
inasmuch as by this means the pieces can remain in existence.
We need not discuss here the relative importance to different animals of
this power of regeneration, but it may be stated, that, while in some
cases it may be necessary to replace the lost part if the animal is to
remain in existence, as when a new head is formed on an earthworm after
the old one was cut off, in other cases the replacement of the lost part
appears to be of minor importance, as in the case of the leg of the
crab. While we are not, for the moment, concerned with the relative
importance of the different adaptations, this question is one of much
importance in other connections and will be considered later.
The protective coloration of some animals, which is the direct result of
a change in color of the animal in response to the surroundings,
furnishes us with some most striking cases of adaptive coloration. A
change of this sort has been recorded in a number of fishes, more
especially in the flounders. The individuals found living on a dark
background are darker than those living on a lighter background; and
when the color of the background is changed it has been observed that
the color of the fish also changes in the same direction. I have
observed a change of this sort from dark to light, or from light to
dark, in the common minnow (_Fundulus_) in accordance with a change of
its background, and the same sort of change appears to take place in
many other fishes.
The change from green to brown and from brown to green in certain tree
frogs and in the lizard (_Anolis_), which is popularly supposed to take
place according to whether the background is green or brown, is not
after all, it appears, connected with the color of the background, but
depends on certain other responses of the animals that have not yet been
satisfactorily made out. If it be claimed that in summer the animal
would generally be warm, and therefore, often green, and that this color
would protect it at this time of year when the surroundings are green,
and in winter brown, when this color is the prevailing one in temperate
regions, then it might appear that the change is of use to the animal;
but if it is true that the same change takes place in some of the
lizards that live in the tropics, where the prevailing color is always
green, it would appear that the result may have no direct relation with
the surroundings. It has been shown in a number of well-authenticated
cases that the pupæ of certain butterflies vary in color within certain
limits in response to the color of the background. When the caterpillar
fixes itself to some surface, and there throws off the outer skin, and
acquires a new one, the color of the latter is influenced by the
background. The result is a better protection to the pupa. The change is
not brought about through the ocelli or eyes, but through the general
surface of the skin, for the same change takes place when the eyes have
been previously covered with a dark pigment.
The growth of plants toward the light may be looked upon as an adaptive
process, since only in the light can they find the conditions necessary
for their life. The extraordinary elongation of shoots and young plants
when grown in the dark may also be considered an adaptation for finding
the light, since in this way a plant, deeply embedded in the ground, may
ultimately reach the surface. Thus while the actual process of
elongation in the dark is not in itself of any use, yet under the
ordinary conditions of its life, this response may be of great benefit
to the plant.
The closing together of the leaves of some plants has been supposed
to protect them from too rapid radiation of heat, and incidentally
this purpose may be fulfilled; but since some tropical plants also
close their leaves during the night, it can hardly be maintained
that the closing has been acquired for this purpose. It has been
suggested that the opening of certain flowers under certain
conditions of light is connected with the visits of insects that
bring about cross-fertilization.
The preceding examples will suffice to give a general idea of what is
meant by adaptation in organisms. That the term includes a large number
of phenomena of very different kinds is apparent. When we have examined
these phenomena further we shall find, I think, that it will be
necessary to put some of them into different categories and treat them
differently. It is probably incorrect to suppose that all processes
useful to the organism have been acquired in the same way, nevertheless,
for the present the term adaptation is sufficiently general, even if
vague, to cover these different groups of cases.
It may be asked, in what respects are these structures and processes of
adaptation different from the ordinary structures and changes that go on
in the organism? Why is the leg of the mole more of an adaptation than
that of a dog? The one is of as much use as the other to its possessor.
What reason can we give for citing the poison of the snake, and not
mentioning in the same connection the other glands of the body? In fact,
the poison gland of the snake is supposed to be a modified superior
labial gland. Why, in short, are not the processes of digestion,
excretion, secretion, the beating of the heart, the ordinary reflex acts
of the nervous system, and the action of the sense-organs, as truly
adaptations as the special cases that have been selected for
illustration. The answer is simply that we are more impressed by those
cases of adaptation that are more unusual, as when an animal departs in
the use of certain structures from the rest of the group to which it
belongs. For example, if all mammals lived underground, ourselves
included, and the fore-legs or arms were used for burrowing, we should
not think this unusual; but if we found an animal using all four legs to
support the body and for purposes of progression, we should, most
likely, think this was an excellent illustration of adaptation.
In other instances the condition is somewhat different. The color of
certain animals may unquestionably be of use to them in concealing them
from their enemies. In other cases the color may not serve this purpose,
or any purpose at all. Thus while in the former case we speak of the
color as an adaptation to the surroundings, in the latter we do not
think of it as having any connection at all with the environment. Even
in the same animal the color of different parts of the body may appear
under this twofold relation. For example, the green color of the skin of
the frog renders it less conspicuous amongst the green plants on the
edge of the stream, but the brilliant orange and black pigment in the
body-cavity cannot be regarded as of any use to the animal.
Adaptations for the Good of the Species
Aside from the class of adaptations that are for the good of the
individual, there is another class connected solely with the
preservation of the race. The organs for reproduction are the most
important examples of this kind. These organs are of no use to the
individual for maintaining its own existence, and, in fact, their
presence may even be deleterious to the animal. The instincts connected
with the use of these organs may lead inevitably to the death of the
individual, as in the case of the California salmon, which, on entering
fresh water in order to deposit its eggs, dies after performing this
act.
The presence of the organs of reproduction in the individual is
obviously connected with the propagation of other individuals. Indeed in
many organisms the life of the individual appears to have for its
purpose the continuation of the race. In a large number of animals the
individual dies after it has deposited its eggs. The most striking case
is that of the May-flies, whose life, as mature individuals, may last
for only a few hours. The eggs are set free by the bursting of the
abdomen, and the insect dies. The male bee also dies after union with
the queen. In some annelids, the body is also said to burst when the
eggs are set free; and in other forms those parts of the body containing
the eggs break off, and, after setting free the eggs, die. These are
extreme cases of what is seen in many animals, namely the replacement of
the old individuals by a new generation; and while in general there is
only a loose connection between the death of the individual and the
consummation of its reproductive power, yet the two run a course so
nearly parallel that several writers have attempted to explain this
connection as one of racial adaptation.
It has also been pointed out that in those higher animals that take care
of their young after birth, the life of the individual does not end with
the period of birth of the young, but extends at least throughout the
time necessary to care for the young. It has even been suggested that
this lengthening of the life period has been acquired on account of its
use to the species. When, however, as in the case of the vertebrates,
the young are born at intervals either in great numbers at a birth, as
in fishes and amphibia, or in lots of twos, threes, or fours, as in many
birds and mammals, or even only one at a time, as in a few birds and in
man, it will be evident that the relation cannot be so simple as has
been supposed. It cannot be assumed in these forms that the end of the
life of the individual is in any way connected with the ripening of the
last eggs, for, on the contrary, hundreds, or even many thousands, of
potential eggs may be present in the ovaries when the animal is
overtaken by old age, and its power of reproduction lost.
In regard to several of the lower animals, we find, in a number of cases
where there are accurate data, that the individual goes on year after
year producing young. Whether they ever grow old, in the sense of losing
their power of reproduction, has not been definitely determined, but
there is, so far as I know, no evidence to show that such a process
takes place, and these animals appear to have the power of reproducing
themselves indefinitely.
The phenomenon of old age (apart from its possible connection with the
cessation of the power of reproduction), which leads to the death of the
individual, has been looked upon by a few writers as an adaptation of
the individual for the good of the species. It has been pointed out by
these writers that the longer an individual lives, the more likely it is
to become damaged, and if along with this its powers of reproduction
diminish, as compared with younger individuals, then it stands in the
way and takes food that might be used by other, younger individuals,
that are better able to carry on the propagation of the race. It is
assumed, therefore, that the life of the individual has been shortened
for the benefit of the race. Whether such a thing is probable is a
question that will also be discussed later. We are chiefly concerned
here only in recording the different groups of phenomena that have been
regarded by biologists as adaptations.
The so-called secondary sexual characters such as the brighter colors of
the males, ornaments of different kinds, crests, color-pattern, tail
feathers, etc., organs of offence and of defence used in fighting
members of the same species, present a rather unique group of
adaptations. These characters are supposed to be of use to the
individual in conquering its rivals, or in attracting the females. They
may be considered as useful to the individual in allowing it to
propagate at the expense of its rivals, but whether the race is thereby
benefited is a question that will be carefully considered later.
The colors of flowers, that is supposed to attract insects, have been
already mentioned. The sweet fluid, or nectar, secreted by many flowers
is sought by insects, which on entering the flowers bring about
cross-fertilization. Thus while the nectar seems to be of no immediate
service to the plant itself, it is useful to the species in bringing
about the fertilization of the flowers. The odors of flowers also serve
to attract insects, and their presence is one of the means by which
insects find the flowers. This also is of advantage to the race.
Organs of Little Use to the Individual
In every organism there are parts of the body whose presence cannot be
of vital importance to the individual. We may leave out of consideration
the reproductive organs, since their presence, as has just been stated,
is connected with the continuation of the race. The rudimentary organs,
so-called, furnish many examples of structures whose presence may be of
little or of no use to the individual; in fact, as in the case of the
appendix in man, the organs may be a source of great danger to the
individual. In this respect the organism is a structure not perfectly
adapted to its conditions of life, since it contains within itself parts
that are of little or of no use, which may even lead to its destruction,
and may often expose it to unnecessary danger. Nevertheless such parts
are surprisingly infrequent, and their presence is usually accounted for
on the supposition that in the past these organs have been of use, and
have only secondarily come to play an insignificant part in the
functions of the organism. Another example of the same thing is found in
the rudimentary eyes of animals living in the dark, such as the mole and
several cave animals, fishes, amphibia, and insects.
There are still other organs, which cannot be looked upon as
rudimentary, yet whose presence can scarcely be considered as essential
to the life of the individual. It is with this class that we are here
chiefly concerned. For instance, the electric organs in some of the rays
and fish can hardly protect the animal from enemies, even when as highly
developed as in the torpedo; and we do not know of any other essential
service that they can perform. Whether the same may be also said of the
phosphorescent organs of many animals is perhaps open in some cases to
doubt, but there can be little question that the light produced by most
of the small marine organisms, such as noctiluca, jellyfish,
ctenophores, copepods, pyrosoma, etc., cannot be of use to these animals
in protecting them from attack. In the case of certain bacteria it seems
quite evident that the production of light can be of no use as such to
them. The production of light may be only a sort of by-product of
changes going on in the organism, and have no relation to outside
conditions. In certain cases, as in the glowworm, it has been supposed
that the display may serve to bring the sexes together; but since the
phosphorescent organs are also present in the larval stages of the
glowworm, and since even the egg itself is said to be phosphorescent, it
is improbable, in these stages at least, that the presence of the light
is of service to the organism.
It has been pointed out that the colors of certain animals may serve to
conceal them and may be regarded as an adaptation; but it is also true
that in many cases the color of the whole animal or the color of special
parts can be of little if any direct use. While it is difficult to show
that the wonderful patterns and magnificent coloration of many of the
larger animals are not of service to the animal, however sceptical we
may be on the subject, yet in the case of many microscopical forms that
are equally brilliantly colored there can be little doubt that the
coloration can be of no special service to them. If it be admitted that
in these small forms the color and the color patterns are not
protective, we should at least be on our guard in ascribing off-hand to
larger forms a protective value in their coloration, unless there is
actual proof that it serves some purpose.
We also see in other cases that the presence of color need not be
connected with any use that it bears as such to the animal. For
instance, the beautiful colors on the inside of the shells of many
marine snails and of bivalve mollusks, can be of no use to the animal
that makes the shell, because as long as the animal is alive this color
cannot be seen from the outside. This being the case let us not jump too
readily to the conclusion that when other shells are colored on the
outer surface that this must be of use to the mollusk.
In regard to the colors of plants, there are many cases of brilliant
coloration, which so far as we can see can be of no service to the
organism. In such forms as the lichens and the toadstools, many of which
are brilliantly colored, it is very doubtful if the color, as such, is
of any use to the plant. The splendid coloring of the leaves in the
autumn is certainly of no service to the trees.
It should not pass unnoticed in this connection that the stems and the
trunks of shrubs and of trees and also many kinds of fruits and nuts are
sometimes highly colored. It is true that some of the latter have been
supposed to owe their color to its usefulness in attracting birds and
other animals which, feeding on the fruit, swallow the seeds, and these,
passing through the digestive tract and falling to the ground, may
germinate. The dissemination of the seeds of such plants is supposed to
be brought about in this way; and since they may be widely disseminated
it may be supposed that it is an advantage to the plant to have
attracted the attention of the fruit-eating birds. On the other hand one
of the most brilliantly colored seeds, the acorn, is too large to pass
through the digestive tracts of birds, and is, in fact, ground to pieces
in the gizzard, and in the case of several mammals that feed on the
acorns, the acorn is crushed by the teeth. It would seem, therefore,
that its coloration is injurious to it rather than the reverse, as it
leads to its destruction. It has been suggested by Darwin that since the
acorns are for a time stored up in the crop of the bird, the passenger
pigeon for example, and since the birds may be caught by hawks and
killed, the seeds in the crop thus become scattered. Consequently it may
be, after all, of use to the oak to produce colored acorns that attract
the attention of these pigeons. This suggestion seems too far-fetched to
consider seriously. In the case of the horse-chestnut the rich brown
color is equally conspicuous, but the nut is too large to be swallowed
by any of the ordinary seed-feeding birds or mammals. Shall we try to
account for its color on the grounds of the poisonous character of the
seed? Has it been acquired as a warning to those animals that have eaten
it once, and been made sick or have died in consequence? I confess to a
personal repugnance to imaginative explanations of this sort, that have
no facts of experience to support them.
Changes in the Organism that are of No Use to the Individual or to the
Race
As an example of a change in the organism that is of no use to it may be
cited the case of the turning white of the hair in old age in man and in
several other mammals. The absorption of bone at the angle of the chin
in man, is another case of a change of no immediate use to the
individual. We also find in many other changes that accompany old age,
processes going on that are of no use to the organism, and which may, in
the end, be the cause of its death. Such changes, for instance, as the
loss of the vigor of the muscles, and of the nervous system, the
weakening of the heart, and partial failure of many of the organs to
carry out their functions. These changes lead sooner or later to the
death of the animal, in consequence of the breaking down of some one
essential organ, or to disease getting an easier foothold in the body.
We have already discussed the possible relation of death as an
adaptation, but the changes just mentioned take place independently of
their relation to the death of the organism as a whole, and show that
some of the normal organic processes are not for the good of the
individual or of the race. In fact, the perversions of some of the most
deeply seated instincts of the species, as in infanticide, while the
outcome of definite processes in the organism, are of obvious
disadvantage to the individual, and the perversion of so deeply seated a
process as the maternal instinct, leading to the destruction of the
young, is manifestly disadvantageous to the race. As soon, however, as
we enter the field of so-called abnormal developments, the adaptive
relation of the organism to its environment is very obscure; and yet, as
in the case of adaptation to poisons, we see that we cannot draw any
sharp line between what we call normal and what we call abnormal
development.
Comparison with Inorganic Phenomena
The preceding examples and discussion give some idea of what is meant by
adaptation in living things. In what respects, it may be asked, do these
adaptations differ from inorganic phenomena? The first group of
inorganic bodies that challenges comparison are machines. These are so
constructed that they may be said to accomplish a definite purpose, and
the question arises whether this purpose can be profitably compared with
the purposefulness of the structure and response of organisms. That the
two cannot be profitably compared is seen at once, when we recall the
fact that the activity of the machine is of no use to it, in the sense
of preserving its integrity. The object of the machine is, in fact, to
perform some useful purpose for the organism that built it, namely, for
man. Furthermore, the activity of the machine only serves to wear it
out, and, therefore, its actions do not assist in preserving its
integrity as do some, at least, of the activities of an animal. It is
true, of course, that in a mechanical sense every action of the organism
leads also to a breaking down of its structure in the same way that a
machine is also worn out by use; but the organism possesses another
property that is absent in the machine, namely, the power of repairing
the loss that it sustains.
One of the most characteristic features of the organism is its power of
self-adjustment, or of regulation, by which it adapts itself to changes
in the environment in such a way that its integrity is maintained. Most
machines have no such regulative power, although, in a sense, the
fly-wheel of an engine regulates the speed, and a water-bath, with a
thermostat, regulates itself to a fixed temperature; but even this
comparison lacks one of the essential features of the regulation seen in
organisms, namely, in that the regulation does not protect the machine
from injury. It may be claimed, however, that the safety valve of an
engine does fulfil this purpose, since it may prevent the engine from
exploding. Here, in fact, we do find better grounds for comparison, but,
when we take into account the relation of the regulations in the
organism to all the other properties of the organism, we see that this
comparison is not very significant. The most essential difference
between a machine and an organism is the power of reproduction possessed
by the latter, which is absent in all machines. Here, however, we meet
with a somewhat paradoxical relation, since the reproductive power of
organisms cannot be looked upon as an adaptation for the continuation of
the individual, but rather for the preservation of a series of
individuals. Hence, in this respect also, we cannot profitably compare
the individual with a machine, but if we make any comparison we should
compare all the individuals that have come from a single one with a
machine. In this sense the power of reproduction is a sort of racial
regulation. A comparison of this sort is obviously empty of real
significance.
The regenerative power of the organism, by means of which it may replace
a lost part, or by means of which a piece may become a new whole, is
also something not present in machines.
In using a machine for comparison we should not leave out of sight the
fact that machines are themselves the work of organisms, and have been
made for some purpose useful to the organism. They may perform the same
purpose for which we would use our own hands, for they differ from parts
of the body mainly in that they are made of different compounds having
different properties, as the above comparisons have shown. But the
regulations of the machine have been added to it by man on account of
their usefulness to himself, and are not properties of the material of
which the machine itself is composed. This shows, I think, the
inappropriateness of making any comparison between these two entirely
different things.
If, then, we find the comparison between machines and organisms
unprofitable, can we find any other things in inorganic nature that can
be better compared with the phenomenon of adaptation of the organism?
The following phenomena have been made the subject of comparison from
time to time. The bendings, which are gradually made by rivers often
lead to a meeting of the loops, so that a direct, new communication is
established, and the course of the river is straightened out. The water
takes, therefore, a more direct course to the sea. It cannot be said,
however, to be of any advantage to the river to straighten its course.
Again, a glacier moulds itself to its bed, and gradually moves around
obstacles to a lower level, but this adaptation of the glacier to the
form of its surroundings cannot be said to be of advantage to the
glacier. On the contrary, the glacier reaches so much the sooner a lower
level where it is melted.
The unusual case of a solid being lighter than the liquid from which it
forms, as seen in the case of ice, has been looked upon as a useful
arrangement, since were the reverse the case all rivers and ponds would
become solid in winter in cold climates, and the polar regions would
become one solid block of ice. But no one will suppose for a moment that
there is any relation between the anomalous condition of the lightness
of ice, and its relation to the winter freezing of streams, ponds, etc.
It has even been suggested that this property of ice was given to it in
order that the animals living in the water might not be killed, which
would be the case if the ice sank to the bottom, but such a method of
interpreting physical phenomena would scarcely commend itself to a
physicist.
The formation of a covering of oxide over the surface of a piece of iron
delays the further process of oxidation, but who will imagine that this
property of iron has been acquired in order to prevent the iron from
being destroyed by oxygen?
If a piece is broken from a crystal, and the crystal is suspended in a
saturated solution of the same substance, new material is deposited over
its whole surface, and, as it grows larger, the broken side is completed
and the crystal assumes its characteristic form. But of what advantage
is it to the crystal whether it is complete or incomplete? In the case
of an animal it is of some importance to be able to complete itself
after injury, because it can then better obtain the food necessary to
keep it alive, or it can better escape its enemies; but this is not the
case with the crystal.
In conclusion, therefore, it is obvious that the adaptations of
organisms are something peculiar to living things, and their obvious
purpose is to maintain the integrity of the individual, or that of the
species to which the individual belongs. We are, therefore, confronted
with the question as to how this peculiarity has come to be associated
with the material out of which living things are made. In subsequent
chapters this will be fully discussed, but before we take up this topic,
it will be necessary to reach some understanding in regard to the theory
of evolution, for the whole subsequent issue will turn upon the question
of the origin of the forms of animals and plants living at the present
time.
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CHAPTER II
THE THEORY OF EVOLUTION
One of the most important considerations in connection with the problem
of adaptation is that in all animals and plants the individuals sooner
or later perish and new generations take their places. Each new
individual is formed, in most cases, by the union of two germ-cells
derived one from each parent. As a result of this process of
intermixing, carried on from generation to generation, all the
individuals would tend to become alike, unless something else should
come in to affect the result.
So far as our actual experience reaches, we find that the succeeding
generations of individuals resemble each other. It is true that no two
individuals are absolutely alike, but if a sufficiently large number are
examined at a given time, they will show about the same variations in
about the same proportionate numbers. Such a group of similar forms,
repeating itself in each generation, is the unit of the systematists,
and is called a species.
It has been said that within each species the individuals differ more or
less from each other, but our experience teaches that in each generation
the same kinds of variations occur, and, moreover, that from any one
individual there may arise in the next generation any one of the
characteristic variations. Certain limitations will have to be made in
regard to this statement, but for the present it will suffice. The Law
of Biogenesis states that each living thing arises from another living
thing; that there is no life without antecedent life, _i.e._ spontaneous
generation does not occur. The law is not concerned with the likeness or
unlikeness of the different individuals that descend from each other.
The theory of evolution includes the same idea, but in addition it has
come to mean nowadays, that there have been changes, as the succeeding
generations have arisen. The transmutation theory, and even the descent
theory, have come to mean nearly the same thing as the theory of
evolution. It is unfortunate that one of these terms cannot be used to
signify simply the repetition, generation after generation, of groups of
similar individuals. The theory of descent might be used to convey only
this idea, but unfortunately it too has come to include also the idea of
change. I shall attempt nevertheless to discriminate between the descent
and the transmutation theory, and use the term _descent theory_ when I
do not wish to convey the idea of change, and _transmutation theory_
when I do wish to emphasize this idea.
On the transmutation theory it is assumed that a group (species) may
give rise to one or more groups of forms differing from their ancestors;
the original group being now replaced by its new kinds of offspring, or
the old and the new may remain in existence at the same time. This
process repeating itself, each or some of the new groups giving rise in
turn to one or more new species, there will be produced a larger group
of species having certain similar characters which are due to their
common descent. Such a group of species is called a genus. The
resemblances of these species is accounted for by their common descent;
but their differences must be due to those factors that have caused them
to depart from the original type. We may now proceed to consider the
evidence on which this idea of transmutation rests.
Evidence in Favor of the Transmutation Theory
EVIDENCE FROM CLASSIFICATION AND FROM COMPARATIVE ANATOMY
It does not require any special study to see that there are certain
groups of animals and of plants that are more like each other than they
are like the members of any other group. It is obvious to every one that
the group known as mammals has a combination of characters not found in
any other group; such, for instance, as a covering of hair, mammary
glands that furnish milk to the young, and a number of other less
distinctive features. These and other common characteristics lead us to
put the mammals into a single class. The birds, again, have certain
common characters such as feathers, a beak without teeth, the
development of a shell around the egg, etc., and on account of these
resemblances we put them into another class. Everywhere in the animal
and plant kingdoms we find large groups of similar forms, such as the
butterflies, the beetles, the annelidan worms, the corals, the snails,
the starfishes, etc.
Within each of these groups we find smaller groups, in each of which
there are again forms more like each other than like those of other
groups. We may call these smaller groups families. Within the families
we find smaller groups, that are more like each other than like any
other groups in the same family, and these we put into genera. Within
the genus we find smaller groups following the same rule, and these are
the species. Here we seem to have reached a limit in many cases, for we
do not always find within the species groups of individuals more like
each other than like other groups. Although we find certain differences
between the individuals of a species, yet the differences are often
inconstant in the sense that amongst the descendants of any individual
there may appear any one of the other variations. If this were the whole
truth, it would seem that we had here reached the limits of
classification, the species being the unit. This, however, is far from
being the case, for, in many species we find smaller groups, often
confined to special localities. These groups are called varieties.
In some cases it appears, especially in plants, these smaller groups of
varieties resemble in many ways the groups of species in other forms,
since they breed true to their kind, even under changed conditions. They
have been recognized as “smaller species” by a number of botanists.
In this connection a point must be brought up that has played an
important rôle in all discussion as to what limits can be set to a
species. As a rule it is found that two distinct species cannot be made
to cross with each other, _i.e._ the eggs of an individual of one
species cannot be fertilized by spermatozoa derived from individuals of
another species; or, at least, if fertilization takes place the embryo
does not develop. In some cases, however, it has been found possible to
cross-fertilize two distinct species, although the offspring is itself
more or less infertile. Even this distinction, however, does not hold
absolutely, for, in a few cases, the offspring of the cross is fertile.
It cannot be maintained, therefore, that this test of infertility
between species invariably holds, although in a negative sense the test
may apply, for if two different forms are infertile, _inter se_, the
result shows that they are distinct species. If they cross they may or
may not be good species, and some other test must be used to decide
their relation.
We should always keep in mind the fact that the individual is the only
reality with which we have to deal, and that the arrangement of these
into species, genera, families, etc., is only a scheme invented by man
for purposes of classification. Thus there is no such thing in nature as
a species, except as a concept of a group of forms more or less alike.
In nature there are no genera, families, orders, etc. These are
inventions of man for purposes of classification.
Having discovered that it is possible to arrange animals and plants in
groups within groups, the question arises as to the meaning of this
relation. Have these facts any other significance than that of a
classification of geometric figures, or of crystals according to the
relations of their axes, or of bodies as to whether they are solids,
liquids, or gases, or even whether they are red, white, or blue?
If we accept the transmutation view, we can offer an explanation of the
grouping of living things. According to the transmutation theory, the
grouping of living things is due to their common descent, and the
greater or less extent to which the different forms have diverged from
each other. It is the belief in this principle that makes the
classification of the biologist appear to be of a different order from
that in any other science; and it is this principle that appears to give
us an insight into a large number of phenomena.
For example, if, as assumed in the theory, a group of individuals
(species) breaks up into two groups, each of these may be supposed to
inherit a large number of common characteristics from their ancestors.
These characters are, of course, the resemblances, and from them we
conclude that the species are related and, therefore, we put them into
the same genus. The differences, as has been said, between the species
must be explained in some other way; but the principle of classification
with which we are here concerned is based simply on the resemblances,
and takes no account of the differences between species.
In this argument it has been tacitly assumed that the transformation of
one species into another, or into more than one, takes place by adding
one or more new characters to those already present, or by changing over
a few characters without altering others. But when we come to examine
any two species whatsoever, we find that they differ, not only in one or
in a few characters, but in a large number of points; perhaps in every
single character. It is true that sometimes the differences are so small
that it is difficult to distinguish between two forms, but even in such
cases the differences, although small, may be as numerous as when they
are more conspicuous. If, then, this is what we really find when we
carefully examine species of animals or of plants, what is meant when we
claim that our classification is based on the characters common to all
of the forms that have descended from the same ancestor? We shall find,
if we press this point that, in one sense, there is no absolute basis of
this sort for our classification, and that we have an unreal system.
If this is admitted, does our boasted system of classification, based as
it is on the principle of descent, give us anything fundamentally
different from an artificial classification? A few illustrations may
make clearer the discussion that follows. If, for example, we take a
definition of the group of vertebrates we read: “The group of craniate
vertebrates includes those animals known as Fishes, Amphibians,
Reptiles, Birds, and Mammals; or in other words, Vertebrates with a
skull, a highly complex brain, a heart of three or four chambers, and
red blood corpuscles.” If we attempt to analyze this definition, we find
it stated that the skull is a characteristic of all vertebrates, but if
we ask what this thing is that is called skull, we find not only that it
is something different in different groups, being cartilaginous in
sharks, and composed of bones in mammals, but that it is not even
identical in any two species of vertebrates. If we try to define it as a
case of harder material around the brain, then it is not something
peculiar to the vertebrates, since the brain of the squid is also
encased in a cartilaginous skull. What has been said of the skull may be
said in substance of the brain, of the heart, and even of the red blood
corpuscles.
If we select another group, we find that the birds present a sharply
defined class with very definite characters. The definition of the group
runs as follows: “Birds are characterized by the presence of feathers,
their fore-limbs are used for flight, the breast-bone is large and
serves for the attachment of the muscles that move the wings; outgrowths
from the lungs extend throughout the body and even into the bones and
serve as air sacs which make the body more buoyant. Only one aortic arch
is present, the right, and the right ovary and oviduct are not
developed. The eyes are large and well developed. Teeth are absent. We
have here a series of strongly marked characteristics such as
distinguish hardly any other class. Moreover, the organization of
existing birds is, in its essential features, singularly uniform; the
entire class presenting less diversity of structure than many orders of
Fishes, Amphibians, and Reptiles.”[1] The feathers are the most unique
features of birds, and are not found in any other group of the animal
kingdom; moreover the plan on which they are formed is essentially the
same throughout the group, yet in no two species are the feathers
identical, but differ not only in form and proportions, but even in the
character of the barbs and hooks for holding the vane together. The
modification of the fore-limbs for flight is another characteristic
feature; yet in some birds, as the ostrich and kiwi, although the wing
has the same general plan as in other birds, it is not used for flight.
In the latter it is so small that it does not project beyond the
feathers, and in some birds, as in the penguins, the wings are used only
as organs for swimming.
Footnote 1:
Parker and Haswell: “Text Book of Zoology.”
In spite of these differences we have no difficulty in recognizing
throughout the group of birds a similarity of plan or structure,
modified though it be in a thousand different ways.
Enough has been said to illustrate what is meant by the similarities of
organisms on which we base our system of classification. When we
conclude from the statement that all vertebrates have a skull that they
owe this to a common descent, we do not mean that a particular structure
has been handed down as a sort of entailed heirloom, but that the
descendants have followed the same plan of structure as that of their
ancestors, and have the brain enclosed in a covering of harder material,
although this material may not have exactly the same form, or be made of
the same substance in all cases. Furthermore while we may recognize that
the cartilaginous skull of the shark is simpler in structure than that
of the cartilaginous-bony skull of the frog, and that the skull of the
frog is simpler than that of the rabbit, yet we should not be justified
in stating, except in a metaphorical sense, that something has been
added to the skull of the shark to make that of the frog, and something
to the latter to make that of the rabbit. On the contrary, while
something may have been added, and the plan made more complicated, the
skull has also been changed throughout in every single part.
There is another point of some importance to be taken into account in
this connection; namely, that each new generation begins life as a
single cell or egg. The egg does not contain any preformed adult
structures that it hands down unaltered, but it is so constructed that,
under constant conditions, the same, or nearly the same, kind of
structure is produced. Should something affect the egg, we can imagine
that it might form a new combination on the same general plan as that of
the old, yet one that differed from the original in every detail of its
structure. It is this idea, I believe, that lies at the base of the
transmutation theory. On some such assumption as this, and on this
alone, can we bring the theory of transmutation into harmony with the
facts of observation.
What has been said in regard to individuals as a whole may be repeated
also in respect to the study of the single organs. Selecting any one
group of the animal or plant kingdom, we find the same organ, or the
same combination of organs present in whole groups of forms. We can
often arrange these organs in definite series passing from the simple to
the complex, or, in case of degeneration, in the reverse order. However
convenient it may be to study the structure of organisms from this point
of view, the artificiality of the procedure will be obvious, since here
also the organs of any two species do not differ from each other in only
one point, but in many, perhaps in all. Therefore to arrange or to
compare them according to any one scheme gives only an incomplete idea
of their structure. We should apply here the same point of view that we
used above in forming a conception of the meaning of the zoological and
botanical systems. We must admit that our scheme is only an ideal, which
corresponds to nothing real in nature, but is an abstraction based on
the results of our experience. It might be a pleasing fancy to imagine
that this ideal scheme corresponds to the plan of structure or of
organization that is in every egg, and furnishes the basis for all the
variations that have come or may come into existence; but we should find
no justification whatsoever for believing that our fiction corresponds
to any such real thing.
To sum up the discussion: we find that the resemblances of animals and
plants can be accounted for on the transmutation theory, not in the way
commonly implied, but in a somewhat different sense. We have found that
the resemblances between the different members of a group are only of a
very general sort, and the structures are not identically the same in
any two species—in fact, perhaps in no two individuals. This conclusion,
however, does not stand in contradiction to the transmutation
hypothesis, because, since each individual begins as an egg which is not
a replica of the original adult from which it is derived, there can be
no identity, but at most a very close similarity. Admitting, then, that
our scheme is an ideal one, we can claim, nevertheless, that on this
basis the facts of classification find a legitimate explanation in the
transmutation theory.
THE GEOLOGICAL EVIDENCE
On the theory of descent, as well as on the theory of transmutation, the
ancestors of all present forms are supposed to have lived at some time
in the past on the surface of the earth. If, therefore, their remains
should have been preserved, we should expect on the descent theory to
find some, at least, of these remains to be like present forms, while on
the transmutation theory we should expect to find most, if not all, of
the ancestral forms to be different from the present ones.
The evidence shows that fossil forms are practically all different from
living forms, and the older they are the greater the difference from
present forms. In general, therefore, it may be said that the evidence
is in favor of the transmutation theory. It can scarcely be claimed that
the evidence is absolutely conclusive, however probable it may appear,
for the problem is complicated in a number of ways.
In the first place, there is convincing evidence that some forms have
been entirely exterminated. Other groups have very few living
representatives, as is the case in the group containing nautilus, and in
that of the crinoids. It is therefore always possible that a given
fossil form may represent an extinct line, and may be only indirectly
connected with forms alive at the present time. Again the historical
record is so broken and incomplete in all but a few cases that its
interpretation is largely a question of probability. We can easily
conceive that it would be only in very exceptional cases that successive
generations of the same form would be buried one above the other, so
that we should find the series unbroken. This is evident not only
because the conditions that were at one time favorable for the
preservation of organic remains might not be favorable at another time,
but also because if the conditions remained the same the organisms
themselves might also remain unchanged. A new form, in fact, would be,
_ex hypothese_, better suited to live in a different environment, and
consequently we should not expect always to find its remains in the same
place as that occupied by the parent species. This possibility of
migration of new forms into a new locality makes the interpretation of
the geological record extremely hazardous.
Nevertheless, if the evolution of the entire animal and plant kingdoms
had taken place within the period between the first deposits of
stratified rocks and the present time, we might still have expected to
find, despite the imperfections of the record, sufficient evidence to
show how the present groups have arisen, and how they are related to one
another. But, unfortunately, at the period when the history of the rocks
begins, nearly all the large groups of animals were in existence, and
some of them, indeed, as the trilobites and the brachiopods, appear to
have reached the zenith of their development.
On the other hand, the subdivisions of the group of vertebrates have
evolved during the period known to us. It is true that the group was
already formed when our knowledge of it begins, but, from the fishes
onwards, the history of the vertebrates is recorded in the rocks. The
highest group of all, the mammals, has arisen within relatively modern
times. The correctness of the transmutation theory could be as well
established by a single group of geological remains as by the entire
animal kingdom. Let us, therefore, examine how far the theory is
substantiated by the paleontological record of the vertebrates. We find
that the earliest vertebrates were fishes, and these were followed
successively by the amphibians, reptiles, birds, and mammals, one of the
last species of all to appear being man himself. There can be little
doubt that this series, with certain limitations to be spoken of in a
moment, represents a progressive series beginning with the simpler forms
and ending with the more complicated. Even did we not know this
geological sequence we would conclude, from the anatomical evidence
alone, that the progression had been in some such order as the
geological record shows. The limitation referred to above is this: that
while the mammals arose later than the birds, we need not suppose that
the mammals arose from the birds, and not even perhaps from the
reptiles, or at least not from reptiles like those living at the present
day. The mammals may in fact, as some anatomists believe, have come
direct from amphibian-like forms. If this is the case, we find the
amphibians giving rise on one hand to reptiles and these to birds, and
on the other hand to mammals.
This case illustrates how careful we should be in interpreting the
record, since two or more separate branches or orders may arise
independently from the same lower group. If the mammals arose from the
amphibians later than did the reptiles, it would be easy to make the
mistake, if the record was incomplete at this stage, of supposing that
the mammals had come directly from the reptiles.
That the birds arose as an offshoot from reptile-like forms is not only
probable on anatomical grounds, but the geological record has furnished
us with forms like archæopteryx, which in many ways appears to stand
midway between the reptiles and birds. This fossil, archæopteryx, has a
bird-like form with feathered wings, and at the same time has a beak
with reptilian teeth, and a long, feathered tail with a core of
vertebræ.
From another point of view we see how difficult may be the
interpretation of the geological record, when we recall that throughout
the entire period of evolution of the vertebrates the fishes,
amphibians, reptiles, and birds remained still in existence, although
they, or some of them, may have at one time given origin to new forms.
In fact, all these groups are alive and in a flourishing condition at
the present time. The fact illustrates another point of importance,
namely, that we must not infer that because a group gives rise to a
higher one, that it itself goes out of existence, being exterminated by
the new form. There may be in fact no relation whatsoever between the
birth of a new group and the extermination of an old one.
On the transmutation theory we should expect to find not only a sequence
of forms, beginning with the simplest and culminating with the more
complex, but also, in the beginning of each new group, forms more or
less intermediate in structure. It is claimed by all paleontologists
that such forms are really found. For example, transitional forms
between the fishes and the amphibia are found in the group of dipnoans,
or lung-fishes, a few of which have survived to the present day. There
are many fossil forms that have characters between those of amphibians
and reptiles, which if not the immediate ancestors of the reptiles, yet
show that at the time when this group is supposed to have arisen
intermediate forms were in existence. The famous archæopteryx remains
have been already referred to above, and it appears in this case that we
have not only an intermediate form, but possibly a transitional one. In
the group of mammals we find that the first forms to appear were the
marsupials, which are undoubtedly primitive members of the group.
The most convincing evidence of transmutation is found in certain series
of forms that appear quite complete. The evolution of the horse series
is the most often cited. As this case will be discussed a little later,
we need not go into it fully here. It will suffice to point out that a
continuous series of forms has been found, that connect the living
horses having a single toe through three-toed, with the five-toed
horses. Moreover, and this is important, this series shows a
transformation not only in one set of structures, but in all other
structures. The fossil horses with three toes are found in the higher
geological layers, and those with more toes in the deeper layers
progressively. In some cases, at least, the fossils have been found in
the same part of the world, so that there is less risk of arranging them
arbitrarily in a series to fit in with the theory.
EVIDENCE FROM DIRECT OBSERVATION AND EXPERIMENT
Within the period of human history we do not know of a single instance
of the transformation of one species into another one, if we apply the
most rigid and extreme tests used to distinguish wild species from each
other.[2] It may be claimed that the theory of descent is lacking,
therefore, in the most essential feature that it needs to place the
theory on a scientific basis. This must be admitted. On the other hand,
the absence of direct observation is not fatal to the hypothesis, for
several reasons. In the first place, it is only within the last few
hundred years that an accurate record of wild animals and plants has
been kept, so that we do not know except for this period whether any new
species have appeared. Again, the chance of observing the change might
not be very great, especially if the change were sudden. We would simply
find a new species, and could not state where it had come from. If, on
the other hand, the change were very slow, it might extend over so many
years that the period would be beyond the life of an individual man. In
only a few cases has it been possible to compare ancient pictures of
animals and plants with their prototypes living at the present time, and
it has turned out in all cases that they are the same. But these have
been almost entirely domesticated forms, where, even if a change had
been found, it might have been ascribed to other factors. In other
cases, as in the mummified remains of a few Egyptian wild animals (which
have also been found to be exactly like the same animals living at the
present day), it was pointed out by Geoffroy Saint-Hilaire that, since
the conditions of the Egyptian climate are the same to-day as they were
two thousand years ago, there is no reason to expect any change would
have taken place. But waiving this assumption, we should not forget that
the theory of evolution does not postulate that a change must take place
in the course of time, but only that it may take place sometimes.
Footnote 2:
The transformation of “smaller species,” described by De Vries, will
be described in a later chapter.
The position that we have here taken in regard to the lack of evidence
as to the transformation of species is, perhaps, extreme, for, as will
be shown in some detail in later chapters, there is abundant evidence
proving that species have been seen to change greatly when the
conditions surrounding them have been changed; but never, as has been
stated, so far, or rather in such a way, that an actual new species that
is infertile with the original form has been produced. Whether, after
all, these changes due to a change in the environment are of the kind
that makes new species, is also a question to be discussed later.
The experimental evidence, in favor of the transformation of species,
relates almost entirely to domesticated forms, and in this case the
conscious agency of man seems, in some cases, to have played an
important part; but here, even with the aid of the factor of isolation,
it cannot be claimed that a single new species has been produced,
although great changes in form have been effected. It is clear,
therefore, that we must, at present, rely on other data, less
satisfactory in all respects, to establish the probability of the theory
of transformation.
MODERN CRITICISM OF THE THEORY OF EVOLUTION
Throughout the whole of the nineteenth century a steady fire of
criticism was directed against the theory of evolution; the names of
Cuvier and of Louis Agassiz stand out preëminent in this connection, yet
the theory has claimed an ever increasing number of adherents, until at
the present time it is rare to find a biologist who does not accept in
one form or another the general principle involved in the theory. The
storm of criticism aroused by the publication of Darwin’s “Origin of
Species,” was directed more against the doctrine of evolution than
against Darwin’s argument for natural selection. The ground has been
gone over so often that there would be little interest in going over it
again. It will be more profitable to turn our attention to the latest
attack on the theory from the ranks of the zoologists themselves.
Fleischmann, in his recent book, “Die Descendenztheorie,” has made a new
assault on the theory of evolution from the three standpoints of
paleontology, comparative anatomy, and embryology. His general method is
to try to show that the recognized leaders in these different branches
of biology have been led to express essentially different views on the
same questions, or rather have compromised the doctrine by the examples
they have given to illustrate it. Fleischmann is fond of bringing
together the antiquated and generally exaggerated views of writers like
Haeckel, and contrasting them with more recent views on the same
subject, without making sufficient allowances for the advances in
knowledge that have taken place. He selects from each field a few
specific examples, by means of which he illustrates the weakness, and
even, as he believes, the falsity of the deductions drawn for the
particular case. For example, the plan of structure of the vertebrates
is dealt with in the following way: In this group the limbs, consisting
typically of a pair of fore-legs and a pair of hind-legs, appear under
the form of cylindrical outgrowths of the body. In the salamander, in
the turtle, in the dog, the cylindrical legs, supporting the body and
serving to support it above the ground, are used also for progression.
The general purpose to which the limbs are put as organs of locomotion
has not interfered with an astonishing number of varieties of structure,
adapted to different conditions of existence, such as the short legs
used for creeping in salamanders, lizards, turtles, crocodiles; the long
and thin legs of good runners, as the hoofed animals; the mobile legs of
the apes used for climbing; and the parachute legs of some squirrels
used for soaring. Even more striking is the great variety of hands and
feet, as seen in the flat, hairy foot of the bear; the fore-foot of the
armadillos, carrying long, sickle-shaped claws; the digging foot of the
mole; the plump foot of the elephant, ending in a broad, flat pad with
nails around the border, and without division into fingers; the hand of
man and of the apes ending with fine and delicate fingers for grasping.
To have discovered a general plan of structure running through such a
great variety of forms was proclaimed a triumph of anatomical study.[3]
Footnote 3:
This paragraph is a free translation of Fleischmann’s text.
A study of the bony structure of the limb shows that typically it
consists of a single proximal bone (the humerus in the upper arm, the
femur in the thigh), followed by two bones running parallel to each
other (the radius and ulna in the arm and the tibia and fibula in the
shank); these are succeeded in the arm by the two series of carpal
bones, and in the leg by the two series of tarsal bones, and these are
followed in each by five longer bones (the metacarpals and metatarsals),
and these again by the series of long bones that lie in the fingers and
toes. Despite the manifold variety of forms, Fleischmann admits that
both the hind- and the fore-limbs are constructed on the same plan
throughout the vertebrates. Even forms like the camel, in which there
are fewer terminal bones, may be brought into the same category by
supposing a reduction of the bones to have taken place, so that three of
the digits have been lost. In the leg of the pig and of the reindeer,
even a greater reduction may be supposed to have taken place.
Fleischmann points out that these facts were supposed to be in full
harmony with the theory of descent.
The analysis of the origin of the foot of the horse gave even better
evidence, it was claimed, in favor of the theory. The foot consists of a
single series of bones corresponding to the middle finger and toe. When,
as sometimes happens, individual horses are found in which in addition
to the single middle finger two smaller lateral fingers with small hoofs
appear, the followers of the descent theory rejoiced to be able to bring
this forward as a confirmation of their doctrine. The occurrence was
explained as a sporadic return to an ancestral form. The naïve
exposition of the laws of inheritance that were supposed to control such
phenomena was accepted without question. And when finally a large number
of fossil remains were found by paleontologists,—remains showing a
gradual increase in the middle finger, and a decrease in size of the
lateral fingers,—it was supposed that the proof was complete; and
anatomists even went so far as to hold that the original ancestor of the
horse was a five-fingered animal.
This same law of type of structure was found to extend to the entire
vertebrate series, and the only plausible explanation appeared to be
that adopted by Darwin and his followers, namely, that the resemblance
is the result of the blood-relationship of the different forms. But a
simple comparison of the skeleton of the limbs if carried out without
theoretical prejudice would show, Fleischmann thinks, that there is only
a common style, or plan of structure, for the vertebrates. This
anatomical result has about the same value as the knowledge of the
different styles of historical architecture—that, for instance, all
large churches of the Gothic period have certain general principles in
common. The believers in the theory of descent have, however, he thinks,
gone beyond the facts, and have concluded that the common plan in
animals is the consequence of a common descent. “I cannot see the
necessity for such a conclusion, and I certainly should unhesitatingly
deny that the common plan of the Gothic churches depended on a common
architect. The illustration is, however, not perfect, because the
influence of the mediæval school of stone-cutters on its wandering
apprentices is well known.”
Fleischmann adds that if the descent theory is true we should expect to
find that if a common plan of structure is present in one set of organs,
as the limbs, it should be present in all other organs as well, but he
does not add that this is generally the case.
The weakness of Fleischmann’s argument is so apparent that we need not
attempt an elaborate refutation. When he says there is no absolute proof
that the common plan of structure must be the result of
blood-relationship, he is not bringing a fatal argument against the
theory of descent, for no one but an enthusiast sees anything more in
the explanation than a very probable theory that appears to account for
the facts. To demand an absolute proof for the theory is to ask for more
than any reasonable advocate of the descent theory claims for it. As I
have tried to show in the preceding pages, the evidence in favor of the
theory of descent is not absolutely demonstrative, but the theory is the
most satisfactory one that has as yet been advanced to account for the
facts. Fleischmann’s reference to the common plan of structure of the
Gothic churches is not very fortunate for his purpose, since he admits
himself that this may be the result of a common tradition handed down
from man to man, a sort of continuity that is not very dissimilar in
principle from that implied in the descent theory; in the latter the
continuity of substance taking the place of the tradition in the other.
Had the plan for each, or even for many of the churches, originated
independently in the mind of each architect, then the similarity in
style would have to be accounted for by a different sort of principle
from that involved in the theory of descent; but as a matter of fact the
historical evidence makes it probable that similar types of architecture
are largely the result of imitation and tradition. Certain variations
may have been added by each architect, but it is just the similarity of
type or plan that is generally supposed to be the outcome of a common
tradition.
Fleischmann’s attempt in the following chapter to belittle Gegenbaur’s
theory of the origin of the five-fingered type of hand from a fin, like
that of a fish, need not detain us, since this theory is obviously only
a special application which like any other may be wrong, without in the
least injuring the general principle of descent. That all phylogenetic
questions are hazardous and difficult is only too obvious to any one
familiar with the literature of the last thirty years.
Fleischmann devotes a long chapter to the geological evidences in
connection with the evolution of the horse, and attempts to throw
ridicule on the conclusions of the paleontologists by emphasizing the
differences of opinion that have been advanced in regard to the descent
of this form. After pointing out that the horse, and its few living
relatives, the ass and the zebra, are unique in the mammalian series in
possessing a single digit, he shows that by the discovery of the fossil
horses the group has been simply enlarged, and now includes horses with
one, three, and five toes. The discovery of the fossil forms was
interpreted by the advocates of the descent theory as a demonstration of
the theory. The series was arranged by paleontologists so that the
five-toed form came first, then those with three and one toe, the last
represented by the living horses. But the matter was not so simple,
Fleischmann points out, as it appeared to be to the earlier writers, for
example to Haeckel, Huxley, Leidy, Cope, Marsh. Different authors came
to express different opinions in regard to the genealogical connection
between the fossil forms. Several writers have tried to show that the
present genus, Equus, has not had a single line of descent, but have
supposed that the European horses and the original American horses had
different lines of ancestry, which may have united only far back in the
genus Epihippus. Fleischmann points out that the arrangement of the
series is open to the criticism that it is arbitrary, and that we could
equally well make up an analogous series beginning with the
five-fingered hand of man, then that of the dog with the thumb
incompletely developed, then the four-fingered hind-foot of the pig
without a big toe and with a weak second and fifth digit, then the foot
of the camel with only two toes, and lastly the foot of the horse with
only one toe. It sounds strange that Fleischmann should make such a
trivial reply as this, and deliberately ignore the all-important
evidence with which he is, of course, as is every zoologist, perfectly
conversant. Not only are there a hundred other points of agreement in
the horse series, but also the geological sequence of the strata, in
which some at least of the series have been found, shows that the
arrangement is not arbitrary, as he implies.
Fleischmann then proceeds to point out that when the evidence from other
parts of the anatomy is taken into account, it becomes evident that all
the known fossil remains of horses cannot be arranged in a single line,
but that there are at least three families or groups recognizable. Many
of these forms are known only from fragments of their skeletons—a few
teeth, for instance, in the case of Merohippus, which on this evidence
alone has been placed at the uniting point of two series. At present
about eight different species of living horses are recognized by
zoologists, and paleontological evidence shows only that many other
species have been in existence, and that even three- and one-toed forms
lived together at the same time.
Fleischmann also enters a protest against the ordinary arrangement of
the fossil genera Eo-, Oro-, Meso-, Merohippus in a series, for these
names stand not for single species, but for groups containing no less
than six species under Protohippus, fourteen under Equus, twelve under
Mesohippus, and twenty under Hipparion. Fleischmann concludes: “The
descent of the horses has not been made out with the precision of an
accurate proof, and it will require a great deal of work before we get
an exact and thorough knowledge of the fossil forms. What a striking
contrast is found on examination between the actual facts and the crude
hopes of the apostles of the descent theory!...”
In so far as this criticism of Fleischmann’s applies to the difficulties
of determining the past history of the horse, it may be granted that he
has scored a point against those who have pretended that the evidence is
simple and conclusive; but we should not fail to remember that this
difficulty has been felt by paleontologists themselves, who have been
the first to call attention to the complexity of the problem, and to the
difficulties of finding out the actual ancestors of the living
representative of the series. And while we may admit that the early
enthusiasts exaggerated, unintentionally, the importance of the few
forms known to them, and went too far in supposing that they had found
the actual series of ancestors of living horses, yet we need not let
this blind us to the importance of the facts themselves. Despite the
fact that it may be difficult and, perhaps, in most cases, impossible,
to arrange the fossil forms in their relations to one another and to
living forms, yet on an unprejudiced view it will be clear, I think,
that so far as the evidence goes it is in full harmony with the theory
of descent. This is especially evident if we turn our attention to a
part of the subject that is almost entirely ignored by Fleischmann, and
yet is of fundamental importance in judging of the result. The series of
forms beginning with the five-toed horses and ending with those having a
single toe has not been brought together haphazard, as Fleischmann’s
comparison might lead one to suppose, but the five-fingered forms are
those from the older rocks, and the three-toed forms from more recent
layers. The value of this kind of evidence might have been open to
greater doubt had the series been made up of forms found scattered over
the whole world, for it is well known how difficult it is to compare in
point of time the rocks of different continents. But in certain parts of
the world, especially in North America, series of fossil horses have
been found in sedimentary deposits that appear to be perfectly
continuous. This series, by itself, and without regard to the point as
to whether in other parts of the world other series may exist, shows
exactly those results which the theory of descent postulates, and we
find here, in all probability, a direct line of descent. While it may be
freely admitted that no such series can demonstrate the theory of
descent with absolute certainty, yet it would be folly to disregard
evidence as clear as this.
In regard to the other point raised by Fleischmann concerning the large
number of species of fossil horses that have existed in past times, it
is obvious that while this greatly increases the difficulty of the
paleontologist it is not an objection to the descent theory. In fact,
our experience with living species would lead us to expect that many
types have been represented at each geological period by a number of
related species that may have inhabited the same country. On the descent
theory, one species only in each geological period could have been in
the line of descent of the present species of horse. The difficulty of
determining which species (if there were several living in a given
epoch) is the ancestor of the horse is increased, but this is not in
itself an objection to the theory.
The descent of birds from flying reptiles is used by Fleischmann as
another point of attack on the transmutation theory. The theory
postulates that the birds have come from ancestors whose fore-legs have
been changed into highly specialized wings. The long vertebrated tail of
the ancestral form is supposed to have become very short, and long
feathers to have grown out from its stump which act as a rudder during
flight. Flying reptiles with winged fore-legs and a long vertebrated
tail have been actually found as fossil remains, as seen in the
pterodactyls and in the famous archæopteryx. The latter, which is
generally regarded either as the immediate ancestor of living birds, or
at least as a closely similar form, possessed a fore-leg having three
fingers ending in claws, and feathers on the forearm similar to those of
modern birds. It had a long tail, like that of a lizard, but with
well-developed feathers along its sides. It had pointed teeth in the
horn-covered jaws. Fleischmann proceeds to point out that the
resemblance of the hand of archæopteryx to that of the reptiles is not
very close, for two fingers are absent as in modern birds. The typical
form of the foot is that of the bird, and is not the simple reptilian
type of structure. Feathers and not scales cover the body, and give no
clew as to how the feathers of birds have arisen. He concludes,
therefore, that archæopteryx, having many true bird-like characters,
such as feathers, union of bones in the foot, etc., has other characters
not possessed by living birds, namely, a long, vertebrated tail, a flat
breastbone, biconcave vertebræ, etc. Therefore, it cannot be regarded as
an intermediate form. Fleischmann does not point out that it is just
these characters that would be postulated on the descent theory for the
ancestor of the birds, if the latter arose from reptiles. Even if it
should turn out that archæopteryx is not the immediate forefather of
living birds, yet the discovery that a form really existed intermediate
in many characters between the reptiles and the birds is a gain for the
transmutation theory. It is from a group having such characters that the
theory postulates that the birds have been evolved, and to have
discovered a member of such a group speaks directly and unmistakably in
favor of the probability of the transmutation theory.
Fleischmann again fails to point out that the geological period in which
the remains of archæopteryx were found, is the one just before that in
which the modern group of birds appeared, and, therefore, exactly the
one in which the theory demands the presence of intermediate forms. This
fact adds important evidence to the view that looks upon archæopteryx as
a form belonging to a group from which living birds have arisen. That a
number of recent paleontologists believe archæopteryx to belong to the
group of birds, rather than to the reptiles, or to an intermediate
group, does not in the least lessen its importance, as Fleischmann
pretends it does, as a form possessing a number of reptilian characters,
such as the transmutation theory postulates for the early ancestors of
the birds.
The origin of the mammalian phylum serves as the text for another attack
on the transmutation theory. Fleischmann points out that the discovery
of the monotremes, including the forms ornithorhynchus and echidna, was
hailed at first as a demonstration of the supposed descent of the
mammals from a reptilian ancestor. The special points of resemblance
between ornithorhynchus and reptiles and birds are the complete fusion
of the skull bones, the great development of the vertebræ of the neck
region, certain similarities in the shoulder girdle, the paired oviducts
opening independently into the last part of the digestive tract
(cloaca), and the presence of a parchment-like shell around the large,
yolk-bearing egg. These are all points of resemblance to reptiles and
birds, and were interpreted as intermediate stages between the latter
groups and the group of mammals. In addition to these intermediate
characters, ornithorhynchus possesses some distinctive, mammalian
features—mammary glands and hair, for instance. Fleischmann takes the
ground, in this case, that there are so many points of difference
between the monotremes and the higher mammals, that it is impossible to
see how from forms like these the higher groups could have arisen, and
that ornithorhynchus cannot be placed as an intermediate form, a link
between saurians and mammals, as the followers of the transmutation
theory maintain. He shows, giving citations, that anatomists themselves
are by no means in accord as to the exact position of ornithorhynchus in
relation to the higher forms.
In reply to this criticism, the same answer made above for archæopteryx
may be repeated here, namely, that because certain optimists have
declared the monotremes to be connecting forms, it does not follow that
the descent theory is untrue, and not even that these forms do not give
support to the theory, if in a less direct way. I doubt if any living
zoologist regards either ornithorhynchus or echidna as the ancestral
form from which the mammals have arisen. But on the other hand it may be
well not to forget that these two forms possess many characters
intermediate between those of mammals and reptiles, and it is from a
group having such intermediate characters that we should expect the
mammals to have arisen. These forms show, if they show nothing else,
that it is possible for a species to combine some of the characters of
the reptiles with those of the mammals; and the transmutation theory
does no more than postulate the existence at one time of such a group,
the different species of which may have differed in a number of points
from the two existing genera of monotremes.
The origin of lung-bearing vertebrates from fishlike ancestors, in which
the swim-bladder has been changed into lungs, has been pointed to by the
advocates of the transmutation theory as receiving confirmation in the
existence of animals like those in the group of dipnoan fishes. In these
animals both gills and a swim-bladder, that can be used as a lung, are
present; and through some such intermediate forms it is generally
supposed that the lung-bearing animals have arisen. Fleischmann argues,
however, that, on account of certain trivial differences in the position
of the duct of the swim-bladder in living species, the supposed
comparison is not to the point; but the issue thus raised is too
unimportant to merit further discussion. Leaving aside also some even
more doubtful criticisms which are made by Fleischmann, and which might
be added to indefinitely without doing more than showing the credulity
of some of the more ardent followers of the transmutation theory, or
else the uncertainty of some of the special applications of the theory,
let us pass to Fleischmann’s criticism of the problem of development.[4]
Footnote 4:
The long argument of Fleischmann in regard to the origin of the
fresh-water snails, as illustrated by the planorbis series, and also
the origin of the nautiloid group, has been recently dealt with fully
by Plate, and, therefore, need not be considered here.
With fine scorn Fleischmann points to the crudity of the ideas of Oken
and of Haeckel in regard to the embryology (or the ontogeny) repeating
the ancestral history (or the phylogeny). We may consider briefly (since
we devote the next chapter almost entirely to the same topic) the
exceptions to this supposed recapitulation, which Fleischmann has
brought together. The young of beetles, flies, and butterflies creep out
of the egg as small worm-like forms of apparently simple organization.
They have a long body, composed of a series of rings; the head is small
and lacks the feelers, and often the faceted eyes. The wings are absent,
and the legs are short. At first sight the larva appears to resemble a
worm, and this led Oken to conclude that the insects appear first in the
form of their ancestors, the segmented worms. If we examine the
structure of the larva more carefully, we shall find that there are a
great many differences between it and the segmented worms; and that even
the youngest larva is indeed a typical insect. The tracheæ, so
characteristic of the group of insects, are present, the structure of
the digestive tract with its Malpighian tubes, the form of the heart,
the structure of the head, as well as the blastema of the reproductive
organs, show in the youngest larva the type of the insects. In other
words the body of the caterpillar is formed on exactly the same
fundamental plan as that of the butterfly.
In regard to the larval forms of other groups we find the same
relations, as, for example, in the amphibians. The young of salamanders,
toads, and frogs leave the egg not in the completed form, but as small
tadpoles adapted to life in the water. A certain resemblance to fish
cannot be denied. They possess a broad tail, gills (rich in blood
vessels) on each side of the neck, and limbs are absent for a long time.
These are characters similar to those of fish, but a more careful
anatomical examination destroys the apparent resemblance. The
superficial resemblances are due to adaptation to the same external
conditions.
Fleischmann ridicules the idea that the young chick resembles at any
stage an adult, ancestral animal; the presence of an open digestive
tract shows how absurd such an idea is. The obvious contradiction is
explained away by embryologists, by supposing that the ancestral adult
stages have been crowded together in order to shorten the period of
development; and that, in addition, larval characters and provisional
organs have appeared in the embryo itself, which confuse and crowd out
the ancestral stages.
In regard to the presence of gill-slits in the embryo of the higher
vertebrates, in the chick, and in man, for example, Fleischmann says: “I
cannot see how it can be shown by exact proof that the gill-slits of the
embryos of the higher vertebrates that remain small and finally
disappear could once have had the power of growing into functional
slits.” With this trite comment the subject is dismissed.
On the whole, Fleischmann’s attack cannot be regarded as having
seriously weakened the theory of evolution. He has done, nevertheless,
good service in recalling the fact that, however probable the theory may
appear, the evidence is indirect and exact proof is still wanting.
Moreover, as I shall attempt to point out in the next chapter, we are
far from having arrived at a satisfactory idea of how the process has
really taken place.
------------------------------------------------------------------------
CHAPTER III
THE THEORY OF EVOLUTION (Continued)
The Evidence from Embryology
THE RECAPITULATION THEORY
At the close of the eighteenth, and more definitely at the beginning of
the nineteenth, century a number of naturalists called attention to the
remarkable resemblance between the embryos of higher animals and the
adult forms of lower animals. This idea was destined to play an
important rôle as one of the most convincing proofs of the theory of
evolution, and it is interesting to examine, in the first place, the
evidence that suggested to these earlier writers the theory that the
embryos of the higher forms pass through the adult stages of the lower
animals.
The first definite reference[5] to the recapitulation view that I have
been able to find is that of Kielmeyer in 1793, which was inspired, he
says, by the resemblance of the tadpole of the frog to an adult fish.[6]
This suggested that the embryo of higher forms corresponds to the adult
stages of lower ones. He adds that man and birds are in their first
stages plantlike.
Footnote 5:
The earlier references of a few embryologists are too vague to have
any bearing on the subject.
Footnote 6:
Autenrieth in 1797 makes the briefest possible reference to some such
principle in speaking of the way in which the nose of the embryo
closes.
Oken in 1805 gave the following fantastic account of this relation:
“Each animal ‘metamorphoses itself’ through all animal forms. The frog
appears first under the form of a mollusk in order to pass from this
stage to a higher one. The tadpole stage is a true snail; it has gills
which hang free at the sides of the body as is the case in _Unio
pictorum_. It has even a byssus, as in Mytilus, in order to cling to the
grass. The tail is nothing else than the foot of the snail. The
metamorphosis of an insect is a repetition of the whole class,
scolopendra, oniscus, julus, spider, crab.”
Walther, in 1808, said: “The human fœtus passes through its
metamorphosis in the cavity of the uterus in such a way that it repeats
all classes of animals, but, remaining permanently in none, develops
more and more into the innate human form. First the embryo has the form
of a worm. It reaches the insect stage just before its metamorphosis.
The origin of the liver, the appearance of the different secretions,
etc., show clearly an advance from the class of the worm into that of
the mollusk.”
Meckel first in 1808, again in 1811, and more fully in 1821 made much
more definite comparisons between the embryos of higher forms and the
adult stages of lower groups. He held that the embryo of higher forms,
before reaching its complete development, passes through many stages
that correspond to those at which the lower animals appear to be checked
through their whole life. In fact the embryos of higher animals, the
mammals, and especially man, correspond in the form of their organs, in
their number, position, and proportionate size to those of the animals
standing below them. The skin is at first, and for a considerable period
of embryonic life, soft, smooth, hairless, as in the zoophytes, medusæ,
many worms, mollusks, fishes, and even in the lower amphibians. Then
comes a period in which it becomes thicker and hairy, when it
corresponds to the skin of the higher animals. It should be especially
noted here, that the fœtus of the negro is more hairy than that of the
European.
The muscular system of the embryo, owing to its lack of union in the
ventral wall, corresponds to the muscles of the shelled, headless
mollusks, whose mantle is open in the same region. Meckel compares the
bones of the higher vertebrates with the simpler bones of the lower
forms, and even with the cartilages of the cephalopod. He points out
that in the early human embryo the nerve cord extends the whole length
of the spinal canal. He compares the simple heart of the embryo with
that of worms, and a later stage, when two chambers are present, with
that of the gasteropod mollusk. The circulation of the blood in the
placenta recalls, he says, the circulation in the skin of the lower
animals. The lobulated form of the kidney in the human embryo is
compared with the adult condition in the fishes and amphibians. The
internal position of the reproductive organs in the higher mammals
recalls the permanent position of these organs in the lower animals. The
posterior end of the body of the human embryo extends backwards as a
tail which later disappears.
Some of these comparisons of Meckel sound very absurd to us nowadays,
especially his comparison between the embryos of the higher vertebrates,
and the adults of worms, crustaceans, spiders, snails, bivalve mollusks,
cephalopods, etc. On the other hand, many of these comparisons are the
same as those that are to be found in modern text-books on embryology;
and we may do well to ask ourselves whether these may not sound equally
absurd a hundred years hence. Why do some of Meckel’s comparisons seem
so naïve, while others have a distinctly modern flavor? In a word, can
we justify the present belief of some embryologists that the embryos of
higher forms repeat the adult stages of lower members of the same group?
It is important to observe that up to this time the comparison had
always been made between the embryo of the higher form and the adult
forms of existing lower animals. The theory of evolution had, so far,
had no influence on the interpretation that was later given to this
resemblance.
Von Baer opposed the theory of recapitulation that had become current
when he wrote in 1828. According to Von Baer, the more nearly related
two animals are, or rather the more nearly similar two forms are (since
Von Baer did not accept the idea of evolution), the more nearly alike is
their development, and so much longer in their development do they
follow in the same path. For example two similar species of pigeons will
follow the same method of development up to almost the last stage of
their formation. The embryos of these two forms will be practically
identical until each produces the special characters of its own species.
On the other hand two animals belonging to different families of the
same phylum will have only the earlier stages in common. Thus, a bird
and a mammal will have the first stages similar, or identical, and then
diverge, the mammal adding the higher characters of its group. The
resemblance is between corresponding embryonic stages and not between
the embryo of the mammal and the adult form of a lower group.
Von Baer was also careful to compare embryos of the same phylum with
each other, and states explicitly that there are no grounds for
comparison between embryos of different groups.[7]
Footnote 7:
In one place Von Baer raises the question whether the egg may not be a
form common to all the phyla.
We shall return again to Von Baer’s interpretation and then discuss its
value from our present point of view.
Despite the different interpretation that Von Baer gave to this doctrine
of resemblance the older view of recapitulation continued to dominate
the thoughts of embryologists throughout the whole of the nineteenth
century.
Louis Agassiz, in the Lowell Lectures of 1848, proposed for the first
time the theory that the embryo of higher forms resembled not so much
lower adult animals living at the present time, as those that lived in
past times. Since Agassiz himself did not accept the theory of
evolution, the interpretation that he gave to the recapitulation theory
did not have the importance that it was destined to have when the
animals that lived in the past came to be looked upon as the ancestors
of existing animals.[8] But with the acceptation of the theory of
evolution, which was largely the outcome of the publication of Darwin’s
“Origin of Species” in 1859, this new interpretation immediately
blossomed forth. In fact, it became almost a part of the new theory to
believe that the embryo of higher forms recapitulated the series of
ancestral adult forms through which the species had passed. The one
addition of any importance to the theory that was added by the Darwinian
school was that the history of the past, as exemplified by the embryonic
development, is often falsified.
Footnote 8:
Carl Vogt in 1842 suggested that fossil species, in their historical
succession, pass through changes similar to those which the embryos of
living forms undergo.
Let us return once more to the facts and see which of them are regarded
at present as demanding an explanation. These facts are not very
numerous and yet sufficiently apparent to attract attention at once when
known.
The most interesting case, and the one that has most often attracted
attention, is the occurrence of gill-clefts in the embryos of reptiles,
birds, and mammals. These appear on each side of the neck in the very
early embryo. Each is formed by a vertical pouch, that grows out from
the wall of the pharynx until it meets the skin, and, fusing with the
latter, the walls of the pouch separate, and a cleft is formed. This
vertical cleft, placing the cavity of the pharynx in communication with
the outside, is the gill-slit. Similar openings in adult fishes put the
pharynx in communication with the exterior, so that water taken through
the mouth passes out at the sides of the neck between the gill filaments
that border the gill-slits. In this way the blood is aerated. The number
of gill-slits that are found in the embryos of different groups of
higher vertebrates, and the number that open to the exterior are
variable; but the number of gill-openings that are present in the adults
of lower vertebrates is also variable. No one who has studied the method
of development of the gill-slits in the lower and higher vertebrates
will doubt for a moment that some kind of relation must subsist between
these structures.
In the lowest adult form of the vertebrates, amphioxus, the gill-system
is used largely as a sieve for procuring food, partly also, perhaps, for
respiration. In the sharks, bony fishes, and lower amphibians, water is
taken in through the mouth, and passes through the gill-slits to the
exterior. As it goes through the slits it passes over the gills, that
stand like fringes on the sides of the slits. The blood that passes in
large quantities through the gills is aerated in this way. In the
embryos of the higher vertebrates the gill-slits may appear even before
the mouth has opened, but in no case is there a passage of water through
the gill-slits, nor is the blood aerated in the gill-region, although it
passes through this part on its way from the heart to the dorsal side of
the digestive tract. It is quite certain that the gill-system of the
embryo performs no respiratory function.[9]
Footnote 9:
This statement is not intended to prejudice the question as to whether
the presence of the gill-slits and arches may be essential to the
formation of other organs.
In the higher amphibians, the frogs for example, we find an interesting
transition. The young embryo, when it emerges from the egg-membranes,
bears three pairs of external gills that project from the gill-arches
into the surrounding water. Later these are absorbed, and a new system
of internal gills, like those of fishes, develops on the gill-arches.
These are used throughout the tadpole stage for respiratory purposes.
When the tadpole is about to leave the water to become a frog, the
internal gills are also absorbed and the gill-clefts close. Lungs then
develop which become the permanent organs of respiration.
There are two points to be noticed in this connection. First, the
external gills, which are the first to develop, do not seem to
correspond to any permanent adult stage of a lower group. Second, the
transition from the tadpole to the frog can only be used by way of
analogy of what is supposed to have taken place ancestrally in the
reptiles, birds, and mammals, since no one will maintain that the frogs
represent a group transitional between the amphibians and the higher
forms. However, since the salamanders also have gills and gill-slits in
the young stages, and lose them when they leave the water to become
adult land forms, this group will better serve to illustrate how the
gill-system has been lost in the higher forms. Not that in this case
either, need we suppose that the forms living to-day represent
ancestral, transitional forms, but only that they indicate how such a
remarkable change from a gill-breathing form, living in the water, might
become transformed into a lung-breathing land form. Such a change is
supposed to have taken place when the ancestors of the reptiles and the
mammals left the water to take up their abode on the land.
The point to which I wish to draw especial attention in this connection
is that in the higher forms the gill-slits appear at a very early stage;
in fact, as early in the mammal as in the salamander or the fish, so
that if we suppose their appearance in the mammal is a repetition of the
adult amphibian stage, then, since this stage appears as early in the
development of the mammal as in the amphibians themselves, the
conclusion is somewhat paradoxical.
The history of the notochord in the vertebrate series gives an
interesting parallel. In amphioxus it is a tough and firm cord that
extends from end to end of the body. On each side of it lie the plates
of muscles. It appears at a very early stage of development as a fold of
the upper wall of the digestive tract. In the cartilaginous fishes the
notochord also appears at a very early stage, and also from the dorsal
wall of the digestive tract. In later embryonic stages it becomes
surrounded by a cartilaginous sheath, or tube, which then segments into
blocks, the vertebræ. The notochord becomes partially obliterated as the
centra of the vertebræ are formed, but traces of it are present even in
adult stages. In the lower amphibians the notochord arises also at an
early stage over and perhaps, in part, from the dorsal wall of the
digestive tract. It is later almost entirely obliterated by the
development of the vertebræ. These vertebræ first appear as a
membraneous tube which breaks up into cartilaginous blocks, and these
are the structures around and in which the bone develops to form the
permanent vertebræ.
In higher forms, reptiles, birds, and mammals, the notochord also
appears at the very beginning of the development, but it is not certain
that we can call the material out of which it forms the dorsal wall of
the archenteron (the amphibians giving, perhaps, intermediate stages).
It becomes surrounded by continuous tissue which breaks up into blocks,
and these become the bases of the vertebræ. The notochord becomes so
nearly obliterated in later stages that only the barest traces of it are
left either in the spaces between, or in, the vertebræ.
In this series we see the higher forms passing through stages similar at
first to those through which the lower forms pass; and it is especially
worthy of note that the embryo mammal begins to produce its notochord at
the very beginning of its development, at a stage, in fact, so far as
comparison is possible, as early as that at which the notochord of
amphioxus develops.
The development of the skull gives a somewhat similar case. The skulls
of sharks and skates are entirely cartilaginous and imperfectly enclose
the brain. The ganoids have added to the cartilaginous skull certain
plates in the dermal layer of the skin. In the higher forms we find the
skull composed of two sets of bones, one set developing from the
cartilage of the first-formed cranium, and the other having a more
superficial origin; the latter are called the membrane bones, and are
supposed to correspond to the dermal plates of the ganoids.
In the development of the kidneys, or nephridia, we find, perhaps,
another parallel, although, owing to recent discoveries, we must be very
cautious in our interpretation. As yet, nothing corresponding to the
nephridia of amphioxus has been discovered in the other vertebrates. Our
comparison must begin, therefore, higher up in the series. In the sharks
and bony fishes the nephridia lie at the anterior end of the
body-cavity. In the amphibia there is present in the young tadpole a
pair of nephridial organs, the head-kidneys, also in the anterior end of
the body-cavity. Later these are replaced by another organ, the
permanent mid-kidney, that develops behind the head-kidney. In reptiles,
birds, and mammals a third nephridial organ, the hind-kidney, develops
later than and posterior to the mid-kidney, and becomes the permanent
organ of excretion. Thus in the development of the nephridial system in
the higher forms we find the same sequence, more or less, that is found
in the series of adult forms mentioned above. The anterior end of the
kidney develops first, then the middle part, and then the most
posterior. The anterior part disappears in the amphibians, the anterior
and the middle parts in the birds and mammals, so that in the latter
groups the permanent kidney is the hind-kidney alone.
The formation of the heart is supposed to offer certain parallels.
Amphioxus is without a definite heart, but there is a ventral blood
vessel beneath the pharynx, which sends blood to the gill-system. This
blood vessel corresponds in position to the heart of other vertebrates.
In sharks we find a thick-walled muscular tube below the pharynx; the
blood enters at its posterior end, flows forward and out at the anterior
end into a blood vessel that sends smaller vessels up through the
gill-arches to the dorsal side.
In the amphibia the heart is a tube, so twisted on itself that the
original posterior end is carried forward to the anterior end, and this
part, the auricle, is divided lengthwise by a partition into a right and
a left side. In the reptiles the ventricle is also partially separated
into two chambers, completely so in the crocodiles. In birds and mammals
the auricular and ventricular septa are complete in the adult, and the
ventral aorta that carries the blood forward from the heart is
completely divided into two vessels, one of which now carries blood to
the lungs. When we examine the development of the heart of a mammal, or
of a bird, we find something like a parallel series of stages,
apparently resembling conditions found in the different groups just
described. The heart is, at first, a straight tube, it then bends on
itself, and a constriction separates the auricular part from the
ventricular, and another the ventricular from the ventral aorta.
Vertical longitudinal partitions then arise, one of which separates the
auricle into two parts, and another the ventricle into two parts, and a
third divides the primitive aorta into two parts. In the early stages
all the blood passes from the single ventral aorta through the
gill-arches to the dorsal side, and it is only after the appearance of
the lung-system that the gill-system is largely obliterated.
We find here, then, a sort of parallel, provided we do not inquire too
particularly into details. This comparison may be justified, at least so
far that the circulation is at first through the arches and is later
partially replaced by the double circulation, the systemic and the
pulmonary.
A few other cases may also be added. The proverbial absence of teeth in
birds applies only to the adult condition, for, as first shown by
Geoffroy Saint-Hilaire, four thickenings, or ridges, develop in the
mouth of the embryo; two in the upper, two in the lower, jaw. These
ridges appear to correspond to those of reptiles and mammals, from which
the teeth develop. It may be said, therefore, that the rudiments of
teeth appear in the embryo of the bird. This might be interpreted to
mean that the embryo repeats the ancestral reptilian stage, or, perhaps,
the ancestral avian stage that had teeth in the beak; but since only the
beginnings of teeth appear, and not the fully formed structures, this
interpretation would clearly overshoot the mark.
The embryo of the baleen whale has teeth that do not break through the
gums and are later absorbed. Since the ancestors of this whale probably
had teeth, as have other whales at the present time, the appearance of
teeth in the embryo has been interpreted as a repetition of the original
condition. Some of the ant-eaters are also toothless, but teeth appear
in the embryo and are lost later. In the ruminants that lack teeth in
the front part of the upper jaw, _e.g._ the cow and the sheep, teeth
develop in the embryo which are subsequently lost.
One interpretation of these facts is that the ancestral adult condition
is repeated by the embryo, but as I have pointed out above in the cases
of the teeth in whales, since the teeth do not reach the adult form, and
do not even break through the gums in some forms, it is obviously
stretching a point to claim that an adult condition is repeated.
Moreover, in the case of the birds only the dental ridges appear, and it
is manifestly absurd to claim in this case that the ancestral adult
condition of the reptiles is repeated.
That a supposed ancestral stage may be entirely lost in the embryo of
higher forms is beautifully shown in the development of some of the
snakes. The snakes are probably derived from lizardlike ancestors, which
had four legs, yet in the development the rudiments of legs do not
appear, and this is the more surprising since a few snakes have small
rudimentary legs. In these, of course, the rudiments of legs must appear
in the embryo, but in the legless forms even the beginnings of the legs
have been lost, or at any rate very nearly so.
Outside the group of vertebrates there are also many cases that have
been interpreted as embryonic repetitions of ancestral stages, but a
brief examination will suffice to show that many of these cases are
doubtful, and others little less than fanciful. A few illustrations will
serve our purpose. The most interesting case is that given by the
history of the nauplius theory.
The free-living larva of the lower crustaceans—water-fleas, barnacles,
copepods, ostracods—emerges from the egg as a small, flattened oval form
with three pairs of appendages. This larva, known as the nauplius,
occurs also in some of the higher crustaceans, not often, it is true, as
a free form, as in penæus, but as an embryonic stage. The occurrence of
this six-legged form throughout the group was interpreted by the
propounders of the nauplius theory as evidence sufficient to establish
the view that it represented the ancestor of the whole group of
Crustacea, which ancestor is, therefore, repeated as an embryonic form.
This hypothesis was accepted by a large number of eminent embryologists.
The history of the collapse of the theory is instructive.
It had also been found in one of the groups of higher crustaceans, the
decapods, containing the crayfish, lobster, and crabs, that another
characteristic larval form was repeated in many cases. This larva is
known as the zoëa. It has a body made up of a fused head and thorax
carrying seven pairs of appendages and of a segmented abdomen of six
segments. The same kind of evidence that justified the formulation of
the nauplius theory would lead us to infer that the zoëa is the ancestor
of the decapods. The later development of the zoëa shows, however, that
it cannot be such an ancestral form, for, in order to reach the full
number of segments characteristic of the decapods, new segments are
intercalated between the cephalothorax and abdomen. In fact, in many
zoëas this intercalated region is already in existence in a rudimentary
condition, and small appendages may even be present. A study of the
comparative anatomy of the crustaceans leaves no grounds for supposing
that the decapods with their twenty-one segments have been evolved from
a thirteen-segmented form like the zoëa by the intercalation of eight
segments in the middle of the body. It follows, if this be admitted, and
it is generally admitted now, that the zoëa does not represent an
original ancestral form at all, but a highly modified new form, as new,
perhaps, as the group of decapods itself. We are forced to conclude,
then, that the presence of a larval form throughout an entire group
cannot be accepted as evidence that it represents an ancestral stage. We
can account for the presence of the zoëa, however, by making a single
supposition, namely, that the ancestor from which the group of decapod
has evolved had a larva like the zoëa, and that this larval form has
been handed down to all of the descendants.
The fate of the zoëa theory cast a shadow over the nauplius theory,
since the two rested on the same sort of evidence. The outcome was, in
fact, that the nauplius theory was also abandoned, and this was seen to
be the more necessary, since a study of the internal anatomy of the
lowest group of crustaceans, the phyllopods, showed that they have
probably come directly from many segmented, annelidian ancestors. The
presence of the nauplius is now generally accounted for by supposing
that it was a larval form of the ancestor from which the group of
crustaceans arose.
The most extreme, and in many ways the most uncritical, application of
the recapitulation theory was that made by Haeckel, more especially his
attempt to reduce all the higher animals to an ancestral double-walled
sac with an opening at one end,—the gastræa. He dignified the
recapitulation theory with an appellation of his own, “The Biogenetic
Law.” Haeckel’s fanciful and extreme application of the older
recapitulation theory has probably done more to bring the theory into
disrepute amongst embryologists than the criticisms of the opponents of
the theory.
In one of the recognized masterpieces of embryological literature, His’s
“Unsere Körperform,” we find the strongest protest that has yet been
made against the Haeckelian pretension that the phylogenetic history is
the “cause” of the ontogenetic series. His writes: “In the entire series
of forms which a developing organism runs through, each form is the
necessary antecedent step of the following. If the embryo is to reach
the complicated end-forms, it must pass, step by step, through the
simpler ones. Each step of the series is the physiological consequence
of the preceding stage and the necessary condition for the following.
Jumps, or short cuts, of the developmental process, are unknown in the
physiological process of development. If embryonic forms are the
inevitable precedents of the mature forms, because the more complicated
forms must pass through the simpler ones, we can understand the fact
that paleontological forms are so often like the embryonic forms of
to-day. The paleontological forms are embryonal, because they have
remained at the lower stage of development, and the present embryos must
pass also through lower stages in order to reach the higher. But it is
by no means necessary for the later, higher forms to pass through
embryonal forms because their ancestors have once existed in this
condition. To take a special case, suppose in the course of generations
a species has increased its length of life gradually from one, two,
three years to eighty years. The last animal would have had ancestors
that lived for one year, two years, three years, etc., up to eighty
years. But who would claim that because the final eighty-year species
must pass necessarily through one, two, three years, etc., that it does
so because its ancestors lived one year, two years, three years, etc.?
The descent theory is correct so far as it maintains that older, simpler
forms have been the forefathers of later complicated forms. In this case
the resemblance of the older, simpler forms to the embryos of later
forms is explained without assuming any law of inheritance whatsoever.
The same resemblance between the older and simpler adult forms, and the
present embryonic forms would even remain intelligible were there no
relation at all between them.”
Interesting and important as is this idea of His, it will not, I think,
be considered by most embryologists as giving an adequate explanation of
many facts that we now possess. It expresses, no doubt, a part of the
truth but not the whole truth.
We come now to a consideration of certain recently ascertained facts
that put, as I shall try to show, the whole question of embryonic
repetition in a new light.
A minute and accurate study of the early stages of division or cleavage
of the egg of annelids has shown a remarkable agreement throughout the
group. The work of E. B. Wilson on nereis, and on a number of other
forms, as well as the subsequent work of Mead, Child, and Treadwell on
other annelids, has shown resemblances in a large number of details,
involving some very complicated processes.[10]
Footnote 10:
On the other hand it should not pass unnoticed that Eisigh as shown in
one form (in which, however, the eggs are under special conditions
being closely packed together) that the usual type of cleavage is
altered.
