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´╗┐Title: On the origin of species - The Origin of Species by means of Natural Selection, 6th Edition
Author: Darwin, Charles, 1809-1882
Language: English
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THE ORIGIN OF SPECIES BY MEANS OF NATURAL SELECTION;

or, the PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE.

By Charles Darwin, M.A., F.R.S.,

Author of "The Descent of Man," etc., etc.


Sixth London Edition, with all Additions and Corrections.

The 6th Edition is often considered the definitive edition.


"But with regard to the material world, we can at least go so far as
this--we can perceive that events are brought about not by insulated
interpositions of Divine power, exerted in each particular case, but by
the establishment of general laws."--Whewell: "Bridgewater Treatise".

"The only distinct meaning of the word 'natural' is STATED, FIXED or
SETTLED; since what is natural as much requires and presupposes an
intelligent agent to render it so, i.e., to effect it continually or at
stated times, as what is supernatural or miraculous does to effect it
for once."--Butler: "Analogy of Revealed Religion".

"To conclude, therefore, let no man out of a weak conceit of sobriety,
or an ill-applied moderation, think or maintain, that a man can search
too far or be too well studied in the book of God's word, or in the book
of God's works; divinity or philosophy; but rather let men endeavour
an endless progress or proficience in both."--Bacon: "Advancement of
Learning".



AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF
SPECIES,

PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION OF THIS WORK.

I will here give a brief sketch of the progress of opinion on the Origin
of Species. Until recently the great majority of naturalists believed
that species were immutable productions, and had been separately
created. This view has been ably maintained by many authors. Some few
naturalists, on the other hand, have believed that species undergo
modification, and that the existing forms of life are the descendants
by true generation of pre existing forms. Passing over allusions to
the subject in the classical writers (Aristotle, in his "Physicae
Auscultationes" (lib.2, cap.8, s.2), after remarking that rain does not
fall in order to make the corn grow, any more than it falls to spoil the
farmer's corn when threshed out of doors, applies the same argument
to organisation; and adds (as translated by Mr. Clair Grece, who first
pointed out the passage to me), "So what hinders the different parts (of
the body) from having this merely accidental relation in nature? as the
teeth, for example, grow by necessity, the front ones sharp, adapted
for dividing, and the grinders flat, and serviceable for masticating
the food; since they were not made for the sake of this, but it was the
result of accident. And in like manner as to other parts in which there
appears to exist an adaptation to an end. Wheresoever, therefore, all
things together (that is all the parts of one whole) happened like as if
they were made for the sake of something, these were preserved,
having been appropriately constituted by an internal spontaneity; and
whatsoever things were not thus constituted, perished and still perish."
We here see the principle of natural selection shadowed forth, but
how little Aristotle fully comprehended the principle, is shown by his
remarks on the formation of the teeth.), the first author who in modern
times has treated it in a scientific spirit was Buffon. But as his
opinions fluctuated greatly at different periods, and as he does not
enter on the causes or means of the transformation of species, I need
not here enter on details.

Lamarck was the first man whose conclusions on the subject excited much
attention. This justly celebrated naturalist first published his views
in 1801; he much enlarged them in 1809 in his "Philosophie Zoologique",
and subsequently, 1815, in the Introduction to his "Hist. Nat. des
Animaux sans Vertebres". In these works he up holds the doctrine that
all species, including man, are descended from other species. He first
did the eminent service of arousing attention to the probability of
all change in the organic, as well as in the inorganic world, being the
result of law, and not of miraculous interposition. Lamarck seems
to have been chiefly led to his conclusion on the gradual change of
species, by the difficulty of distinguishing species and varieties,
by the almost perfect gradation of forms in certain groups, and by
the analogy of domestic productions. With respect to the means of
modification, he attributed something to the direct action of the
physical conditions of life, something to the crossing of already
existing forms, and much to use and disuse, that is, to the effects of
habit. To this latter agency he seems to attribute all the beautiful
adaptations in nature; such as the long neck of the giraffe for
browsing on the branches of trees. But he likewise believed in a law
of progressive development, and as all the forms of life thus tend to
progress, in order to account for the existence at the present day of
simple productions, he maintains that such forms are now spontaneously
generated. (I have taken the date of the first publication of Lamarck
from Isidore Geoffroy Saint-Hilaire's ("Hist. Nat. Generale", tom. ii.
page 405, 1859) excellent history of opinion on this subject. In
this work a full account is given of Buffon's conclusions on the same
subject. It is curious how largely my grandfather, Dr. Erasmus Darwin,
anticipated the views and erroneous grounds of opinion of Lamarck in
his "Zoonomia" (vol. i. pages 500-510), published in 1794. According to
Isid. Geoffroy there is no doubt that Goethe was an extreme partisan of
similar views, as shown in the introduction to a work written in 1794
and 1795, but not published till long afterward; he has pointedly
remarked ("Goethe als Naturforscher", von Dr. Karl Meding, s. 34) that
the future question for naturalists will be how, for instance, cattle
got their horns and not for what they are used. It is rather a singular
instance of the manner in which similar views arise at about the same
time, that Goethe in Germany, Dr. Darwin in England, and Geoffroy
Saint-Hilaire (as we shall immediately see) in France, came to the same
conclusion on the origin of species, in the years 1794-5.)