Not only is the same method of cleavage found in most annelids, but the
same identical form of division is also present in many of the mollusks,
as shown especially by the work of Conklin, Lillie, and Holmes. This
resemblance has been discussed at some length by those who have worked
out these results in the two groups. The general conclusion reached by
them is that the only possible interpretation of the phenomenon is that
some sort of genetic connection must exist between the different forms;
and while not explicitly stated, yet there is not much doubt that some
at least of these authors have had in mind the view that the annelids
and mollusks are descended from common ancestors whose eggs segmented as
do those of most of the mollusks and annelids of the present day. This
conclusion is, I believe, of more far-reaching importance than has been
supposed, and may furnish the key that will unlock the whole question of
the resemblance of embryos to supposed ancestral forms. It is a most
fortunate circumstance that in the case of this cell lineage the facts
are of such a kind as to preclude the possibility that the stages in
common could ever have been ancestral adult stages. If this be granted
then only two interpretations are possible: the results are due either
to a coincidence, or to a common embryonic form that is repeated in the
embryo of many of the descendants. That the similarity is not due to a
coincidence is made probable from the number and the complexities of the
cleavage stages.
I believe that we can extend this same interpretation to all other cases
of embryonic resemblance. It will explain the occurrence of gill-slits
in the embryo of the bird, and the presence of a notochord in the higher
forms in exactly the same way as the cleavage stages are explained. But
how, it may be asked, can we explain the apparent resemblance between
the embryo of the higher form and the adult of lower groups. The answer
is that this resemblance is deceptive, and in so far as there is a
resemblance it depends on the resemblance of the adult of the lower form
to its own embryonic stages with which we can really make a comparison.
The gill-slits of the embryo of the chick are to be compared, not with
those of the adult fish, but with those of the embryo of the fish. It is
a significant fact, in this connection, that the gill-slits appear as
early in the embryo of the fish as they do in the bird! The notochord of
the embryo bird is comparable with that of the embryo of amphioxus, and
not with the persistent notochord in the adult amphioxus. Here also it
is of the first importance to find that the notochord appears both in
the embryo bird and in amphioxus at the very beginning of the
development. The embryo bird is not fishlike except in so far as there
are certain organs in the embryo fish that are retained in the adult
form. The embryo bird bears the same relation to the embryo fish that
the early segmentation stages of the mollusk bear to the early
segmentation stages of the annelid. There are certain obvious
resemblances between this view and that of Von Baer, but there are also
some fundamental differences between the two conceptions.
Von Baer thought that within each group the embryonic development is the
same up to a certain point. He supposed that the characters of the group
are the first to appear, then those of the order, class, family, genus,
and, finally, of the species. He supposed that two similar species would
follow the same method of development until the very last stage was
reached, when each would then add the final touches that give the
individual its specific character. We may call this the theory of
embryonic parallelism. Here there is an important difference between my
view and that of Von Baer, for I should not expect to find the two
embryos of any two species identical at any stage of their development,
but at most there might exist a close resemblance between them.
Von Baer’s statement appears to be erroneous from a modern point of view
in the following respects. We know that in certain large groups some
forms develop in a very different way from that followed by other
members of the group, as shown by the cephalopods, for instance, in the
group of mollusks. Again, it is entirely arbitrary to assume that the
group-characters are the first to appear, and then successively those of
the order, family, genus, species. Finally, as has been said above, we
do not find the early embryos of a group identical; for with a
sufficient knowledge of the development it is always possible to
distinguish between the embryos of different species, as well as between
the adults, only it is more difficult to do so, because the embryonic
forms are simpler. The most fundamental difference between the view of
Von Baer and modern views is due to our acceptation of the theory of
evolution which seems to make it possible to get a deeper insight into
the meaning of the repetition, that carries us far ahead of Von Baer’s
position. For with the acceptance of this doctrine we have an
interpretation of how it is possible for the embryonic stages of most
members of a group to have the same form, although they are not
identical. There has been a continuous, although divergent, stream of
living material, carrying along with it the substance out of which the
similar embryonic forms are made. As the stream of embryonic material
divided into different paths it has also changed many of the details,
sometimes even all; but nevertheless it has often retained the same
general method of development that is associated with its particular
composition. We find the likeness, in the sense of similarity of plan,
accounted for by the inheritance of the same sort of substance; the
differences in the development must be accounted for in some other way.
Among modern writers Hurst alone has advanced a view that is similar in
several respects to that which I have here defended. It may be well to
give his statement, since it brings out certain points of resemblance
with, as well as certain differences from, my own view.[11] He says:
“Direct observation has shown that, when an animal species _varies_
(_i.e._ _becomes_ unlike what it was before) in adult structure, those
stages in the development which are nearest the adult undergo a similar,
but usually smaller, change. This is shown in domestic species by the
observations of Darwin, and the result is in exact harmony with the
well-known law of Von Baer, which refers to natural species, both nearly
related and widely dissimilar. Von Baer’s observations as well as
Darwin’s, and as well as those of every student who has ever compared
the embryos of two vertebrate species, may be summarized as follows:—
Footnote 11:
Hurst, C. H., “Biological Theories, III,” “The Recapitulation Theory,”
_Natural Science_, Vol. ii., 1893.
“Animals which, though related, are very similar in the adult state,
resemble each other more closely in early stages of development, often,
indeed, so closely as to be indistinguishable in those early stages. As
development proceeds _in such species_, the differences between the two
embryos compared become more and more pronounced.” On this point, which
is an essential one, I cannot agree with Hurst; for I do not think that
the facts show that the early stages of two related forms are
necessarily more and more alike the farther back we go. The resemblance
that is sometimes so striking in the earlier stages is due to the fewer
points there are for comparison, and to the less development of the
parts then present. Hurst continues: “If similar comparisons could be
instituted between the ancestral species and its much modified
descendants, there is no reason for doubting that a similar result would
be reached. This, indeed, has been done in the case of some breeds of
pigeons, which we have excellent reasons for believing to be descended
from _Columba livia_. True, _C. livia_ is not a very remote ancestor,
but I do not think that will vitiate the argument. Let me quote Darwin
verbatim: ‘As we have conclusive evidence that the breeds of the pigeon
are descended from a single wild species, I have compared the young
within twelve hours after being hatched; I have carefully measured the
proportions (but will not here give the details) of the beak, width of
mouth, length of nostril, and of eyelid, size of feet, and length of leg
in the wild, parent species, in pouters, fantails, runts, barbs,
dragons, carriers, and tumblers. Now some of these birds when mature
differ in so extraordinary a manner in the length and form of the beak,
and in other characters, that they would certainly have been ranked as
distinct genera if found in a state of nature. But when the nestling
birds of these several breeds were placed in a row, though most of them
could just be distinguished, the proportional differences in the above
specified points were incomparably less than in the full-grown birds.
Some characteristic points of difference—for instance, that of the width
of the mouth—could hardly be detected in the young. But there was one
remarkable exception to this rule, for the young of the short-faced
tumbler differed from the young of the wild-rock pigeon, and of the
other breeds in almost exactly the same proportions as in the adult
state.’”
Hurst concludes that: “The more the adult structure comes to be unlike
the adult structure of the ancestors, the more do the late stages of
development undergo a modification of the same kind. This is not mere
dogma, but it is a simple paraphrase of Von Baer’s law. It is proved
true not only by the observations of Von Baer and of Darwin, already
referred to, but by the direct observation of every one who takes the
trouble to compare the embryos of any two vertebrates, provided only he
will be content to see what actually lies before him and not the
phantasms which the recapitulation theory may have printed on his
imagination.”
The growth of the antlers of stags is cited by Hurst in order to
illustrate that what has been interpreted as a recapitulation may have a
different interpretation. “Each stag develops a new pair of antlers in
each successive year, and each pair of antlers is larger than the pair
produced in the previous year. This yearly increase in the size of the
antlers has been put forward as an example of an ontogenetic record of
past evolution. I, however, deny that it is such a record.”
“The series of ancestors may have possessed larger antlers in each
generation than in the generation before it. It is not an occasional
accidental parallelism between the ontogeny and the phylogeny which I
deny, but the causal relation between the two. Had the ancestors had
larger antlers than the existing ones, there is no justification for the
assumption that existing stags would acquire antlers of which each pair,
in later years, would be smaller than those of the previous year.”
Hurst concludes: “There are many breeds of hornless sheep, but they do
not bear large horns in early years and then shed them. If a rudiment
ever appears in the embryo of such sheep, its growth is very early
arrested.” The case of the appendix in man might have been cited here as
a case in point. It is supposed to have been larger in the ancestors of
man, but we do not find it appearing full size in the embryo and later
becoming rudimentary. The preceding statements will show that, while
Hurst’s view is similar in some respects to my own, yet it differs in
one fundamental respect from it, and in this regard he approaches more
nearly to the theory of Von Baer.
Hertwig has recently raised some new points of issue in regard to the
recapitulation theory, and since he may appear to have penetrated
farther than most other embryologists of the present time, it will be
necessary to examine his view somewhat carefully. He speaks of the
germ-cell (egg, or spermatozoön) as a species-cell, because it contains,
in its finer organization, the essential features of the species to
which it belongs. There are as many of these kinds of cells as there are
different kinds of animals and plants. Since the bodies of the higher
animals have developed from these species-cells, so the latter must have
passed in their phylogeny through a corresponding development from a
simple to a more and more complex cell-structure. “Our doctrine is, that
the species-cell, even as the adult, many-celled representative of the
species, has passed through a progressive, and, indeed, in general a
corresponding development in the course of phylogeny. This view appears
to stand in contradiction to the biogenetic law. According to the
formula that Haeckel has maintained, the germ development is an epitome
of the genealogy; or the ontogeny is a recapitulation of the phylogeny;
or, more fully, the series of forms through which the individual
organism passes during its development from the egg-cell to the finished
condition is a short, compressed repetition of the longer series of
forms which the forefathers of the same organism, or the stem-form of
the species, has passed through, from the earliest appearance of
organisms to the present time.” “Haeckel admits that the parallel may be
obliterated, since much may be absent in the ontogeny that formerly
existed in the phylogeny. If the ontogeny were complete, we could trace
the whole ancestry.” Hertwig states further, that “The theory of
biogenesis[12] makes it necessary to change Haeckel’s expression of the
biogenetic law, so that a contradiction contained in it may be removed.
We must drop the expression ‘repetition of the form of extinct
forefathers,’ and put in its place the repetition of forms which are
necessary for organic development, and lead from the simple to the
complex. This conception may be illustrated by the egg-cell.”
Footnote 12:
This term, by which Hertwig designates a particular view of his own,
has been already preoccupied in a much wider sense by Huxley to mean
that all life comes from preëxisting life. Hertwig means by the theory
of biogenesis that as the egg develops there is a constant interchange
between itself and its surroundings.
Since each organism begins its life as an egg we must not suppose that
the primitive conditions of the time, when only single-celled amœbas
existed on our planet, are repeated. The egg-cell of a living mammal is
not, according to Hertwig’s hypothesis, an indifferent structure without
much specialization like an amœba, but is an extraordinarily complex
end-product of a long historical process, which the organized substance
has passed through. If the egg of a mammal is different from that of a
reptile, or of an amphibian, because in its organization it contains the
basis of a mammal, just so much more must it be different from the
hypothetical one-celled amœba, which has no other characteristics than
those that go to make up an amœba. Expressed more generally, the
developmental process in the many-celled organisms begins, not where it
began in primitive times, but as the representation of the highest point
which the organization has at present reached. The development commences
with the egg, because it is the elemental and fundamental form in which
organic life is represented in connection with the reproductive process,
and also because it contains in itself the properties of the species in
its primordia.
“The egg-cell of the present time, and its one-celled predecessor in the
phylogenetic history, the amœba, are only comparable in so far as they
fall under the common definition of the cell, but beyond this they are
extraordinarily different from each other.”
“The phyletic series must be divided into two different kinds of
processes:—First. The evolution of the species-cell, which is a steady
advance from a simple to a complex organization. Second. The
periodically repeated development of the many-celled individual out of
the single cell, representative of the species (or the individual
ontogeny), which in general follows the same rules as the preceding
ontogeny, but is each time somewhat modified according to the amount to
which the species-cell has itself been changed in the phylogeny. Similar
restricting and explanatory additions to the biogenetic law, like those
stated here for the one-celled stage, must be made in other directions.
Undoubtedly there exists in a certain sense a parallel between the
phylogenetic, and the ontogenetic, development.
“On the basis of the general developmental hypothesis on which we stand,
all forms which in the chain of ancestors were end-products of the
individual development are now passed through by their descendants as
embryonic stages, and so in a certain degree are recapitulated. We also
admit that the embryonic forms of higher animals have many points of
comparison with the mature forms of related groups standing lower in the
system.
“Nevertheless, a deeper insight into the conditions relating to these
resemblances shows that there are very important differences that should
not be overlooked. Three points need to be mentioned: 1. The
cell-material which in the ancestral chain gives the basis for each
ontogenetic process is each time a different material as far as concerns
its finer organization and primordia. Indeed, the differences become
greater the farther apart the links of the original chain become. This
thought may be formulated in another way: The same ontogenetic stages
that repeat themselves periodically in the course of the phylogeny
always contain at bottom a somewhat different cell-material. From this
the second rule follows as a consequence. 2. Between the mature end-form
of an ancestor and the corresponding embryonic form of a widely remote
descendant (let us say between the phylogenetic gastræa and the
embryonic gastrula stage of a living mammal, according to the
terminology of Haeckel) there exists an important difference, namely,
that the latter is supplied with numerous primordia which are absent in
the other, and which force it to proceed to the realization of its
developmental process. The gastrula, therefore, as the bearer of
important latent forces, is an entirely different thing from the
gastræa, which has already reached the goal of its development. 3. In
the third place, at each stage of the ontogeny outer and inner factors
are at work, in fact even more intensely than in the fully formed
organism. Each smallest change that acts anew in this way at the
beginning of the ontogeny can start an impulse leading to more extensive
changes in later stages. Thus the presence of yolk and its method of
distribution in the egg alone suffice to bring about important changes
in the cleavage, and in the formation of the germ-layers, the blastula,
and gastrula stages,” etc. “Moreover, the embryo may adapt itself to
special conditions of embryonic life, and produce organs of an ephemeral
nature like the amnion, chorion, and placenta.”
“A comparison of ontogenetic with antecedent phylogenetic stages must
always keep in view the fact that the action of external and internal
factors has brought about considerable changes in the ontogenetic
system, and, indeed, in a generally advancing direction, so that in
reality a later condition can never correspond to a preceding one.”
Hertwig sums up his conclusion in the statement that ontogenetic stages
give us, therefore, a greatly changed picture of the phylogenetic series
of adult ancestors. “The two correspond not according to their actual
contents but only as to their form.” Hertwig also repeats His’s idea,
that the reason that certain kinds of form repeat themselves in the
development of animals with a great constancy depends principally on
this, that they supply the necessary conditions under which alone the
following higher stage of the ontogeny can be formed. The development,
for instance, begins with the division of the egg, because this is the
only way that a one-celled condition can give rise to a many-celled
form. Again, the organs can be formed only when groups of cells have
made a closer union with one another. Thus the gastrula must begin with
the antecedent blastula, etc. Definite forms are, despite all modifying
influences, held to firmly, because by their presence the complicated
end-stages can be reached in the simplest and most suitable way.
Thus Hertwig adopts here a little from one doctrine and there a little
from another, and between his attempt to reinstate the old biogenetic
law of Haeckel, and to adopt a more modern point of view, he brings
together a rather curious collection of statements which are not any too
well coördinated. Take, for example, his description of the relation
between Haeckel’s gastræa and the embryonic gastrula stage. The latter
he maintains is a repetition of the other, but only in form, not in
actual contents. And in another connection we are told that the cause of
this repetition is that the gastrula is the simplest way in which the
later stages can be reached, and, therefore, it has been retained. It
seems to me that Hertwig has undertaken an unnecessary and impossible
task when he attempts to adjust the old recapitulation theory to more
modern standards. His statement that the egg is entirely different from
its amœba prototype is, of course, only the view generally held by all
embryologists. His mystical statement that the embryonic form _repeats
the ancestral adult stage in its form, but not in its contents_, will
scarcely recommend itself as a model of clear thinking. Can we be asked
to believe for instance that a young chick repeats the ancestral adult
fish form but not the contents of the fish?
In conclusion, then, it seems to me that _the idea that adult ancestral
stages have been pushed back into the embryo, and that the embryo
recapitulates in part these ancestral adult stages is in principle
false_. The resemblance between the embryos of higher forms and the
adults of lower forms is due, as I have tried to show, to the presence
in the embryos of the lower groups of certain organs that remain in the
adult forms of this group. It is only the embryonic stages of the two
groups that we are justified in comparing; and their resemblances are
explained on the assumption that there has been an ancestral adult form
having these embryonic stages in its development and these stages have
been handed down to the divergent lines of its descendants.
Since we have come to associate with the name of the recapitulation
theory the idea of the recurrence of an ancestral adult form, it may be
better to find a substitute for this term. I suggest, therefore, for the
view, that the embryos of the higher group repeat the modified form of
the embryos of the lower groups, the term, the theory of embryonic
repetition, or, more briefly, the repetition theory.
Conclusions
In the light of the preceding discussion concerning the evidence in
favor of the transmutation theory, we may now proceed to sum up our
general conclusions, and at the same time discuss some further
possibilities in regard to the descent theory.
The most widely accepted view in regard to the theory of organic
evolution is that which looks upon the resemblances between the members
of a group as due to their common descent from one original species that
has broken up, as it were, into a number of new forms. Strictly applied,
this means that all the vertebrates have come from one original species,
all the mollusks from another, the echinoderms from a third, etc. Even
farther back there may have been a common ancestral species for any two
of the large groups, as, for example, the annelids and the mollusks; and
if the relationship of all the many-celled forms be looked upon as
probable, then they too have originated from one ancestral species.
Many zoologists appear to hesitate to apply strictly this fundamental
idea contained in the transmutation theory, because, perhaps, they feel
that it does not fit in with their general experience of living forms.
Yet there can be no doubt that it is the primary conception of the
transmutation theory. This is, however, not the whole question, for we
must further consider the number of individuals of a species that are
involved.
In some species there are smaller groups of individuals that are more
like one another than like other individuals of the same species. Such
groups are called varieties, and are often associated with certain
localities, or with a special environment. In the latter case they are
called local varieties. Some of these appear to breed true, not only
when kept under the same conditions, but even when transferred to a new
environment. Others change with the environment. It is not improbable
that the varieties are of a different kind in these two cases, as shown
by their different behavior when put under new and different
surroundings. The variety that owes its peculiarities, not to the
immediate environment, but to some internal condition independent of the
surroundings, is recognized by some biologists as a smaller species.
Such species appear to be commoner in plants than in animals, although
it is possible that this only means that more cases have been found by
the botanists, owing to the greater ease with which plants can be
handled. These smaller species, in contradistinction to the ordinary
Linnæan species, differ from the latter in the smaller amount of
differences between the groups, and probably also in that they freely
interbreed, and leave fertile descendants; but whether this is only on
account of the smaller differences between them than between larger
species, or because of some more fundamental difference in the kind of
variation that gives rise to these two kinds of groups, we do not know.
These smaller species, or constant varieties, as we may call them, may
be looked upon as incipient Linnæan species, which, by further
variations of the same, or of other sorts, may end by giving rise to
true species. A genus composed of several species might be formed in
this way, and then, if each species again broke up into a number of new
groups, each such group would now be recognized as a genus, and the
group of genera would form a family, etc. The process continuing, a
whole class, or order, or even phylum, might be the result of this
process that began in a single species.
But we must look still farther, and inquire whether the start was made
from a single individual, that began to vary, or from a number of
individuals, or even from all the individuals, of a species. If we
suppose the result to depend on some external cause that affects all the
individuals of a species alike, then it might appear that the species,
or at least as many individuals of a species as are affected, will give
the starting-point for the new group. But if the new variation arises
not directly as a response to some change in the surroundings, then it
might appear in one or in a few individuals at a time. Let us consider
what the results might be under these two heads.
If amongst the descendants of a single individual a new form or a number
of new forms were to arise, then, if they represented only a variety,
they would cross with the other forms like the parent species; and,
under these conditions, it is generally assumed that the new variety
would be swamped. If, however, the new forms have the value of new
species, then, _ex hypothese_, they are no longer fertile with the
original forms, and might perpetuate themselves by self-fertilization,
as would be possible in some of the higher plants, and in those animals
that are bisexual. But as a rule even bisexual forms are not
self-fertilized, and, therefore, unless a number of offspring arose from
the same form the chance of propagation would be small.
If, however, a number of new forms appeared at the same time and left a
number of descendants, then the probability that the new group might
perpetuate itself is greater, and the chance that such a group would
arise is in proportion to the number of individuals that varied in the
same direction simultaneously. In this case the new species has not come
from a single individual or even from a pair of individuals, but from a
number of individuals that have varied more or less in the same
direction.
This point of view puts the descent theory in a somewhat unforeseen
light, for we cannot assume in such a case that the similarities of the
members of even the same species are due to direct descent from an
original ancestor, because there are supposed to have been a number of
ancestors that have all changed in the same direction. The question is
further complicated by the fact that the new individuals begin to
interbreed, so that their descendants come to have, after a time, the
common blood, so to speak, of all the new forms. If with each union
there is a blending of the substances of the individuals, there will
result in the end a common substance representing the commingled racial
germ-plasm.
A new starting-point is then reached, and new species may continue to be
formed out of this homogeneous material. Thus, in a sense, we have
reached a position which, although it appears at first quite different
from the ordinary view, yet, after all, gives us the same standpoint as
that assumed by the transmutation theory; for, while the latter assumes
that the resemblances of the members of a group are due to descent from
the same original form, and often by implication from a single
individual, we have here reached the conclusion that it is only a
common, commingled germ-plasm that is the common inheritance.
When we examine almost any group of living animals or plants, whether
they are low or high in organization, we find that it is composed of a
great many different species, and so far as geology gives any answer, we
find that this must have been true in the past also. Why, then, do we
suppose that all the members of the higher groups have come from a
single original species or variety? Why may not all, or many, of the
similar species of the lower group have changed into the species of the
higher group,—species for species? If this happened, the resemblance of
the new species of the group could be accounted for on the supposition
that their ancestors were also like one another. The likeness would not
be due, then, to a common descent, and it would be false to attempt to
explain their likeness as due to a common inheritance. But before going
farther, it may be well to inquire to what the resemblances of the
individuals of the original species were due; for, if they have come
from an older group that has given rise to divergent lines of descent,
then we are only removing the explanation one step farther back. If this
original group has come from numerous species of a still older group,
and this, in turn, from an older one still, then we must go back to the
first forms of life that appeared on the globe, and suppose that the
individuals of these primitive forms are the originals of the species
that we find living to-day. For instance, it is thinkable that each
species of vertebrate arose from a single group of the earliest forms of
life that appeared on the surface of the earth. If this were the case,
there must have been as many different kinds of species of the original
group as there are species alive at the present time, and throughout all
the past. This view finds no support from our knowledge of fossil
remains, and, although it may be admitted that this knowledge is very
incomplete, yet, if the process of evolution had taken place as sketched
out above, we should expect, at least, to have found some traces of it
amongst fossil forms. Since this question is an historical one, we can,
at best, only expect to decide which of all the possible suggestions is
the more probable.
We conclude, then, that it is more probable that the vertebrates, the
mollusks, the insects, the crustaceans, the annelids, the cœlenterates,
and the sponges, etc., have come each from a single original species.
Their resemblances are due to a common inheritance from a common
ancestral species. Even if it be probable that at the time when the
group of vertebrates arose from a single species, there were in
existence other closely related species, yet we must suppose, if we
adhere to our point of view, that these other related species have had
nothing to do with the group of vertebrates, but that they have died
out. Moreover, we must suppose that each order, each class of
vertebrate, has come from a single original species; each family has had
a similar origin, as well as each genus, but, of course, at different
periods of time. Let us not shrink from carrying this principle to its
most extreme point, for, unless the principle is absolutely true, then
our much boasted explanation of the resemblances of forms in the same
group will be thrown into hopeless confusion.
Let us ask another question in this connection. If a single species gave
rise to a group of new species that represented the first vertebrates,
they would have formed the first genus; and if the descendants of these
diverged again so that new genera were formed, then a group which we
should call a family would have been formed.
As the divergence went on, an _order_ would be developed, and then a
_class_, and then a _phylum_. The common characters possessed by the
members of this phylum would have been present in the original species
that began to diverge. Hence, we find the definition of the phylum
containing only those points that are the features possessed by all of
the descendants, and in the same way we should try to construct the
definition of each of the subordinate groups. This is the ideal of the
principle of classification based on the theory of descent with
divergence. If we admit the possibility of the other view that I have
mentioned above, or of any other of the numerous possibilities that will
readily suggest themselves, then we must be prepared to give up some of
the most attractive features of the explanation of resemblance as due to
descent.
That all biologists believe strictly in divergent descent, to the
exclusion of any other processes, is not the case. And, as I have said
before, since we are dealing with an historical question, it would be
very unwise, in our present ignorance on many points, to pretend that we
have any direct proof of the explanation that we find generally given to
account for the resemblances of the species of a group to each other. At
most we can claim that it is the simplest point of view, and that most
biologists believe it to be also the most probable. It has been
suggested that, in some cases, the new forms that arise from two or more
species run a parallel course. If the original forms from which they
came were very much alike, it would soon be impossible to say what the
parentage of a particular form was; that is, to which of the two
original forms it belonged. It has also been suggested that even a
convergence has at times taken place, so that the descendants of
different species have become more alike than the original forms, _at
least in some one or more respects_. This last limitation is the saving
clause, for species differ in so many points that, even when they
converge in a few, it is unlikely that they will do so in all, and,
therefore, the deception may be discovered by the acute observer. One
famous paleontologist has gone so far even as to suppose that a species
may change its generic characters, so that it goes over bodily into a
new genus without losing its specific characters. If such things do
occur, then our classifications may well be the laughing-stock of
Nature.
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CHAPTER IV
DARWIN’S THEORIES OF ARTIFICIAL AND OF NATURAL SELECTION
The Principle of Selection
Darwin’s theory of natural selection is preëminently a theory of
adaptation. It appears, in fact, better suited to explain this
phenomenon than that of the “origin of species.” Darwin prepared his
reader for the ideas contained in the theory of natural selection by a
brief consideration of the results of artificial selection; and since
the key to the situation is, I believe, to be found in just this
supposed resemblance, we cannot do better than examine the theories in
the order followed by Darwin himself.
One of the means by which the artificial races of animals and plants
have been formed by man is selection. The breeder picks out individuals
having a certain peculiarity, and allows them to breed together. He
hopes to find among the offspring, not only individuals like the parent
forms, but also some that have the special peculiarity even more
strongly developed. If such are found, they are isolated and allowed to
breed, and in the next generation it is hoped to find one or more new
individuals that show still more developed the special character that is
sought. This process, repeated through a number of generations, is
supposed to have led to the formation of many of our various forms of
domesticated animals and plants.
This heaping up as a result of the union of similar individuals cannot
for a moment be supposed to be the outcome of the addition of the two
variations to each other. Such an idea is counter to all the most
familiar facts of inheritance. For instance, when two similar forms
unite, we do not find that the young show all the characters of the
mother plus all those of the father, _i.e._ each peculiarity that is the
same in both, increased twofold. On the contrary, the young are in the
vast majority of cases not essentially different from either parent.
A more thorough examination of the facts shows that the problem is by no
means so simple as the preceding general statement might lead one to
suppose, for our experience shows that it is not always possible to
increase all variations by selection, and, furthermore, there is very
soon found a limit, even in favorable cases, to the extent to which the
process can be carried. The most important point appears to be the
nature of the variations themselves which may arise from different
causes, and which have different values in relation to the possibility
of their continuation.
We may begin, therefore, by following Darwin in his analysis of
variation, as given in the opening chapter of the “Origin of Species.”
He thinks that the great amount of variation shown by domesticated
animals and plants is due, in the first place, to the new conditions of
life to which they are exposed, and also to the lack of uniformity of
these conditions. Darwin thinks, also, that there is some probability
that this variability is due, in part, to an excess of food. “It seems
clear that organic beings must be exposed during several generations to
new conditions to cause any great amount of variation, and that when the
organization has once begun to vary, it generally continues varying for
many generations. No case is on record of a variable organism ceasing to
vary under cultivation. Our oldest cultivated plants, such as wheat,
still yield new varieties; our oldest domesticated animals are still
capable of rapid improvement or modification.” In this statement of
Darwin, full of significance, we must be careful to notice that he does
not mean to imply, when he states that an organism that has once begun
to vary continues to vary for many generations, that this continuous
variation is always in the same direction, but only that new
combinations, scattering in all directions, continue to appear.
The nature of the organism seemed to Darwin to be a more important
factor in the origin of new variations than the external conditions,
“for nearly similar variations sometimes arise under, as far as we can
judge, dissimilar conditions; and, on the other hand, dissimilar
variations arise under conditions which appear to be nearly uniform.”
The following statement is important in connection with the origin of
“definite” variations. “Each of the endless variations which we see in
the plumage of our fowls must have had some efficient cause; and if the
same causes were to act uniformly during a long series of generations on
many individuals, all probably would be modified in the same direction.”
Here we find an explicit statement in regard to the accumulation of
variation in a given direction as the result of an external agent, but
Darwin hastens to add: “Indefinite variability is a much more common
result of changed conditions than definite variability, and has probably
played a more important part in the formation of our domestic races. We
see indefinite variability in the endless slight peculiarities which
distinguish the individuals of the same species, and which cannot be
accounted for by inheritance from either parent or from some more remote
ancestor. Even strongly marked differences occasionally appear in the
young of the same litter, and in seedlings from the same seed capsule.
At long intervals of time, out of millions of individuals reared in the
same country and fed on nearly the same food, deviations of structure so
strongly pronounced as to deserve to be called monstrosities arise; but
monstrosities cannot be separated by any distinct line from slighter
variations.”
Another cause of variation, Darwin believes, is in the inherited effect
of “habit and of the use and disuse of parts,” or what is generally
known as the Lamarckian factor of heredity. Darwin believes that changes
in the body of the parent, that are the result of the use or of the
disuse of a part, may be transmitted to the descendants, and cites a
number of cases which he credits to this process. As we shall deal more
fully with this topic in another chapter, we may treat it here quite
briefly. As an example of the inheritance of disuse, Darwin gives the
following case: “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 this change may be
safely attributed to the domestic duck flying much less and walking more
than its wild parents.” The great and inherited development of the
udders of cows and of goats in countries where they are habitually
milked, in comparison with these organs in other countries, is given as
another instance of the effect of use. “Not one of our domestic animals
can be named that in some country has not drooping ears, and the view
has been suggested that the drooping is due to the disuse of the muscles
of the ears from the animals being seldom much alarmed.”
It need scarcely be pointed out here, that, in the first case given,
those ducks would have been most likely to remain in confinement that
had less well-developed wings, and hence at the start artificial
selection may have served to bring about the result. The great
development of the udders of cows and of goats is obviously connected
with the greater milk-giving qualities of these animals, which may have
been selected for this purpose.
Another “law” of variation recognized by Darwin is what is called
correlated variation. For example, it has been found that cats which are
entirely white and have blue eyes are generally deaf, and this is stated
to be confined to the males. The teeth of hairless dogs are imperfect;
pigeons with feathered feet have skin between the outer toes, and those
with short beaks have small feet, and _vice versa_.
Another source of variation is that of reversion, or the reappearance in
the offspring of characters once possessed by the ancestors. Finally,
Darwin thinks that a source of variation is to be found in modifications
due to the influence of a previous union with another male, or, as it is
generally called, telegony. As an example Darwin cites the famous case
of Lord Morton’s mare. “A nearly purely bred Arabian chestnut mare bore
a hybrid to a quagga. She subsequently produced two colts by a black
Arabian horse. These colts were partially dun-colored and were striped
on the legs more plainly than the real hybrid or even than the
quagga.”[13] This case, however, is not above suspicion, since it is
well known that stripes often appear on young horses, and the careful
analysis made later by Ewart, as well as his other experiments on the
possibility of the transmission of influences of this sort, puts the
whole matter in a very dubious light.
These citations show that Darwin recognized quite a number of sources of
variation, and, although he freely admits that “our ignorance of the
laws of variation is profound,” yet some at least of these sources of
variation are very questionable. Be this as it may, it is important to
emphasize that Darwin recognized two main sources of variation,—one of
which is the indefinite, or fluctuating, variability that appears
constantly in domesticated animals and plants, and the other, definite
variability, or a change in a definite direction, that can often be
traced to the direct action of the environment on the parent or on its
reproductive cells. It is the former, _i.e._ the fluctuating
variability, that, according to Darwin, has been used by the breeder to
produce most of our domestic races. In regard to the other source of
variation, the definite kind, we must analyze the facts more closely.
Footnote 13:
“Animals and Plants under Domestication,” Chap. IX.
A definite change in the surroundings might bring about a definite
change in the next generation, because the new condition acts either on
the developing organism, or on the egg itself from which the individual
develops. The distinction may be one of importance, for, if the new
condition only effects the developing organism directly, then, when the
influence is removed, there should be a return to the former condition;
but if the egg itself is affected, so that it is fundamentally changed,
then the effect might persist even if the animal were returned to its
former environment. More important still is Darwin’s recognition of the
cumulative effect in a given direction of external influences, for a new
variation, that was slight at first, might, through prolonged action,
continue to become more developed without any other processes affecting
the organism.
From the Darwinian point of view, however, the all-important source for
the origin of new forms is the fluctuating variation, which is made use
of both in the process of artificial and of natural selection. We may
now proceed to inquire how this is supposed to take place.
It has been stated that, by means of artificial selection, Darwin
believes the breeder has produced the greater number of domesticated
animals and plants. The most important question is what sort of
variations he has made use of in order to produce his result. Has he
made use of the fluctuating variations, or of the definite ones? It is
difficult, if not impossible, to answer this question in most cases,
because the breeder does not always distinguish between the two. There
can be little question, however, that he may sometimes have made use of
the definite kinds, whether these are the outcome of external or of
internal influences. The question has been seriously raised only in
recent years, and we are still uncertain how far we can accumulate and
fix a variation that is of the fluctuating kind. In a few cases it has
been found that the upper limit is soon reached, as shown by De Vries’s
experiments with clover, and it is always possible that a definite
variation of the right sort may arise at any stage of the process. If
this should occur, then a new standard is introduced from which, as from
a new base, variations fluctuating in the desired direction may be
selected.
This question, before all others, ought to be settled before we begin to
speculate further as to what selection is able to accomplish.
Darwin’s theory is often stated in such a general way that it would be
applicable to either sort of variation; but if definite variation can go
on accumulating without selection, then possibly we could account for
evolution without supposing any other process to intervene. Under these
circumstances all that could be claimed for selection would be the
destruction of those variations incapable of living, or of competing
with other forms. Hence the process of selection would have an entirely
negative value.
The way in which domesticated animals and plants have originated is
explained by Darwin in the following significant passage:—
“Let us now briefly consider the steps by which domestic races have been
produced, either from one or from several allied species. Some effect
may be attributed to the direct and definite action of the external
conditions of life, and some to habit; but he would be a bold man who
would account by such agencies for the differences between a dray- and
race-horse, a greyhound and bloodhound, a carrier and tumbler pigeon.
One of the most remarkable features in our domesticated races is that we
see in them adaptation, not indeed to the animal’s or plant’s own good,
but to man’s use or fancy. Some variations useful to him have probably
arisen suddenly, or by one step; many botanists, for instance, believe
that the fuller’s-teasel, with its hooks, which cannot be rivalled by
any mechanical contrivance, is only a variety of the wild Dipsacus; and
this amount of change may have suddenly arisen in a seedling. So it has
probably been with the turnspit dog; and this is known to have been the
case with the ancon sheep. But when we compare the dray-horse and
race-horse, the dromedary and camel, the various breeds of sheep fitted
either for cultivated land or mountain pasture, with the wool of one
breed good for one purpose, and that of another breed for another
purpose; when we compare the many breeds of dogs, each good for man in
different ways; when we compare the game-cock, so pertinacious in
battle, with other breeds so little quarrelsome, with ‘everlasting
layers’ which never desire to sit, and with the bantam so small and
elegant; when we compare the host of agricultural, culinary, orchard,
and flower-garden races of plants, most useful to man at different
seasons and for different purposes, or so beautiful in his eyes, we
must, I think, look further than to mere variability. We cannot suppose
that all the breeds were suddenly produced as perfect and as useful as
we now see them; indeed, in many cases, we know that this has not been
their history. The key is man’s power of accumulative selection: nature
gives successive variations; man adds them up in certain directions
useful to him. In this sense he may be said to have made for himself
useful breeds.”
Darwin also gives the following striking examples, which make probable
the view that domestic forms have really been made by man selecting
those variations that are useful to him:—
“In regard to plants, there is another means of observing the
accumulated effects of selection—namely, by comparing the diversity of
flowers in the different varieties of the same species in the
flower-garden; the diversity of leaves, pods, or tubers, or whatever
part is valued, in the kitchen-garden, in comparison with the flowers of
the same varieties; and the diversity of fruit of the same species in
the orchard, in comparison with the leaves and flowers of the same set
of varieties. See how different the leaves of the cabbage are, and how
extremely alike the flowers; how unlike the flowers of the heartsease
are, and how alike the leaves; how much the fruit of the different kinds
of gooseberries differ in size, color, shape, and hairiness, and yet the
flowers present very slight differences. It is not that the varieties
which differ largely in some one point do not differ at all in other
points; this is hardly ever,—I speak after careful observation,—perhaps
never, the case. The law of correlated variation, the importance of
which should never be overlooked, will insure some differences; but, as
a general rule, it cannot be doubted that the continued selection of
slight variations, either in the leaves, the flowers, or the fruit, will
produce races differing from each other chiefly in these characters.”
Exception may perhaps be taken to the concluding sentence, for,
interesting as the facts here recorded certainly are, it does not
necessarily follow that all domestic products have arisen “by the
continued selection of slight variations,” however probable the
conclusion may appear. Darwin also believes that a process of
“unconscious selection” has given even more important “results than
methodical selection.” By unconscious selection is meant the outcome of
“every one trying to possess and breed from best individual animals.”
“Thus a man who intends keeping pointers naturally tries to get as good
dogs as he can, and afterwards breeds from his own best dogs, but he has
no wish, or expectation of permanently altering the breed. Nevertheless
we may infer that this process, continued during centuries, would
improve and modify any breed.... There is reason to believe that the
King Charles spaniel has been unconsciously modified to a large extent
since the time of that monarch.”
The enormous length of time required to produce new species by the
selection of fluctuating variations is everywhere admitted by Darwin;
nowhere perhaps more strikingly than in the following statement: “If it
has taken centuries or thousands of years to improve or modify most of
our plants up to their present standard of usefulness to man, we can
understand how it is that neither Australia, the Cape of Good Hope, nor
any other region inhabited by quite uncivilized man has afforded us a
single plant worth culture. It is not that these countries, so rich in
species, do not by a strange chance possess the aboriginal stocks of any
useful plants, but that the native plants have not been improved by
continued selection up to a standard of perfection comparable with that
acquired by the plants in countries anciently civilized.”
In reply to this, it may be said that if the selection of fluctuating
variations leads to an accumulation in the given direction, it is not
apparent why it should take thousands of years to produce a new race, or
require such a high degree of skill as Darwin supposes the breeder to
possess.
The conditions favorable to artificial selection are, according to
Darwin: 1. The possession of a large number of individuals, for in this
way the chance of the desired variation appearing is increased. 2.
Prevention of intercrossing, such as results when the land is enclosed,
so that new forms may be kept apart. 3. Changed conditions, as
introducing variability. 4. The intercrossing of aboriginally distinct
species. 5. The intercrossing of new breeds, “but the importance of
intercrossing has been much exaggerated.” 6. In plants propagation of
bud variations by means of cuttings. The chapter concludes with the
statement, “Over all these causes of Change, the accumulative action of
Selection, whether applied methodically and quickly, or unconsciously
and slowly, but more efficiently, seems to have been the predominant
Power.”
Variability, Darwin says, is governed by many unknown laws, and the
final result is “infinitely complex.” If this is so, we may at least
hesitate before we accept the statement that selection of fluctuating
variations has been the only principle that has brought about these
results. This is a most important point, for, as we shall see, the
central question in the theory of natural selection has come to be
whether by the accumulation of fluctuating variations a new species
could ever be produced. If it be admitted that the evidence from
artificial selection is far from convincing, in showing that selection
of fluctuating variations could have been the main source, even in the
formation of new races, we need not be prejudiced in favor of such a
process, when we come to examine the formation of species in nature.
There are still other questions raised in this same chapter that demand
serious consideration. Darwin writes as follows:—
“When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with closely allied species, we
generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races often have
a somewhat monstrous character; by which I mean, that, although
differing from each other, and from other species of the same genus, in
several trifling respects, they often differ in an extreme degree in
some one part, both when compared one with another, and more especially
when compared with the species under nature to which they are nearest
allied. With these exceptions (and with that of the perfect fertility of
varieties when crossed,—a subject hereafter to be discussed), domestic
races of the same species differ from each other in the same manner as
do the closely allied species of the same genus in a state of nature,
but the differences in most cases are less in degree. This must be
admitted as true, for the domestic races of many animals and plants have
been ranked by some competent judges as the descendants of aboriginally
distinct species, and by other competent judges as mere varieties. If
any well-marked distinction existed between a domestic race and a
species, this source of doubt would not so perpetually recur.”
The point here raised in regard to the systematic value of the new forms
is the question that first demands our attention. We must exclude all
those cases in which several original species have been blended to make
a new form, because the results are too complicated to make use of at
present. The domesticated races of dogs appear to have had such a
multiple origin, the origin of horses is in doubt; but the domesticated
pigeons, ducks, rabbits, and fowls are supposed, by Darwin, to have come
each from one original wild species. The great variety of the domestic
pigeons gives perhaps the most striking illustration of changes that
have taken place under domestication; and Darwin lays great stress on
the evidence from this source.
It seems probable in this case, (1) that all the different races of
pigeons have come from one original species; (2) that the structural
differences are in some respects as great as those recognized by
systematists as specifically distinct; (3) that the different races
breed true to their kind; (4) that the result has been reached mainly by
selecting and isolating variations that have appeared under
domestication, and that probably some, at least, of these variations
were fluctuating ones.
Does not this grant all that Darwin contends for? In one sense, yes; in
another, no! The results appear to show that by artificial selection of
some kind a group of new forms may be produced that in many respects
resemble a natural family, or a genus; but if this is to be interpreted
to mean that the result is the same as that by which natural groups have
arisen, then I think that there are good reasons for dissenting from
such a conclusion. Moreover, we must not grant too readily that the
different races of pigeons have arisen by the selection of _fluctuating
variations_ alone, for this is not established with any great degree of
probability by the evidence.
In regard to the first point we find that one of the most striking
differences between species in nature is their infertility, and the
infertility of their offspring when intercrossed. This is a very general
rule, so far as we know. In regard to the different races of
domesticated forms, the most significant fact is that, no matter how
different they may be, they are perfectly fertile _inter se_. In this
respect, as well as in others, there are important differences between
domesticated races and wild species. The further difference, that has
been pointed out by a number of writers, should also not pass unnoticed,
namely, that the domestic forms differ from each other in the extreme
development of some one character, and not in a large number of less
conspicuous characters, as is the case in wild species.
These considerations show that, interesting and suggestive as are the
facts of artificial selection, they fail to demonstrate the main point
for which they are used by Darwin. With the most rigorous attention to
the process of artificial selection, new species comparable in all
respects to wild ones have not been formed, even in those cases in which
the variation has been carried farthest (where the history of the forms
is most completely known).
There is another point on which emphasis should be laid. If by selecting
the most extreme forms in each generation and breeding from them the
standard can be raised, it might appear that we could go on indefinitely
in the same direction, and produce, for instance, pigeons with legs five
metres long, and with necks of corresponding length. But experience has
shown that this cannot be done. As Darwin frequently remarks, the
breeder is entirely helpless until the desired variation appears. It
seems possible, by selecting the more extreme of the fluctuating
variations in each generation, that a higher plane of variation is
established, and even that more extreme forms are likely to arise for a
few generations; but, even if this is the case, a limit is soon reached
beyond which it is impossible to go.
The facts of observation show, that when a new variety appears its
descendants are more likely, on the average, to produce proportionately
more individuals that show the same variation, and some even that may go
still farther in the same direction. If these latter are chosen to be
the parents of the next generation, then once more the offspring may
show the same advance; but little by little the advance slows down,
until before very long it may cease altogether. Unless, then, a new kind
of variation appears, or a new standard of variation develops of a
different kind, the result of selection of fluctuating variations has
reached its limit. Our experience seems, therefore, to teach us that
selection of fluctuating variations leads us to only a certain point,
and then stops in this direction. We get no evidence from the facts in
favor of the view that the process, if carried on for a long time, could
ever produce such great changes, or the kind of changes, as those seen
in wild animals and plants.
Variation and Competition in Nature
Darwin rests his theory on the small individual variations which occur
in nature, as the following quotation shows:—
“It may be doubted whether sudden and considerable deviations of
structure such as we occasionally see in our domestic productions, more
especially with plants, are ever permanently propagated in a state of
nature. Almost every part of every organic being is so beautifully
related to its complex conditions of life that it seems as improbable
that any part should have been suddenly produced perfect, as that a
complex machine should have been invented by man in a perfect state.
Under domestication monstrosities sometimes occur which resemble normal
structures in widely different animals. Thus pigs have occasionally been
born with a sort of proboscis, and if any wild species of the same genus
had naturally possessed a proboscis, it might have been argued that this
had appeared as a monstrosity; but I have as yet failed to find, after
diligent search, cases of monstrosities resembling normal structures in
nearly allied forms, and these alone bear on the question. If monstrous
forms of this kind ever do appear in a state of nature and are capable
of reproduction (which is not always the case), as they occur rarely and
singly, their preservation would depend on unusually favorable
circumstances. They would, also, during the first and succeeding
generations cross with the ordinary form, and thus their abnormal
character would almost inevitably be lost.”
It is clear that Darwin does not think that the sudden and large
variations that sometimes occur furnish the basis for natural selection,
and the final statement in the last citation (which was added in later
editions of the “Origin of Species”), to the effect that if such
monstrous variations appeared as single or occasional variations they
would be lost by intercrossing implies that, in general, single
variations would likewise be lost unless they appeared in a sufficient
number of individuals to maintain themselves against the swamping
effects of intercrossing.
It is necessary to quote again, in order to show that, in some cases at
least, Darwin believed selection plays little or no part in the origin
and maintenance of certain peculiarities that are of no use to the
species. “There is one point connected with individual differences,
which is extremely perplexing: I refer to those genera which have been
called protean or ‘polymorphic,’ in which the species present an
inordinate amount of variation. With respect to many of these forms,
hardly two naturalists agree, whether to rank them as species or as
varieties. We may instance Rubus, Rosa, and Hieracium amongst plants,
several genera of insects and of Brachiopod shells. In most polymorphic
genera some of the species have fixed and definite characters. Genera
which are polymorphic in one country seem to be, with a few exceptions,
polymorphic in other countries, and likewise, judging from Brachiopod
shells, at former periods of time. These facts are very perplexing, for
they seem to show that this kind of variability is independent of the
conditions of life. I am inclined to suspect that we see, at least in
some of these polymorphic genera, variations which are of no service or
disservice to the species, and which consequently have not been seized
on by selection to act on and accumulate, in the same manner as man
accumulates in any given direction individual differences in his
domesticated productions. These individual differences generally affect
what naturalists consider unimportant parts; but I could show by a long
catalogue of facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of view, sometimes
vary in the individuals of the same species. I am convinced that the
most experienced naturalist would be surprised at the number of cases of
variability, even in important parts of structure, which he could
collect on good authority, as I have collected, during a course of
years.”
After pointing out that naturalists have no definite standard to
determine whether a group of individuals is a variety or a species,
Darwin makes the highly important admissions contained in the following
paragraph: “Hence, I look at individual differences, though of small
interest to the systematist, as of the highest importance for us, as
being the first steps toward such slight varieties as are barely thought
worth recording in works on natural history. And I look at varieties
which are in any degree more distinct and permanent, as steps toward
more strongly marked and permanent varieties; and at the latter, as
leading to subspecies, and then to species. The passage from one stage
of difference to another may, in many cases, be the simple result of the
nature of the organism and of the different physical conditions to which
it has long been exposed; but with respect to the more important and
adaptive characters, the passage from one stage of difference to another
may be safely attributed to the cumulative action of natural selection,
hereafter to be explained, and to the effects of the increased use or
disuse of parts. A well-marked variety may therefore be called an
incipient species; but whether this belief is justifiable must be judged
by the weight of the various facts and considerations to be given
throughout this work.”
In this paragraph attention should be called especially, first, to the
statement in respect to the origin of varieties, which are said to arise
through individual differences. It is not clear whether these
differences are supposed to have appeared first in one, or in a few
individuals, or in large numbers at the same time. Again, especial note
should be made of the striking admission, that the passage from one
stage to another may, in many cases, be the simple result of the nature
of the organism and of the physical conditions surrounding it; but with
respect to the more important and adaptive differences, natural
selection “may safely” be supposed to have intervened. Is it to be
wondered at that Darwin’s critics have sometimes accused him of playing
fast and loose with the origin of varieties? And since this question is
fundamental for the theory of natural selection, it is much to be
regretted that Darwin leaves the matter in such a hazy condition. It may
be said that, at the time when he wrote, he made the best of the
evidence in regard to the origin of varieties. Be this as it may, a
theory standing on no better foundations than this is not likely to be
found satisfactory at the present time.
We come now to the most important chapters, the third and the fourth, of
the “Origin of Species,” dealing with “the struggle for existence,”
“natural selection,” or the “survival of the fittest.” Behind these
fatal phrases, which have become almost household words, lurk many
dangers for the unwary.
“It has been seen in the last chapter that amongst organic beings in a
state of nature there is some individual variability: indeed I am not
aware that this has ever been disputed. It is immaterial for us whether
a multitude of doubtful forms be called species or subspecies or
varieties; what rank, for instance, the two or three hundred doubtful
forms of British plants are entitled to hold, if the existence of any
well-marked varieties be admitted. But the mere existence of individual
variability and of some few well-marked varieties, though necessary as
the foundation for the work, helps us but little in understanding how
species arise in nature. How have all those exquisite adaptions of one
part of the organization to another part, and to the conditions of life,
and of one organic being to another being, been perfected? We see these
beautiful coadaptions most plainly in the woodpecker and the mistletoe;
and only a little less plainly in the humblest parasite which clings to
the hairs of a quadruped or feathers of a bird; in the structure of the
beetle which dives through the water; in the plumed seed which is wafted
by the gentlest breeze; in short, we see beautiful adaptions everywhere
and in every part of the organic world.
“Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next
chapter, follow from the struggle for life. Owing to this struggle,
variations, however slight and from whatever cause proceeding, if they
be in any degree profitable to the individuals of a species, in their
infinitely complex relations to other organic beings and to their
physical conditions of life, will tend to the preservation of such
individuals, and will generally be inherited by the offspring. The
offspring, also, will thus have a better chance of surviving, for, of
the many individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by which each
slight variation, if useful, is preserved, by the term Natural
Selection, in order to mark its relation to man’s power of selection.
But the expression often used by Mr. Herbert Spencer of the Survival of
the Fittest is more accurate, and is sometimes equally convenient. We
have seen that man by selection can certainly produce great results, and
can adapt organic beings to his own uses, through the accumulation of
slight but useful variations, given to him by the hand of Nature. But
Natural Selection, as we shall hereafter see, is a power incessantly
ready for action, and is as immeasurably superior to man’s feeble
efforts, as the works of Nature are to those of Art.”
Darwin gives the following explicit statement of the way in which he
intends the term “struggle for existence” to be understood: “I should
premise that I use this term in a large and metaphorical sense,
including dependence of one being on another, and including (which is
more important) not only the life of the individual, but success in
leaving progeny. Two canine animals, in time of dearth, may be truly
said to struggle with each other which shall get food and live. But a
plant on the edge of a desert is said to struggle for life against the
drought, though more properly it should be said to be dependent on the
moisture. A plant which actually produces a thousand seeds of which only
one on an average comes to maturity may be more truly said to struggle
with the plants of the same and other kinds which already clothe the
ground. The mistletoe is dependent on the apple, and a few other trees,
but can only in a far-fetched sense be said to struggle with these
trees, for if too many of these parasites grow on the same tree, it
languishes and dies. But several seedling mistletoes, growing close
together on the same branch, may more truly be said to struggle with
each other. As the mistletoe is disseminated by birds, its existence
depends on them, and it may metaphorically be said to struggle with
other fruit-bearing plants, in tempting the birds to devour and thus
disseminate its seeds. In these several senses, which pass into each
other, I use for convenience’ sake the general term ‘Struggle for
Existence.’”
A number of writers have objected to the general and often vague way in
which Darwin makes use of this phrase; but it does not seem to me that
this is a serious objection, provided we are on our guard as to what the
outcome will be in each case. In each instance we must consider the
question on its own merits, and if it is found convenient to have a
sufficiently general and non-committal term, such as the “struggle for
existence,” to include all cases, I see no serious objection to the use
of such an expression, although it is true the outcome has been that it
has become a catchword, that is used too often by those who have no
knowledge of its contents.
Were it not that each animal and plant gives birth, on an average, to
more than two offspring, the species would soon become exterminated by
accidents, etc. We find in some of the lower animals, and in some of the
higher plants, that thousands and even millions of eggs are produced by
a single individual in the course of its life. A single nematode may lay
sixty million eggs, and a tapeworm one thousand million. A starfish may
produce about thirty-nine million eggs, a salmon may contain fifteen
thousand, and a large shad as many as one hundred thousand. The queen of
a termite nest is said to lay eighty thousand eggs a day.
In the higher vertebrates the number of young is considerably less, but
since the young stages are passed within the body of the parent,
proportionately more of them reach maturity, so that even in man the
population may be doubled in twenty-five years, and in the elephant,
slowest breeder of all animals, Darwin has calculated that, if it begins
breeding when about thirty years old and goes on until ninety years,
bringing forth six young in the interval, after 750 years there will be
nearly nineteen million elephants alive which have descended from the
first pair.
Obviously, then, if all the descendants of all the individuals of a
species were to remain alive, the world would be over-crowded in a very
short time, and the want of room would in itself lead to the destruction
of countless individuals, if for no other reason than lack of food. We
can easily carry out on a small scale an experiment that shows how the
overstocking, resulting from favorable conditions, comes about, and how
it checks itself. If we make a meat broth suitable for the life of a
particular bacterium, and sow in the broth a very few individuals, we
find in the course of several days the fluid swarming with the
descendants of the original individuals. Thus it has been shown that, if
we start with a few hundred bacteria, there will be five thousand after
twenty-four hours, and twenty thousand, forty-eight hours later; and
after four days they are beyond calculation.
Cohn found that a single bacterium produces two individuals in one hour,
and four in two hours, and if they continue to multiply at this rate
there will be produced at the end of three days 4,772 billions of
descendants. If these are reduced to weight, they would weigh
seventy-five hundred tons. Thus when the conditions are favorable,
bacteria are able to increase at such an enormous rate that they could
cover the surface of the earth in a very few days. The reason that they
do not go on increasing at this rate is that they soon exhaust the food
supply, and the rate of increase slows down, and will finally cease
altogether. If the bacteria were dependent on a continuous supply of
food, they would perish after the supply had been exhausted, so that the
rapid rate of multiplication would serve only to bring the career of the
organism to an untimely end. If the weaker individuals were to die
first, the products of their disintegration might serve to nourish the
stronger individuals; hunger coming on again, the next weakest might
die; and the same process continuing, we might imagine that the bacteria
were finally reduced to a single one which would then die in turn for
lack of food. Like a starving shipload of men, reduced by hunger to
cannibalism, the life of some and finally of the last individual might
be prolonged in the hope of rescue, but if this did not arrive, the last
and perhaps the strongest individual would perish. But this is not what
we find occurring in these lower organisms, for, as a rule, they
gradually cease to increase when the food supply becomes lessened, and
their activities slow down. Finally, when the food is gone, they pass
into a resting stage, in which condition they can remain dormant for a
long time, even for years. If they should again find themselves in
favorable surroundings, they become active, and begin once more their
round of multiplication. We cannot follow the individuals in such a
culture of bacteria, but there is nothing to be seen that suggests a
struggle for existence, if this idea conveys the impression of the
destruction of certain individuals by competition with others. In fact,
the results are in some respects exactly the reverse. Millions of
individuals are present at the time when the food supply becomes
exhausted, and they all pass into a protected resting stage.
The enormous rate of increase in this case finds its counterpart in
higher animals when the food supply, or the absence of enemies, allows a
species to multiply at its maximum rate of increase. The introduction of
rabbits into Australia was followed by an enormous increase in a few
years, and the introduction of the English sparrow into the United
States has had a similar result. But in no country can such a process
continue beyond a certain point, because, in the first place, the
scarcity of food will begin to keep the birth-rate down, and in the
second place, the increase in numbers may lead to an increase in the
number of its enemies, or even induce other forms to feed on it.
Crowding will also give an opportunity for the spread of disease, which
again may check the increase. Sooner or later a sort of ever shifting
balance will be reached for each species, and after this, if the
conditions remain the same, the number of individuals will keep
approximately constant.
Darwin admits that the “causes which check the natural tendency of each
species to increase are most obscure.” “We know not exactly what the
checks are even in a single instance.” This admission may well put us on
our guard against a too ready acceptation of a theory in which the whole
issue turns on just this very point, namely, the nature of the checks to
increase. Darwin gives the following general cases to show what some of
the checks to increase are. He states that eggs and very young animals
and seeds suffer more than the adults; that “the amount of food for each
species of course gives the extreme limit to which each can increase;
but very frequently it is not the obtaining food, but the serving as
prey to other animals which determines the average numbers of a species.
Thus, there seems to be little doubt that the stock of partridges,
grouse, and hares on any large estate depends largely on the destruction
of the vermin.” “On the other hand, in some cases, as with the elephant,
none are destroyed by beasts of prey; for even the tiger in India most
rarely dares to attack a young elephant protected by its dam.” “Climate
plays an important part in determining the average number of a species,
and periodical seasons of extreme cold or drought seem to be the most
effective of all checks.” “The action of climate seems at first sight to
be quite independent of the struggle for existence; but in so far as
climate acts in reducing food, it brings on the most severe struggle
between the individuals, whether of the same, or of distinct species
which subsist on the same kind of food.”
We need not follow Darwin through his account of how complex are the
relations of all animals and plants to one another in the struggle for
existence, for, if true, it only goes to show more plainly how
impossible it is to establish any safe scientific hypothesis, where the
conditions are so complex and so impossible to estimate. To show that
the young Scotch fir in an enclosed pasture is kept down by the browsing
of the cattle, and in other parts of the world, Paraguay for instance,
the number of cattle is determined by insects, and that the increase of
these flies is _probably_ habitually checked by other insects, leads to
a bewilderingly complex set of conditions. We cannot do better than to
quote Darwin’s conclusion: “Hence, if certain insectivorous birds were
to decrease in Paraguay, the parasitic insects would probably increase;
and this would lessen the number of the navel-frequenting flies—then
cattle and horses would become feral, and this would certainly greatly
alter (as indeed I have observed in parts of South America) the
vegetation: this again would largely affect the insects; and this, as we
have just seen in Staffordshire, the insectivorous birds, and so onwards
in ever increasing circles of complexity. Not that under nature the
relations will ever be as simple as this. Battle within battle must be
continually recurring with varying success; and yet in the long run the
forces are so nicely balanced, that the face of nature remains for long
periods of time uniform, though assuredly the merest trifle would give
the victory to one organic being over another. Nevertheless, so profound
is our ignorance, and so high our presumption, that we marvel when we
hear of the extinction of an organic being; and as we do not see the
cause, we invoke cataclysms to desolate the world, or invent laws on the
duration of the forms of life!”
The effect of the struggle for existence in determining _the
distribution of species_ is well illustrated in the following cases:—
“As the species of the same genus usually have, though by no means
invariably, much similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between them, if
they come into competition with each other, than between the species of
distinct genera. We see this in the recent extension over parts of the
United States of one species of swallow having caused the decrease of
another species. The recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush. How frequently we
hear of one species of rat taking the place of another species under the
most different climates! In Russia the small Asiatic cockroach has
everywhere driven before it its great congener. In Australia the
imported hive-bee is rapidly exterminating the small, stingless native
bee. One species of charlock has been known to supplant another species;
and so in other cases. We can dimly see why the competition should be
most severe between allied forms, which fill nearly the same place in
the economy of nature; but probably in no one case could we precisely
say why one species has been victorious over another in the great battle
of life.”
All this goes to show, if it really shows anything at all, that the
distribution of a species is determined, in part, by its relation to
other animals and plants—a truism that is recognized by every
naturalist. The statement has no necessary bearing on the origin of new
species through competition, as the incautious reader might infer. Not
that I mean in any way to imply that Darwin intended to produce this
effect on the reader; but Darwin is not always careful to discriminate
as to the full bearing of the interesting illustrations with which his
book so richly abounds.
At the end of his treatment of the subject, Darwin emphasizes once more
how little we know about the subject of the struggle for existence.
“It is good thus to try in imagination to give to any one species an
advantage over another. Probably in no single instance should we know
what to do. This ought to convince us of our ignorance on the mutual
relations of all organic beings; a conviction as necessary, as it is
difficult, to acquire. All that we can do, is to keep steadily in mind
that each organic being is striving to increase in a geometrical ratio;
that each at some period of its life, during some season of the year,
during each generation or at intervals, has to struggle for life and to
suffer great destruction. When we reflect on this struggle, we may
console ourselves with the full belief, that the war of nature is not
incessant, that no fear is felt, that death is generally prompt, and
that the vigorous, the healthy, and the happy survive and multiply.”
The kindliness of heart that prompted the concluding sentence may arouse
our admiration for the humanity of the writer, but need not, therefore,
dull our criticism of his theory. For whether no fear is felt, and
whether death is prompt or slow, has no bearing on the question at
issue—except as it prepares the gentle reader to accept the dreadful
calamity of nature, pictured in this battle for existence, and make more
contented with their lot “the vigorous, the healthy, and the happy.”
The Theory of Natural Selection
We have already anticipated, to some extent, Darwin’s conclusion in
regard to the outcome of the competition of animals and plants. This
result is supposed to lead to the survival of the fittest. The
competition is carried out by nature, who is personified as selecting
those forms for further experiments that have won in the struggle for
existence.
“Can the principle of selection, which we have seen is so potent in the
hands of man, apply under Nature? I think we shall see that it can act
most efficiently. Let the endless number of slight variations and
individual differences occurring in our domestic productions, and, in a
lesser degree, in those under Nature, be borne in mind; as well as the
strength of the hereditary tendency. Can it, then, be thought
improbable, seeing that variations useful to man have undoubtedly
occurred, that other variations useful in some way to each being in the
great and complex battle for life, should occur in the course of many
successive generations? If such do occur can we doubt (remembering how
many more individuals are born than can possibly survive) that
individuals having any advantage, however slight, over others, would
have the best chance of surviving and of procreating their kind? On the
other hand, we may feel sure that any variation in the least degree
injurious would be rigidly destroyed.”
The process of natural selection is defined as follows, “The
preservation of favorable individual differences and variations and the
destruction of those that are injurious I have called Natural Selection
or the Survival of the Fittest.” And immediately there follows the
significant statement, that, “Variations neither useful nor injurious
would not be affected by natural selection, and would be left either a
fluctuating element, as perhaps we see in certain polymorphic species,
or would ultimately become fixed, owing to the nature of the organism
and the nature of the conditions.” It will be seen from this quotation,
as well as from others already given, that Darwin leaves many structures
outside of the pale of natural selection, and uses his theory to explain
only those cases that are of sufficient use to be decisive in the life
and death struggle of the individuals with each other and with the
surrounding conditions.
Darwin states that we can best understand “the probable course of
natural selection by taking the case of a country undergoing some slight
physical change, for instance, of climate. The proportional numbers of
its inhabitants will almost immediately undergo a change, and some
species will probably become extinct. We may conclude, from what we have
seen of the intimate and complex manner in which the inhabitants of each
country are bound together, that any change in the numerical proportions
of the inhabitants, independency of the change of climate itself, would
seriously affect the others.... In such cases, slight modifications,
which in any way favored the individuals of any species, by better
adapting them to their altered conditions, would tend to be preserved;
and natural selection would have free scope for the work of
improvement.”
The first half of the first of these two quotations seems so plausible,
that without further thought we may be tempted to give a ready assent to
the second, yet the whole issue is contained in this statement. In the
abstract, it undoubtedly appears true that any slightly useful
modification might tend to be preserved. Whether it will, in reality, be
preserved must depend on many things that should be taken into account.
This question will come up later for further consideration; but it
should be pointed out here, that, even assuming that one or more
individuals happen to possess a favorable variation, it by no means
follows that natural selection would have free scope for the work of
improvement, because the question of the inheritance of this variation,
and of its accumulation and building up through successive generations,
must be determined before we can be expected to give assent to this
argument, that appears so attractive when stated in an abstract and
vague way.
Darwin again makes the statement that under the term _variation_ it must
never be forgotten that mere individual differences are meant. “As a man
can produce a great result with his domestic animals and plants by
adding up in any given direction individual differences, so could
natural selection, but far more easily from having incomparably longer
time for action.” Too much emphasis cannot be laid on the fact that
Darwin believed that selection takes place amongst the small individual
differences that we find in animals and plants. Some of his followers,
as we shall see, are apt to put into the background this fundamental
conception of Darwin’s view. His constant comparison between the results
of artificial and natural selection leaves no room for doubt as to his
meaning. Darwin himself seems, at times, not unconscious of the weakness
of this comparison. He says: “How fleeting are the wishes and efforts of
man! how short his time! and consequently how poor will be his results,
compared with those accumulated by Nature during whole geological
periods. Can we wonder then that Nature’s productions should be far
‘truer’ in character than man’s productions; that they should be
infinitely better adapted to the most complex conditions of life, and
should plainly bear the stamp of far higher workmanship?” We should not
lose sight of the fact that even after the most rigorous selective
process has been brought to bear on organisms, namely, by isolation
under domestication, we do not apparently find ourselves gradually
approaching nearer and nearer to the formation of new species, but we
find, on the contrary, that we have produced something quite different.
In the light of this truth, the relation between the two selective
theories may appear quite different from the interpretation that Darwin
gives of it. We may well doubt whether nature does select so much better
than does man, and whether she has ever _made_ new species in this way.
We come now to a point that touches the theory of natural selection in a
very vital spot.
“It may be well here to remark that with all beings there must be much
fortuitous destruction, which can have little or no influence on the
course of natural selection. For instance, a vast number of eggs or
seeds are annually devoured, and these could be modified through natural
selection only if they varied in some manner which protected them from
their enemies. Yet many of these eggs or seeds would perhaps, if not
destroyed, have yielded individuals better adapted to their conditions
of life than any of those which happened to survive. So again a vast
number of mature animals and plants, whether or not they be the best
adapted to their conditions, must be annually destroyed by accidental
causes, which would not be in the least degree mitigated by certain
changes of structure or constitution which would in other ways be
beneficial to the species. But let the destruction of the adults be ever
so heavy, if the number which can exist in any district be not wholly
kept down by such causes,—or again let the destruction of eggs or seeds
be so great that only a hundredth or a thousandth part are
developed,—yet of those which do survive, the best adapted individuals,
supposing that there is any variability in a favorable direction, will
tend to propagate their kind in larger numbers than the less well
adapted. If the numbers be wholly kept down by the causes just
indicated, as will often have been the case, natural selection will be
powerless in certain beneficial directions; but this is no valid
objection to its efficiency at other times and in other ways; for we are
far from having any reason to suppose that many species ever undergo
modification and improvement at the same time in the same area.”
Some of the admissions made in this paragraph have an important bearing
on the theory of natural selection. Far from supposing that fortuitous
destruction would have no influence on the course of natural selection,
it can be shown that it would have a most disastrous effect. In many
cases the destruction comes in the form of a catastrophe to the
individuals, so that small differences in structure, whether
advantageous or not, are utterly unavailing. Our experience shows us
that a destruction of this sort is going on around us all the time, and
accounts in large part for the way in which the majority of animals and
plants are destroyed. Unless, for example, a seed happen to fall on a
place suitable for its growth, it will perish without respect to a
slight advantage it may have over other seeds of its kind. Of the
thousands of eggs laid by one starfish, chance alone will decide whether
one or another embryo is destroyed by larger animals, or if they escape
this danger, the majority of them may be carried out to sea, where it
will not be of the least avail if one individual has a slight advantage
over the others. Darwin admits this, but adds that, if only a thousandth
part is developed, yet of those that do survive the best adapted
individuals will tend to propagate their kind in larger numbers than the
less well adapted. The argument is not, however, so simple as it appears
to be on the surface. I pass over, for the present, the apparent
inconsequence in this statement that the best adapted individuals will
tend to propagate their kind in larger numbers. It is not by any means
certain that this is the case. Darwin’s meaning is, however, fairly
clear, and can be interpreted to mean this: after the fortuitous
destruction has finished, there will be a further competition of the
survivors amongst themselves and with the surrounding conditions. In
this higher competition, which is less severe, small individual
differences suffice to determine the survival of certain individuals.
These are, therefore, selected.
In this argument it is assumed that a second competition takes place
after the first destruction of individuals has occurred, and this
presupposes that more individuals reach maturity than there is room for
in the economy of nature. But we do not know to what extent this takes
place. If only as many mature as can survive, then the second
competition does not take place. If, on the other hand, fewer mature
than there is room for, then again competition does not take place. And
if at all times selection is not rigorously carried out, everything may
be lost that has been so laboriously gained. We see then that the result
that Darwin imagines would take place, can be carried out only when more
individuals reach maturity than there is room for (if it is a case of
competition with one another), or that escape their enemies (if it is a
question of competition with other forms).
It is instructive to consider some of the examples that Darwin has given
to illustrate how the process of natural selection is carried out. The
first example is the imaginary case of a species of wolf, the
individuals of which secure their prey sometimes by craft, sometimes by
strength, and sometimes by fleetness. If the prey captured by the first
two methods should fail, then all the wolves would be obliged to capture
their food by fleetness, and consequently the fleetest alone would
survive. “I can see no more reason to doubt that this would be the
result than that man should improve the fleetness of his greyhounds.”
But even if the fleetness of the race could be kept up in this way, it
does not follow that a new species of wolf would be formed in
consequence, as Darwin implies. His own comment on this illustration is,
perhaps, the best criticism that can be made.
“It should be observed that, in the above illustration, I speak of the
slimmest individual wolves, and not of any single strongly marked
variation having been preserved. In former editions of this work I
sometimes spoke as if this latter alternative had frequently occurred. I
saw the great importance of individual differences, and this led me
fully to discuss the results of unconscious selection by man, which
depends on the preservation of all the more or less valuable
individuals, and on the destruction of the worst. I saw, also, that the
preservation in a state of nature of any occasional deviation of
structure, such as a monstrosity, would be a rare event; and that, if at
first preserved, it would generally be lost by subsequent intercrossing
with ordinary individuals. Nevertheless, until reading an able and
valuable article in the _North British Review_ (1867), I did not
appreciate how rarely single variations, whether slight or strongly
marked, could be perpetuated. The author takes the case of a pair of
animals, producing during their lifetime two hundred offspring, of
which, from various causes of destruction, only two on an average
survive to procreate their kind. This is rather an extreme estimate for
most of the higher animals, but by no means so for many of the lower
organisms. He then shows that if a single individual were born, which
varied in some manner, giving it twice as good a chance of life as that
of the other individuals, yet the chances would be strongly against its
survival. Supposing it to survive and to breed, and that half its young
inherited the favourable variation; still, as the reviewer goes on to
show, the young would have only a slightly better chance of surviving
and breeding; and this chance would go on decreasing in the succeeding
generations. The justice of these remarks cannot, I think, be disputed.
If, for instance, a bird of some kind could procure its food more easily
by having its beak curved, and if one were born with its beak strongly
curved, and which consequently flourished, nevertheless there would be a
very poor chance of this one individual perpetuating its kind to the
exclusion of the common form; but there can hardly be a doubt, judging
by what we see taking place under domestication, that this result would
follow from the preservation during many generations of a large number
of individuals with more or less strongly curved beaks, and from the
destruction of a still larger number with the straightest beaks.”
There then follows what, I believe, is one of the most significant
admissions in the “Origin of Species”:—
“It should not, however, be overlooked that certain rather strongly
marked variations, which no one would rank as mere individual
differences, frequently recur owing to a similar organization being
similarly acted on—of which fact numerous instances could be given with
our domestic productions. In such cases, if the varying individual did
not actually transmit to its offspring its newly acquired character, it
would undoubtedly transmit to them, as long as the existing conditions
remained the same, a still stronger tendency to vary in the same manner.
There can also be little doubt that the tendency to vary in the same
manner has often been so strong that all the individuals of the same
species have been similarly modified without the aid of any form of
selection. Or only a third, fifth, or tenth part of the individuals may
have been thus affected, of which fact several instances could be given.
Thus Graba estimates that about one-fifth of the guillemots in the Faroe
Islands consist of a variety so well marked, that it was formerly ranked
as a distinct species under the name of _Uria lacrymans_. In cases of
this kind, if the variation were of a beneficial nature, the original
form would soon be supplanted by the modified form, through the survival
of the fittest.”
Do not the admissions in this paragraph almost amount to a withdrawal of
much that has preceded in regard to the survival of fluctuating,
individual differences? In the last edition, from which we have just
quoted, Darwin, in response to the criticisms which his book met,
inserted here and there statements that are in many ways in
contradiction to the statements in the first edition, and yet the
earlier statements have been allowed to stand for the most part.
The next example is also worthy of careful examination, since it appears
to prove too much:—
“It may be worth while to give another and more complex illustration of
the action of natural selection. Certain plants excrete sweet juice,
apparently for the sake of eliminating something injurious from the sap:
this is effected, for instance, by glands at the base of the stipules in
some Leguminosæ, and at the backs of the leaves of the common laurel.
This juice, though small in quantity, is greedily sought by insects; but
their visits do not in any way benefit the plant. Now, let us suppose
that the juice or nectar was excreted from the inside of the flowers of
a certain number of plants of any species. Insects in seeking the nectar
would get dusted with pollen, and would often transport it from one
flower to another. The flowers of two distinct individuals of the same
species would thus get crossed; the act of crossing, as can be fully
proved, gives rise to vigorous seedlings, which consequently would have
the best chance of flourishing and surviving. The plants which produced
flowers with the largest glands or nectaries, excreting most nectar,
would oftenest be visited by insects, and would oftenest be crossed; and
so in the long run would gain the upper hand and form a local variety.”
The reader will notice that the sweet juice or nectar secreted by
certain plants is supposed to have first appeared independently of the
action of natural selection. Why then account for its presence in
flowers as the outcome of an entirely different process? If the nectar
is eagerly sought for by insects, without the plant benefiting in any
way by their visitations, why give a different explanation of its origin
in flowers where it is of benefit to the plant?
Darwin carries his illustration further: “When our plant, by the above
process long continued, had been rendered highly attractive to insects,
they would unintentionally, on their part, regularly carry pollen from
flower to flower; and that they do this effectually, I could easily show
by many striking facts. I will give only one, as likewise illustrating
one step in the separation of the sexes of plants.... As soon as the
plant had been rendered so highly attractive to insects that pollen was
regularly carried from flower to flower, another process might commence.
No naturalist doubts the advantage of what has been called the
‘physiological division of labour’; hence we may believe that it would
be advantageous to a plant to produce stamens alone in one flower or on
one whole plant, and pistils alone in another flower or on another
plant. In plants under culture and placed under new conditions of life,
sometimes the male organs and sometimes the female organs become more or
less impotent; now if we suppose this to occur in ever so slight a
degree under nature, then, as pollen is already carried regularly from
flower to flower, and as a more complete separation of the sexes of our
plant would be advantageous on the principle of the division of labour,
individuals with this tendency more and more increased would be
continually favoured or selected, until at last a complete separation of
the sexes might be effected. It would take up too much space to show the
various steps, through dimorphism and other means, by which the
separation of the sexes in plants of various kinds is apparently now in
progress; but I may add that some of the species of holly in North
America are, according to Asa Gray, in an exactly intermediate
condition, or, as he expresses it, are more or less diœciously
polygamous.”
From this it will be seen that Darwin supposes that the separation of
the sexes in some of the higher plants has been brought about by natural
selection. Despite the supposed advantage of the so-called “division of
labor,” one may, I venture to suggest, be sceptical as to whether the
separation of the sexes can be explained in this way. The whole case is
largely supposititious, since in most of the higher hermaphroditic
plants and in nearly all hermaphroditic animals the sexual products
ripen at different times in the same individual. Hence there is no basis
for the assumption that unless the sexes are separated there will be
self-fertilization. Shall we assume that this difference in time of
ripening of the two kinds of sex-cells is also the outcome of natural
selection, and that there has existed an earlier stage in all animals
and plants, that now have different times for the ripening of their
sexual elements, a time when these products ripened simultaneously? I
doubt if even a Darwinian would give such loose rein to his fancy.
But this is not yet the whole story that Darwin has made out in this
connection, for he continues:—
“Let us now turn to the nectar-feeding insects; we may suppose the
plant, of which we have been slowly increasing the nectar by continued
selection, to be a common plant; and that certain insects depended in
main part on its nectar for food. I could give many facts showing how
anxious bees are to save time: for instance, their habit of cutting
holes and sucking the nectar at the bases of certain flowers, which with
a very little more trouble, they can enter by the mouth. Bearing such
facts in mind, it may be believed that under certain circumstances
individual differences in the curvature or length of the proboscis,
etc., too slight to be appreciated by us, might profit a bee or other
insect, so that certain individuals would be able to obtain their food
more quickly than others; and thus the communities to which they
belonged would flourish and throw off many swarms inheriting the same
peculiarities.”
Aside from the general criticism that will suggest itself here also, it
should be pointed out that even if “certain individuals” of the bees had
slightly longer proboscides, this would, in the case of the hive-bees at
least, be of no avail, since they do not reproduce, and hence leave no
descendants with longer mouth-parts. Of course, it may be replied that
those colonies in which the queens produce more of the long-proboscis
kind of worker would have an advantage over other colonies not having so
many individuals of this sort. It would then be a competition of one
colony with another, as Darwin supposes to take place in colonial forms.
But whether slight differences of this sort would lead to the
elimination of the least well-endowed colonies is entirely a matter of
speculation. Since there are flowers with corolla-tubes of all lengths,
we can readily suppose that if one kind of flower excluded individuals
of certain colonies, they would search elsewhere for their nectar rather
than perish. While different races might arise in this way, the process
would not be the survival of the fittest, but a process of adaptation to
a new environment.
We come now to a topic on which Darwin lays much stress: the divergence
of character. He tries to show how the “lesser differences between the
varieties become augmented into the greater differences between
species.”
“Mere chance, as we may call it, might cause one variety to differ in
some character from its parents, and the offspring of this variety again
to differ from its parent in the very same character and in a greater
degree; but this alone would never account for so habitual and large a
degree of difference as that between the species of the same genus. As
has always been my practice, I have sought light on this head from our
domestic productions.”
Then, after pointing out that under domestication two different races,
the race-horse and the dray-horse, for instance, might arise by
selecting different sorts of variations, Darwin inquires:—
“But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently (though it was a long
time before I saw how), from the simple circumstance that the more
diversified the descendants from any one species become in structure,
constitution, and habits, by so much will they be better enabled to
seize on many and widely diversified places in the polity of nature, and
so be enabled to increase in numbers.”
Here we touch on one of the fundamental principles of the doctrine of
evolution. It is intimated that the new form of animal or plant first
appears (without regard to any kind of selection), and then finds that
place in nature where it can remain in existence and propagate its kind.
Darwin refers here, of course, only to the less extensive variations,
the individual or fluctuating kind; but as we shall discuss at greater
length in another place, this same process, if extended to other kinds
of variation, may give us an explanation of evolution without
competition, or selection, or destruction of the individuals of the same
kind taking place at all.
------------------------------------------------------------------------
CHAPTER V
THE THEORY OF NATURAL SELECTION (_Continued_)
Objections to the Theory of Natural Selection
Although in the preceding chapter a number of criticisms have been made
of the special parts of the theory of natural selection, there still
remain to be considered some further objections that have been made
since the first publication of the theory. It is a fortunate
circumstance from every point of view that Darwin himself was able in
the later editions of the “Origin of Species” to reply to those
criticisms that he thought of sufficient importance. He says:—
“Long before the reader has arrived at this part of my work, a crowd of
difficulties will have occurred to him. Some of them are so serious that
to this day I can hardly reflect on them without being in some degree
staggered; but, to the best of my judgment, the greater number are only
apparent, and those that are real are not, I think, fatal to the
theory.”
The first difficulty is this: “Why, if species have descended from other
species by fine gradations, do we not everywhere see innumerable
transitional forms? Why is not all nature in confusion, instead of the
species being, as we see them, well defined?”
The answer that Darwin gives is, that by competition the new form will
crowd out its own less-improved parent form, and other less-favored
forms. But is this a sufficient or satisfactory answer? If we recall
what Darwin has said on the advantage that those forms will have, in
which a great number of new variations appear to fit them to the great
diversity of natural conditions, and if we recall the gradations that
exist in external conditions, I think we shall find that Darwin’s reply
fails to give a satisfactory answer to the question.
It is well known, and Darwin himself has commented on it, that the same
species often remains constant under very diverse external conditions,
both inorganic and organic. Hence I think the explanation fails, in so
far as it is based on the accumulation by selection of small individual
variations that are supposed to give the individuals some slight
advantage under each set of external conditions. Darwin admits that
“this difficulty for a long time quite confounded me. But I think it can
be in large part explained.” The first explanation that is offered is
that areas now continuous may not have been so in the past. This may be
true in places, but the great continents have had continuous areas for a
long time, and Darwin frankly acknowledges that he “will pass over this
way of explaining the difficulty.” The second attempt is based on the
supposed narrowness of the area, where two species, descended from a
common parent, overlap. In this region the change is often very abrupt,
and Darwin adds:—
“To those who look at climate and the physical conditions of life as the
all-important elements of distribution, these facts ought to cause
surprise, as climate and height or depth graduate away insensibly. But
when we bear in mind that almost every species, even in its metropolis,
would increase immensely in numbers, were it not for other competing
species; that nearly all either prey on or serve as prey for others; in
short, that each organic being is either directly or indirectly related
in the most important manner to other organic beings,—we see that the
range of the inhabitants of any country by no means exclusively depends
on insensibly changing physical conditions, but in a large part on the
presence of other species, on which it lives, or by which it is
destroyed, or with which it comes into competition; and as these species
are already defined objects, not blending one into another by insensible
gradations, the range of any one species, depending as it does on the
range of others, will tend to be sharply defined.”
Here we have a _petitio principii_. The sharp definition of species,
that we started out to account for, is explained by the sharp definition
of other species!
A third part of the explanation is that, owing to the relative fewness
of individuals at the confines of the range during the fluctuations of
their enemies, or of their prey, or in the nature of the seasons, they
would be extremely liable to utter extermination. If this were really
the case, then new species themselves which, on the theory, are at first
few in numbers ought to be exterminated. On the whole, then, it does not
appear that Darwin has been very successful in his attempt to meet this
objection to the theory.