Geoffroy Saint-Hilaire, as is stated in his "Life", written by his
son, suspected, as early as 1795, that what we call species are various
degenerations of the same type. It was not until 1828 that he published
his conviction that the same forms have not been perpetuated since
the origin of all things. Geoffroy seems to have relied chiefly on the
conditions of life, or the "monde ambiant" as the cause of change. He
was cautious in drawing conclusions, and did not believe that existing
species are now undergoing modification; and, as his son adds, "C'est
donc un probleme a reserver entierement a l'avenir, suppose meme que
l'avenir doive avoir prise sur lui."

In 1813 Dr. W.C. Wells read before the Royal Society "An Account of a
White Female, part of whose skin resembles that of a Negro"; but his
paper was not published until his famous "Two Essays upon Dew and Single
Vision" appeared in 1818. In this paper he distinctly recognises the
principle of natural selection, and this is the first recognition which
has been indicated; but he applies it only to the races of man, and to
certain characters alone. After remarking that negroes and mulattoes
enjoy an immunity from certain tropical diseases, he observes, firstly,
that all animals tend to vary in some degree, and, secondly, that
agriculturists improve their domesticated animals by selection; and
then, he adds, but what is done in this latter case "by art, seems to
be done with equal efficacy, though more slowly, by nature, in the
formation of varieties of mankind, fitted for the country which they
inhabit. Of the accidental varieties of man, which would occur among
the first few and scattered inhabitants of the middle regions of Africa,
some one would be better fitted than others to bear the diseases of the
country. This race would consequently multiply, while the others would
decrease; not only from their in ability to sustain the attacks of
disease, but from their incapacity of contending with their more
vigorous neighbours. The colour of this vigorous race I take for
granted, from what has been already said, would be dark. But the same
disposition to form varieties still existing, a darker and a darker race
would in the course of time occur: and as the darkest would be the best
fitted for the climate, this would at length become the most prevalent,
if not the only race, in the particular country in which it had
originated." He then extends these same views to the white inhabitants
of colder climates. I am indebted to Mr. Rowley, of the United States,
for having called my attention, through Mr. Brace, to the above passage
of Dr. Wells' work.

The Hon. and Rev. W. Herbert, afterward Dean of Manchester, in the
fourth volume of the "Horticultural Transactions", 1822, and in his
work on the "Amaryllidaceae" (1837, pages 19, 339), declares that
"horticultural experiments have established, beyond the possibility of
refutation, that botanical species are only a higher and more permanent
class of varieties." He extends the same view to animals. The dean
believes that single species of each genus were created in an originally
highly plastic condition, and that these have produced, chiefly by
inter-crossing, but likewise by variation, all our existing species.

In 1826 Professor Grant, in the concluding paragraph in his well-known
paper ("Edinburgh Philosophical Journal", vol. XIV, page 283) on the
Spongilla, clearly declares his belief that species are descended
from other species, and that they become improved in the course of
modification. This same view was given in his Fifty-fifth Lecture,
published in the "Lancet" in 1834.

In 1831 Mr. Patrick Matthew published his work on "Naval Timber and
Arboriculture", in which he gives precisely the same view on the origin
of species as that (presently to be alluded to) propounded by Mr.
Wallace and myself in the "Linnean Journal", and as that enlarged in
the present volume. Unfortunately the view was given by Mr. Matthew very
briefly in scattered passages in an appendix to a work on a different
subject, so that it remained unnoticed until Mr. Matthew himself drew
attention to it in the "Gardeners' Chronicle", on April 7, 1860. The
differences of Mr. Matthew's views from mine are not of much importance:
he seems to consider that the world was nearly depopulated at successive
periods, and then restocked; and he gives as an alternative, that new
forms may be generated "without the presence of any mold or germ of
former aggregates." I am not sure that I understand some passages; but
it seems that he attributes much influence to the direct action of
the conditions of life. He clearly saw, however, the full force of the
principle of natural selection.