Darwin tries to meet the objection, that organs of extreme perfection
and complication cannot be accounted for by natural selection, as
follows:—
“To suppose that the eye with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different
amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I freely
confess, absurd in the highest degree.”
The following sketch that Darwin gives to show how he imagined the
vertebrate eye to have been formed is very instructive, as illustrating
how he supposed that natural selection acts:—
“If we must compare the eye to an optical instrument, we ought in
imagination to take a thick layer of transparent tissue, with spaces
filled with fluid, and with a nerve sensitive to light beneath, and then
suppose every part of this layer to be continually changing slowly in
density, so as to separate into layers of different densities and
thicknesses, placed at different distances from each other, and with the
surfaces of each layer slowly changing in form. Further we must suppose
that there is a power, represented by natural selection or the survival
of the fittest, always intently watching each slight alteration in the
transparent layers; and carefully preserving each which, under varied
circumstances, in any way or in any degree, tends to produce a
distincter image. We must suppose each new state of the instrument to be
multiplied by the million; each to be preserved until a better one is
produced, and then the old ones to be all destroyed. In living bodies,
variation will cause the slight alterations, generation will multiply
them almost infinitely, and natural selection will pick out with
unerring skill each improvement. Let this process go on for millions of
years; and during each year on millions of individuals of many kinds;
and may we not believe that a living optical instrument might thus be
formed as superior to one of glass, as the works of the Creator are to
those of man.”
We may conclude in Darwin’s own words:—
“To arrive, however, at a just conclusion regarding the formation of the
eye, with all its marvellous yet not absolutely perfect characters, it
is indispensable that the reason should conquer the imagination; but I
have felt the difficulty far too keenly to be surprised at others
hesitating to extend the principle of natural selection to so startling
a length.”
The electric organs, present in several fish, offer a case of special
difficulty to the selection theory. When well developed, as in the
Torpedo and in Gymnotus, it is conceivable that it may serve as an organ
of defence, but in other forms the shock is so weak that it is not to be
supposed that it can have any such function. Romanes, who in many ways
was one of the stanchest followers of Darwin, admits that, so far as he
can see, the evolution of the electric organs cannot be explained by the
selection theory. Darwin offers no explanation, but bases his defence on
the grounds that we do not know of what use this organ can be to the
animal.
Darwin also refers to the phosphorescent, or luminous, organs as a
supposed case of difficulty for his theory.
“The luminous organs which occur in a few insects, belonging to widely
different families, and which are situated in different parts of the
body, offer, under our present state of ignorance, a difficulty almost
exactly parallel with that of the electric organs.”
In this case also, as in that of the electric organs, the structures
appear in entirely different parts of the body of the insect in
different species, so that their occurrence in this group cannot be
accounted for on a common descent. In whatever way they have arisen,
they must have evolved independently in different species. Darwin
advances no explanation of the origin of the luminous organs, but states
that they “offer under our present state of ignorance a difficulty
almost exactly parallel with that of the electric organs.” It will be
noticed that the difficulty referred to rests on the assumption that
since the organs are well developed they must have some important use!
We may next consider “organs of little apparent importance as affected
by natural selection.” Darwin says:—
“As natural selection acts by life and death,—by the survival of the
fittest, and by the destruction of the less well-fitted individuals,—I
have sometimes felt great difficulty in understanding the origin or
formation of parts of little importance; almost as great, though of a
very different kind, as in the case of the most perfect and complex
organs.”
His answers to this difficulty are: (1) we are too ignorant “in regard
to the whole economy of any one organic being to say what slight
modifications would be of importance or not,”—thus such apparently
trifling characters as the down on fruit, or the colors of the skin and
hair of quadrupeds, which from being correlated with constitutional
differences or from determining the attacks of insects might be acted on
by natural selection; (2) organs now of trifling importance have in some
cases been of high importance to an early progenitor; (3) the changed
conditions of life may account for some of the useless organs; (4)
reversion accounts for others; (5) the complex laws of growth account
for still others, such as correlation, compensation of the pressure of
one part on another, etc.; (6) the action of sexual selection is
responsible for many characters not to be explained by natural
selection. Admitting that there may be cases that can be accounted for
on one or the other of these six possibilities, yet there can be no
doubt that there are still a considerable number of specific characters
that cannot be explained in any of these ways. I do not think that
Darwin has by any means met this objection, even if all these six
possibilities be admitted as generally valid.
Amongst the “miscellaneous objections” to his theory that Darwin
considers we may select the most important cases. The following
paragraph has been sometimes quoted by later writers to show that Darwin
saw, to a certain extent, the insufficiency of fluctuating variations as
a basis for selection. What he calls here “spontaneous variability”
refers to sudden and extensive variations, or what we may call
discontinuous variations. “In the earlier editions of this work I
underrated, as it now seems probable, the frequency and importance of
modifications due to spontaneous variability. But it is impossible to
attribute to this cause the innumerable structures which are so well
adapted to the habits of life of each species. I can no more believe in
this, that the well-adapted form of a race-horse or greyhound, which
before the principle of selection by man was well understood, excited so
much surprise in the minds of the older naturalists, can thus be
explained.”
Darwin appears to mean by the latter part of this statement, that he
cannot believe that such sudden and great variations as have caused a
peach tree to produce nectarines can account for the wonderful
adaptations of organisms; but it is not really necessary to suppose that
this would often occur, for the same result could be reached by several
stages, even if the discontinuous variations had been small, and had
appeared in many individuals simultaneously. After showing that in a
number of flowers, especially of the Compositæ and Umbelliferæ, the
individual flowers in the closely crowded heads are sometimes formed on
a different type, Darwin concludes: “In these several cases, with the
exception of that of the well-developed ray-florets, which are of
service in making the flowers conspicuous to insects, natural selection
cannot, as far as we can judge, have come into play, or only in a quite
subordinate manner. All these modifications follow from the relative
position and interaction of the parts; and it can hardly be doubted that
if all the flowers and leaves on the same plant had been subjected to
the same external and internal condition, as are the flowers and leaves
in certain positions, all would have been modified in the same manner.”
Further on we meet with the following remarkable statement: “But when,
from the nature of the organism and of the conditions, modifications
have been induced which are unimportant for the welfare of the species,
they may be, and apparently often have been, transmitted in nearly the
same state to numerous, otherwise modified, descendants. It cannot have
been of much importance to the greater number of mammals, birds, or
reptiles, whether they were clothed with hair, feathers, or scales; yet
hair has been transmitted to almost all mammals, feathers to all birds,
and scales to all true reptiles. A structure, whatever it may be, which
is common to many allied forms, is ranked by us as of high systematic
importance, and consequently is often assumed to be of high vital
importance to the species. Thus, as I am inclined to believe,
morphological differences, which we consider as important,—such as the
arrangement of the leaves, the divisions of the flower or of the
ovarium, the position of the ovules, etc.,—first appeared in many cases
as fluctuating variations, which sooner or later became constant through
the nature of the organism and of the surrounding conditions, as well as
through the intercrossing of distinct individuals, but not through
natural selection; for as these morphological characters do not affect
the welfare of the species, any slight deviations in them could not have
been governed or accumulated through this latter agency. It is a strange
result which we thus arrive at, namely, that characters of slight vital
importance to the species are the most important to the systematist;
but, as we shall hereafter see when we treat of the genetic principle of
classification, this is by no means so paradoxical as it may at first
appear.”
If all this be granted, it is once more evident that the only variations
that come under the action of selection are the limited number that are
of vital importance to the organism. How little the theory of natural
selection can be used to explain the origin of species will be apparent
from the above quotation. This is, of course, not an argument against
the theory itself, which would still be one of vast importance if it
explained adaptive characters alone; but enough has been said, I think,
to show that it is improbable that the origin of adaptive and
non-adaptive characters are to be explained by entirely different
principles.
In reply to a criticism of Mivart, Darwin makes the further admission as
to the insufficiency of the theory of natural selection: “When
discussing special cases, Mr. Mivart passes over the effects of the
increased use and disuse of parts, which I have always maintained to be
highly important, and have treated in my ‘Variation under Domestication’
at greater length than, as I believe, any other writer. He likewise
often assumes that I attribute nothing to variation, independent of
natural selection, whereas in the work just referred to I have collected
a greater number of well-established cases than is to be found in any
other work known to me.” If this is admitted, and if it can be shown
that the evidence in favor of the inheritance of acquired characters is
very doubtful at best, may we not conclude that Mivart’s criticisms have
sometimes hit the mark?
The following objection appears to be a veritable stumbling-block to the
theory. Flatfishes and soles lie on one side, and do not stand in a
vertical position as do other fish. Some species lie on one side and
some on the other, and some species contain both right-sided and
left-sided individuals. In connection with this unusual habit we find a
striking change in the structure. The eye that would be on the under
side has shifted, so that it has come to lie on the upper side of the
head, _i.e._ both eyes lie on the same side,—a condition found in no
other vertebrate. As a result of the shifting of the eye, the bones of
the skull have also become profoundly modified. The young fish that
emerge from the egg swim at first upright, as do ordinary fish, and only
after they have led a free existence for some time do they turn to one
side and sink to the bottom. Unless the under eye moved to the upper
side it would be of no use to the flatfish, and might even be a source
of injury. Mivart points out that a sudden, spontaneous transformation
in the position of eye is hardly conceivable, and to this Darwin, of
course, assents. Mivart adds: “If the transit was gradual, then how such
transit of one eye a minute fraction of the journey towards the other
side of the head could benefit the individual is, indeed, far from
clear. It seems even that such an incipient transformation must rather
have been injurious.” Darwin’s reply is characteristic:—
“We thus see that the first stages of the transit of the eye from one
side of the head to the other, which Mr. Mivart considers would be
injurious, may be attributed to the habit, no doubt beneficial to the
individual and to the species, of endeavoring to look upwards with both
eyes, whilst resting on one side at the bottom. We may also attribute to
the inherited effects of use the fact of the mouth in several kinds of
flatfish being bent towards the lower surface, with the jaw-bones
stronger and more effective on this, the eyeless side of the head, than
on the other side, for the sake, as Dr. Traquair supposes, of feeding
with ease on the ground. Disuse, on the other hand, will account for the
less developed condition of the whole inferior half of the body,
including the lateral fins; though Yarrell thinks that the reduced size
of these fins is advantageous to the fish, as ‘there is so much less
room for their action, than with the larger fins above.’ Perhaps the
lesser number of teeth in the proportion of four to seven in the upper
halves of the two jaws of the plaice, to twenty-five to thirty in the
lower halves, may likewise be accounted for by disuse. From the
colorless state of the ventral surface of most fishes and of many other
animals, we may reasonably suppose that the absence of color in flatfish
on the side, whether it be the right or left, which is undermost, is due
to the exclusion of light.”
By falling back on the theory of inheritance of acquired characters
Darwin tacitly admits the incompetence of natural selection to explain
the evolution of the flatfish. If the latter theory prove incorrect, it
must then be admitted that the evolution of the flatfishes cannot be
accounted for by either of the two main theories on which Darwin relies.
Mivart further points out that the beginning stages of the mammary
glands cannot be explained by Darwin’s theory. To which Darwin replies,
that an American naturalist, Mr. Lockwood, believes from what he has
seen of the development of the young of the pipe-fish (Hippocampus) that
“they are nourished by a secretion from the cutaneous glands of the sac”
in which the young are enclosed. This can scarcely be said to be a
satisfactory reply; for, if it is true that this is the case for the
pipe-fish,—and I cannot find on inquiry that this statement has been
confirmed,—it is still rather speculative to suppose that the ancestral
mammals nourished their young by secreting a fluid into the marsupial
sac around the embryos.
Darwin deals with instincts of animals in the same way as he deals with
their structures. After pointing out that instincts are variable, and
that the variations are hereditary, he proceeds to show how selection
may act by picking out those individuals possessing the more favorable
instincts. In other words, the theory of natural selection is applied to
functions, as well as to structure. Darwin makes use here also of the
Lamarckian factor of inheritance, and concludes that “in most cases
habit and selection have probably both occurred.”
A few examples will sufficiently serve to illustrate Darwin’s meaning.
The first case given is that of the cuckoo, which lays its eggs in the
nests of other birds, where they are hatched and the young reared by
their foster-parents. The starting-point for such a perversion of the
ordinary habits of birds is to be found, he thinks, in the occasional
deposition of eggs in the nests of other birds, which has at times been
observed for a number of species. For instance, this has been seen in
the American cuckoo, which ordinarily builds a nest of its own. It is
recorded and believed to be true that the young English cuckoo, when
only two or three days old, ejects from the nest the offspring of its
foster-parents, and this “strange and odious instinct” is supposed by
Darwin to have been acquired in order that the young cuckoo might get
more food, and that the young bird has acquired during successive
generations the strength and structure necessary for the work of
ejection. This is of course largely speculative, and it is by no means
obvious that it was a greater benefit to the cuckoo to have other birds
rear its young than to do so itself. We can equally well imagine, since
this is the turn the argument takes, that the occasional instinct to
deposit eggs in the nests of other birds would be disadvantageous, and
could not have been acquired by the selection of a fluctuating instinct
of this sort. We have no right to assume, that because a new habit has
been acquired, that it is a more advantageous one than the one that has
been lost. All that we can legitimately infer is, that, although the
normal instinct has been changed into another, the race has still been
able to remain in existence. The same conclusion applies to the case of
_Molothrus bonariensis_, cited by Darwin, and is here even more
obvious:—
“Some species of Molothrus, a widely distinct genus of American birds,
allied to our starlings, have parasitic habits like those of the cuckoo;
and the species present an interesting gradation in the perfection of
their instincts. The sexes of _Molothrus badius_ are stated by an
excellent observer, Mr. Hudson, sometimes to live promiscuously together
in flocks, and sometimes to pair. They either build a nest of their own,
or seize on one belonging to some other bird, occasionally throwing out
the nestlings of the stranger. They either lay their eggs in the nest
thus appropriated, or oddly enough build one for themselves on the top
of it. They usually sit on their own eggs and rear their own young; but
Mr. Hudson says it is probable that they are occasionally parasitic, for
he has seen the young of this species following old birds of a distinct
kind and clamoring to be fed by them. The parasitic habits of another
species of Molothrus, the _M. bonariensis_, are much more highly
developed than those of the last, but are still far from perfect. This
bird, as far as is known, invariably lays its eggs in the nest of
strangers; but it is remarkable that several together sometimes commence
to build an irregular untidy nest of their own, placed in singularly
ill-adapted situations, as on the leaves of a large thistle. They never,
however, as far as Mr. Hudson has ascertained, complete a nest for
themselves. They often lay so many eggs—from fifteen to twenty—in the
same foster-nest, that few or none can possibly be hatched. They have,
moreover, the extraordinary habit of pecking holes in the eggs, whether
of their own species or of their foster-parents, which they find in the
appropriated nests. They drop also many eggs on the bare ground, which
are thus wasted.”
Can we possibly be expected to believe that it has been to the advantage
of this species to give up its original regular method of incubating its
own eggs, and acquire such a haphazard, new method? Does not the
explanation prove too much, rather than give support to Darwin’s
hypothesis? Is it not better to conclude, that despite the disadvantages
entailed by a change in the original instincts, the species is still
able to remain in existence?
Darwin points out, in the case of the slave-making ants, that the
slave-making instinct may have arisen in the first instance by ants
carrying pupæ, that they have captured, into their own nests. Later this
habit might become fixed, and, finally, after passing through several
stages of development, the ants might become absolutely dependent on
their slaves. It is also supposed that those colonies in which this
instinct was better developed would survive in competition with other
colonies of the same species on account of the supposed advantage of
owning slaves. In this way natural selection steps in and perfects the
process.
It is far from proven, or even made probable, that a species of ant that
becomes gradually dependent on its slaves is more likely to survive than
other colonies that are not so dependent. All we can be certain of is
that with slaves they have still been able to maintain their own.
Moreover, we must not forget that it is not enough to show that a
particular habit might be useful to a species, but it should also be
shown that it is of sufficient importance, at every stage of its
evolution, to give a decisive advantage in the “struggle for existence.”
For unless a life and death struggle takes place between the different
colonies, natural selection is powerless to bring about its supposed
results. And who will be bold enough to affirm that the presence of
slaves in a nest will give victory to that colony in competition with
its neighbors? Has the history of mankind taught us that the
slave-making countries have exterminated the countries without slaves?
Is the question so simple as this? May not the degeneration of the
masters more than compensate for the acquirement of slaves, and may not
the loss of life in obtaining slaves more than counterbalance the
advantage of the slaves after they are captured? In the face of these
possibilities it is not surprising to find that Darwin, when summing up
the chapter, makes the following admission: “I do not pretend that the
facts in this chapter strengthen in any degree my theory; but none of
the cases of difficulty, to the best of my judgment, annihilate it.”
Darwin, with his usual frankness, adds:—
“No doubt many instincts of very difficult explanation could be opposed
to the theory of natural selection,—cases, in which we cannot see how an
instinct could have originated; cases, in which no intermediate
gradations are known to exist; cases of instincts of such trifling
importance, that they could hardly have been acted on by natural
selection; cases of instincts almost identically the same in animals so
remote in the scale of nature, that we cannot account for their
similarity by inheritance from a common progenitor, and consequently
must believe that they were independently acquired through natural
selection. I will not here enter on these several cases, but will
confine myself to one special difficulty, which at first appeared to me
insuperable, and actually fatal to the whole theory. I allude to the
neuters or sterile females in insect communities; for these neuters
often differ widely in instinct and in structure from both the males and
fertile females, and yet, from being sterile, they cannot propagate
their kind.
“The subject well deserves to be discussed at great length, but I will
here take only a single case, that of working or sterile ants. How the
workers have been rendered sterile is a difficulty; but not much greater
than that of any other striking modification of structure; for it can be
shown that some insects and other articulate animals in a state of
nature occasionally become sterile; and if such insects had been social,
and it had been profitable to the community that a number should have
been annually born capable of work, but incapable of procreation, I can
see no especial difficulty in this having been effected through natural
selection. But I must pass over this preliminary difficulty. The great
difficulty lies in the working ants differing widely from both the males
and the fertile females in structure, as in the shape of the thorax, and
in being destitute of wings and sometimes of eyes, and in instinct. As
far as instinct alone is concerned, the wonderful difference in this
respect between the workers and the perfect females, would have been
better exemplified by the hive-bee. If a working ant or other neuter
insect had been an ordinary animal, I should have unhesitatingly assumed
that all its characters had been slowly acquired through natural
selection; namely, by individuals having been born with slight
profitable modifications, which were inherited by the offspring; and
that these again varied and again were selected, and so onwards. But
with the working ant we have an insect differing greatly from its
parents, yet absolutely sterile; so that it could never have transmitted
successively acquired modifications of structure or instinct to its
progeny. It may well be asked, how is it possible to reconcile this case
with the theory of natural selection?”
Darwin’s answer is that the differences of structure are correlated with
certain ages and with the two sexes, but this is obviously only shifting
the difficulty, not meeting it. He concludes, “I can see no great
difficulty in any character becoming correlated with the sterile
condition of certain members of the insect communities, the difficulty
lies in understanding how such correlated modifications of structure
could have been slowly accumulated by natural selection.” “This
difficulty, though appearing insuperable, is lessened, or, as I believe,
disappears, when it is remembered that selection may be applied to the
family, as well as to the individual, and may thus give the desired
end.”
Darwin did not fail to see that there is a further difficulty even
greater than the one just mentioned. He says: “But we have not as yet
touched on the acme of the difficulty; namely, the fact that the neuters
of several ants differ, not only from the fertile females and males, but
from each other, sometimes to an almost incredible degree, and are thus
divided into two or even three castes. The castes, moreover, do not
commonly graduate into each other, but are perfectly well defined; being
as distinct from each other as are any two species of the same genus, or
rather as any two genera of the same family. Thus in Eciton, there are
working and soldier neuters, with jaws and instincts extraordinarily
different: in Cryptocerus, the workers of one caste alone carry a
wonderful sort of shield on their heads, the use of which is quite
unknown: in the Mexican Myrmecocystus, the workers of one caste never
leave the nest; they are fed by the workers of another caste, and they
have an enormously developed abdomen which secretes a sort of honey,
supplying the place of that excreted by the aphides, or the domestic
cattle as they may be called, which our European ants guard and
imprison.”
“It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate the theory. In the simpler
case of neuter insects all of one caste, which, as I believe, have been
rendered different from the fertile males and females through natural
selection, we may conclude from the analogy of ordinary variations, that
the successive, slight, profitable modifications did not first arise in
all the neuters in the same nest, but in some few alone; and that by the
survival of the communities with females which produced most neuters
having the advantageous modification, all the neuters ultimately came to
be thus characterized. According to this view we ought occasionally to
find in the same nest neuter insects, presenting gradations of
structure; and this we do find, even not rarely, considering how few
neuter insects out of Europe have been carefully examined.”
From this the conclusion is reached:—
“With these facts before me, I believe that natural selection, by acting
on the fertile ants or parents, could form a species which should
regularly produce neuters, all of large size with one form of jaw, or
all of small size with widely different jaws; or lastly, and this is the
greatest difficulty, one set of workers of one size and structure, and
simultaneously another set of workers of a different size and
structure;— a graduated series having first been formed, as in the case
of the driver ant, and then the extreme forms having been produced in
greater and greater numbers, through the survival of the parents which
generated them, until none with an intermediate structure were produced.
“I have now explained how, as I believe, the wonderful fact of two
distinctly defined castes of sterile workers existing in the same nest,
both widely different from each other and from their parents, has
originated. We can see how useful their production may have been to a
social community of ants, on the same principle that the division of
labor is useful to civilized man. Ants, however, work by inherited
instincts and by inherited organs or tools, whilst man works by acquired
knowledge and manufactured instruments. But I must confess, that, with
all my faith in natural selection, I should never have anticipated that
this principle could have been efficient in so high a degree, had not
the case of these neuter insects led me to this conclusion. I have,
therefore, discussed this case, at some little but wholly insufficient
length, in order to show the power of natural selection, and likewise
because this is by far the most serious special difficulty which my
theory has encountered. The case, also, is very interesting, as it
proves that with animals, as with plants, any amount of modification may
be effected by the accumulation of numerous, slight, spontaneous
variations, which are in any way profitable, without exercise or habit
having been brought into play. For peculiar habits confined to the
workers or sterile females, however long they might be followed, could
not possibly affect the males and fertile females, which alone leave
descendants. I am surprised that no one has hitherto advanced this
demonstrative case of neuter insects, against the well-known doctrine of
inherited habit, as advanced by Lamarck.”
We may dissent at once from Darwin’s statement which, he thinks, “proves
that any amount of modification may be affected by the accumulation of
numerous slight variations which are in any way profitable without
exercise or habit having been brought into play”; we may dissent if for
no other reason than that this begs the whole point at issue, and is not
proven. It does not follow because in some colonies all intermediate
stages of neuters exist, that in other colonies, where no such
intermediate stages are present, these have been slowly weeded out by
natural selection, causing to disappear all colonies slightly below the
mark. It is this that begs the question. Because we can imagine that
intermediate stages between the different castes may have been present,
it neither follows that such fluctuating variations have been the basis
for the evolution of the more sharply defined types, nor that the
imagined advantage of such a change would have led through competition
to the extermination of the other colonies. However much we may admire
the skill with which Darwin tried to meet this difficulty, let us not
put down the results to the good of the theory, but rather repeat once
more Darwin’s own words at the end of this chapter, to the effect that
the facts do not strengthen the theory.
Sterility between Species
The care with which Darwin examined every bearing of his theory is
nowhere better exemplified than in his treatment of the question of
sterility between the individuals of different species. It would be so
obviously to the advantage of the selection theory if it were true that
sterility between species had been acquired by selection in order to
prevent intercrossing, that it would have been easy for a less cautious
thinker to have fallen into the error of supposing that sterility might
have been acquired in this way. Tempting as such a view appears, Darwin
was not caught by the specious argument, as the opening sentence in the
chapter of hybridism shows:—
“The view commonly entertained by naturalists is that species, when
intercrossed, have been specially endowed with sterility, in order to
prevent their confusion. This view certainly seems at first highly
probable, for species living together could hardly have been kept
distinct had they been capable of freely crossing. The subject is in
many ways important for us, more especially as the sterility of species
when first crossed, and that of their hybrid offspring, cannot have been
acquired, as I shall show, by the preservation of successive profitable
degrees of sterility. It is an incidental result of differences in the
reproductive systems of the parent species.”
In dealing with this subject Darwin points out that we must be careful
to distinguish between “the sterility of species when first crossed, and
the sterility of hybrids produced from them.” In the former case, the
reproductive organs of each individual are in a perfectly normal
condition, while hybrids appear to be generally impotent owing to some
imperfection in the reproductive organs themselves. They are not
perfectly fertile, as a rule, either with each other, or with either of
the parent forms.
In striking contrast to the sterility between species is the fertility
of varieties. If, as Darwin believes, varieties are incipient species,
we should certainly expect to find them becoming less and less fertile
with other fraternal varieties, or with the parent forms in proportion
as they become more different. Yet experience appears to teach exactly
the opposite; but the question is not a simple one, and the results are
not so conclusive as appears at first sight. Let us first see how Darwin
met this obvious contradiction to his view.
In the first place, he points out that all species are not infertile
when crossed with other species. The sterility of various species, when
crossed, is so different in degree, and graduates away so insensibly,
and the fertility of pure species is so easily affected by various
circumstances, that it is most difficult to say where perfect fertility
ends and sterility begins. “It can thus be shown that neither sterility
nor fertility afford any certain distinction between species and
varieties.” Darwin cites several cases in plants in which crosses
between species have been successfully accomplished. The following
remarkable results are also recorded: “Individual plants in certain
species of Lobelia, Verbascum, and Passiflora can easily be fertilized
by pollen from a distinct species, but not by pollen from the same
plant, though this pollen can be proved to be perfectly sound by
fertilizing other plants or species. In the genus Hippeastrum, in
Corydalis as shown by Professor Hildebrand, in various orchids as shown
by Mr. Scott and Fritz Müller, all the individuals are in this peculiar
condition. So that with some species, certain abnormal individuals, and
in other species all the individuals, can actually be hybridized much
more readily than they can be fertilized by pollen from the same
individual plant!”[14]
Footnote 14:
A somewhat parallel case has recently been discovered by Castle for
the hermaphroditic ascidian _Ciona intestinalis_. In this case the
spermatozoa of any individual fail to fertilize the eggs of the same
individual, although they will fertilize the eggs of any other
individual.
In regard to animals, Darwin concludes that “if the genera of animals
are as distinct from each other as are the genera of plants, then we may
infer that animals more widely distinct in the scale of nature can be
crossed more easily than in the case of plants; but the hybrids
themselves are, I think, more sterile.”
The most significant fact in this connection is that the more widely
different two species are, so that they are placed in different
families, so much the less probable is it that cross-fertilization will
produce any result. From this condition of infertility there may be
traced a gradation between less different forms of the same genus to
almost complete, or even complete, fertility between closely similar
species. Darwin further points out that: “The hybrids raised from two
species which are very difficult to cross, and which rarely produce any
offspring, are generally very sterile; but the parallelism between the
difficulty of making a first cross, and the sterility of the hybrids
thus produced—two classes of facts which are generally confounded
together—is by no means strict. There are many cases, in which two pure
species, as in the genus Verbascum, can be united with unusual facility,
and produce numerous hybrid offspring, yet these hybrids are remarkably
sterile. On the other hand, there are species which can be crossed very
rarely, or with extreme difficulty, but the hybrids, when at last
produced, are very fertile. Even within the limits of the same genus,
for instance in Dianthus, these two opposite cases occur.”
In regard to reciprocal crosses Darwin makes the following important
statements: “The diversity of the result in reciprocal crosses between
the same two species was long ago observed by Kölreuter. To give an
instance: _Mirabilis jalapa_ can easily be fertilized by the pollen of
_M. longiflora_, and the hybrids thus produced are sufficiently fertile;
but Kölreuter tried more than two hundred times, during eight following
years, to fertilize reciprocally _M. longiflora_ with the pollen of _M.
jalapa_, and utterly failed.”
A formal interpretation of this difference can be easily imagined. The
infertility in one direction may be due to some physical difficulty met
with in penetrating the stigma, or style. For instance, the tissue in
one species may be too compact, or the style too long. Pflüger, who
carried out a large number of experiments by cross-fertilizing different
species of frogs, reached the conclusion that the spermatozoa having
small and pointed heads could cross-fertilize more kinds of eggs, than
could the spermatozoa with large blunt heads. This is probably due to
the ability of the smaller spermatozoa to penetrate the jelly around the
eggs, or the pores in the surface of the egg itself. But there are also
other sides to this question, as recent results have shown, for, even if
a foreign spermatozoon can enter an egg, it does not follow that the
development of the egg will take place. Here the difficulty is due to
some obscure processes in the egg itself. Now that we know more of the
nicely balanced combinations that take place during fertilization of the
egg, and during the process of cell division, we can easily see that if
the processes were in the least different in the two species it might be
impossible to combine them in a single act.
“Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we
must suppose it would be equally important to keep from blending
together?”
“The foregoing rules and facts, on the other hand, appear to me clearly
to indicate that the sterility both of first crosses and of hybrids is
simply incidental or dependent on unknown differences in their
reproductive systems; the differences being of so peculiar and limited a
nature, that, in reciprocal crosses between the same two species, the
male sexual element of the one will often freely act on the female
sexual element of the other, but not in a reversed direction.”
Does Darwin give here a satisfactory answer to the difficulty that he
started out to explain away? On the whole, the reader will admit, I
think, that he has fairly met the situation, in so far as he has shown
that there is no absolute line of demarcation between the power of
intercrossing of varieties and races, and of species. It is also
_extremely important to have found that the difficulties increase, so to
speak, even beyond the limits of the species_; since species, belonging
to different genera, are as a rule more difficult to intercross than
when they belong to the same genus. The further question, as to whether
there are differences in respect to the power of intercrossing between
different kinds of varieties, such as those dependent on selection of
fluctuating variations, of local conditions, of mutations, etc., is far
from being settled at the present time.
That this property of species is useful to them, in the somewhat unusual
sense that it keeps them from freely mingling with other species, is
true; but, as has been said, this would be a rather peculiar kind of
adaptation. If, however, it be claimed that this property is useful to
species, as Darwin himself claims, then, as he also points out, it is a
useful acquirement that cannot have arisen through natural selection. It
is not difficult to show why this must be so. If two varieties were to
some extent at the start less fertile, _inter se_, than with their own
kind, the only way in which they could become more infertile through
selection would be by selecting those individuals in each generation
that are still more infertile, but the forms of this sort would, _ex
hypothese_, become less numerous than the descendants of each species
itself, which would, therefore, supplant the less fertile ones.
Darwin’s own statement in regard to this point is as follows:—
“At one time it appeared to me probable, as it has to others, that the
sterility of first crosses and of hybrids might have been slowly
acquired through the natural selection of slightly lessened degrees of
fertility, which, like any other variation, spontaneously appeared in
certain individuals of one variety when crossed with those of another
variety. For it would clearly be advantageous to two varieties or
incipient species, if they could be kept from blending, on the same
principle that, when man is selecting at the same time two varieties, it
is necessary that he should keep them separate.
“In considering the probability of natural selection having come into
action, in rendering species mutually sterile, the greatest difficulty
will be found to lie in the existence of many graduated steps from
slightly lessened fertility to absolute sterility. It may be admitted
that it would profit an incipient species, if it were rendered in some
slight degree sterile when crossed with its parent form or with some
other variety; for thus fewer bastardized and deteriorated offspring
would be produced to commingle their blood with the new species in
process of formation. But he who will take the trouble to reflect on the
steps by which this first degree of sterility could be increased through
natural selection to that high degree which is common with so many
species, and which is universal with species which have been
differentiated to a generic or family rank, will find the subject
extraordinarily complex. After mature reflection it seems to me that
this could not have been effected through natural selection. Take the
case of any two species which, when crossed, produced few and sterile
offspring; now, what is there which could favor the survival of those
individuals which happened to be endowed in a slightly higher degree
with mutual infertility, and which thus approached by one small step
toward absolute sterility? Yet an advance of this kind, if the theory of
natural selection be brought to bear, must have incessantly occurred
with many species, for a multitude are mutually quite barren.”
Darwin points out the interesting parallel existing between the results
of intercrossing, and those of grafting together parts of different
species.
“As the capacity of one plant to be grafted or budded on another is
unimportant for their welfare in a state of nature, I presume that no
one will suppose that this capacity is a _specially_ endowed quality,
but will admit that it is incidental on differences in the laws of
growth of the two plants. We can sometimes see the reason why one tree
will not take on another, from differences in their rate of growth, in
the hardness of their wood, in the period of the flow or nature of their
sap, etc.; but in a multitude of cases we can assign no reason whatever.
Great diversity in the size of two plants, one being woody and the other
herbaceous, one being evergreen and the other deciduous, and adapted to
widely different climates, do not always prevent the two grafting
together. As in hybridization, so with grafting, the capacity is limited
by systematic affinity, for no one has been able to graft together trees
belonging to quite distinct families; and, on the other hand, closely
allied species, and varieties of the same species, can usually, but not
invariably, be grafted with ease. But this capacity, as in
hybridization, is by no means absolutely governed by systematic
affinity. Although many distinct genera within the same family have been
grafted together, in other cases species of the same genus will not take
on each other. The pear can be grafted far more readily on the quince,
which is ranked as a distant genus, than on the apple, which is a member
of the same genus. Even different varieties of the pear take with
different degrees of facility on the quince; so do different varieties
of the apricot and peach on certain varieties of the plum.”
“We thus see, that although there is a clear and great difference
between the mere adhesion of grafted stocks, and the union of the male
and female elements in the act of reproduction, yet that there is a rude
degree of parallelism in the results of grafting and of crossing of
distinct species. And we must look at the curious and complex laws
governing the facility with which trees can be grafted on each other as
incidental on unknown differences in their vegetative systems, so I
believe that the still more complex laws governing the facility of first
crosses are incidental on unknown differences in their reproductive
systems.... The facts by no means seem to indicate that the greater or
lesser difficulty of either grafting or crossing various species has
been a special endowment; although in the case of crossing, the
difficulty is as important for the endurance and stability of specific
forms, as in the case of grafting it is unimportant for their welfare.”
Weismann’s Germinal Selection
We cannot do better, in bringing this long criticism of the Darwinian
theory to an end, than by considering the way in which Weismann has
attempted in his paper on “Germinal Selection” to solve one of the
“patent contradictions” of the selection theory. He calls attention, in
doing so, to what he regards as a vital weakness of the theory in the
form in which it was left by Darwin himself. Weismann says:—
“The basal idea of the essay—the existence of Germinal Selection—was
propounded by me some time since,[15] but it is here for the first time
fully set forth and tentatively shown to be the necessary complement of
the process of selection. Knowing this factor, we remove, it seems to
me, the patent contradiction of the assumption that the general fitness
of organisms, or the adaptations _necessary_ to their existence, are
produced by _accidental_ variations—a contradiction which formed a
serious stumbling-block to the theory of selection. Though still
assuming that the _primary_ variations are ‘accidental,’ I yet hope to
have demonstrated that an interior mechanism exists which compels them
to go on increasing in a definite direction, the moment selection
intervenes. _Definitely directed variation exists_, but not predestined
variation, running on independently of the life conditions of the
organism, as Nägeli, to mention the most extreme advocate of this
doctrine, has assumed; on the contrary, the variation is such as is
elicited and controlled by those conditions themselves, though
indirectly.”
Footnote 15:
_Neue Gedanken zur Vererbungsfrage, eine Antwort an Herbert Spencer_,
Jena, 1895.
“The real aim of the present essay is to rehabilitate the principle of
selection. If I should succeed in reinstating this principle in its
emperilled rights, it would be a source of extreme satisfaction to me;
for I am so thoroughly convinced of its indispensability as to believe
that its demolition would be synonymous with the renunciation of all
inquiry concerning the causal relation of vital phenomena. If we could
understand the adaptations of nature, whose number is infinite, only
upon the assumption of a teleological principle, then, I think, there
would be little inducement to trouble ourselves about the causal
connection of the stages of ontogenesis, for no good reason would exist
for excluding teleological principles from this field. Their
introduction, however, is the ruin of science.”[16]
Footnote 16:
Translated by J. McCormack. The Open Court Publishing Company. The
following quotations are also taken from this translation.
Weismann states that those critics who maintain that selection cannot
create, but only reject, “fail to see that precisely through this
rejection its creative efficacy is asserted.” There is raised here,
though not for the first time, a point that is of no small importance
for both Darwinians and anti-Darwinians to consider; for, without
further examination, it is by no means self-evident, as Weismann
implies, that by exterminating all variations that are below the average
the standard of successive generations could ever be raised beyond the
most extreme fluctuating variation. At least this appears to be the case
if individual, fluctuating variations be the sort selected, and it is to
this kind of variation to which Weismann presumably refers. Without
discussing this point here, let us examine further what Weismann has to
say. He thinks that while in each form there may be a very large number
of possible variations, yet there are also impossible variations as
well, which do not appear. “The cogency, the irresistible cogency as I
take it, of the principle of selection is precisely its capacity of
explaining why fit structures always arise, and this certainly is the
great problem of life.” Weismann points out that it is a remarkable fact
that to-day, after science has been in possession of this principle for
something over thirty years, “during which time she has busily occupied
herself with its scope, the estimation in which the theory is held
should be on the decline.” “It would be easy to enumerate a long list of
living writers who assign to it a subordinate part only in evolution, or
none at all.” “Even Huxley implicitly, yet distinctly, intimated a doubt
regarding the principle of selection when he said: ‘Even if the
Darwinian hypothesis were swept away, evolution would still stand where
it is.’ Therefore he, too, regarded it as not impossible that this
hypothesis should disappear from among the great explanatory principles
by which we seek to approach nearer to the secrets of nature.”
Weismann is not, however, of this opinion, and believes that the present
depression is only transient, because it is only a reaction against a
theory that had been exalted to the highest pinnacle. He thinks that the
principle of selection is not overestimated, but that naturalists
imagined too quickly that they understood its workings. “On the
contrary, the deeper they penetrated into its workings the clearer it
appeared that something was lacking, that the action of the principle,
though upon the whole clear and representable, yet when carefully looked
into encountered numerous difficulties, which were formidable, for the
reason that we were unsuccessful in tracing out the actual details of
the individual process, and, therefore, in _fixing_ the phenomenon as it
actually occurred. We can state in no single case how great a variation
must be to have selective value, nor how frequently it must occur to
acquire stability. We do not know when and whether a desired useful
variation really occurs, nor on what its appearance depends; and we have
no means of ascertaining the space of time required for the fulfilment
of the selective processes of nature, and hence cannot calculate the
exact number of such processes that do and can take place at the same
time in the same species. Yet all this is necessary if we wish to follow
out the precise details of a given case.
“But perhaps the most discouraging circumstance of all is, that we can
assert in scarcely a single actual instance in nature whether an
observed variation is useful or not—a drawback that I distinctly
emphasized some time ago. Nor is there much hope of betterment in this
respect, for think how impossible it would be for us to observe all the
individuals of a species in all their acts of life, be their habitat
ever so limited—and to observe all this with a precision enabling us to
say that this or that variation possessed selective value, that is, was
a decisive factor in determining the existence of the species.”
“And thus it is everywhere. Even in the most indubitable cases of
adaptation as, for instance, in that of the striking protective coloring
of many butterflies, the sole ground of inference that the species on
the whole is adequately adapted to its conditions of life, is the simple
fact that the species is, to all appearances, preserved undiminished,
but the inference is not at all permissible that just this protective
coloring has selective value for the species, that is, if it were
lacking, the species would necessarily have perished.”
Few opponents of Darwinism could give a more pessimistic account of the
accomplishments of the theory of natural selection than this, by one of
the leaders of the modern school: “Discouraging, therefore, as it may be
that the control of nature in her minutest details is here gainsaid us,
yet it were equivalent to sacrificing the gold to the dross, if simply
from our inability to follow out the details of the individual case we
should renounce altogether the principle of selection, or should
proclaim it as only subsidiary, on the ground that we believe the
protective coloring of the butterfly is not a protective coloring, but a
combination of colors inevitably resulting from internal causes. The
protective coloring remains a protective coloring whether at the time in
question it is or is not necessary for the species; and it arose as
protective coloring—arose not because it was a constitutional necessity
of the animal’s organism that here a red and there a white, black, or
yellow spot should be produced, but because it was advantageous, because
it was necessary for the animal. There is only one explanation possible
for such patent adaptations, and that is selection. What is more, no
other natural way of their originating is conceivable, for we have no
right to assume teleological forces in the domain of natural phenomena.”
Weismann states that he does not accept Eimers’s view that the markings
of the wings of the butterflies of the genus Papilio are due to a
process of evolution in a direct line, independent of external causes.
“On the contrary, I believe it can be clearly proved that the wing of
the butterfly is a tablet on which Nature has inscribed everything she
has deemed advantageous to the preservation and welfare of her
creatures, and nothing else; or, to abandon the simile, that these color
patterns have not proceeded from inward evolutional forces but are the
result of selection. At least in all places where we do understand their
biological significance these patterns are constituted and distributed
over the wing exactly as utility would require.”
Again: “I should be far from maintaining that the markings arose
unconformably to law. Here, as elsewhere, the dominance of law is
certain. But I take it, that the laws involved, that is, the
physiological conditions of the variation, here are without exception
subservient to the ends of a higher power—utility; and that it is
utility primarily that determines the kind of colors, spots, streaks,
and bands that shall originate, as also their place and mode of
disposition. The laws come into consideration only to the extent of
conditioning the quality of the constructive materials—the variations,
out of which selection fashions the designs in question. And this also
is subject to important restrictions, as will appear in the sequel.”
This conclusion contains all that the most ardent Darwinian could ask.
He rejects the idea that internal laws alone could have produced the
result, because:—
“If internal laws controlled the markings on butterflies’ wings, we
should expect that some general rule could be established, requiring
that the upper and under surfaces of the wings should be alike or that
they should be different, or that the fore wings should be colored the
same as or differently from the hind wings, etc. But in reality all
possible kinds of combinations occur simultaneously, and no rule holds
throughout. Or, it might be supposed that bright colors should occur
only on the upper surface or only on the under surface, or on the fore
wings or only on the hind wings. But the fact is they occur
indiscriminately, now here, now there, and no one method of appearance
is uniform throughout all the species. But the fitness of the various
distributions of colors is apparent, and the moment we apply the
principle of utility we know why in the diurnal butterflies the upper
surface alone is usually variegated and the under surface protectively
colored, or why in the nocturnal butterflies the fore wings have the
appearance of bark, of old wood, or of a leaf, whilst the hind wings,
which are covered when resting, alone are brilliantly colored. On this
theory we also understand the exceptions to these rules. We comprehend
why Danaids, Heliconids, Euploids, and Acracids, in fact all diurnal
butterflies offensive to the taste and smell, are mostly brightly marked
and equally so on both surfaces, whilst all species not thus exempt from
persecution have the protective coloring on the under surface and are
frequently quite differently colored there from what they are on the
upper.
“In any event, the supposed formative laws are not obligatory.
Dispensations from them can be issued and are issued _whenever utility
requires it_.”
Dispensations from the laws of growth! Does not a philosophy of this
sort seem to carry us back into the dark ages? Is this the best that the
Darwinian school can do to protect itself against the difficulties into
which its chief disciple confesses it has fallen?
Weismann lays great emphasis on the case of the Indian leaf-butterfly,
_Kallima inachis_; and points out that the leaf markings are executed
“in absolute independence of the other uniformities governing the wing.”
“The venation of the wing is utterly ignored by the leaf markings, and
its surface is treated as a _tabula rasa_ upon which anything
conceivable can be drawn. In other words, we are presented here with a
_bilaterally symmetrical_ figure engraved on a surface which is
essentially _radially symmetrical_ in its divisions.
“I lay unusual stress upon this point because it shows that we are
dealing here with one of those cases which cannot be explained by
mechanical, that is, by natural means, unless natural selection actually
exists and is actually competent to create new properties; for the
Lamarckian principle is excluded here _ab initio_, seeing that we are
dealing with a formation which is only passive in its effects: the leaf
markings are effectual simply by their existence and not by any function
which they perform; they are present in flight as well as at rest,
during the absence of a danger, as well as during the approach of an
enemy.
“Nor are we helped here by the assumption of _purely internal motive
forces_, which Nägeli, Askenasy, and others have put forward as
supplying a _mechanical_ force of evolution. It is impossible to regard
the coincidence of an Indian butterfly with the leaf of a tree now
growing in an Indian forest as fortuitous, as a _lusus naturæ_. Assuming
this seemingly mechanical force, therefore, we should be led back
inevitably to a teleological principle which produces adaptive
characters and which must have deposited the directive principle in the
very first germ of terrestrial organisms, so that after untold ages at a
definite time and place the illusive leaf markings should be developed.
The assumption of preëstablished harmony between the evolution of the
ancestral line of the tree with its prefigurative leaf, and that of the
butterfly with its imitating wing, is absolutely necessary here, as I
pointed out many years ago, but as is constantly forgotten by the
promulgators of the theory of internal evolutionary forces.”
Weismann concludes, therefore, that for his present purpose it suffices
to show “that cases exist wherein all natural explanations except that
of selection fail us,” and he then proceeds to point out that even the
natural selection of Darwin and of Wallace also fail to give us a
reasonable explanation of how, for example, the markings on the wings of
the Kallima butterfly have come about. The main reason that he gives to
show that this is the case rests on the difficulty of the assumption
that the right variations should always be present in the right place.
Here “is the insurmountable barrier for the explanatory power of the
principle [natural selection] for who, or what, is to be our guarantee
that the dark scales shall appear at the exact spots on the wing where
the midrib of the leaf must grow? And that later dark scales shall
appear at the exact spots to which the midrib must be prolonged? And
that still later such dark scales shall appear at the places whence the
lateral ribs start, and that here also a definite acute angle shall be
preserved.” Thus the philosopher in his closet multiplies and magnifies
the difficulties for which he is about to offer a panacea. Had the same
amount of labor been spent in testing whether the life of this butterfly
is so closely dependent on the exact imitation of the leaf, we might
have been spared the pains of this elaborate exordium. There are at
least some grounds for suspicion that the whole case of Kallima is “made
up.” If this should prove true, it will be a bad day for the Darwinians,
unless they fall back on Weismann’s statement that their theory is
insufficient to prove a single case!
Weismann has used Kallima only as the most instructive illustration. The
objections that are here evident are found not only in the cases of
protective coloration, but “are applicable in all cases where the
process of selection is concerned. Take, for example, the case of
instincts that are called into action only once in life, as the pupal
performances of insects, the fabrication of cocoons, etc. How is it that
the useful variations were always present here?” Weismann concludes that
“something is still wanting to the selection theory of Darwin and
Wallace, which it is obligatory on us to discover, if we possibly can,
and without which selection as yet offers no complete explanation of the
phyletic processes of transformation.” Weismann’s first step in the
solution of the difficulty is contained in the following statement:—
“My inference is a very simple one: if we are forced by the facts on all
hands to the assumption that the useful variations which render
selection possible are always present, then, _some profound connection
must exist between the utility of a variation and its actual
appearance_, or, in other words, _the direction of the variation of a
part must be determined by utility_, and we shall have to see whether
facts exist that confirm our conjecture.”
Weismann finds the solution in the method by which the breeder has
obtained his results in artificial selection. For instance, the
long-tailed variety of the domestic cock of Japan owes its existence, it
is claimed, to skilful selection, and not at all to the circumstance
that, at some period of the race’s history, a cock with tail-feathers
six feet in length suddenly and spasmodically appeared.
Weismann continues: “Now what does this mean? Simply that the hereditary
diathesis, the germinal constitution (the _Anlage_) of the breed was
changed in the respect in question, and our conclusion from this and
numerous similar facts of artificial selection runs as follows: _by the
selection alone of the plus or minus variations of a character is the
constant modification of that character in the plus or minus direction
determined_. Obviously the hereditary _diminution_ of a part is also
effected by the simple selection of the individuals in each generation
possessing the smallest parts, as is proved, for example, by the tiny
bills and feet of numerous breeds of doves. We may assert, therefore, in
general terms: a definitely directed progressive variation of a given
part is produced by continued selection in that definite direction. This
is no hypothesis, but a direct inference from the facts and may also be
expressed as follows: _by a selection of the kind referred to the germ
is progressively modified in a manner corresponding with the production
of a definitely directed progressive variation of the part_.”
So far there is nothing essentially new offered, since Darwin often
tacitly recognized that the standard of variation could be raised in
this way, and in some places he has made definite statements that this
will take place. Weismann thinks that after each selection, fluctuation
will then occur around a higher average (mode). He says “that this is a
fact,” and is proved by the case of the Japanese cock. It need scarcely
be pointed out that it is an assumption, based on what is supposed to
have taken place in this bird, and is not a “fact.”
Weismann continues: “But the question remains, _why_ is this the fact?”
He believes his hypothesis of the existence of determinants in the germ
gives a satisfactory answer to this “why.” “According to this theory
every independent and hereditarily variable part is represented in the
germ by a _determinant_, whose size and power of assimilation
corresponds to the size and vigor of the part. These determinants
multiply as do all vital units by growth and division, and necessarily
they increase rapidly in every individual, and the more rapidly the
greater the quantity of the germinal cells the individual produces. And
since there is no more reason for excluding irregularities of passive
nutrition, and of the supply of nutriment in these minute,
microscopically invisible parts, than there is in the larger visible
parts of the cells, tissues, and organs, consequently the descendants of
a determinant can never all be exactly alike in size and capacity of
assimilation, but they will oscillate in this respect to and fro about
the maternal determinant as about their zero point, and will be partly
greater, partly smaller, and partly of the same size as that. In these
oscillations, now, the material for further selection is presented, and
in the inevitable fluctuations of the nutrient supply, I see the reason
why every step attained immediately becomes the zero point of new
fluctuations, and consequently why the size of a part can be augmented
or diminished by selection without limit, solely by the displacement of
the zero point of variation as the result of selection.”
The best illustration of this process of germinal selection is found,
Weismann believes, in the case of the degeneration of organs. “For in
most retrogressive processes _active_ selection in Darwin’s sense plays
no part, and advocates of the Lamarckian principle, as above remarked,
have rightly denied that active selection, that is, the selection of
individuals possessing the useless organ in its most reduced state, is
sufficient to explain the process of degeneration. I, for my part, have
never assumed this, and have on this very account enunciated the
_principle of panmixia_. Now, although this, as I have still no reason
for doubting, is a perfectly correct principle, which really does have
an essential and indispensable share in the process of retrogression,
still it is not _alone_ sufficient for a full explanation of the
phenomena. My opponents, in advancing this objection, were right, to the
extent indicated, and as I expressly acknowledge, although they were
unable to substitute anything positive in its stead or to render my
explanation complete. The very fact of the cessation of control over the
organ is sufficient to explain its _degeneration_, that is, its
deterioration, the disharmony of its parts, but not the fact which
actually and always occurs where an organ has become useless—viz., _its
gradual and unceasing diminution continuing for thousands and thousands
of years and culminating in its final and absolute effacement_.”
If then neither selection of persons nor the cessation of personal
selection can explain the phenomenon, we must look elsewhere for the
answer. This Weismann finds in the application of Roux’s hypothesis of
the struggle of the parts to obtain nourishment.
“The production of the long tail-feathers of the Japanese cock does not
repose solely on the displacement directly effected by personal
selection, of the zero point of variation upward, but that _it is also
fostered and strengthened by germinal selection_. Were that not so, the
phenomena of the transmutation of species, in so far as fresh growth and
the enlargement and complication of organs already present are
concerned, _would not be a whit more intelligible than they were
before_.”
Thus Weismann has piled up one hypothesis on another as though he could
save the integrity of the theory of natural selection by adding new
speculative matter to it. The most unfortunate feature is that the new
speculation is skilfully removed from the field of verification, and
invisible germs whose sole functions are those which Weismann’s
imagination bestows on them, are brought forward as though they could
supply the deficiencies of Darwin’s theory. This is, indeed, the old
method of the philosophizers of nature. An imaginary system has been
invented which attempts to explain all difficulties, and if it fails,
then new inventions are to be thought of. Thus we see where the theory
of the selection of fluctuating germs has led one of the most widely
known disciples of the Darwinian theory.
The worst feature of the situation is not so much that Weismann has
advanced new hypotheses unsupported by experimental evidence, but that
the speculation is of such a kind that it is, from its very nature,
unverifiable, and therefore useless. Weismann is mistaken when he
assumes that many zoologists object to his methods because they are
largely speculative. The real reason is that the speculation is so often
of a kind that cannot be tested by observation or by experiment.
------------------------------------------------------------------------
CHAPTER VI
DARWIN’S THEORY OF SEXUAL SELECTION
Sexual Selection
The theory of sexual selection was formulated by Darwin, even in the
first edition of the “Origin of Species,” but was developed at much
greater length in “The Descent of Man.” “This form of selection depends,
not on a struggle for existence in relation to other organic beings or
to external conditions, but on a struggle between the individuals of one
sex, generally the males, for the possession of the other sex. The
result is not death to the unsuccessful competitor, but few or no
offspring. Sexual selection is, therefore, less rigorous than natural
selection. Generally the most vigorous males, those which are best
fitted for their place in nature, will leave most progeny. But in many
cases victory depends, not so much on general vigor, as on having
special weapons, confined to the male sex. A hornless stag or spurless
cock would have a poor chance of leaving numerous offspring. Sexual
selection, by always allowing the victor to breed, might surely give
indomitable courage, length to the spur, and strength to the wing to
strike in the spurred leg in nearly the same manner as the brutal
cock-fighter by the careful selection of his best cocks.” It is
important to notice that the theory of sexual selection is admittedly an
extension of the selection principle into a new field. Having accounted
for domesticated animals and plants by artificial selection, and for the
adaptations of wild species by natural selection, there remained only to
account for the secondary sexual differences between the sexes by the
principle of sexual selection.
There are two ways in which Darwin supposes sexual selection to act: (1)
through competition of the individuals of the same sex with each
other,—the strongest or best-equipped for fighting or for finding the
individuals of the other sex gaining an advantage; (2) through selection
by the individuals of one sex of certain preferred individuals of the
other sex.
The first category is natural selection applied to the members of one
sex in competition with each other, although the result does not lead to
the death of the unsuccessful individual, but excludes it from leaving
progeny. In the second category a new element is introduced, namely, the
_selective power_ of the individuals of one sex, usually the female. It
is this part that adds a distinctly new element to Darwin’s other two
theories of selection, and it is this part that we naturally think of as
the theory of sexual selection _par excellence_. Darwin makes, however,
no sharp distinction between these two sides of his theory, but includes
both under the heading of sexual selection.
In order to get the theory itself before us in as concrete form as
possible, let us examine some of the cases that Darwin has given to show
how he supposes the process to be carried out.
“There are many other structures and instincts which must have been
developed through sexual selection—such as the weapons of offence and
the means of defence of the males for fighting with and driving away
their rivals—their courage and pugnacity—their various ornaments—their
contrivances for producing vocal or instrumental music—and their glands
for emitting odors, most of these latter structures serving only to
allure or excite the female. It is clear that these characters are the
result of sexual and not of ordinary selection, since unarmed,
unornamented, or unattractive males would succeed equally well in the
battle for life and in leaving a numerous progeny, but for the presence
of better-endowed males. We may infer that this would be the case,
because the females, which are unarmed and unornamented, are able to
survive and procreate their kind. Secondary sexual characters of the
kind just referred to will be fully discussed in the following chapters,
as being in many respects interesting, but especially as depending on
the will, choice, and rivalry of the individuals of either sex. When we
behold two males fighting for the possession of the female, or several
male birds displaying their gorgeous plumage, and performing strange
antics before an assembled body of females, we cannot doubt that, though
led by instinct, they know what they are about, and consciously exert
their mental and bodily powers.”
This general statement gives an idea of the class of phenomena that
Darwin proposes to explain by the theory of sexual selection. The close
resemblance between this process and that of artificial selection may be
gathered from the following statement:—
“Just as man can improve the breed of his game-cocks by the selection of
those birds which are victorious in the cockpit, so it appears that the
strongest and most vigorous males, or those provided with the best
weapons, have prevailed under nature, and have led to the improvement of
the natural breed or species. A slight degree of variability leading to
some advantage, however slight, in reiterated deadly contests would
suffice for the work of sexual selection; and it is certain that
secondary sexual characters are eminently variable. Just as man can give
beauty, according to his standard of taste, to his male poultry, or more
strictly can modify the beauty originally acquired by the parent
species, can give to the Sebright bantam a new and elegant plumage, an
erect and peculiar carriage—so it appears that female birds in a state
of nature have, by a long selection of the more attractive males, added
to their beauty or other attractive qualities. No doubt this implies
powers of discrimination and taste on the part of the female which will
at first appear extremely improbable; but by the facts to be adduced
hereafter, I hope to be able to show that the females actually have
these powers. When, however, it is said that the lower animals have a
sense of beauty, it must not be supposed that such sense is comparable
with that of a cultivated man, with his multiform and complex associated
ideas. A more just comparison would be between the taste for the
beautiful in animals, and that in the lowest savages, who admire and
deck themselves with any brilliant, glittering, or curious object.”
Darwin did not close his eyes to the difficulties which the theory had
to contend against. One of the most formidable of these objections is
described in the following words: “Our difficulty in regard to sexual
selection lies in understanding how it is that the males which conquer
other males, or those which prove the most attractive to the females,
leave a greater number of offspring to inherit their superiority than
their beaten and less attractive rivals. Unless this result does follow,
the characters which give to certain males an advantage over others
could not be perfected and augmented through sexual selection. When the
sexes exist in exactly equal numbers, the worst-endowed males will
(except where polygamy prevails) ultimately find females, and leave as
many offspring, as well fitted for their general habits of life, as the
best-endowed males. From various facts and considerations, I formerly
inferred that with most animals, in which secondary sexual characters
are well developed, the males considerably exceeded the females in
number; but this is not by any means always true. If the males were to
the females as two to one, or as three to two, or even in a somewhat
lower ratio, the whole affair would be simple; for the better-armed or
more attractive males would leave the largest number of offspring. But
after investigating, as far as possible, the numerical proportion of the
sexes, I do not believe that any great inequality in number commonly
exists. In most cases sexual selection appears to have been effective in
the following manner.
“Let us take any species, a bird for instance, and divide the females
inhabiting a district into two equal bodies, the one consisting of the
more vigorous and better-nourished individuals, and the other of the
less vigorous and healthy. The former, there can be little doubt, would
be ready to breed in the spring before the others; and this is the
opinion of Mr. Jenner Weir, who has carefully attended to the habits of
birds during many years. There can also be no doubt that the most
vigorous, best-nourished and earliest breeders would on an average
succeed in rearing the largest number of fine offspring. The males, as
we have seen, are generally ready to breed before the females; the
strongest, and with some species the best-armed of the males, drive away
the weaker; and the former would then unite with the more vigorous and
better-nourished females, because they are the first to breed. Such
vigorous pairs would surely rear a larger number of offspring than the
retarded females, which would be compelled to unite with the conquered
and less powerful males, supposing the sexes to be numerically equal;
and this is all that is wanted to add, in the course of successive
generations, to the size, strength and courage of the males, or to
improve their weapons.”
I shall comment later on the points here raised, but we should not let
this opportunity pass without noticing, that even if the pairing were to
follow according to the method here imagined, still the argument breaks
down at the critical point, for there is no evidence that the more
precocious females would rear a larger number of offspring than the more
normal females, or even those that breed somewhat later.
The greater eagerness of the males which has been observed in so many
different classes of animals is accounted for as follows:—
“But it is difficult to understand why the males of species, of which
the progenitors were primordially free, should invariably have acquired
the habit of approaching the females, instead of being approached by
them. But in all cases, in order that the males should seek efficiently,
it would be necessary that they should be endowed with strong passions;
and the acquirement of such passions would naturally follow from the
more eager leaving a larger number of offspring than the less eager.”
Thus we are led to the rather complex conclusion, that the more eager
males will leave more descendants, and those that are better endowed
with ornaments will be the ones selected. But unless it can be shown
that there is some connection between greater eagerness and better
ornamentation, it might often occur that the less ornamented were the
more eager individuals, in which case there would be an apparent
conflict between the two acquirements.
After giving some cases of the greater variability of the males, in
respect to characters that are not connected with sexual selection, and
presumably not the result of any kind of selection, Darwin concludes:
“Through the action of sexual and natural selection male animals have
been rendered in very many instances widely different from their
females; but independently of selection the two sexes, from differing
constitutionally, tend to vary in a somewhat different manner. The
female has to expend much organic matter in the formation of her ova,
whereas the male expends much force in fierce contests with his rivals,
in wandering about in search of the female, in exerting his voice,
pouring out odoriferous secretions, etc.: and this expenditure is
generally concentrated within a short period. The great vigor of the
male during the season of love seems often to intensify his colors,
independently of any marked difference from the female. In mankind, and
even as low down in the organic scale as in the Lepidoptera, the
temperature of the body is higher in the male than in the female,
accompanied in the case of man by a slower pulse. On the whole, the
expenditure of matter and force by the two sexes is probably nearly
equal, though effected in very different ways and at different rates.”
Again: “From the causes just specified, the two sexes can hardly fail to
differ somewhat in constitution, at least during the breeding season;
and although they may be subjected to exactly the same conditions, they
will tend to vary in a different manner. If such variations are of no
service to either sex, they will not be accumulated and increased by
sexual or natural selection. Nevertheless, they may become permanent if
the exciting cause acts permanently; and in accordance with a frequent
form of inheritance they may be transmitted to that sex alone in which
they first appeared. In this case, the two sexes will come to present
permanent, yet unimportant, differences of character. For instance, Mr.
Allen shows that with a large number of birds inhabiting the northern
and southern United States, the specimens from the south are
darker-colored than those from the north; and this seems to be the
direct result of the difference in temperature, light, etc., between the
two regions. Now, in some few cases, the two sexes of the same species
appear to have been differently affected; in the _Agelæus phœniceus_ the
males have had their colors greatly intensified in the south; whereas
with _Cardinalis virginianus_ it is the females which have been thus
affected: with _Quiscalus major_ the females have been rendered
extremely variable in tint, whilst the males remain nearly uniform.”
The admissions contained in this statement would seem to jeopardize the
entire question, for, if it is admitted that, on account of the
difference in the constitution of the two sexes, the influence of the
surrounding conditions would produce a different effect on them, it
would seem that there is no need whatsoever for the theory of sexual
selection. What Darwin is probably attempting to show is that the
material for the further action of sexual selection is already given;
but the question may well be asked, if the external conditions have done
so much, why may they not have gone farther and produced the entire
result?
Darwin makes the following suggestion to account for those cases in
which the female is the more highly colored:—
“A few exceptional cases occur in various classes of animals, in which
the females instead of the males have acquired well-pronounced secondary
sexual characters, such as brighter colors, greater size, strength, or
pugnacity. With birds there has sometimes been a complete transposition
of the ordinary characters proper to each sex; the females having become
the more eager in courtship, the males remaining comparatively passive,
but apparently selecting the more attractive females, as we may infer
from the results. Certain hen birds have thus been rendered more highly
colored or otherwise ornamented, as well as more powerful and pugnacious
than the cocks; these characters being transmitted to the female
offspring alone.”
Then follows immediately the discussion as to whether a double process
of sexual selection may not be supposed to go on at the same time. “It
may be suggested that in some cases a double process of selection has
been carried on; that the males have selected the more attractive
females, and the latter the more attractive males. This process,
however, though it might lead to the modification of both sexes, would
not make the one sex different from the other, unless indeed their
tastes for the beautiful differed; but this is a supposition too
improbable to be worth considering in the case of any animal, excepting
man. There are, however, many animals in which the sexes resemble each
other, both being furnished with the same ornaments, which analogy would
lead us to attribute to the agency of sexual selection. In such cases it
may be suggested with more plausibility, that there has been a double or
mutual process of sexual selection; the more vigorous and precocious
females selecting the more attractive and vigorous males, the latter
rejecting all except the more attractive females. But from what we know
of the habits of animals, this view is hardly probable, for the male is
generally eager to pair with any female. It is more probable that the
ornaments common to both sexes were acquired by one sex, generally the
male, and then transmitted to the offspring of both sexes. If, indeed,
during a lengthened period the males of any species were greatly to
exceed the females in number, and then during another lengthened period,
but under different conditions, the reverse were to occur, a double but
not simultaneous process of sexual selection might easily be carried on,
by which the two sexes might be rendered widely different.”
The improbability of such a process is so manifest that the suggestion
can scarcely be looked upon as anything more than pure speculation. We
shall have occasion later to return to the same subject, and point out
its bearing more explicitly.
Nearly the whole animal kingdom is passed in review by Darwin from the
point of view of the sexual selection theory. There is brought together
a large number of extremely interesting facts, and if the theory did no
more than merely hold them together, it has served, in this respect, a
useful end. We may select some of the most instructive cases by way of
illustrating the theory.
In many of the lower animals in which the sexes are separated, and these
alone, of course, can be supposed to come within the range of the
theory, there are no striking differences between the sexes, in regard
to ornamentation, although in other respects differences may exist.
“Moreover it is almost certain that these animals have too imperfect
senses and much too low mental powers, to appreciate each other’s beauty
or other attractions, or to feel rivalry.
“Hence in these classes or subkingdoms, such as the Protozoa,
Cœlenterata, Echinodermata, Scolecida, secondary sexual characters, of
the kind which we have to consider, do not occur; and this fact agrees
with the belief that such characters in the higher classes have been
acquired through sexual selection, which depends on the will, desire,
and choice of either sex.”
There are some cases, however, in which animals low in the scale show a
difference in the ornamentation of the two sexes. A few cases have been
recorded in the roundworms, where different shades of the same tint
distinguish the sexes. In the annelids the sexes are sometimes so
different, that, as Darwin remarks, they have been placed in different
genera and even families, “yet the differences do not seem to be of the
kind which can be safely attributed to sexual selection.” In regard to
the nemertian worms, although they “vie in variety and beauty of
coloring with any other group in the invertebrate series,” yet McIntosh
states that he “cannot discover that these colors are of any service.”
In the copepods, belonging to the group of lower Crustacea, Darwin
excludes those cases in which the males alone “are furnished with
perfect swimming legs, antennæ, and sense-organs; the females being
destitute of these organs, with their bodies often consisting of a mere
distorted mass,” because these extraordinary differences between the two
sexes are no doubt related to their widely different habits of life.
Nevertheless, it is important to observe that such extreme differences
may exist in cases where sexual selection cannot come in, because of the
absence of eyes in the female.
In regard to another copepod, Saphirina, Darwin points out that the
males are furnished with minute scales, which exhibit beautiful changing
colors, and these are absent in the females; yet he states that it would
be extremely rash to conclude that these curious organs serve to attract
the females. Differences in the sexes are also found in one species of
Squilla, and a species of Gelasimus. In the latter case Darwin thinks
that the difference is probably due to sexual selection. In addition to
these cases, recorded by Darwin, there may be added the two remarkable
cases, shown in our Figure 2 A, B, of _Calocalanus pavo_, the female of
which has a gorgeous tail worthy of a peacock, and of _Calocalanus
plumulosus_, in which one of the setæ of the tail is drawn out into a
long featherlike structure. In the former, the male is much more
modestly adorned, as shown in Figure 2 C; in the latter species the male
is unknown.
[Illustration:
Fig. 2.—A male of the copepod, _Calocalanus plumulosus_.
B and C, a male and a female of _Calocalanus pavo_. (After
Giesbrecht.)]
In spiders, where as a rule the sexes do not differ much from each other
in color, the males are often of a darker shade than the females. “In
some species, however, the difference is conspicuous; thus the female of
_Sparassus smaragdulus_ is dullish green, whilst the adult male has the
abdomen of a fine yellow with three longitudinal stripes of rich red.”
Darwin believes that sexual selection must take place in this group,
because Canestrini has observed that the males fight for the possession
of the females. He has also stated that the males pay court to the
female, and that she rejects some of the males who court her, and
sometimes devours them, until finally one is chosen. Darwin believed, on
this evidence, that the difference in color of the sexes had been
acquired by sexual selection, “though we have here not the best kind of
evidence—the display by the male of his ornaments.” This evidence has,
however, now been supplied through the interesting observations of Mr.
and Mrs. Peckham. These accurate observers have studied the courtship of
the male, and observed that during the process, he twists and turns his
body in such a way as to show to best advantage his colors to the
female. From their account this certainly appears to be the result of
his movements, but whether this is really the case, and whether the
female makes any choice amongst her suitors, according to whether they
are more or less brilliantly marked, we are absolutely ignorant. The
following account given by Darwin should not pass unnoticed:—
“The male is generally much smaller than the female, sometimes to an
extraordinary degree, and he is forced to be extremely cautious in
making his advances, as the female often carries her coyness to a
dangerous pitch. De Geer saw a male that ‘in the midst of his
preparatory caresses was seized by the object of his attentions,
enveloped by her in a web and then devoured, a sight which, as he adds,
filled him with horror and indignation.’ The Rev. O. P. Cambridge
accounts in the following manner for the extreme smallness of the male
in the genus Nephila. ‘M. Vinson gives a graphic account of the agile
way in which the diminutive male escapes from the ferocity of the
female, by gliding about and playing hide and seek over her body and
along her gigantic limbs: in such a pursuit it is evident that the
chances of escape would be in favor of the smallest males, while the
larger ones would fall early victims; thus gradually a diminutive race
of males would be selected, until at last they would dwindle to the
smallest possible size compatible with the exercise of their generative
functions,—in fact probably to the size we now see them, _i.e._ so small
as to be a sort of parasite upon the female, and either beneath her
notice, or too agile and too small for her to catch without great
difficulty.’”
It is certainly surprising to find Darwin ascribing even this difference
in size between the sexes to the action of selection. Is it not a little
ludicrous to suppose that the females have reduced the males to a size
too small for them to catch?
There are many cases known in the animal kingdom where there is a
difference in size between the two sexes, especially in the group of
insects; but I doubt very much if they are to be accounted for as the
result of sexual selection. In some of these cases Darwin accounts for
the larger size of the female, on account of the large number of eggs
which she has to carry. In other insects where the male is larger, as in
the stag-beetle, the size is ascribed to the conflicts of the males,
leading to the survival of the larger individuals. In still other cases,
where the males are larger, but do not fight, an explanation is
admittedly wanting; but it is suggested that here there would be no
necessity for the males to be smaller than the females in order to
mature before them (as is supposed to happen in other species), for in
these cases the individuals are not short-lived, and there would be
ample time for pairing. Again, although the males of nearly all bees are
smaller than the females, yet the reverse is true in those forms in
which the females are fertilized during the marriage flight. The
explanation offered is that in these forms the male carries the female,
and this is assumed to require greater size on his part. This loose way
of guessing, as to a possible explanation, is characteristic of the
whole hypothesis of sexual selection. First one, and then another, guess
is made as to the causes of the differences between the sexes. It is not
shown in a single one of the instances that the postulated cause has
really had anything to do with the differences in question; and the
attempt to show that the theory is probable, by pointing out the large
number of cases which it appears to account for, is weakened to a very
great degree by the number of exceptional cases, for which an equally
ready explanation of a different kind is forthcoming. This way of giving
loose rein to the imagination has been the bane of the method that has
followed hard on the track of Darwin’s hypothesis, and for which his
example has been in no small measure responsible. Thus, in the case just
quoted, there are no less than four distinct conjectures made to account
for the differences in size between the sexes, and each guess involves
an entirely different set of processes. Considering the complicated
relation of the life of organisms, it may be doubted if any of the
imagined processes could bring about the result, and certainly not a
single one has been shown to be a real, or a sufficient, cause in the
evolutionary process. Neither the actuality of the postulated causes,
nor their application to a particular case, has been shown to exist.
In the Diptera, or flies, Wallace records one interesting case of sexual
difference in the genus Elaphomyia of New Guinea, in which the males are
furnished with horns, which the females lack. Darwin writes:—
“The horns spring from beneath the eyes, and curiously resemble those of
a stag, being either branched or palmated. In one of the species, they
equal the whole body in length. They might be thought to be adapted for
fighting, but as in one species they are of a beautiful pink color,
edged with black, with a pale central stripe, and as these insects have
altogether a very elegant appearance, it is perhaps more probable that
they serve as ornaments.”
Presumably, therefore, Darwin means these colored horns have been
acquired by sexual selection.
In the Hemiptera, or bugs, both sexes of some species are “beautifully
colored,” and as the members of the group are often unpalatable to other
animals, the color in this case is supposed to act as a warning signal.
In other cases it is stated, however, that “a small pink and green
species” could hardly be distinguished from the buds on the trunks of
the lime trees which this insect frequents. In this case the color
appears “to be directly protective.” Thus without any means of forming a
correct judgment, the color of one animal is supposed to be the result
of natural selection, since it resembles its surroundings, but of sexual
selection if the color is present or more pronounced in one sex. Where
neither view can easily be applied, the color is ascribed in a general
way to the nature of the organism.
In respect to the group of Hymenoptera, or bees, Darwin records the
following cases:—
“In this order slight differences in color, according to sex, are
common, but conspicuous differences are rare except in the family of
bees; yet both sexes of certain groups are so brilliantly colored—for
instance in Chrysis, in which vermilion and metallic greens prevail—that
we are tempted to attribute the result to sexual selection. In the
Ichneumonidæ, according to Mr. Walsh, the males are almost universally
lighter-colored than the females. On the other hand, in the
Tenthredinidæ the males are generally darker than the females. In the
Siricidæ the sexes frequently differ; thus the male of _Sirex juvencus_
is banded with orange, whilst the female is dark purple; but it is
difficult to say which sex is the more ornamented.”
In other families of bees, differences in the color of the sexes have
been recorded, and since the males have been seen fighting for the
possession(?) of the females, and since bees are known to recognize
differences in color, Darwin believes that:—
“In some species the more beautiful males appear to have been selected
by the females; and in others the more beautiful females by the males.
Consequently in certain genera, the males of the several species differ
much in appearance, whilst the females are almost indistinguishable; in
other genera the reverse occurs. H. Müller believes that the colors
gained by one sex through sexual selection have often been transferred
in a variable degree to the other sex, just as the pollen-collecting
apparatus of the female has often been transferred to the male, to whom
it is absolutely useless.”
Although in beetles the sexes are generally colored alike, yet in some
of the longicorns there are exceptions to the rule. “Most of these
insects are large and splendidly colored. The males in the genus
Pyrodes, which I saw in Mr. Bates’s collection, are generally redder but
rather duller than the females, the latter being colored of a more or
less splendid golden-green. On the other hand, in one species the male
is golden-green, the female being richly tinted with red and purple. In
the genus Esmeralda the sexes differ so greatly in color that they have
been ranked as distinct species; in one species both are of a beautiful
shining green, but the male has a red thorax. On the whole, as far as I
could judge, the females of those Prionidæ, in which the sexes differ,
are colored more richly than the males, and this does not accord with
the common rule in regard to color, when acquired through sexual
selection.”
The great horns that rise from the heads of many male beetles are very
striking cases of sexual difference, and Darwin compares them to the
horns of stags and of the rhinoceros. They “are wonderful from their
size and shapes.” Darwin offers the following conjecture as to their
meaning: “The extraordinary size of the horns, and their widely
different structure in closely allied forms, indicate that they have
been formed for some purpose; but their excessive variability in the
males of the same species leads to the inference that this purpose
cannot be of a definite nature. The horns do not show marks of friction,
as if used for any ordinary work. Some authors suppose that as the males
wander about much more than the females, they require horns as a defence
against their enemies; but as the horns are often blunt, they do not
seem well adapted for defence. The most obvious conjecture is that they
are used by the males for fighting together; but the males have never
been observed to fight; nor could Mr. Bates, after a careful examination
of numerous species, find any sufficient evidence, in their mutilated or
broken condition, of their having been thus used. If the males had been
habitual fighters, the size of their bodies would probably have been
increased through sexual selection, so as to have exceeded that of the
females; but Mr. Bates, after comparing the two sexes in above a hundred
species of the Copridæ, did not find any marked difference in this
respect amongst well-developed individuals. In Lethrus, moreover, a
beetle belonging to the same great division of the lamellicorns, the
males are known to fight, but are not provided with horns, though their
mandibles are much larger than those of the female.”
“The conclusion that the horns have been acquired as ornaments is that
which best agrees with the fact of their having been so immensely, yet
not fixedly, developed,—as shown by their extreme variability in the
same species, and by their extreme diversity in closely allied species.
This view will at first appear extremely improbable; but we shall
hereafter find with many animals standing much higher in the scale,
namely fishes, amphibians, reptiles and birds, that various kinds of
crests, knobs, horns and combs have been developed apparently for this
sole purpose.”
It is asking a great deal to suppose that animals, so dull and sluggish
as these beetles, are endowed with a sufficient æsthetic discrimination
to select in each generation those males whose horns are a little longer
than the average. The resemblance of the horns to those of stags is, as
Darwin points out, obvious, but in the latter case also it remains to be
proven that they are the result of sexual selection, as Darwin believes
to be the case; but the evidence for this belief is not much better, as
we shall see in the case of the antlers of deer, than it is in these
beetles.
In regard to butterflies, the males and females are both often equally
brilliantly colored; in other species the differences in the sexes are
very striking. Darwin states:—
“Even within the same genus we often find species presenting
extraordinary differences between the sexes, whilst others have their
sexes closely alike.” The fine colors of the wings of many moths are
also supposed by Darwin to have arisen through sexual selection,
although the colors are usually on the lower wings, which are covered
during the day by the less ornamented upper wings. It is assumed that,
since the moths often begin to fly at dusk, their colors might at this
time be seen and appreciated by the other sex. It should not be
overlooked, however, that, in the case of some of the most highly
colored moths, it is known that the males find the females through the
sense of smell. Moreover, although moths are often finely colored,
Darwin points out that “it is a singular fact that no British moths
which are brilliantly colored, and, as far as I can discover, hardly any
foreign species, differ much in color according to sex; though this is
the case with many brilliant butterflies.”
Yet Darwin does not hesitate to conclude: “From the several foregoing
facts it is impossible to admit that the brilliant colors of
butterflies, and of some few moths, have commonly been acquired for the
sake of protection. We have seen that their colors and elegant patterns
are arranged and exhibited as if for display. Hence I am led to believe
that the females prefer or are most excited by the more brilliant males;
for on any other supposition the males would, as far as we can see, be
ornamented to no purpose. We know that ants and certain lamellicorn
beetles are capable of feeling an attachment for each other, and that
ants recognize their fellows after an interval of several months. Hence
there is no abstract improbability in the Lepidoptera, which probably
stand nearly or quite as high in the scale as these insects, having
sufficient mental capacity to admire bright colors. They certainly
discover flowers by color.”
So far as the evidence of ants having an attachment for each other is
concerned, we may eliminate this part of the argument, since the
evidence on which the statement is based is now regarded as only showing
that ants recognize each other by their sense of smell, which resides in
the antennæ. Hence the so-called fondling means only that the ants are
trying by smell to determine the odor of the other individual.
Darwin points out a number of cases in which the females are more
brightly colored than the males, and for such cases he reverses the
process of selection, supposing that the males have been discriminating,
and have not “gladly accepted any female.” No explanation is offered to
account for this reversal of instinct, in fact, no evidence to show that
such a reversal really exists. Darwin points out that in most cases the
male insect carries the female during the period of union, while in two
species of butterflies, _Colias edusa_ and _hyale_, the females carry
the males, and the females are here the more highly colored. He suggests
that since in this case “the females take the more active part in the
final marriage ceremony, so we may suppose that they likewise do so in
the wooing; and in this case we can understand how it is that they have
been rendered the more beautiful.”
A most significant fact in connection with the difference in sexual
coloration of butterflies did not escape Darwin’s attention.
“Whilst reflecting on the beauty of many butterflies, it occurred to me
that some caterpillars were splendidly colored; and as sexual selection
could not possibly have here acted, it appeared rash to attribute the
beauty of the mature insect to this agency, unless the bright colors of
their larvæ could be somehow explained. In the first place, it may be
observed that the colors of caterpillars do not stand in any close
correlation with those of the mature insect. Secondly, their bright
colors do not serve in any ordinary manner as a protection. Mr. Bates
informs me, as an instance of this, that the most conspicuous
caterpillar which he ever beheld (that of a Sphinx) lived on the large
green leaves of a tree on the open llanos of South America; it was about
four inches in length, transversely banded with black and yellow, and
with its head, legs, and tail of a bright red. Hence it caught the eye
of any one who passed by, even at the distance of many yards, and no
doubt that of every passing bird.”
Darwin applied to Wallace for a solution of this difficulty, and
received the reply that he “thought it probable that conspicuously
colored caterpillars were protected by having a nauseous taste; but as
their skin is extremely tender, and as their intestines readily protrude
from a wound, a slight peck from the beak of a bird would be as fatal to
them as if they had been devoured. Hence, as Mr. Wallace remarks,
‘distastefulness alone would be insufficient to protect a caterpillar
unless some outward sign indicated to its would-be destroyer that its
prey was a disgusting morsel.’ Under these circumstances it would be
highly advantageous to a caterpillar to be instantaneously and certainly
recognized as unpalatable by all birds and other animals. Thus the most
gaudy colors would be serviceable, and might have been gained by
variation and the survival of the most easily recognized individuals.”
It need scarcely be pointed out that an occasional peck can scarcely be
supposed to have led to the splendid development of color shown by some
caterpillars, and Darwin confesses that at first sight this hypothesis
appears bold, but nevertheless he believes that it will be found to be
true. He adds, “We cannot, however, at present thus explain the elegant
diversity in the colors of many caterpillars.”
A most important fact in this connection should not be overlooked,
namely, that in the caterpillar stage the sexual organs are so little
developed that it is generally impossible at this time to distinguish
between the sexes, unless a microscopic examination is made. This gives
us, perhaps, a clew as to the difference between the mature sexual
forms. These differences are connected with difference of sex itself.
This conclusion also fits in well with the fact that during the period
when the sexual organs are at the height of their development the
individuals are most brilliantly colored. The primary cause of the
brilliant color of many animals concerns us here only secondarily, for,
since it is known that many of the lower forms are no less brilliantly
and elaborately colored than are the sexes of the higher forms, it is
not surprising that the sexes themselves sometimes differ in this
respect.
Organs for producing sounds of different sorts are present in some
insects, and these organs Darwin includes under the head of secondary
sexual organs. In the group of Hemiptera, or bugs, the cicadas are the
most familiar species that produce sounds. The noise is made by the
males; the females are quite mute.
“With respect to the object of the music, Dr. Hartman, in speaking of
the _Cicada septemdecim_ of the United States, says, ‘the drums are now
(June 6th and 7th, 1851) heard in all directions. This I believe to be
the marital summons from the males. Standing in thick chestnut sprouts
about as high as my head, where hundreds were around me, I observed the
females coming around the drumming males.’ He adds, ‘this season
(August, 1868) a dwarf pear-tree in my garden produced about fifty larvæ
of _C. pruinosa_; and I several times noticed the females to alight near
a male while he was uttering his clanging notes.’ Fritz Müller writes to
me from S. Brazil that he has often listened to a musical contest
between two or three males of a species with a particularly loud voice,
seated at a considerable distance from each other: as soon as one had
finished his song, another immediately began, and then another. As there
is so much rivalry between the males, it is probable that the females
not only find them by their sounds, but that, like female birds, they
are excited or allured by the male with the most attractive voice.”
In the flies the following cases are given by Darwin:—
“That the males of some Diptera fight together is certain; for Professor
Westwood has several times seen this with the Tipulæ. The males of other
Diptera apparently try to win the females by their music: H. Müller
watched for some time two males of an Eristalis courting a female; they
hovered above her, and flew from side to side, making a high humming
noise at the same time. Gnats and mosquitoes (Culicidæ) also seem to
attract each other by humming; and Professor Mayer has recently
ascertained that the hairs on the antennæ of the male vibrate in unison
with the notes of a tuning-fork, within the range of the sounds emitted
by the female.”
In the crickets, grasshoppers, and locusts, the males “are remarkable
for their musical powers”; and it is generally assumed that the sounds
serve to call or to excite the female. In these forms the noise is made
by rubbing the wings over each other or the legs against the
wing-covers.
In some of these forms both sexes have stridulating organs, and in one
case they differ to a certain extent from each other. “Hence we cannot
suppose that they have been transferred from the male to the female, as
appears to have been the case with the secondary sexual characters of
many other animals. They must have been independently developed in the
two sexes, which no doubt mutually call to each other during the season
of love.”
Some beetles also possess rasping organs in different parts of the body,
but they cannot produce much noise by this means.
“We thus see that in the different coleopterous families the
stridulating organs are wonderfully diversified in position, but not
much in structure. Within the same family some species are provided with
these organs, and others are destitute of them. This diversity is
intelligible, if we suppose that originally various beetles made a
shuffling or hissing noise by the rubbing together of any hard and rough
parts of their bodies, which happened to be in contact; and that from
the noise thus produced being in some way useful, the rough surfaces
were gradually developed into regular stridulating organs. Some beetles
as they move, now produce, either intentionally or unintentionally, a
shuffling noise, without possessing any proper organs for the purpose.”
Darwin says that he expected from analogy to find in this group also
differences in the sexes, but none such were found, although in some
cases the males alone possess certain characters or have them more
highly developed.
It is important not to forget, when considering all questions connected
with sexual selection, that in order for the result to be successful it
is not only necessary that the female respond to the noises and music of
the other sex, but that she choose the suitor that makes the greatest,
or the most pleasing, noise. If the stridulating organs are only used by
the animals in finding each other, then the case might be considered as
coming under the head of natural selection. If this be granted, then it
may be claimed, and apparently Darwin is inclined to adopt this view,
that those males that make the most noise will be more likely to be
heard, and possibly approached. They will, therefore, be more likely to
leave descendants. We have already considered this question when dealing
with the theory of natural selection in the preceding chapter and need
not go over the ground again. This much may, however, be said again,
that even if it is probable that these organs are of use to the animals
in finding each other, and this seems not improbable, it does not follow
that the organs have been acquired through selection for this purpose.
Darwin finds his best examples of secondary sexual characters in the
group of vertebrates, and since in this group the intelligence is of a
higher order than in the other groups, the argument that the female
chooses the more pleasing suitor is made to appear more plausible.
The elongation of the lower jaw that occurs in a few fishes at the
breeding season is regarded as a secondary sexual character. On the
other hand, Darwin recognizes the following difficulty in regard to the
size of the males:—
“In regard to size, M. Carbonnier maintains that the female of almost
all fishes is larger than the male; and Dr. Günther does not know of a
single instance in which the male is actually larger than the female.
With some cyprinodonts the male is not even half as large. As in many
kinds of fishes the males habitually fight together, it is surprising
that they have not generally become larger and stronger than the females
through the effects of sexual selection. The males suffer from their
small size, for, according to M. Carbonnier, they are liable to be
devoured by the females of their own species when carnivorous, and no
doubt by other species. Increased size must be in some manner of more
importance to the females, than strength and size are to the males for
fighting with other males; and this perhaps is to allow of the
production of a vast number of ova.”
The last sentence implies that this particular case is to be explained
by the females becoming larger on account of the number of eggs that
they are to produce. But why was not the same explanation offered in the
case of the spiders? It is this uncertain way of applying any
explanation that suggests itself, that puts the whole method in an
unfortunate light.
In many species of fish the males are brighter in color than the
females. In the case of _Callionymus lyra_, Darwin states:—
“When fresh caught from the sea the body is yellow of various shades,
striped and spotted with vivid blue on the head; the dorsal fins are
pale brown with dark longitudinal bands, the ventral, caudal, and anal
fins being bluish black. The female, or sordid dragonet, was considered
by Linnæus, and by many subsequent naturalists, as a distinct species;
it is of a dingy reddish brown, with the dorsal fin brown and the other
fins white. The sexes differ also in the proportional size of the head
and mouth, and in the position of the eyes; but the most striking
difference is the extraordinary elongation in the male of the dorsal
fin. Mr. W. Saville Kent remarks that this ‘singular appendage appears
from my observations of the species in confinement, to be subservient to
the same end as the wattles, crests, and other abnormal adjuncts of the
male in gallinaceous birds, for the purpose of fascinating their
mates.’”
In the case of another fish, _Cottus scorpius_, there is also a great
difference between the sexes, and here the males become very brilliant
only at the breeding season. In other fishes, in which the sexes are
colored alike, the males may become more brilliant during the breeding
season. This, too, is explained by Darwin on the assumption that those
males that have varied at the breeding season, so as to become more
brightly colored, have been chosen in preference to the other males.
A few cases are cited in which it has been observed that the males
appear to exhibit themselves before the females, as in the following
case of the Chinese Macropus:—
“The males are most beautifully colored, more so than the females.
During the breeding season they contend for the possession of the
females; and, in the act of courtship, expand their fins, which are
spotted and ornamented with brightly colored rays, in the same manner,
according to M. Carbonnier, as the peacock. They then also bound about
the females with much vivacity, and appear by ‘l’étalage de leurs vives
couleurs chercher à attirer l’attention des femelles, lesquelles ne
paraissaient indifférentes à ce manége, elles nageaient avec une molle
lenteur vers les mâles et semblaient se complaire dans leur voisinage.’”
In this connection Darwin makes the following general statement:—
“The males sedulously court the females, and in one case, as we have
seen, take pains in displaying their beauty before them. Can it be
believed that they would thus act to no purpose during their courtship?
And this would be the case, unless the females exert some choice and
select those males which please or excite them most. If the female
exerts such choice, all the above facts on the ornamentation of the
males become at once intelligible by the aid of sexual selection.”
While it may readily be granted that display of the male may have for
its purpose the excitement of the female, it is another question as to
whether she will be more excited by the more beautiful suitor. The
attentions of the male may be supposed to have a purpose, even if the
female does not choose the more beautiful of her suitors. It is this
last proposition, so necessary for the theory of sexual selection, that
seems improbable. But even if it were probable, there are, as we shall
see, other difficulties to be overcome before we should be justified in
accepting Darwin’s statement quoted above, concerning the results of
sexual selection.
In regard to those species of fish in which both sexes are equally
ornamented, Darwin returns once more to his hypothesis that the color of
the male, acquired through sexual selection, may be transmitted to the
other sex, and then, as if in doubt on this point, he adds, that it may
be the result of the “nature of the tissues and of the surrounding
conditions.” He even makes the suggestion, somewhat further on, that the
colors may be warning, although it is confessedly unknown that these
fish are distasteful to fish-devouring animals.
In amphibians the crest on the back of the male triton, which becomes
colored along its edge, is described as a secondary sexual character.
The vocal sacs, present in some species of frogs, are found sometimes in
both sexes, but more highly developed in the males. In other species, as
in the toad, it is the male alone that sings. In the reptiles we find
that the two sexes of the turtles are colored alike, and this holds also
for the crocodiles. Some male turtles make sounds at the breeding
season, and the same is true for the crocodiles, the males of which are
said to make a “prodigous display.” In snakes the males are smaller, as
a rule, than the females, and the colors are more strongly pronounced,
and although some snakes are very brilliantly colored, Darwin puts this
down either to protective coloration, or to mimicry of other kinds of
snakes. But surely the extremely brilliant colors of many snakes cannot
be accounted for in any of these ways. The cause of the color of the
venomous kinds, that are supposed to be imitated by the others, “remains
to be explained and this may perhaps be sexual selection.”
“It does not, however, follow because snakes have some reasoning power,
strong passions and mutual affection, that they should likewise be
endowed with sufficient taste to admire brilliant colors in their
partners, so as to lead to the adornment of the species through sexual
selection. Nevertheless, it is difficult to account in any other manner
for the extreme beauty of certain species; for instance, of the
coral-snakes of South America, which are of a rich red with black and
yellow transverse bands.”
In lizards the erectile crests of the male _Anolis_, the brilliant
throat patches of _Sitaria minor_, which is colored blue, black, and
red, the skinny appendages present on the throat of the little lizards
of the genus Draco, which in the beauty of their colors baffle
description, are given as cases of sexual adornment. In the last case
cited the ornaments are present, however, in both sexes. The remarkable
horns in the males of different species of chameleons are imagined to
have been acquired through the battle of the males with each other.
In the group of birds we find some of the most striking cases of
secondary sexual differences. The spurs, combs, wattles, horns,
air-filled sacs, topknots, feathers with naked shafts, plumes, and
greatly elongated feathers are all secondary sexual characters. The
songs of the males, the rattling together of the quills of the peacock,
the drumming of the grouse, and the booming sounds made by the night
jars while on the wing, are further examples of secondary sexual
differences. The odor of the male of the Australian musk duck is also
put in the same category.
The pugnacity of many male birds is well known, and it is imagined that
one of the results of the competition of the individuals of the same sex
with each other has led to the development of the organs of defence and
offence. The males that have been successful in these battles are then
supposed to mate with the best females. In this way those secondary
sexual differences, connected with the encounters of the males, are
supposed to have been formed. Darwin states in this connection:—
“Even with the most pugnacious species it is probable that the pairing
does not depend exclusively on the mere strength and courage of the
male; for such males are generally decorated with various ornaments,
which often become more brilliant during the breeding season, and which
are sedulously displayed before the females. The males also endeavor to
charm or excite their mates by love-notes, songs, and antics; and the
courtship is, in many instances, a prolonged affair. Hence it is not
probable that the females are indifferent to the charms of the opposite
sex, or that they are invariably compelled to yield to the victorious
males.”
Thus a double process of selection is imagined to take place; one, the
outcome of a competition of the males with each other, and the other,
through a choice of the more successful males by the females, the more
beautiful being supposed to be chosen.
It may be well not to lose sight of the fact that unless the selection
is severe in each generation, its good effects will be lost, as has been
stated in connection with the theory of natural selection. Still more
important is the consideration that unless the same variations appear at
the same time, in many of the surviving males, the results will be lost
through crossing. These statements will show that the difficulties of
the theory are by no means small, and when we are asked to believe
further that another process still has been superimposed on this one,
namely, the selection of the more beautiful males by the females, we can
appreciate how great are the difficulties that must be overcome in order
that the process may be carried out.
The love-antics and dances of male birds at the breeding season furnish
many curious data. The phenomena are imagined by Darwin to be connected
with sexual selection, for in the dances the males are supposed to
exhibit their ornaments to the females who are imagined to choose the
suitor that is most to their taste.
Hudson, who has studied the habits of birds in the field, asks some very
pertinent questions in connection with their performances of different
kinds. “What relation that we can see or imagine to the passion of love
and the business of courtship have these dancing and vocal performances
in nine cases out of ten? In such cases, for instance, as that of the
scissortail tyrant-bird, and its pyrotechnic displays, when a number of
couples leave their nests containing eggs and young to join in a wild
aërial dance; the mad exhibitions of ypecahas and ibises and the
jacana’s beautiful exhibition of grouped wings; the triplet dances of
the spur-winged lapwing, to perform which two birds already mated are
compelled to call in a third bird to complete the set; the harmonious
duets of the oven-birds and the duets and choruses of nearly all the
wood-hewers, and the wing-slapping aërial displays of the whistling
widgeons,—will it be seriously contended that the female of this species
makes choice of the male able to administer the most vigorous and
artistic slaps?”
“The believer in the theory would put all these cases lightly aside to
cite the case of the male cow-bird practising antics before the female,
and drawing a wide circle of melody around her, etc.... And this was in
substance what Darwin did.” “How unfair the argument is based on these
carefully selected cases gathered from all regions of the globe and
often not properly reported is seen when we turn to the book of nature
and closely consider the habits and actions of all the species
inhabiting any _one_ district.” Hudson concludes that he is convinced
that any one who will note the actions of animals for himself will reach
the conviction, that “conscious sexual selection on the part of the
female is not the cause of music and dancing performances in birds, nor
of the brighter colors and ornaments that distinguish the male.”
The differences in color in the sexes of birds are classified by Darwin
as follows: (1) when the males are ornamented exclusively or in a much
higher degree than the females; (2) when both sexes are highly
ornamented; (3) when the female is more brightly colored. A few examples
of each sort may be chosen for illustration.
“In regard to color, hardly anything need here be said, for every one
knows how splendid are the tints of many birds, and how harmoniously
they are combined. The colors are often metallic and iridescent.
Circular spots are sometimes surrounded by one or more differently
shaded zones, and are thus converted into ocelli. Nor need much be said
on the wonderful difference between the sexes of many birds. The common
peacock offers a striking instance. Female birds of paradise are
obscurely colored and destitute of all ornaments, whilst the males are
probably the most highly decorated of all birds, and in so many
different ways, that they must be seen to be appreciated. The elongated
and golden-orange plumes which spring from beneath the wings of the
_Paradisea apoda_, when vertically erected and made to vibrate, are
described as forming a sort of halo, in the centre of which the head
‘looks like a little emerald sun, with its rays formed by the two
plumes.’”
Male humming-birds are almost as splendidly colored as are the birds of
paradise, some having the feathers modified in a truly extraordinary
way. “Almost every part of their plumage has been taken advantage of,
and modified; and the modifications have been carried, as Mr. Gould
showed me, to a wonderful extreme in some species belonging to nearly
every subgroup. Such cases are curiously like those which we see in our
fancy breeds, reared by man for the sake of ornament: certain
individuals originally varied in one character, and other individuals of
the same species in other characters; and these have been seized on by
man and much augmented—as shown by the tail of the fantail pigeon, the
hood of the jacobin, the beak and wattle of the carrier, and so forth.
The sole difference between these cases is that in the one the result is
due to man’s selection, whilst in the other, as with humming-birds,
birds of paradise, etc., it is due to the selection by the females of
the more beautiful males.”
A remarkable bird of South America, the bell-bird, has a peculiar note
that “can be distinguished at the distance of nearly three miles and
astonishes every one who hears it.... The male is pure white, whilst the
female is dusky-green; and white is a very rare color in terrestrial
species of moderate size and inoffensive habits. The male, also, as
described by Waterton, has a spiral tube, nearly three inches in length,
which rises from the base of the beak. It is jet-black, dotted over with
minute downy feathers. This tube can be inflated with air, through a
communication with the palate; and when not inflated hangs down on one
side. The genus consists of four species, the males of which are very
distinct, whilst the females, as described by Mr. Sclater in a very
interesting paper, closely resemble each other, thus offering an
excellent instance of the common rule that within the same group the
males differ much more from each other than do the females. In a second
species (_C. nudicollis_) the male is likewise snow-white, with the
exception of a large space of naked skin on the throat and round the
eyes, which during the breeding season is of a fine green color. In a
third species (_C. tricarunculatus_) the head and neck alone of the male
are white, the rest of the body being chestnut-brown, and the male of
this species is provided with three filamentous projections half as long
as the body—one rising from the base of the beak, and the two others
from the corners of the mouth.”
The most familiar case of sexual difference amongst North American birds
is that of the scarlet tanager, in which the male is scarlet with
jet-black wings, while the female is an inconspicuous yellow-green
color. Amongst domesticated animals the peafowl shows the most beautiful
case of sexual differences. The magnificent tail of the male can be
lifted up, so as to be seen to best advantage when the male faces the
observer. Moreover the wild form, living in the forests of India, has
the same gorgeous train.
The male Argus pheasant has a remarkable series of spots, or ocelli, on
the secondary wing-covers. They are concealed until the male displays
them before the female. Darwin states that, while it may seem incredible
that such elegant shading and exquisite patterns could have been the
outcome of the taste of the female, yet the extraordinary attitude
assumed by the male during courtship appears entirely purposeless,
unless it be supposed that he is attempting to charm the female by a
display of his ornamentation.
Let us pass to the second class of cases, in which both sexes are
similarly and brightly colored, and in which the young have a plumage
different from the adults. For example, the male and the female of the
splendid scarlet ibis are alike, whilst the young are brown. The males
and females of many finely colored herons are ornamented alike, and this
plumage, Darwin admits, has a nuptial character. He even tries to
explain this by the curious assumption, that while the color has been
acquired through the selection of the males by the females, the results
attained in this way have been transmitted to both sexes. We find here
another example of the method so often employed by Darwin. When he meets
with facts that are not in conformity with the theory, he proceeds to
make a new assumption without establishing its validity. Thus, to assume
that in all cases where the sexes are colored differently, the
characters acquired by the males have been transmitted only to the same
sex, and in those cases where the sexes are colored alike the
transmission has been to both sexes, is most arbitrary.
In other cases, which are commoner than the last, the male and female
have the same color, and the young in their first plumage resemble the
adults. Darwin admits that here the facts are so complex that his
conclusions are doubtful. The following account of the tree-sparrow
shows how vague are the principles involved in the entire discussion in
relation to transmission:—
“Now with the tree-sparrow (_P. montanus_) both sexes and the young
closely resemble the male of the house-sparrow; so that they have all
been modified in the same manner, and all depart from the typical
coloring of their early progenitor. This may have been effected by a
male ancestor of the tree-sparrow having varied, firstly, when nearly
mature; or secondly, whilst quite young, and by having in either case
transmitted his modified plumage to the females and the young; or,
thirdly, he may have varied when adult and transmitted his plumage to
both adult sexes, and, owing to the failure of the law of inheritance at
corresponding ages, at some subsequent period to his young.”
The further admissions made in the following quotation are also
significant:—
“The plumage of certain birds goes on increasing in beauty during many
years after they are fully mature; this is the case with the train of
the peacock, with some of the birds of paradise, and with the crest and
plumes of certain herons, for instance, the _Ardea ludovicana_. But it
is doubtful whether the continued development of such feathers is the
result of the selection of successive beneficial variations (though this
is the most probable view with birds of paradise) or merely of
continuous growth. Most fishes continue increasing in size, as long as
they are in good health and have plenty of food; and a somewhat similar
law may prevail with the plumes of birds.”
We need not follow Darwin through his discussion of those cases in which
the adults have a winter and a summer dress and the young resemble the
one or the other in plumage, or are different from either. The
discussion of these cases, confessedly very complex, adds nothing to our
understanding of the theory, and little but conjecture is offered to
account for the facts.
The extreme to which even conjecture can be carried may be gathered from
the following quotation, taken from the section dealing with cases in
which the young in their first plumage differ from each other according
to sex, the young males resembling more or less closely the adult males,
and the young females more or less closely the adult females:
“Two humming-birds belonging to the genus Eustephanus, both beautifully
colored, inhabit the small island of Juan Fernandez, and have always
been ranked as specifically distinct. But it has lately been ascertained
that the one which is of a rich chestnut-brown color with a golden-red
head, is the male, whilst the other, which is elegantly variegated with
green and white with a metallic-green head, is the female. Now the young
from the first somewhat resemble the adults of the corresponding sex,
the resemblance gradually becoming more and more complete.
“In considering this last case, if as before we take the plumage of the
young as our guide, it would appear that both sexes have been rendered
beautiful independently; and not that one sex has partially transferred
its beauty to the other. The male apparently has acquired his bright
colors through sexual selection in the same manner as, for instance, the
peacock or pheasant in our first class of cases; and the female in the
same manner as the female Rhynchæa or Turnix in our second class of
cases. But there is much difficulty in understanding how this could have
been effected at the same time with the two sexes of the same species.
Mr. Salvin states, as we have seen in the eighth chapter, that with
certain humming-birds the males greatly exceed the females in number,
whilst with other species inhabiting the same country the females
greatly exceed the males. If, then, we might assume that during some
former lengthened period the males of the Juan Fernandez species had
greatly exceeded the females in number, but that during another
lengthened period the females had far exceeded the males, we could
understand how the males at one time, and the females at another, might
have been rendered beautiful by the selection of the brighter-colored
individuals of either sex; both sexes transmitting their characters to
their young at a rather earlier age than usual. Whether this is the true
explanation I will not pretend to say; but the case is too remarkable to
be passed over without notice.”
The third group of cases include those in which the females are more
brightly colored, or more ornamented, than the males. These cases are
rare, and the differences between the sexes are never so great as when
the male is the more highly colored. Wallace thinks that since in these
cases the male incubates the eggs his less conspicuous colors have been
acquired through natural selection. In the genus Turnix the female is
larger than the male, and lacks the black on the throat and neck, and
the plumage as a whole is lighter than that of the male. The natives
assert that the females after laying their eggs associate in flocks, and
leave the males to do the incubating; and from other evidence Darwin
thinks that this is true.
In three species of painted snipe the females “are not only larger but
much more richly colored than the males,” and the trachea is more
convoluted in some species. “There is also reason to believe that the
male undertakes the duty of incubation.” In the dotterel plover the
female is larger and somewhat more strongly colored. The males take at
least a share in the incubation. In the common cassowary the female is
larger and the skin of the head more brightly colored than in the male.
The female is pugnacious during the breeding season and the male sits on
the eggs. The female emu is large and has a crest. She is more
courageous and pugilistic and makes a deep, hollow, guttural boom. The
male is more docile and can only hiss or croak. He not only incubates
the eggs, but defends the young against their own mother. “So that with
this emu we have a complete reversal not only of the parental and
incubating instincts, but of the usual moral qualities of the two sexes;
the females being savage, quarrelsome, and noisy, the males gentle and
good. The case is very different with the African ostrich, for the male
is somewhat larger than the female and has finer plumes with more
strongly contrasted colors; nevertheless he undertakes the whole duty of
incubation.”
Darwin attempts to explain these reversals of instincts on the
assumption that the males have turned the tables on the females, and
have themselves done the selecting; and incidentally, it may be pointed
out in passing, they have had to pay the penalty by incubating the eggs.
In the group of mammals, Darwin thinks that the male wins the female by
conquering other males rather than by charming her through his display.
The males, even when unarmed, engage in desperate conflicts with each
other, and sometimes kill, but more often only wound, their fellows. The
secondary sexual characters of the males have been acquired, therefore,
by natural selection applied to one sex, and less frequently through the
choice of the female. Since we are here more especially concerned with
the latter class of phenomena, we may examine only a few cases under the
first head.
The horns of stags are used by them in their conflicts with each other;
the tusks of the elephant make this animal the most dangerous in the
world, when in must. The horns of bulls, the canine teeth of many
mammals, the tusks of the walrus, are further examples of organs which
have been, according to Darwin, acquired through the competitions of the
males with each other.
The voices of mammals are used for various purposes, “as a signal of
danger, as a call from one member of the troup to another, and from the
mother to her lost offspring, or from the latter for protection.”
“Almost all male animals use their voices much more during the rutting
season than at any other time; and some, as the giraffe and porcupine,
are said to be completely mute excepting at this season. As the throats
(_i.e._ the larynx and thyroid bodies) of stags periodically become
enlarged at the beginning of the breeding season, it might be thought
that their powerful voices must be somehow of high importance to them;
but this is very doubtful. From information given to me by two
experienced observers, Mr. McNeill and Sir P. Egerton, it seems that
young stags under three years old do not roar or bellow; and that the
old ones begin bellowing at the commencement of the breeding season, at
first only occasionally and moderately, whilst they restlessly wander
about in search of the females. Their battles are prefaced by loud and
prolonged bellowing, but during the actual conflict they are silent.
Animals of all kinds which habitually use their voices utter various
noises under any strong emotion, as when enraged and preparing to fight;
but this may merely be the result of nervous excitement, which leads to
the spasmodic contraction of almost all the muscles of the body, as when
a man grinds his teeth and clenches his fists in rage or agony. No doubt
stags challenge each other to mortal combat by bellowing; but those with
the more powerful voices, unless at the same time the stronger,
better-armed, and more courageous, would not gain any advantage over
their rivals.”
“Some writers suggest that the bellowing serves as a call to the female;
but the experienced observers above quoted inform me that female deer do
not search for the male, though the males search eagerly for the
females, as indeed might be expected from what we know of the habits of
other male quadrupeds. The voice of the female, on the other hand,
quickly brings to her one or more stags, as is well known to the hunters
who in wild countries imitate her cry.
“As the case stands, the loud voice of the stag during the breeding
season does not seem to be of any special service to him, either during
his courtship or battles, or in any other way. But may we not believe
that the frequent use of the voice, under the strong excitement of love,
jealousy, and rage, continued during many generations, may at last have
produced an inherited effect on the vocal organs of the stag, as well as
of other male animals? This appears to me, in our present state of
knowledge, the most probable view.”
Here once more we find that Darwin makes use, as a sort of last resort,
of the principle of the inheritance of acquired characters. As long as
the theory of selection, in any of its forms, appears to offer a
satisfactory solution, we find the facts used in support of this theory,
but as soon as a difficulty arises the Lamarckian theory is brought to
the front. It is this shifting, as we have already more than once
pointed out, that shows how little real basis there is for the theory of
sexual selection.
The male gorilla has a tremendous voice, and he has, as has also the
orang, a laryngeal sac. One species of gibbon has the power of producing
a correct octave of musical notes.
“The vocal organs of the American _Mycetes caraya_ are one-third larger
in the male than in the female, and are wonderfully powerful. These
monkeys in warm weather make the forests resound at morning and evening
with their overwhelming voices. The males begin the dreadful concert,
and often continue it during many hours, the females sometimes joining
in with their less powerful voices. An excellent observer, Rengger,
could not perceive that they were excited to begin by any special cause;
he thinks that, like many birds, they delight in their own music, and
try to excel each other. Whether most of the foregoing monkeys have
acquired their powerful voices in order to beat their rivals and charm
the females—or whether the vocal organs have been strengthened and
enlarged through the inherited effects of long-continued use without any
particular good being thus gained—I will not pretend to say; but the
former view, at least in the case of the _Hylobates agilis_, seems the
most probable.”
The odor of some mammals is confined to, or more developed, in the
males; but in some forms, as in the skunk, it is present in both sexes.
In the shrew mice, abdominal scent glands are present, but since these
mice are rejected by birds of prey, their glands probably serve to
protect them; “nevertheless the glands become enlarged in the males
during the breeding season.” In many other quadrupeds the scent glands
are of the same size in both sexes, and their function is unknown.
“In other species the glands are confined to the males, or are more
developed than in the females; and they almost always become more active
during the rutting season. At this period the glands on the sides of the
face of the male elephant enlarge, and emit a secretion having a strong
musky odor. The males, and rarely the females, of many kinds of bats
have glands and protrudable sacs situated in various parts; and it is
believed that these are odoriferous.
“The rank effluvium of the male goat is well known, and that of certain
male deer is wonderfully strong and persistent. Besides the general
odor, permeating the whole body of certain ruminants (for instance, _Bos
moschatus_) in the breeding season, many deer, antelopes, sheep, and
goats, possess odoriferous glands in various situations, more especially
on their faces. The so-called tear-sacs, or suborbital pits, come under
this head. These glands secrete a semifluid fetid matter which is
sometimes so copious as to stain the whole face, as I have myself seen
in an antelope. They are ‘usually larger in the male than in the female,
and their development is checked by castration.’ According to Desmarest
they are altogether absent in the female of _Antilope subgutturosa_.
Hence, there can be no doubt that they stand in close relation with the
reproductive functions. They are also sometimes present, and sometimes
absent, in nearly allied forms. In the adult male musk-deer (_Moschus
moschiferus_), a naked space round the tail is bedewed with an
odoriferous fluid, whilst in the adult female and in the male until two
years old, this space is covered with hair, and is not odoriferous.”
Darwin believes in these cases that the odor serves to attract the
females. He admits that here, “active and long-continued use cannot have
come into play as in the case of the vocal organs.” He concludes,
therefore, that “the odor emitted must be of considerable importance to
the male, inasmuch as large and complex glands, furnished with muscles
for everting the sac, and for closing or opening the orifice, have in
some cases been developed. The development of these organs is
intelligible through sexual selection, if the most odoriferous males are
the most successful in winning the females, and in leaving offspring to
inherit their gradually perfected glands and colors.”
There is sometimes a difference in the mammals in the hair of the two
sexes both in amount and in color. In some species of goats the males
have a beard, in others it is present in both sexes. The bull, but not
the cow, has curly hair on the forehead. In some monkeys the beard is
confined to the male, as in the orang; in other species it is only
larger in the males.
“The males of various members of the ox family (Bovidæ), and of certain
antelopes, are furnished with a dewlap, or great fold of skin on the
neck, which is much less developed in the female.
“Now, what must we conclude with respect to such sexual differences as
these? No one will pretend that the beards of certain male goats, or the
dewlap of the bull, or the crests of hair along the backs of certain
male antelopes, are of any use to them in their ordinary habits.
“Must we attribute all these appendages of hair or skin to mere
purposeless variability in the male? It cannot be denied that this is
possible; for in many domesticated quadrupeds, certain characters,
apparently not derived through reversion from any wild parent form, are
confined to the males, or are more developed in them than in the
females—for instance, the hump on the male zebu cattle of India, the
tail of fat-tailed rams, the arched outline of the forehead in the males
of several breeds of sheep, and, lastly, the mane, the long hairs on the
hind-legs, and the dewlap of the male of the Berbura goat.”
In these cases and in others that Darwin cites, which seem clearly to
indicate that some of these secondary sexual characters are not the
result of sexual selection, he concludes, “that they must be due to
simple variability, together with sexually limited inheritance.
“Hence it appears reasonable to extend this same view to all analogous
cases with animals in a state of nature. Nevertheless I cannot persuade
myself that it generally holds good, as in the case of the extraordinary
development of hair on the throat and fore-legs of the male Ammotragus,
or in that of the immense beard of the male Pithecia. Such study as I
have been able to give to nature makes me believe that parts or organs
which are highly developed, were acquired at some period for a special
purpose. With those antelopes in which the adult male is more strongly
colored than the female, and with those monkeys in which the hair on the
face is elegantly arranged and colored in a diversified manner, it seems
probable that the crests and tufts of hair were gained as ornaments; and
this I know is the opinion of some naturalists. If this be correct,
there can be little doubt that they were gained, or at least modified
through sexual selection; but how far the same view may be extended to
other mammals is doubtful.”
The astonishing colors in some of the monkeys cannot be passed over
without comment.
“In the beautiful _Cercopithecus diana_, the head of the adult male is
of an intense black, whilst that of the female is dark gray; in the
former the fur between the thighs is of an elegant fawn-color, in the
latter it is paler.
“In the _Cercopithecus cynosurus_ and _griseoviridis_ one part of the
body, which is confined to the male sex, is of the most brilliant blue
or green, and contrasts strikingly with the naked skin on the hinder
part of the body, which is vivid red.
“Lastly, in the baboon family, the adult male of _Cynocephalus
hamadryas_ differs from the female not only by his immense mane, but
slightly in the color of the hair and of the naked callosities. In the
drill (_C. leucophæus_) the females and young are much paler-colored,
with less green, than the adult males. No other member in the whole
class of mammals is colored in so extraordinary a manner as the adult
male mandrill (_C. mormon_). The face at this age becomes of a fine
blue, with the ridge and tip of the nose of the most brilliant red.
According to some authors, the face is also marked with whitish stripes,
and is shaded in parts with black, but the colors appear to be variable.
On the forehead there is a crest of hair, and on the chin a yellow
beard. ‘Toutes les parties supérieures de leurs cuisses et le grand
espace nu de leurs fesses sont également colorés du rouge le plus vif,
avec un mélange de bleu qui ne manque réellement pas d’élégance.’ When
the animal is excited all the naked parts become much more vividly
tinted.”
Darwin sums up the evidence in regard to the differences in color
between the male and female in the following statement:—
“I have now given all the cases known to me of a difference in color
between the sexes of mammals. Some of these may be the result of
variations confined to one sex and transmitted to the same sex, without
any good being gained, and therefore without the aid of selection. We
have instances of this with our domesticated animals, as in the males of
certain cats being rusty-red, whilst the females are tortoise-shell
colored. Analogous cases occur in nature: Mr. Bartlett has seen many
black varieties of the jaguar, leopard, vulpine phalanger, and wombat;
and he is certain that all or nearly all these animals, were males. On
the other hand, with wolves, foxes, and apparently American squirrels,
both sexes are occasionally born black. Hence it is quite possible that
with some mammals a difference in color between the sexes, especially
when this is congenital, may simply be the result, without the aid of
selection, of the occurrence of one or more variations, which from the
first were sexually limited in their transmission. Nevertheless it is
improbable that the diversified, vivid, and contrasted colors of certain
quadrupeds, for instance, of the above monkeys and antelopes, can thus
be accounted for.”
Finally, the case of man must be considered from the point of view of
sexual selection, for Darwin claims that man has acquired a number of
his secondary sexual characters in this way. For instance, the beard is
an excellent case of a secondary sexual character. Darwin’s
interpretation is that the beard has been retained, or even developed,
through the selection by the females of those males that had this
outgrowth best developed. Conversely, the absence of hair on the face of
the female is supposed by Darwin to have been brought about by men
selecting those women having less hair on their faces. The greater
intellect, energy, courage, pugnacity, and size of man are the outcome
of the competition of the males with each other, since the individual
excelling in these qualities will be able to select the most desirable
wife, or wives, and it is assumed will, therefore, leave more
descendants. The standard of beauty has been kept up by men selecting
the most beautiful women in each generation (the fate of the other
married women is ignored), and this beauty is supposed to have been
transmitted primarily to their daughters, but also to their sons.
Although all these forms of selection are imagined to be acting in man,
either alternately or simultaneously, yet Darwin recognizes in man a
number of checks to the action of sexual selection: amongst savages, the
so-called communal marriages; second, infanticide, generally of the
young females, which appears in some races to be practised to an
astonishing degree; third, early betrothals; fourth, the holding of
women as slaves.
When we recall that selection to be effective can only be carried out
under very exacting conditions, we cannot but be appalled at the demands
made here on our credulity. The choice of the women has produced the
beard of man, the choice of man the absence of a beard in women; the
competition of the males with each other is leading at the same time to
the development of at least half a dozen qualities that are supposed to
be male specialities, and while all this is going on the results are
being checked sometimes by one means, sometimes by another. Moreover,
even this is not all that we are asked to accept, for there are several
other qualities of the male that are put down as secondary sexual
characters. For example, let us examine what Darwin has to say in regard
to the development of the voice, and of singing in man.
In man the vocal cords are about a third longer than in woman and his
voice deeper. Emasculation arrests the development of the vocal
apparatus, and the voice remains like that of a woman. This difference
between the sexes, Darwin thinks, is due probably to long-continued use
by the male “under the excitement of love, rage, and jealousy.” In other
words, an appeal is again made to the Lamarckian theory, and in this
case to explain the origin of an organ that conforms to all the
requirements of the secondary sexual characters.
“The capacity and love for singing, or music, though not a sexual
character in man,” in the sense of being confined to one sex, yet is
supposed to have arisen through sexual selection in the following way:
“Human song is generally admitted to be the basis or origin of
instrumental music. As neither the enjoyment nor the capacity of
producing musical notes are faculties of the least use to man in
reference to his daily habits of life, they must be ranked amongst the
most mysterious with which he is endowed.”
Man is supposed to have possessed this faculty of song from a very
remote time, and even the most savage races make musical sounds,
although we do not enjoy their music, or they ours.
“We see that the musical faculties, which are not wholly deficient in
any race, are capable of prompt and high development, for Hottentots and
Negroes have become excellent musicians, although in their native
countries they rarely practise anything that we should consider music.
Hence the capacity for high musical development, which the savage races
of man possess, may be due either to the practice by our semi-human
progenitors of some rude form of music, or simply to their having
acquired the proper vocal organs for a different purpose. But in this
latter case we must assume, as in the above instance of parrots, and as
seems to occur with many animals, that they already possessed some sense
of melody.”
Darwin sums up the evidence in the two following statements, the
insufficiency of which to explain the phenomena is I think only too
obvious: “All these facts in respect to music and impassioned speech
become intelligible to a certain extent, if we assume that musical tones
and rhythm were used by our half-human ancestors, during the season of
courtship, when animals of all kinds are excited not only by love, but
by the strong passions of jealousy, rivalry, and triumph. From the
deeply laid principle of inherited associations, musical tones in this
case would be likely to call up vaguely and indefinitely the strong
emotions of a long past age.” Thus the difficulty is shifted to the
shoulders of our long-lost savage ancestors; or even, in fact, to our
simian forefathers, as the following paragraph indicates:—
“As the males of several quadrumanous animals have their vocal organs
much more developed than in the females, and as a gibbon, one of the
anthropomorphous apes, pours forth a whole octave of musical notes and
may be said to sing, it appears probable that the progenitors of man,
either the males or females or both sexes, before acquiring the power of
expressing their mutual love in articulate language, endeavored to charm
each other with musical notes and rhythm. So little is known about the
use of the voice by the Quadrumana during the season of love, that we
have no means of judging whether the habit of singing was first acquired
by our male or female ancestors. Women are generally thought to possess
sweeter voices than men, and as far as this serves as any guide, we may
infer that they first acquired musical powers in order to attract the
other sex. But if so, this must have occurred long ago, before our
ancestors had become sufficiently human to treat and value their women
merely as useful slaves. The impassioned orator, bard, or musician, when
with his varied tones and cadences he excites the strongest emotions in
his hearers, little suspects that he uses the same means by which his
half-human ancestors long ago aroused each other’s ardent passions
during their courtship and rivalry.”
We have now examined in some detail the evidence that Darwin has brought
forward in support of his hypothesis of sexual selection. A running
comment has been made while considering the individual cases, but it may
be well to sum up the matter by briefly indicating the reasons why the
hypothesis seems incompetent to explain the facts.
General Criticism of the Theory of Sexual Selection
1. Some of the objections that apply to the theory of natural selection
apply also with equal force to the theory of sexual selection in so far
as the results in both cases are supposed to be the outcome of the
selection of individual, or fluctuating, variations. If these variations
appear in only a few individuals, their perpetuation is not possible,
since they will soon disappear through crossing. It would be, of course,
preposterous to suppose that at any one time only those few individuals
pair and leave descendants that have the secondary sexual characters
developed to the highest point, but if something of this sort does not
occur, the extreme of fluctuating variations cannot be maintained. Even
if half of the individuals are selected in each generation, the
accumulation of a variation in a given direction could not go very far.
The assumption, however, that only half of all the individuals that
reach maturity breed, and that all of these are chosen on account of the
special development of their secondary sexual characters, seems
preposterous. Furthermore, if it is assumed that the high development of
the new character appears in a large number of individuals, then it is
not improbable that its continued appearance might be accounted for
without bringing in, at all, the hypothesis of sexual selection.
2. But even supposing that the females select the most beautiful males,
then, since in the vast majority of higher animals the males and the
females are in equal numbers, the others will also be able to unite with
each other in pairs after this first selection has taken place. Nothing
will therefore be gained in the next generation. It is interesting to
see how Darwin attempts to meet this argument. He tries to show in the
case of birds, that there are always unpaired individuals, but since the
few facts that he has been able to collect show that there are as many
additional females as males, the argument proves too much. A few species
are polygamous, one male having a number of female birds; but on this
basis we can only account, at best, for the development through
competition of the organs of offence and defence used to keep away the
weaker males. Yet it is just amongst these birds that we often find the
ornamental characters well developed. In fact, since all the females in
such cases are selected, and since they will transmit the characters of
all the males, it is evident that the secondary sexual characters could
not be formed in the way imagined.
3. If the female fails to select only the more ornamental males, no
result will follow. It has not been shown that she is capable of making
such a choice, and in the lower forms particularly, it does not seem
probable that this is done. The argument that Darwin often employs,
namely, that unless she does select, the display of the males before her
is meaningless, is not to the point. So far as we can detect the “cause”
of the display of the male, it appears to be due to his own excitement;
and even if we go so far as to admit that the “purpose” is to attract
the other sex, it still does not in the least follow that the most
ornamental male is selected, and unless this occurs the display has no
bearing on the hypothesis of sexual selection.
4. The two forms of sexual selection, namely, competition of the males
with one another (really one form of natural selection), and the
selection of the most ornamental or gifted individuals, are both used by
Darwin to explain secondary sexual characters, the one for organs of
offence and defence, and the other for ornamental characters. If we
fully appreciate the difficulties that any theory of selection meets
with, we shall realize how extraordinarily complex the action must be,
when two such processes are carried out at the same time, or even during
alternating periods.
5. It has been objected to Darwin’s theory of sexual selection, that he
suddenly reverses its mode of action to explain those cases in which the
female is the stronger and more ornamented sex; but if, as Darwin shows,
the instincts of the male have also changed, and have become more like
those of the female, I can see no inherent difficulty in this way of
applying the theory. A much more serious objection, it seems to me, is
that the male is supposed to select the female for one set of
characteristics, and the female to select the male for another set. It
sounds a little strange to suppose that women have caused the beard of
man to develop by selecting the best-bearded individuals, and the
compliment has been returned by the males selecting the females that
have the least amount of beard. It is also assumed that the results of
the selection are transmitted to one sex only. Unless, in fact, the
character in question were from the beginning peculiar to only one sex
as to its inheritance, the two sexes might go on forever selecting at
cross-purposes, and the result would be nothing.
6. The development, or the presence, of the æsthetic feeling in the
selecting sex is not accounted for on the theory. There is just as much
need to explain why the females are gifted with an appreciation of the
beautiful, as that the beautiful colors develop in the males. Shall we
assume that still another process of selection is going on, as a result
of which those females are selected by the males that appreciate their
unusual beauty, or that those females whose taste has soared a little
higher than that of the average (a variation of this sort having
appeared) select males to correspond, and thus the two continue heaping
up the ornaments on one side and the appreciation of these ornaments on
the other? No doubt an interesting fiction could be built up along these
lines, but would any one believe it, and, if he did, could he prove it?
Darwin assumes that the appreciation on the part of the female is always
present, and he thus simplifies, in appearance, the problem, but he
leaves half of it unexplained.
7. It has been pointed out, that it is important to distinguish between
the possible excitement of the female by the display or antics of the
male, and the selection of the more beautiful or agile performer. Darwin
himself records a few cases, which plainly show that the more beautiful
is not always the more successful. It has also been suggested that the
battles of the males are sometimes sham performances, and even when they
are real, if the less vigorous do not remain to be destroyed but run
away, they live to find mates of their own. In fact, the conduct of the
males at the breeding season appears to be much more the outcome of
their own excitement than an attempt to attract the females.
8. There is another side to the question, the importance of which is so
great, that it is surprising that Darwin has not taken any notice of it.
If, in order to bring about, or even maintain, the results of sexual
selection, such a tremendous elimination of individuals must take place,
it is surprising that natural selection would not counteract this by
destroying those species in which a process, so useless for the welfare
of the species, is going on. It is curious that this has not been
realized by those who believe in both of these two hypotheses.
9. What has just been said applies also with almost equal force to the
development of such structures as the horns of deer, bison, antelopes,
and the brilliant colors of many insects and birds. If in nature,
competition between species takes place on the scale that the Darwinian
theory of natural selection postulates, such forms, if they are much
exposed, would be needlessly reduced in numbers in the process of
acquiring these structures. So many individuals would have been at such
a disadvantage in breeding, that if competition is as severe as the
theory of natural selection postulates, these species could hardly be
expected to compete successfully with other species in which sexual
selection was not taking place.
10. Darwin admits that, in certain cases, external conditions may have
acted directly to produce the colors in certain forms, and if these were
not injurious he thinks they might have become constant. Such cases are
left unexplained in the sense that they are not supposed to be
adaptations to anything in particular. That colors produced in this way
might afterward be found useful, irrespective of how they arose, is
admitted as one of the ways in which sexual differences may have arisen.
11. It is baffling to find Darwin resorting to the Lamarckian
explanation in those cases in which the improbability of the hypothesis
of sexual selection is manifest. If either principle is true, we should
expect it to apply to all phenomena of the same sort; yet Darwin makes
use of the Lamarckian principle, in the hypothesis of sexual selection,
only when difficulties arise.
12. In attempting to explain the development of the musical sense in
man, it is clear that the hypothesis of sexual selection fails to give a
satisfactory explanation. To suppose that the genius of a Beethoven or
of a Mozart could have been the result of a process of sexual selection
is too absurd to discuss. Neither the power of appreciation nor of
expression in music could possibly have been the outcome of such a
process, and it does not materially help the problem to refer it back to
a troop of monkeys making the woods hideous with their cries.
We come now to some of the special cases to which Darwin’s hypothesis
has been applied.
13. In one case at least, it is stated that a bird living on the ground
might have acquired the color of the upper surface of the body through
natural selection, while the under surface of the males of the same
species might have become ornamented through the action of sexual
selection. Thus in one and the same individual the two processes are
supposed to have been at work, and it does not lessen the difficulty
very much by supposing the two processes to have been carried out at
different times, because it is evident that what had been gained at one
time by one process might become lost while the color of certain parts
was being acquired through the other process.
14. Darwin points out that “the plumage of certain birds goes on
increasing in beauty during many years after they are fully mature,” as
in the peacock, and in some of the birds of paradise, and with the
plumes and crests of some herons. This is explained as possibly merely
the result of “continued growth.” The improbability of selection is
manifest in these cases, but if “continued growth” can accomplish this
much, why may not the whole process be also the outcome of such growth?
At any rate, whatever the explanation is, it is important to find a case
of a secondary sexual character that the hypothesis obviously is
insufficient to explain.
15. It is admitted in a number of cases, as in the stag for instance,
that, although the larynx of the male is enlarged, this is not, in all
probability, the outcome of sexual selection, but in other forms this
same enlargement is ascribed to the selection process.
16. It is admitted that in none of the highly colored British moths is
there much difference according to sex, although when a difference of
color is found in butterflies this is put down to the action of sexual
selection. If such wonderful colors as those of moths can arise without
the action of selection, why make a special explanation for those cases
in which this difference is associated with sex?
17. It is well known that birds sing at other times of the year than at
the breeding season, and an attempt is made to account for this in that
birds take pleasure in practising those instincts that they make use of
at other times, as the cat plays with the captive mouse. Does not this
suggest that, if they had certain instincts, they would be more likely
to employ them at the times when their vitality or excitement is at its
highest without regard to the way in which they have come by them?
18. The color of the iris of the eyes of many species of hornbills is
said to be an intense crimson in the males, and white in the females. In
the male condor the eye is yellowish brown, and in the female a bright
red. Darwin admits that it is doubtful if this difference is the result
of sexual selection, since in the latter case the lining of the mouth is
black in the males, and flesh-colored in the females, which does not
affect the external beauty. Yet if these colors were more extensive and
on the exterior, there can be little doubt that they would have been
explained as due to sexual selection.
19. When the females in certain species of birds differ more from each
other than they do from their respective males, the case is compared to
“those inexplicable ones, which occur independently of man’s selection
in certain sub-breeds of the game-fowl, in which the females are very
different, whilst the males can hardly be distinguished.” Here then is a
case of difference in color associated with sex, but not the outcome of
sexual selection.
20. The long hairs on the throat of the stag are said possibly to be of
use to him when hunted, since the dogs generally seize him by the
throat, “but it is not probable that the hairs were specially developed
for this purpose; otherwise the young and the females would have been
equally protected.” Here also is a sexual difference that can scarcely
be ascribed to selection.
Some cases of differences in color between the sexes “may be the result
of variations confined to one sex, and transmitted to the same sex
without any good being gained, and, therefore, without the aid of
selection. We have instances of this with our domesticated animals, as
in the males of certain cats being rusty-red while the females are
tortoise-shell colored. Analogous cases occur in nature: Mr. Bartlett
has seen many black varieties of the jaguar, leopard, vulpine phalanger,
and wombat; and he is certain that all or nearly all of these animals
were males.” If changes of this sort occur, associated with one sex, why
is there any need of a special explanation in other cases of difference?
* * * * *
In the light of the many difficulties that the theory of sexual
selection meets with, I think we shall be justified in rejecting it as
an explanation of the secondary sexual differences amongst animals.
Other attempts to explain these differences have been equally
unsuccessful. Thus Wallace accounts for them as due to the excessive
vigor of the male, but Darwin’s reply to Wallace appears to show that
this is not the cause of the difference. He points out that, while the
hypothesis might appear plausible in the case of color, it is not so
evident in the case of other secondary sexual characters, such, for
instance, as the musical apparatus of the males of certain insects, and
the difference in the size of the larynx of certain birds and mammals.
Darwin’s theory served to draw attention to a large number of most
interesting differences between the sexes, and, even if it prove to be a
fiction, it has done much good in bringing before us an array of
important facts in regard to differences in secondary sexual characters.
More than this I do not believe it has done. The theory meets with fatal
objections at every turn.
In a later chapter the question will be more fully discussed as to the
sense in which these secondary sexual differences may be looked upon as
adaptations.
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CHAPTER VII
THE INHERITANCE OF ACQUIRED CHARACTERS AS A FACTOR IN EVOLUTION
Lamarck’s Theory
One of the most striking and peculiar characteristics of living things
is that through use a part is able to carry out a particular function
better than before, and in some cases the use of the part leads to its
increase in size. Conversely, disuse leads to the decrease of a part in
size. We are perfectly familiar with this process in ourselves as
applied to our nervous system and muscles.
It is not surprising that the idea should have arisen that, if the
results of the use of a part are inherited by the next generation, the
adaptation of organisms might be explained in this way. The presence of
the organs of touch, in those parts of the body that are more likely to
come into contact with foreign bodies, offers a striking parallel to the
perfecting of the sensation of touch that can be brought about through
the use of any part. The development of eyes only on the exposed parts
of the body, as on the tentacles of the sedentary annelids, or along the
margin of the mantle of a bivalve mollusk, suggests that there may be
some direct connection between their presence in these regions and the
effect of light on the parts. In fact, ever since the time of Lamarck,
there have been many zoologists who have claimed that many of the
adaptations of organisms have arisen in this way, that is, through the
inheritance of the characters acquired through use. In general this
theory is summed up in the phrase, “the inheritance of acquired
characters.”
This view is prominently associated with the name of Lamarck, who held,
however, a different view in regard to the origin of some of the other
structures of the organism. Moreover, Erasmus Darwin, even before
Lamarck, had suggested the principle of the inheritance of acquired
characters.
As has just been said, Lamarck held that the inheritance of acquired
characters was only one of the ways in which animals have become
changed, and he clearly stated that in the case of all plants and of
some of the lower animals the change (evolution) which he supposed them
to undergo was due to the general influence of the environment. Since
plants and the lower animals (as he supposed) have no central nervous
system, or at least no such well-defined nervous system as have the
higher animals, Lamarck thought that they could not have evolved in the
same way as have the higher animals. We now know that, so far as the
lower animals, at least, are concerned, there was no need for such a
distinction, since many of their responses are like those of the higher
animals. This distinction that Lamarck made is responsible, no doubt,
for a misconception that was long held in regard to a part of his views.
It is often stated that he supposed the desire of the animal for a
particular part has led to the development of that part; while in
reality he only maintained the desire to use a particular organ to
fulfil some want led to its better development through exercise, and the
result was inherited. Lamarck also supposed that the _decrease_ in use
of a part which leads to its decrease in size accounts for the
degeneration of organs.
Lamarck first advanced his theory in 1801, when he cited the following
examples in its favor. A bird, driven through want to the water to find
its food, will separate its toes when they strike the water. The skin
uniting the bases of the toes will be stretched in consequence, and in
this way the broad membrane between the toes of ducks and geese has been
acquired. The toes of a bird that is in the habit of perching on a tree
become elongated in consequence of becoming stretched, hence has arisen
the foot with the long toes characteristic of arboreal birds.
Shore-birds, “which do not care to swim,” but must approach the water in
order to obtain food, will be in danger of sinking into the mud, “but,
wishing to act so that their body shall not fall into the liquid, they
will contract the habit of extending and lengthening their legs.” Hence
have arisen the stiltlike legs of shore-birds.
These ideas were more fully elaborated in the following year. He added
the further examples: Our dray-horses have arisen through the use to
which they have been put, and the race-horse also, which has been used
in a different way. Cultivated plants, on the contrary, are the result
of the new environment to which they have been subjected.
In the “Philosophic Zoologique,” published in 1809, Lamarck has much
more fully developed his theory. Here he combats strenuously the idea
that species are fixed. His point of view may be judged by the following
propositions, which he believes can be established:—
1. That all organized bodies of our globe are veritable productions of
nature, which she has successively produced in the course of a long
time.
2. That in her progress nature began, and begins still every day, to
produce the simplest organisms, and that she still produces directly the
same primitive kinds of organizations. This process has been called
spontaneous generation.
3. That the first beginning of animals and of plants takes place in
favorable localities and under favorable circumstances. An organic
movement having once established their production, they have of
necessity gradually developed their organs, and have become diversified
in the course of time.
4. That the power of growth of each part of the body being inherited as
a consequence of the first effect of life, different modes of
multiplication and of regeneration have arisen, and these have been
conserved.
5. That with the aid of sufficient time and of favorable circumstances
the changes that have taken place on the surface of the globe have
called forth new structures and new habits, and in consequence have
modified the organs of the body, and made animals and plants such as we
see them at the present day.
6. Finally, as a result of these changes that living bodies have been
forced to undergo, species have been formed, but these species have only
a relative constancy, and are not as ancient as is nature herself. If
the environment remains the same, species also remain the same, as is
exemplified by the animals living at present in Egypt, which are exactly
like those living there in ancient times.
Lamarck concludes that the appearance of stability is always mistaken by
the layman for the reality, because, in general, every one judges things
relatively to himself. In fact, species are not absolutely constant, but
are so only temporarily. “The influence of the environment is continuous
and always active, but its effects may only be recognized after a long
time.” The irregularity and the complexity of the organization of
animals is the outcome of the infinitely diversified circumstances to
which they have been subjected. These changes, Lamarck claims, do not
directly cause modifications in the form of animals,[17] but bring about
changes in their needs, and changes in their needs bring about changes
in their actions. If the needs remain the same, the acquired actions
become habits. These habitual actions lead to the use of certain parts
in preference to others, and this in turn to an alteration in form and
structure. The individuals so changed breed together and leave
descendants that inherit the acquired modification.
Footnote 17:
This is clearly meant to be applied only in the case of higher
animals.
Curiously enough, Lamarck follows up this argument by citing some cases
amongst plants that have been changed directly by the action of the
environment. He says that since plants have no motions they have
consequently no habits, but they are developed by changes in their
nutrition, etc., and this brings about the superiority of some of the
vital movements over others.
Amongst domestic animals Lamarck cites the case of the dog, that has
come from a wild form like the wolf, but having been carried into
different countries has acquired different and new habits, and this has
led to the formation of new races, such as the bulldog, greyhound,
pug-dog, spaniel, etc.
Lamarck’s argument shifts so often back and forth from animals to
plants, that it is clear that in his own mind he did not see any
important difference between the action of the environment on plants,
and the use of the organs of the animal. He gives in this same
connection his oft-quoted summary of what he calls the two laws of
nature “which observation always establishes.”
First Law. In every animal, that has not passed beyond the term of its
development, the frequent and sustained use of any organ strengthens it,
develops it, increases its size, and gives it strength proportionate to
the length of time of its employment. On the other hand, the continued
lack of use of the same organ sensibly weakens it; it deteriorates, and
its faculties diminish progressively until at last it disappears.
Second Law. Nature preserves everything that she has caused the
individual to acquire or to lose by the influence of the circumstances
to which the race has been for a long time exposed, and consequently by
the influence of the predominant use of certain organs (or in
consequence of its continued disuse). She does this by the generation of
new individuals which are produced with the newly acquired organs. This
occurs, provided that the acquired changes were common to the two sexes,
or to the individuals that produced the new forms.
These laws are, Lamarck says, fundamental truths which cannot be
misunderstood except by those who have never observed or followed nature
in her operations. He insists that it is a mistake to suppose that the
parts are responsible for the functions, for it is easy to demonstrate
that it is the needs and uses of the organs that have caused the parts
to develop.
If it is supposed, he continues, that these laws are hypothetical, they
may be demonstrated by the following facts: The adult baleen whale is
without teeth, although in the fœtus teeth are present, concealed in the
jaws. The loss of the teeth is the result of the whale swallowing its
food without first masticating it. The ant-eater is also without teeth,
and has also the habit of swallowing its food without chewing it. The
mole has very small eyes, and this is the result of its having made very
little use of them, since its habits are subterranean. Another animal,
the aspalax, has only the rudiments of eyes, and has almost completely
lost the power of sight. This animal also lives underground like the
mole.
Proteus, an aquatic salamander living in deep caves, has only
rudimentary eyes. In these latter cases it is the disuse of the eye that
has led to its degeneration. This is proven, Lamarck adds, by the fact
that the organs of hearing are never in this condition, because sound
vibrations penetrate everywhere, even into the densest bodies.
It is a part of the plan of organization of the reptiles that they have
four legs; but the snakes, although belonging to this group, have no
legs. This absence of legs is explained by their having acquired the
habit of gliding over the ground, and of concealing themselves in the
grass. Owing to their repeated effort to elongate themselves, in order
to pass through narrow spaces, their bodies have become drawn out. Under
these circumstances legs would be useless, since long ones would
interfere with their motion, and short ones could not move their long
bodies. Since the plan of organization limits the snakes to only four
legs, and since this number would be useless, they have disappeared.
Many insects are destitute of wings, although wings are a part of the
plan of organization of this group. They are absent only in those forms
whose habits render wings useless, consequently they have disappeared
through disuse.
The preceding cases are those in which the disuse of an organ has led to
its degeneration. The following cases are cited to show that by use an
organ increases in size. The formation of the web in the feet of
water-birds has already been given as a character which Lamarck supposes
to have been acquired through use; also the case of shore-birds, which,
by an effort to elongate their legs, have actually made them so in the
course of time. The necks of water-birds are also long on account of
their having been stretched in the efforts to catch fish. The long
tongues of the ant-eater, of the woodpecker, and of humming-birds are
the result of use, and the long, forked tongue of serpents has come from
their using their tongue to feel objects in front of them.
Fishes that have acquired the habit of living in shallow water,
flounders, soles, etc., have been forced to swim on their sides in order
to approach nearer to the shore. Since more light comes from above than
from below, the eye on the under side, straining to turn to the light,
has finally migrated to the upper side.
The habit of eating great quantities of food, which distends the
digestive organs, has caused the bodies of herbivorous quadrupeds to
become large, as seen in the elephant, the rhinoceros, oxen, horses, and
buffaloes. The habit of standing for a long time on their feet has
caused some animals to develop hard, thick hoofs. Herbivorous animals,
that inhabit countries where they are constantly subjected to attack, as
deer and antelopes for example, are forced to escape by rapid flight,
and in consequence their bodies have become slenderer and their legs
thinner. The horns, antlers, and protuberances that many of these
animals possess are the results of their butting each other when
angered.
“The long neck and the form of the giraffe offer a curious case. We know
that the giraffe is the tallest of all animals. It inhabits the centre
of Africa, living in those localities where the earth is nearly always
dry and without herbage. It is obliged to browse on the foliage of
trees, and this leads to its stretching continually upwards. As a result
of this habit, carried on for a long time, in all the individuals of the
race, the anterior limbs have become longer than the posterior, and its
neck has also lengthened, so that the giraffe without rising on its
hind-legs stretches up its neck and can reach to the height of six
metres.”
The curved claws of the carnivora have arisen from the necessity of
grasping their prey. The power of retracting the claws has also been
acquired by the effort to draw them in when running over hard ground.
The abdominal pouch of the kangaroo, in which the young are carried,
opens anteriorly, and this has led to the animal standing erect so that
its young are not injured. In consequence, the fore-legs have become
shorter through disuse, and the hind-legs have become stronger through
use. The tail, which is also used as a support, has become enormously
thick at its base.
The sloth has been compelled to seek refuge in the trees, and has taken
up its abode permanently there, feeding on leaves. Its movements are
limited to those involved in crawling along the limbs in order to reach
the leaves. After feeding it remains inactive and sluggish, these habits
being provoked by the heat of the climate. The results of its mode of
life have been to cause the arms to become elongated due to the habit of
the sloth of grasping the limbs of the tree; the claws of the fingers
and toes have also become long and hooked in order to retain their hold.
The digits that do not make any individual movements have lost the power
to do so, and have become fused, and can only be bent in and
straightened out. The thighs, being bent out to clasp the larger
branches, have caused the pelvis to widen, and, in consequence, the
cotyloid cavities have become directed backward. Many of the bones of
the skeleton have become fused, as a result of the immobility of the
animal.
Lamarck says, that “Nature, in producing, successively, all the species
of animals, beginning with the most imperfect, or the most simple, and
terminating with the most perfect, has gradually complicated their
organization. These animals becoming scattered throughout the habitable
regions of the globe each species has received from the influences of
its surroundings its present habits, and the modifications of the parts
the use of which we recognize.”
Such are Lamarck’s views and a fairly complete statement of the facts
from which he draws his conclusions. His illustrations appear naïve, and
often not a little ludicrous, but it must be admitted that, despite
their absurdities, his theory appears in some cases to account
wonderfully well for the facts. The long legs of wading birds, the long
neck and disproportionately long fore-legs of the giraffe, the structure
of the sloth, and particularly the degeneration of the eyes of animals
living in the dark, seem to find a simple explanation in the principle
of the inheritance of acquired characters. But the crucial point of the
entire theory is passed over in silence, or rather is taken for granted
by Lamarck, namely, the inheritance in the offspring of the characters
acquired through use or disuse in the parent. He does not even discuss
this topic, but in several places states unreservedly that the increase
or decrease of a part reappears in the next generation. It is here that
Lamarck’s theory has been attacked in more modern times, for as soon as
experimental proof was demanded to show that the results of use or of
disuse of an organ is inherited, no such proof was forthcoming. Yet the
theory is one that has the great merit of being capable of experimental
test, and it is astonishing to find that, with the immense amount that
has been written by his followers, so few attempts have been made to
give the theory a thorough test. The few results that have been obtained
are not, however, favorable to the theory, but almost the only attempts
at experiment that have been made in this direction have been those of
mutilating certain parts; and were it not for popular belief to the
effect that such mutilations are inherited, one would least expect to
get evidence for or against the theory in this direction. Lamarck
himself believed that the changes were slowly acquired, and I think
modern Lamarckians are justified in claiming that the validity of the
theory can only be tested by experiments in which the organism is
subjected to influences extending over a considerable period, although
Lamarck appears to have believed that the first results may appear quite
soon. Before expressing any opinion in regard to the probability of the
theory, let us examine what the followers of Lamarck have contributed in
the way of evidence to the theory, rather than the applications that
they have made of the theory. We shall also find it profitable to
consider some of the modern criticism, to which the theory has been
subjected.
Despite the contempt with which Darwin referred to Lamarck’s theory, he
himself, as we have seen, often made use of the principle of the
inheritance of acquired characters, and even employed the same
illustrations cited by Lamarck. Darwin seems to have misunderstood
Lamarck’s view, and to have accepted the current opinion that Lamarck
supposed an animal acquired a new organ by desiring or needing it.
Darwin says, “Heaven forefend me from Lamarck’s nonsense of a tendency
to progressive adaptation from the slow willing of the animals.” Darwin
speaks of Lamarck as stating that animals will that the egg shall be a
particular form so as to become attached to particular objects.
Lamarck’s latest biographer, Packard, says he is unable to find any
statements of this sort in Lamarck’s writings.
The following cases that Darwin tried to explain through the inheritance
of acquired characters are exactly like those to which Lamarck applied
his theory. The bones of the wing of the domestic duck weigh less than
those of the wild duck, and the bones of the leg more. Darwin believes
this is due to the effects of the inheritance of acquired characters.
The drooping ears of many domestic mammals are also explained by him as
a result of disuse—“the animals being seldom much alarmed.” In speaking
of the male of the beetle, _Onites apelles_, Darwin quotes Kirby to the
effect that the tarsi are so habitually lost that the species has been
described without this part of the foot. In the sacred beetle of Egypt
the tarsus is totally absent. Hence he concludes that the absence of
tarsi in the sacred beetle, and the rudimentary condition of the tarsus
in others, is probably the result of disuse, rather than a case of
inheritance of a mutilation. Darwin grants that “the evidence that
accidental mutilations can be inherited is at present not decisive, but
the remarkable case observed by Brown-Séquard in guinea-pigs of the
inherited effects of operations should make us cautious in denying this
tendency.”
The wingless condition of several insects inhabiting oceanic islands has
come about, Darwin thinks, through disuse. The ostrich also, owing to
its increase in size, made less use of its wings and more use of its
legs, with the result that its wings degenerated and its legs got
stronger. The rudimentary condition of the eyes of the mole is the
result of disuse, “aided perhaps by natural selection.” Many of the
animals inhabiting the caves of Kentucky and of Carniola are blind, and
this is ascribed to disuse. “As it is difficult to imagine that the
eyes, though useless, could be in any way injurious to animals living in
darkness, their loss may be attributed to disuse.” The long neck of the
giraffe Darwin attributes partly to natural selection and partly to use.
These references will suffice to show that Darwin is in full accord with
the main argument of Lamarck. In fact, the curious hypothesis of
pangenesis that Darwin advanced was invented partly to account for the
inheritance of acquired characters. Despite the hesitancy that Darwin
himself felt in advancing this view, and contrary to Huxley’s advice, he
at last published his provisional hypothesis of pangenesis in the
twenty-seventh chapter of his “Animals and Plants under Domestication.”
Darwin’s Hypothesis of Pangenesis
The study of bud variation, of the various forms of inheritance, and of
reproduction and of the causes of variation, led him, Darwin says, to
the belief that these subjects stand in some sort of relation to each
other. He says: “I have been led, or rather forced, to form a view which
to a certain extent connects these facts by a tangible method. Every one
would wish to explain to himself, even in an imperfect manner, how it is
possible for a character possessed by some remote ancestor suddenly to
reappear in the offspring; how the effects of increased or decreased use
of a limb can be transmitted to the child; how the male sexual element
can act not solely on the ovules, but occasionally on the mother form;
how a hybrid can be produced by the union of the cellular tissue of two
plants independently of the organs of generation; how a limb can be
reproduced on the exact line of amputation, with neither too much nor
too little added; how the same organism may be produced by such widely
different processes, as budding and true seminal generation; and,
lastly, how of two allied forms, one passes in the course of its
development through the most complex metamorphoses, and the other does
not do so, though when mature both are alike in every detail of
structure. I am aware that my view is merely a provisional hypothesis or
speculation; but, until a better one be advanced, it will serve to bring
together a multitude of facts which are at present left disconnected by
any efficient cause.”
In presenting the hypothesis of pangenesis Darwin begins by enumerating
the different kinds of sexual and asexual processes of reproduction, for
which he hopes to offer a provisional explanation. Here we find
mentioned various methods of budding and self-division, regeneration,
parthenogenesis, sexual reproduction, and the inheritance of acquired
characters. It is with the last only that we are here chiefly concerned;
in fact, the need of an hypothesis _of this sort_ to explain the other
kinds of inheritance is by no means evident. There are, however, two
other phenomena, besides that of the supposed inheritance of acquired
characters, to which the hypothesis of pangenesis might appear to apply
specially, namely, the effect of foreign pollen on the tissues of the
mother plant, and the supposed influence of the union with the first
male on the subsequent young (telegony). It is, however, far from being
shown that any influence of this latter kind really occurs, despite the
fact that it is generally believed in by breeders.
It is important to observe that Darwin proposes to explain on the
hypothesis of pangenesis, not only the inheritance of characters
acquired through use, but also the decrease of structures through
disuse; and this applies, not only to the structure, but to function as
well, as when the intelligence of the dog is explained through his
association with man, and the tameness of the domestic rabbits through
their long confinement. In the following quotation these points are
referred to: “How can the use or disuse of a particular limb or of the
brain affect a small aggregate of reproductive cells, seated in a
distant part of the body, in such a manner that the being developed from
these cells inherits the characters of either one or both parents? Even
an imperfect answer to this question would be satisfactory.”
Coming now to the theory, we find that it consists of one chief
assumption and several minor ones. “It is universally admitted that the
cells or units of the body increase by self-division or proliferation,
retaining the same nature, and that they ultimately become converted
into the various tissues and substances of the body. But besides this
means of increase I assume that the units throw off minute granules
which are dispersed throughout the whole system; that these, when
supplied with proper nutriment, multiply by self-division, and are
ultimately developed into units like those from which they were
originally derived. These granules may be called gemmules. They are
collected from all parts of the system to constitute the sexual
elements, and their development in the next generation forms a new
being; but they are likewise capable of transmission in a dormant state
to future generations, and may then be developed.... Gemmules are
supposed to be thrown off by every unit, not only during the adult
state, but during each stage of development of every organism; but not
necessarily during the continued existence of the same unit. Lastly, I
assume that the gemmules in their dormant state have a mutual affinity
for each other, leading to their aggregation into buds, or into the
sexual elements. Hence, it is not the reproductive organs, or buds,
which generate new organisms, but the units of which each individual is
composed. These assumptions constitute the provisional hypothesis which
I have called Pangenesis.”
It will be noticed that the first assumption is that the cells throw off
minute gemmules or granules. The second assumption is that these are
collected in the reproductive organs, or in buds, or in regenerating
parts; the third assumption is that the gemmules may lie dormant through
several generations; the fourth, that the development of the
reproductive cells is not so much the development of the cell itself,
but of the gemmules that have collected in it. The fifth assumption is
that the gemmules are thrown off at all stages of development; the
sixth, that in their dormant state they have a mutual affinity for each
other; the seventh, that there may be a sort of continual competition in
the germ-cells between the original gemmules and the new ones, and,
according to which win, the old or the new form develops. Thus we see on
closer analysis that the pangenesis hypothesis is made up of a goodly
number of different assumptions. At least half a dozen imaginary
properties are ascribed to the imaginary gemmules, and these attributes
are all essential to the working of the hypothesis.
Some of the more obvious objections to the hypothesis have been stated
by Darwin himself. Such, for instance, as our ignorance at what stage in
their history the body-cells are capable of throwing off gemmules, and
whether they collect only at certain times in the reproductive organs,
as the increased flow of blood to these organs at certain seasons might
seem to indicate. Nor have we any evidence that they are carried by the
blood at all. The experiment of Galton, of transfusing the blood of one
animal into another, and finding that this produced no effect on the
young that were born later, might be interpreted to mean that gemmules
are not transported by the blood; but this kind of experiment is
inconclusive, especially in the light of recent results on the effect of
the blood of one animal on that of another.
A part of the evidence on which Darwin relied to support his theory has
been shown to be incorrect by later work. Thus the assumption that more
than a single pollen grain, or more than one spermatozoon, is necessary
in some cases for fertilization, is certainly wrong. In most cases, in
fact, the entrance of more than one spermatozoon into the egg is
disastrous to the development. The cases referred to by Darwin can
probably be explained by the difficulty that some of the pollen grains,
or spermatozoa, may have in penetrating the egg, or to the immaturity or
impotence of some of the male germ-cells, and not to the need of more
than one to accomplish the true fertilization.
Darwin’s idea that the small number of gemmules in the unfertilized egg
may account for the lack of power of such eggs to develop until they are
fertilized, has been shown to be incorrect by recent results in
experimental embryology. We now know that many different kinds of
stimuli have the power to start the development of the egg. Moreover, we
also know that if a single spermatozoon is supplied with a piece of
egg-protoplasm without a nucleus, it suffices to cause this piece of
protoplasm to develop.
In the case of regeneration, which Darwin also tries to explain on the
pangenesis hypothesis, we find that there is no need at all for an
hypothesis of this sort; and there are a number of facts in connection
with regeneration that are not in harmony with the hypothesis. For
instance, when a part is cut off, the same part is regenerated; but
under these circumstances it cannot be imagined that the part removed
supplies the gemmules for the new part. Darwin tries to meet this
objection by the assumption that every part of the body contains
gemmules from every other part. But it has been shown that if a limb of
the newt is completely extirpated, a new limb does not regenerate; and
there is no reason why it should not do so on Darwin’s assumption that
germs of the limb exist throughout the body.
The best-authenticated cases of the influence of the male on the tissues
of the female are those in plants, where one species, or variety, is
fertilized by another. Thus, if the orange is fertilized by the pollen
of the lemon, the fruit may have the color and flavor of the lemon. Now
the fruit is a product of the tissues of the ovary of the female, and
not a part of the seedling that develops in the fruit from the
cross-fertilized egg-cell. Analogous cases are recorded for the bean,
whose pods may have their color influenced by fertilizing the flower
with pollen of another variety having pods of a different color. In
these cases we do not know whether the color of the fruit is influenced
directly by the foreign pollen, or whether the influence is through the
embryo that develops from the egg-cell. The action may appear to be the
same, however, in either case; but because it seems probable here that
there is some sort of influence of one tissue on another, let us not too
readily conclude that this is brought about through any such imaginary
bodies as gemmules. It may be directly caused, for instance, by some
chemical substance produced in the young hybrid plant. If this is the
case, the result would not be different in kind from that of certain
flowers whose color may be influenced by certain chemical substances in
the soil.
In the cases amongst animals, where the maternal tissues are believed to
be influenced by a previous union with the male, as in the oft-cited
case of Lord Morton’s mare, a reëxamination of the evidence by Ewart has
shown that the case is not demonstrated, and not even probable. Several
years ago I tried to test this view in the case of mice. A white mouse
was first bred to a dark male house-mouse, and the next time to a white
mouse, but none of the offspring from the second union showed any trace
of black. If the spermatozoa of the dark mouse are hypodermically
injected into the body-cavity of the female, the subsequent young from a
white male show no evidence that the male cells have had any influence
on the ovary.
The following facts, spoken of by Darwin himself, are not in favor of
his hypothesis of pangenesis: “But it appears at first sight a fatal
objection to our hypothesis that a part of an organ may be removed
during several successive generations, and if the operation be not
followed by disease, the lost part reappears in the offspring. Dogs and
horses formerly had their tails docked during many generations without
any inherited effect; although, as we have seen, there is some reason to
believe that the tailless conditions of certain sheep-dogs is due to
such inheritance.” The answer that Darwin gives is that the gemmules
themselves, that were once derived from the part, are still present in
other parts of the body, and it is from these that the organs in the
next generation may be derived. But Darwin fails to point out that, if
this were the case, it must also be true for those cases in which an
organ is no longer used. Its decrease in size in successive generations
cannot be due to its disuse, for the rest of the body would supply the
necessary gemmules to keep it at its full state of development. Thus, in
trying to meet an obvious objection to his hypothesis, Darwin brings
forward a new view that is fatal to another part of his hypothesis.
The following cases, also given by Darwin, are admitted by him to be
inexplicable on his hypothesis: “With respect to variations due to
reversion, there is a similar difference between plants propagated from
buds and seeds. Many varieties can be propagated securely by buds, but
generally or invariably revert to their parent forms by seed. So, also,
hybridized plants can be multiplied to any extent by buds, but are
continually liable to reversion by seed,—that is, to the loss of their
hybrid or intermediate character. I can offer no satisfactory
explanation of these facts. Plants with variegated leaves, phloxes with
striped flowers, barberries with seedless fruit, can all be securely
propagated by buds taken from the stem or branches; but buds from the
roots of these plants almost invariably lose their character and revert
to their former condition. This latter fact is also inexplicable, unless
buds developed from the roots are as distinct from those on the stem, as
is one bud on the stem from another, and we know that these latter
behave like independent organisms.” As Darwin here states, these facts
appear to be directly contradictory to his hypothesis, and he makes no
effort to account for them.
The entire question of the possibility of the inheritance of acquired
characters is itself at present far from being on a satisfactory basis,
as we shall try to show; and Darwin’s attempt at an explanation, in his
chapter on pangenesis, does not put the matter in a much more
satisfactory condition.
The Neo-Lamarckian School
Let us now turn our attention to a school that has grown up in modern
times, the members of which call themselves Neo-Lamarckians. Let us see
if they have supplied the essential evidence that is required to
establish the Lamarckian view, namely, that characters acquired by the
individual are transmitted to the offspring.
Lamarck’s views were adopted by Herbert Spencer, and play an important
rôle in his “Principles of Biology” (1866-1871), and even a more
conspicuous part in his later writings. In the former he cites, amongst
other cases, that of “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.” If tricks like this are
inheritable is it not surprising that more puppies do not stand on their
hind-legs?
The larger hands of the laboring classes in England are supposed to be
inherited by their children, and the smaller hands of the leisure
classes are supposed to be the result of the disuse of the hands by
their ancestors; but even if these statements in regard to size are
true, there are many other conceivable causes that may have led to this
result.
Short-sightedness appears more often, it is said, in those classes of
society that make most use of their eyes in reading and in writing; but
if we ask for experimental evidence to show that this is due to
inheritance, and not due to the children spoiling their eyes at school,
there is none forthcoming. The problem is by no means so simple as the
uninitiated may be led to believe.
Spencer thinks that “some of the best illustrations of functional
heredity are furnished by mental characteristics.” He cites the musical
faculty as one that could not have been acquired by natural selection,
and must have arisen through the inheritance of acquired modifications.
The explanation offered is “that the habitual association of certain
cadences of speech with certain emotions has clearly established in the
race an organized and inherited connection between such cadences and
such emotions, ... and that by the continued hearing and practice of
melody there has been gained and transmitted an increasing musical
sensibility.” But a statement that the results have been acquired in
this way does not supply the proof which the theory is in need of;
neither does it follow that, because the results cannot be explained by
the theory of natural selection, therefore, they must be explained by
the Lamarckian theory.
The clearest proofs that Spencer finds of the inheritance of acquired
characters are in the well-known experiments of Brown-Séquard. These
experiments will be more fully discussed below. Amongst the other morbid
processes that Spencer thinks furnish evidence in favor of this view,
are cases of a tendency to gout, the occurrence of mental tricks,
musical prodigies, liability to consumption, in all of which cases the
fundamental distinction between the inheritance of an acquired character
and the inherited tendency toward a particular malady is totally
ignored.
Twenty-seven years later (in 1893) Spencer took up the open challenge of
the anti-Lamarckian writers, and by bringing forward a number of new
_arguments_ attempted to reinstate the principle of the inheritance of
acquired characters. His first illustration is drawn from the
distribution of the sense of touch in different parts of our bodies.
Weber’s experiments have shown that if the sharp points of a pair of
compasses are applied to the tips of the forefingers, the sensation of
two separate points is given when the points are only one-twelfth of an
inch apart, and if the points are moved nearer together, they give the
sensation of only one point. The inner surfaces of the second joints of
the fingers can only distinguish two points when they are one-sixth of
an inch apart. The innermost joints are less discriminating, and are
about equal in the power of discrimination to the tip of the nose. The
end of the big toe, the palm of the hand, and the cheek discriminate
only about one-fifth as well as do the tips of the fingers. The back of
the hand and the top of the head distinguish only about one-fifteenth as
well as the finger-tips. The front of the thigh, near the knee, is
somewhat less sensitive than the back of the hand. On the breast the
points of the compasses must be separated by more than an inch and a
half in order to give two sensations. In the middle of the back the
points must be separated by two and a half inches, or more, in order to
give two separate impressions.
What is the meaning of these differences, Spencer asks. If natural
selection has brought about the result, then it must be shown that
“these degrees of endowment have advantaged the possessor to such an
extent that not infrequently life has been directly or indirectly
preserved by it.” He asks if this, or anything approaching this, result
could have occurred.
That the superior perceptiveness of the forefinger-tip might have arisen
through selection is admitted by Spencer, but how could this have been
the case, he asks, for the middle of the back, and for the face? The tip
of the nose has three times more power of discrimination than the lower
part of the forehead. Why should the front of the thigh near the knee be
twice as perceptive as in the middle of the thigh; and why should the
middle of the back and of the neck and the middle of the forearm and of
the thigh stand at such low levels? Is it possible, Spencer asks again,
that natural selection has determined these relations, and if not, how
can they be explained? His reply is that the differences can all be
accounted for on the theory of the inheritance of use, for it is evident
that “these gradations in tactile perceptiveness correspond with the
gradations in the tactual exercise of the parts.” Except from contact
with the clothing the body receives hardly any touch sensations from
outside, and this accounts for its small power of discrimination. The
greater sensitiveness of the chest and abdomen, as compared with the
back, is due to these regions being more frequently touched by the
hands, and is also owing to inheritance from more remote ancestors, in
which the lower surface of the body was more likely to have come in
contact with foreign objects than was the back. The middle of the
forearm and of the thigh are also less exposed than the knee and the
hand, and have correspondingly the power of tactile discrimination less
well developed.
Weber showed that the tip of the tongue is more sensitive than any other
part of the body, for it can distinguish between two points only one
twenty-fourth of an inch apart. Obviously, Spencer says, natural
selection cannot account for such extreme delicacy of touch, because,
even if it were useful for the tongue to distinguish objects by touch,
this power could never be of vital importance to the animal. It cannot
even be supposed that such delicacy is necessary for the power of
speech.
The sensitiveness of the tongue can be accounted for, however, Spencer
claims, as the result of the constant use of the tongue in exploring the
cavity of the mouth. It is continually moving about, and touching now
one part, and now another, of the mouth cavity. “No advantage is gained.
It is simply that the tongue’s position renders perpetual exploration
almost inevitable.” No other explanation of the facts seemed possible to
Spencer.
Two questions will at once suggest themselves. First, can it be shown
that the sensitiveness to touch in various parts of the body is the
result of individual experience? Have we learned to discriminate in
those parts of the body that are most often brought into contact with
surrounding objects? Even the power of discrimination in the tips of the
fingers can be improved, as Spencer himself has shown, in the case of
the blind, and of skilled compositors. Can we account in this way for
the power of discrimination in various parts of the body? In other
words, if, beginning in infancy, the middle of the back constantly came
into contact with surrounding objects, would this region become as
sensitive as the tips of the fingers? The experiment has not, of course,
been carried out, but it is not probable that it would succeed. I
venture this opinion on the ground of the relative number of the nerves
and of the organs of touch on the back, as compared with those of the
finger-tips. But, it will be asked, will not the number of the
sense-organs become greater if a part is continually used by the
individual? It is improbable that much improvement could be brought
about in this way. The improvement that takes place through experience
is probably not so much the result of the development of more
sense-organs, as of better discrimination in the sensation, because the
increased power can be very quickly acquired.
An examination of the relative abundance of touch-spots in the skin
shows that they are much more numerous in regions of greater
sensitiveness. The following table, taken from Sherrington’s account of
sense-organs in Schaefer’s “Textbook of Physiology,” gives the smallest
distance that two points, simultaneously applied, can be recognized as
such (and not simply as one impression) in different regions.
Mm.
Tip of tongue 1.1
Volar surface of 2.3
ungual phalanx of
finger
Red surface of lip 4.5
Volar face of second 4.5
phalanx
Dorsal face of third 6.8
phalanx
Side of tongue 9.0
Third line of tongue, 9.0
27 mm. from tip
Plantar face of ungual 11.3
phalanx of first toe
Palm 11.3
Back of second phalanx 11.3
of finger
Forehead 22.6
Back of ankle 22.6
Back of hand 31.6
Forearm, leg 40.6
Dorsum of foot 40.6
Outer sternum 45.1
Back of neck 54.1
Middle of back 67.1
Upper arm, thigh 67.1
The great difference in the sensitiveness of the skin in the different
regions is very striking, and if, as seems probable, about the same
proportionate difference is found at birth, then the degree of
sensibility of the different regions is inborn, and is not the result of
each individual experience. Until it can be shown that more of the
sense-organs develop in any special part, as the result of the increased
use of the part, we have no real basis on which to establish, even as
probable, the Lamarckian view.