The celebrated geologist and naturalist, Von Buch, in his excellent
"Description Physique des Isles Canaries" (1836, page 147), clearly
expresses his belief that varieties slowly become changed into permanent
species, which are no longer capable of intercrossing.

Rafinesque, in his "New Flora of North America", published in 1836,
wrote (page 6) as follows: "All species might have been varieties once,
and many varieties are gradually becoming species by assuming constant
and peculiar characters;" but further on (page 18) he adds, "except the
original types or ancestors of the genus."

In 1843-44 Professor Haldeman ("Boston Journal of Nat. Hist. U. States",
vol. iv, page 468) has ably given the arguments for and against the
hypothesis of the development and modification of species: he seems to
lean toward the side of change.

The "Vestiges of Creation" appeared in 1844. In the tenth and much
improved edition (1853) the anonymous author says (page 155): "The
proposition determined on after much consideration is, that the several
series of animated beings, from the simplest and oldest up to the
highest and most recent, are, under the providence of God, the results,
FIRST, of an impulse which has been imparted to the forms of life,
advancing them, in definite times, by generation, through grades of
organisation terminating in the highest dicotyledons and vertebrata,
these grades being few in number, and generally marked by intervals
of organic character, which we find to be a practical difficulty in
ascertaining affinities; SECOND, of another impulse connected with the
vital forces, tending, in the course of generations, to modify organic
structures in accordance with external circumstances, as food, the
nature of the habitat, and the meteoric agencies, these being the
'adaptations' of the natural theologian." The author apparently believes
that organisation progresses by sudden leaps, but that the effects
produced by the conditions of life are gradual. He argues with much
force on general grounds that species are not immutable productions.
But I cannot see how the two supposed "impulses" account in a scientific
sense for the numerous and beautiful coadaptations which we see
throughout nature; I cannot see that we thus gain any insight how, for
instance, a woodpecker has become adapted to its peculiar habits of
life. The work, from its powerful and brilliant style, though displaying
in the early editions little accurate knowledge and a great want of
scientific caution, immediately had a very wide circulation. In my
opinion it has done excellent service in this country in calling
attention to the subject, in removing prejudice, and in thus preparing
the ground for the reception of analogous views.

In 1846 the veteran geologist M.J. d'Omalius d'Halloy published in an
excellent though short paper ("Bulletins de l'Acad. Roy. Bruxelles",
tom. xiii, page 581) his opinion that it is more probable that new
species have been produced by descent with modification than that they
have been separately created: the author first promulgated this opinion
in 1831.

Professor Owen, in 1849 ("Nature of Limbs", page 86), wrote as follows:
"The archetypal idea was manifested in the flesh under diverse such
modifications, upon this planet, long prior to the existence of those
animal species that actually exemplify it. To what natural laws or
secondary causes the orderly succession and progression of such organic
phenomena may have been committed, we, as yet, are ignorant." In his
address to the British Association, in 1858, he speaks (page li) of "the
axiom of the continuous operation of creative power, or of the ordained
becoming of living things." Further on (page xc), after referring
to geographical distribution, he adds, "These phenomena shake our
confidence in the conclusion that the Apteryx of New Zealand and the
Red Grouse of England were distinct creations in and for those islands
respectively. Always, also, it may be well to bear in mind that by the
word 'creation' the zoologist means 'a process he knows not what.'" He
amplifies this idea by adding that when such cases as that of the Red
Grouse are "enumerated by the zoologist as evidence of distinct creation
of the bird in and for such islands, he chiefly expresses that he
knows not how the Red Grouse came to be there, and there exclusively;
signifying also, by this mode of expressing such ignorance, his belief
that both the bird and the islands owed their origin to a great first
Creative Cause." If we interpret these sentences given in the same
address, one by the other, it appears that this eminent philosopher felt
in 1858 his confidence shaken that the Apteryx and the Red Grouse first
appeared in their respective homes "he knew not how," or by some process
"he knew not what."