But, after all, is the distribution of the sense-organs exactly that
which we should expect on the Lamarckian theory? Has not Spencer taken
too much for granted in this direction? The lower part of the forearm
(represented by 15) we should expect to be more sensitive than the
protected surface of the eyelid (11.3), but this is not the case. The
forehead (22.6) is much less sensitive than the forearm, and only half
as sensitive as the eyelid. The knee (36.1) is still less sensitive than
any of these other parts, and this does not in the least accord with the
theory, since in its constant moving forward it must be continually
coming into contact with foreign bodies. The fact that the back is as
insensitive as the upper arm (67.7) can hardly be accredited in favor of
the theory. The great difference between the lower third of the forearm
on the ulnar surface (15) and the upper arm (67.7) seems out of all
proportion to what we should expect on the theory. And is it not a
little odd that the end of the nose should be so highly sensitive?
There is another point that we cannot afford to neglect in this
connection. It is known that in addition to touch-spots there are warm
and cold spots in the skin, which produce, when touched, the sensation
of warmth, or of cold, respectively, and not the sensation of touch. The
degree of sensitiveness of different regions of the body throws an
interesting side-light on Spencer’s argument.
The warm spots are much fewer than the cold spots. The spots are
arranged in short lines radiating from centres which coincide with
hairs. The number of these spots varies a good deal, even in the same
region of the skin. If the sensitiveness of the skin is tested, the
following results will be obtained. The list includes twelve grades of
sensitiveness, beginning with the places giving the lowest maximum of
intensity. About one hundred square areas were tested in each region.
COLD SENSATIONS
1. Tips of fingers and toes, malleoli, ankle.
2. Other parts of digits, tip of nose, olecranon.
3. Glabella, chin, palm, gums.
4. Occiput, patella, wrist.
5. Clavicle, neck, forehead, tongue.
6. Buttocks, upper eyelid.
7. Lower eyelid, popliteal space, sole, cheek.
8. Inner aspect of thigh, arm above elbow.
9. The intercostal spaces along axillary line.
10. Mammary areola.
11. Nipple, flank.
12. Certain areas of the loins and abdomen.
WARMTH SENSATIONS
0. Lower gum, mucosa of cheek, cornea.
1. Tips of fingers and toes, cavity of mouth, conjunctiva, and
patella.
2. Remaining surface of digits, middle of forehead, olecranon.
3. Glabella, chin, clavicle.
4. Palm, buttock, popliteal space.
5. Neck.
6. Back.
7. Lower eyelid, cheek.
8. Nipple, loin.
These two tables show the great differences in the range of
sensitiveness to cold and to warmth in different parts of the body. I
doubt if any one will attempt to show that these differences of range of
sensation can be accounted for either by natural selection or by the
Lamarckian hypothesis.
Of course, it does not necessarily follow that, because this is true for
the warm and cold spots, that it must also be true for the tactile
organs; but I think that the fact of such a great difference in the
responsiveness to cold and to warmth in different parts of the body
should put us on our guard against a too ready acceptation of Spencer’s
argument. More especially is this seen to be necessary, when, as has
been shown above, the distribution of the touch-organs themselves by no
means closely corresponds to what we should expect, if they have
developed in response to contact, as Spencer maintains.
The other main argument advanced by Spencer to fortify the theory of the
inheritance of acquired characters, and at the same time to show the
inadequacy of the theory of natural selection, is based on the idea of
what he calls the “coöperation of the parts” that is required in order
to carry out any special act. Spencer contends that “the relative powers
of coöperative parts cannot be adjusted solely by the survival of the
fittest, and especially where the parts are numerous and the coöperation
complex.”
Spencer illustrates his point by the case of the extinct Irish elk,
whose immensely developed horns weighed over a hundredweight. The horns,
together with the massive skull, could not have been supported by the
outstretched neck without many and great changes of the muscles and
bones of the neck and of the fore-part of the body. Unless, for
instance, the fore-legs had been also strengthened, there would be
failure in fighting and in locomotion. Since “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 the parts that were for a time useless,—parts, moreover,
which would revert to their original sizes before the other needful
variations occurred.”
The answer made to this argument was that coördinating parts vary
together. In reply to which Spencer points to the following cases, which
show that this is not so: The blind crayfish in the Kentucky caves have
lost their eyes, but not the stalks that carry them. Again, the normal
relation between the length of tongue and of beak in some varieties of
pigeons is lost. The greater decrease in the jaws in some species of pet
dogs than of the number of their teeth has caused the teeth to become
crowded.[18] “I then argued that if coöperative parts, small in number,
and so closely associated as these are, do not vary together, it is
unwarrantable to allege that coöperative parts, which are very numerous
and remote from one another, vary together.” Spencer puts himself here
into the position of seriously maintaining that, because some
coöperative parts do not vary together, therefore no coöperative parts
have ever done so, and he has taken this position in the face of some
well-known cases in which certain parts have been found to vary
together.
Footnote 18:
It is curious that Spencer does not see that this case is as much
against his point as in favor of it, since the _unused_ teeth did not
also degenerate.
In this same connection Spencer brings up the familiar _pièce de
résistance_ of the Lamarckian school, the giraffe. He recognizes that
the chief traits in the structure of this animal are the result of
natural selection, since its efforts to reach higher branches could not
be the cause of the lengthening of the legs. But “the coadaptation of
the parts, required to make the giraffe’s structure useful, is much
greater than at first appears.” For example, the bones and the muscles
of the hind-legs have been also altered, and Spencer argues that it is
“impossible to believe” that all parts of the hind-quarters could have
been coadapted to one another, and to all parts of the fore-quarters. A
lack of coadaptation of a single muscle “would cause fatal results when
high speed had to be maintained while escaping from an enemy.”
Spencer claims that, since 1886, when he first published this argument,
nothing like an adequate response has been made; and I think he might
have added that an adequate answer is not likely to be forthcoming,
since nothing short of a demonstration of how the giraffe really evolved
is likely to be considered as sufficient. Wallace’s reply, that the
changes in question could have been brought about by natural selection,
since similar changes have been brought about by artificial selection,
is regarded as inadequate by Spencer, since it assumes a parallel which
does not exist. Nevertheless, Wallace’s reply contains, in my opinion,
the kernel of the explanation, in so far as it assumes that congenital
variation[19] may suffice to account for the origin of a form even as
bizarre as that of the giraffe. The ancon ram and the turnspit dog were
marked departures from the normal types, and yet their parts were
sufficiently coördinated for them to carry out the usual modes of
progression. It would not have been difficult, if we adopted Spencer’s
mode of arguing, to show that these new forms could not possibly have
arisen as the result of congenital variations.
Footnote 19:
Wallace assumes fluctuating variation to suffice, but in this I cannot
agree with him.
Again, it might be argued that the large, powerful dray-horse could not
have arisen through a series of variations from the ordinary horse,
because, even if variations in the right direction occurred in the
fore-quarters, it is unlikely that similar variations would occur in the
hind-quarters, etc. Yet the feat has been accomplished, and while it is
difficult to prove that the inheritance of acquired characters has not
had a hand in the process, it is improbable that this has been the case,
but rather that artificial selection of some kind of variations has been
the factor at work.
So long as the Lamarckian theory is supported by arguments like these,
it can never hope to be established with anything more than a certain
degree of probability. If it is correct, then its demonstration must
come from experiment. This brings us to a consideration of the
experimental evidence which has been supposed by some writers to give
conclusive proof of the validity of the theory.
The best direct evidence in favor of the Lamarckian argument is that
furnished by the experiments of Brown-Séquard. He found, as the result
of injury to the nervous system of guinea-pigs, that epilepsy appeared
in the adult animal, and that young born from these epileptic parents
became also epileptic. Still more important was his discovery that,
after an operation on the nerves, as a result of which certain organs,
the ear or the leg, for instance, are affected, the same affection
appears in the young born from such parents. These results of
Brown-Séquard have been vouched for by two of his assistants, and his
results in regard to the inheritance of epilepsy have been confirmed by
Obersteiner, and by Luciani on dogs. Equally important is their later
confirmation, as far as the main facts go, by Romanes.
Brown-Séquard gives the following summary of his results. I follow
Romanes’ translation in his book on “Darwin and after Darwin,” where
there is also given a careful analysis of Brown-Séquard’s results, as
well as the outcome of the experiments of Romanes himself. The summary
is as follows:—
1. “Appearance of epilepsy in animals born of parents which had been
rendered epileptic by an injury to the spinal cord.
2. Appearance of epilepsy also in animals born of parents which had been
rendered epileptic by section of the sciatic nerve.
3. A change in the shape of the ear in animals born of parents in which
such a change was the effect of a division of the cervical sympathetic
nerve.
4. Partial closure of the eyelids in animals born of parents in which
that state of the eyelids had been caused either by section of the
cervical sympathetic nerve, or the removal of the superior cervical
ganglion.
5. Exophthalmia in animals born of parents in which an injury to the
restiform body had produced that protrusion of the eyeball. This
interesting fact I have witnessed a good many times, and seen the
transmission of the morbid state of the eye continue through four
generations. In these animals modified by heredity, the two eyes
generally protruded, although in the parents usually only one showed
exophthalmia, the lesion having been made in most cases only on one of
the corpora restiformia.
6. Hæmatoma and dry gangrene of the ears in animals born of parents in
which these ear alterations had been caused by an injury to the
restiform body near the nib of the calamus.
7. Absence of two toes out of the three of the hind-leg, and sometimes
of the three, in animals whose parents had eaten up their hind-leg toes,
which had become anæsthetic from a section of the sciatic nerve alone,
or of that nerve and also of the crural. Sometimes, instead of complete
absence of the toes, only a part of one or two or three was missing in
the young, although in the parent not only the toes, but the whole foot
was absent (partly eaten off, partly destroyed by inflammation,
ulceration, or gangrene).
8. Appearance of various morbid states of the skin and hair of the neck
and face in animals born of parents having had similar alterations in
the same parts as effects of an injury to the sciatic nerve.”
Romanes, who later went over the same ground, in part under the
immediate direction of Brown-Séquard himself, has made some important
observations in regard to these results, many of which he was able to
confirm.
He did not repeat the experiment of cutting the cord, but he found that,
to produce epilepsy, it was only necessary to cut the sciatic nerve. The
“epileptiform habit” does not appear in the animal until some time after
the operation; it lasts for some weeks or months, and then disappears.
The attacks are not brought on spontaneously, but by “irritating a small
area of the skin behind the ear on the same side of the body as that on
which the sciatic nerve had been divided.” The attack lasts for only a
few minutes, and during it the animal is convulsed and unconscious.
Romanes thinks that the injury to the sciatic nerve, or to the spinal
cord, produces some sort of a change in the cerebral centres, “and that
it is this change—whatever it is, and in whatever part of the brain it
takes place—which causes the remarkable phenomena in question.”
In regard to Brown-Séquard’s statements, made in the 3d and the 4th
paragraphs, in respect to the results of the operation of cutting the
cervical sympathetic, Romanes had not confirmed the results when his
manuscript went to press; but soon afterward, after Romanes’ death, a
note was printed in _Nature_ by Dr. Hill, announcing that two
guinea-pigs from Romanes’ experiment had been born, “both of which
exhibited a well-marked droop of the upper eyelid. These guinea-pigs
were the offspring of a male and female in both of which I had produced
for Dr. Romanes, some months earlier, a droop of the left upper eyelid
by division of the left cervical sympathetic nerve. This result is a
corroboration of the series of Brown-Séquard experiments on the
inheritance of acquired characters.”
Romanes states that he also found that injury to a particular spot of
the restiform bodies is quickly followed by a protrusion of the eye on
the same side, and further, that he had “also had many cases in which
some of the progeny of parents thus affected have shown considerable
protrusion of the eyeballs of both sides, and this seemingly abnormal
protrusion has occasionally been transmitted to the next generation.
Nevertheless, I am far from satisfied that this latter fact is anything
more than an accidental coincidence.” This reservation is made on the
ground that the protrusion in the young is never so great as in the
parents, and also because there is amongst guinea-pigs a considerable
amount of individual variation in the degree of prominence of the
eyeballs. Romanes, while unwilling to deny that an “obviously abnormal
amount of protrusion, due to the operation, may be inherited in lesser
degree,” is also unwilling to affirm so important a conclusion on the
basis of these experiments alone.
In regard to Brown-Séquard’s 6th statement, Romanes found after injury
to the restiform body that hæmatoma and dry gangrene may supervene,
either several weeks after the operation, or at any subsequent time,
even many months afterward. The disease usually affects the upper parts
of both ears, and may then gradually extend downward until nearly the
whole ear is involved. “As regards the progeny of animals thus affected
in some cases, but by no means in all, a similarly morbid state of the
ears may arise apparently at any time in the life history of the
individual. But I have observed that in cases where two or more
individuals _of the same litter_ develop this diseased condition, they
usually do so at about the same time, even though this may be months
after birth, and therefore after the animals are fully grown.” Moreover,
the morbid process never extends so far in the young as it does in the
parents, and “it almost always affects the middle third of the ear.”
Several of the progeny from this first generation, which had apparently
inherited the disease, but had not themselves been directly operated
upon, showed a portion of the ear consumed apparently by the same
disease. Romanes then gives the following significant analysis of this
result. Since a different part of the ear of the progeny is affected,
and also a “very much less quantity thereof,” it might seem that the
result was due either to a mere coincidence, or to the transmission of
microbes. But he goes on to say, that he fairly well excluded both of
these possibilities, for, in the first place, he has never observed “the
very peculiar process in the ears, or in any other parts of guinea-pigs
which have neither themselves had the restiform bodies injured, nor been
born of parents thus mutilated.” In regard to microbes, Romanes tried to
infect the ears of normal guinea-pigs by first scarifying these parts,
and then rubbing them with the diseased surfaces of the ears of affected
guinea-pigs. In not a single case was the disease produced.
Romanes concludes that these “results in large measure corroborate the
statements of Brown-Séquard; and it is only fair to add that he told me
they were the results which he had himself obtained most frequently, but
that he had also met with many cases where the diseased condition of the
ears in parents affected the same parts in their progeny and also
occurred in more equal degrees.”
We come now to the remarkable conclusion given in Brown-Séquard’s 7th
statement, in regard to the absence of toes in animals whose parents had
eaten off their own hind toes and even parts of their legs. Romanes got
neuroses in the animals operated upon, and found that the toes might be
eaten off; but none of the young showed any defect in these parts.
Furthermore, Romanes repeated the same operation upon the descendants
through six successive generations, so as to produce, if possible, a
cumulative effect, but no inheritance of the mutilation was observed.
“On the other hand, Brown-Séquard informed me that he had observed this
inherited absence of toes only in about one or two per cent of cases.”
It is possible, therefore, Romanes adds, that his own experiments were
not sufficiently numerous to have obtained such cases.
In this connection I may give an account of some observations that I
made while carrying out some experiments in telegony with mice. I found
in one litter of mice that when the young came out of the nest they were
tailless. The same thing happened again when the second litter was
produced, but this time I made my observations sooner, and examined the
young mice immediately after birth. I found that the mother had bitten
off, and presumably eaten, the tails of her offspring at the time of
birth. Had I been carrying on a series of experiments to see if, when
the tails of the parents were cut off, the young inherit the defect, I
might have been led into the error of supposing that I had found such a
case in these mice. If this idiosyncrasy of the mother had reappeared in
any of her descendants, the tails might have disappeared in succeeding
generations. This perversion of the maternal instincts is not difficult
to understand, when we recall that the female mouse bites off the
navel-string of each of her young as they are born, and at the same time
eats the afterbirth. Her instinct was carried further in this case, and
the projecting tail was also removed.
Is it not possible that something of this sort took place in
Brown-Séquard’s experiment? The fact that the adults had eaten off their
own feet might be brought forward to indicate the possibility of a
perverted instinct in this case also. At least my observation shows a
possible source of error that must be guarded against in future work on
this subject.
In regard to the 8th statement of Brown-Séquard, as to various morbid
states of the skin, Romanes did not test this, because the facts which
it alleges did not seem of a sufficiently definite character.
These experiments of Brown-Séquard, and of those who have repeated them,
may appear to give a brilliant experimental confirmation of the
Lamarckian position; yet I think, if I were a Lamarckian, I should feel
very uncomfortable to have the best evidence in support of the theory
come from this source, because there are a number of facts in the
results that make them appear as though they might, after all, be the
outcome of a transmitted disease, as Weismann claims, rather than the
inheritance of an acquired character. Until we know more of the
pathology of epilepsy, it may be well not to lay too great emphasis on
these experiments. It should not be overlooked that during the long time
that the embryo is nourished in the uterus of the mother, there is ample
opportunity given for the transmission of material, or possibly even of
bacteria. If it should prove true that epilepsy is due to some substance
present in the nervous system, such substances could get there during
the uterine life of the embryo. Even if this were the case, it may be
claimed that it does not give an explanation of the local reappearance
of the disease in the offspring. But here also we must be on our guard,
for it is possible that only certain regions of the body are susceptible
to a given disease; and it has by no means been shown that the local
defect itself is inherited, but only the disease. Romanes insists that a
very special operation is necessary to bring about certain forms of
transmission.
It is well also to keep in mind the fact, that if this sort of effect is
inherited, then we must be prepared to accept as a possibility that
other kinds of injury to the parent may be transmitted to the offspring.
It would be of great disadvantage to animals if they were to inherit the
injuries that their parents have suffered in the course of their lives.
In fact, we might expect to find many plants and animals born in a
dreadful state of mutilation as a result of inheritances of this sort.
Thus, while the Lamarckians try to show that, on their principle,
characters for the good of the species may be acquired, they must also
be prepared, if they accept this kind of evidence, to grant that immense
harm may also result from its action. I do not urge this as an argument
against the theory itself, but point it out simply as one of the
consequences of the theory.
It has been shown quite recently, by Charrin, Delamare, and Moussu, that
when, after the operation of laparotomy on a pregnant rabbit or
guinea-pig, the kidney or the liver has become diseased, the offspring
sometimes show similar affections in the corresponding organs (kidney or
liver). The result is due, the authors think, to some substance set free
from the diseased kidney of the parent that affects the kidney of the
young in the uterus. By injecting into the blood of a pregnant animal
fresh extracts from the kidney of another animal, the authors believe
that the kidney of the young are also affected. It will be observed that
this transmission of an acquired character appears to be different from
that of transmission through the egg; for it is the developing, or
developed organ itself, that is acted upon. The results throw an
interesting light on the cases of epilepsy described by Brown-Séquard,
since they show that the diseased condition of the parent may be
transmitted to the later embryonic stages. May not, therefore,
Brown-Séquard’s results be also explained as due to direct transmission
from the organs of the parent to the similar organs of the young in the
uterus?
There is another series of experiments of a different sort that has been
used as an argument in favor of the Lamarckian view. These are the
results that Cunningham has obtained on young flatfish. He put the very
young fish, while still bilaterally symmetrical (in which stage the
pigment is equally developed on both sides of the body) into aquaria
lighted from below. He found that when the young fish begins to undergo
its metamorphosis, the pigment gradually disappears on one side, as it
would have done under normal conditions, _i.e._ when they are lighted
from above. If, however, the fish are kept for some time longer, lighted
from below, the pigment begins to come back again. “The first fact
proves that the disappearance of the pigment-cells from the lower side
in the metamorphosis is an hereditary character, and not a change
produced in each individual by the withdrawal of the lower side from the
action of light. On the other hand, the experiments show that the
absence of pigment-cells from the lower side throughout life is due to
the fact that light does not act upon that side, for, when it is allowed
to act, pigment-cells appear. It seems to me that the only reasonable
conclusion from these facts is, that the disappearance of pigment-cells
was originally due to the absence of light, and that the change has now
become hereditary. The pigment-cells produced by the action of light on
the lower side are in all respects similar to those normally present on
the upper side of the fish. If the disappearance of the pigment-cells
were due entirely to a variation of the germ-plasm, no external
influence could cause them to reappear, and, on the other hand, if there
were no hereditary tendency, the coloration of the lower side of the
flatfish when exposed would be rapid and complete.”[20]
Footnote 20:
_Natural Science_, October, 1893.
This evidence might be convincing were it not weakened by two or three
assumptions. In the first place, it is not shown that if the loss of
color on the lower side had been the result of the inheritance of an
acquired character that the results seen in Cunningham’s experiment
would follow as a consequence. Thus one of the starting-points of the
argument really begs the whole question. In the second place, it is
unproven that, had the loss of color of the lower side been the result
of a variation of the germ-plasm, no external influence could cause it
to reappear. In this connection there is another fact that has a bearing
on the point here raised. In some species of flatfish the right side is
turned down, and in other species the left. Occasionally an individual
is found in a right-sided species that is left-sided, and in such cases
the color is also reversed. Now, to explain this in the way suggested by
Cunningham, we should be obliged to assume that some of the ancestors
acquired the loss of pigment on one side of the body, and others on the
other side according to which side was turned down. This supposition
might be appealed to to give us an explanation of the occasional
reversal of the symmetry as a rare occurrence at the present time; but
the argument is so transparently improbable that, I believe, the
Lamarckian school would hesitate to make use of it, yet, in principle,
it is about the same as that Cunningham has followed above.
If, on the other hand, we suppose the difference in color of the two
sides to have been the result of a germ-variation, we need only suppose
that this was of such a kind that the color of the under side is only in
a latent condition, and if an external factor can cause a reaction to
take place on the light side, it is not surprising that this should call
forth the latent color patterns. The result can be given at least a
formal explanation on the theory that the original change was a
germ-variation.
We come now to the evidence derived from paleontology. A number of
evolutionists, more especially of the American school, have tried to
show that the evolution of a number of groups can best be accounted for
on the theory of the inheritance of acquired characters. A point that we
must always bear in mind is that evolution in a direct line need not
necessarily be the outcome of Lamarckian factors. Some of our leading
paleontologists, Cope, Hyatt, Scott, Osborn, have been strongly
impressed by the paleontological evidence in favor of the view that
evolution has often been in direct lines; and some, at least, of these
investigators have been led to conclude that only the Lamarckian factor
of the inheritance of acquired characters can give a sufficient
explanation of the facts. Paleontologists have been much impressed by
the fact that evolution has been along the lines which we might imagine
that it would follow if the effects of use and of disuse are inherited.
There is, however, no proof that this is the case, although there are a
number of instances to which this mode of explanation appears to give
the readiest solution. But, as has been said before, it is not this kind
of evidence that the theory is in need of, since Lamarck himself gave an
ample supply of illustrations. What we need is clear evidence that this
sort of inheritance is possible, and, from the very nature of the case,
it is just this evidence that fossil remains can never supply.
The same criticism may be made of the work of Ryder, Packard, Dali,
Jackson, Eimer, Cunningham, Semper, De Varigny, and others of the
Lamarckian school. Despite the large number of cases that they have
collected, which appear to them to be most easily explained on the
assumption of the inheritance of acquired characters, the proof that
such inheritance is possible is not forthcoming. Why not then spend a
small part of the energy, that has been used to expound the theory, in
demonstrating that such a thing is really possible? One of the chief
virtues of the Lamarckian theory is that it is capable of experimental
verification or contradiction, and who can be expected to furnish such
proof if not the Neo-Lamarckians?
We may fairly sum up our position in regard to the theory of the
inheritance of acquired characters in the verdict of “not proven.” I am
not sure that we should not be justified at present in claiming that the
theory is unnecessary and even improbable.
------------------------------------------------------------------------
CHAPTER VIII
CONTINUOUS AND DISCONTINUOUS VARIATION AND HEREDITY
The two terms _continuous_ and _discontinuous variation_ refer to the
succession or inheritance of the variations rather than to the actual
conditions amongst a group of individuals living at the same time; but
this distinction has only a subordinate value. The term _fluctuating_,
or _individual variation_, expresses more nearly the conditions of the
individuals of a species at any one time, and the continuation of this
sort of difference is the continuous variation spoken of above. The
discontinuous variations are probably of the same nature as those that
have been called mutations, and what Darwin sometimes called sports, or
single variations, or definite variations.
Continuous Variation
If we examine a number of individuals of the same species, we find that
no two of them are exactly alike in all particulars. If, however, we
arrange them according to some one character, for example, according to
the height, we find that there is a gradation more or less perfect from
one end of the series to the other. Thus, if we were to take at random a
hundred men, and stand them in line arranged according to their height,
the tops of their heads, if joined, would form a nearly continuous line;
the line will, of course, incline downward from the tallest to the
shortest man. This illustrates individual variation. An arrangement of
this kind fails to bring out one of the most important facts connected
with individual differences. If the line is more carefully examined, it
will be found that somewhere near the middle the men are much more
nearly of the same height, or rather there are more men having about the
same height than there are near the ends of the line. Another
arrangement will bring this out better. If we stand in a line all the
men from 60 to 61.9 inches, and in another parallel line all those
between 62 and 63.9, then those between 64 and 65.9, then between 66 and
67.9 inches in__ height, etc., it will be found that there are more men
in some of these lines than in others. The longest line will be that
containing the men of about 65 inches; the two lines formed out of men
on each side of this one will contain somewhat fewer men, and the next
ones fewer still, and so on. If we looked at our new group of men from
above, we should have a figure triangular in outline, the so-called
frequency polygon, Figure 3 B. With a larger amount of data of this sort
it is possible to construct a curve, the curve of frequency, Figure 3 A.
In order to obtain this curve of frequency, it is of course not
necessary to actually put the individuals in line, but the curve can be
drawn on paper from the measurements. We sort out the measurements into
classes as in the case given above. The classes are laid off at regular
intervals along a base-line by placing points at definite intervals.
Perpendiculars are then erected at each point, the height of each being
proportional to the frequency with which each class occurs. If now we
join the tops of these perpendiculars, the curve of frequency is the
result.
[Illustration:
Fig. 3.—Curves of frequency, etc.
A, normal curve.
B, showing the method of arranging individuals in lines containing
similar kinds of individuals.
C, curve that is skew to the right.
D, polygon of frequencies of horns of rhinoceros beetles.
(After Davenport.)]
“In arranging the individuals it will be found, as has been said, that
certain groups contain more individuals. They will form the longest
line. This value that occurs with the greatest frequency is called the
mode. The position of this modal class in the polygon is one of the
points of importance, and the spread of the polygon at its base is
another. A polygon with a low mode and a broad range means great
variability. The range may, however, be much affected by a single
individual standing far removed from the rest, so that a polygon
containing such an individual might appear to show greater variation
than really exists. Therefore we need a measure of variability that
shall take into account the departures of all the individuals from the
mode. One such measure is the arithmetical average of all the departures
from the mean in both directions; and this measure has been widely
employed. At present another method is preferred, namely, the square
root of the squared departures. This measure is called the standard
deviation. The standard deviation is of great importance, because it is
the index of variability.”[21]
Footnote 21:
Davenport, C. B., “The Statistical Study of Biological Problems,”
_Popular Science Monthly_, September, 1900.
Of the different kinds of polygons there are two main sorts, the simple
and the complex. The former have only a single mode, the latter have
more than one mode. Some simple polygons lie symmetrically on each side
of the mode, Figure 3 A; others are unsymmetrical or skew, Figure 3 B.
The skew polygon generally extends out on one side farther than on the
other. It has been suggested that when a polygon is symmetrical the
species is not changing, and when skew that the species is evolving in
the direction of the longer base. This assumes that the sort of
variation measured by these curves is of the kind of which evolution is
made up, but this is a question that we must further consider. How far
the change indicated by the skew curve may be carried is also another
point for further examination.
A complex polygon of variation, Figure 3 D, has been sometimes
interpreted to mean that two subgroups exist in a species, as is well
shown in the case of the rhinoceros beetle described by Bateson. Two
kinds of male individuals exist, some with long horns, others with short
horns; each with a mode of its own, the two polygons overlapping. Other
complex polygons may be due to changes occurring at different times in
the life of the individual, as old age, for example.
If, instead of examining the variations of the individuals of the race,
we study the variations in the different organs of the same individual,
we find in many cases that certain organs vary together. Thus the right
and the left leg nearly always vary in the same direction, also the
first joints of the index and middle fingers, and the stature and the
forearm. On the other hand, the length of the clavicle and that of the
humerus do not vary together to the same extent; and the breadth and
height of the skull even less so.
════════════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════
No. of Veins│ 10│ 11│ 12│ 13│ 14│ 15│ 16│ 17│ 18│ 19│ 20│ 21│ 22
────────────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────
First Tree │ —│ —│ —│ —│ —│ 1│ 4│ 7│ 9│ 4│ 1│ —│ —
Second Tree │ —│ —│ —│ 3│ 4│ 9│ 8│ 2│ —│ —│ —│ —│ —
════════════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════
We may also study those cases in which a particular organ is repeated a
number of times in the same individual, as are the leaves of a tree. If
the leaves of the same tree are examined in respect, for example, to the
number of veins that each contains, we find that the number varies, and
that the results give a variation polygon exactly like that when
different individuals are compared with one another. Let us take the
illustration given by Pearson. He counted the veins on each side of the
midrib of the leaves of the beech. If a number of leaves be collected
from one tree, and the same number from another, and if all those having
fifteen veins are put in one vertical column, and all those with sixteen
in another, as shown in the following table, it will be found that each
tree has a mode of its own. Thus in the first tree the mode is
represented by nine individuals having eighteen veins, and in the second
by nine individuals having fifteen veins. So far as this character is
concerned we might have interchanged certain of the individual leaves,
but we could not have interchanged the two series. They are _individual_
to the two trees. Now in what does this individuality consist? Clearly
there are most leaves in one tree with eighteen ribs, and most in the
other with fifteen ribs.
If we contrast these results with those obtained by picking at random a
large number of leaves from different beech trees, we have no longer
types of individuals, but racial characters. Pearson has given the
following table to illustrate these points:
Frequency of Different Types of Beech Leaves
════════════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════
No. of Veins│ 10│ 11│ 12│ 13│ 14│ 15│ 16│ 17│ 18│ 19│ 20│ 21│ 22
────────────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────
Frequency │ 1│ 7│ 34│ 110│ 318│ 479│ 595│ 516│ 307│ 181│ 36│ 15│ 1
════════════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════
Thus the mode for beech trees in general is sixteen; but, as shown in
the other table, this mode does not correspond with either of the two
individual modes here ascertained. The illustration shows that the
racial mode may differ from the individual mode. There are also cases
known in which the mode of a group of individuals living in one locality
is different from that of another group living in another locality. This
difference may be a constant one from year to year, although so slight,
that unless actual measurements are made, the difference cannot be
detected, because of the overlapping of the individuals from different
localities. If evolution took place by slow changes of this sort, it
might be possible to detect its action, even when very slow, by means of
measurements made on a large number of individuals. At least this has
been suggested by those who believe new species may result from changes
of this sort.
There is some evidence showing that by selecting particular individuals
of a series, and breeding from them, the mode may be changed in the
direction of selection. Thus it has been stated by Davenport that the
descendants of twelve- and thirteen-rayed daisies give a polygon with a
skewness of +1.92; while the descendants of twenty-one-rayed plants give
a polygon with a skewness of -.13.
Pearson has described very concisely the possibilities involved in the
selective action of the environment. He states that if we examine the
frequency distribution of a set of organisms that have just become
mature, and later make a similar examination on the same number of
individuals (but not the same individuals) during the period of
reproduction, we shall probably find that a change has taken place which
may have been due to selection of some sort. The same thing might be
found in the next generation, and, if it did, this would indicate that
“selection does not necessarily mean a permanent or a progressive
change.” The selection in this imaginary case would be purely periodic
and suffice only to maintain a given race under given conditions. “Each
new adolescent generation is not the product of the entire preceding
generation, but only of selected individuals. This is certainly the case
for civilized man, in which case twenty-six per cent of the married
population produce fifty per cent of the next generation.”
Pearson believes that “if a race has been long under the same
environment it is probable that only periodic selection is at work,
maintaining its stability. Change the environment and a secular change
takes place, the deviations from the mode previously destroyed giving
the requisite material.” “Clearly periods of rapidly changing
environment, of great climatological and geological change, are likely
to be associated with most marked secular selection. To show that there
is little or no change year by year in the types of rabbit and wild
poppy in our English fields, or of daphnia in our English ponds, is to
put forward no great argument for the inefficiency of natural selection.
Take the rabbit to Australia, the wild poppy to the Cape, the daphnia
into the laboratory, and change their temperature, their food supply,
and the chemical constituents of water and air, and then the existence
of no secular selection would indeed be a valid argument against the
Darwinian theory of evolution.” In regard to the last point, it should
be noted that, even if under the changed conditions a change in the mode
took place, as Pearson assumes, it does not follow necessarily that
selection has had anything to do with it, but the environment may have
directly changed the forms. Furthermore, and this is the essential
point, even if selection does act to the extent of changing the mode, we
should not be justified in concluding that this sort of change could go
on increasing as long as the selection lasts. All that might happen
would be to keep the species up to the highest point to which
fluctuating variation can be held. This need not lead to the formation
of new species, or direct the course of evolution.
Pearson points out further that, even if we suppose that a secular
change is produced in a new environment, we cannot explain how species
may break up into two or more races that are relatively infertile.
Suppose two groups of individuals, subjected to different environments,
become isolated geographically. Two local races will be produced.
“Isolation may account for the origin of local races, but never for the
origin of species unless it is accompanied by a differential fertility.”
In other words, Pearson thinks that, unless the reproductive organs are
correlated with other organs, in such a way that as these organs change
the interracial fertility of the germ-cells is altered, so that in the
two changed groups the individuals are no longer interfertile, new
species cannot be accounted for, since their mutual infertility is one
of their most characteristic features. “Without a barrier to
intercrossing during differentiation the origin of species seems
inexplicable.”
We need not discuss the various suggestions that have been made to
explain this difficulty, none of which, as Pearson points out, have been
satisfactory. He himself believes that a process of segregation of like
individuals must occur, during the incipient stages at least, in the
formation of species. Afterwards a correlation may exist between the new
organs and the germ-cells, of such a sort that a relative or an absolute
sterility between the incipient species is attained. After this
condition has been reached the two new species may freely intermix
without a return to the primitive type, since they are no longer fertile
_inter se_. It seems to me, also, that this would be an essential
requisite if we assume that species are slowly formed out of races from
individual differences, as Pearson supposes to be the case. There are,
however, other possibilities that Pearson does not take into account,
namely, that from the very beginning the change may be so great that the
new form is not fertile with the original one; and there is also another
possibility as well, that, although the new and the old forms are
fertile, the hybrids may be like one or the other parent, as in several
cases to be given later. Not that I mean to say that in either of these
two ways can we really offer a solution of the question of infertility,
for, from the evidence that we possess, it appears improbable that the
infertility of species _inter se_ has been the outcome of either of
these causes.
In support of his main thesis Pearson gives certain data in respect to
preferential mating in the human race. By this is meant that selection
of certain types of individuals is more likely to take place, and also
that the fertility of certain types of individuals is greater than that
of other types. The calculations are based on stature, color of hair,
and of eyes. The results appear to show in all cases examined that there
is a slight tendency to form new races as the result of the more
frequent selection of certain kinds of individuals. But even if this is
the case, what more do the results show than that local races may be
formed,—races having a certain mode for height, for color of eyes or of
hair? That changes of this kind can be brought about we knew already
without any elaborate measurements, yet we should not conclude from this
that new species will be formed by a continuation of the process.
Pearson writes: “As to the problem of evolution itself we are learning
to see it under a new light. Natural selection, combined with sexual
selection [by which Pearson means segregation of certain types through
individual selection] and heredity, is actually at work changing types.
We have quantitative evidence of its effects in many directions.” Yes!
but no evidence that selection of this sort can do anything more than
keep up the type to the upper limit attained in each generation by
fluctuating variations. Pearson adds, “Variations do not occur
accidentally, or in isolated instances; autogamic and assortive mating
are realities, and the problem of the near future is not whether
Darwinism is a reality, but what is quantitively the rate at which it is
working and has worked.” This statement expresses no more than Pearson’s
conviction that the process of evolution has taken place by means of
selection. He ignores other possibilities, which if established may put
the whole question in a very different light.
Heredity and Continuous Variation
It has been to a certain extent assumed in the preceding pages that both
parents are alike, or, if different, that they have an equal influence
on the offspring. This may be true in many cases for certain
characteristics. Thus a son from a tall father and a short mother may be
intermediate in height, or if the father is white and the mother black,
the children are mulattoes. But other characters rarely or never blend.
In such cases the offspring is more like one or the other parent, in
which case the inheritance is said to be exclusive. Thus if one parent
has blue eyes and the other black, some of the children may have black
eyes and others blue. There are also cases of particular inheritance
where there may be patches of color, some like the color of one parent,
some like that of the other parent. The latter two kinds of inheritance
will be more especially considered in the subsequent part of this
chapter; for the present we are here chiefly concerned with blended
characters.
How much in such cases does each parent contribute to the offspring?
This has been expressed by Galton in his law of ancestral heredity. This
law takes into account not only the two parents, but also the four
grandparents, and the eight great-grandparents, etc. There will be 1024
in the tenth generation. These 1024 individuals may be taken as a fair
sample of the general population, provided there has not been much
interbreeding. Are we then to look upon the individual as the fused or
blended product of the population a few generations back? If this were
true, should we not expect to find all the individuals of a community
very much alike, except for the fluctuating variations close around the
mode?
As a result of his studies on the stature of man, and on the coat color
of the Basset hounds, Galton has shown that the inheritance from the
parents can be represented by the fraction 1/2; that is one-half of the
peculiarities of the individual comes from the two parents. The four
grandparents together count for 1/4 of the total inheritance, the
great-grandparents 1/8, and so on, giving the series 1/2, 1/4, 1/8.
Pearson, taking certain other points into consideration, believes the
following series more fully represents the inheritance from the
ancestors, .3, .15, .075, .0375, etc. He concludes that, “if Darwinism
be the true view of evolution, _i.e._ if we are to describe evolution by
natural selection combined with heredity, then the law which gives us
definitely and concisely the type of the offspring in terms of the
ancestral peculiarities is at once the foundation stone of biology and
the basis upon which heredity becomes an exact branch of science.”
The preceding statements give some idea of what would occur in a
community in which no selection was taking place. The results will be
quite different, although the same general law of inheritance will hold,
if selection takes place in each generation. If, for instance, selection
takes place, the offspring after four generations will have .93 of the
selected character, and without further selection will not regress, but
breed true to this type.[22] “After six generations of selection the
offspring will, selection being suspended, breed true to under two per
cent divergence from the previously selected type.”
Footnote 22:
In this statement the earlier ancestors are assumed to be identical
with the general type of the population.
If, however, we do not assume that the ancestors were mediocre, it is
found that after six generations of selection the offspring will breed
true to the selected type within one per cent of its value. Thus, if
selection were to act on a race of men having a mode of 5 feet 9 inches,
and the 6-foot men were selected in each generation, then in six
generations this type would be permanently established, and this change
could be effected in two hundred years.[23]
Footnote 23:
Quoted from Pearson’s “Grammar of Science.”
Thus we have exact data as to what will happen on the average when
blended, fluctuating variations are selected. Important as such data
must always be to give us accurate information as to what will occur if
things are left to “chance” variations, yet if it should prove true that
evolution has not been the outcome of chance, then the method is
entirely useless to determine how evolution has occurred.
More important than a knowledge of what, according to the theory of
chances, fluctuating variations will do, will be information that would
tell us what changes will take place in each individual. In this field
we may hope to obtain data no less quantitative than those of chance
variations, but of a different kind. A study of some of the results of
discontinuous variation will show my meaning more clearly.
Discontinuous Variation
Galton, in his book on “Natural Inheritance,” points out that “the
theory of natural selection might dispense with a restriction for which
it is difficult to see either the need or the justification, namely,
that the course of evolution always proceeds by steps that are severally
minute and that become effective only through accumulation.” An apparent
reason, it is suggested, for this common belief “is founded on the fact
that whenever search is made for intermediate forms between widely
divergent varieties, whether they are of plants or of animals, of
weapons or utensils, of customs, religion, or language, or of any other
product of evolution, a long and orderly series can usually be made out,
each member of which differs in an almost imperceptible degree from the
adjacent specimens. But it does not at all follow because these
intermediate forms have been found to exist, that they were the very
stages that were passed through in the course of evolution. Counter
evidence exists in abundance, not only of the appearance of considerable
sports, but of their remarkable stability in hereditary transmission.”
Comparing such an apparently continuous series with machines, Galton
concludes, “If, however, all the variations of any machine that had ever
been invented were selected and arranged in a museum, each would differ
so little from its neighbors as to suggest the fallacious inference that
the successive inventions of that machine had progressed by means of a
very large number of hardly discernible steps.”
Bateson, also, in his “Materials for the Study of Variation,” speaks of
the two possible ways in which variations may arise. He points out that
it has been tacitly assumed that the transitions have been continuous,
and that this assumption has introduced many gratuitous difficulties.
Chief of these is the difficulty that in their initial and imperfect
stages many variations would be useless. “Of the objections that have
been brought against the Theory of Natural Selection, this is by far the
most serious.” He continues: “The same objection may be expressed in a
form which is more correct and comprehensive. We have seen that the
differences between species on the whole are Specific, and are
differences of kind forming a discontinuous Series, while the
diversities of environment to which they are subject are, on the whole,
differences of degree, and form a continuous Series; it is, therefore,
hard to see how the environmental differences can thus be made in any
sense the directing cause of Specific differences, which by the Theory
of Natural Selection they should be. This objection of course includes
that of the utility of minimal Variations.”
“Now the strength of this objection lies wholly in the supposed
continuity of the process of Variation. We see all organized nature
arranged in a discontinuous series of groups differing from each other
by differences which are Specific; on the other hand, we see the diverse
environments to which these forms are subject passing insensibly into
each other. We must admit, then, that if the steps by which the diverse
forms of life have varied from each other have been insensible,—if, in
fact, the forms ever made up a continuous series,—these forms cannot
have been broken into a discontinuous series of groups by a continuous
environment, whether acting directly as Lamarck would have, or as
selective agent as Darwin would have. This supposition has been
generally made and admitted, but in the absence of evidence as to
Variation it is nevertheless a gratuitous assumption, and, as a matter
of fact, when the evidence as to Variation is studied, it will be found
to be in a great measure unfounded.”
There is a fair number of cases on record in which discontinuous
variations have been seen to take place. Darwin himself has given a
number of excellent examples, and Bateson, in the volume referred to
above, has brought together a large and valuable collection of facts of
this kind.
Some of the most remarkable of these instances have been already
referred to and need only be mentioned here. The black-shouldered
peacock, the ancon ram, the turnspit dog, the merino sheep, tailless and
hornless animals, are all cases in point. In several of these it has
been discovered that the young inherit the peculiarities of their
parents if the new variations are bred together; and what is more
striking, if the new variation is crossed with the parent form, the
young are like one or the other parent, and not intermediate in
character. This latter point raises a question of fundamental importance
in connection with the origin of species.
Darwin states that he knows of _no cases in which, when different
species or even strongly marked varieties are crossed, the hybrids are
like one form or the other_. They show, he believes, always a blending
of the peculiarities of the two parents. He then makes the following
significant statement: “All the characters above enumerated which are
transmitted in a perfect state to some of the offspring and not to
others—such as distinct colors, nakedness of skin, smoothness of leaves,
absence of horns or tail, additional toes, pelorism, dwarfed structure,
etc., have all been known to appear suddenly in individual animals or
plants. From this fact, and from the several slight, aggregated
differences which distinguish domestic races and species from each
other, not being liable to this peculiar form of transmission, we may
conclude that it is in some way connected with the sudden appearance of
the characters in question.”
Darwin has, incidentally, raised here a question of the most
far-reaching import. If it should prove true, as he believes, that
inheritance of this kind of discontinuous variation is also
discontinuous, and that we do not get the same result when distinct
species are intercrossed, or even when well-marked domestic races are
interbred, then he has, indeed, placed a great obstacle in the path of
those who have tried to show that new species have arisen through
discontinuous variation of this sort.
If wild species, when crossed, give almost invariably intermediate
forms, then it may appear that we are going against the only evidence
that we can hope to obtain if we claim that discontinuous variation, of
the kind that sports are made of, has supplied the material for
evolution. If, furthermore, when distinct races of domesticated animals
are crossed, we do not get discontinuous inheritance, it might, perhaps,
with justness be claimed that this instance is paralleled by what takes
place when wild species are crossed. And if domesticated forms have been
largely the result of the selection of fluctuating variations, as Darwin
believes, then a strong case is apparently made out in favor of Darwin’s
view that continuous variation has given the material for the process of
evolution in nature. Whether selection or some other factor has directed
the formation of the new species would not, of course, be shown, nor
would it make any difference in the present connection.
Before we attempt to reach a conclusion on this point let us analyze the
facts somewhat more closely.
In the first place, a number of these cases of discontinuous variation
are of the nature of abnormalities. The appearance of extra fingers or
toes in man and other mammals is an example of this sort. This
abnormality is, if inherited at all, inherited completely; that is, if
present the extra digit is perfect, and never appears in an intermediate
condition, even when one of the parents was without it. The most obvious
interpretation of this fact is that when the material out of which the
fingers are to develop is divided up, or separated into its component
parts, one more part than usual is laid down. Similarly, when a flower
belonging to the triradiate type gives rise to a quadriradiate form,—as
sometimes occurs,—the new variation seems to depend simply on the
material being subdivided once more than usual; perhaps because a little
more of it is present, or because it has a somewhat different shape. My
reasons for making a surmise of this sort are based on certain
experimental facts in connection with the regeneration of animals. It
has been shown in several cases that it is possible to produce more than
the normal number of parts by simply dividing the material so that each
part becomes more or less a new whole, and the total number of parts
into which the material becomes subdivided is increased. It seems not
improbable that phenomena of this sort have occurred in the course of
evolution, although it is, of course, possible that those characters
that define species do not belong to this class of variation. To take an
example. There are nine neck-vertebræ in some birds, but in the swan the
number is twenty-five. We cannot suppose that the ancestor of the swan
gradually added enough materially to make up one new vertebra and then
another, but at least one new whole vertebra was added at a time; and we
know several cases in which the number of vertebræ in the neck has
suddenly been increased by the addition of one more than normal, and the
new vertebra is perfectly formed from the first.
In cases of this sort we can easily understand that the inheritance must
be either of one kind or the other, since intermediate conditions are
impossible, when it comes to the question of one or not one; but if one
individual had one and another six vertebræ, then it would be
theoretically possible for the hybrid to have three.
This brings us to a question that should have been spoken of before in
regard to the inheritance of discontinuous variation. It sometimes
occurs that a variation, which appears in other respects to be
discontinuous, is inherited in a blended form. Thus the two kinds of
variation may not always be so sharply separated as one might be led to
believe. There may be two different kinds of discontinuous variation in
respect to inheritance, or there may be variations that are only to a
greater or a less extent inherited discontinuously; and it seems not
improbable that both kinds occur.
This diversion may not appear to have brought us any nearer to the
solution of the difficulty that Darwin’s statement has emphasized,
except in so far as it may show that the lines are not so sharply drawn
as may have seemed to be the case. The solution of the difficulty is, I
believe, as follows:—
_The discontinuity referred to by Darwin relates to cases in which only
a single step (or mutation) has been taken, and it is a question of
inheritance of one or not one. If, however, six successive steps should
be taken in the same direction, then when such a form is crossed with
the original form, the hybrid may inherit only three of the steps and
stand exactly midway between the parent forms; or it may inherit four,
or five, or three, or two steps and stand correspondingly nearer to the
one_ _or to the other parent. Thus while it may not be possible to halve
a single step (hence one-sided inheritance), yet when more than one step
has been taken the inheritance may be divided. There is every evidence
that most of the Linnæan (wild) species that Darwin refers to have
diverged from the parent form, and from each other, by a number of
successive steps; hence on crossing, the hybrid often stands somewhere
between the two parent forms. On this basis not only can we meet
Darwin’s objection, but the point of view gives an interesting insight
into the problem of inheritance and the formation of species._
The whole question of inheritance has assumed a new aspect; first on
account of the work of De Vries in regard to the appearance of
discontinuous variation in plants; and secondly, on account of the
remarkable discoveries of Gregor Mendel as to the laws of inheritance of
discontinuous variations. Mendel’s work, although done in 1865, was long
neglected, and its importance has only been appreciated in the last few
years. We shall take up Mendel’s work first, and then that of De Vries.
Mendel’s Law[24]
Footnote 24:
Bateson, in his book on “Mendel’s Principles of Heredity,” has given
an admirable presentation of Mendel’s results. I have relied largely
on this in my account.
The importance of Mendel’s results and their wide application is
apparent from the results in recent years of De Vries, Correns,
Tschermak, Bateson, Castle, and others. Mendel carried out his
experiments on the pea, _Pisum sativum_. Twenty-two varieties were used,
which had been proven by experiment to be pure breeds. When crossed they
gave perfectly fertile offspring. Whether they all have the value of
varieties of a single species, or are different subspecies, or even
independent species, is of little consequence so far as Mendel’s
experiments are concerned. The flower of the pea is especially suitable
for experiments of this kind. It cannot be accidentally fertilized by
foreign pollen, because the reproductive organs are inclosed in the keel
of the flower, and, as a rule, the anthers burst and cover the stigma of
the same flower with its own pollen before the flower opens. In order to
cross-fertilize the plants it is necessary to open the young buds before
the anthers are mature and carefully remove all the anthers. Foreign
pollen may be then, or later, introduced.
The principle involved in Mendel’s law may be first stated in a
theoretical case, from which a certain complication that appears in the
actual results may be removed.
If _A_ represent a variety having a certain character, and _B_ another
variety in which the same character is different, let us say in color,
and if these two individuals, one of each kind, are crossed, the hybrid
may be represented by _H_. If a number of these hybrids are bred
together, their descendants will be of three kinds; some will be like
the grandparent, _A_, in regard to the special character that we are
following, some will be like the other grandparent, _B_, and others will
be like the hybrid parent, _H_. Moreover, there will be twice as many
with the character _H_, as with _A_, or with _B_.
A B
↘ ↙
H
↙|↘
A | B
↙ | ↘
A | B
H
↙|↘
A | B
↙ | ↘
A | B
H
↙|↘
A | B
H
If now we proceed to let these _A_’s breed together, it will be found
that their descendants are all _A_, forever. If the _B_’s are bred
together they produce only _B_’s. But when the _H_’s are bred together
they give rise to _H_’s, _A_’s, and _B_’s, as shown in the accompanying
diagram. In each generation, the _A_’s will also breed true, the _B_’s
true, but the _H_’s will give rise to the three kinds again, and always
in the same proportion.
Thus it is seen that the hybrid individuals continue to give off the
pure original forms, in regard to the special character under
consideration. The numerical relation between the numbers is also a
striking fact. Its explanation is, however, quite simple, and will be
given later.
In the actual experiment the results appear somewhat more complicated
because the hybrid cannot be distinguished from one of the original
parents, but the results really conform exactly to the imaginary case
given above. The accompanying diagram will make clearer the account that
follows.
A B
↘ ↙
↘ ↙
A(B)
↙ | ↘
↙ | ↘
A | B
|
A(B)
↙ | ↘
↙ | ↘
A | B
|
A(B)
↙ | ↘
↙ | ↘
A | B
|
A(B)
The hybrid, _A_(_B_), produced by crossing _A_ and _B_ is like _A_ so
far as the special character that we will consider is concerned. In
reality the character that _A_ stands for is only dominant, that is, it
has been inherited discontinuously, while the other character,
represented by _B_, is latent, or recessive as Mendel calls it.
Therefore, in the table, it is included in parentheses. If the hybrids,
represented by this form _A_(_B_), are bred together, there are produced
two kinds of individuals, _A_’s and _B_’s, of which there are three
times as many _A_’s as _B_’s. It has been found, however, that some of
these _A_’s are pure forms, as indicated by the _A_ on the left in our
table, while the others, as shown by their subsequent history, are
hybrids, _A_(_B_). There are also twice as many of these _A_(_B_)’s as
of the pure _A_’s (or of the _B_’s). Thus the results are really the
same as in our imaginary case, only obscured by the fact that the _A_’s
and the _A_(_B_)’s are exactly alike to us in respect to the character
chosen. We see also why there appear to be three times as many _A_’s as
_B_’s. In reality the results are 1 _A_, 2 _A_(_B_), 1 _B_.
In subsequent generations the results are the same as in this one, the
_A_’s giving rise only to _A_, the _B_’s to _B_, and the _A_(_B_)’s
continuing to split up into the three forms, as shown in our diagram.
Mendel found the same law to hold for all the characters he examined,
including such different ones as the form of the seed, color of
seed-albumen, coloring of seed-coat, form of the ripe pods, position of
flowers, and length of stem.
Mendel also carried out a series of experiments in which several
differentiating characters are associated. In the first experiment the
parental plants (varieties) differed in the form of the seed and in the
color of the albumen. The two characters of the seed plant are
designated by the capital letters _A_ and _B_; and of the pollen plant
by small _a_ and _b_. The hybrids will be, of course, combinations of
these, although only certain characters may dominate. Thus in the
experiments, the parents are _AB_ (seed plant) and _ab_ (pollen plant),
with the following seed characters:—
Seed parent {A form round Pollen parent {a form angular
_AB_ {B albumen yellow _ab_ {b albumen green
When these two forms were crossed the seeds appeared round and yellow
like those of the parent, _AB_, _i.e._ these two characters dominated in
the hybrid.
The seeds were sown, and in turn yielded plants which when
self-fertilized gave four kinds of seeds (which frequently all appeared
in the same pod). Thus 556 seeds were produced by 15 plants, having the
following characters:—
_AB_ 315 round and yellow
_Ab_ 101 angular and yellow
_aB_ 108 round and green
_ab_ 32 angular and green
These figures stand almost in the relation of 9 : 3 : 3 : 1.
These seeds were sown again in the following year and gave:—
From the round yellow seeds:—
_AB_ 38 round and yellow seeds
_ABb_ 65 round yellow and green seeds
_AaB_ 60 round yellow and angular yellow seeds
_AaBb_ 138 round yellow and green, angular yellow and green
seeds
From the angular yellow seeds:—
_aB_ 28 angular yellow seeds
_aBb_ 68 angular yellow and green seeds
From the round green seeds:—
_Ab_ 35 round green seeds
_Aab_ 67 round angular seeds
From the angular green seeds:—
_ab_ 30 angular green seeds
Thus there were 9 different kinds of seeds produced. There had been
separated out at this time 38 individuals like the parent seed plant,
_AB_, and 30 like the parent pollen plant, _ab_. Since these had come
from similar seeds of the preceding generation they may be looked upon
as pure at this time. The forms _Ab_ and _aB_ are also constant forms
which do not subsequently vary. The remainder are still mixed or hybrid
in character. By successive self-fertilizations it is possible gradually
to separate out from these the pure types of which they are compounded.
Without going into further detail it may be stated that the offspring of
the parent hybrids, having two pairs of differentiating characters, are
represented by the series:—
_AB_ _Ab_ _aB_ _ab_ 2_ABb_ 2_aBb_ 2_Aab_ 2_ABa_ 2_AaBb_
This series is really a combination of the two series:—
_A_ + 2_Aa_ + _a_
_B_ + 2_Bb_ + _b_
Mendel even went farther, and used two parent varieties having three
differentiating characters, as follows:—
_ABC seed parent_ _abc pollen plant_
{ A form round { a form angular
{ B albumen yellow { b albumen green
{ C seed-coat grey { c seed-coat white
brown
The results, as may be imagined, were quite complex, but can be
expressed by combining these series:—
_A_ + 2_Aa_ + _a_
_B_ + 2_Bb_ + _b_
_C_ + 2_Cc_ + _c_
In regard to the two latter experiments, in which two and three
characters respectively were used, it is interesting to point out that
the form of the hybrid more nearly approaches “to that one of the
parental plants which possesses the greatest number of dominant
characters.” If, for instance, the seed plant has short stem, terminal
white flowers, and simply inflated pods; the pollen plant, on the other
hand, a long stem, violet-red flowers distributed along the stem, and
constricted pods,—then the hybrid resembles the seed parent only in the
form of the pod; in its other characters it agrees with the pollen
plant. From this we may conclude that, if two varieties differing in a
large number of characters are crossed, the hybrid might get some of its
dominant characters from one parent, and other dominant characters from
the other parent, so that, unless the individual characters themselves
were studied, it might appear that the hybrids are intermediate between
the two parents, while in reality they are only combinations of the
dominant characters of the two forms. But even this is not the whole
question.
Mendel points out that, from knowing the characters of the two parent
forms (or varieties), one could not prophesy what the hybrid would be
like without making the actual trial. Which of the characters of the two
parent forms will be the dominant ones, and which recessive, can only be
determined by experiment. Moreover, the hybrid characters are something
peculiar to the hybrid itself, and to itself alone, and not simply the
combination of the characters of the two forms. Thus in one case a
hybrid from a tall and a short variety of pea was even taller than the
taller parent variety. Bateson lays much emphasis on this point,
believing it to be an important consideration in all questions relating
to hybridization and inheritance.
The theoretical interpretation that Mendel has put upon his results is
so extremely simple that there can be little doubt that he has hit on
the real explanation. The results can be accounted for if we suppose
that the hybrid produces egg-cells and pollen-cells, each of which is
the bearer of only one of the alternative characters, dominant or
recessive as the case may be. If this is the case, and if on an average
there are the same number of egg-cells and pollen-cells, having one or
the other of these kinds of characters, then on a random assortment
meeting of egg-cells and pollen-cells, Mendel’s law would follow. For,
25 per cent of dominant pollen grains would meet with 25 per cent
dominant egg-cells; 25 per cent recessive pollen grains would meet with
25 per cent recessive egg-cells; while the remaining 50 per cent of each
kind would meet each other. Or, as Mendel showed by the following
scheme:—
A A a a
| / |
| × |
| / |
A A a a
Or more simply by this scheme:—
A a
| /|
| × |
|/
A a
Mendel’s results have received confirmation by a number of more recent
workers, and while in some cases the results appear to be complicated by
other factors, yet there can remain little doubt that Mendel has
discovered one of the fundamental laws of heredity.
It has been found that there are some cases in which the sort of
inheritance postulated by Mendel’s law does not seem to hold, and, in
fact, Mendel himself spoke of such cases. He found that some kinds of
hybrids do not break up in later generations into the parent forms. He
also points out that in cases of discontinuity the variations in each
character must be separately regarded. In most experiments in crossing,
forms are chosen which differ from each other in a multitude of
characters, some of which are continuous and others discontinuous, some
capable of blending with their contraries while others are not. The
observer in attempting to discover any regularity is confused by the
complications thus introduced. Mendel’s law could only appear in such
cases by the use of an overwhelming number of examples which are beyond
the possibilities of experiment.[25]
Footnote 25:
This statement is largely taken from Bateson’s book.
Let us now examine the bearing of these discoveries on the questions of
variation which were raised in the preceding pages. It should be pointed
out, however, that it would be premature to do more than indicate, in
the most general way, the application of these conclusions. The chief
value of Mendel’s results lies in their relation to the theory of
inheritance rather than to that of evolution.
In the first place, Mendel’s results indicate that we cannot make any
such sharp distinction as Darwin does between the results of inheritance
of discontinuous and of continuous variations. As Mendel’s results show,
it is the separate characters that must be considered in each case, and
not simply the sum total of characters.
The more general objection that Darwin has made may appear to hold,
nevertheless. He thinks that the evolution of animals and plants cannot
rest primarily on the appearance of discontinuous variations, because
they occur rarely and would be swamped by intercrossing. If Mendel’s law
applies to such cases, that is, if a cross were made between such a
sport and the original form, the hybrid in this case, if
self-fertilized, would begin to split up into the two original forms.
But, on the other hand, it could very rarely happen that the hybrid did
fertilize its own eggs, and, unless this occurred, the hybrid, by
crossing with the parent forms in each generation, would soon lose all
its characters inherited from its “sport” ancestor. Unless, therefore,
other individuals gave rise to sports at the same time, there would be
little chance of producing new species in this way. We see then that
discontinuity in itself, unless it involved infertility with the parent
species, of which there is no evidence, cannot be made the basis for a
theory of evolution, any more than can individual differences, for the
swamping effect of intercrossing would in both cases soon obliterate the
new form. If, however, a species begins to give rise to a large number
of individuals of the same kind through a process of discontinuous
variation, then it may happen that a new form may establish itself,
either because it is adapted to live under conditions somewhat different
from the parent form, so that the dangers of intercrossing are lessened,
or because the new form may absorb the old one. It is also clear, from
what has gone before, that the new form can only cease to be fertile
with the parent form, or with its sister forms, after it has undergone
such a number of changes that it is no longer able to combine the
differences in a new individual. This result will depend both on the
kinds of the new characters, as well as the amounts of their difference.
This brings us to a consideration of the results of De Vries, who has
studied the first steps in the formation of new species in the
“mutations” of the evening primrose.
The Mutation Theory of De Vries
De Vries defines the mutation theory as the conception that “the
characters of the organism are made up of elements (‘Einheiten’) that
are sharply separated from each other. These elements can be combined in
groups, and in related species the same combinations of elements recur.
Transitional forms like those that are so common in the external
features of animals and plants do not exist between the elements
themselves, any more than they do between the elements of the chemist.”
This principle leads, De Vries says, in the domain of the descent theory
to the conception that species have arisen from each other, not
continuously, but by steps. Each new step results from a new combination
as compared with the old one, and the new forms are thereby completely
and sharply separated from the species from which they have come. The
new species is all at once there; it has arisen from the parent form
without visible preparation and without transitional steps.
The mutation theory stands in sharp contrast to the selection theory.
The latter uses as its starting-point the common form of variability
known as individual or fluctuating variation; but according to the
mutation theory there are two kinds of variation that are entirely
different from each other. “The fluctuating variation can, as I hope to
show, not overstep the bounds of the species, even after the most
prolonged selection,—much less can this kind of variation lead to the
production of new, constant characters.” Each peculiarity of the
organism has arisen from a preceding one, not through the common form of
variation, but through a sudden change that may be quite small but is
perfectly definite. This kind of variability that produces new species,
De Vries calls mutability; the change itself he calls a mutation. The
best-known examples of mutations are those which Darwin called “single
variations” or “sports.”
De Vries recognizes the following kinds of variation:—
First, the polymorphic forms of the systematists. The ordinary groups
which, following Linnæus, we call species, are according to De Vries
collective groups, which are the outcome of mutations. Many such Linnæan
species include small series of related forms, and sometimes even large
numbers of such forms. These are as distinctly and completely separated
from each other as are the best species. Generally these small groups
are called varieties, or subspecies,—varieties when they are separated
by a single striking character, subspecies when they differ in the
totality of their characters, in the so-called habitus.
These groups have already been recognized by some investigators as
elementary species, and have been given corresponding binary names. Thus
there are recognized two hundred elementary species of the form formerly
called _Draba verna_.
When brought under cultivation these elementary species are constant in
character and transmit their peculiarities truly. They are not local
races in the sense that they are the outcome in each generation of
special external conditions. Many other Linnæan species are in this
respect like _Draba verna_, and most varieties, De Vries thinks, are
really elementary species.
Second, the polymorphism due to intercrossing is the outcome of
different combinations of hereditary qualities. There are here, De Vries
says, two important classes of facts to be kept strictly
apart,—scientific experiment, and the results of the gardener and of the
cultivator. The experimenter chooses for crossing, species as little
variable as possible; the gardener and cultivator on the other hand
prefer to cross forms of which one at least is variable, because the
variations may be transmitted to the hybrid, and in this way a new form
be produced.
New elementary characters arise in experiments in crossing only through
variability, not through crossing itself.
Third, variability in the ordinary sense, that is, individual
variability, includes those differences between the individual organs
that follow Quetelet’s theory of chance. This kind of variability is
characterized by its presence at all times, in all groups of
individuals.
De Vries recalls Galton’s apt comparison between variability and a
polyhedron which can roll from one face to another. When it comes to
rest on any particular face, it is in stable equilibrium. Small
vibrations or disturbances may make it oscillate, but it returns always
to the same face. These oscillations are like the fluctuating
variations. A greater disturbance may cause the polyhedron to roll over
on to a new face, where it comes to rest again, only showing the ever
present fluctuations around its new centre. The new position corresponds
to a mutation. It may appear from our familiarity with the great changes
that we associate with the idea of discontinuous variability, that a
mutation must also involve a considerable change. Such, however, De
Vries says, is not the case. In fact, numerous mutations are smaller
than the extremes of fluctuating variation. For example, the different
elementary species of _Draba verna_ are less different from each other
than the forms of leaves on a tree. The essential differences between
the two kinds of variation is that the mutation is constant, while the
continuous variation fluctuates back and forth.
The following example is given by De Vries to illustrate the general
point of view in regard to varieties and species. The species _Oxalis
corniculata_ is a “collective” species that lives in New Zealand. It has
been described as having seven well-characterized varieties which do not
live together or have intermediate forms. If we knew only this group,
there would be no question that there are seven good species. But in
other countries intermediate forms exist, which exactly bridge over the
differences between the seven New Zealand forms. For this reason all the
forms have been united in a single species.
Another example is that of the fern, _Lomaria procera_, from New
Zealand, Australia, South Africa, and South America. If the forms from
only one country be considered, they appear to be different species; but
if all the forms from the different parts of the world be taken into
account, they constitute a connected group, and are united into one
large species.
It will be seen, therefore, that the limits of a collective species are
determined solely by the deficiencies in the genealogical tree of the
elementary species. If all the elementary species in one country were
destroyed, then the forms living in other countries that had been
previously held together because of those which have now been destroyed,
would, after the destruction, become true species. In other words: “The
Linnæan species are formed by the disappearance of other elementary
species, which at first connected all forms. This mode of origin is a
purely historical process, and can never become the subject of
experimental investigation.” Spencer’s famous expression, the “survival
of the fittest,” is incomplete, and should read the “survival of the
fittest species.” It is, therefore, not the study of Linnæan species
that has a physiological interest, but it is the study of the elementary
species of which the Linnæan species are made up, that furnishes the
all-important problem for experimental study.
De Vries gives a critical analysis of a number of cases in which new
races have been formed under domestication. He shows very convincingly
that, whenever the result has been the outcome of the selection of
fluctuating variations, the product that is formed can only be kept to
its highest point of development by the most rigid and ever watchful
care. If selection ceases for only a few generations, the new form sinks
back at once to its original level. Many of our cultivated plants have
really arisen, not by selection of this sort, but by mutations; and
there are a number of recorded cases where the first and sudden
appearance of a new form has been observed. In such cases as these there
is no need for selection, for if left to themselves there is no return
to the original form. If, however, after a new mutation has appeared in
this way, we subject its fluctuating variations to selection, we can
keep the new form up to its most extreme limit, but can do nothing more.
Another means, frequently employed, by which new varieties have been
formed is by bringing together different elementary species under
cultivation. For instance, there are a large number of wild elementary
species of apples, and De Vries believes that our different races of
apples owe their origin in part to these different wild forms. Crossing,
cultivation, and selection have done the rest.
De Vries points out some of the inconsistencies of those who have
attempted to discriminate between varieties and species. The only rule
that can be adhered to is that a variety differs from a species to which
it belongs in only one or in a few characters. Most so-called varieties
in nature are really elementary species, which differ from their nearest
relatives, not in one character only, but in nearly all their
characters. There is no ground, De Vries states, for believing them to
be varieties. If it is found inconvenient to rank them under the names
of the old Linnæan species, it will be better, perhaps, to treat them as
subspecies, but De Vries prefers to call them elementary species.
In regard to the distribution of species in nature, it may be generally
stated that the larger the geographical domain so much the larger is the
number of elementary species. They are found to be heaped up in the
centre of their area of distribution, but are more scattered at the
periphery.
In any one locality each Linnæan species has as a rule only one or a few
elementary species. The larger the area the more numerous the forms.
From France alone Jordan had brought together in his garden 50
elementary species of _Draba verna_. From England, Italy, and Austria
there could be added 150 more. This polymorphism is, De Vries thinks, a
general phenomenon, although the number of forms is seldom so great as
in this case.
Amongst animals this great variety of forms is not often met with, yet
amongst the mammalia and birds of North America there are many cases of
local forms or races, some of which at least are probably mutations.
This can only be proven, however, by actually transferring the forms to
new localities in order to find out if they retain their original
characters, or become changed into another form. It seems not improbable
that many of the forms are not the outcome of the external conditions
under which the animal now lives, but would perpetuate themselves in a
new environment.
From the evidence that his results have given, De Vries believes it is
probable that mutation has occurred in all directions. In the same way
that Darwin supposed that individual or fluctuating variations are
scattering, so also De Vries believes that the new forms that arise
through mutation are scattering. On this point it seems to me that De
Vries may be too much prejudiced by his results with the evening
primrose. If, as he supposes, many forms, generally ranked as varieties,
are really elementary species, it seems more probable that the mutation
of a form may often be limited to the production of one or of only a
very few new forms. The single variations, or sports, point even more
strongly in favor of this interpretation. Moreover, the general problem
of evolution from a purely theoretical point of view is very much
simplified, if we assume that the kinds of mutating forms may often be
very limited, and that mutations may often continue to occur in a direct
line. On this last point, De Vries argues that the evidence from
paleontology cannot be trusted, for all that we can conclude from fossil
remains is that certain mutations have dominated, and have been
sufficiently abundant to leave a record. In other words, the conditions
may have been such that only certain forms could find a foothold.
De Vries asks whether there are for each species periods of mutation
when many and great changes take place, and periods when relatively
little change occurs. The evidence upon which to form an opinion is
scanty, but De Vries is inclined to think that such periods do occur. It
is at least certain from our experience that there are long periods when
we do not see new forms arising, while at other times, although we know
very few of them, epidemics of change may take place. The mutative
period which De Vries found in the evening primrose is the best-known
example of such a period of active mutation. Equally important for the
descent theory is the idea that the same mutation may appear time after
time. There is good evidence to show that this really occurs, and in
consequence the chances for the perpetuation of such a form are greatly
increased. Delbœuf, who advocated this idea of the repeated reappearance
of a new form, has also attempted to show that if this occurs the new
form may become established without selection of any kind taking
place,—the time required depending upon the frequency with which the new
form appears. This law of Delbœuf, De Vries believes, is correct from
the point of view of the mutation theory. It explains, in a very simple
way, the existence of numerous species-characters that are entirely
useless, such, for instance, as exist between the different elementary
species of _Draba verna_. “According to the selection theory only useful
characters can survive; according to the mutation theory, useless
characters also may survive, and even those that may be hurtful to a
small degree.”
We may now proceed to examine the evidence from which De Vries has been
led to the general conclusions given in the preceding pages. De Vries
found at Hilversam, near Amsterdam, a locality where a number of plants
of the evening primrose, _Œnothera lamarckiana_, grow in large numbers.
This plant is an American form that has been imported into Europe. It
often escapes from cultivation, as is the case at Hilversam, where for
ten years it had been growing wild. Its rapid increase in numbers in the
course of a few years may be one of the causes that has led to the
appearance of a mutation period. The escaped plants showed fluctuating
variations in nearly all of their organs. They also had produced a
number of abnormal forms. Some of the plants came to maturity in one
year, others in two, or in rare cases in three, years.
A year after the first finding of these plants De Vries observed two
well-characterized forms, which he at once recognized as new elementary
species. One of these was _O. brevistylis_, which occurred only as
female plants. The other new species was a smooth-leafed form with a
more beautiful foliage than _O. lamarckiana_. This is _O. lævifola_. It
was found that both of these new forms bred true from self-fertilized
seeds. At first only a few specimens were found, each form in a
particular part of the field, which looks as though each might have come
from the seeds of a single plant.
ŒNOTHERA LAMARCKIANA
Elementary Species
===============+======+======+======+======+========+========+====+========
GENERATION |GIGAS |ALBIDA|OBLON-|RUBRI-|LAMARCK-|NANNELLA|LATA|SCINTIL-
| | |GATA |NERVIS| IANA | | | LANS
===============+======+======+======+======+========+========+====+========
|8 Gener. |
VIII | 1899 | 5 1 0 1700 21 1
|annual | ‘———————————v—————————’
| |
|7 Gener. |
VII | 1898 | 9 0 3000 11
|annual | ‘———————v——————’
| |
|6 Gener. |
VI | 1897 | 11 29 3 1800 9 5 1
|annual | ‘———————————v—————————————’
| |
|5 Gener. |
V | 1896 | 25 135 20 8000 49 142 6
|annual | ‘———————————v—————————————’
| |
|4 Gener. |
IV | 1895 | 1 15 176 8 14000 60 73 1
|annual | ‘—————————————v—————————————’
| |
|3 Gener. |
III | 1890-91 | 1 10000 3 3
|biennial | ‘———v——————————’
| |
|2 Gener. |
II | 1888-89 | 15000 5 5
|biennial | ‘—v——————————’
| |
|1 Gener. |
I | 1886-87 | 9
|biennial |
=====+=========+===========================================================
These two new forms, as well as the common _O. lamarckiana_, were
collected, and from these plants there have arisen the three groups or
families of elementary species that De Vries has studied. In his garden
other new forms also arose from those that had been brought under
cultivation. The largest group and the most important one is that from
the original _O. lamarckiana_ form. The accompanying table shows the
mutations that arose between 1887 and 1899 from these plants. The seeds
were selected in each case from self-fertilized plants of the
_lamarckiana_ form, so that the new plants appearing in each horizontal
line are the descendants in each generation of _lamarckiana_ parents. It
will be observed that the species, _O. oblongata_, appeared again and
again in considerable numbers, and the same is true for several of the
other forms also. Only the two species, _O. gigas_ and _O. scintillans_,
appeared very rarely.
Thus De Vries had, in his seven generations, about fifty thousand
plants, and about eight hundred of these were mutations. When the
flowers of the new forms were artificially fertilized with pollen from
the flowers on the same plant, or of the same kind of plant, they gave
rise to forms like themselves, thus showing that they are true
elementary species.[26] It is also a point of some interest to observe
that all these forms differed from each other in a large number of
particulars.
Footnote 26:
_O. lata_ is always female, and cannot, therefore, be self-fertilized.
When crossed with _O. lamarckiana_ there is produced fifteen to twenty
per cent of pure _lata_ individuals.
Only one form, _O. scintillans_, that appeared eight times, is not
constant as are the other species. When self-fertilized its seeds
produce always three other forms, _O. scintillans_, _O. oblongata_, and
_O. lamarckiana_. It differs in this respect from all the other
elementary species, which mutate not more than once in ten thousand
individuals.
From the seeds of one of the new forms, _O. lævifolia_, collected in the
field, plants were reared, some of which were _O. lamarckiana_ and
others _O. lævifolia_. They were allowed to grow together, and their
descendants gave rise to the same forms found in the _lamarckiana_
family, described above, namely, _O. lata_, _elliptica_, _nannella_,
_rubrinervis_, and also two new species, _O. spatulata_ and
_leptocarpa_.
In the _lata_ family, only female flowers are produced, and, therefore,
in order to obtain seeds they were fertilized with pollen from other
species. Here also appeared some of the new species, already mentioned,
namely, _albida_, _nannella_, _lata_, _oblongata_, _rubrinervis_, and
also two new species, _elliptica_ and _subovata_.
De Vries also watched the field from which the original forms were
obtained, and found there many of the new species that appeared under
cultivation. These were found, however, only as weak young plants that
rarely flowered. Five of the new forms were seen either in the Hilversam
field, or else raised from seeds that had been collected there. These
facts show that the new species are not due to cultivation, and that
they arise year after year from the seeds of the parent form, _O.
lamarckiana_.
Conclusions
From the evidence given in the preceding pages it appears that the line
between fluctuating variations and mutations may be sharply drawn. If we
assume that mutations have furnished the material for the process of
evolution, the whole problem appears in a different light from that in
which it was placed by Darwin when he assumed that the fluctuating
variations are the kind which give the material for evolution.
From the point of view of the mutation theory, species are no longer
looked upon as having been slowly built up through the selection of
individual variations, but the elementary species, at least, appear at a
single advance, and fully formed. This need not necessarily mean that
great changes have suddenly taken place, and in this respect the
mutation theory is in accord with Darwin’s view that _extreme_ forms
that rarely appear, “sports,” have not furnished the material for the
process of evolution.
As De Vries has pointed out, each mutation may be different from the
parent form in only a slight degree for each point, although all the
points may be different. The most unique feature of these mutations is
the constancy with which the new form is inherited. It is this fact, not
previously fully appreciated, that De Vries’s work has brought
prominently into the foreground. There is another point of great
interest in this connection. Many of the groups that Darwin recognized
as varieties correspond to the elementary species of De Vries. These
varieties, Darwin thought, are the first stages in the formations of
species, and, in fact, cannot be separated from species in most cases.
The main difference between the selection theory and the mutation theory
is that the one supposes these varieties to arise through selection of
individual variations, the other supposes that they have arisen
spontaneously and at once from the original form. The development of
these varieties into new species is again supposed, on the Darwinian
theory, to be the result of further selection, on the mutation theory,
the result of the appearance of new mutations.
In consequence of this difference in the two theories, it will not be
difficult to show that the mutation theory escapes some of the gravest
difficulties that the Darwinian theory has encountered. Some of the
advantages of the mutation theory may be briefly mentioned here.
1. Since the mutations appear fully formed from the beginning, there is
no difficulty in accounting for the incipient stages in the development
of an organ, and since the organ may persist, even when it has no value
to the race, it may become further developed by later mutations and may
come to have finally an important relation to the life of the
individual.
2. The new mutations may appear in large numbers, and of the different
kinds those will persist that can get a foothold. On account of the
large number of times that the same mutations appear, the danger of
becoming swamped through crossing with the original form will be
lessened in proportion to the number of new individuals that arise.
3. If the time of reaching maturity in the new form is different from
that in the parent forms, then the new species will be kept from
crossing with the parent form, and since this new character will be
present from the beginning, the new form will have much better chances
of surviving than if a difference in time of reaching maturity had to be
gradually acquired.
4. The new species that appear may be in some cases already adapted to
live, in a different environment from that occupied by the parent form;
and if so, it will be isolated from the beginning, which will be an
advantage in avoiding the bad effects of intercrossing.
5. It is well known that the differences between related species
consists largely in differences of unimportant organs, and this is in
harmony with the mutation theory, but one of the real difficulties of
the selection theory.
6. Useless or even slightly injurious characters may appear as
mutations, and if they do not seriously affect the perpetuation of the
race, they may persist.
In Chapters X and XI, an attempt will be made to point out in detail the
advantages which the mutation theory has over the Darwinian theory.
------------------------------------------------------------------------
CHAPTER IX
EVOLUTION AS THE RESULT OF EXTERNAL AND INTERNAL FACTORS
We come now to a consideration of other theories that have been advanced
to account for the evolution of new forms; and in so far as these new
forms are adapted to their environment, the theories will bear directly
on the question of the origin of adaptive variations. One school of
transformationists has made the external world and the changes taking
place in it the source of new variations. Another school believes that
the changes arise within the organism itself. We may examine these two
points of view in turn.
The Effect of External Influences
We have already seen that Lamarck held as a part of his doctrine of
transformation that the changes in the external world, the environment,
bring about, directly, changes in the organism, and he believed that all
plants and many of the lower animals have evolved as the result of a
reaction of this sort. This idea did not originate with Lamarck,
however, since before him Buffon had advanced the same hypothesis, and
there cannot be much doubt that Lamarck borrowed from his patron,
Buffon, this part of his theory of evolution.
This idea of the influence of the external world as a factor inducing
changes in the organism has come, however, to be associated especially
with the name of Geoffroy Saint-Hilaire, whose period of activity,
although overlapping, came after that of Lamarck. The central idea of
Geoffroy’s view was that species of animals and plants undergo change as
the environment changes; and it is important to note, in passing, that
he did not suppose that these changes were always for the benefit of the
individual, _i.e._ they were not always adaptive. If they were not, the
forms became extinct. So long as the conditions remain constant, the
species remains constant; and he found an answer in this to Cuvier’s
argument, in respect to the similarity between the animals living at
present in Egypt and those discovered embalmed along with mummies at
least two thousand years old. Geoffroy Saint-Hilaire said, that since
the climatic conditions of Egypt had remained exactly the same during
all these years, the animals of Egypt would also have remained
unchanged.
Geoffroy’s views were largely influenced by his studies in systematic
zoology and by his conception of a unity of plan running through the
entire animal kingdom. His study of embryology and paleontology had led
him to believe that present forms have descended from other organisms
living in the past, and in this connection his discovery of teeth in the
jaws of the embryo of the baleen whale and also his discovery of the
embryonic dental ridges in the upper and in the lower jaws of birds,
were used with effect in supporting the theory of change or evolution.
Lastly, his remarkable work in the study of abnormal forms prepared the
way for his conception of sudden and great changes, which he believed
organisms capable of undergoing. He went so far in fact, in one
instance, as to suppose that it was not impossible that a bird might
have issued fully equipped from the egg of a crocodile. Such an extreme
statement, which seems to us nowadays only laughable, need not prejudice
us against the more moderate parts of his speculation.
His study of the fossil gavials found near Caen led him to believe that
they are quite distinct from living crocodiles. He asked whether these
old forms may not represent a link in the chain that connects, without
interruption, the older inhabitants of the earth with animals living at
the present time. Without positively affirming that this is the case, he
did not hesitate to state that a transformation of this sort seemed
possible to him. He said: “I think that the process of respiration
constitutes an acquirement so important in the ‘disposition’ of the
forms of animals, that it is not at all necessary to suppose that the
surrounding respiratory gases become modified quickly and in large
amount in order that the animal may become slowly modified. The
prolonged action of time would ordinarily suffice, but if combined with
a cataclysm, the result would be so much the better.”
He supposed that in the course of time respiration becomes difficult and
finally impossible as far as certain systems of organs are concerned.
The necessity then arises and creates another arrangement, perfecting or
altering the existing structures. Modifications, fortunate or fatal, are
created which through propagation are continued, and which, if
fortunate, influence all the rest of the organization. But if the
modifications are injurious to the animals in which they have appeared,
the animals cease to exist, and are replaced by others having a
different form, and one suited to the new circumstances.
The comparison between the stages of development of the individual and
the evolution of the species was strongly impressed on the mind of
Geoffroy. He says: “We see, each year, the spectacle of the
transformation in organization from one class into another. A batrachian
is at first a fish under the name of a tadpole, then a reptile
(amphibian) under that of a frog.” “The development, or the result of
the transformation, is brought about by the combined action of light and
of oxygen; and the change in the body of the animal takes place by the
production of new blood-vessels, whose development follows the law of
the balancing of organs, in the sense, that if the circulating fluids
precipitate themselves into new channels there remains less in the old
vessels.” By preventing tadpoles from leaving the water, Geoffroy claims
that it has been shown that they can be prevented from changing into
frogs. The main point that Geoffroy attempts to establish is no doubt
fairly clear, but the way in which he supposes the change to be effected
is not so clear, and his ideas as to the way in which new change may be
perpetuated in the next generation are, from our more modern point of
view, extremely hazy. It is perhaps not altogether fair to judge his
view from the standpoint of the origin of adaptive structures, but
rather as an attempt to explain the causes that have brought about the
evolution of the organic world.
During the remainder of the nineteenth century there accumulated a large
number of facts in relation to the action of the external conditions in
bringing about changes in animals and plants. Much of this evidence is
of importance in dealing with the question of the origin of organic
adaptation.
The first class of facts in this connection is that of geographical
variation in animals and plants. It will be impossible here to do more
than select some of the most important cases. De Varigny, in his book on
“Experimental Evolution,” has brought together a large number of facts
of this kind, and from his account the following illustrations have been
selected. He says: “When the small brown honey-bee from High Burgundy is
transported into Bresse—although not very distant—it soon becomes larger
and assumes a yellow color; this happens even in the second generation.”
It is also pointed out that the roots of the beet, carrot, and radish
are colorless in their wild natural state, but when brought under
cultivation they become red, yellow, etc. Vilmorin has noted that the
red, yellow, and violet colors of carrots appear only some time after
the wild forms have been brought under cultivation. Moquin-Tandon has
seen “gentians which are blue in valleys become white on mountains.”
Other cases also are on record in which the colors of a plant are
dependent on external conditions.
The sizes of plants and animals are also often directly traceable to
certain external conditions; the change is generally connected with the
amount of food obtainable. “Generally speaking,” De Varigny says,
“insular animals are smaller than their continental congeners. In the
Canary Islands the oxen of one of the smallest islands are smaller than
those on the others, although all belong to the same breed, and the
horses are also smaller, and the indigenous inhabitants are in the same
case, although belonging to a tall race. It would seem that in Malta
elephants were very small,—fossil elephants, of course,—and that during
the Roman period the island was noted for a dwarf breed of dogs, which
was named after its birthplace, according to Strabo. In Corsica, also,
horses and oxen are very small, and _Cervus corsicanus_, the indigenous
deer, is quite reduced in dimensions; ... and lastly, the small
dimensions of the Falkland horses—imported from Spain in 1764—are
familiar to all. The dwarf rabbits of Porto Santo described by Darwin
may also be cited as a case in point.”
These facts, interesting as they are, will, no doubt, have to be more
carefully examined before the evidence can have great value, for it is
not clear what factor or factors have produced the decrease in size of
these animals.
The following cases show more clearly the immediate effect of the
environment: “Many animals, when transferred to warm climates, lose
their wool, or their hairy covering is much reduced. In some parts of
the warmer regions of the earth, sheep have no wool, but merely hairs
like those of dogs. Similarly, as Roulin notices, poultry have, in
Columbia, lost their feathers, and while the young are at first covered
with a black and delicate down, they lose it in great part as they grow,
and the adult fowls nearly realize Plato’s realistic description of
man—a biped without feathers. Conversely, many animals when transferred
from warm to cold climates acquire a thicker covering; dogs and horses,
for instance, becoming covered with wool.”
A number of kinds of snails that were supposed to belong to different
species have been found, on further examination, to be only varieties
due to the environment. “Locard has discovered through experiments that
_L. turgida_ and _elophila_ are mere varieties—due to environment—of the
common _Lymnæa stagnalis_.” He says, “These are not new species, but
merely common aspects of a common type, which is capable of modification
and of adaptation according to the nature of the media in which it has
to live.” It has also been shown by Bateson that similar changes occur
in _Cardium edule_, and other lamellibranchs are known to vary according
to the nature of the water in which they live.
In regard to plants, the influence of the environment has long been
known to produce an effect on the form, color, etc., of the individuals.
“The common dandelion (_Taraxacum densleonis_) has in dry soil leaves
which are much more irregular and incised, while they are hardly dentate
in marshy stations, where it is called _Taraxacum palustre_.
“Individuals growing near the seashore differ markedly from those
growing far inland. Similarly, species such as some Ranunculi, which can
live under water as well as in air, exhibit marked differences when
considered in their different stations, as is well known to all. These
differences may be important enough to induce botanists to believe in
the existence of two different species when there is only one.”
An interesting case is that of _Daphnia rectirostris_, a small
crustacean living sometimes in fresh water, at other times in water
containing salt and also in salt lakes. There are two forms,
corresponding to the conditions under which they live, and it is said
that the differences are of a kind that suffice to separate species from
each other. In another crustacean, _Branchipus ferox_, the form differs
in a number of points, according to whether it lives in salt or in fresh
water. Schmankewitsch says that, had he not found all transitional
forms, and observed the transformation in cultures, he would have
regarded the two forms as separate species. The oft-quoted case of
Artemia furnishes a very striking example of the influence of the
environment. _Artemia salina_ lives in water whose concentration varies
between 5 and 12 degrees of saltness. When the amount of salt is
increased to 12 degrees, the animal shows certain characteristics like
those of _Artemia milhausenii_, which may live in water having 24 to 25
degrees of saltness. The form _A. salina_ may be further completely
changed into that of _A. milhausenii_ by increasing the amount of salt
to the latter amount.
Among domesticated animals and plants—a few instances of which have been
already referred to—we find a large number of cases in which a change in
the environment produces definite changes in the organism. Darwin has
made a most valuable collection of facts of this kind in his “Animals
and Plants under Domestication.” He believes that domesticated forms are
much more variable than wild ones, and that this is due, in part, to
their being protected from competition, and to their having been removed
from their natural conditions and even from their native country. “In
conformity with this, all our domesticated productions without exception
vary far more than natural species. The hive-bee, which feeds itself,
and follows in most respects its natural habits of life, is the least
variable of all domesticated animals.... Hardly a single plant can be
named, which has long been cultivated and propagated by seed, that is
not highly variable.” “Bud-variation ... shows us that variability may
be quite independent of seminal reproduction, and likewise of reversion
to long-lost ancestral characters. No one will maintain that the sudden
appearance of a moss-rose on a Provence rose is a return to a former
state, ... nor can the appearance of nectarines on peach trees be
accounted for on the principle of reversion.” It is said that
bud-variations are also much more frequent on cultivated than on wild
plants.
Darwin adds: “These general considerations alone render it probable that
variability of every kind is directly or indirectly caused by changed
conditions of life. Or to put the case under another point of view, if
it were possible to expose all the individuals of a species during many
generations to absolutely uniform conditions of life, there would be no
variability.”
In some cases it has been observed that, in passing from one part of a
continent to another, many or all of the forms of the same group and
even of different groups change in the same way. Allen’s account of the
variations in North American birds and mammals furnishes a number of
striking examples of this kind of change. He finds that, as a rule, the
birds and mammals of North America increase in size from the south
northward. This is true, not only for the individuals of the same
species, but generally the largest species of each genus are in the
north. There are some exceptions, however, in which the increase in size
is in the opposite direction. The explanation of this is that the
largest individuals are almost invariably found in the region where the
group to which the species belongs receives its greatest numerical
development. This Allen interprets as the hypothetical “centre of
distribution of the species,” which is in most cases doubtless also its
original centre of dispersal. If the species has arisen in the north,
then the northern forms are the largest; but if it arose in the south,
the reverse is the case. Thus, most of the species of North America that
live north of Mexico are supposed to have had a northern origin, as
shown by the circumpolar distribution of some of them and by the
relationship of others to Old World species; and in these the largest
individuals of the species of a genus are northern. Conversely, in the
exceptional cases of increase in size toward the south, it can be shown
that the forms have probably had a southern origin.
The Canidæ (wolves and foxes) have their largest representatives, the
world over, in the north. “In North America the family is represented by
six species, the smallest of which (speaking generally) are southern and
the largest northern.” The three species that have the widest ranges
(the gray wolf, the common fox, and the gray fox) show the most marked
differences in size. The skull, for instance, of “the common wolf is
fully one-fifth larger in the northern parts of British America and
Alaska than it is in northern Mexico, where it finds the southern limit
of its habitat. Between the largest northern skull and the largest
southern skull there is a difference of about thirty-five per cent of
the mean size. Specimens from the intermediate region show a gradual
intergradation between the extremes, although many of the examples from
the upper Missouri country are nearly as large as those from the extreme
north.” The common fox is about one-tenth larger, on the average, in
Alaska than it is in New England. The gray fox, whose habitat extends
from Pennsylvania southward to Yucatan, has an average length of skull
of about five inches in the north, and less than four in Central
America—about ten per cent difference.
The Felidæ, or cats, “reach their greatest development as respects both
the number and the size of the species in the intertropical regions.
This family has sent a single typical representative, the panther
(_Felis concolor_), north of Mexico, and this ranges only to about the
northern boundary of the United States. The other North American
representatives of the family are the lynxes, which in some of their
varieties range from Alaska to Mexico.” Although they vary greatly in
different localities in color and in length and texture of pelage, they
do not vary as to the size of their skulls. On the other hand the
panther (and the ocelots) greatly increases in size southward, “or
toward the metropolis of the family.”
Other carnivora that increase in size northward are the badger, the
marten, the fisher, the wolverine, and the ermine, which are all
northern types.
Deer are also larger in the north; in the Virginia deer the annually
deciduous antlers of immense size reach their greatest development in
the north. The northern race of flying squirrels is one-half larger than
the southern, “yet the two extremes are found to pass so gradually one
into the other, that it is hardly possible to define even a southern and
a northern geographical race.” The species ranges from the arctic
regions to Central America.
In birds also similar relations exist, but there is less often an
increase in size northward. In species whose breeding station covers a
wide range of latitude, the northern birds are not only smaller, but
have quite different colors, as is markedly the case in the common
quail, the meadow-lark, the purple grackle, the red-winged blackbird,
the flicker, the towhee bunting, the Carolina dove, and in numerous
other species. The same difference is also quite apparent in the blue
jay, the crow, in most of the woodpeckers, in the titmice, numerous
sparrows, and several warblers and thrushes. The variation often amounts
to from ten to fifteen per cent of the average size of the species.
Allen also states that certain parts of the animal may vary
proportionately more than the general size, there being an apparent
tendency for peripheral parts to enlarge toward the warmer regions,
_i.e._ toward the south. “In mammals which have the external ears
largely developed—as in the wolves, foxes, some of the deer, and
especially the hares—the larger size of this organ in southern as
compared with northern individuals of the same species, is often
strikingly apparent.” It is even more apparent in species inhabiting
open plains. The ears of the gray rabbit of the plains of western
Arizona are twice the size of those of the Eastern states.
In birds the bill especially, but also the claws and tail, is larger in
the south. In passing from New England southward to Florida the bill in
slender-billed forms becomes larger, longer, more attenuated, and more
decurved; while in short-billed forms the southern individuals have
thicker and larger bills, although the birds themselves are smaller.
The remarkable changes and gradations of color in birds in different
parts of North America are very instructive, and the important results
obtained by American ornithologists form an interesting chapter in
zoology. The evidence would convince the most sceptical of the
difficulty of distinguishing between Linnæan species. It is not
surprising to find in this connection a leading ornithologist
exclaiming, “if there really are such things as species.” The
differences here noted are mainly from east to west. We may briefly
review here a few striking cases selected from Coues’s “Key to North
American Birds.”
The flicker, or golden-winged woodpecker (_Colaptes auratus_), has a
wide distribution in eastern North America. It is replaced in western
North America (from the Rocky Mountains to the Pacific) by _C.
mexicanus_. In the intermediate regions, Missouri and the Rocky Mountain
region, the characters of the two are blended in every conceivable
degree in different specimens. “Perhaps it is a hybrid, and perhaps it
is a transitional form, and doubtless there are no such things as
species in Nature.... In the west you will find specimens _auratus_ on
one side of the body, _mexicanus_ on the other.” There is a third form,
_C. chrysoides_, with the wings and tail as in _auratus_, and the head
as in _mexicanus_, that lives in the valley of the Colorado River, Lower
California, and southward.
In regard to the song-sparrow (_Melospiza_), Coues writes: “The type of
the genus is the familiar and beloved song-sparrow, a bird of constant
characters in the east, but in the west is split into numerous
geographical races, some of them looking so different from typical
_fasciata_ that they have been considered as distinct species, and even
placed in other genera. This differentiation affects not only their
color, but the size, relative proportions of parts, and particularly the
shape of the bill; and it is sometimes so great, as in the case of _M.
cinerea_, that less dissimilar looking birds are commonly assigned to
different genera. Nevertheless the gradation is complete, and affected
by imperceptible degrees.... The several degrees of likeness and
unlikeness may be thrown into true relief better by some such
expressions as the following, than by formal antithetical phrases: (1)
The common eastern bird commonly modified in the interior into the
duller colored (2) _fallax_. This in the Pacific watershed, more
decidedly modified by deeper coloration,—broader black streaks in (3)
_hermanni_, with its diminutive local race (4) _samuelis_, and more
ruddy shades in (5) _guttata_ northward, increasing in intensity with
increased size in (6) _rafina_. Then the remarkable (7) _cinerea_,
insulated much further apart than any of the others. A former American
school would probably have made four ‘good species,’ (1) _fasciata_, (2)
_samuelis_, (3) _rafina_, (4) _cinerea_.”
Somewhat similar relations are found in three other genera of finches.
Thus Passerella is “imperfectly differentiated”; Junco is represented by
one eastern species, but in the west the stock splits up into numerous
forms, “all of which intergrade with each other and with the eastern
bird. Almost all late writers have taken a hand at Junco, shuffling them
about in the vain attempt to decide which are ‘species’ and which
‘varieties.’ All are either or both, as we may elect to consider them.”
In the distribution of the genus Pipilo similar relations are found.
There is an eastern form much more distinct from the western forms than
these are from each other.
Finally may be mentioned the curious variations in screech-owls of the
genus Scops. This owl has two strikingly different plumages—a mottled
gray and a reddish brown, which, although very distinct when fully
developed, yet “are entirely independent of age, season, or sex.” There
is an eastern form, _Scops asio_, that extends west to the Rocky
Mountains. There is a northwestern form, _S. kennicotti_, which in its
red phase is quite different from _S. asio_, but in its gray plumage is
very similar. The California form, _S. benderii_, is not known to have a
red phase, and the gray phase is quite different from that of _S. asio_,
but like the last form. The Colorado form, _S. maxwellæ_, has no red
phase, “but on the contrary the whole plumage is very pale, almost as if
bleached, the difference evident in the nestlings even.” The Texas form,
_S. maselli_, has both phases, and is very similar to _S. asio_. The
Florida form is smaller and colored like _S. asio_. The red phase is the
frequent, if not the usual, one. The flammulated form, _S. fiammula_, is
“a very _small species_, with much the general aspect of an ungrown _S.
asio_.” This is the southwestern form, easily distinguished on account
of its small size and color from the other forms.
These examples might be greatly increased, but they will suffice, I
think, to convince one of the difficulty of giving a sharp definition to
“species.” The facts speak strongly in favor of the transmutation
theory, and show us how a species may become separated under different
conditions into a number of new forms, which would be counted as new
different species, if the intermediate forms were exterminated.
In discussing the nature of the changes that bring about variability,
Darwin remarks: “From a remote period to the present day, under climates
and circumstances as different as it is possible to conceive, organic
beings of all kinds, when domesticated or cultivated, have varied. We
see this with the many domestic races of quadrupeds and birds belonging
to different orders, with goldfish and silkworms, with plants of many
kinds, raised in various quarters of the world. In the deserts of
northern Africa the date-palm has yielded thirty-eight varieties; in the
fertile plains of India it is notorious how many varieties of rice and
of a host of other plants exist; in a single Polynesian island,
twenty-four varieties of the breadfruit, the same number of the banana,
and twenty-two varieties of the arum, are cultivated by the natives. The
mulberry tree of India and Europe has yielded many varieties serving as
food for the silkworm; and in China sixty-three varieties of the bamboo
are used for various domestic purposes. These facts, and innumerable
others which could be added, indicate that a change of almost any kind
in the conditions of life suffices to cause variability—different
changes acting on different organisms.”
Darwin thinks that a change in climate alone is not one of the potent
causes of variability, because the native country of a plant, where it
has been longest cultivated, is where it has oftenest given rise to the
greatest number of varieties. He thinks it also doubtful that a change
in food is an important source of variability, since the domestic pigeon
has varied more than any other species of fowl, yet the food has been
always nearly the same. This is also true for cattle and sheep, whose
food is probably much less varied in kind than in the wild species.
Another point of interest is raised by Darwin. He thinks, as do others
also, that the influence of a change in the conditions is cumulative, in
the sense that it may not appear until the species has been subjected to
it for several generations. Darwin states that universal experience
shows that when new plants are first introduced into gardens they do not
vary, but after several generations they will begin to vary to a greater
or less extent. In a few cases, as in that of the dahlia, the zinnia,
the Swan River daisy, and the Scotch rose, it is known that the new
variations only appeared after a time. The following statement by Salter
is then quoted, “Every one knows that the chief difficulty is in
breaking through the original form and color of the species, and every
one will be on the lookout for any natural sport, either from seed or
branch; that being once obtained, however trifling the change may be,
the result depends on himself.” Jonghe is also quoted to the effect that
“there is another principle, namely, that the more a type has entered
into a state of variation, the greater is the tendency to continue doing
so, and the more it has varied from the original type, the more is it
disposed to vary still further.” Darwin also quotes with approval the
opinion of the most celebrated horticulturist of France, Vilmorin, who
maintained that “when any particular variation is desired, the first
step is to get the plant to vary in any manner whatever, and to go on
selecting the most variable individuals, even though they vary in the
wrong direction; for the fixed character of the species being once
broken, the desired variation will sooner or later appear.”
Darwin also cites a few cases where animals have changed quite quickly
when brought under domestication. Turkeys raised from the eggs of wild
species lose their metallic tints, and become spotted with white in the
third generation. Wild ducks lose their true plumage after a few
generations. “The white collar around the neck of the mallard becomes
much broader and more irregular, and white feathers appear in the
duckling’s wings. They increase also in size of body.” In these cases it
appears that several generations were necessary in order to bring about
a marked change in the original type, but the Australian dingoes, bred
in the Zoological Gardens, produced puppies which were in the first
generation marked with white and other colors.
The following cases from De Varigny are also very striking. The dwarf
trees from Japan, for the most part conifers, which may be a hundred
years old and not be more than three feet high, are in part the result
“of mechanical processes which prevent the spreading of the branches,
and in part of a starving process which consists in cutting most roots
and in keeping the plant in poor soil.”
As an example of the sudden appearance of a new variation the following
case is interesting. A variety of begonia is recorded as having appeared
quite suddenly at a number of places at the same time. In another case a
narcissus which had met with adverse circumstances, and had then been
supplied with a chemical manure in some quantity, began to bear double
flowers.
Amongst animals the following cases of the appearance of sudden
variations are pointed out by De Varigny. “In Paraguay, during the last
century (1770), a bull was born without horns, although his ancestry was
well provided with these appendages, and his progeny was also hornless,
although at first he was mated with horned cows. If the horned and the
hornless were met in fossil state, we would certainly wonder at not
finding specimens provided with semi-degenerate horns, and representing
the link between both, and if we were told that the hornless variety may
have arisen suddenly, we should not believe it and we should be wrong.
In South America also, between the sixteenth and eighteenth centuries
the niata breed of oxen sprang into life, and this breed of bulldog oxen
has thriven and become a new race. So in the San Paulo provinces of
Brazil, a new breed of oxen suddenly appeared which was provided with
truly enormous horns, the breed of franqueiros, as they are called. The
mauchamp breed of sheep owes its origin to a single lamb that was born
in 1828 from merino parents, but whose wool, instead of being curly like
that of its parents, remained quite smooth. This sudden variation is
often met with, and in France has been noticed in different herds.”
The ancon race of sheep originated in 1791 from a ram born in
Massachusetts having short crooked legs and a long back. From this one
ram by crossing, at first with common sheep, the ancon race has been
produced. “When crossed with other breeds the offspring, with rare
exception, instead of being intermediate in character, perfectly
resemble either parent; even one of twins has resembled one parent and
the second the other.”
Two especially remarkable cases remain to be described. These are the
Porto Santo rabbit and the japanned peacock. Darwin has given a full
account of both of these cases. “The rabbits which have become feral on
the island of Porto Santo, near Madeira, deserve a fuller account. In
1418 or 1419 J. Gonzales Zarco happened to have a female rabbit on board
which had produced young during the voyage, and he turned them all out
on the island. These animals soon increased so rapidly that they became
a nuisance, and actually caused the abandonment of the settlement.
Thirty-seven years subsequently, Cada Mosto describes them as
innumerable; nor is this surprising, as the island was not inhabited by
any beast of prey, or by any terrestrial mammal. We do not know the
character of the mother rabbit; but it was probably the common domestic
kind. The Spanish peninsula, whence Zarco sailed, is known to have
abounded with the common wild species at the most remote historical
period; and as these rabbits were taken on board for food, it is
improbable that they should have been of any peculiar breed. That the
breed was well domesticated is shown by the doe having littered during
the voyage. Mr. Wollaston, at my request, brought two of these feral
rabbits in spirits of wine; and, subsequently, Mr. W. Haywood sent home
three more specimens in brine and two alive. These seven specimens,
though caught at different periods, closely resemble each other. They
were full-grown, as shown, by the state of their bones. Although the
conditions of life in Porto Santo are evidently highly favorable to
rabbits, as proven by their extraordinarily rapid increase, yet they
differ conspicuously in their small size from the wild English
rabbit.... In color the Porto Santo rabbit differs considerably from the
common rabbit; the upper surface is redder, and is rarely interspersed
with any black or black-tipped hairs. The throat and certain parts of
the under surface, instead of being pure white, are generally gray or
leaden color. But the most remarkable difference is in the ears and
tail. I have examined many fresh English rabbits, and the large
collection of skins in the British Museum from various countries, and
all have the upper surface of the tail and the tips of the ears clothed
with blackish gray fur; and this is given in most works as one of the
specific characters of the rabbit. Now in the seven Porto Santo rabbits
the upper surface of the tail was reddish brown, and the tips of the
ears had no trace of the black edging. But here we meet with a singular
circumstance: in June, 1861, I examined two of these rabbits recently
sent to the Zoological Gardens and their tails and ears were colored as
just described; but when one of their dead bodies was sent to me in
February, 1863, the ears were plainly edged, and the upper surface of
the tail was covered with blackish gray fur, and the whole body was much
less red; so that under the English climate this individual rabbit had
recovered the proper color of its fur in rather less than four years.”
Another striking case of sudden variation is found in the peacock. It is
all the more remarkable because this bird has hardly varied at all under
domestication, and is almost exactly like the wild species living in
India to-day. Darwin states: “There is one strange fact with respect to
the peacock, namely, the occasional appearance in England of the
‘japanned’ or ‘black-shouldered’ kind. This form has lately been named,
on the high authority of Mr. Slater, as a distinct species, viz. _Pavo
nigripennis_, which he believes will hereafter be found wild in some
country, but not in India, where it is certainly unknown. The males of
these japanned birds differ conspicuously from the common peacock in the
color of their secondary wing-feathers, scapulars, wing-coverts, and
thighs, and are, I think, more beautiful; they are rather smaller than
the common sort, and are always beaten by them in their battles, as I
hear from the Hon. A. S. G. Canning. The females are much paler-colored
than those of the common kind. Both sexes, as Mr. Canning informs me,
are white when they leave the egg, and they differ from the young of the
white variety only in having a peculiar pinkish tinge on their wings.
These japanned birds, though appearing suddenly in flocks of the common
kind, propagate their kind quite truly.”
In two cases, in which these birds had appeared quite suddenly in flocks
of the ordinary kind, it is recorded that “though a smaller and weaker
bird, it increased to the extinction of the previously existing breed.”
Here we have certainly a remarkable case of a new species suddenly
appearing and replacing the ordinary form, although the birds are
smaller, and _are beaten in their battles_.
Darwin has given an admirably clear statement of his opinion as to the
_causes of variability_ in the opening paragraph of his chapter dealing
with this topic in his “Animals and Plants.” Some authors, he says,
“look at variability as a necessary contingent on reproduction, and as
much an original law as growth or inheritance. Others have of late
encouraged, perhaps unintentionally, this view by speaking of
inheritance and variability as equal and antagonistic principles. Pallas
maintained, and he has had some followers, that variability depends
exclusively on the crossing of primordially distinct forms. Other
authors attribute variability to an excess of food, and with animals, to
an excess relatively to the amount of exercise taken, or again, to the
effects of a more genial climate. That these causes are all effective is
highly probable. But we must, I think, take a broader view, and conclude
that organic beings, when subjected during several generations to any
change whatever in their condition, tend to vary; the kind of variation
which ensues depending in most cases in a far higher degree on the
nature of the constitution of the being, than on the nature of the
changed conditions.”
Most naturalists will agree, in all probability, with this conclusion of
Darwin’s. The examples cited in the preceding pages have shown that
there are several ways in which the organisms may respond to the
environment. In some cases it appears to affect all the individuals in
the same way; in other cases it appears to cause them to fluctuate in
many directions; and in still other cases, without any recognizable
change in the external conditions, new forms may suddenly appear, often
of a perfectly definite type, that depart widely from the parent form.
For the theory of evolution it is a point of the first importance to
determine which of these modes of variation has supplied the basis for
evolution. Moreover, we are here especially concerned with the question
of how adaptive variations arise. Without attempting to decide for the
present between these different kinds of variability, let us examine
certain cases in which an immediate and adaptive response to the
environment has been described as taking place.
Responsive Changes in the Organism that adapt it to the New Environment
There is some experimental evidence showing that sometimes organisms
respond directly and adaptively to certain changes in the environment.
Few as the facts are, they require very careful consideration in our
present examination. The most striking, perhaps, is the acclimatization
to different temperatures. It has been found that while few active
organisms can withstand a temperature over 45 degrees C., and that for
very many 40 degrees is a fatal point, yet, on the other hand, there are
organisms that live in certain hot springs where the temperature is very
high. Thus, to give a few examples, there are some of the lower plants,
nostocs and protococcus forms, that live in the geysers of California at
a temperature of 93 degrees C., or nearly that of boiling water.
Leptothrix is found in the Carlsbad springs, that have a temperature of
44 to 54 degrees. Oscillaria have been found in the Yellowstone Park in
water between 54 and 68 degrees, and in the hot springs in the
Philippines at 71 degrees, and on Ischia at 85 degrees, and in Iceland
at 98 degrees.
It is probable from recent observations of Setchel that most of the
temperatures are too high, since he finds that the water at the edge of
hot springs is many degrees lower than that in the middle parts.
The snail, _Physa acuta_, has been found in France living at a
temperature of 35 to 36 degrees; another snail, Paludina, at Abano,
Padua, at 50 degrees. Rotifers have been found at Carlsbad at 45 to 54
degrees; Anguillidæ at Ischia at 81 degrees; _Cypris balnearia_, a
crustacean at Hammam-Meckhoutin, at 81 degrees; frogs at the baths of
“Pise” at 38 degrees.
Now, there can be little doubt that these forms have had ancestors that
were like the other members of the group, and would have been killed had
they been put at once into water of these high temperatures, therefore
it seems highly probable that these forms have become specially adapted
to live in these warm waters. It is, therefore, interesting to find that
it has been possible to acclimatize animals experimentally to a
temperature much above that which would be fatal to them if subjected
directly to it. Dutrochet (in 1817) found that if the plant, nitella,
was put into water at 27 degrees, the currents in the protoplasm were
stopped, but soon began again. If put now into water at 34 degrees they
again stopped moving, but in a quarter of an hour began once more. If
then put into water at 40 degrees the currents again slowed down, but
began again later.
Dallinger (in 1880) made a most remarkable series of experiments on
flagellate protozoans. He kept them in a warm oven, beginning at first
at a temperature of 16.6 degrees C. “He employed the first four months
in raising the temperature 5.5 degrees. This, however, was not
necessary, since the rise to 21 degrees can be made rapidly, but for
success in higher temperatures it is best to proceed slowly from the
beginning. When the temperature had been raised to 23 degrees, the
organisms began dying, but soon ceased, and after two months the
temperature was raised half a degree more, and eventually to 25.5
degrees. Here the organisms began to succumb again, and it was necessary
repeatedly to lower the temperature slightly, and then to advance it to
25.5 degrees, until, after several weeks, unfavorable appearances
ceased. For eight months the temperature could not be raised from this
_stationary point_ a quarter of a degree without unfavorable
appearances. During several years, proceeding by slow stages, Dallinger
succeeded in raising the organisms up to a temperature of 70 degrees C.,
at which the experiment was ended by an accident.”[27]
Footnote 27:
Quoted from Davenport’s “Experimental Morphology.”
Davenport and Castle carried out a series of experiments on the egg of
the toad, in which they tried to acclimatize the eggs to a temperature
higher than normal. Recently laid eggs were used; one lot kept at a
temperature of 15 degrees C., the other at 24-25 degrees C. Both lots
developed normally. At the end of four weeks the temperature point at
which the tadpoles were killed was determined. Those reared at a
temperature of 15 degrees C. died at 41 degrees C., or below; those
reared at 24-25 degrees C. sustained a temperature 10 degrees higher; no
tadpole dying in this set under 43 degrees C. “This increased capacity
for resistance was not produced by the dying off of the less resistant
individuals, for no death occurred in these experiments during the
gradual elevation of the temperatures in the cultures.” The increased
resistance was due, therefore, to a change in the protoplasm of the
individuals. It was also determined that the acquired resistance was
only very gradually lost (after seventeen days’ sojourn in cooler
water). The explanation of this result may be due, in part, to the
protoplasm containing less water at higher temperatures, for it is known
that while the white of egg (albumen) coagulates at 56 degrees C. in
aqueous solution; with only 18 per cent of water it coagulates between
80 degrees and 90 degrees C.; and with 6 per cent, at 145 degrees C.;
and without water between 100 degrees and 170 degrees C.
It has long been known that organisms in the dry condition resist a much
higher temperature. The damp uredospore is killed at 58.5 degrees to 60
degrees C.; but dry spores withstand 128 degrees C. It is also known
that organisms may become acclimatized to cold through loss of water,
but we lack exact experimental data to show to what extent this can be
carried.
There are also some experiments that go to show that animals may become
attuned to certain amounts of light, but the facts in this connection
will be described in another chapter.
Some important results have been obtained by accustoming organisms to
solutions containing various amounts of salts. A number of cases of this
sort are given by De Varigny. It has been found that littoral marine
animals that live where the water may become diluted by the rain, or by
rivers, survive better when put into fresh water than do animals living
farther from the shore. Thus the oyster, the mussel, and the snail,
Patella, withstand immersion in fresh water better than other animals
that live farther out at sea. The reverse is also true; fresh-water
forms, such as Lymnæa, Physa, Paludina, and others may be slowly
acclimatized to water containing more salt. The forms mentioned above
could be brought by degrees into water containing 4 per cent of salt,
which would have killed the animals if they had been brought suddenly
into it. Similar results have been obtained for amœba.
It has been shown that certain rotifers and tardigrades, and also some
unicellular animals, that live in pools and ponds that are liable to
become dry, withstand desiccation, while other members of the same
groups, living in the sea, do not possess this power of resistance.
Cases of this sort are usually explained as cases of adaptation, but it
has not been shown experimentally that resistance to drying can be
acquired by a process of acclimatization to this condition. The case is
also in some respects different from the preceding, since intermediate
conditions are less likely to be met with, or to be of sufficiently long
duration for the animal to become acclimatized to them. It seems more
probable, in such cases, that these forms have been able to live in such
precarious conditions from the beginning because they could resist the
effects of drying, not that they have slowly acquired this power.
Finally, there must be discussed the question of the acclimatization to
poisons, to which an individual may be rendered partially immune. The
point of special importance in this connection is that the animal may be
said to respond adaptively to a large number of substances, which it has
never met before in its individual history, or to which its ancestors
have never been subjected. It may become slowly adapted to many
different kinds of injurious substances. These cases are amongst the
most important adaptive individual responses with which we are familiar,
and the point cannot be too much emphasized that organisms have this
latent capacity without ever having had an opportunity to acquire it
through experience.
The preceding groups of phenomena, included under the general heading of
individual acclimatization, have one striking thing in common, namely,
that a physiological adaptation is brought about without a corresponding
change in form, although we must suppose that the structure has been
altered in certain respects at least. The form of the individual remains
the same as before, but so far as its powers of resistance are concerned
it is a very different being.
In regard to the perpetuation of the advantages gained by means of this
power of adaptation, it is clear in those cases in which the young are
nourished during their embryonic life by the mother, that, in this way,
the young may be rendered immune to a certain extent, and there are
instances of this sort recorded, especially in the case of some
bacterial diseases. Whether this power can also be transmitted through
the egg, in those instances in which the egg itself is set free and
development takes place outside the body, has not been shown. In any
case, the effect appears not to be a permanent one and will wear off
when the particular poison no longer acts. It is improbable, therefore,
that any permanent contribution to the race could be gained in this way.
Adaptations of this sort, while of the highest importance to the
individual, can have produced little direct effect on the evolution of
new forms, although it may have been often of paramount importance to
the individuals to be able to adapt themselves, or rather to become able
to resist the effect of injurious substances. The important fact in this
connection is the wonderful latent power possessed by all animals. So
many, and of such different kinds, are the substances to which they may
become immune, that it is inconceivable that this property of the
organism could ever have been acquired through experience, no matter how
probable it may be made to appear that this might have occurred in
certain cases of fatal bacterial diseases. And if not, in so many other
cases, why invent a special explanation for the few cases?
We may defer the general discussion of the rôle that external factors
have played in the adaptation of organisms, until we have examined some
of the theories which attribute changes to internal factors. The idea
that something innate in the living substance itself has served as the
basis for evolution has given rise to a number of different hypotheses.
That of the botanist Nägeli is one of the most elaborately worked out
theories of this sort that has been proposed, and may be examined by way
of illustration.
Nägeli’s Perfecting Principle
Nägeli used the term _completing principle_ (“Vervollkommungsprincip”)
to express a tendency toward perfection and specialization.
Short-sighted writers, he says, have pretended to see in the use of this
principle something mystical, but on the contrary it is intended that
the term shall be employed in a purely physical sense. It represents the
law of inertia in the organic realm. Once set in motion, the
developmental process cannot stand still, but must advance in its own
direction. Perfection, or completion, means nothing else than the
advance to complicated structure, “but since persons are likely to
attach more meaning to the word _perfection_ than is intended, it would
perhaps be better to replace it with the less objectionable word
_progression_.”
Nägeli says that Darwin, having in view only the condition of
adaptation, designates that as more complete which gives its possessor
an advantage in the battle for existence. Nägeli claims that this is not
the only criterion that applies to organisms, and it leaves out the most
important part of the phenomenon. There are two kinds of completeness
which we should keep distinctly apart: (1) the completeness of
organization characterized by the complication of the structure and the
most far-reaching specialization of the parts; (2) the completeness of
the adaptation, present at each stage in the organization, which
consists in the most advantageous development of the organism (under
existing conditions) that is possible with a given complication of
structure and a given division of functions.
The first of these conceptions Nägeli always calls “completeness”
(Vollkommenheit), for want of a simpler and better expression; the
second he calls adaptation. By way of illustrating the difference
between the two, the following examples may be given. The unicellular
plants and the moulds are excellently adapted each to its conditions of
life, but they are much less complete in structure than an apple tree,
or a grape vine. The rotifers and the leeches are well adapted to their
station, but in completeness of structure they are much simpler than the
vertebrates.
If we consider only organization and division of labor as the work of
the completing principle, and leave for the moment adaptation out of
account, we may form the following picture of the rise of the organic
world. From the inorganic world there arose the simplest organic being
thinkable, being little more than a drop of substance. If this underwent
any change at all, it would have been necessarily in the direction of
greater complication of structure; and this would constitute the first
step in the upward direction. In this way Nägeli imagines the process
once begun would continue. When the movement has reached a certain
point, it must continue in the same direction. The organic kingdom
consists, therefore, of many treelike branches, which have had a common
starting-point. Not only does he suppose that organisms were once
spontaneously generated, and began their first upward course of
development, but the process has been repeated over and over again, and
each time new series have been started on the upward course. The organic
kingdom is made up, therefore, of all degrees of organization, and all
these have had their origins in the series of past forms that arose and
began their upward course at different times in the past. Those that are
the highest forms at the present time represent the oldest series that
successfully developed; the lowest forms living at the present time are
the last that have appeared on the scene of action.
Organisms, as has been said, are distinguished from one another, not
only in that one is simpler and another more complicated, but also in
that those standing at the same stage of organization are unequally
differentiated in their functions and in their structure, which is
connected primarily with certain external relations which Nägeli calls
adaptations.
Adaptation appears at each stage of the organization, which stage is,
for a given environment, the most advantageous expression of the main
type that was itself produced by internal causes. For this condition of
adaptation, a sufficient cause is demanded, and this is, as Nägeli tries
to show later, the result of the inherited response to the environment.
In many cases this cause will continue to act until complete adaptation
is gained; in other cases, the external conditions give a direction
only, and the organism itself continues the movement to its more perfect
condition.
The difference between the conception of the organic kingdom as the
outcome of mechanical causes on the one hand, or of competition and
extermination on the other hand, can be best brought out, Nägeli thinks,
by the following comparison of the two respective methods of action.
There might have been no competition, and no consequent extermination in
the plant kingdom, if from the beginning the surface of the earth had
continually grown larger in proportion as living things increased in
numbers, and if animals had not appeared to destroy the plants. Under
these conditions each germ could then have found room and food, and have
unfolded itself without hinderance. If now, as is assumed to be the case
on the Darwinian theory, individual variations had been in all
directions, the developmental movement could not have gone beyond its
own beginnings, and the first-formed plants would have remained swinging
now on one side and now on another of the point first reached. The whole
plant kingdom would have remained in its entirety at its first stage of
evolution, that is, it would never have advanced beyond the stage of a
naked drop of plasma with or without a membrane. But, according to the
further Darwinian conception, competition, leading to extermination, is
capable of bringing such a condition to a higher stage of development,
since it is assumed that those individuals which vary in a beneficial
direction would have an advantage over those that have not taken such a
step, or have made a step backward.
If, on the other hand, under the above-mentioned conditions of
unrestricted development, without competition, variations were
determined by “_mechanical principles_,” then, according to Nägeli’s
view, all plant forms that now exist would still have evolved, and would
be found living at the present time, but along with all those that now
exist there would be still other forms in countless numbers. These would
represent those forms which have been suppressed. On Nägeli’s view
competition and suppression do not produce new forms, but only weed out
the intermediate forms. He says without competition the plant kingdom
would be like the Milky Way; in consequence of competition the plant
kingdom is like the firmament studded with bright stars.
The plant kingdom may also be compared to a branched tree, the ends of
whose branches represent living species. This tree has an inordinate
power of growth, and if left to itself it would produce an impenetrable
tangle of interwoven branches. The gardener prevents this crowding by
cutting away some of the parts, and thus gives to the tree distinct
branches and twigs. The tree would be the same without the watchful
trimming of the gardener, but without definite form.
Nägeli states: “From my earlier researches I believe that the external
influences are small in comparison to the internal ones. I shall speak
here only of the influences of climate and of food, which are generally
described as the causes of change, without however any one’s having
really determined whether or not a definite result can be brought about
by these factors. Later I shall speak of a special class of external
influences which, according to my view, bring forth beyond a doubt
adaptive changes.”
The external influence of climate and of food act only as transitory
factors. A rich food supply produces fat, lack of food leads to
leanness, a warm summer makes a plant more aromatic, and its fruit
sweeter; a cold year means less odor and sour fruit. Of two similar
seeds the one sown in rich soil will produce a plant with many branches
and abundance of flowers; the other, planted in sandy soil, will produce
a plant without branches, with few flowers, and with small leaves. The
seeds from these two plants will behave in exactly the same way; they
have inherited none of the differences of their parents. Influences of
this sort, even if extending over many generations, have no permanent
effect. Alpine plants that have lived since the ice age under the same
conditions, and have the characters of true high-mountain plants, lose
these characters completely during the first summer, if transplanted to
the plains. Moreover, it makes no difference whether the seed or the
whole plant itself be transferred. In place of the dwarfed, unbranched
growth, and the reduced number of organs, the plant when transferred to
the plains shoots up in height, branches strongly, and produces numerous
leaves and flowers. The plants retain their new characters as long as
they live in the plain without any other new variation being observed in
them.
Other characteristics also, which arise from different kinds of external
influences due to different localities, such as dampness and shade, a
swampy region, or different geological substrata, last only so long as
the external conditions last.
These transient peculiarities make up the characters of local varieties.
That they have no permanency is intelligible, since they exhibit no new
characters, but the change consists mainly in the over- or
under-development of those peculiarities that are dependent on external
influences. The effect of these influences may be compared to an elastic
rod, which, however much it may be distorted by external circumstances,
returns again to its original form as soon as released.
Besides these temporary changes, due to external influences, there are
many cases known in which the same plant lives under very diverse
conditions and yet remains exactly the same. For example, the species of
_Rhododendron ferragineum_ lives on archæan mountains and especially
where the soil is poor in calcium. Another species, _Rhododendron
hirsutum_ is found especially on soil rich in calcium. The difference in
the two species has been supposed to depend on differences in the soil,
and if so, we would imagine that, if transplanted for a long time, the
one should change in the direction of the other. Yet it is known that
the rusty rhododendron may be found in all sorts of localities, even on
dry, sunny, calcareous rocks of the Apennines and of the Jura, and
despite its residence in these localities, since the glacial epoch, no
change whatever has taken place.
Single varieties of the large and variable genus of _Hieracium_ have
lived since the glacial period in the high regions of the Alps,
Carpathians, and in the far north, and also in the plains of different
geological formations, but these varieties have remained exactly the
same, although on all sides there are transitional forms leading from
these to other varieties.
Some parasitic species also furnish excellent illustrations of the same
principle. Besides the several species of Orobanchia and of the
parasitic moulds, the mistletoe deserves special mention. It lives on
both birch and apple trees and on both presents exactly the same
appearance; and even if it is true that mistletoe growing on conifers
presents certain small deviations in its character, it is still doubtful
whether, if transferred to the birch or apple tree, it would not lose
these differences, thus indicating that they are not permanent.
It is a fact of general observation that, on the one hand, the same
variety occurs in different localities and under different surroundings,
and, on the other hand, that slightly different varieties live together
in the same place and therefore under the same external conditions. It
is evident, then, that food conditions have neither originated the
differences nor kept them up. The rarer cases in which in different
localities different varieties exist show nothing, because competition
and suppression keep certain varieties from developing where it would be
possible otherwise for them to exist.
Nägeli says his conclusion may be tested from another point of view. If
food conditions, as is generally supposed, have a definite, _i.e._ a
permanent, effect on the organism, then all organisms living under the
same conditions should show the same characters. Indeed, it has been
claimed in some instances that this is actually the case. Thus it is
stated that dry localities cause plants to become hairy, and that
absence of hairiness is met with in shady localities. This may apply to
certain species, but in other cases exactly the reverse is true, and
even the same species behaves differently in different regions, as in
_Hieracium_. And so it is with all characteristics which are ascribed to
external influences. As soon as it is supposed a discovery has been made
in this direction, we may rest assured that in other cases the reverse
will be found to hold. We have had, in respect to the influence of the
outer world on organisms, the same experience as with the rules for the
weather,—when we come to examine the facts critically there are found to
be as many exceptions as confirmations of the rule.
If climatic influence has a definite effect, the entire flora of a
special locality ought to have the same peculiarities, but this stands
in contradiction to all the results of experience. The character of the
vegetation is not determined by the environment of the plants but by
their prehistoric origin, and as the result of competition. Nägeli
concludes his discussion with the statement that all of our experience
goes to show that the effects of external influences (climate and food)
appear at once, and their results last only as long as the influences
themselves last, and are then lost, leaving nothing permanent behind.
This is true even when the external influences have lasted for a long
time,—since the glacial epoch, for instance. We find, he claims, nothing
that supports the view that such influences are inherited.
If we next examine the question of changes from _internal causes_,
Nägeli claims that here also observation and research fail to show the
origin of a new species, or even of a new variety from external causes.
In the organic world little change has taken place, he believes, since
the glacial epoch. Many varieties have even remained the same throughout
the whole intervening time; and while it cannot be doubted that new
varieties have also been formed, yet the cause of their origin cannot be
empirically demonstrated. The permanent, hereditary characters, of whose
origin we know something from experience, belong to the individual
changes which have appeared under cultivation in the formation of
domestic races. These are for the most part the result of crossing. So
far as we have any definite information as to the origin of the changes,
they are the result of inner, and never of external, causes. We
recognize that this must be the case, since under the same external
conditions individuals behave differently—in the same flower-bud some
seeds give rise to plants like the parent, others to altered ones. The
strawberry with a single leaflet, instead of three, arose in the last
century in a single individual amongst many other ordinary plants. From
the ten seeds of a pear Van Mons obtained as many different kinds of
pears. The most conclusive proof of the action of inner causes is most
clearly seen when the branches of the same plant differ. In Geneva a
horse-chestnut bore a branch with “filled” flowers, and from this
branch, by means of cuttings, this variation has been carried over all
Europe. In the Botanic Garden at Munich there is a beech with small
divided leaves; but one of its branches produces the common broad
undivided leaves. Many such examples have been recorded which can only
be explained by assuming that a cell, or a group of cells, like those
from which the other branches arose, have become changed in some unknown
way as the result of inner causes. The properties that are permanent and
inherited are contained in the idioplasm, which the parent transmits to
its offspring. A cause that permanently transforms the organism must
also transform the idioplasm. How powerless, in comparison to internal
causes, the external causes are is shown most conclusively in grafting.
The graft, although it receives its nourishment through the stock, which
may be another species, remains itself unchanged.
Nägeli makes the following interesting comparison between the
development of the individual from an egg, and the evolution, or
development, of the phylum. No one will doubt that the egg during the
entire time of its process of transformation is guided by internal
factors. Each successive stage follows with mechanical necessity from
the preceding. If an animal can develop from inner causes from a drop of
plasma, why should not the entire evolutionary process have also been
the outcome of developmental inner causes? He admits that there is a
difference in the two cases in that the plasma that forms the egg has
come from another animal, and contains all the properties of the
individual in a primordial condition. In the other case we must suppose
that the original drop of plasma did not contain at first the primordium
of definite structures, but only the ability to form such. Logically the
difference is unimportant. The main point is that in the primordium of
the germ a special peculiarity of the substance is present which by
forming new substances grows, and changes as it grows, and the one
change of necessity excites the next until finally a highly organized
being is the result.
Nägeli discusses a question in this connection, which, he says, has been
unnecessarily confused in the descent theory. Since we are entirely in
the dark as to how much time has been required for the formation of
phyla, so also are we ignorant as to how long it may have taken for each
step in advance. We may err equally in ascribing too much and too little
time to the process. It is, moreover, not necessary that for every step
the same amount of time should have been required. On the contrary, the
probability is that recognizable changes may at times follow each other
rapidly, and then for a time come to a standstill,—just as in the
development of the individual there are periods of more rapid and others
of less rapid change.
A more difficult problem than that relating to the sort of changes the
external influences bring about in the organism, is the question as to
how they effect the organism, or how they act on it mechanically. This,
as is well known, was answered by Darwin, who regards all organization
as a problem of adaptation: only those chance variations surviving which
are capable of existence, the others being destroyed. On this theory
external influences have only a negative or a passive action, namely, in
setting aside the unadapted individuals. Nägeli, on the other hand,
looks upon some kinds of external conditions as directly giving rise to
the adaptive characters of the organism. This is accomplished, he
supposes, in the following ways: two kinds of influence are recognized;
_the direct action_, which, as in inorganic nature, comes to an end when
the external influences come to an end, as when cold diminishes the
chemical actions in the plant; and _the indirect action_, generally
known as a stimulus, which starts a series of molecular motions,
invisible to us, but which we recognize only in their effects. Very
often the stimulus starts only a reflex action, usually at the place of
application.
A stimulus acting for but a short time produces no lasting effect on the
idioplasm. A person stung by a wasp suffers no permanent effect from the
injury. But if a stimulus acts for a long time, and through a large
number of generations, then it may, even if of small strength, so change
the _idioplasm_, that a tendency or disposition capable of being seen
may be the result. This appears to be the case in regard to the action
of light, which causes certain parts of the plant to turn toward it and
others away from it; also for the action of gravity, which determines
the downward direction of the roots. It may be claimed, perhaps, that
these are the results of direct influence and not of an internal
response, but this is not the case; for some plants act in exactly the
opposite way, and send a stem downward, as in the case of the
cleistogamous flowers of _Cardamine chenopodifolia_; and other plants
turn away from the light. This means that the idioplasm behaves
differently in different plants in response to the same stimulus.
Concerning the more visible effects of adaptation, Nägeli states that in
regard to some of them there can be no question as to how they must have
arisen. Protection against cold, by the formation of a thick coat of
hair, is the direct result of the action of the cold on the skin of the
animal. The different weapons of offence and of defence, horns, spurs,
tusks, etc., have arisen, he maintains, through stimulus to those parts
of the body where these structures arise.
The causes of the other adaptations, especially of those occurring in
plants, are less obvious. Land plants protect themselves from drying by
forming a layer of cork over the surface. The most primitive plants were
water plants, which acclimated themselves little by little to moist, and
then to dry, air. When they first emerged from the water the drying
acted as a stimulus on the surface, and caused it to harden in the same
way as a drop of glue hardens. This hardening in turn acted as a
stimulus, causing a chemical transformation of the surface into a corky
substance. This effect was inherited, and in this way the power to form
cork originated.
Land plants have, in addition to the soft parts, the hard bast and wood
which serves the mechanical purpose of supporting the soft tissues and
protecting them from being injured. The arrangement of the hard parts is
such as to suggest that they are the result of the action of pressures
and tensions on the plant, for the strongest cells are found where there
is most need for them. It is easy to imagine, Nägeli adds, that this
important arrangement of the tissues is the result of external forces
which brought about the result in these parts.
Nägeli accounts for the origin of twining plants as follows. Being
overshadowed by other plants, the stem will grow rapidly in the damp
air. Coming in contact with the stems of other plants, the delicate stem
is stimulated on one side, and grows around the point of contact. This
tendency becomes inherited, and the habit to twine is ultimately
established.
The difference in the two sides of leaves is explained by Nägeli as the
result of the difference in the illumination of the two sides. This
influence of light on the leaf has been inherited. The formation of the
tubular corolla that is seen in many plants visited by insects is
explained as the result of the stimulus produced by the insects in
looking for the pollen. The increase in the length of the proboscis of
the insect is the result of the animal straining to reach the bottom of
the ever elongating tube of the corolla. “The tubular corolla and the
proboscis of the insect appear as though made for each other. Both have
slowly developed to their present condition, the long tube from a short
tube and the long proboscis from a short one.” Thus, by purely
Lamarckian principles, Nägeli attempts to account for many of the
adaptations between the organism and the outer world. But if this takes
place, where is there left any room for the action for his so-called
perfecting principle? Nägeli proceeds to show how he supposes that the
two work together.
As a result of inner causes the organism would pass through a series of
perfectly definite stages, J, J^1, J^2. But if, at any stage, external
influences produced an effect on the organism so that the arrangement of
the idioplasm changes in response, a new adaptation is produced. In this
way new characters, not inherent in the idioplasm, may be added, and old
ones be changed or lost. “In order not to be misunderstood in regard to
the completing or perfecting principle I will add, that I ascribe to it
no determinate action in the organism, neither in producing the long
neck of the giraffe, nor the prehensile tail of the ape, neither the
claws of the crab, nor the decoration of the bird of paradise. These
structures are the outcome of both factors. I cannot picture to myself
how external causes alone, and just as little how internal causes alone,
could have changed a monad into a man.” But Nägeli goes on to say, that
if at any stage of organization one of the two causes should cease to
act, the other could only produce certain limited results. Thus, if
external causes alone acted, the organization would remain at the same
stage of completeness, but might become adapted to all kinds of external
conditions—a worm, for instance, would not develop into a fish, but
would remain a worm forever, although it might change its worm structure
in many ways in response to external stimuli. If, on the other hand,
only the completing principle acted, then without changing its
adaptations the number of the cells and the size of the organs might be
increased, and functions that were formerly united might become
separated. Thus, without altering the character of the organism, a more
highly developed (in the sense of being more specialized) organism would
appear.
Nägeli, as we have just seen, has attempted to build up a conception of
nature based on two assumptions, neither of which has been demonstrated
to be an actual principle of development. His hypothesis appears,
therefore, entirely arbitrary and speculative to a high degree. Even if
it were conceivable that two such principles as these control the
evolution of organisms, it still requires a good deal of imagination to
conceive how the two go on working together. Moreover, it is highly
probable that whole groups have evolved in the direction of greater
simplification, as seen especially in the case of those groups that have
become degenerate. To what principle can we refer processes of this
sort?
It is certainly a strange conclusion this, at which Nägeli finally
arrives, for, after strenuously combating the idea that the external
factors of climate and of food have influence in producing new species,
he does not hesitate to ascribe all sorts of imaginary influences to
other external causes. The apparent contradiction is due, perhaps, to
the fact that his experience with actual species led him to deny that
the direct action of the environment produces permanent changes, while
in theory he saw the necessity of adding to his perfecting principle
some other factor to explain the adaptations of the new forms produced
by inner causes. Nägeli seems to have felt strongly the impossibility of
explaining the process of evolution and of adaptation as the outcome of
the selection of chance variations, now in this direction, now in that.
He seems to have felt that there must be something within the organism
that is driving it ever upward, and he attempts to avoid the
teleological element, which such a conception is almost certain to
introduce, by postulating the inheritance of the effects of
long-continued action of the environment, in so far as certain factors
in the environment produce a response in the organism. Nevertheless,
this combination is not one that is likely to commend itself, aside from
the fact that the assumptions have no evidence to support them. Despite
Nägeli’s protest that his principles are purely physical, and that there
is nothing mystical in his point of view, it must be admitted that his
conception, as a whole, is so vague and difficult in its application
that it probably deserves the neglect which it generally receives.
Nägeli’s wide experience with living plants convinced him that there is
something in the organism over and beyond the influence of the external
world that causes organisms to change; and we cannot afford, I think, to
despise his judgment on this point, although we need not follow him to
the length of supposing that this internal influence is a “force”
driving the organism forward in the direction of ever greater
complexity. A more moderate estimate would be that the organism often
changes through influences that appear to us to be internal, and while
some of the changes are merely fluctuating or chance variations, there
are others that appear to be more limited in number, but perfectly
definite and permanent in character. It is the latter, which, I believe,
we can safely accredit to internal factors, and which may be compared to
Nägeli’s internal causes, but this is far from assuming that these
changes are in the direction of greater completeness or perfection, or
that evolution would take place independently of the action of external
agencies.
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CHAPTER X
THE ORIGIN OF THE DIFFERENT KINDS OF ADAPTATIONS
In the present chapter we may first consider, from the point of view of
discontinuous variations as contrasted with the theory of the selection
of individual variations, the structural adaptations of animals and
plants, _i.e._ those cases in which the organism has a definite form
that adapts it to live in a particular environment. In the second place,
we may consider those adaptations that are the result of the adjustment
of each individual to its surroundings. In subsequent chapters the
adaptations connected with the responses of the nervous system and with
the process of sexual reproduction will be considered.
It should be stated here, at the outset, that the term _mutation_ will
be used in the following chapters in a very general way, and it is not
intended that the word shall convey only the idea which De Vries
attaches to it; it is used rather as synonymous with _discontinuous and
also definite variation_ of all kinds. The term will be used to include
“the single variations” of Darwin, “sports,” and even orthogenic
variation, if this has been definite or discontinuous.
Form and Symmetry
Almost without exception, animals and plants have definite and
characteristic forms. In other words, they are not amorphous masses of
substance. The members of each species conform, more or less, to a sort
of ideal type. Our first problem is to examine in what sense the form
itself may be looked upon as an adaptation to the surroundings.
It is a well-recognized fact that the forms of many animals appear to
stand in a definite relation to the environment. For instance, animals
that move in definite directions in relation to their structure have the
anterior and the posterior ends quite different, and it is evident that
these ends stand in quite different relations to surrounding objects;
while, on the other hand, the two sides of the body which are, as a
rule, subjected to the same influences are nearly exactly alike. The
dorsal and the ventral surfaces of the body are generally exposed to
very different external conditions, and are quite different in
structure.
The relation is so obvious in most cases that it might lead one quite
readily to conclude that the form of the animal had been moulded by its
surroundings. Yet this first impression probably gives an entirely wrong
conception of how such a relation has been acquired. Before we attempt
to discuss this question, let us examine some typical examples.
A radial type of structure is often found in fixed forms, and in some
floating forms, like the jellyfish. In a fixed form, a sea-anemone, for
instance, the conditions around the free end and the fixed end of the
body are entirely different, and we find that these two ends are also
different. The free end contains the special sense-organs, the mouth,
tentacles, etc.; while the fixed end contains the organ for attachment.
It is evident that the free end is exposed to the same conditions in all
directions, and it may seem probable that this will account for the
radial symmetry of the anemone. There are also a few free forms, the
sea-urchin for instance, that have a radial symmetry. Whether their
ancestors were fixed forms, for which there is some evidence, we do not
know definitely; but, even if this is true, it does not affect the main
point, namely, that, although at present free to move, the sea-urchin is
radially symmetrical. But when we examine its method of locomotion, we
find that it moves indifferently in any direction over a solid surface;
that is, it keeps its oral face against a solid object, and moves over
the surface in any direction. Under these circumstances the same
external conditions will act equally upon all sides of the body. In
contrast to these common sea-urchins, there are two other related
groups, in which, although traces of a well-marked radial symmetry are
found, the external form has been so changed that a secondary bilateral
form has been superimposed on it. These are the groups of the
clypeasters and the spatangoids, and it is generally supposed that their
forefathers were radially symmetrical forms like the ordinary forms of
sea-urchins. These bilateral forms move in the direction of their plane
of symmetry, but we have no means of knowing whether they first became
bilateral and, in consequence, now move in the direction of the median
plane, or whether they acquired the habit of moving in one direction,
and in consequence acquired a bilateral symmetry. It seems more probable
that the form changed first, for otherwise it is difficult to see why a
change of movement in one direction should ever have taken place.
The radially symmetrical form is characteristic of many flowers that
stand on the ends of their stalks. They also will be subjected to
similar external influences in all directions. Many flowers, on the
other hand, are bilaterally symmetrical. Some of these forms are of such
a sort that they are generally interpreted as having been acquired in
connection with the visits of insects. Be this as it may, it is still
not clear why, if the flowers are terminal, insects should not approach
them equally from every direction. If the flowers are not terminal, as,
in fact, many of them are not, their relation to the surroundings is
bilateral with respect to internal as well as to external conditions.
The former, rather than the latter, may have produced the bilateral form
of the flower. Here also we meet with the problem as to whether the
flowers, being lateral in position, have assumed a bilateral form
because their internal relations were bilateral; or whether an external
relation, for example, the visits of insects, has been the principle
cause of their becoming bilateral.
[Illustration:
Fig. 4.—A, right and left claws of lobster;
B, of the fiddler-crab; and
C, of Alpheus.]
In some bilateral forms the right and left sides may be unsymmetrical in
certain organs. Right and left handedness in man is the most familiar
example, although the structural difference on which this rests is not
very obvious. More striking is the difference in the two big claws of
the lobster (Fig. 4 A). One of the two claws is flat and has a fine
saw-toothed edge. The other is thicker and has rounded knobs instead of
teeth. It is said that these two claws are used by the lobster for
different purposes,—the heavy one for crushing and for holding on, and
the narrower for cutting up the food. If this is true, then we find a
symmetrical organism becoming unsymmetrical, and in consequence it takes
advantage of its asymmetry by using its right and left claws for
different purposes.
More striking still is the difference in the size of the right and left
claws in a related form, Alpheus—a crayfish-like form that lives in the
sea. With the larger claw (Fig. 4 C) it makes a clicking sound that can
be heard for a long distance. In some of the crabs the difference in the
size of the two claws is enormous, as in the male fiddler-crab, for
example (Fig. 4 B). One of the claws is so big and unwieldy that it must
put the animal at a distinct disadvantage. Its use is unknown, although
it has been suggested that it is a secondary sexual character.
The asymmetry of the body of the snail is very conspicuous, at least so
far as certain organs are concerned. The foot on which the animal crawls
and the head have preserved their bilaterality; but the visceral mass of
the animal, contained in the spirally wound shell, lying on the middle
of the upper surface of the foot, is twisted into a spiral form. Many of
the organs of one side of the body are atrophied. The gill, the kidney,
the reproductive organ, and one of the auricles of the heart have
completely, or almost completely, disappeared. The cause of this loss
seems to be connected with the spiral twist of the visceral mass. One of
the consequences of the twisting has been to bring the organs of the
left side of the body around the posterior end until they come to lie on
the right side, the organs of the original right side being carried
forward and there atrophying.
There is another remarkable fact connected with the asymmetry of the
snail. In some species, _Helix pomatia_, for example, the twist has been
toward the right, _i.e._ in the direction which the hands of a watch
follow when the face is turned upward toward the observer. Individuals
twisted in this direction are called dextral. Occasionally there is
found an individual with the spiral in the opposite direction
(sinistral), and in this the conditions of the internal organs are
exactly reversed. It is the left set of organs that is now atrophied,
and the right set that is functional. Such changes appear suddenly.
Organs of one side of the body that have not been functional for many
generations may become fully developed. Moreover, Lang has shown that
when a sinistral form breeds with a normal dextral form, or even when
sinistral forms are bred with each other, the young are practically all
of the ordinary type.
An attempt has been made to connect these facts with the mode of
development of the mollusks. It is known that the eggs of a number of
gasteropod mollusks segment in a perfectly definite manner. A sort of
spiral cleavage is followed by the formation of a large mesodermal cell
from the left posterior yolk-cell. From this mesodermal cell nearly all
the mesodermal organs of the body are formed. Thus it may appear that
the spiral form of the snail is connected with the spiral form of the
cleavage. In a few species of marine and fresh-water snails the cleavage
spiral is reversed, and the mesoderm arises from the right posterior
yolk-cell. It has been shown in several cases that the snail coming from
such an egg is twisted in the reverse direction from that of ordinary
snails.
It has been suggested, therefore, that the occasional sinistral
individual of Helix arises from an egg cleaving in the reverse
direction, and there is nothing improbable in an assumption of this
kind. No attempt has been made as yet to explain why, in some cases, the
cleavage spiral is turned in one direction, and in other cases in the
reverse direction; but even leaving this unaccounted for, the assumption
of the unusual form of Helix being the result of a reversal of the
cleavage throws some light as to how it is possible for the complete
reversal of the organs of the adult to arise. If it is assumed that in
the early embryo the cells on each side of the median line are alike,
and at this time capable of forming adult structures, a simple change of
the spiral from right to left might determine on which side of the
middle line the mesodermal cell would lie, and its presence on one side
rather than on the other might determine which side of the embryo would
develop, and which would not. This possibility removes much of the
mystery which may appear to surround a sudden change of this sort.
It seems to me that we shall not go far wrong if we assume that it is
largely a matter of indifference whether an individual snail is a
right-handed or a left-handed form, as far as its relation to the
environment is concerned. One form would have as good a chance for
existing as the other. If this is granted, we may conclude that, while
in most species a perfectly definite type is found, a right or a left
spiral, yet neither the one nor the other has been acquired on account
of its relation to the environment. This conclusion does not, of course,
commit us in any way as to whether the spiral form of the visceral mass
has been acquired in relation to the environment, but only to the view
that, if a spiral form is to be produced, it is indifferent which way it
turns. From the evolutionary point of view this conclusion is of some
importance, since it indicates that one of the alternatives has been
adopted and has become practically constant in most cases without
selection having had anything to do with it.
Somewhat similar conditions are found in the flounders and soles. As is
well known, these fishes lie upon one side of the body on the bottom of
the ocean. Some species, with the rarest exceptions to be mentioned in a
moment, lie always on the right side, others on the left side. A few
species are indifferently right or left. At rare intervals a left-sided
form is found in a right-sided species, and conversely, a right-sided
form in a left-sided species. In such cases the reversed type is as
perfectly developed in all respects as the normal form, but with a
complete reversal of its right and left sides.
When the young flounders leave the egg, they swim in an upright
position, as do ordinary fishes, with both sides equally developed.
There cannot be any doubt that the ancestors of these fish were
bilaterally symmetrical. Therefore, within the group, both right-handed
and left-handed forms have appeared. It seems to me highly improbable
that if a right-handed form had been slowly evolved through the
selection of favorable variations in this direction, the end result
could be suddenly reversed, and a perfect left-sided form appear.
Moreover, as has been pointed out, the intermediate stages would have
been at a great disadvantage as compared with the parent, and this would
lead to their extermination on the selection theory. If, however, we
suppose that a variation of this sort appeared at once, and was fixed,—a
mutation in other words,—and that whether or not it had an advantage
over the parent form, it could still continue to exist, and propagate
its kind, then we avoid the chief difficulty of the selection theory.
Moreover, we can imagine, at least, that if this variation appeared in
the germ and was, in its essential nature, something like the relation
seen in the snail, the occasional reversal of the relations of the parts
presents no great difficulty.
In this same connection may be mentioned a curious fact first discovered
by Przibram and later confirmed by others. If the leg carrying the large
claw of a crustacean be removed, then, at the next moult, the leg of the
other side that had been the smaller first leg becomes the new big one;
and the new leg that has regenerated from the place where the big one
was cut off becomes the smaller one.
Wilson has suggested that both claws in the young crustacean have the
power to become either sort. We do not know what decides the matter in
the adult, after the removal of one of the claws. Some slight difference
may turn the balance one way or the other, so that the smaller claw
grows into the larger one. At any rate, there is seen a latent power
like that in the egg of the snail. Zeleny has found a similar relation
to exist for the big and the little opercula of the marine worm,
Hydroides.
Let us consider now the more general questions involved in these
symmetrical and asymmetrical relations between the organism and its
environment. In what sense, it may be asked, is the symmetry of a form
an adaptation to its environment? That the kind of symmetry gives to the
animal in many cases a certain advantage in relation to its environment
is so evident that I think it will not be questioned. The main question
is how this relation is supposed to have been attained. Three points of
view suggest themselves: First, that the form has resulted directly from
the action of the environment upon the organism. This is the Lamarckian
point of view, which we rejected as improbable. Second, that the form
has been slowly acquired by selecting those individual variations that
best suited it to a given set of surrounding conditions. This is the
Darwinian view, which we also reject. The third, that the origin of the
form has had nothing to do with the environment, but appeared
independently of it. Having, however, appeared, it has been able to
perpetuate itself under certain conditions.
It should be pointed out that the Darwinian view does not suppose that
the environment actually produces any of the new variations which it
selects after they have appeared, but in so far as the environment
selects individual differences it is supposed to determine the direction
in which evolution takes place. On the theory that evolution has taken
place independently of selection, this latter is not supposed to be the
case; the finished products, so to speak, are offered to the
environment; and if they pass muster, even ever so badly, they may
continue to propagate themselves.
The asymmetrical form of certain animals living in a symmetrical
environment might be used as an argument to show that the relation of
symmetry between an animal and its environment can easily be overstepped
without danger. The enormous claw of the fiddler-crab must throw the
animal out of all symmetrical relation with its environment, and yet the
species flourishes. The snail carries around a spiral hump that is
entirely out of symmetrical relation with the surroundings of a snail.
These facts, few though they are, yet suffice to show, I believe, that
the relation of symmetry between the organism and its environment may
be, and is no doubt in many cases, more perfect than the requirements of
the situation demand. The fact that animals made unsymmetrical through
injuries (as when a crab loses several legs on one side, or a worm its
head) can still remain in existence in their natural environment, is in
favor of the view that I have just stated. By this I do not mean to
maintain that a symmetrical form does not have, on the whole, an
advantage over the same form rendered asymmetrical, but that this
relation need not have in all forms a selective value, and if not, then
it cannot be the outcome of a process of natural selection.
To sum up: it appears probable that the laws determining the symmetry of
a form are the outcome of internal factors, and are not the result
either of the direct action of the environment, or of a selective
process. The finished products and not the different imperfect stages in
such a process, are what the inner organization offers to the
environment. While the symmetry or asymmetry may be one of the numerous
conditions which determine whether a form can persist or not, yet we
find that the symmetrical relations may be in some cases more perfect
than the environment actually demands; and in other cases, although the
form may place the organism at a certain disadvantage, it may still be
able to exist in certain localities.
Mutual Adaptation of Colonial Forms
In the white ants, true ants, and bees, we find certain individuals of
the community specialized in such a way that their modifications stand
in certain useful relations to other members of the community. Amongst
the bees, the workers collect the food, make the comb, and look after
the young. The queen does little more than lay eggs, and the drone’s
only function is to fertilize the queen. In the true ants there are,
besides the workers and the queen and the males, the soldier caste.
These have large thick heads and large strong jaws. On the Darwinian
theory it is assumed that this caste must have an important rôle to
play, for otherwise their presence as a distinct group of forms cannot
be accounted for; but I do not believe it is necessary to find an excuse
for their existence in their supposed utility. From the point of view of
the mutation theory, their real value may be very small, but so long as
their actual presence is not entirely fatal to the community they may be
endured.
In regard to these forms, Sharp writes:[28] “The soldiers are not alike
in any two species of Termitidæ, so far as we know, and it seems
impossible to ascribe the differences that exist between the soldiers of
different species of Termitidæ to special adaptations for the work they
have to perform.” “On the whole, it would be more correct to say that
the soldiers are very dissimilar in spite of their having to perform
similar work, than to state that they are dissimilar in conformity with
the different tasks they carry on.” The soldiers have the same instincts
as the workers, and do the same kinds of things to a certain extent.
“The soldiers are not such effective combatants as the workers are.”
Statements such as these indicate very strongly that the origin of this
caste can have very little to do with its importance as a specialized
part of the community.
Footnote 28:
“The Cambridge Natural History,” Vol. V, 1895.
The differences between the castes have gone so far in some of these
groups that the majority of the members of the community have even lost
the power to reproduce their kind, and this function has devolved upon
the queen, whose sole duty is to reproduce the different castes of which
the community is composed. This specialization carries with it the idea
of the individuals being adapted to each other, so that, taken all
together, they form a whole, capable of maintaining and reproducing
itself. It does not seem that we must necessarily look upon this union
as the result of competition leading to a death struggle between
different colonies, so that only those have survived in each generation
that carried the work of specialization one step farther. All that is
required is to suppose that such specialization has appeared in a group
of forms living together, and the group has been able to perpetuate
itself. We do not find that all other members of the two great groups to
which the white ants and true ants belong have been crowded out because
these colonial forms have been evolved. Neither need we suppose that
during the evolution of these colonial species there has been a death
struggle accompanying each stage in the evolution. If the members of a
colonial group began to give rise to different forms through mutations,
and if it happened that some of the combinations formed in this way were
capable of living together, and perpetuating the group, this is all that
is required for such a condition to persist.
The relation of the parents to the offspring presents in some groups a
somewhat parallel case to that of these colonial forms. Not only are
some of the fundamental instincts of the parents changed, but structures
may be present in the parents whose only use is in connection with the
young. The marsupial pouch of the kangaroo, in which the immature young
are carried and suckled, is a case in point, and the mammary glands of
the Mammalia furnish another illustration.
Adaptations of these kinds are clearly connected with the perpetuation
of the race. In the case of the mammals the young are born so immature
that they are dependent on the parental organs, just spoken of, for
their existence. Could we follow this relation through its evolutionary
stages, it would no doubt furnish us with important data, but
unfortunately we can do no more than guess how this relation became
established. The changes in the young and in the parent may have been
intimately connected at each stage, or more or less independent. If we
suppose the mammary glands to have appeared first, they might have been
utilized by the young in order to procure food. Their presence would
then make it possible for the young to be born in an immature condition,
as is the case with the young of many of the mammals. But this is pure
guessing, and until we know more of the actual process of evolution in
this case, it is unprofitable to speculate.
Degeneration
In almost every group of the animal kingdom there are forms that are
recognized as degenerate. This degeneration is usually associated with
the habitat of the animal. In many cases it can be shown with much
probability that these degenerate forms have descended from members of
the group that are not degenerate. We find there is a loss of those
organs that are not useful to the organism in its new environment. The
degeneration may involve nearly the whole organization (except as a rule
the reproductive system), as seen in the tapeworm, or only certain
organs of the body, as the eyes in cave animals. A few examples will
bring the main facts before us.
A parasitic existence is nearly always associated with degeneration.
Under these conditions, food can generally be obtained without
difficulty, at the expense of the host, and apparently associated with
this there is a degeneration, and even a complete loss of so important
an organ as the digestive tract. Thus the tapeworm has lost all traces
of its digestive tract, absorbing the already digested matter of its
host through its body wall. Some of the roundworms, that live in the
alimentary tracts of other animals, may have their digestive organs
reduced. In Trichina, this degeneration has gone so far that the
digestive tract is represented, in part, by a single line of endoderm
cells, pierced by a cavity. The digestive organs are also absent in
certain male rotifers, which are parasitic on the females, and these
organs are also very degenerate in the male of _Bonellia_, a gephyrean
worm. A parasitic snail, _Entoscolax ludwigii_, has its digestive
apparatus reduced to a sucking tube ending in a blind sac. The rest of
the tract has completely degenerated. The remarkable parasitic
crustacean, _Sacculina carcini_, looks like a tumor attached to the
under surface of the abdomen of a crab. It has neither mouth nor
digestive tract, and absorbs nourishment from the crab through rootlike
outgrowths that penetrate the body. From its development alone we know
that it is a degenerate barnacle.
There seems to be in all these cases an apparent connection between the
absence of the digestive tract and the presence of an abundant supply of
food, that has already been partly digested by the host. Put in a
different way, we may say that the presence of this food has furnished
the environment in which an animal may live that has a rudimentary
digestive tract.
An interesting case of degeneration is found in the rudimentary mouth
parts of the insects known as May-flies, or ephemerids. Some of these
species live in the adult condition for only a few hours, only long
enough to unite and deposit their eggs. In the adult stage the insects
do not take any food. In this case the degeneration is obviously not
connected with the presence of food, but apparently with the shortness
of the adult life.
One of the most familiar cases of degeneration is blindness, associated
with life in the dark. The most striking cases are those of cave
animals, but this is only an extreme example of what is found everywhere
amongst animals that live concealed during the day under stones, etc.
The blind fish and the blind crayfish of the Mammoth Cave, the blind
proteus of the caves of Carniola, the blind mole that burrows
underground, the blind larvæ of many insects that live in the dark, are
examples most often cited. Some nocturnal animals, like the earthworm,
have no eyes, although they are still able to distinguish light; and
some of the deep-sea animals, that live below the depth to which light
penetrates, have degenerate eyes. The workers of some ants, that remain
in the nests, are blind, but the males and the queens of these forms
have well-developed eyes, although the eyes may be of use to them at
only one short period of their life, namely, at the time of the marriage
flight. This fact is significant and is underestimated by those who
believe that disuse accounts for the degeneration of organs.
The wings of the ostrich and of the kiwi are rudimentary structures no
longer used for flight, and many insects, belonging to several different
orders, have lost their wings, as seen in fleas, some kinds of bugs, and
moths, and even in some grasshoppers.
A curious case of degeneration is found in the abdomen of the hermit
crab, which is protected by the appropriated shell of a snail. The
appendages of one side of the abdomen have nearly disappeared in the
male, although in the female the abdominal appendages are used to carry
the eggs as in other decapod crustaceans. The abdomen, instead of being
covered by a hard cuticle, as in other members of this group, is soft
and unprotected except by the shell of the snail.
Cases of these kinds could be added to almost indefinitely, and the
explanation of these degenerate structures has been a source of
contention amongst zoologists for a long time. The most obvious
interpretation is that the degeneration has been the result of disuse.
But as I have already discussed this question, and given my reasons for
regarding it as improbable that degeneration has arisen in this way, we
need not further consider this point here.
The selectionists have offered several suggestions to account for
degeneration. In fact, this has been one of the difficulties that has
given them most concern. They have suggested, for example, that when an
organ is no longer of use to its possessor it would become a source of
danger, and hence would be removed through natural selection. They have
also suggested that since such organs draw on the general food supply
they would place their possessor at a disadvantage, and hence would be
removed. Weismann has attempted to meet the difficulty by his theory of
“Panmixia,” or universal crossing, by which means the useless structures
are imagined to be eliminated.
These attempts will suffice to point out the straits to which the
Darwinians have found themselves reduced, and we have by no means
exhausted the list of suggestions that have been made. Let us see, if,
on any other view, we can avoid some of the difficulties that the
selection theory has encountered.
In the first place we shall be justified, I think, in eliminating
competition as a factor in the process, since the admission that an
organ has become useless carries with it the idea that it has no longer
a selective value. If, in its useless condition, it is no longer greatly
injurious, as is probably, though not necessarily always, the case, then
selection cannot enter into the problem. If in parasitism we assume that
an animal finds a lodgement in another animal, where it is able to
exist, we may have the first stage of the process introduced at once. If
under these conditions a mutation appeared, involving some of the organs
that are no longer essential to the life of the individual in its new
environment, the new mutation may persist. We need not suppose that the
original form becomes crowded out, but only that a more degenerate form
has come into existence. As a matter of fact we find in most groups, in
which degenerate forms exist, a number of different stages in the
degeneration in different species. Mutation after mutation might follow
until many of the original organs have disappeared. The connection that
appears to exist between the degeneration of a special part and the
environment in which the animal lives finds its explanation simply in
the fact that the environment makes possible the existence of that sort
of mutation in it. We do not know, as yet, whether through mutative
changes an organ can completely disappear, although this seems probable
from the fact that in a few cases mutations are known to have arisen in
which a given part is entirely functionless. If we could assume that, a
mutation in the direction of degeneration being once established,
further mutations in the same direction would probably occur, the
problem would be much simplified; but we lack data, at present, to
establish this view.
In the case of blind animals it seems probable that the transition has
taken place in such forms as had already established themselves in
places more or less removed from the light. Such forms as had the habit
of hiding away under stones, or in the ground, living partly in and
partly out of the light, might, if a mutation appeared of such a sort
that amongst other changes the eyes were less developed, still be
capable of leading an existence in the dark, while it might be
impossible for them to exist any longer with weakened vision in the
light. If such a process took place, the habitat of the new form would
be limited, or in other words it would be confined to the locality to
which it finds itself adapted; not that it has become adapted to the
environment through competition with the original species, or, in fact,
with any other.
Thus, from the point of view that is here taken, an animal does not
become degenerate because it becomes parasitic, but the environment
being given, some forms have found their way there; in fact, we may
almost say, have been forced there, for these degenerate forms can only
exist under such conditions.
In conclusion, this much at least can be claimed for the mutation
theory; that it meets with no serious difficulty in connection with the
phenomena of degeneration. It meets with no difficulty, because it makes
no pretence to explain the origin of adaptations, but can account for
the occurrence of degenerate forms, if it is admitted that these appear
as mutations, or as definite variations. Let us, however, not close our
eyes to the fact that there is still much to be explained in respect to
the degeneration of animals and plants. It is far from my purpose to
apply the mutation theory to all adaptations; in fact, it will not be
difficult to show that there are many adaptations whose existence can
have nothing directly to do with the mutation theory.
Protective Coloration
That many species of animals are protected by their resemblance to their
environment no one will probably deny. That we are ignorant in all cases
as to how far this protection is necessary for the maintenance of the
species must be admitted. That some of the resemblances that have been
pointed out have been given fictitious value, I believe very probable.
Resemblance in color between the organism and its environment has given
to the modern selectionist some of his most valuable arguments, but we
should be on our guard against supposing that, because an animal may be
protected by its color, the color has been acquired on this account. On
the supposition that the animal has become adapted by degrees, and
through selection, we meet with all the objections that have been urged,
in general, against the theory of natural selection. But if we assume
here also that mutations have occurred without relation to the
environment, and, having once appeared, determined in some cases the
distribution of the species, we have at least a simple hypothesis that
appears to explain the facts. If it be claimed that the resemblance is,
in some cases, too close for us to suppose that it has arisen
independently of the environment, it may be pointed out that it has not
been shown that such a close resemblance is at all necessary for the
continued existence of the species, and hence the argument is likely to
prove too much. For instance, the most remarkable case of resemblance is
that of Kallima, but in the light of a recent statement by Dean it may
be seriously asked whether there is absolute need of such a close
resemblance to a leaf. Even if it be admitted that to a certain extent
the butterfly is at times protected by its resemblance to a leaf, it is
not improbable that it could exist almost equally well without such a
close resemblance. If this is true, natural selection could never have
brought about such a close imitation of a leaf. Cases like these of
over-adaptation are not unaccountable on the theory of mutation, for on
this view the adaptation may be far ahead of what the actual
requirements for protection demand. We meet occasionally, I think,
throughout the living world with resemblances that can have no such
interpretation, and a number of the kinds of adaptations to be described
in this chapter show the same relation.
Some of the cases of mimicry appear also to fall under this head;
although I do not doubt that many so-called cases of mimicry are purely
imaginary, in the sense that the resemblance has not been acquired on
account of its relation to the animal imitated. There is no need to
question that in some cases animals may be protected by their
resemblance to other animals, but it does not follow, despite the
vigorous assertions of some modern Darwinians, that this imitation has
been the result of selection. Until it can be shown that the imitating
species is dependent on its close imitation for its existence, the
evidence is unconvincing; and even if, in some cases, this should prove
to be the case, it does not follow that natural selection has brought
about the result, or even that it is the most plausible explanation that
we have to account for the results. The mutation theory gives, in such
cases, an equally good explanation, and at the same time avoids some of
the difficulties that appear fatal to the selection theory.
What has been said against the theory of mimicry might be repeated in
much stronger terms against the hypothesis of warning colors.
It seems to me, in this connection, that the imagination of the
selectionist has sometimes been allowed to “run wild”; and while it may
be true that in some cases the colors may serve as a signal to the
possible enemies of the animal, it seems strange that it has been
thought necessary to explain the origin of such colors as the result of
natural selection. Indeed, some of these warning colors appear
unnecessarily conspicuous for the purpose they have to perform. In other
words, it does not seem plausible that an animal already protected
should need to be so conspicuous. If we stop for a moment to consider
what an enormous amount of destruction must have occurred, according to
Darwin’s theory, in order to bring this warning coloration to its
supposed state of perfection, we may well hesitate before committing
ourselves to such an extreme view.
That gaudy colors have appeared or been present in animals that are
protected in other ways is not improbable, when we consider the rôle
that color plays everywhere in nature. That the presence of such colors
may, to a certain limited extent, protect its possessor may be admitted
without in any degree supposing that natural selection has directed the
evolution of such color, or that it has been acquired through a life and
death struggle of the individuals of the species.
Sexual Dimorphism[29] and Trimorphism
Footnote 29:
This term is used here in the sense employed by Darwin. The same term
is sometimes used for those cases in which the male departs very
greatly from the female in form.
It has been found in a few species of animals and plants that two or
more forms of one sex may exist, and here we find a condition that
appears to be far more readily explained on the mutation theory than on
any other. The most important cases, perhaps, are those in plants, but
there are also similar cases known amongst animals, and these will be
given first.
There is a North American butterfly, _Papilio turnus_, that appears
under at least two forms. In the eastern United States the male has
yellow wings with black stripes. There are two kinds of females, one of
which resembles the male except that she has also an orange “eye-spot”;
the other female is much blacker, and this variety is found particularly
in the south and west. The species is dimorphic, therefore, mainly in
the latter regions.
The cases of seasonal dimorphism offer somewhat similar illustrations.
The European butterfly, _Vanessa levana-prorsa_, has a spring generation
(_levana_) with a yellow and black pattern on the upper surface of the
wings. The summer generation (_prorsa_) has black wings “with a broad
white transverse band, and delicate yellow lines running parallel to the
margins.” These two types are sharply separated, and their differences
in color do not appear to be associated with any special protection that
it confers on the bearer. These facts in regard to Vanessa seem to
indicate that differences may arise that are perfectly well marked and
sharply defined, which yet appear to be without any useful significance.
We meet with cases in which the same animal has at different times of
year different colors, as seen in the summer and winter plumage of the
ptarmigan. There is no direct evidence to show how this seasonable
change has been brought about; but from the facts in regard to Vanessa
we can see that it might have been at least possible for the white
winter plumage, for instance, to have appeared without respect to any
advantage it conferred on the animal, but after it had appeared it may
have been to a certain degree useful to its possessor.
[Illustration:
Fig. 5.—A, long-styled, and
B, short-styled, forms of _Primula veris_.
C, D, E, the three forms of the trimorphic flower of _Lythrum
salicaria_, with petals and calyx removed on near side. (After
Darwin.)]
Amongst plants there are some very interesting cases of dimorphism and
trimorphism in the structure of the flowers. Darwin has studied some of
these cases with great care, and has made out some important points in
regard to their powers of cross-fertilization.[30] The common European
cowslip, _Primula veris_, var. _officinalis_, is found under two forms,
Figure 5 A and B, which are about equally abundant. In one the style is
long so that the stigma borne on its end comes to the top of the tube of
the corolla. The stamens in this form stand about halfway up the tube.