This address was delivered after the papers by Mr. Wallace and myself on
the Origin of Species, presently to be referred to, had been read before
the Linnean Society. When the first edition of this work was published,
I was so completely deceived, as were many others, by such expressions
as "the continuous operation of creative power," that I included
Professor Owen with other palaeontologists as being firmly convinced
of the immutability of species; but it appears ("Anat. of Vertebrates",
vol. iii, page 796) that this was on my part a preposterous error. In
the last edition of this work I inferred, and the inference still seems
to me perfectly just, from a passage beginning with the words "no doubt
the type-form," etc.(Ibid., vol. i, page xxxv), that Professor Owen
admitted that natural selection may have done something in the formation
of a new species; but this it appears (Ibid., vol. iii. page 798)
is inaccurate and without evidence. I also gave some extracts from a
correspondence between Professor Owen and the editor of the "London
Review", from which it appeared manifest to the editor as well as to
myself, that Professor Owen claimed to have promulgated the theory of
natural selection before I had done so; and I expressed my surprise
and satisfaction at this announcement; but as far as it is possible to
understand certain recently published passages (Ibid., vol. iii. page
798) I have either partially or wholly again fallen into error. It
is consolatory to me that others find Professor Owen's controversial
writings as difficult to understand and to reconcile with each other,
as I do. As far as the mere enunciation of the principle of natural
selection is concerned, it is quite immaterial whether or not Professor
Owen preceded me, for both of us, as shown in this historical sketch,
were long ago preceded by Dr. Wells and Mr. Matthews.

M. Isidore Geoffroy Saint-Hilaire, in his lectures delivered in 1850 (of
which a Resume appeared in the "Revue et Mag. de Zoolog.", Jan., 1851),
briefly gives his reason for believing that specific characters "sont
fixes, pour chaque espece, tant qu'elle se perpetue au milieu des memes
circonstances: ils se modifient, si les circonstances ambiantes viennent
a changer. En resume, L'OBSERVATION des animaux sauvages demontre deja
la variabilite LIMITEE des especes. Les EXPERIENCES sur les animaux
sauvages devenus domestiques, et sur les animaux domestiques redevenus
sauvages, la demontrent plus clairment encore. Ces memes experiences
prouvent, de plus, que les differences produites peuvent etre de VALEUR
GENERIQUE." In his "Hist. Nat. Generale" (tom. ii, page 430, 1859) he
amplifies analogous conclusions.

From a circular lately issued it appears that Dr. Freke, in 1851
("Dublin Medical Press", page 322), propounded the doctrine that all
organic beings have descended from one primordial form. His grounds of
belief and treatment of the subject are wholly different from mine;
but as Dr. Freke has now (1861) published his Essay on the "Origin of
Species by means of Organic Affinity", the difficult attempt to give any
idea of his views would be superfluous on my part.

Mr. Herbert Spencer, in an Essay (originally published in the "Leader",
March, 1852, and republished in his "Essays", in 1858), has contrasted
the theories of the Creation and the Development of organic beings
with remarkable skill and force. He argues from the analogy of domestic
productions, from the changes which the embryos of many species undergo,
from the difficulty of distinguishing species and varieties, and from
the principle of general gradation, that species have been modified;
and he attributes the modification to the change of circumstances.
The author (1855) has also treated Psychology on the principle of the
necessary acquirement of each mental power and capacity by gradation.

In 1852 M. Naudin, a distinguished botanist, expressly stated, in an
admirable paper on the Origin of Species ("Revue Horticole", page 102;
since partly republished in the "Nouvelles Archives du Museum", tom. i,
page 171), his belief that species are formed in an analogous manner as
varieties are under cultivation; and the latter process he attributes to
man's power of selection. But he does not show how selection acts under
nature. He believes, like Dean Herbert, that species, when nascent,
were more plastic than at present. He lays weight on what he calls the
principle of finality, "puissance mysterieuse, indeterminee; fatalite
pour les uns; pour les autres volonte providentielle, dont l'action
incessante sur les etres vivantes determine, a toutes les epoques de
l'existence du monde, la forme, le volume, et la duree de chacun d'eux,
en raison de sa destinee dans l'ordre de choses dont il fait partie.
C'est cette puissance qui harmonise chaque membre a l'ensemble, en
l'appropriant a la fonction qu'il doit remplir dans l'organisme general
de la nature, fonction qui est pour lui sa raison d'etre." (From
references in Bronn's "Untersuchungen uber die Entwickelungs-Gesetze",
it appears that the celebrated botanist and palaeontologist Unger
published, in 1852, his belief that species undergo development and
modification. Dalton, likewise, in Pander and Dalton's work on Fossil
Sloths, expressed, in 1821, a similar belief. Similar views have, as is
well known, been maintained by Oken in his mystical "Natur-Philosophie".
From other references in Godron's work "Sur l'Espece", it seems that
Bory St. Vincent, Burdach, Poiret and Fries, have all admitted that
new species are continually being produced. I may add, that of the
thirty-four authors named in this Historical Sketch, who believe in
the modification of species, or at least disbelieve in separate acts
of creation, twenty-seven have written on special branches of natural
history or geology.)