This is called the long-styled form. The other kind, known as the
short-styled form, has a style only half as long as the tube of the
corolla, and the stamens are attached around the upper end of the tube
near its opening. In other words, the position of the end of the style
(the stigma) and that of the stamens is exactly reversed in the two
forms. The corolla is also somewhat differently shaped in the two forms,
and the expanded part of the tube above the stamens is larger in the
long-styled than in the short-styled form. Another difference is found
in the stigma, which is globular in the long-styled, and depressed on
its top in the short-styled, form. The papillæ on the former are twice
as long as those on the short-styled form. The most important difference
is found in the size of the pollen grains. These are larger in the
long-styled form, being in the two cases in the proportion of 100 to 67.
The shape of the grains is also different. Furthermore, the long-styled
form tends to flower before the other kind, but the short-styled form
produces more seeds. The ovules in the long-styled form, even when
unfertilized, are considerably larger than those of the short-styled,
and this, Darwin suggests, may be connected with the fact that fewer
seeds are produced, since there is less room for them. The important
point for our present consideration is that intermediate forms do not
exist, although there are fluctuating variations about the two types.
Moreover, the two kinds of flowers never appear on the same plant.
Darwin tried the effect of fertilizing the long-styled flowers with the
pollen from the same flower or from other long-styled flowers. Unions of
this sort he calls illegitimate, for reasons that will appear later. He
also fertilized the long-styled flowers with pollen from short-styled
forms. A union of this sort is called legitimate. Conversely, the
short-styled forms were fertilized with their own pollen or with that
from another short-styled form. This is also an illegitimate union.
Short-styled forms fertilized with pollen from long-styled forms give
again legitimate unions.
Footnote 30:
Many of the facts as to the occurrence of these cases were known
before Darwin worked on them; but very little had been ascertained in
regard to the sexual relation between the dimorphic and trimorphic
forms, and it was here that Darwin obtained his most interesting
results.
The outcome of these different crossings are most curious. In the table,
page 364, the results of the four combinations are given. It will be
seen at once that the legitimate unions give more capsules, and the
seeds weigh more, than in the illegitimate unions.
The behavior of the offspring from seeds of legitimate and
illegitimate origin is even more astonishing. Darwin found in _Primula
veris_ (the form just described) that the seeds from the short-styled
form fertilized with pollen from the same form germinated so badly
that he obtained only 14 plants, of which 9 were short-styled and 5
long-styled. The long-styled form fertilized with its own-styled
pollen produced “in the first generation 3 long-styled plants. From
their seed 53 long-styled grandchildren were produced; from their seed
4 long-styled great-grandchildren; from their seed 20 long-styled
great-great-grandchildren; and lastly, from their seed 8 long-styled
and 2 short-styled great-great-great-grandchildren.”
══════════════╤═════════╤═════════╤═════════╤═════════╤═════════
│Number of│ │ Maximum │ Minimum │ Average
Nature of │ Flowers │Number of│of Seeds │of Seeds │ No. of
Union │Fertilized│ Seed │ in any │ in any │Seeds per
│ │Capsules │ one │ one │ Capsule
│ │ │ Capsule │ Capsule │
──────────────┼─────────┼─────────┼─────────┼─────────┼─────────
Long-styled │ 10 │ 6 │ 62 │ 34 │ 46.5
form by │ │ │ │ │
pollen of │ │ │ │ │
short-styled│ │ │ │ │
form: │ │ │ │ │
Legitimate │ │ │ │ │
union. │ │ │ │ │
──────────────┼─────────┼─────────┼─────────┼─────────┼─────────
Long-styled │ 20 │ 4 │ 49 │ 2 │ 27.7
form by │ │ │ │ │
own-form │ │ │ │ │
pollen: │ │ │ │ │
Illegitimate│ │ │ │ │
union. │ │ │ │ │
──────────────┼─────────┼─────────┼─────────┼─────────┼─────────
Short-styled │ 10 │ 8 │ 61 │ 37 │ 47.7
form by │ │ │ │ │
pollen of │ │ │ │ │
long-styled │ │ │ │ │
form: │ │ │ │ │
Legitimate │ │ │ │ │
union. │ │ │ │ │
──────────────┼─────────┼─────────┼─────────┼─────────┼─────────
Short-styled │ 17 │ 3 │ 19 │ 6 │ 12.1
form by │ │ │ │ │
own-form │ │ │ │ │
pollen: │ │ │ │ │
Illegitimate│ │ │ │ │
union. │ │ │ │ │
──────────────┼─────────┼─────────┼─────────┼─────────┼─────────
The two │ 20 │ 14 │ 62 │ 37 │ 47.1
legitimate │ │ │ │ │
unions │ │ │ │ │
together. │ │ │ │ │
──────────────┼─────────┼─────────┼─────────┼─────────┼─────────
The two │ 30 │ 7 │ 49 │ 2 │ 35.5
illegitimate│ │ │ │ │
unions │ │ │ │ │
together. │ │ │ │ │
══════════════╧═════════╧═════════╧═════════╧═════════╧═════════
From other long-styled plants, fertilized with their own-form pollen, 72
plants were raised, which were made up of 68 long-styled and 4
short-styled. In all, 162 illegitimate unions of this sort produced 156
long-styled and 6 short-styled plants. It is evident from these results
that the long-form pistils, fertilized with pollen from flowers of the
same pistil-form (from other individuals as a rule), tend to produce the
same form as their parents, although occasionally the other form. The
fertility of these plants from an illegitimate union is found to be very
low. Darwin observed that sometimes the male and female organs of these
plants were in a very deteriorated condition. It is interesting to
notice, in this connection, that in another species, _Primula sinensis_,
illegitimate plants from long-styled parents were vigorous, but the
flowers were small and more like the wild form. They were, however,
perfectly fertile.
Illegitimate plants from short-styled parents were dwarfed in stature,
and often had a weakly constitution. They were not very fertile _inter
se_, and remarkably infertile when legitimately fertilized. This kind of
result, where a difference in the power of mutual intercrossing exists
between two forms, recalls in many ways the difference in the results of
crossing of different species of animals and plants, especially those
cases in which a cross can be made in one way more successfully than in
the other.
The heterostyled trimorphic plants, of which _Lythrum salicaria_, Figure
5 C, D, E, may be taken as an example, are even more remarkable. There
are three different kinds of flowers: in one the pistil is long and
there is a medium and a short set of stamens; in another the pistil is
of intermediate length and there is a long set and a short set of
stamens; in the third kind the pistil is short, and there is a medium
and a long set of stamens. There are possible only six sorts of
legitimate unions between these three sets of flowers. No less than
twelve kinds of illegitimate unions may occur. In regard to the
difference in the sizes of the pollen grains, those from the long-styled
form are the largest, from the mid-styled form next, and from the
short-styled form the smallest. The extreme difference is as 100 to 60.
“Nothing shows more clearly the extraordinary complexity of the
reproductive system of this plant than the necessity of making eighteen
distinct unions in order to ascertain the relative fertilizing power of
the three forms.” Darwin tried the effect of each of these combinations,
making 223 unions in all. The results are surprising. Comparing the
outcome of the six legitimate unions with the twelve illegitimate ones,
the following results were obtained:—
═══════════════╤══════════╤══════════╤══════════╤══════════
│Number of │ │ │ Average
│ Flowers │Number of │ Average │ No. of
Nature of Union│Fertilized│ Capsules │ No. of │Seeds per
│ │ Produced │Seeds per │ Flower
│ │ │ Capsule │Fertilized
───────────────┼──────────┼──────────┼──────────┼──────────
The 6 │ 75 │ 56 │ 96.29 │ 71.89
legitimate │ │ │ │
unions │ │ │ │
───────────────┼──────────┼──────────┼──────────┼──────────
The 12 │ 146 │ 36 │ 44.72 │ 11.03
illegitimate │ │ │ │
unions │ │ │ │
═══════════════╧══════════╧══════════╧══════════╧══════════
This table shows that the fertility of the legitimate to that of the
illegitimate is as 100 to 33, as judged by the flowers that produced
capsules; and as 100 to 46 as judged by the average number of seeds per
capsule. It is evident, therefore, that “it is only the pollen from the
longest stamens that can fully fertilize the longest pistil; only that
from the mid-length stamens, the mid-length pistil; and only that from
the shortest stamens, the shortest pistil.”
Darwin tries to connect this fact with the visits of insects to the
flowers. He says: “And now we can comprehend the meaning of the almost
exact correspondence in length between the pistil in each form and of a
set of six stamens in two of the other forms; for the stigma of each
form is thus rubbed against that part of the insect’s body which becomes
charged with the proper pollen.” A further conclusion that Darwin draws
is “that the greater the inequality in length between the pistil and the
set of stamens, the pollen of which is employed for its fertilization,
by so much is the sterility the more increased.” Darwin also makes the
following significant comment on the problem here involved: “The
correspondence in length between the pistil in each form, and a set of
stamens in the other two forms, is probably the direct result of
adaptation, as it is of the highest service to the species by leading to
full and legitimate fertilization.” He points out, on the other hand,
that the increased sterility of the illegitimate unions, in proportion
to the inequality in length between the pistil and the stamens employed,
can be of no service at all. Neither can this relation have any
connection with the facility for self-fertilization. “We are led,
therefore, to conclude that the rule of increased sterility in
accordance with increased inequality in length between the pistils and
stamens is a purposeless result, incidental on those changes through
which the species has passed in acquiring certain characters fitted to
insure the legitimate fertilization of the three flowers.”
In regard to the plants that were raised from the seeds from legitimate
and illegitimate unions, Darwin found in Lythrum that of twelve
illegitimate unions two were completely barren, and nearly all showed
lessened fertility; only one approached complete fertility. Darwin lays
much emphasis on the close resemblance in the sterility of the
illegitimate unions, and the sterility of different species when
crossed. In both cases every degree of sterility is met with, “from very
slightly lessened fertility to absolute barrenness.” The importance of
this comparison cannot, I think, be overestimated, for, if admitted, it
indicates clearly that the infertility between species cannot be used as
a criterion of their distinctness, because here, in individuals
belonging to the same species, we find sterility between pistils and
stamens of different lengths. If, as I shall urge below, we must
consider these different forms of Primula the results of a mutation, and
not the outcome of selection as Darwin supposed, then this relation in
regard to infertility becomes a point of great interest.
This brings us to the central point of our examination of these cases of
dimorphism and trimorphism. How have these forms arisen? Darwin tries to
account for them as follows: Since heterostyled plants occur in fourteen
different families of plants, it is probable that this condition has
been acquired independently in each family, and “that it can be acquired
without any great difficulty.” The first step in the process he imagines
to have been due to great variability in the length of the pistil and
stamens, or of the pistil alone. Flowers in which there is a great deal
of variation of this sort are known. “As most plants are occasionally
cross-fertilized by the aid of insects, we may assume that this was the
case with our supposed varying plant; but that it would have been
beneficial to it to have been more regularly cross-fertilized.” “This
would have been better accomplished if the stigma and the stamens stood
at the same level; but as the stamens and pistil are supposed to have
varied much in length, and to be still varying, it might well happen
that they could be reduced much more easily through natural selection
into two sets of different lengths in different individuals than all to
the same length and level in all individuals.” By means of these
assumptions, improbable as they may appear, Darwin tries to explain
these cases of dimorphism. But when we attempt to apply the same
argument to the trimorphic forms, it is manifestly absurd to pretend
that three such sharply defined types could ever have been formed as the
result of natural selection. But we have not even yet touched the chief
difficulty, as Darwin himself points out. “The essential character of a
heterostyled plant is that an individual of one form cannot fully
fertilize, or be fertilized by, an individual of the same form, but only
by one belonging to another form.” This result Darwin admits cannot be
explained by the selection theory, for, as he says, “How can it be any
advantage to a plant to be sterile with half of its brethren, that is,
with the individuals belonging to the same form?” He concludes that this
sterility between the individuals of the same form is an incidental and
purposeless result. “Inner constitutional differences” between the
individuals is the only suggestion that is offered to account for the
phenomenon. In other words, it is clearly apparent that the attempt to
apply the theory of selection has here broken down, and it is a
fortunate circumstance that the Lamarckian theory cannot here be brought
to the rescue, as it so often is in Darwin’s writings, when the theory
of natural selection fails to give a sufficient explanation.
On the other hand, this is one of the cases that seem to fit in
excellently with the mutation theory, for if these two forms of the
primrose should appear, as mutations, and if, as is the case, they do
not blend when crossed, but are equally inherited, they would both
continue to exist as we find them to-day. Whether the similar forms were
infertile with each other would be determined at the outset by the
nature of the individual variation, and if, despite this obvious
disadvantage, the forms could still continue to propagate themselves,
the new dimorphic form would remain in existence. Darwin cannot explain
the origin of dimorphic forms and trimorphic forms unless he can show
that there is some advantage in having two forms, and as we have seen,
he fails completely to show that there is an advantage. On the other
hand, the result might have been reached on the mutation theory, even if
the dimorphic and trimorphic forms were placed at a greater disadvantage
than were the parent forms. In such a case fewer individuals might
appear, or find a foothold; but as long as the race could be kept up the
new forms would remain in existence. Thus, while no attempt is made to
explain what has always been, and may possibly long remain, inexplicable
to us, namely, the origin of the new form itself, yet granting that such
new forms may sometimes appear spontaneously, they may be able to
establish themselves, regardless of whether they are a little more or a
little less well adapted to the environment than were their parent
forms. If it should appear that the question is begged by the assumption
that mutations such as these may appear (at one step or by a series of
steps is immaterial), it should not be forgotten that the whole
Darwinian theory itself also rests on the spontaneous appearance of
fluctuating variations, whose origin it does not pretend to explain. In
this respect both theories are on the same footing, but where the
Darwinian theory meets with difficulties at every turn by assuming that
new forms are built up through the action of selection, the mutation
theory escapes most of these difficulties, because it applies no such
rigid test as that of selection to account for the presence of new
forms.
Length of Life as an Adaptation
It has been pointed out in the first chapter that the length of life of
the individual has been supposed by some of the most enthusiastic
followers of Darwin to be determined by the relation of the individual
to the species as a whole. In other words, the doctrine of utility has
been applied here also, on the ground that it would be detrimental to
the species to have part of the individuals live on to a time when they
can no longer propagate the race or protect the young. It is assumed
that those varieties or groups of individuals (unfortunately not sharply
defined) would have the best chance to survive in which the parent forms
died as soon as they had lost the power to produce new individuals.
Sometimes interwoven with this idea there is another, namely, that
_death itself_ has been acquired because it was more profitable to
supplant the old and the injured individuals by new ones, than to have
the old forms survive, and thus deprive the reproducing individuals of
some of the common food supply.
This insidious form that the selection theory has taken in the hands of
its would-be advocates only serves to show to what extremes its
disciples are willing to push it. On the whole it would be folly to
pursue such a will-o’-the-wisp, when the theory can be examined in much
more tangible examples. If in these cases it can be shown to be
improbable, the remaining superstructure of quasi-mystical hypothesis
will fall without more ado.
That the problem of the length of life may be a real one for
physiological investigation will be granted, no doubt, without
discussion, and that in some cases the length of life and the coming to
maturity of the germ-cell may be, in some way, physiologically connected
seems not improbable; but that this relation has been regulated by the
competition of species with each other can scarcely be seriously
maintained. I will not pretend to say whether the mutation theory can or
cannot be made to appear to give the semblance of an explanation of the
length of life in each species, but it seems to me fairly certain that
this is one of the questions which we are not yet in a position to
attempt to consider on any theory of evolution.
Organs of Extreme Perfection
It has often been pointed out that certain organs may be more perfectly
developed than the requirements of the surroundings strictly demand. At
least we have no good reasons to suppose in some cases that constant
selection is keeping certain organs at the highest possible point of
development, yet, on the Darwinian theory, as soon as selection ceases
to be operative the level of perfection must sink to that which the
exigencies of the situation demand. The problem may be expressed in a
different way. Does the animal or plant ever possess organs that are
more perfectly adapted than the absolute requirements demand? If such
organs are the result of fluctuating variations, they will be unable to
maintain themselves in subsequent generations without a constant process
of selection going on. If, on the other hand, the organs have arisen as
mutations, they may become permanently established without respect to
the degree of perfection of their adaptation. We can see, therefore,
that cases of extreme perfection meet with no difficulty on the mutation
theory, while they have proven one of the stumbling-blocks to the
selection theory.
There are, in fact, many structures in the animal and plant kingdoms
that appear to be more perfect than the requirements seem to demand. The
exact symmetry of many forms appears in some cases to be unnecessarily
perfect. The perfection of the hand of man, the development of his vocal
organs, and certain qualities of his brain, as his musical and
mathematical powers, seem to go beyond the required limits. It is not,
of course, that these things may not be of some use, but that their
development appears to have gone beyond what selection requires of these
parts.
Closely related to this group of phenomena are those cases in which
certain organs are well developed, but which can scarcely be of use to
the animal in proportion to their elaboration. The electric organs of
several fishes and skates are excellent examples of this sort of
structures. The phosphorescent organs do not appear, in some forms at
least, to be useful in proportion to their development. The selection
theory fails completely to explain the building up of organs of this
kind, but on the mutation theory there is no difficulty at all in
accounting for the presence of even highly developed organs that are of
little or of no use to the individual. If the organs appeared in the
first place as mutations, and their presence was not injurious to the
extent of interfering seriously with the existence and propagation of
the new form, this new form may remain in existence, and if the
mutations continued in the same direction, the organs might become more
perfect, and highly developed. The whole class of secondary sexual
organs may belong to this category, but a discussion of these organs
will be deferred to the following section.
Secondary Sexual Organs as Adaptations
In the sixth chapter we have examined at some length Darwin’s
interpretation of the secondary sexual characters. His explanation has
been found insufficient in many cases to account for the conditions.
That these organs do play in some cases a role in the relation of the
sexes to each other may be freely admitted. In other words, in some
animals the organs in one sex appear in the light of adaptations to
certain instincts in the other sex. It would, perhaps, appear to
simplify the problem to deny outright that any such relation exists; but
I think, in the light of the evidence that we have, this procedure would
be like that of the proverbial ostrich, which is supposed to stick its
head in the sand in order to escape an anticipated danger. If we assumed
this agnostic position, we might attempt to account for the appearance
of secondary sexual organs as mutations that had appeared in one sex,
and had no immediate connection with the other sex; and, so long as
these organs were not directly and seriously injurious, we might assume
that the animals in which such structures had appeared might be able to
exist. But, on the other hand, I think that an examination of the
evidence will show that this way out of the difficulty is not very
satisfactory, for the organs in question appear, in some cases at least,
to be closely connected with certain definite responses in the other
sex. Moreover, as Darwin has so insistently pointed out, the action of
the males is of such a sort that it is evidently associated with the
presence of the secondary sexual organs which they often display before
the other sex. Furthermore, the greater and often exclusive development
of these organs during the sexual period distinctly points to them as in
some way connected with the relation of the sexes to each other. And
finally, there is a small, although not entirely convincing, body of
evidence, indicating that the female is influenced by the action of the
male; but I do not think that this evidence shows that she selects one
individual at the expense of all other rivals. We meet here with a
problem that is as profoundly interesting as it is obscure. In fact, if
we admit that this relation exists we have a double set of conditions to
deal with: first, the development in the males of certain secondary
sexual organs; and secondly, the instinct to display these organs. The
supposed influence of the display on the female may also have to be
taken into account, although, for all we know to the contrary, the same
results might follow were there no secondary sexual character at all, as
is, in fact, the case in most animals.
I have a strong suspicion that much that has been written on this
subject is imaginative, and in large part fictitious; so that it may,
after all, be the wisest course not to attempt to explain how this
relation has arisen until we have a more definite conception of what we
are really called upon to explain. For example, when we see a gorgeously
bedecked male displaying himself before a female, we feel that his
finery must have been acquired for this very purpose. On the other hand,
when we see an unornamented male also making definite movements before
the female, we do not feel called upon to explain the origin of his
colors. Now, it is not improbable that the ornaments of the first
individual have not been acquired in order to display them before the
female, and this view seems to me the more probable. From this
standpoint our problem is at least much simplified. What we need to
account for is only that the male is excited to undergo certain
movements in the presence of the female, and possibly that the female
may be influenced by the result. That this view is the more profitable
is indicated by the occurrence of secondary sexual characters in the
lower forms, as in the insects and crustaceans, in which it appears
almost inconceivable that the ornamentation could have been acquired in
connection with the æsthetic taste of the other sex. It does not seem to
me that the conditions in the higher animals call for any other
explanation than that which applies to these lower forms.
My position may be summed up in the statement, that, while in some cases
there appears to be a connection between the presence in one sex of
secondary sexual organs and their effect on the other sex, yet their
origin cannot be explained on account of this connection.
Individual Adjustments as Adaptations
As pointed out in the first chapter, there is a group of adaptations,
obviously including several quite different kinds of phenomena, that can
at least be conveniently brought together under the general rubric of
individual adjustments or regulations. A few examples of these will
serve to show in what sense they may be looked upon as adaptations, and
how they may be regarded from the evolutionary point of view.
Color Changes as Individual Adaptations
The change in color of certain fish in response to the color of the
background, the change in color of some chrysalides also in response to
their surroundings, appears to be of some use to the animals in
protecting them from their enemies. The change in color from green to
brown and from brown back to green in several lizards and in some tree
frogs is popularly supposed to be in response to the color of the
surroundings, but a more searching examination has shown that, in some
cases at least, the response has nothing to do with the color of the
background.
In the first cases mentioned above, in which the response appears to be
of some advantage to the animal, the question may be asked, how have
such responses arisen? The selection theory assumes that those animals
that responded at first to a slight degree in a favorable direction have
escaped, and this process being repeated, the power to change has been
gradually built up. The mutation theory will also account for the result
by assuming the response to have appeared as a new quality, but it has
been preserved, not because it has been of vital importance to its
possessor, but simply because the species possessing it has been able to
survive, perhaps in some cases even more easily, although this is not
essential. Even if the change were of no direct benefit, or even
injurious to a slight degree, it might have been retained, as appears in
fact to be the case in the change of color of the green lizards.
Increase of Organs through Use and Decrease through Disuse
We meet here with one of the most characteristic and unique features of
living things as contrasted with non-living things. We shall have to
dismiss at once the idea that we can explain this attribute of organisms
by either the selection or the mutation theory; for we find animals
possessing this power that could never be supposed to have acquired it
by any experience to which they have been subjected; and since it
appears to be so universally present, we cannot account for it as a
chance mutation that may have appeared in each species. No doubt Wolff
had responses of this kind in view when he made the rather sweeping
statement that purposeful adaptation is the most characteristic feature
of living things. The statement appears to contain a large amount of
truth, if confined to the present group of phenomena.
This power of self-regulation may confer a great benefit on its
possessor. The increase in the size and strength of the muscles through
use may give the animal just those qualities that make its existence
easier. The increase in the power of vision, or at least of visual
discrimination through use, of the power of smell and of taste, of
hearing and of touch, are familiar examples of this phenomenon.
However much we may be tempted to speculate as to how this property of
the animal may have been acquired, we lack the evidence which would
justify us in formulating even a working hypothesis. It may be that when
we come to know more of what the process of contraction of the muscle
involves, the possibility of its development as a consequence of its use
may be found to be a very simple phenomenon that requires no special
explanation at all to account for its existence in the individual,
further than that the muscles are of such a kind that this is a
necessary physical result of their action. But until we know more of the
physiology involved in the process, it is idle to speculate about the
origin of the phenomenon.
Reactions of the Organism to Poisons, etc.
In this case also we meet with a number of responses for whose origin we
can give not the shadow of an explanation. On the other hand, the cases
are significant in so far as a number of them show quite clearly that
the response cannot have been acquired through the experience of the
organism, or the selection of those individuals that have best resisted
the particular poison. This is true, because in a number of cases the
poison is a substance that the animal cannot possibly have met with
during the ordinary course of its life, or of that of its ancestors. It
may be argued, it is true, that in the case of the poisons produced by
certain bacteria the power of resistance has been acquired through the
survival of the less susceptible, or more resistant, individuals.
Improbable as this may be in some cases, it does not, even if it were
true, alter the real issue, for it can be shown, as has just been said,
that the same power of responding adaptively is sometimes shown in cases
of poisons that are new to the animal.
There is no question that different individuals respond in very
different degrees to these poisonous substances, and it is easy to
imagine in the case of contagious diseases that a sort of selective
process might go on that would bring the race up to the highest point to
which fluctuating variations could be carried, even to complete
immunity; but even if this were the case, it seems to be true that the
moment the selection stopped the race would sink back to the former
condition.
All this touches only indirectly the main point that we have under
consideration, namely, the existence of this power of resistance in
cases where it cannot have been the result of any educative process.
Since the responses to new poisons do not appear to be in principle
different from the responses to those to which the organism may have
possibly been subjected at times in the past, we shall probably not go
far wrong if we treat all cases on the same general footing. Whether the
power of adaptation to certain substances, such as nicotine, morphine,
cocaine, arsenic, alcohol, etc., is brought about by the formation of a
counter-substance is as yet unproven. And while it seems not improbable
that in some of these instances it may turn out that this is the case,
especially for poisons of plant origin, it is better to suspend judgment
on this point until each case has been established.
In recent years it has been shown that the animal body has the power of
making counter-substances when a very large number of different kinds of
things are introduced into the blood. We seem to be here on the
threshold of a field for discovery which may, if opened up, give us an
insight into some of the most remarkable phenomena of adaptation shown
by living things.
It has already been pointed out that it appears to be almost a _reductio
ad absurdum_ to speak of animals adapting themselves to poisonous
substances. It is curious, too, that in man at least the use of these
substances may arouse a craving for the poison, or at any rate the
individual may become so dependent on the poison that the depression
following its disuse may lead to a desire for a repetition of the dose.
The two questions that are raised here must be kept apart, for the
adaptation of the individual to the poison and the so-called craving for
it may depend on quite different factors. Nevertheless, it seems to be
true in the case of morphine and of arsenic, and probably for some other
substances as well, that if their use is suddenly stopped the individual
may die in consequence. In this respect the organism behaves exactly as
it does to an environment to which it has become adapted.
Regeneration
Many animals are able to replace lost parts, and all of them can heal
wounds and mend injuries. This power is obviously of great advantage to
them, and it has been supposed by Darwin, and more especially by his
followers, that the power has been acquired through natural selection.
It is not difficult to show that regeneration could not, in many cases,
and presumably in none, have been acquired in this way. Since I have
treated this subject at some length recently in my book on
“Regeneration,” I shall attempt to do no more here than indicate the
outline of the argument.
The Darwinians believe that, if some individuals of a species have the
power to replace a part that is lost better than have other individuals,
it would follow that those would survive that regenerate best, and in
this way after a time the power to regenerate perfectly would be
acquired.
But the matter is by no means so simple as may appear from this
statement. In the first place, it is a matter of common observation that
all the individuals of a species are never injured in the same part of
the body at the same time. In those cases in which it is known that a
special part is often injured, an examination has shown that there are
not more than ten per cent of individuals that are injured at any one
time, and in the case of the vast majority of animals this estimate is
much too great. Thus there will be very little chance for competition of
the injured individuals in each generation with each other, and the
effects that are imagined to be gained as a result would be entirely
lost by crossing with the uninjured individuals. But it is not necessary
to consider this possibility, since there is another fact that shows at
once that the power to regenerate could not have been gained through
selection. The number of uninjured individuals in each generation will
be much greater than the injured ones, and these will have so great an
advantage over the injured individuals that, if competition approached
the degree assumed by the selectionists, the injured individuals should
be exterminated. A slight advantage gained through better powers of
regeneration would be of little avail in competition, as compared with
the competition with the uninjured individuals. Since selection is
powerless to accomplish its end without competition, and since with
competition all the injured individuals would be eliminated, it is clear
that an appeal cannot be made to selection to explain the power of
regeneration.
In many cases the power of regeneration could not have been slowly
acquired through selection, since the intermediate steps would be of no
use. Unless, for example, a limb regenerated from the beginning almost
completely, the result would be of no use to the animal. If the limb did
regenerate completely the first time it was injured, then the selection
hypothesis becomes superfluous.
There are also a few cases known in which a process of regeneration
takes place that is of no use to the animal. If, for instance, the
earthworm (_Allolobophora fœtida_) be cut in two in the middle, the
posterior piece regenerates at its anterior cut end, not a head, but a
tail. Not by the widest stretch of the imagination can such a result be
accounted for on the selection theory. Again, we find the reverse case,
as it were, in certain planarians. If the head of _Planaria lugubris_ is
cut off just behind the eyes, there develops at the cut surface of this
head-piece another head turned in the opposite direction. Here again we
have the regeneration of a perfect structure, but one that is entirely
useless to the individual. The development of an antenna in place of an
eye in the shrimp, when the eye stalk is cut off near its base, is
another instance of the occurrence of a perfectly constant process, but
one that is of no use to the organism.
When we recall that in some organisms regeneration takes place in almost
every part of the body, it does not seem possible that this power could
have been acquired by selection. And when we find that many internal
organs regenerate, that can rarely or never be injured without the
animal perishing, it seems impossible that this can be ascribed to the
principle of natural selection.
It has also been found that if the first two cells of the egg of a
number of animals, jellyfish, sea-urchins, salamanders, etc., be
separated, each will produce an entire animal. In some of these cases it
is inconceivable that the process could ever have been acquired through
selection, because the cells themselves can be separated only by very
special and artificial means.
These, and other reasons, indicate with certainty that regeneration
cannot be explained by the theory of natural selection.
------------------------------------------------------------------------
CHAPTER XI
TROPISMS AND INSTINCTS AS ADAPTATIONS
Of the different kinds of adaptation none are more remarkable than those
connected with the immediate responses of organisms to external agents.
These responses are usually thought of as associated with the nervous
system; and while in the higher forms the nervous system plays an
important role in the reaction, yet in many cases it is little more than
the shortest path between the point stimulated and the muscles that
contract; and in the lower animals, where we find just as definite
responses, there may be no distinct nervous system, as in the protozoa,
for instance.
Many of the so-called instincts of animals have been shown in recent
years to be little more than direct responses to external agents. Many
of these instincts are for the good of the individual, and must be
looked upon as adaptations. For example: if a frog is placed in a jar of
water, and the temperature of the water lowered, the frog will remain at
the top until the water reaches 8 degrees C., when it will dive down to
the bottom of the jar; and, if the temperature is further lowered, it
will remain there until the water becomes warmer again, when it will
come to the surface again. It is clear that, under the ordinary
conditions of life of the frog, this reaction is useful to it, since it
leads the animal to go to the bottom of the pond on the approach of cold
weather, and thus to avoid being frozen at the surface.
Another illustration of an instinct that is a simple response to light
is shown by the earthworm. During the day the worm remains in its
burrow, but on dark nights it comes out of its hole, and lies stretched
out on the surface of the ground. It procures its food at this time, and
the union of the individuals takes place. In the early morning the worm
retires into its burrow.
This habit of the earthworm is the direct result of its reaction to
light. It crawls away from ordinary light as bright as that of diffuse
daylight, and, indeed, from light very much fainter than that of
daylight. If, however, the light be decreased to a certain point, the
worm will then turn and crawl toward the source of light. This lower
limit has been found by Adams to be about that of .001 candle-metre.
This corresponds to the amount of light of a dark night, and gives an
explanation of why the worm leaves its burrow only at night, and also
why it crawls back on the approach of dawn. It is also obvious that this
response is useful to the animal, for if it left the burrow during the
day, it would quickly fall a prey to birds.
The blow-fly lays its eggs on decaying meat, on which the larvæ feed.
The fly is drawn to the meat by its sense of smell, a simple and direct
response to a chemical compound given off by the meat. The maggot that
lives in the decaying meat is also attracted by the same odor, as Loeb
has shown, and will not leave the meat, or even a spot on a piece of
glass that has been smeared with the juice of the meat, so long as the
odor remains. Here again the life of the race depends on the proper
response to an external agent, and the case is all the more interesting,
since the response of the fly to the meat is of no immediate use to the
fly itself, but to the maggot that hatches from the egg of the fly.
The movement toward or from a stimulating agent is, in some cases,
brought about in the following way. Suppose an earthworm is lying in
complete darkness, and light be thrown upon it from one side. The worm
turns its head, as it thrusts it forward, to the side away from the
light; and as it again moves forward, it continues to bend its head away
from the light, until it is crawling directly away from the source. When
the light first strikes the worm, the two sides will be differently
illuminated. This causes a bending of the head, as it stretches forward,
toward the side of less illumination, and the bending is due to a
stronger contraction of some of the muscles on the less illuminated
side; at least the reaction appears to be due to a simple response of
this kind. When the body has been so far turned that the two sides are
equally illuminated, the muscles of the two sides will contract equally,
and the movement will be straight forward and away from the light. If
the reaction is as simple as this (which is in principle the explanation
advanced by Loeb), the result is a simple reflex act, and need not
involve any consciousness or intentional action on the part of the worm
to crawl away from the light. In fact, the same reaction takes place
when the brain is removed, not so quickly or definitely, it is true, but
this may be due to the removal of the anterior segments of the worm, in
which part the skin appears to be more sensitive to light than
elsewhere.
Another factor that plays an important rôle in the habits of the
earthworm is the response to contact,—the so-called stereotropism. If,
in crawling over a flat surface, the worm comes in contact with a
crevice, it will crawl along it, and refuse to leave until the end is
reached. The contact holds the worm as strongly as though it were
actually pulled into the crevice. It can be forced to leave a crevice
only by strong sunlight, and then it does not do so at once. If the worm
crawls into a small glass tube, it is also held there by its response to
contact, and the smaller the tube, the more difficult is it to make the
worm leave by throwing strong sunlight upon it.
Loeb has found that when winged aphids, the sexual forms, are collected
in a tube, and the tube is kept in a room, the aphids crawl toward the
light. This happens in ordinary diffuse light, as well as in lamplight.
It is stated that the animals orientate themselves towards the light
more quickly when it is strong than when it is weak. They turn their
bodies toward the light, and then move forward in the direction from
which the rays come. It can be shown by a simple experiment that the
aphids are turned by the _direction_ of the light, and not by its
intensity. If they are placed in a tube, and the tube laid obliquely
before a window in such a way that the direct sunlight falls only on the
inner end of the tube, the aphids will, if started at the inner end of
the tube, first crawl toward the outer surface of the tube, and then
wander along this wall, passing out of the region of sunlight into the
end of the tube nearest the window, where they come to rest at the end.
They have moved constantly towards the direction from which the rays
come, passing, as it were, from ray to ray, but each time toward a ray
nearer the source of the light.
If the tube be turned toward the window, and the window end be covered
with blue glass, the aphids crawl into this end of the tube, as they
would have done had the tube been uncovered. If, on the other hand, the
end of the tube be covered with red glass, they do not crawl into the
part of the tube that is covered, unless they are very sensitive to
light. Even in the latter case they may remain scattered in the red
part, and do not all accumulate at the end, as they do when blue glass
is used. In other words, while they respond to blue as they do to
ordinary light, they behave toward red as they do towards a very faint
light.
In diffuse daylight the aphids, as has been said, crawl toward the
light, but if they come suddenly into the sunlight they begin to fly.
Thus they remain on the food-plant until the sun strikes it, and then
they fly away.
The aphid also shows another response; it is negatively geotropic,
_i.e._ it tends to crawl upward against gravity. If placed on an
inclined, or on a vertical, surface, it will crawl upward. Such an
experiment is best made in the dark, since in the light the aphid also
responds to the light. If put on a window it crawls upward never
downward.
Aphids are also sensitive to heat. If they are placed in a darkened tube
and put near a stove, they crawl away from the warmer end; but if they
are acted upon by the light at the same time, they will be more strongly
attracted by the light than repulsed by the heat. We thus see that there
are at least three external agents that determine the movements of this
animal, and its ordinary behavior is determined by a combination of
these, or by that one that acts so strongly as to overpower the others.
The swarming of the male and female ants is also largely directed by the
influence of light. Loeb observed that when the direct sunlight fell
full upon a nest in a wall the sexual forms emerged, and then flew away.
Other nests in the ground were affected earlier in the day, because the
sun reached them first. These ants, when tested, were found to respond
to light in the same way as do the aphids. The wingless forms, or worker
ants, do not show this response, and the winged forms soon lose their
strong response to light after they have left the nest. Thus we see that
the heliotropism is here connected with a certain stage in the
development of the individual; and this is useful to the species, as it
leads the winged queens and males to leave the nest, and form new
colonies. Even the loss of response that takes place later may be looked
upon as beneficial to the species, since the queens do not leave the
nest after they have once established it.
It is familiar to every one that many of the night-flying insects are
attracted to a lamplight, and since those that fly most rapidly may be
actually carried into the flame before they can turn aside, it may seem
that such a response is worse than useless to them. The result must be
considered, however, in connection with other conditions of their life.
The following experiments carried out by Loeb on moths show some of the
responses of these insects to light.
Night-flying moths were placed in a box and exposed in a room to
ordinary light. As twilight approached the moths became active and began
to fly always toward the window side of the box. They were positively
heliotropic to light of this intensity. If let out of the case, they
flew toward the window, where they remained even during the whole of the
next day, fully exposed to light. If the moth is disturbed in the
daytime, so that it flies, it goes always toward the light, and never
away from it. These facts show that the moth is always positively
heliotropic, and also that the flight toward the lamp is a natural
response, misapplied in this case. That the moths do not fly by day is
due to another factor, namely, the alternation in the degree of their
sensitiveness at different times. But this condition alone does not seem
to account fully for all the facts.
If the moths are given the alternative of flying toward the evening
light, or toward the lamp, they always go toward the brighter light.
Thus if, when they swarm at dusk, they are set free in the middle of the
room, at the back of which a lamp is burning, the moths fly toward the
window. If, however, they are set free within a metre of the lamp, they
fly toward it.
The explanation that Loeb offers of the habit of these moths to fly only
in the evening is, that, although they are at all times positively
heliotropic, they respond to light only in the evening. In other words,
it is assumed that there is a periodic change in their sensitiveness to
light, which corresponds with the change from day to night. Loeb says
that, just as certain flowers open only at night, and others only during
the day, so do moths become more responsive in the evening, and
butterflies during the day. Both moths and butterflies are positively
heliotropic, and the sensitiveness of moths to light may be even greater
in the evening than is that of butterflies, for the light of the evening
to which the moth reacts is less than the minimal to which the butterfly
responds.
Moths appear to pass into a sort of sleep during the day, while
butterflies are quiescent only at night. The periodicity of the sleeping
time continues, at least for several days, when the insects are kept in
the dark. For instance, moths kept in the dark become restless as the
evening approaches, as Réaumur observed long ago. It has been found in
plants that this sort of periodicity may continue for several days, but
gradually disappears if the plants are kept in the dark. By using
artificial light, and exposing the plants to it during the night, and
putting them in the dark during the day, a new periodicity, alternating
with the former one, may be induced; and this will continue for some
days if the plants are then kept continually in the dark.
Loeb tried the experiment of exposing the quiescent moths suddenly to a
lower intensity of light, in order to see if they would respond equally
well at any time of day. It was found that if the change was made in the
forenoon, between six o’clock and noon, it was not possible to awaken
the moths by a sudden decrease in the intensity of the light. But it was
possible to do so in the afternoon, long before the appearance of dusk.
It appears, therefore, that in this species, _Sphinx euphorbiæ_, it is
possible to influence the period of awakening by decreasing the
intensity of light, but this can be done only near the natural period of
awakening. It seems to me that this awaking of a positively heliotropic
animal by decreasing the light needs to be further investigated.
The day butterflies are also positively heliotropic. Butterflies of the
species _Papilio machaon_, that have been raised from the pupa, remain
quietly on the window in the diffuse daylight of a bright day. They can
be carried around on the finger without leaving it, but the moment they
come into the direct rays of the sun they fly away.
Butterflies that have just emerged from their pupa case exhibit a marked
negative geotropic reaction, and this appears to be connected with the
necessity of unfolding their wings at this time. Loeb says that the same
cause that determines the direction of the falling stone and the paths
of the planets, namely, gravity, also directs the actions of the
butterfly that has just left its pupa case. The geotropic response is
especially strong at first. The animal wanders around until it reaches a
vertical wall, which it immediately ascends, straight upward, and
remains hanging at the top until its wings have unfolded. A similar
response occurs in the final stage of the larva of the May-fly, which
leaves the water and crawls up a blade of grass, or other vertical
support, and there, bursting the pupa skin, it dries its wings and flies
away. That this is a reaction to gravity and not to light is shown by
Loeb’s observation, that their empty skins are sometimes observed under
a bridge where the light does not come from above. “This observation on
the larva of the May-fly contradicts the assumption that the ‘purpose’
of the geotropic response of the butterfly is that it may the better
unfold its new wings, for in the ephemerid larva the negative geotropism
appears at a time when no wings are present.” On the other hand, it
should not be overlooked that the reaction is important for the May-fly
larva in other ways, because it leads the larva to leave the water at
the right period, and come out into the air, where the flying insect can
more safely emerge.
It is not without interest to find that caterpillars exhibit some of the
same reaction shown by butterflies. Loeb has made numerous experiments
with the caterpillars of _Porthesia chrysorrhœa_. The caterpillars of
this moth collect together in the autumn and spin a web or nest in which
they pass the winter. If they are taken from the nest and brought into a
warm room, they will orientate themselves to the light, and also crawl
toward it. If placed in a tube, they crawl to the upper side of the
glass and then along this side toward the light. If a covering is placed
over the end of the tube that is turned toward the window, the
caterpillars will crawl only as far as the edge of the cloth. They also
react negatively to gravity. If kept in a dark room, they will crawl
upward to the top of the receptacle in which they are enclosed. If
subjected to the influences of both light and gravity, they respond more
strongly to the light. The caterpillars also show a contact reaction.
They tend to collect on convex sides or on corners and angles of solid
bodies. They may even pile up one on top of the other in response to
this reaction; the convex side of a quiescent animal acting on another
animal crawling over it as any convex surface would do and holding the
animal fast.
These three kinds of reactions determine the instincts of these
caterpillars. In the spring, when they become warm, they leave the nest.
Positive heliotropism and negative geotropism compel them to crawl
upward to the tops of the branches of the trees, and there the contact
reaction with the small buds holds them fast in this place. That they
are not attracted to the end of the branches by the food that they find
there is shown by placing buds in the bottom of the tubes in which the
caterpillars are contained. The caterpillars remain at the top of the
tube, although food is within easy reach. If, however, they are placed
directly on the buds, the contact reaction will hold them there, and
they will not crawl farther upward. Curiously enough, as soon as the
caterpillars have fed and the time for shedding approaches, the
responsiveness to light and to gravity decreases, and at the time of
shedding they do not respond at all to these agents. These same
caterpillars react also to warmth above a certain point. In a dark tube
placed near a stove, the caterpillars collect at the end farthest away
from the source of the heat. They react to light best at a temperature
between 20 and 30 degrees C., and above this temperature point they
become restless and wander about.
The very close connection between the reactions of this caterpillar and
its mode of life is perfectly obvious. The entire series of changes
seems to have for its “purpose” the survival of the individual by
bringing it to the place where it will find its food. It may seem
natural to conclude that these responses have been acquired for this
very purpose, but let us not too quickly jump at this obvious conclusion
until the whole subject has been more fully examined.
The upward and downward movements of some pelagic animals have been
shown to depend on certain tropic responses. Every student of marine
zoology is familiar with the fact that many animals come to the surface
at night, and go down at the approach of daylight. It has been shown
that this migration is due largely to a response to light. Light can
penetrate to only about four hundred metres in sea-water, and there is
complete darkness below this level. It has been shown that the swimming
larvæ of one of the barnacles is positively heliotropic in a weak light,
but negatively heliotropic in a stronger light. Animals having responses
like these will come to the surface as the light fades away in the
evening and remain there until the light becomes too bright in the
following morning. They will then become negatively heliotropic and
begin to go down. When they reach a level where the intensity of the
light is such that they become positively heliotropic, they will turn
and start upward again. Thus during the day they will keep below the
surface, remaining in the region where they change from positive to
negative, and _vice versa_.
It would not be difficult to imagine that this upward and downward
migration of pelagic animals is useful to them, but, on the other hand,
it may be equally well imagined that the response may be injurious to
them. Thus it might be supposed that certain forms could procure their
food by coming to the surface at night, and avoid their enemies by going
down during the day. But it is difficult to see why organisms that serve
as prey should not have acquired exactly the opposite tropisms in order
to escape.
Some of these marine forms are also geotropic. Loeb has determined that
“the same circumstances that make the animals negatively heliotropic
also make them positively geotropic, and _vice versa_.” It was found,
for instance, that the larva of the marine worm Polygordius is
negatively geotropic at a low temperature, while at a higher temperature
it is positively geotropic. This response would drive the animals upward
when the water becomes too cold, and back again if the surface water
becomes too warm; but whether the response is so adjusted that the
animals keep, as far as possible, in water of that temperature that is
best for their development, we do not know. We can easily imagine that
within wide limits this is the case.
The change from positive to negative can also be brought about in other
ways. One of the most striking cases of this sort is that described by
Towle in one of the small crustaceans, _Cypridopsis vidua_. It was found
that after an animal had been picked up in a pipette its response was
always positive; that is, it swam toward the light, no matter what its
previous condition had been. The disturbance caused by picking the
animal up induced always a positive response towards light. If the light
were moved, the Cypridopsis followed the light. In this way it could be
kept positive for some time, but if it came to rest, or if it came into
contact with the sides or end of the trough, it became, after a short
time, negatively heliotropic, and remained negative as long as it could
be kept in motion, without being disturbed, or coming into contact with
a solid object. If when positive it were allowed to reach the glass at
the end of the trough, it would swim about there, knocking against the
glass, and then soon turn and swim away from the light. If the light
were shifted while the negative animal was in the middle of the trough,
it would turn and swim directly away, as before, from the source of
light. It could be kept in this negative state as long as it did not
come into contact with the ends.
It appears that the positive condition in Cypridopsis is of short
duration, and ceases after a while either as a response to contact or
without any observable external factor causing the change.
This crustacean lives at the bottom of pools, amongst water-plants, and
here also, no doubt, the same change from one to the other reaction
takes place. What possible advantage it may be to the animal to be kept
continually changing in this way is not at all obvious, nor, in fact,
are we obliged to assume that this reaction may be of any special use to
it. Indeed, it is far from obvious how the change that causes the animal
to swim toward the light when it is disturbed could be of the least
advantage to it.
In another crustacean, one of the marine copepods, _Labidocera æstiva_,
it has been shown by Parker that the male and female react in a somewhat
different way both to light and to gravity. The females are strongly
negatively geotropic, and this sends them up to the top of the water.
The males are very slightly negatively geotropic. The females are
strongly positively heliotropic toward light of low intensity; the males
show the same response to a less degree. To strong light the females are
negative and the males are indifferent. On the other hand, the males are
attracted to the females, probably in response to some chemical
substance diffusing from the females, since the males show the same
reaction when the females are enclosed in an opaque tube through whose
ends a diffusion of substances may take place. This crustacean frequents
the surface of the ocean from sunset to sunrise. During the day it
retires to deeper water. Its migrations can be explained as follows: The
females come to the surface at night, because they are positively
heliotropic to weak light, and also because they are negatively
geotropic. They go down during the day, because they react to bright
light more strongly than to gravity. The males follow the females,
largely because they react positively chemotactically toward the
females.
Some other animals respond in a somewhat different way to light, as
shown by the fresh-water planarians. These animals remain during the day
under stones, where the amount of light is relatively less than outside.
If they are placed in a dish in the light in front of a window, they
crawl away from the light, but when they reach the back of the dish they
do not come to rest, but continue to crawl around the sides of the dish
even toward the light. The light makes the worms restless, and while
they show a negative response as long as they are perfectly free to move
away from the light, they will not come to rest when they come to the
back of the dish if they are there still in the light, because the
irritating action of the light on them is stronger than its directive
action. If, however, in crawling about they come accidentally into a
place less bright than that in which they have been, they stop, and will
not leave this somewhat darker spot for a brighter one, although they
might leave the newly found spot for one still less bright.
At night the planarians come out and wander around, which increases
their chance of finding food, although it would not be strictly correct
to say that they come out in search of food. If, however, food is placed
near them, a piece of a worm, for example, they will turn toward it,
being directed apparently by a sense of smell, or rather of taste.
The heliotropic responses of the planarians appear to be of use to them,
causing them to hide away in the daytime, and to come out only after
dark, when their motions will not discover them to possible enemies. But
some of the planarians are protected in other ways, so that they will
not be eaten by fish, probably owing to a bad taste; so that it is not
so apparent that they are in real need of the protection that their
heliotropic response brings to them. Their turning towards their food
is, however, beyond question of great advantage to them, for in this way
they can find food that they cannot detect in any other way.
The unicellular plants were amongst the first organisms whose tropic
responses were studied, and the classical work of Strasburger gave the
impetus to much of the later work. In recent years the unicellular
animals, the protozoans, have been carefully studied, more especially by
Jennings. His results show that the reactions in these animals are
different in some important respects from those met with in higher
forms. For instance, most of the free-swimming infusoria are
unsymmetrical, as are also many of the flagellate forms, and as they
move forward they rotate freely on a longitudinal axis. It is therefore
impossible that they could orientate themselves as do the higher animals
that have been described above, and we should not expect these Protozoa
to react in the same way. In fact, Jennings shows that they exhibit a
different mode of response. Paramœcium offers a typical case. As it
moves forward it rotates toward the aboral side of the body. As a result
of the asymmetry of the body, the path followed, as it revolves on its
own axis, is that of a spiral. Did the animal not rotate, as it swims
forward, its asymmetrical form would cause it to move in a circle, but
its rotation causes, as has been said, the course to be that of a
spiral, and the general direction of movement is forward.[31] The
rotation of a paramœcium on its axis is in turn caused by the oblique
stroke of the cilia that cover the surface of the body. Their action
when reversed causes the animal to rotate backward.
Footnote 31:
The same result is attained by a bullet that is caused by the rifling
to rotate as it moves forward.
If a drop of weak acid be put into the water in which the paramœcia are
swimming,—for instance, in the water between a cover-slip and a
slide,—it will be found, after a time, that many individuals have
collected in the drop. It was at first supposed that the paramœcia are
attracted by the diffusion of the acid in the water, and turn toward the
source of the chemical stimulus; but Jennings has shown that this is not
the way in which the aggregation is brought about. If the individuals
are watched, it will be found that they swim forward in a spiral path
without regard to the position of the drop of acid. If one happens, by
chance, to run into the drop, there is no reaction as it enters, but
when it reaches the other side of the drop, and comes into contact with
the water on this side, it suddenly reacts. It stops, backs into the
middle of the drop, rotates somewhat toward the aboral side (_i.e._ away
from the vestibule), and then starts forward again, only to repeat the
action on coming into contact with the edge of the drop again. The
paramœcium has been caught in a veritable trap. All paramœcia that
chance to swim into the drop will also be caught, until finally a large
number will accumulate in the region. The result shows, that, in passing
from ordinary water into a weak acid, no reaction takes place; but
having once entered the acid, the animal reacts on coming into contact
with the water again.
On the other hand, there are some substances to which the paramœcium may
be said to be negatively chemotropic. If a drop of a weak alkaline
solution be put into water in which paramœcium is swimming, an
individual that happens to run against it reacts at once. It stops
instantly, backs off, revolving in the opposite direction, turns
somewhat to one side, and swims forward again. The chances are that it
will again hit the drop, in which case it repeats the same reaction,
turning again to one side. If it continues to react in this way, it
will, in the course of time, turn so far that when it swims forward it
will miss the edge of the drop, and then continue on its way. If an
individual were put into an alkaline drop, it would leave it, because it
would not react when it passed from inside the drop into the surrounding
water.
Unicellular animals react to other things besides differences in the
chemical composition of different parts of a solution. In many cases
they react to light, swimming toward or away from it according to
whether they are positively or negatively heliotropic. If they are
positively heliotropic, and while swimming run into a shadow, they react
as they would on coming into contact with a drop of acid. Since they
rotate as they swim forward, we cannot explain their orientation as in
the case of other animals that hold a fixed vertical position. If we
assume that the two ends of the body are differently affected by the
light, for which there is some evidence, we can perhaps in this way
account for their turning toward, or away from, the source of light.
Changes in the osmotic pressure of the different parts of the fluid,
mechanical stimulation produced by jarring, extremes of heat and of
cold, all cause this same characteristic reaction in Paramœcium; and
this accounts for their behavior toward these agents that are so
different in other respects.
Paramœcia, as well as other protozoans, show a contact response. They
fix themselves to certain kinds of solid bodies. If, for example, a
small bit of bacterial slime is put into the water, the paramœcia
collect around it in crowds, and eat the bacteria; but they will collect
in the same way around almost any solid. On coming in contact with
bodies having a certain physical texture, the cilia covering the
paramœcium stop moving, only those in the oral groove continuing to
strike backward. The animal comes to rest, pressed against the solid
body. If one or more paramœcia remain in the same place, they set free
carbon dioxide, as a result of their respiratory processes. There is
formed around them a region containing more of this acid than does the
surrounding water. If other moving paramœcia swim, by chance, into this
region, they are caught, and as a result an accumulation of individuals
will take place. The more that collect the larger will the area become,
and thus large numbers may be ultimately entrapped in a region where
there is formed a substance that, from analogy with other animals, we
should expect to be injurious.
The question as to how far these responses of the unicellular forms are
of advantage to them is difficult to decide, for while, as in the above
case, the response appears to be injurious rather than useful, yet under
other conditions the same response may be eminently advantageous. In
other cases, as when the paramœcia back away, and then swim forward
again, only to repeat the process, the act appears to be such a stupid
way of avoiding an obstacle that the reaction hardly appears to us in
the light of a very perfect adaptation. If we saw a higher animal trying
to get around a wall by butting its head into it until the end was
finally reached, we should probably not look upon that animal as well
adapted for avoiding obstacles.
Bacteria, which are generally looked upon as unicellular plants, appear,
despite the earlier statements to the contrary, to react in much the
same way as do the protozoans, according to the recent work of Rothert,
and of Jennings and Crosby. The bacteria do not seem to turn toward or
away from chemical substances, but they collect in regions containing
certain substances in much the same way as do the protozoans. The
collecting of bacteria in regions where oxygen is present has been known
for some time, but it appears from more recent results that they are not
attracted toward the oxygen, but by accidentally swimming into a region
containing more oxygen they are held there in the same way as is
paramœcium in a drop of acid. On the other hand bacteria do not enter a
drop of salt solution, or of acids, or of alkalies. They react
negatively to all such substances. Some kinds of bacteria have a
flagellum at each end, and swim indifferently in either direction. If
they meet with something that stimulates them, as they move forward,
they swim away in the opposite direction, and continue to move in the
new direction until something causes again a reversal of their movement.
In this respect their mode of reaction seems of greater advantage than
that followed by paramœcium.
Another instinct, that appears to be due to a tropic response, is the
definite time of day at which some marine animals deposit their eggs.
The primitive fish, Amphioxus, sets free its eggs and sperm only in the
late afternoon. A jellyfish, Gonionema, also lays its eggs as the light
begins to grow less in the late afternoon, and in this case it has been
found that the process can be hastened if the animals are placed in the
dark some hours before their regular time of laying. There is no
evidence that this habit is of any advantage to the animal. We may
imagine, if we like, that the early stages may meet with less risk at
night, but this is not probable, for it is at this time that countless
marine organisms come to the surface, and it would seem that the chance
of the eggs being destroyed would then be much greater. It is more
probable that the response is of no immediate advantage to the animals
that exhibit it, although in particular cases it may happen to be so.
This response recalls the diurnal opening and closing of certain
flowers. The flowers of the night-blooming cereus open only in the dusk
of evening, and then emit their strong fragrance. Other flowers open
only in the daytime, and some only in bright sunlight. It is sometimes
pointed out that it is of advantage to some of these flowers to open at
a certain time, since the particular insects that are best suited to
fertilize them may then be abroad. This may often be the case, but we
cannot but suspect that in other cases it may be a matter of little
importance. In special instances it may be that the time of opening of
the flowers is of importance to the species; but even if this is so,
there is no need to assume that the response has been gradually acquired
for this particular purpose. If it were characteristic of a new form to
open at a particular time, and there were insects in search of food at
this time that would be likely to fertilize the plant, then the plant
would be capable of existing; but this is quite different from supposing
that the plant developed this particular response, because this was the
most advantageous time of day for the fertilization of its flowers.
We can apply this same point of view, I believe, to many of the
remarkable series of tropisms shown by plants, whose whole existence in
some cases is closely connected with definite reactions to their
environment. Let us examine some of these cases.
When a seed germinates, the young stem is negatively geotropic, and, in
consequence, as it elongates it turns upward towards the light that is
necessary for its later growth. The root, on the contrary, is positively
geotropic, and, in consequence, it is carried downward in the ground.
Both responses are in this case of the highest importance to the
seedling, for in this way its principal organs are carried into that
environment to which they are especially adapted. It matters very little
how the seed lies in the ground, since the stem when it emerges will
grow upward and the root downward. The young stem, when it emerges from
the soil, will turn toward the light if the illumination comes from one
side, and this also may often be of advantage to the plant, since it
turns toward the source from which it gets its energy. The leaves also
turn their broad surfaces toward the light, and as a result they are
able to make use of a greater amount of the energy of the sunlight. The
turning is due to one side of the stem growing more slowly than the
opposite side, and it is true, in general, that plants grow faster at
night than in the daylight. Very bright light will in some cases
actually stop all growth for a time. Thus we see that this bending of
the stem toward the light and the turning of the leaves to face the
light are only parts of the general relation of the whole plant toward
the light.
Negative heliotropism is much less frequent in plants. It has been
observed in aërial roots, in many roots that are ordinarily buried in
the ground, in anchoring tendrils that serve as holdfasts, and even in
the stems of certain climbers. In all of these cases, and more
especially in the case of the climbers, the reaction is obviously of
advantage to the plant; and it is significant to find, in plants that
climb by tendrils carrying adhering disks, that there is a reversal of
the ordinary heliotropism shown by homologous organs in other plants.
There is an obvious adaptation in the behavior of the tendril, since its
growth away from the more illuminated side is just the sort of reaction
that is likely to bring it into contact with a solid body.
In this connection it is important to observe that these reactions to
light are perfectly definite, being either positive or negative under
given conditions, and therefore there is at present nothing to indicate
that there has been a gradual transformation from positive to negative,
or _vice versa_. It seems to me much more probable that when the
structural change took place, that converted the plant into a climber,
there appeared a new heliotropic response associated with the other
change. In other words, both appeared together in the new organ, and
neither was gradually acquired by picking out fluctuating variations.
The leaves of plants also show a sort of transverse heliotropic
response. It has been found, for example, that the leaves of Malva will
turn completely over if illuminated by a mirror from below. A curious
case of change of heliotropism is found in the flower stalks of Linaria.
They are at first positively heliotropic, but after the flower has been
fertilized the stalk becomes negatively heliotropic. As the stalks
continue to grow longer, they push the fruits into the crevices of the
rocks on which the plants grow, and in this way insure the lodgement of
the seeds. Here we have an excellent example showing that the negative
heliotropism of the flower stalk could scarcely have been acquired by
slight changes in the final direction, for only the complete change is
useful to the plant. Intermediate steps would have no special value.
As has been pointed out in the case of the seedling plant, the main stem
responds positively and the roots negatively to gravity. In addition to
this, the lateral position taken by the lateral roots and branches and
by underground stems are also, in part, due to a geotropic response. In
this case also the effect is produced by the increased growth on the
upper side when the response is positive, and on the lower when it is
negative. Leaves also assume a transverse position in response to the
action of gravity, or at least they make a definite angle with the
direction of its action.
The most striking case of geotropic response is seen in plants that
climb up the stems of other plants. The twining around the support is
the result of a geotropic response of the sides of the stem. The young
seedling plant stands at first erect. As its end grows it begins to
curve to one side in an oblique position, and this is due to an increase
in growth on one side of the apex of the shoot. As a result the stem
bends toward the other side. Not only does the end “sweep round in a
circle like the hands of a watch,” but it rotates on its long axis as it
revolves. As a result of this rotation “the part of the stem subjected
to the action of the lateral geotropism is constantly changing; and the
revolving movement once begun, must continue, as no position of
equilibrium can be attained.” This movement will carry the end around
any support, not too thick, that the stem touches.
Most climbers turn to the left, _i.e._ against the hands of a watch,
others are dextral, and a few climb either way.[32] Strasburger states
that whenever any external force, or substance, is important to the
vital activity of the plant or any of its organs, there will also be
found to be developed a corresponding irritability to their influence.
Roots in dry soil are diverted to more favorable positions by the
presence of greater quantities of moisture. This may, I venture to
suggest, be putting the cart before the horse. The plant may be only
able to exist whose responses are suited to certain external conditions,
and these determine the limits of distribution of the plant or the
places in which it is found.
Footnote 32:
These cases recall the spiral growth of the shell of the snail, but
the spiral in the latter is due to some other factor.
A number of plants climb in a different way, and show another sort of
tropism. Those that climb by means of tendrils twist their tendrils
about any support that they happen to come in contact with, and thus the
plant is able to lift its weak stem, step by step, into the air. The
twining of the tendrils is due to contact, which causes a cessation of
growth at the points of contact. The growth of the opposite side
continues, and thus the tendril bends about its support. In the grape
and in ampelopsis the tendril is a modified branch. The stalk of the
leaves in a few plants, as in Lophospermum, act as tendrils. Other
climbers are able to ascend vertical walls owing to the presence of
disks, whose secretions hold the tendril firmly against the support, as
in ampelopsis.
It is interesting to find in practically all these cases that, whatever
the stimulus may be, the results are reached in the same way, namely, by
one part growing faster than another. The fact of importance in this
connection is that the plant is so constructed that the response is
often beneficial to the organism.
Before leaving this subject there is one set of responses to be referred
to that is not the result of growth. Certain movements are brought about
by the change in the turgidity of certain organs. The small lateral
leaflets of _Desmodium gyrans_ make circling movements in one to three
minutes. No apparent benefit results from their action. The terminal
leaflets of _Trifolium pratense_ oscillate in periods of two to four
hours, but do so only in the dark; in the light the leaflets assume a
rigid position. There is nothing in the process to suggest that the
movement is useful to the plant, and yet it appears to be as definite as
are those cases in which the response is of vital importance. Had these
movements been of use, their origin would, no doubt, have been explained
because of their usefulness, and the conclusion would have been wrong.
The leaves of the Mimosa respond, when touched, and it cannot be
supposed that this is of any great advantage to the plant. The sleep
movements of many plants are also due to the effect of light. In some
cases the leaflets are brought together with their upper surfaces in
contact with one another; in other cases the lower surfaces are brought
together. Darwin supposed that these sleep movements served to protect
the leaves from a too rapid loss of heat through radiation, but it has
been pointed out that tropical plants exhibit the same responses. We
have here another admirable instance of the danger of concluding that
because we can imagine an advantage of a certain change, that the change
has, therefore, been acquired because of the advantage. In the Mimosa
not only do the leaflets close together, but the whole leaf drops down
if the stimulus is strong. Other plants also show in a less degree the
same movements, Robinia and Oxalis for instance, and certainly in these
latter the result does not appear to be of any advantage to the plants.
The preceding account of some of the tropisms in animals and plants will
serve to give an idea of how certain movements are direct responses to
the environment. Some of the reactions appear to be necessary for the
life of the individual, others seem to be of less importance, and a few
of no use at all. Yet the latter appear to be as definite and
well-marked as are the useful responses. I think the conviction will
impress itself on any one who examines critically the facts, that we are
not warranted in applying one explanation to those responses that are of
use, and another to those that are of little or of no value. Inasmuch as
the Darwinian theory fails to account for the origin of organs of little
or of no value, it is doubtful if it is needed to explain the origin of
the useful responses. If, on the other hand, we assume that the _origin_
of the responses has nothing to do with their value to the organism, we
meet with no difficulty in those cases in which the response is of
little or of no use to the organism. That great numbers of responses are
of benefit to the organism that exhibits them can be accounted for on
the grounds that those new species, that have appeared, that have useful
responses, are more likely, in the long run, to survive, than are those
that do not respond adaptively.
We may now examine some of the more complicated responses and instincts,
more especially those of the higher animals. Some of these are pure
tropisms, _i.e._ definite responses or reactions to an external exciting
agent; others may be, in part, the result of individual experience,
involving memory; others, combinations of the two; and still others may
depend on a more complex reaction in the central nervous system of the
animal. These cases can be best understood by means of a few
illustrations.
As an example of a simple action may be cited a well-known reflex after
cutting the nerve-cord of the frog, or after destroying the brain. If
the frog is held up, and its side tickled, the leg is drawn up to rub
the place touched. To accomplish this requires a beautifully adjusted
system of movements, yet the act seems to be a direct reflex, involving
only the spinal cord.
An example of a somewhat more complex reflex is the biting off of the
navel-string by the mother in rodents and other mammals; an act
eminently useful to the young animal, although of no importance to the
mother herself. The protection of the young by their parents from the
attacks of other animals appears to be a somewhat complex instinct, and
it is interesting to note that the protection is extended to the young
only so long as they are in need of it, and as soon as they are able to
shift for themselves the maternal protection is withdrawn.
The instinct of the young chick to seize in its beak any small moving
object is a simple and useful reflex action, but if the object should
happen to be a bee which stings the chick, another bee or similar insect
will not be seized. Here we see that a reflex has been changed, and
changed with amazing quickness. Moreover, the chick has learnt to
associate this experience with a particular sort of moving object. It is
this power to benefit by the result of a brief experience that is one of
the most advantageous properties of the organism.
Young chicks first show a drinking reflex if by chance their beaks are
wet by water. At once the head is lifted up, and the drop of water
passes down the throat. In this way the chick first learns the meaning
of water, and no doubt soon comes to associate it with its own condition
of thirst. The sight of water produces no effect on the inexperienced
chick, and it may even stand with its feet in the water without
drinking; but as soon as it touches, by chance, the water with its beak,
the reflex, or rather the set of reflexes is started.
A more complicated instinct is that shown by the spider in making its
web. In some cases the young are born from eggs laid in the preceding
summer, and can have had, therefore, no experience of what a web is
like; and yet, when they come to build this wonderfully complex
structure, they do so in a manner that is strictly characteristic of the
species.
The formation of the comb by bees, in which process, with a minimum of
wax, they secure a maximum number of small storehouses in which to keep
their honey and rear their young, is often cited as a remarkable case of
adaptation.
There has been some discussion as to whether birds build their nests in
imitation of the nest in which they were reared, or whether they do so
independently of any such experience. There can be no doubt, however,
that in some birds neither memory nor imitation can play any important
part in the result, and that they build their nests as instinctively as
spiders make webs.
These instincts of spiders, bees, and birds appear to be more complex
than the reflexes and tropisms that were first described. Whether they
are really so, or only combinations of simple responses, we do not yet
know. That they have come suddenly into existence as we now find them
does not seem probable, but this does not mean that they must have been
slowly acquired as the result of selection. The mutation theory also
assumes that the steps of advance may have been small.
Our account may be concluded with the recital of some instincts, chosen
almost at random, that serve to show some other adaptations which are
the result of these inborn responses.
It is known that ants travel long distances from their nests, and yet
return with unerring accuracy. It has been shown that they are able to
do this through a marvellous sense of smell. The track left by the ant,
as it leaves the nest, serves as a trail in returning to the
starting-point. Moreover, it appears that the ant can pick out her own
trail, even when it has been crossed by that of other ants. This means
that she can distinguish the odor of her own trail from that of other
members of the colony. The sense-organs by means of which the odor is
detected lie in the antennæ. This fact accounts for certain actions of
ants that have been described as showing that they have an affection for
each other. Two ants, meeting, pat each other with their antennæ. In
this way they are quickly able to distinguish members of their own nest
from those of other nests. If they are of the same nest, they separate
quietly; if of other nests, they may fight. If an ant from one nest is
put into another nest, it is instantly attacked and killed—an act that
appears to be injurious rather than useful, for the ant might become a
valuable member of the new colony. If, however, an ant is first immersed
in the blood of a member of the community into which she is to be
introduced, she will not be attacked, and may soon become a part of the
new community. By her baptism of blood she has no doubt acquired
temporarily the odor of the new nest, and by the time that this has worn
off she will have acquired this odor by association, and become thereby
a member of another colony.
Numerous stories have been related of cases in which an ant, having
found food, returns to the nest with as much of it as she can carry, and
when she comes out again brings with her a number of other ants. This
has been interpreted to mean that in some mysterious way the ant
communicates her discovery to her fellow-ants. A simpler explanation is
probably more correct. The odor of the food, or of the trail, serves as
a stimulus to other ants, that follow to the place where the first ant
goes for a new supply of the food. The fact that the first individual
returns to the supply of food seems to indicate that the ant has memory,
and this is obviously of advantage to her and to the whole colony.
The peculiar habits of some of the solitary wasps, of stinging the
caterpillar or other insect which they store up as food for their young,
is often quoted as a wonderful case of adaptive instinct. The poison
that is injected into the wound paralyzes the caterpillar, but as a rule
does not kill it, so that it remains motionless, but in a fresh state to
serve as food for the young that hatch from the egg of the wasp. A
careful study of this instinct by Mr. and Mrs. Peckham has shown
convincingly that the act is not carried out with the precision formerly
supposed. It had been claimed that the sting is thrust into the
caterpillar on the lower side, a ventral ganglion being pierced, the
poison acting with almost instantaneous effect. But it may be questioned
whether this is really necessary, and whether the same end might not be
gained, although not quite so instantaneously, if the caterpillar were
pierced in almost any other part of the body. Can we be seriously asked
to believe that this instinct has been perfected by the destruction of
those individuals (or of their descendants) that have not pierced the
caterpillar in exactly the middle of a segment of the anterior ventral
surface? It seems to me that the argument proves too much from the
selectionist’s point of view. If the wasp pierced the caterpillar in the
middle of its back, we should have passed over the act without comment;
but since the injection is usually made on the ventral side, and since
we know that the nervous system lies in this position, it has been
assumed that the act is carried out in this way, in order that the
poison may penetrate the nervous system more quickly. Yet a fuller
knowledge may show that there is really no necessity for such precision.
A curious response is the so-called death-feigning instinct shown by a
number of animals, especially by certain insects, but even by some
mammals and birds. Certain insects, if touched, draw in their legs, let
go their hold, and fall to the ground, if they happen to be on a plant.
It is not unusual to meet with the statement that this habit has been
acquired because it is useful to the insect, since it may often escape
in this way from an enemy. This does not appear on closer examination to
be always the case, and sometimes as much harm as good may result, or
what is more probable, neither much advantage, nor disadvantage, is the
outcome. This can, of course, only be determined in each particular case
from a knowledge of the whole life of a species and of the enemies that
are likely to injure it.
Hudson has recorded[33] a number of cases of this death-feigning
instinct in higher animals, and attributes it to violent emotion, or
fear, that produces a sort of swoon. He describes the gaucho boys’
method, in La Plata, of catching the silver-bill by throwing a stick or
a stone at it, and then rushing toward the bird, “when it sits perfectly
still, disabled by fear, and allows itself to be taken.” He also states
that one of the foxes (_Canis azaræ_) and one of the opossums
(_Didelphys azaræ_) “are strangely subject to the death-simulating
swoon.”
Footnote 33:
“The Naturalist in La Plata.”
Hudson remarks that it seems strange that animals so well prepared to
defend themselves should possess this “safeguard.” When caught or run
down by dogs, the fox fights savagely at first, but after a time its
efforts stop, it relaxes, and it drops to the ground. The animal appears
dead, and Hudson states that the dogs are “constantly taken in by it.”
He has seen the gauchos try the most barbarous tricks on a captive fox
in this condition, and, despite the mutilations to which it was
subjected, it did not wince. If, however, the observer draws a little
away from the animal, “a slight opening of the eye may be detected, and
finally, when left to himself, he does not recover and start up like an
animal that has been stunned, but cautiously raises his head at first
and only gets up when his foes are at a safe distance.” Hudson, coming
once suddenly upon a young fox, saw it swoon at his approach, and
although it was lashed with a whip it did not move.
The common partridge of the pampas of La Plata (_Hothura maculosa_)
shows this death-feigning instinct in a very marked degree. “When
captured, after a few violent struggles to escape, it drops its head,
gasps two or three times, and to all appearance dies.” But if it is
released it is off in an instant. The animal is excessively timid, and
if frightened, may actually die simply from terror. If they are chased,
and can find no thicket or burrow into which to escape, “they actually
drop down dead on the plain. Probably when they feign death in their
captor’s hand they are in reality very near to death.”
In this latter instance it must appear very improbable that we are
dealing with an instinct that has been built up by slow degrees on
account of the benefit accruing at each stage to the individual. In
fact, it appears that the instinct is in this case of really no use at
all to the animal, for there can scarcely be any question of an escape
by this action. Yet so far as we can judge it is the same instinct shown
by other animals, and it is not logical to account for its origin in one
case on the grounds of its usefulness, when we cannot apply the
explanation in the other cases. If this be admitted, we have another
illustration of the importance of keeping apart the origin of an
instinct or of a structure and the fact of its usefulness or
non-usefulness to the organism. Thus under certain conditions this
death-feigning instinct might really be of use to the animal, while
under other conditions and in other animals it may be of no advantage at
all, and in still other conditions it may be a positive injury to its
possessor. Perhaps we need not go outside of our own experience to find
a parallel case, for the state of fright into which imminent danger may
throw an individual may deprive him for the moment of the proper use of
those very mental qualities of which he stands in this crisis in
greatest need.
The peculiar behavior of cattle caused by the smell of blood is another
case of an instinct whose usefulness to its possessors is far from
apparent. It is known that cattle and horses and several wild animals
become violently excited by the smell of blood. Hudson gives a vivid
account of a scene witnessed by himself, the animals congregating, “and
moving around in a dense mass, bellowing continually.” Those animals
that forced their way into the centre of the mass where the blood was
“pawed the earth and dug it up with their horns, and trampled each other
down in their frantic excitement.”
This action leads us to a consideration of the behavior of animals
toward companions in distress. “Herbivorous animals at such times will
trample and gore the distressed one to death. In the case of wolves and
other savage-tempered carnivorous species the distressed fellow is
frequently torn to pieces and devoured on the spot.” If any one will be
bold enough to claim in this case that this habit has been acquired
because of advantage to the pack, _i.e._ if it be imagined that the pack
gains more by feeding on a weak member than by letting him take his
chances of recovery, it may be pointed out in reply that cattle also
destroy their weak or injured, but do not devour them, and the same
statement holds for birds, where the same instinct has often been
observed. Romanes has suggested that the instinct of destroying the weak
or injured members is of use because such members are a source of danger
to the rest of the herd; but Hudson points out that it is not so much
the weak and sickly members of the herd that are attacked in this way,
as those that are injured, and concludes, “the instinct is not only
useless, but actually detrimental.” He suggests that these “wild
abnormal movements of social animals” are a sort of aberration, so “that
in turning against a distressed fellow they oppose themselves to the law
of being.” Yet whether we gain anything by calling this action aberrant
or abnormal, the important fact remains that it is a definite response
under certain external conditions, and is shown by all the individuals
of the species.
The preceding illustrations of reactions that go to make up the
so-called instincts of animals may be separated into those that are
essential to the life of the individual or of the race, those that are
of some apparent use, although not absolutely essential, and a few of no
use at all, and fewer still that appear to be even injurious. If the
latter reactions take place only rarely, as appears often to be the
case, they are not sufficiently harmful to cause the destruction of the
race. The evidence points to the conclusion, I believe, that the origin
of these tropisms and instincts cannot be accounted for on the ground of
their benefit to the individual or to the race; and it does not seem
reasonable to make up one explanation for the origin of those that are
essential, and another for those that are of little use or even of no
use at all.
From what has been already said more than once, while discussing each
particular case, the simplest course appears to be in all instances to
look upon these instincts as having appeared independently of the use to
which they may be put, and not as having been built up by selection of
the individual variations that happen to give an organism some advantage
over its fellows in a life and death struggle. It appears reasonable to
deal with the origin of tropisms and instincts in general in the same
way as in dealing with structures; for, after all, the tropism is only
the outcome of some material or structural basis in the organism.
No attempt has been made here to interpret the more complex reactions of
the nervous system, for until we can get some insight into the meaning
of the simpler processes, we are on safer ground in dealing with these
first.
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CHAPTER XII
SEX AS AN ADAPTATION
In what sense may the separation of all the individuals of a species
into two kinds of individuals, male and female, be called an adaptation?
Does any advantage result to the species that would not come from a
non-sexual method of reproduction? Many attempts have been made to
answer these questions, but with what success I shall now try to show.
There are four principal questions that must be considered:—
I. The different kinds of sexual individuals in the animal and plant
kingdoms.
II. The historical question as to the evolution of separate sexes.
III. The factors that determine the sex in each individual developing
from an egg.
IV. The question as to whether any advantage is gained by having each
new individual produced by the union of two germ-cells, or by having the
germ-cells carried by two kinds of individuals.
While our main problem is concerned with the last of these topics, yet
there would be little hope of giving a complete answer to it unless we
could get some answer to the first three questions.
The Different Kinds of Sexual Individuals
Amongst the unicellular animals and plants the fusion of two (or more)
individuals into a single one is generally regarded as the simplest, and
possibly also the most primitive, method of sexual reproduction. Two
amœbas, or amœba-like bodies, thus flow together, as it were, to produce
a new individual.
In the more highly specialized unicellular animals, the processes are
different. Thus in vorticella, a small, active individual unites with a
larger fixed individual. The protoplasm fuses into a common mass, and a
very complicated series of changes is passed through by the nucleus. In
paramœcium, a free-swimming form very much like vorticella, two
individuals that are alike unite only temporarily, and after an
interchange of nuclear material they separate.
In the lower plants, and more especially in some of the simple
aggregates or colonial forms, there are found a number of stages between
species in which the uniting individuals are alike, and those in which
they are different. There are several species whose individuals appear
to be exactly alike; and other species in which the only apparent
difference between the individuals that fuse together is one of size;
and still other species in which there are larger resting or passive
individuals, and smaller active individuals that unite with the larger
ones. In several of the higher groups, including the green algæ and
seaweeds, we find similar series, which give evidence of having arisen
independently of each other. If we are really justified in arranging the
members of these groups in series, beginning with the simpler cases and
ending with those showing a complete differentiation into two kinds of
germ-cells, we seem to get some light as to the way in which the change
has come about. It should not be forgotten, however, that it does not
follow because we can arrange such a series without any large gaps in
its continuity, that the more complex conditions have been gradually
formed in exactly this way from the simplest conditions.
So far we have spoken mainly of those cases in which the forms are
unicellular, or of many-celled species in which all the cells of the
individual resolve themselves into one or the other kind of germ-cells.
This occurs, however, only in the lowest forms. A step higher we find
that only a part of the cells of the colony are set aside for purposes
of reproduction. The cells surrounding these germ-cells may form
distinct organs, which may show certain differences according to whether
they contain male or female germ-cells. When these two kinds of cells
are produced by two separate individuals, the individuals themselves may
be different in other parts of the body, as well as in the reproductive
organs.
When this condition is reached, we have individuals that we call males
and females, because, although they do not themselves unite to form new
individuals, they produce one or the other kind of germ-cell. It is the
germ-cells alone that now combine to form the new individual.
Amongst living groups of animals we find no such complete series of
forms as exist in plants, and the transition from the one-celled to the
many-celled forms is also more abrupt. On the other hand, we find an
astonishing variety of ways in which the reproduction is accomplished,
and several ways in which the germ-cells are carried by the sexual
individuals. Let us examine some of the more typical conditions under
the following headings: (1) sexes separate; (2) sexes united in the same
individual; (3) parthenogenetic forms; (4) exceptional methods of
propagation.
1. _Sexes Separate; Unisexual Forms._[34]—Although the animals with
which we are more familiar have the sexes separate, this is far from
being universal amongst animals and plants; and, in fact, can scarcely
be said to be even the rule. When the sexes are separate they may be
externally alike, and this is especially true for those species that do
not unite, but set free their eggs and spermatozoa in the water, as
fish, frogs, corals, starfish, jellyfish, and many other forms. In other
animals there are sometimes other secondary differences in the sexes
besides those connected with the organs of reproduction. Such
differences are found, as we have seen, in insects, in some spiders,
crustaceans, and in many birds and mammals. In a few cases the
difference between the sexes is very great, especially when the female
is parasitic and the male free, as in some of the crustaceans. In some
other cases the male is parasitic on the female. Thus in Bonellia the
male is microscopic in size, being in length only one-hundredth part of
the female. In _Hydatina senta_ the male is only about a third as large
as the female. It has no digestive tract, and lives only a few days. In
another rotifer the males are mere sacs enclosing the male reproductive
organs.
Footnote 34:
Geddes and Thompson’s “The Evolution of Sex” has been freely used in
the preparation of this part of this chapter.
2. _Hermaphroditic Forms._—There are many species of animals and plants
in which each individual contains both the male and the female organs of
reproduction, and there are whole groups in which only these
hermaphroditic forms occur. Thus in the ctenophors the eggs develop
along one side of each radial canal and spermatozoa along the other. The
group of flatworms is almost exclusively hermaphroditic. The earthworms
and the leeches have only these bisexual forms, and in the mollusks,
while a few groups have separate sexes, yet certain groups of
gasteropods and of bivalve forms are entirely hermaphroditic.
In the common garden snail, although there are two sets of sexual ducts
closely united, yet from the same reproductive sac both eggs and sperm
are produced. The barnacles and the ascidians are for the most part
hermaphroditic forms. Many other examples might be cited, but these will
suffice to show that it is by no means unusual in the animal kingdom for
the same individual to produce both male and female germ-cells. However,
one of the most striking facts in this connection is that
self-fertilization seldom takes place, so that the result is the same in
certain respects as though separate sexes existed. This point will come
up later for further consideration.
3. _Parthenogenetic Reproduction._—It has long been known that, in some
cases, eggs that are not fertilized will begin to develop and may even
produce new individuals. Tichomiroff showed that by rubbing with a brush
the unfertilized eggs of the silkworm moth, a larger percentage would
produce caterpillars than if they were not rubbed. During the last few
years it has been shown that the development of a non-fertilized egg may
be started in a number of ways. Such, for example, as by certain
solutions of salt or of sugar, by subjecting the eggs to cold, or by
simply shaking them.
There are certain groups of animals in which the males appear only at
regular (in others at irregular) intervals. In their absence the females
produce eggs that develop without being fertilized, _i.e._
parthenogenetically. The following examples will serve to show some of
the principal ways in which this “virgin reproduction” takes place. In
the group of rotifers the males are generally smaller than the females
and are usually also degenerate. In some species, although degenerate
males are present, they are unnecessary, since parthenogenesis is the
rule. In still other species no males exist and the eggs develop,
therefore, without being fertilized. In some of the lower crustaceans
parthenogenesis occurs in varying degrees. In Apus males may be entirely
absent at times in certain localities, and at other times a few, or even
very many, males may appear. Some species of ostracod crustaceans seem
to be purely parthenogenetic; others reproduce by means of fertilized
eggs; and others by an alternation of the two processes. The crustaceans
of the genus _Daphnia_ produce two kinds of eggs. The summer eggs are
small, and have a thin shell. These eggs develop without being
fertilized, but in the autumn both male and female individuals develop
from these unfertilized eggs, and the eggs of the female, the so-called
winter eggs, are fertilized. These are also larger than the summer eggs,
have thicker shells, and are much more resistant to unfavorable
conditions. They give rise in the following spring to females only, and
these are the parthenogenetic individuals that continue to produce
during the summer new parthenogenetic eggs.
It is within the group of insects that some of the most remarkable cases
of parthenogenesis that we know are found. In the moth, _Psyche helix_,
only females are present, as a rule, but rarely males have been found.
In another moth, _Solenobia trinquetrella_, the female reproduces by
parthenogenesis, but at times males appear and may then be even more
numerous than the females. In the gall-wasps parthenogenetic generations
may alternate with a sexual generation, and it is interesting to note
that the sexual and the parthenogenetic generations are so different
that they were supposed to belong to separate species, until it was
found that they were only alternate generations of the same species.
The aphids or plant-lice reproduce during the summer by parthenogenesis,
but in the autumn winged males and females appear, and fertilized winter
eggs are produced. From these eggs there develop, in the following
spring, the wingless parthenogenetic summer forms, which produce the
successive generations of the wingless forms. As many as fourteen summer
broods may be produced. By keeping the aphids in a warm temperature and
supplying them with plenty of moist food, it has been possible to
continue the parthenogenetic reproduction of the wingless forms for
years. As many as fifty successive broods have been produced in this
way. It has not been entirely determined whether it is the temperature
or a change in the amount, or kind, of food that causes the appearance
of the winged males and females, although it seems fairly certain that
diminution in the food, or in the amount of water contained in it, is
the chief cause of the change.
In the honey-bee the remarkable fact has been well established that
fertilized eggs give rise only to females (queens and workers), while
unfertilized eggs develop into males. Whether a fertilized egg becomes a
queen or a worker (sterile female) depends solely on the kind of food
that is given to the young larva, and this is determined, in a sense,
entirely by the bees themselves.
In plants also there are many cases of parthenogenesis known. Some
species of Chara when kept under certain conditions produce only female
organs, and seem to produce new plants parthenogenetically. In this case
it appears that the same conditions that caused the plants to produce
only female organs may also lead to the development of the egg-cells
without fertilization. In fact it is only by a combination of this kind
that parthenogenesis could arise. The result is similar when the eggs of
insects produce only females whose eggs are capable of parthenogenetic
development. If a case should arise in which only females appeared whose
eggs did not possess the power of parthenogenetic development, the
species would die out.
In the green alga, Spirogyra, it has been found that if conjugation of
two cells is prevented, a single cell may become a parthenogenetic cell.
In a number of parasitic fungi the male organs appear to be degenerate,
and from the female organs parthenogenetic development takes place. A
small number of flowering plants are also capable of parthenogenetic
reproduction.
There is a peculiarity in the development of the parthenogenetic eggs of
animals that will be more fully discussed later, but may be mentioned
here. Ordinarily an egg that becomes fertilized gives off two polar
bodies, but in a number of cases in which parthenogenetic development
occurs it has been found that only one polar body is given off. It is
supposed that in such cases one polar body is retained, and that it
plays the same part as the entrance of the spermatozoon of the male.
4. _Exceptional Cases._—Occasionally in a species that is unisexual an
individual is found that is bisexual. The male of the toad, _Pelobates
fuscus_, has frequently a rudimentary ovary in front of the testis. The
same thing has been found in several species of fish. In Serranus, a
testis is present in the wall of the ovary, and the eggs are said to be
fertilized by the spermatozoa of the same individual. In frogs it has
been occasionally found that ovary and testis may be associated in the
same individual, or a testis may be present on one side, and a testis
with an anterior ovarian portion on the other. Cases like these lead up
to those in which the body itself may also show a mosaic of
sex-characters, and it is noticeable that when this occurs there is
nearly always a change in the reproductive organs also. Thus butterflies
have been found with the wings and the body of one side colored like the
male and the other side like the female. Similar cases have also been
found in bees and ants. Bees have been found with the anterior part of
the body of one sex and posterior part of another!
The preceding cases illustrate, in different ways, the fact that in the
same individual both kinds of reproductive organs may suddenly appear,
although it is the rule in such species that only one set develops.
Conversely, there are cases known, especially amongst plants, in which
individuals, that usually produce male and female organs (or more
strictly spores of two kinds from which these organs develop), produce
under special conditions only one or the other kind. Facts like these
have led to the belief that each individual is potentially bisexual, but
in all unisexual forms one sex predominates, and the other remains
latent. This idea has been the starting-point for nearly all modern
theories of sex.
An excellent illustration of this theory is found in those cases in
which the same individual may be male at one time and female at another.
For instance, it is said that in one of the species of starfish
(_Asterina gibbosa_) the individuals at Roscoff are males for one or two
years, and then become females. At Banyuls they are males for the first
two or three years, and then become females; while at Naples some are
always males, others females, some hermaphrodites, others transitional
as in the cases just given. In one of the isopod crustaceans,
_Angiostomum_, the young individuals are males and the older females. In
_Myzostomum glabrum_ the young animal is at first hermaphroditic, then
there is a functional male condition, followed by a hermaphroditic
condition, and finally a functional female phase, during which the male
reproductive organs disappear.
The flowers of most of the flowering plants have both stamens and
pistils, which contain the two kinds of spores out of which the male and
female germ-cells are formed. The stamens become mature before the
pistils, as a rule, but in some cases the reverse is the case. This
difference in the time of ripening of the two organs is often spoken of
as an adaptation which prevents self-fertilization. The latter is
supposed to be less advantageous than cross-fertilization. This question
will be more fully considered later.
Before we come to an examination of the question of the adaptations
involved in the cases in which the sexes are separate, and the different
times at which the sex-cells are ripened, it will be profitable first to
examine the question as to what determines in the egg or young whether a
male or a female or a hermaphroditic form shall arise.
The Determination of Sex
A large number of views have been advanced as to what determines whether
an egg will give rise to a male or to a female individual. The central
question is whether the fertilized egg has its sex already determined,
or whether it is indifferent; and if the latter, what external factor or
factors determine the sex of the embryo. Let us first examine the view
that some external factor determines the sex of the individual, and then
the evidence pointing in the opposite direction. Among the different
causes suggested as determining the sex of the embryo, that of the
condition of the egg itself at the time of fertilization has been
imagined to be an important factor in the result. Another similar view
holds that the condition of the spermatozoon plays the same rôle. For
instance, it has been suggested that if the egg is fertilized soon after
it leaves the ovary, it produces a female, but if the fertilization is
delayed, a male is produced. It has also been suggested that the
relative age of the male and the female parents produces an effect in
determining the sex of the young. There is no satisfactory evidence,
however, showing that this is really the case.
Another view suggested is that the sex is determined by the more
vigorous parent; but again there is no proof that this is the case, and
it would be a difficult point to establish, since as Geddes and Thompson
point out, what is meant by greater vigor is capable of many
interpretations. Somewhat similar is the idea that if the conditions are
favorable, the embryo develops further, as it were, and becomes a male;
but there are several facts indicating that this view is untenable.
Düsing maintains that several of these factors may play a part in
determining the sex of the embryo, and if this be true, the problem
becomes a very complex one. He also suggests that there are
self-regulative influences of such a kind that, when one sex becomes
less numerous, the conditions imposed in consequence on the other sex
are such as to bring the number back to the normal condition; but this
idea is far from being established. The fact that in some species there
are generally more individuals of one sex than of the other shows that
this balance is not equally adjusted in such forms.
Of far greater value than these speculations as to the origin of sex are
the experiments that appear to show that nutrition is an important
factor in determining sex. Some of the earlier experiments in this
direction are those of Born and of Yung. By feeding one set of tadpoles
with beef, Yung found the percentage of females that developed to be
greatly increased, and a similar increase was observed when the tadpoles
were fed on the flesh of fish. An even greater effect was produced by
using the flesh of frogs, the percentage rising to 92 females in every
hundred. These results have been given a different interpretation by
Pflüger and by others, and, as will be pointed out later, there is a
possible source of error that may invalidate them.
Somewhat similar results have been obtained by Nussbaum for one of the
rotifers. He found that if the rotifer is abundantly fed in early life,
it produces _female eggs_, that is, larger eggs that become females;
while if sparingly fed, it produces only small eggs, from which males
develop. It has been claimed also in mammals, and even in man, that sex
is to some extent determined by the nourishment of the individual.
Some experiments made by Mrs. Treat with caterpillars seemed to show
that if the caterpillars were well nourished more female moths were
produced, and if starved before pupation more males emerged. But Riley
has pointed out that since the larger female caterpillars require more
food they will starve sooner than the males, and, in consequence, it may
appear that proportionately more male butterflies are born when the
caterpillars are subjected to a starvation diet. This point of view is
important in putting us on our guard against hastily supposing that food
may directly determine sex. Unless the entire number of individuals
present at the beginning of the experiment is taken into account, the
results may be misleading, because the conditions may be more fatal to
one sex than to the other.
In some of the hymenopterous insects, the bees for example, it has been
discovered that the sex of the embryo is determined by the entrance, or
lack of entrance, of the spermatozoon. In the honey-bee all the
fertilized eggs produce females and the unfertilized eggs males. The
same relation is probably true also in the case of ants and of wasps. In
the saw-flies, the conditions are very remarkable. Sharp gives the
following account of some of these forms:[35]—“It is a rule in this
family that males are very much less numerous than females, and there
are some species in which no males have been discovered. This would not
be of itself evidence of the occurrence of parthenogenesis, but this has
been placed beyond doubt by taking females bred in confinement,
obtaining unfertilized eggs from them, and rearing the larvæ produced
from the eggs. This has been done by numerous observers with curious
results. In many cases the parthenogenetic progeny, or a portion of it,
dies without attaining full maturity. This may or may not be due to
constitutional weakness, arising from the parthenogenetic state.
Cameron, who has made extensive observations on this subject, thinks
that the parthenogenesis does involve constitutional weakness, fewer of
the parthenogenetic young reaching maturity. This, he suggests, may be
compensated for—when the parthenogenetic progeny is all of the female
sex—by the fact that all those that grow up are producers of eggs. In
many cases the parthenogenetic young of Tenthredinidæ are of the male
sex, and sometimes the abnormal progeny is of both sexes. In the case of
one species—the common currant-fly, _Nematus ribesii_—the
parthenogenetic progeny is nearly, but not quite always, entirely of the
male sex; this has been ascertained again and again, and it is
impossible to suggest in these cases any advantage to the species to
compensate for constitutional parthenogenetic weakness. On the whole, it
appears most probable that the parthenogenesis, and the special sex
produced by it, whether male or female, are due to physiological
conditions of which we know little, and that the species continues in
spite of the parthenogenesis rather than profits by it. It is worthy of
remark that one of the species in which parthenogenesis with the
production of males occurs—_Nematus ribesii_—is perhaps the most
abundant of saw-flies.”
Footnote 35:
“The Cambridge Natural History,” Vol. V, “Insects,” by David Sharp.
It has been pointed out that in a number of species of animals and
plants only parthenogenetic females are present at certain times. In a
sense this means a preponderance of one sex, but since the eggs are
adapted only to this kind of development, it may be claimed that the
conditions in such cases are somewhat different from those in which eggs
that would be normally fertilized may develop in the absence of
fertilization. Nevertheless, it is generally supposed that the actual
state of affairs is about the same. It is usually assumed, and no doubt
with much probability, that these parthenogenetic forms have evolved
from a group which originally had both male and female forms. One of the
most striking facts in this connection is that in the groups to which
these parthenogenetic species belong there are, as a rule, other species
with occasional parthenogenesis, and in some of these the males are also
fewer in number than the females.
In the aphids, the parthenogenetic eggs give rise during the summer to
parthenogenetic females, but in the autumn the parthenogenetic eggs give
rise without fertilization both to males and to females. It appears,
therefore, that we can form no general rule as to a relation between
fertilization and the determination of sex. While in certain cases, as
in the bees, there appears to be a direct connection between these two,
in other cases, as in that of the aphids just mentioned, there is no
such relation apparent.
Geddes and Thompson have advocated a view in regard to sex which at best
can only serve as a sort of analogy under which the two forms of sex may
be considered, rather than as a legitimate explanation of the phenomenon
of sex. They rest their view on the idea that living material is
continually breaking down and building up. An animal in which there is
an excess of the breaking-down process is a male, and one that is more
constructive is a female. Furthermore, whichever process is in the
excess during development determines the sex of the individual. Thus, if
conditions are very favorable, there will be more females produced; but
if, on the other hand, there is an excess of the breaking-down process,
males are produced. So far, the process is conceived as a purely
physiological one, but to this the authors then apply the selection
hypothesis, which, they suppose, acts as a sort of break or regulation
of the physiological processes, or in other words as a directive agent.
They state: “Yet the sexual dimorphism, in the main, and in detail, has
an adaptive significance, also securing the advantages of
cross-fertilization and the like, and is, therefore, to some extent the
result of the continual action of natural selection, though this may, of
course, check variation in one form as well as favor it in another.”
Disregarding this last addition, with which Geddes and Thompson think it
necessary to burden their theory, let us return to the physiological
side of the hypothesis. Their idea appears to me a sort of symbolism
rather than a scientific attempt to explain sex. If their view had a
real value, it ought to be possible to determine the sex of the
developing organism with precision by regulating the conditions of its
growth, and yet we cannot do this, nor do the authors make any claim of
being able to do so. The hypothesis lacks the only support that can give
it scientific standing, the proof of experiment.
There have been made, from time to time, a number of attempts to show
that the sex of the embryo is predetermined in the egg, and is not
determined later by external circumstances. In recent years this view
has come more to the front, despite the apparent experimental evidence
which seemed in one or two cases to point to the opposite view. One of
the most complete analyses of the question is that of Cuénot, who has
attempted to show that the sex of the embryo is determined in the egg,
before or at the time of fertilization. He has also examined critically
the evidence that appeared to show that external conditions, acting on
the embryo, may determine the sex, and has pointed out some possible
sources of error that had been overlooked. The best-known case is that
of the tadpole of the frog, but Cuénot shows not only that there are
chances of error in this experiment as carried out, but also, by his own
experiments and observations, that the facts themselves are not above
suspicion. He points out that at the age at which some of the tadpoles
were when the examination was made, it was not always possible to tell
definitely the sex of the individual, and least of all by means of the
size alone of the reproductive organs, as was supposed, in one case at
least, to be sufficient. In his own experiments he did not find an
excess of one sex over the other as a result of feeding.
Cuénot points out that Brocadello found that the larger eggs laid by the
silkworm give rise to from 88 to 95 per cent of females, and the small
eggs to from 88 to 92 per cent of males. Joseph has confirmed this for
_Ocneria dispar_, and Cuénot himself also reached this conclusion.
Korschelt found that the large eggs of Dinophilus produced females and
the small ones males. Cuénot experimented with three species of flies,
and found that when the maggots were well nourished the number of the
individuals of the two sexes was about equal, and when poorly nourished
there were a few more females in two cases, and in another about the
same number of males and females.
It has been claimed that the condition of nourishment of the mother may
determine the number of eggs of a particular sex, but Cuénot found, in
three species of flies which he raised, that there was a slight response
in the opposite direction. He concludes that the condition of the mother
is not a factor in the determination of sex.
The first egg of the two laid in each set by the pigeon is said, as a
rule, to produce a male, and the second a female. Both Flourens and
Cuénot found this to be the case in the few instances that they
examined, but Cuénot has shown that this does not always happen. Even
when this occurs, it has not been determined whether the result depends
on something in the egg itself, that causes a male egg to be set free
first, or on some external condition that determines that the first egg
shall become a male. It has been claimed that the age of the
spermatozoon might in this and in other cases determine the result; but
Gerbe has shown that if the domestic hen is isolated for fifteen days
after union with the male, she will continue to produce fertile eggs
from which both sexes are produced, without showing any relation between
the time the eggs are laid and the particular sex that develops.
Cuénot does not discuss whether sex is determined by the nucleus or by
the protoplasm, but if, as he thinks probable, the size of the egg is a
determining factor, it would appear that the protoplasm must be the
chief agent. Even if this were the case it would still be possible that
the size of the egg itself might be connected with some action on the
part of the nucleus. If, as seems probable, identical twins come from
halves of the same egg, then, since they are of the same sex, the
absolute amount of protoplasm cannot be a factor in sex determination.
[Illustration:
Fig. 6.—Diagram showing the maturation of the egg.]
As a basis for the discussion that follows, certain processes that take
place during the maturation divisions of the egg and of the spermatozoon
must be briefly noticed. After the egg leaves the ovary it extrudes a
minute body called the first polar body (Fig. 6 B, C, D). This process
of extrusion is really a cell division accompanied by the regular
mitotic division of the nucleus; but since one of the products of the
division, the polar body, is extremely small, the meaning of the process
was not at first understood. The half of the nucleus, that remains in
the egg, divides again, and one of its halves is thrown out into a
second polar body (Fig. 6 E, F, G)). Meanwhile, the first polar body has
divided into two equal parts, so that we find now three polar bodies and
the egg (Fig. 6 G)). A strictly analogous process takes place in the
formation of the spermatozoa (Fig. 7 B-F). The mother-cell of the
spermatozoon divides into two parts, which are equal in this case (Fig.
7 B-D). Each of these then divides again (Fig. 7 E, F), producing four
cells that are comparable to the three polar bodies and the mature egg.
Each of the four becomes a functional spermatozoon (Fig. 7 G, H). Thus
while in the maturation of the egg only the egg itself is capable of
development, in the case of the male cells all four products of the two
maturation divisions are functional.
[Illustration:
Fig. 7.—Diagram showing the maturation of the spermatozoon.]
Now, in certain cases of parthenogenesis, it has been found that one of
the polar bodies may not be given off, but, remaining in the egg, its
nucleus reunites with the egg nucleus, and thus takes the place of the
spermatozoon, which does exactly the same thing when it fertilizes the
egg, _i.e._ the nucleus of the spermatozoon unites with the nucleus of
the egg. This fact in regard to the action of the polar body in
fertilization is not as surprising as appears at first sight, for if
each of the polar bodies is equivalent to a spermatozoon, the
fertilization of the egg by one of its own polar bodies conforms to
theory.
There is a considerable body of evidence showing that in many eggs at
one of the two maturation divisions the chromatin rods derived from the
nucleus are divided crosswise (Fig. 6 B, C). The same thing occurs at
one of the two divisions in the formation of the spermatozoon (Fig. 7 B,
C). At the other division to form the other polar body (or the other
sperm-cell) the chromatin rods appear to be split lengthwise, as in
ordinary cell division (Fig. 6 E, F, G). In recent years the
_cross-division_ of the chromatin rods has attracted a great deal of
notice, and Weismann in particular drew attention to the possible
importance of this kind of division.
There is another fact that gives this division especial significance. It
has been discovered that the number of chromosomes that appears in each
dividing cell of the organism is a constant number, but it has also been
discovered that the egg, before extruding its polar bodies, and the
mother-cell of the spermatozoon (Figs. 6, 7 B), contain exactly half of
the number of chromosomes that are characteristic of the body-cells of
the same animal (Figs. 6, 7 A). Now there is good evidence to show that
the reduction in number is due to the chromosomes uniting sometimes end
to end in pairs, as shown in Figures A and B. Furthermore, it has been
suggested that at one of the maturation divisions, when the chromosomes
divide crosswise, the united chromosomes are separated (Figs. 6, 7 B,
C), so that one remains in the egg and the other goes out into the polar
body. The same thing is supposed to occur at one of the maturation
divisions of the sperm mother-cell. A further consideration of capital
importance in this connection has been advocated by Montgomery and by
Sutton, namely, that, when the chromosomes unite in pairs, a chromosome
from one parent unites with one from the other parent. Consequently at
one of the two reduction divisions maternal and paternal chromosomes may
separate again, some to go to one cell, some to the other.
When the spermatozoon enters the egg it brings into the egg as many new
chromosomes as the egg itself possesses at this time, and the two
nuclei, uniting into a single one, furnish the total number of
chromosomes characteristic of the animal that develops from the egg. At
first the chromosomes that are brought in by the spermatozoon lie at one
side of the fused nucleus, and those from the egg itself at the other
side. This arrangement appears, however, in some cases at least, to be
lost later. At every division of the nucleus, each chromosome divides
and sends a half to each of the daughter-nuclei. Thus every cell in the
body contains as many paternal as maternal chromosomes. This statement
also applies to the first cells that go into the reproductive organs,
some of which become the mother-cells of the germ-cells. Later, however,
in the history of the germ-cells,—just before the maturation
divisions,—these chromosomes are supposed to unite in pairs, end to end,
as explained above, to give the reduced number. Later there follows the
separation of these paired chromosomes at one of the two maturation
divisions. If at this time all the paternal chromosomes should pass to
one pole, and all the maternal to the other, the germ-cell ceases to be
mixed, and becomes purely paternal or maternal. If this ever occurs, the
problem of heredity may become simplified, and even the question of sex
may be indirectly involved; but it has not been established that, when
the reduced number of chromosomes is formed, there is a strict union
between the paternal and maternal chromosomes, and if not, the
subsequent separation is probably not along these lines. If, however,
the chromosomes contain different qualities, as Boveri believes, there
may be two kinds of eggs, and two kinds of spermatozoa in regard to
_each particular character_. It is this last assumption only that is
made in Mendel’s theory of the purity of the germ-cells.
Several attempts have been made at different times to connect the facts
in regard to the extrusion of the polar bodies with those involved in
the determination of sex. Minot suggested, in 1877, that the egg ejects
by means of the polar bodies its male elements, which are again received
in the fertilization of the egg by the spermatozoon. The same idea has
also been expressed by others. It has been objected to this view that
one polar body ought to suffice, and that no similar throwing out of
part of its substance is found in the process of formation of the
spermatozoon, which should, on the hypothesis, throw out its female
elements. It would seem, on first thought, that this view might find
support in the idea expressed above, namely, that in one of the polar
bodies half of the chromosomes pass out, so that there is conceivably a
separation of the maternal from the paternal. If this were the case also
in the spermatozoa, then two of each four would be paternal and two
maternal. This is, however, a very different thing from supposing them
to be male and female, for it by no means follows, because the
chromosomes correspond to those of the father or of the mother in the
sum of their characters, that they are, therefore, also male or female
in regard to sex.
It has been pointed out already, that in most parthenogenetic eggs only
one polar body is extruded. There are, it is true, a few apparent
exceptions to this rule, but in most cases it is certain that only one
is extruded. In several cases the beginning of the formation of the
second maturation division of the nucleus takes place, but after the
chromosomes have divided they come together again in the nucleus. If
each polar body be interpreted as equivalent to a spermatozoon, then
this result is rather a process of self-fertilization than true
parthenogenesis. It is, nevertheless, true that in some cases
development seems to go on after both polar bodies have been extruded.
Moreover, it has been found possible to cause the eggs of the sea-urchin
to begin their development by artificial solutions after they have
extruded both polar bodies. A single spermatozoon may also produce an
embryo if it enters a piece of egg-protoplasm without a nucleus. The
last instance is a case of male parthenogenesis, and if the theory of
the equivalency of spermatozoon and egg be correct, this is what should
occur.
Quite recently, Cuénot, Beard, Castle, and Lenhossek have contended that
the differentiation of sex is the outcome of internal factors. They
think that the view that sex is determined by external agents is
fundamentally erroneous. The fallacies that have given rise to this
conception, Castle points out, are, first, that in animals that
reproduce sometimes by parthenogenesis and sometimes by fertilized eggs,
the former process is favored by good nutrition and the latter by poor
nutrition. This only means, in reality, Castle thinks, that
parthenogenetic reproduction is favored by external conditions, and this
kind of reproduction, he thinks, is a thing _sui generis_, and not to be
compared to the formation of more females in the sexual forms of
reproduction. There is no proof, however, that this is anything more
than a superficial distinction, and it ignores the fact that in ordinary
cases the females sometimes lay parthenogenetic eggs which differ, as
far as we can see, from eggs that are destined to be fertilized in no
important respect. More significant, it seems to me, is the fact that
only parthenogenetic females develop the following spring from the
fertilized eggs of the last generation of the autumn series, whose
origin is described to be due to lack of food. We find, in the case of
aphids, that unfertilized parthenogenetic eggs and also fertilized eggs
give rise to females only, while a change in the amount of food causes
the parthenogenetic eggs to give rise both to males and to females. This
point is not, I think, fully met by Castle, for even if the change in
food does not, as he claims, cause only one sex to appear, yet lack of
food does seem to account for the appearance of the males at least.
The other fallacy, mentioned by Cuénot, is that the excess of males that
has been observed when the food supply is limited is due to the early
death of a larger percentage of females, which require more food, but
this still fails to account for the excess of females when more food is
given, provided Yung’s experiments on tadpoles are correct. It may be,
however, in the light of Pflüger’s results, that there has been some
mistake in the experiments themselves.
We may now proceed to examine Castle’s argument, attempting to show in
what way sex is predetermined in the embryo. His hypothesis rests on the
three following premises: “(1) the idea of Darwin, that in animals and
plants of either sex the characters of the opposite sex are latent; (2)
the idea of Mendel, that in the formation of the gametes [germ-cells] of
hybrids a segregation of the parental characters takes place, and when
in fertilization different segregated characters meet, one will
dominate, the other become latent or recessive; (3) the idea of
Weismann, that in the maturation of egg and spermatozoon a segregation
is attended by a visible reduction in the number of chromosomes in the
germinal nuclei.”
Expressed in a somewhat more general way, Castle suggests that each egg
and each spermatozoon is either a male or a female germ-cell (and not a
mixture of the two), and when a female egg is fertilized by a male
spermatozoon, or _vice versa_, the individual is a sexual hybrid with
one sex dominating and the other latent. The assumption that there are
two kinds of eggs, male and female, and two kinds of spermatozoa, male
and female, is not supported by any direct or experimental evidence.
Moreover, in order to carry out the hypothesis, it is necessary to make
the further assumption that a female egg can only be fertilized by a
male spermatozoon, and a male egg by a female spermatozoon. While such a
view is contrary to all our previous ideas, yet it must be admitted that
there are no facts which disprove directly that such a selection on the
part of the germ-cells takes place. If these two suppositions be
granted, then Castle’s hypothesis is as follows:—
In order that half of the individuals shall become males and half
females it is necessary to assume that in some individuals the male
element dominates and in others the female, and since each fertilized
egg contains both male and female elements, it is necessary to assume
that either the egg or the spermatozoon contains the dominating element.
Castle supposes that in hermaphroditic organisms the two characters
“exist in the balanced relationship in which they were received from the
parents,” but, as has just been stated, in unisexual forms one or the
other sex dominates, except of course in those rare cases, as in the
bees and ants, where half of the body may bear the characters of one
sex, and the other half that of the other sex.
In parthenogenetic species the female character is supposed to be
uniformly stronger, so that it dominates in every contest, “for the
fertilized egg in such species develops invariably into a female.” Under
certain circumstances, as Castle points out, the parthenogenetic female
produces both males and females, and this is also true in the occasional
development of the unfertilized egg of the silkworm moth, and of the
gypsy moth, in which both male and female individuals are produced by
parthenogenesis. These facts show that even in unfertilized eggs both
sexes are potentially present; but this might be interpreted to mean
that some eggs are male and some female, rather than that each egg has
the possibility of both kinds of development. If, however, one polar
body is retained in these parthenogenetic eggs, then _ex hypothese_ each
egg would contain the potentialities of both sexes (if the polar body
were of the opposite sex character). It seems necessary to make this
assumption because in some parthenogenetic forms males and females may
be produced later by each individual, as in the aphids, and this could
not occur if we assume that some parthenogenetic eggs are purely male
and some female.
Castle assumes, in fact, that in animals like daphnids and rotifers one
polar body only is extruded, and the other (the second) is retained in
the egg, and hence the potentiality of producing males is present. In
the honey-bee, on the contrary, Castle assumes that both polar bodies
are extruded in the unfertilized egg (and there are some observations
that support this idea), and since only males are produced from these,
he believes it is the female element that has been sent out into the
second polar body. This hypothesis is necessary, because Castle assumes
that when both elements are present in the bee’s eggs, the female
element dominates. “Hence, if the egg which has formed two polar cells
develops without fertilization, it must develop into a male. But if such
an egg is fertilized, it invariably forms a parthenogenetic female ♀
(♂), that is, an individual in which the male character is recessive.
Accordingly the _functional_ spermatozoon must in such cases invariably
bear the _female character_, and this is invariably dominant over the
male character when the two meet in fertilization.”
If it should prove generally true that the size of the egg is one of the
factors determining the sex, we have still the further question to
consider as to whether some eggs are bigger because they are already
female, or whether all eggs that go beyond a certain size are females,
and all those that fail to reach this are males. If this is the case, an
animal might produce more females if the external conditions were
favorable to the growth of the eggs, and if in some cases these large
eggs were capable of developing, parthenogenetic races might become
established. Should, however, the conditions for nutrition become less
favorable, some of the eggs might fall below the former size and produce
males. It is not apparent, however, why all the fertilized autumn eggs
of the aphids should give rise to females, for although these eggs are
known to be larger than the summer eggs, yet they are produced under
unfavorable conditions.
The preceding discussion will show how far we still are from knowing
what factors determine sex. Castle’s argument well illustrates how many
assumptions must be made in order to make possible the view that sex is
a predetermined quality of each germ-cell. Even if these assumptions
were admissible, we still return to the old idea that the _fertilized
egg_ has both possibilities, and something determines which shall
dominate. Until we have ascertained definitely by experimental work
whether the sex in some forms can be determined by external conditions,
it is almost worthless to speculate further. Whatever decision is
reached, the conclusion will have an immediate bearing on the question
to be next discussed. Meanwhile, we can at least examine some of the
theories that have been advanced as to what advantage, if any, has been
gained by having the individuals of many classes divided into two kinds,
male and female.
Sex as a Phenomenon of Adaptation
Of what advantage is it to have the individuals of many species
separated into males and females? It is obviously a disadvantage from
the point of view of propagation to have half of the individuals
incapable of producing young, and the other half also incapable of doing
so, as a rule, unless the eggs are fertilized by the other sex. Is there
any compensation gained because each new individual arises from two
parents instead of from one? Many answers have been attempted to these
questions.
At the outset it should be recognized that we are by no means forced to
assume, as is so often done, that because there is this separation of
the sexes it must have arisen on account of its advantage to the
species. Whether the result may be of some benefit regardless of how it
arose, may be an entirely different question. It would be extremely
difficult to weigh the relative advantages (if there are any) and
disadvantages (that are obvious as pointed out above), nor is it
probable that in this way we can hope to get a final answer to our
problem. We may begin by examining some of the modern hypotheses that
have been advanced in this connection.
Darwin has brought together a large number of facts which appear to show
the beneficial effects of the union of germ-cells from two different
individuals. Conversely, it is very generally believed, both by breeders
and by some experimenters, that self-fertilization in the case of
hermaphroditic forms leads, in many cases, though apparently not in all,
to the production of less vigorous offspring. Darwin’s general position
is that it is an advantage to the offspring to have been derived from
two parents rather than to have come from the union of the germ-cells of
the same individual, and he sees, in the manifold contrivances in
hermaphroditic animals and plants to insure cross-fertilization, an
adaptation for this purpose.
This question of whether self-fertilization is less advantageous than
cross-fertilization is, however, a different question from that of
whether _non-sexual_ methods of reproduction are less advantageous than
sexual ones. Since some plants, like the banana, have been propagated
for a very long time solely by non-sexual methods without any obvious
detriment to them, it is at first sight not easy to see what other
advantage could be gained by the sexual method. The case of the banana
shows that some forms do not require a sexual method of propagation.
Other forms, however, are so constituted, as we find them, that they
cannot reproduce at the present time except by the sexual method. In
other words, the latter are now adapted, as it were, to the sexual
method, and there is no longer any choice between the two methods. The
question of whether a non-sexual form might do better if it had another
method of propagation is not, perhaps, a profitable question to discuss.
What we really need to know is whether or not the sexual method was once
acquired, because it was an advantage to a particular organism, or to
the species to reproduce in this way. It is assumed by many writers that
this was the case, but whether they have sufficient ground for forming
such an opinion is our chief concern here. On the other hand, it is
conceivable, at least, that if the sexual method once became
established, it might continue without respect to any superiority it
gave over other methods, and might finally become a necessary condition
for the propagation of particular species. Thus the method would become
essential to propagation without respect to whether the species lost
more than it gained. Whichever way the balance should turn, it might
make little difference, so long as the species was still able to
propagate itself.
Brooks made the interesting and ingenious suggestion that the separation
of the sexes has been brought about as a sort of specialization of the
individuals in two directions. The male cells are supposed to accumulate
the newly acquired characters, and represent, therefore, the progressive
element in evolution. The female cells are the conservative element,
holding on to what has been gained in the past. It does not seem
probable, in the light of more recent work, that this is the function of
the two sexes, and it is unlikely that we could account for the origin
of the two sexes through the supposed advantage that such a
specialization might bring about. A number of writers, Galton, Van
Beneden, Bütschli, Maupas, and others, have looked at the process of
sexual reproduction as a sort of renewal of youth, or rejuvenescence of
the individuals. There is certainly a good deal in the process to
suggest that something of this sort takes place, although we must be on
our guard against assuming that the rejuvenescence is anything more than
the fulfilment of a necessary stage in the life history. Weismann has
ridiculed this suggestion on the ground that it is inconceivable that
two organisms, decrepit with old age, could renew their youth by
uniting. Two spent rockets, he says, cannot be imagined to form a new
one by combining. There is apparent soundness in this argument, if the
implication is taken in a narrow physical sense. If, on the other hand,
the egg is so constituted that at a certain stage in its development an
outside change is required to introduce a new phase, then the conception
of rejuvenescence does not appear in quite so absurd a light.
This hypothesis of rejuvenescence is based mainly on certain processes
that take place in the life history of some of the unicellular animals.
Let us now see what this evidence is. The results of certain experiments
carried out by Maupas on some of the ciliate protozoans have been
fruitful in arousing discussion as to the ultimate meaning of the sexual
process. Maupas’ experiments consisted in isolating single individuals,
and in following the history of the descendants that were produced
non-sexually by division. He found that the descendants of an individual
kept on dividing, but showed no tendency to unite with each other. After
a large number of generations had been passed through (in _Stylonychia
pustulata_, between 128 and 175; in _Leucophys patula_, 300 to 450; and
in _Onychodromus grandis_, 140 to 230 generations), the division began
to slow down, and finally came to a standstill. Maupas found that if he
took one of these run-down individuals, and placed it with another in
the same condition from another culture, that had had a different
parentage, the two would unite and the so-called process of conjugation
take place. This process consists for the species used, in the temporary
union and partial fusion of the protoplasm of the two individuals, of an
interchange of micronuclei, and of a fusion, in each individual, of the
micronucleus received from the other individual with one of its own. The
individuals then separate, and a new nucleus (or nuclei) is formed out
of the fused pair.
The individuals in question, in which this interchange of micronuclei
has taken place, undergo a change, and behave differently from what they
did before. They feed, become larger and less vacuolated, and are more
active. They soon begin once more to divide. Maupas found that an
individual that has conjugated will run through a new cycle of
divisions, which will, however, after a time also slow down, unless
conjugation with another individual having a different history takes
place. If conjugation is prevented, the individual will die after a
time. These results seemed to show that the division phase of the life
history cannot go on indefinitely, and that through conjugation the
individual is again brought back to the starting-point.
Quite recently Calkins has carried out a somewhat similar series of
experiments, which have an important bearing on the interpretation of
Maupas’ results. The experiment of isolating an individual and tracing
the career of its descendants was repeated with the following results:
two series were started, the original forms coming from different
localities. Of their eight descendants four of each were isolated. The
remaining four of each set were kept together as stock material. The
rate of division was taken as the measure of vitality. The animals
divided more or less regularly from February to July. After each
division (or sometimes after two divisions) the individuals were
separated. About the 30th of July the paramœcia began to die “at an
alarming rate, indicating that a period of depression had apparently set
in, or degeneration in Maupas’ sense.” Up to this time the animals had
been living in hay infusion, renewed every few days, from which they
obtained the bacteria on which they feed. Calkins tried the effect of
putting the weakened paramœcia into a new environment. Infusion of
vegetables gave no good results, but meat infusions proved successful.
“The first experiment with the latter was with teased liver, which was
added to the usual hay infusion. The result was very gratifying, for the
organisms began immediately to grow and to divide, the rate of division
rising from five to nine divisions in successive ten-day periods.” This
beneficial effect was not lasting, however, and after ten days the
paramœcia began to die off faster than before, and the renewed
application of the liver extract failed to revive them. A number of
other extracts were then tried without effect. Finally they were
transferred to the clear extract of lean beef in tap water. The effect
of this medium was interesting, for, although it restored the weakened
vitality, there was no rapid increase in the rate of division, as when
first treated with the teased liver. The infusoria were, however, now
large and vigorous, and did not die unless transferred from the beef
medium to the usual hay infusion. “When this was attempted, they would
become abnormally active and would finally die. The division rate
gradually increased during the month of August until, in the last ten
days, they averaged six generations. Finally, in September, the attempts
to get them back on the old diet of hay infusion were successful, and
then the division rate went up at once to twelve times in ten days, and
a month later they were dividing at the rate of fifty times a month.”
“These cultures went on well until December, when the paramœcia began to
die again. They were saved once more with the beef extract, and when
returned later to the hay infusion continued through another cycle of
almost three months. Some of these were treated, once a week for
twenty-four hours, with the beef extract, and while the two sets ran a
parallel course at first, those kept continuously in the hay infusion
died after a time, but those that had been put once a week into the beef
extract (which had been stopped, however, in March) continued their high
rate of division throughout the period of decline of their sister cells,
and did not show signs of diminished vitality until the first period in
June.” At this time their rate of division increased rapidly. They were
put back into the beef extract, but it failed now to have a beneficial
effect, and the animals continued to die at a rapid rate. To judge from
the appearance of the organisms the new decline was due to a different
cause; for, while in the former periods the food vacuoles contained
undigested food, at this period the interior was free from food masses.
The protoplasm became granular and different from that of a healthy
individual. None of the former remedies were now of any avail. “When the
last of the _B_-series stock had died in the five hundred and seventieth
generation (June 16th), it looked as though the cultures were about to
come to an end.” Extract of the brain and of the pancreas were then
tried. To this a favorable response took place at once. The organisms
became normal in appearance and began to divide. After forty-eight
hours’ treatment they were returned to the usual hay infusion. Here they
continued to multiply and reached on June 28th the six hundred and
sixty-fifth generation.
There can be no doubt that the periods of depression that appear in
these infusoria kept in cultures can be successfully passed if the
animals are introduced into a new environment. Without a change of this
sort they will die. Calkins thinks that the effect is produced, not by
the new kind of food that is supplied, but by the presence of certain
chemical compounds. The beef extract “does not have a direct stimulating
effect upon the digestive process and upon division, for, while the
organisms are immersed in it, there is a very slow division rate; when
transferred again to the hay infusion, however, they divide more rapidly
than before.”
This brings us back to the idea of the “renewal of youth” through
conjugation. Maupas claimed that union of individuals having the same
immediate descent is profitless. Calkins suggests that this is due to
the similarity in the chemical composition of the protoplasm of the two
individuals. When in nature two individuals that have lived under
somewhat different conditions conjugate, the result should be
beneficial, since there takes place the commingling of different
protoplasms.
Calkins’s work has shown that by means of certain substances much the
same effect can be produced as that which is supposed to follow from the
conjugation of two unrelated individuals. The presumption, therefore, is
in favor of the view that the two results may be brought about in the
same way, although we should be careful against a too ready acceptation
of this plausible argument; for we have ample evidence to show that many
closely similar (if not identical) responses of organisms may be brought
about by very different agencies. The experiments seem to indicate that
paramœcium might go on indefinitely reproducing by division, provided
its environment is changed from time to time. If this is true, it is
conceivable that the same thing is accomplished through conjugation. In
the light of this possible interpretation much of the mystery connected
with the term _rejuvenescence_ is removed, for we see that there is
nothing in the process itself except that it brings the organism into a
new relation with other substances. Difficult as it assuredly is to
understand how this benefits the animal, the experimental fact shows,
nevertheless, that such a change is for its good. That there is really
nothing in the process of conjugation itself apart from the difference
in the constitution of the conjugating individuals is shown by the
result that the union of individuals having the same history and kept
under the same conditions is of no benefit.
Can we apply this same conception to the process of fertilization in the
higher animals and plants? Is the substance of which their bodies are
made of such a sort that it cannot go on living indefinitely under the
same conditions, but must at times be supplied with a new environment?
If this could be established, we could see the advantage of sexual
reproduction over the non-sexual method. It would be extremely rash at
present to make a generalization of this kind, for there are many forms
known in which the only method of propagation that exists is the
non-sexual one. In other words, there are no grounds for the assumption
that this is a necessary condition for all kinds of protoplasm, but only
for certain kinds.
In the insects, crustaceans, rotifers, and in some plants there are a
few species whose egg develops without fertilization. This makes it
appear probable that the particular kind of protoplasm of these animals
does not absolutely require union from time to time with the protoplasm
of another individual having a somewhat different constitution.
There is also an interesting parallel between the effects of solutions
on the protozoans in Calkins’s experiments and certain results that have
been obtained in artificial parthenogenesis. It has been stated, that by
brushing the unfertilized eggs of the silkworm moth a larger percentage
will develop parthenogenetically; and more recently it has been shown by
Matthews that by agitation of the water in which the unfertilized eggs
of the starfish have been placed many of them will begin their
development. It was first shown by Richard Hertwig that by putting the
unfertilized eggs of the sea-urchin in strychnine solutions, they will
begin to segment, and I obtained the same results much better by placing
the eggs in solutions of magnesium chloride. Loeb then succeeded in
carrying the development to a later stage by using a different strength
of the same solution, as well as by other solutions. Under the most
favorable circumstances some of the eggs may produce larvæ that seem
normal in all respects, but whether they can develop into adult
sea-urchins has not yet been shown.
These results indicate that one at least of the factors of fertilization
is the stimulus given to the egg. On the other hand, the lack of vigor
shown by many eggs that have been artificially fertilized indicates that
some other result is also accomplished by the normal method of
fertilization that is here absent. This may mean no more than that as
yet we have not found all the conditions necessary to supply the place
of the spermatozoon.
In our study of the phenomena of adaptation we have found that sometimes
the adaptation is for the benefit of the individual and at other times
for the benefit of the species. May it not be true also that the process
of sexual reproduction has more to do with a benefit conferred on the
race rather than on the individual? In fact, Weismann has elaborated a
view based on the conception that the process of sexual reproduction is
beneficial to the race rather than to the individual. His idea, however,
is not so much that the result is of direct benefit to a particular
species, as it is advantageous to the formation of new species from the
original one. In a sense this amounts, perhaps, to nearly the same
thing, but in another sense the idea involves a somewhat different point
of view.
According to his view “the deeper significance of conjugation” and of
sexual reproduction is concerned “with the mingling of the hereditary
tendencies of two individuals.” In this way, through the different
combinations that are formed, variations which he supposes are
indispensable for the action of natural selection originate. The purpose
of the sexual process is solely, according to Weismann, to supply the
variations for natural selection. If it be asked how this process has
been acquired for the purpose of supplying natural selection with the
material on which it can work, we find the following reply given by
Weismann. “But if amphimixis [by which he means the union of sex-cells
from different individuals] is not absolutely necessary, the rarity of
purely parthenogenetic reproduction shows that it must have a widespread
and deep significance. Its benefits are not to be sought in the single
individual; for organisms can arise by agamic methods, without thereby
suffering any loss of vital energy; amphimixis must rather be
advantageous for the maintenance and modification of species. As soon as
we admit that amphimixis confers some such benefits, it is clear that
the latter must be augmented, as the method appears more frequently in
the course of generations; hence we are led to inquire how nature can
best have undertaken to give this amphimixis the widest possible range
in the organic world.” Nature, Weismann says, could find no more
effectual means of bringing about the union of the sexual cells than by
rendering them incapable of developing alone. “The male germ-cells,
being specially adapted for seeking and entering the ovum, are, as a
rule, so ill provided with nutriment that their unaided development into
an individual would be impossible; but with the ovum it is otherwise,
and accordingly the ‘reduction division’ removes half the germ-plasm and
the power of developing is withdrawn.” It can scarcely be claimed, in
the light of more recent discoveries, that the reduction division takes
place in order to prevent the development of the ovum, for how then
could we explain the corresponding division of the male germ-cells?
Whatever means has been employed to bring about the process of sexual
reproduction, the guiding principle is supposed by Weismann to be
natural selection as stated in the following paragraph: “If we regard
amphimixis as an adaptation of the highest importance, the phenomenon
can be explained in a simple way. I only assume that amphimixis is of
advantage in the phyletic development of life, and furthermore that it
is beneficial in maintaining the level of adaptation, which has been
once attained, in every single organism; for this is as dependent upon
the continuous activity of natural selection as the coming of new
species. According to the frequency with which amphimixis recurs in the
life of a species, is the efficiency with which the species is
maintained; since so much the more easily will it adapt itself to new
conditions of life, and thus become modified.”
Thus we reach the somewhat startling conclusion that through natural
selection the germ-cells and their protozoan prototypes have been
rendered incapable, through natural selection, of reproducing by
non-sexual methods, in order that variations may be supplied for the
farther action of this same process of natural selection. The
speculation has the appearance of arguing in a circle, although if it
were worth the attempt an ingenious mind might perhaps succeed in
showing that such a thing is not logically inconceivable.
It seems strange that a claim of this sort should have been made, when
it is so apparent that the most immediate effect of intercrossing is to
swamp all variations that depart from the average. Even if it were true
that new combinations of characters would arise through the union of the
germ-cells of two different animals, it is certainly true that in the
case of fluctuating variations this new combination would be lost by
later crossing with average individuals. Moreover, it is well known that
variations occur amongst forms that are produced asexually. On the
whole, it does not seem to be a satisfactory solution of the problem to
assume that sexual reproduction has been acquired in order to supply
natural selection with material on which it may work.
Our examination of the suggestions that have been made and of the
speculation indulged in, as to what benefit the process of sexual
reproduction confers on the animals and plants that make use of this
method of propagation, has failed to show convincingly that any
advantage to the individual or to the species is the outcome. This may
mean, either that there is no advantage, or that we have as yet failed
to understand the meaning of the phenomenon. The only light that has
been thrown on the question is that a certain amount of renewed vigor is
a consequence of this process, but we cannot explain how this takes
place. There is also the suggestion that the union of different cells
produces the same beneficial effect as a change in the conditions of
life produces on the organism. The bad effects of close interbreeding
that seem sometimes to follow is explicable on this view. This, it seems
to me, is the most plausible solution of the question that has been
advanced; but, even if this should prove to be the correct view, we need
not assume that the process has been acquired on account of this
advantage, for there is nothing to show that it has been acquired in
this way.
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CHAPTER XIII
SUMMARY AND GENERAL CONCLUSIONS
The question of the origin of the adaptations with which the last three
chapters have so largely dealt is one of the most difficult problems in
the whole range of biology, and yet it is one whose immense interest has
tempted philosophers in the past, and will no doubt continue to excite
the imagination of biologists for many years to come. No pretence has
been made in the preceding pages to account for the cause of a single
useful variation. We have examined the evidence, and from this we
believe the assumption justified that such variations do sometimes
appear. The more fundamental question as to the origin of these
variations has not been taken up, except in those cases in which the
adaptive response appeared directly in connection with a known external
cause. But these kinds of responses do not appear to have been the
source of the other adaptations of the organic world. Our discussion has
been largely confined to the problem of the widespread occurrence of
adaptation in living things, and to the most probable kinds of known
variations that could have given rise to these adaptations. But, to
repeat, we have made no attempt to account for the causes or the origin
of the different kinds of variation.
Nägeli, in speaking of the methods of the earlier theorists in Germany,
remarks with much acumen: “We might have expected that after the period
of the Nature-philosophizers, which in Germany crippled the best forces
that might have been used for the advance of the science, we should have
learnt something from experience, and have carefully guarded the field
of real scientific work from philosophical speculation. But the outcome
has shown that, in general, the philosophical, philological, and
æsthetic expression always gets the upper hand, and a fundamental and
exact treatment of scientific questions remains limited to a small
circle. The public at large always shows a distinct preference for the
so-called idealistic, poetic, and speculative modes of expression.” The
truth of this statement can scarcely be doubted when in our own time we
have seen more than once the same method employed with great public
applause. Nowhere is this more apparent than in the writings of many of
the followers of Darwin in respect to the adaptations of living things.
To imagine that a particular organ is useful to its possessor, and to
account for its origin because of the imagined benefit conferred, is the
general procedure of the followers of this school. Although protests
have from time to time been raised against this unwarrantable way of
settling the matter, they have been largely ignored and forgotten. The
fallacy of the argument has, for example, been admirably pointed out by
Bateson in the following statement:[36] “In examining cases of variation
I have not thought it necessary to speculate on the usefulness or
harmfulness of the variations described. For reasons given in Section II
such speculation, whether applied to normal structures or to variation,
is barren and profitless. If any one is curious on these questions of
Adaptation, he may easily thus exercise his imagination. In any case of
Variation there are a hundred ways in which it may be beneficial or
detrimental. For instance, if the ‘hairy’ variety of the moor-hen became
established on an island, as many strange varieties have been, I do not
doubt that ingenious persons would invite us to see how the hairiness
fitted the bird in some special way for life in that island in
particular. Their contention would be hard to deny, for on this class of
speculation the only limitations are those of the ingenuity of the
author. While the only test of utility is the success of the organism,
even this does not indicate the utility of one part of the economy, but
rather the net fitness of the whole.” Keeping in mind the admonitions
contained in the two preceding quotations, let us pass in review and
attempt to analyze more fully the different points that have been
considered in the preceding chapters.
Footnote 36:
“Materials for the Study of Variation.”
It has been pointed out that the evidence in favor of the theory of
evolution appears to establish this theory with great probability,
although a closer examination shows that we are almost completely in the
dark as to how the process has come about. For example, we have not yet
been able to determine whether the great groups of animals and plants
owe their resemblance to descent from a single original species or from
a large number of species. The former view is more plausible, because on
it we appear to be furnished with a better explanation of resemblances
as due to divergence of character. Yet even here a closer scrutiny of
the homologies of comparative anatomy shows that this explanation may be
more apparent than real. If discontinuous variation represents the steps
by which evolution has taken place, the artificiality of the explanation
is apparent, at least to a certain degree.
Admitting that the theory of evolution is the most probable view that we
have to account for the facts, we next meet with two further
questions,—the origin of species and the meaning of adaptation. These
are two separate and distinct questions, and not one and the same as the
Darwinian theory claims. The fact that all organisms are more or less
adapted to live in some environment appears from our examination to have
no direct connection with the origin of the adaptation, for, in the
first place, it seems probable that, in general, organisms do not
respond adaptively to the environment and produce new species in this
way; and, in the second place, there is no evidence to show that
variation from internal causes is so regulated that only adaptive
structures arise (although only adaptive ones may survive).
Our general conclusion is then as follows: A species does not arise from
another one because it is better adapted. Selection, in other words,
does not account for the origin of new species; and adaptation cannot be
taken as the measure of a species.
It may sound like a commonplace to state that only those individuals
survive and propagate themselves that can find some place in nature
where they can exist and leave descendants; and yet this statement may
contain all that it is necessary to assume, in order to account for the
fact that organisms are, on the whole, adapted. Let us see how this view
differs from the Darwinian statement of the origination of new forms
through a process of natural selection.
According to Darwin’s view of the origin of species, each new species is
gradually formed out of an older one, because of the advantage that the
new individual may have over the parent form. Each step forward is
acquired, because it better adapts the organism to the old, or to a new
set of conditions. In contrast to this, I have urged that the formation
of the new species is, as a rule, quite independent of its adaptive
value in regard to the parent species. But after it has appeared, its
survival will depend upon whether it can find a place in nature where it
can exist and leave descendants. If it should be well adapted to an
environment, it will be represented in it by a large number of
individuals. If it is poorly adapted, it may only barely succeed in
existing, and leave correspondingly fewer descendants. If its
adaptiveness falls below a certain point, it can never get a permanent
foothold, however often it may appear. Thus the test of survival
determines which species can remain in existence, and which cannot, but
new species are not manufactured in this way. How far subsequent
variations may be supposed to be determined by the survival of certain
species and the destruction of others will be discussed presently.
The difference between the two points of view that we are contrasting
can be best brought out by considering the two other kinds of selection
which Darwin supposes to have been at work; namely, artificial and
sexual selection.
Darwin thinks that the results of artificial selection are brought about
by the breeder picking out fluctuating variations. It appears that he
has probably overestimated the extent to which this process can be
carried; for while there can be no doubt that a certain standard, or
fixity of type, can be obtained by selecting fluctuating variations, yet
it now seems quite certain that the extent to which this can be carried
is very limited. It appears that other factors have also played an
important rôle; amongst these the occasional appearance of discontinuous
variation, also the bringing under cultivation of the numerous “smaller
species” of De Vries, or the so-called “single variations” of Darwin.
Further, the effects of intercrossing in all combinations of the above
forms of variations, followed by the selection of certain of the new
forms obtained, has been largely employed, and also the direct influence
of food and of other external conditions, which may be necessary to keep
the race up to a certain standard, have played a part in some cases. The
outcome is, therefore, by no means so simple as one might infer from
Darwin’s treatment of the subject in his “Origin of Species.” For these
reasons, as well as for others that have been given, it will be evident
that the process of artificial selection cannot be expected to give a
very clear idea of how natural selection could act.
It is, however, the process of sexual selection that brings out in the
strongest contrast the difference between Darwin’s main idea of natural
selection and the law of the survival of species. In sexual selection
the competition is supposed to be always between the individuals of the
same species and of the same sex. There can be no doubt in one’s mind,
after reading “The Descent of Man,” that Darwin held firmly to the
belief that the individual differences, or fluctuating variations,
furnish the material for selection. In this way it could never happen
that two competing species could exterminate each other, because in the
one the males were better adorned, or killed each other off on a larger
scale, owing to the presence of special weapons of warfare. It is clear
that on the law of the survival of species, secondary sexual characters
cannot be supposed to have evolved because of their value. Their origin
is totally inexplicable on this view. In fact, the presence of the
ornaments must be in some cases injurious to the existence of the
species. The interpretation of this means, I think, that individual
competition cannot be as severe as Darwin believed, and cannot lead to
the results that he imagined it does. For this reason it seemed
important to make as careful an examination of the claims of the theory
of sexual selection as possible, and I hope that the outcome of the
examination has shown quite definitely that the theory is incompetent to
account for the facts that it claims to explain. It is certain in this
case that we are dealing with a phenomenon that must be studied quite
apart from any selective value that the secondary sexual organs may
have. If this is granted, it will be seen that there is here a wide
field for experimental investigation that is practically untouched.
It is evident that the first step that will clear the way to a fuller
understanding of the problem of evolution must be a more thorough
examination of the question of variation. Darwin himself fully
appreciated this fact, yet until within the last fifteen years the study
of variation has been largely neglected. With a fuller knowledge of the
nature of fluctuating variation as the outcome of the studies of Galton,
Pearson, De Vries, and others, and with a fuller knowledge of the
possibilities of discontinuous variation as emphasized by Bateson and by
De Vries, and, further, with a better knowledge of some of the laws of
inheritance in these cases, we have begun to get a different conception
of how evolution has come about. It may be well, therefore, to go once
more over the main points in regard to the different kinds of variation.
While it has been found that no two individuals of a species are exactly
alike, yet, taken as a group, the variations appear as though they
followed the law of chance. The descendants of the group show the same
differences. Thus the group _as a whole_ appears constant, while the
individuals fluctuate continually in all directions. This is what we
understand by fluctuating variation. If the external conditions are
changed, a new “_mode_” may appear, but the change is generally only a
temporary one, and lasts only as long as the new conditions remain.
Thus, while the direct influence of the environment may show for a time,
the result is transient. Even if it were permanent, there is no evidence
that the adaptation of organisms could be accounted for in this way
unless the response were useful. It appears that this sometimes really
occurs, especially in responses to temperature, to moisture, to the
amount of salts in solution, to poisonous substances, etc. In this way,
one kind of adaptation is brought about, but there is no evidence that
the great number of structural adaptations have thus arisen.
The Lamarckian principle of the inheritance of acquired characters has
also been supposed by many writers to be an important source of adaptive
variation. An examination of this theory is not found to inspire
confidence. We have, therefore, eliminated this hypothesis on the ground
that it lacks evidence in its favor, and also because it appears
improbable that in this way many of the adaptations in organisms could
have been acquired.
Finally, there is the group of discontinuous variations. Of these there
may be several kinds, and there is some evidence showing that there are
such. For the present we may include all the different sorts under the
term _mutation_, meaning that the new character or group of characters
suddenly appears, and is inherited in its new form. From the results of
De Vries it appears that mutations are sometimes scattering, at least in
the case of the evening primrose. From such scattering mutations, the
smaller species or varieties (in so far as these do not depend on local
conditions) arise. There is here an important point of agreement with
Darwin’s idea in regard to evolution, inasmuch as he supposed that
varieties are incipient species. Our point of view is different,
however, in that we do not suppose these varieties (mutations) to have
been gradually formed out of fluctuating variations by a process of
selection, but to have arisen at once by a single mutation. It also
appears that in some cases a single new mutation may develop in a
species. We may suppose that the new form might in such a case supplant
the parent species by absorbing it, or both may go on living side by
side, as will be more likely the case if they are adapted to somewhat
different conditions.
A number of writers have supposed that evolution marches steadily
forward toward its final goal, which may even lead in some cases to the
final but inevitable destruction of the species. By certain writers this
view has been called orthogenesis, although at other times the idea is
not so much that there is advance in a straight line, as advance in all
directions. This appears to be Nägeli’s view. It gives a splendid
picture of the organic world, as irresistibly marching toward its
goal,—a relentless process in some cases, leading to final annihilation,
a beneficent process in other cases, leading to the fullest perfection
of form of which the type is capable. Compared with the vacillating
progress which is supposed to be the outcome of individual selection,
this view of progression has a grandeur that appeals directly to the
imagination. We must be guided, however, by evidence, rather than by
sentiment. The case will, moreover, bear closer scrutiny. If evolution
has indeed taken place by the survival of a series of mutations, whose
origin has no connection with their value, does not this in the end
amount to nearly the same thing as maintaining that evolution of
organisms has been a steady progress forward,—a progress undirected by
external forces, but the outcome of internal development? Admitting that
innumerable creations have been lopped off, because they could find no
foothold, yet, as Nägeli points out, the result is that, instead of a
dense tangle of forms, there has been left relatively few that have been
found capable of existing,—those that have found some place in which
they can live and leave progeny. From this point of view it may appear,
at first thought, that the idea of evolution through mutations involves
a fundamentally different view from that of the Darwinian school of
selection; but in so far as selection also depends on the spontaneous
appearance of fluctuating variations, the same point of view is to some
extent involved,—only the steps are supposed to be smaller. This point
is usually ignored and passed over in silence by the Darwinians, but, as
Wigand has pointed out, it makes very little difference whether the
stages in the process of evolution are imagined to be very small or
somewhat larger, so long as they are spontaneous. Selection does not do
more than determine the survival of what is offered to it, and does not
create anything new.
It is true that if the fluctuating variations that are selected be
connected by very slight differences with an almost continuous series of
other forms, and if little by little such a series be advanced in a
given direction by selection, we get the idea of a continuity, whose
advance is determined by selection. It is this conception that appears
to give the theory of natural selection a creative power, which in
reality it does not possess, and certainly not in the modified form in
which the theory was finally left by Darwin. For Darwin found himself
forced to admit that, unless a very considerable number of individuals
varied at the same time and in the same direction, the formation of new
species could not take place, and this idea of many individuals varying
at the same time, and in the same direction, at once involves the
conception that evolution moves forward by some force residing in the
organism, driving it forwards or backwards. Instability comes, perhaps,
nearer to expressing this idea than any other term, and yet to evolve
from a protozoan to a man implies the idea of something more than simple
unstableness.
The idea that Weismann has touched upon in this connection, namely, that
the survival of a given form determines the future course of evolution
for that form, is very plausible, and also fits in well with the results
of our experience in the field of the inheritance of variations. We see
new variations or mutations departing in some or in many characters from
the original type, apparently by new combinations or perturbations of
those already present. We never expect to see a bird emerge from the egg
of an alligator. Thus it appears that by the survival of certain forms
the future course of evolution is determined in so far as the new types
of mutation are thereby limited. Weismann means, however, that in this
way new plus or minus steps will be indefinitely determined amongst the
new fluctuating variations, but this statement is contradicted by our
experience of the results of artificial selection. The upper limit does
not keep on pushing out indefinitely in the direction determined by the
first selection, but is soon brought to a standstill. So that, as far as
Weismann’s hypothesis is concerned, the idea appears to have no special
value. On the other hand, this idea may be fruitful if applied to
mutations, but here unfortunately we have not sufficient experience to
guide us, and we do not know definitely whether a new character that
appears as a mutation will be more likely, in subsequent mutations, to
go on increasing in some of the descendants. Thus, while the mutation
theory must assume that some new characters will go on heaping up, we
lack the experimental evidence to show that this really occurs. It would
be also equally important to determine, whether, if after several
mutations have successively appeared in the same direction, there would
be an established tendency to go on in the same direction in some of the
future mutations. But here again we must wait until we have more data
before we attempt to build up a theory on such a basis.
The attacks on the Darwinian school by the followers of the modern
school of experimentalists are with few exceptions based on the
assumption that the natural selectionists pretend that their principle
is a sort of creative force,—a factor in evolution in the sense of being
an active agent. This assumption of the selectionists has led many of
them to ignore a fundamental weakness of their theory, namely, the
origin of the variations themselves, although Darwin did not overlook or
ignore this side of the problem, or fail to realize its importance, as
have some of his more ardent, but less critical, followers. They have
contented themselves, as a rule, with pointing out that certain
structures are useful, and this has seemed to them sufficient proof that
the structures must have been acquired because of their value. In
contrast to this complacency of the selectionists, we find here and
there naturalists who have, from time to time, insisted that the
scientific problem of evolution is not to be found in the principle of
selection, but in the origin of the variations themselves. It will be
clear, from what has been said, that this is our position also, and for
us adaptation itself does not appear to be any more a problem that can
be examined by scientific methods, than the lack of adaptation. The
causes of the change of whatever kind should be our immediate quest. The
destruction of the unfit, because they can find no place where they can
exist, does not explain the origin of the fit.
Over and beyond the primary question of the _origin_ of the adaptive, or
non-adaptive, structure is the fact that we find that the great majority
of animals and plants show distinct evidence of being suited or adapted
to live in a special environment, _i.e._ their structure and their
responses are such that they can live and leave descendants behind them.
I can see but two ways in which to account for this condition, either
(1) teleologically, by assuming that only adaptive variations arise, or
(2) by the survival of only those mutations that are sufficiently
adapted to get a foothold. Against the former view is to be urged that
the evidence shows quite clearly that variations (mutations) arise that
are not adaptive. On the latter view the dual nature of the problem that
we have to deal with becomes evident, for we assume that, while the
origin of the adaptive structures must be due to purely physical
principles in the widest sense, yet whether an organism that arises in
this way shall persist depends on whether it can find a suitable
environment. This latter is in one sense selection, although the word
has come to have a different significance, and, therefore, I prefer to
use the term _survival of species_.
The origin of a new form and its survival after it has appeared have
been often confused by the Darwinian school and have given the critics
of this school a fair chance for ridiculing the selection theory. The
Darwinian school has supposed that it could explain the origin of
adaptations on the basis of their usefulness. In this it seems to me
they are wrong. Their opponents, on the other hand, have, I believe,
gone too far when they state that the present condition of animals and
plants can be explained without applying the test of survival, or in a
broad sense the principle of selection amongst species.
It will be clear, therefore, in spite of the criticism that I have not
hesitated to apply to many of the phases of the selection theory,
especially in relation to the selection of the individuals of a species,
that I am not unappreciative of the great value of that part of Darwin’s
idea which claims that the _condition_ of the organic world, as we find
it, cannot be accounted for entirely without applying the principle of
selection in one form or another. This idea will remain, I think, a most
important contribution to the theory of evolution. We may sum up our
position categorically in the following statements:
Animals and plants are not changed in this or in that part in order to
become better adjusted to a given environment, as the Darwinian theory
postulates. Species exist that are in some respects very poorly adapted
to the environment in which they must live. If competition were as
severe as the selection theory assumes, this imperfection would not
exist.
In other cases a structure may be more perfect than the requirements of
selection demand. We must admit, therefore, that we cannot measure the
organic world by the measure of utility alone. If it be granted that
selection is not a moulding force in the organic world, we can more
easily understand how both less perfection and greater perfection may be
present than the demands of survival require.
If we suppose that new mutations and “definitely” inherited variations
suddenly appear, some of which will find an environment to which they
are more or less well fitted, we can see how evolution may have gone on
without assuming new species have been formed through a process of
competition. Nature’s supreme test is survival. She makes new forms to
bring them to this test through mutation, and does not remodel old forms
through a process of individual selection.
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INDEX
Acclimatization, 319.
Acorn, 24.
Acracids, 160.
Adaptation, definition of, 1.
Adjustments, individual, 12.
Agassiz, 1, 44, 61.
Agelæus, 173.
Alcohol, 13.
Algæ, red, 9.
Alkaloids, 13.
Allen, 173, 307-310.
Allolobophora, 380.
Alpheus, 344.
Ammophila, 5.
Ammotragus, 208.
Ampelopsis, 403.
Amphimixis, 448-449.
Amphioxus, 399.
Ancon race, 315-316.
Angiostomum, 422.
Anguillidæ, 320.
Annelids, 19, 20.
Anolis, 10, 194.
Ant-eater, 227, 228.
Antelope, 6, 206, 208.
Antitoxin, 14.
Ants, 141-146, 354, 386, 407.
Aphids, 384-386, 419, 426.
Apus, 418.
Archæopteryx, 41, 42, 53, 54.
Ardea, 200.
Argus pheasant, 199.
Aristolochia Clematitis, 10, 11, 12.
Arsenic, 13.
Artemia, 306.
Ascidians, 417.
Askenasy, 161.
Aspalax, 227.
Asterina, 421-422.
Autenrieth, 58.
Baboon, 209.
Bacteria, 14, 15, 111, 398.
Baer, Von, 60, 61, 74, 75.
Bamboo, 313.
Barnacles, 417.
Bartlett, 209, 220.
Bat, 2.
Bates, 183, 186.
Bateson, 273, 278, 453.
Beard, 210, 211, 216.
Beard, J., 435.
Bee, 2, 3, 19, 143, 179, 303, 350, 406, 420, 421, 425, 438.
Beethoven, 218.
Beetles, 182, 183, 189.
Bell-bird, 198.
Beneden, Van, 441.
Berbura goat, 208.
Biogenetic Law, 71.
Birds, 6, 407;
definition of group, 36;
evolution of, 41;
instincts of young, 4;
nest, 4;
of paradise, 6;
teeth of, 301;
variation in, 309-312.
Blind animals, 354.
Blow-fly, 383.
Bonellia, 353, 417.
Born, 424.
Bos, 206.
Boveri, 433.
Bovidæ, 207.
Branchipus, 306.
Brocadello, 428.
Brooks, 441.
Brown-Séquard, 232, 241, 250-257.
Buffon, 300.
Bull, 207, 315.
Bütschli, 441.
Butterfly, 3, 184, 389.
Cactus, 10.
Caffein, 13.
California salmon, 19.
Calkins, 443-447.
Callionymus, 191.
Calocalanus, 177.
Cameron, 425.
Canestrini, 178.
Canidæ, 308.
Canis, 410.
Carbonnier, 190, 192.
Cardamine, 335.
Cardinalis, 173.
Cardium, 305.
Cassowary, 202.
Castle, 148, 321, 435, 438.
Caterpillar, 5, 8, 186.
Cattle, 411.
Cats, 209.
Cercopithecus, 208.
Cervus, 304.
Chara, 420.
Charrin, 257.
Chick, 57, 406.
Child, 72.
Chinese pheasants, 6.
Chlorophyl, 9.
Cicadas, 187, 188.
Ciona, 148.
Classification, 31-36.
Classification, scheme of, 38.
Cockatoo, 6.
Colaptes, 310.
Colias, 185.
Colonial forms, 127.
Color, 18, 19, 24, 133, 375.
Coloration, 5, 6, 7, 23, 357-360.
Columba livia, 76.
Comb of bees, 4.
Communal marriages, 210.
Competition, 104, 112, 119, 120, 121, 122, 123.
Compositæ, 130.
Cones, 310.
Conklin, 72.
Cope, 49, 259.
Copridæ, 183.
Coral-snakes, 194.
Correlated variation, 94.
Correlation, 134.
Cottus, 191.
Crab, 15, 248, 344, 354.
Crickets, 188.
Crocodiles, 193.
Crosby, 398.
Cross-fertilization, 21.
Crossing of species, 148, 149, 150.
Crystal, 29.
Cryptocerus, 144.
Ctenophors, 417.
Cuckoo, 139, 140, 141.
Cuénot, 427-428, 435.
Culicidæ, 188.
Cunningham, 257-260.
Cuvier, 44, 301.
Cynocephalus, 209.
Cypridopsis, 392-393.
Cyprinodonts, 190.
Cypris, 320.
Dall, 260.
Dallinger, 320.
Danaids, 160.
Dances, 195.
Daphnia, 305, 418.
Darwin, C., numerous references throughout the text.
Darwin, Erasmus, 223.
Date-palm, 313.
Davenport, 264, 266, 321.
Dean, 358.
Death, 370.
Death, feigning, 410, 411.
Deer, 309.
Degeneration, 165.
Delamare, 257.
Descent theory, 31-35.
Desmarest, 206.
Desmodium, 403.
Dianthus, 149.
Didelphys, 410.
Dimorphism, 360.
Dingoes, 314.
Dinophilus, 428.
Diptera, 180, 188.
Divergence of character, 127, 128.
Dog, 226.
Draba, 288, 289, 290, 292, 294.
Draco, 194.
Dragonet, 191.
Drill, 209.
Ducks, 94, 314.
Düsing, 423.
Dutrochet, 320.
Earthworm, 380, 382, 383, 384, 417.
Echidna, 54.
Eciton, 144.
Egerton, 204.
Egg, 429-430, 432.
Eggs, number of, 19.
Egypt, animals of, 225.
Egyptian remains of animals, 43, 44.
Eimer, 158, 260.
Eisig, 72.
Electric organs, 22, 132, 372.
Elephant, 110-111, 206, 304.
Emu, 202.
Entoscolax, 353.
Epihippus, 50.
Equus, 50.
Eristales, 188.
Esmeralda, 182.
Euploids, 160.
Eustephanus, 201.
Evolution, 29.
Ewart, 238.
Exercise, 12.
External conditions, 130.
Eye, 13, 131, 132.
Eye, evolution of, 131, 132.
Eye, of flatfish, 137.
Fayal Islands, 124.
Felidæ, 308.
Felis, 308.
Fish, change of color, 16.
Fishes, 7.
Fishes, secondary sexual character of, 190.
Flatfish, 137, 138.
Flatworms, 417.
Fleischmann, 45-57.
Flounders, 228, 346, 347.
Flowers, 9, 17, 21, 342, 399, 422, 429.
Fly, 428.
Foot of horse, 47.
Forel, 5.
Fossil horses, 52.
Foxes, 210, 410.
Franqueiros cattle, 315.
Frogs, 193, 320, 382, 405.
Frogs, cross-fertilization, 150.
Fruit, down of, 133.
Fundulus, 16.
Galton, 236, 270-272, 289, 441.
Gavials, 301.
Geddes and Thompson, 417, 423, 426, 427.
Geer, De, 178.
Gegenbaur, 49.
Gelasimus, 177.
Geoffroy St.-Hilaire, 44, 67, 300-303.
Geological evidence, 39.
Gerbe, 429.
Germinal selection, 154.
Gibbon, 213.
Gill-clefts, 62, 63, 64, 73.
Giraffe, 6, 203, 229, 248-249.
Glacier, 28.
Glowworm, 23.
Goat, 206, 208.
Gonionema, 399.
Gorilla, 205.
Gothic period, 47, 48.
Gould, 197.
Graba, 124, 125.
Grafting, 153.
Grasshoppers, 8, 188.
Gray, 126.
Greyhound, 134.
Growth of plants, 17.
Guillemots, 124.
Guinea-pigs, 232.
Günther, 190.
Gymnotus, 132.
Haeckel, 48, 49, 56, 70, 71, 79, 80, 82.
Hartman, 187.
Heart, 66, 67.
Heliconids, 160.
Helix, 344, 345-346.
Hemiptera, 181.
Heredity, 270.
Hermaphroditic animals, 126.
Hertwig, O., 78, 79, 80, 81, 82, 83.
Hertwig, R., 447.
Hieracium, 330, 331.
Hildebrand, 148.
Hill, 252.
Hipparion, 51.
Hippeastrum, 148.
His, 71, 72.
Holmes, 72.
Hornbills, 219.
Horns, 229.
Horse, 42, 224.
Horse-chestnut, 24.
Hothura, 410.
Hottentots, 212.
Hudson, 140, 195, 409-412.
Humming-birds, 6, 197, 228.
Hurst, 75, 76, 77, 78.
Huxley, 49, 156, 233.
Hyatt, 259.
Hybrids, 149, 239.
Hydatina, 417.
Hydroides, 348.
Hylobates, 205.
Hymenoptera, 181.
Ice, 28.
Ichneumonidæ, 181.
Idioplasm, 335.
Immunity, 13.
India cattle, 208.
Infanticide, 25.
Inorganic adaptations, 26.
Insectivorous plants, 10.
Insects, coloration of, 7;
wingless, 228.
Instinct, 25, 139, 140, 141.
Irish elk, 247.
Jackson, 260.
Jaguar, 209.
Japanese cock, 163.
Jennings, 395.
Jonghe, 314.
Jordan, 292.
Joseph, 428.
Junco, 311.
Kallima, 7, 161, 162, 358.
Kangaroo, 229, 351.
Katydid, 8.
Kent, W. Saville, 191.
Kidneys, 66.
Kielmeyer, 58.
Kirby, 232.
Kiwi, 354.
Kölreuter, 149.
Korschelt, 428.
Labidocera, 393.
Lamarck, 146, 222-233.
Lamarckian factor, 94, 205, 211, 222, 458.
Lang, 345.
Law of Biogenesis, 30.
Leaf, resemblance to, 7.
Leaves, closing of, 11.
Leeches, 417.
Leguminosæ, 124.
Leidy, 46.
Length of life, 20.
Lenhossek, 435.
Leopard, 209.
Lepidoptera, 172.
Leptothrix, 320.
Leucophys, 442.
Lichen, 9.
Lillie, 72.
Limbs of vertebrates, 46.
Linaria, 401.
Linnæan species, 83.
Linnæus, 191.
Lion, 6.
Lizards, 7, 16, 17, 193.
Lobelia, 148.
Lobster, 343.
Lockwood, 138.
Locusts, 188.
Loeb, 383-392, 447.
Lomaria, 290.
Lowell lectures, 61.
Lumbriculus, 15.
Luminous organs, 133.
Lymnæa, 305, 322.
Lythrum, 363-370.
Machines, 26, 27, 28.
McIntosh, 176.
McNeill, 204.
Macropus, 192.
Malva, 401.
Mammalia, origin, 54, 202.
Man, 210.
Marsh, 49.
Matthews, 447.
Mauchamp, 315.
Maupas, 441, 442, 445.
May-flies, 19, 353, 389.
Mead, 72.
Meckel, 59, 60.
Melanism, 209.
Melospiza, 311.
Mendel, 278-286, 433, 436.
Mesohippus, 51.
Mimosa, 404.
Minnow, 16.
Minot, 433.
Mirabilis, 149, 150.
Mivart, 136, 137.
Mole, 1, 2, 18, 227.
Mole-cricket, 1, 2.
Molothrus, 140.
Monkeys, 207, 208.
Mons, Van, 332.
Montgomery, 432.
Moor-hen, 453.
Moquin-Tandon, 303.
Morton, Lord, 238.
Moschus, 206.
Moths, 184, 387, 388.
Moussu, 257.
Mozart, 218.
Mulberry, 313.
Müller, 182, 188.
Müller, Fritz, 148.
Muscles, 12.
Mycetes, 205.
Myzostomum, 422.
Nägeli, 161, 325-339, 459.
Natural selection, 104-107, 108, 109, 110, etc.;
definition of, 117.
Nauplius, 69.
Nectar, 124.
Nectar-feeding insects, 126, 127.
Nectarines, 134.
Negroes, 212.
Nematode, number of eggs, 110.
Nematus, 425.
Nemertian worms, 176.
Neo-Lamarckians, 240, 259-260.
Nepenthes, 10.
Nephela, 178.
Nest of birds, 4, 407-408.
Neuters, 142.
Nicotine, 13.
Nostocs, 320.
Notochord, 64, 65.
Nussbaum, 424.
Ocneria, 428.
Œnothera, 294-297.
Oken, 56, 58.
Old age, 21, 25.
Onites, 232.
Onychodromus, 442.
Opossum, 410.
Organs of little use, 22.
“Origin of Species,” 129.
Ornithorynchus, 54.
Orobanchia, 330.
Osborn, 259.
Oscillaria, 320.
Ostrich, 203, 354.
Oxalis, 290, 404.
Oxen, 304.
Oxide, 29.
Packard, 231, 260.
Paludina, 320, 322.
Pangenesis, 233-240.
Papilio, 158, 360, 388;
polyxenes, 3.
Paradisea, 197.
Paramæcium, 395-398, 442-447.
Parasitism, 352-353.
Parker, 393.
Parrots, 6.
Partridge, 410.
Passerella, 311.
Passiflora, 148.
Pavo, 317.
Peach, 134.
Peacock, 200, 317-318.
Peafowl, 198.
Pearson, 265, 267, 268-270.
Peas, 281-286.
Peckham, 178, 408.
Pelobates, 421.
Pflüger, 424, 430.
Phosphorescent organs, 22, 133.
Physa, 320, 322.
Pigeons, selection in, 102.
Pipilo, 311.
Pisum, 278.
Pithecia, 208.
Planaria, 380.
Planarians, 394.
Plants, 403, 415;
color of, 24;
influence of light, 17.
Plato, 304.
Plover, 202.
Poisons, 13, 14, 15, 18, 20, 377.
Polar bear, 6.
Pollen, 2, 125.
Polygon, 262.
Porthesia, 389.
Primula, 361-365.
Prionidæ, 182.
Probosces of insects, 127.
Protective coloration, 5, 6, 16, 158, 159.
Proteus, 227.
Protohippus, 51.
Przibram, 347.
Psyche, 419.
Ptarmigan, 5.
Pyrodes, 182.
Quetelet, 289.
Quiscalus, major, 173.
Rabbit, Porto Santo, 316-317.
Rabbits, 304.
Rabbits in Australia, 112.
Race-horse, 134.
Ranunculus, 305.
Ray-florets, 135.
Rays, electric organs of, 22.
Réaumur, 388.
Recapitulation theory, 58-83.
Reduction division, 432-433.
Regeneration, 15, 16, 27, 379.
Regulations, 27, 28.
Reproductive organs, 19.
Reptiles, fossil, 52, 53.
Rengger, 205.
Rhododendron, 330.
Rhynchæa, 201.
Riley, 424.
Rivers, 28.
Robinia, 404.
Romanes, 132, 250-256, 412.
Rose, 307.
Rothert, 398.
Rotifers, 118, 353, 424.
Roulin, 304.
Roundworms, 176, 353.
Rudimentary organs, 22.
Ryder, 260.
Sacculina, 353.
Sachs, 10.
Salmon, 19.
Salter, 314.
Salvin, 201.
Saphirina, 176.
Savages, 210.
Saw-flies, 425.
Scarlet tanager, 198.
Schaefer, 244.
Sclater, 198.
Scops, 312.
Scott, 148, 259.
Sea-anemone, 341.
Sea-urchin, 341.
Secondary sexual characters, 21.
Selection, 116.
Selection, artificial, 91, 92, 96, 97, 98.
Self-fertilization, 126.
Semper, 260.
Setchel, 320.
Sexual characters, secondary, 372-374.
Sexual selection, 167.
Sharp, 350, 425.
Sheep, 208.
Sherrington, 244.
Shrew mice, 206.
Silkworm, 428, 447.
Silver-bill, 410.
Sirex, 181.
Siricidæ, 181.
Sitaria, 194.
Skin, thickening of, 12, 13.
Skull, 37, 65.
Skunk, 3.
Slaves of ants, 141.
Sleep in plants, 404.
Sloth, 229.
Snail, 417.
Snails, color of, 23.
Snakes, 14, 193-194, 227.
Snowy owl, 6.
Solenobia, 419.
Soles, 137, 228.
Sparassus, 178.
Sparrow, 200;
English, 112.
Species, 31, 32, 33;
adaptation for good of, 19;
sharp separation of, 131.
Spencer, 240-246, 247, 290.
Spermatozoa, 150, 430-433.
Sphinx, 186, 388.
Spiders, 177-178, 179, 406;
web, 3.
Spirogyra, 420.
Spontaneous variability, 134.
Spores, 322.
Squilla, 177.
Squirrels, 210.
Stag-beetle, 179.
Stags, 203-204, 219.
Sterility, 147-152.
Strasburger, 395.
Stridulating organs, 188, 189.
Struggle for existence, 109, 110.
Stylonychia, 442.
Survival of the fittest, 107, 108, 109, 117.
Sutton, 432.
Swallow, 115.
Sweating, 12.
Tadpole, 321, 428.
Tail, 2.
Tanager, 6.
Tapeworm, 353;
number of eggs, 110.
Taraxacum, 305.
Tear-sacs, 206.
Teeth, bird’s, 67, 68.
Telegony, 95, 234, 237, 238, 239.
Tenthredinidæ, 181, 425.
Termite, number of eggs, 110.
Termitidæ, 350.
Thrush, 115.
Tipulæ, 188.
Toad, 7.
Torpedo, 132.
Towle, 392.
Transitional forms, 42.
Transmutation theory, 31, 34.
Traquair, 138.
Treadwell, 72.
Treat, 424.
Tree-frogs, 7.
Trichina, 353.
Trifolium, 404.
Triton, 193.
Turkeys, 314.
Turnix, 201, 202.
Turtles, 193.
Umbelliferæ, 135.
Uria lacrymans, 124.
Utricularia, 10,
Vanessa, 360.
Variability, 92, 93, 95, 96, 318-319.
Variation, 261, 340.
Variation, fluctuating, 100, 118, 123.
Variation under domestication, 136.
Varieties, 106, 107, 148.
Varigny, De, 303-306, 314-315, 322.
Venus fly-trap, 9.
Verbascum, 148, 149.
Vertebrates, evolution of, 40, 45.
Vilmorin, 303, 314.
Vinson, 178.
Vries, De, 97, 278, 289-298, 340.
Vulpine, 209.
Wallace, 7, 162, 186, 202, 221, 249.
Walrus, 203.
Walsh, 181.
Walther, 59.
Wasp, 3, 5, 408, 409.
Waterton, 198.
Web, spider’s, 3, 4.
Weir, 171.
Weismann, 154-166, 441, 448-450.
Westwood, 188.
Whale, 227, 301.
Wilson, E. B., 72.
Wing of bat, 2.
Wolf, 308, 376.
Wolves, 412.
Women, 210.
Woodpecker, 228.
Wounds, healing of, 15.
Yarrell, 138.
Yung, 424, 436.
Zebu cattle, 208.
Zeleny, 348.
Zoea, 69, 70.
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Transcriber’s note:
○ Missing or obscured punctuation was silently corrected.
○ Typographical errors were silently corrected.
○ Inconsistent spelling and hyphenation were made consistent only
when a predominant form was found in this book.
○ The cover image was created by the transcriber and is placed in
the public domain.
○ Some character-based illusrations were re-drawn using different
characters.
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