In 1853 a celebrated geologist, Count Keyserling ("Bulletin de la Soc.
Geolog.", 2nd Ser., tom. x, page 357), suggested that as new diseases,
supposed to have been caused by some miasma have arisen and spread over
the world, so at certain periods the germs of existing species may have
been chemically affected by circumambient molecules of a particular
nature, and thus have given rise to new forms.

In this same year, 1853, Dr. Schaaffhausen published an excellent
pamphlet ("Verhand. des Naturhist. Vereins der Preuss. Rheinlands",
etc.), in which he maintains the development of organic forms on the
earth. He infers that many species have kept true for long periods,
whereas a few have become modified. The distinction of species he
explains by the destruction of intermediate graduated forms. "Thus
living plants and animals are not separated from the extinct by new
creations, but are to be regarded as their descendants through continued
reproduction."

A well-known French botanist, M. Lecoq, writes in 1854 ("Etudes sur
Geograph." Bot. tom. i, page 250), "On voit que nos recherches sur la
fixite ou la variation de l'espece, nous conduisent directement aux
idees emises par deux hommes justement celebres, Geoffroy Saint-Hilaire
et Goethe." Some other passages scattered through M. Lecoq's large
work make it a little doubtful how far he extends his views on the
modification of species.

The "Philosophy of Creation" has been treated in a masterly manner by
the Rev. Baden Powell, in his "Essays on the Unity of Worlds", 1855.
Nothing can be more striking than the manner in which he shows that the
introduction of new species is "a regular, not a casual phenomenon," or,
as Sir John Herschel expresses it, "a natural in contradistinction to a
miraculous process."

The third volume of the "Journal of the Linnean Society" contains
papers, read July 1, 1858, by Mr. Wallace and myself, in which, as
stated in the introductory remarks to this volume, the theory of
Natural Selection is promulgated by Mr. Wallace with admirable force and
clearness.

Von Baer, toward whom all zoologists feel so profound a respect,
expressed about the year 1859 (see Prof. Rudolph Wagner,
"Zoologisch-Anthropologische Untersuchungen", 1861, s. 51) his
conviction, chiefly grounded on the laws of geographical distribution,
that forms now perfectly distinct have descended from a single
parent-form.

In June, 1859, Professor Huxley gave a lecture before the Royal
Institution on the "Persistent Types of Animal Life". Referring to such
cases, he remarks, "It is difficult to comprehend the meaning of such
facts as these, if we suppose that each species of animal and plant, or
each great type of organisation, was formed and placed upon the surface
of the globe at long intervals by a distinct act of creative power; and
it is well to recollect that such an assumption is as unsupported by
tradition or revelation as it is opposed to the general analogy of
nature. If, on the other hand, we view "Persistent Types" in relation to
that hypothesis which supposes the species living at any time to be the
result of the gradual modification of pre-existing species, a hypothesis
which, though unproven, and sadly damaged by some of its supporters,
is yet the only one to which physiology lends any countenance; their
existence would seem to show that the amount of modification which
living beings have undergone during geological time is but very small in
relation to the whole series of changes which they have suffered."

In December, 1859, Dr. Hooker published his "Introduction to the
Australian Flora". In the first part of this great work he admits the
truth of the descent and modification of species, and supports this
doctrine by many original observations.

The first edition of this work was published on November 24, 1859, and
the second edition on January 7, 1860.



CONTENTS.


  INTRODUCTION


  CHAPTER I.

  VARIATION UNDER DOMESTICATION.

  Causes of Variability--Effects of Habit and the use or disuse of
  Parts--Correlated Variation--Inheritance--Character of Domestic
  Varieties--Difficulty of distinguishing between Varieties and
  Species--Origin of Domestic Varieties from one or more Species--Domestic
  Pigeons, their Differences and Origin--Principles of Selection,
  anciently followed, their Effects--Methodical and Unconscious
  Selection--Unknown Origin of our Domestic Productions--Circumstances
  favourable to Man's power of Selection.


  CHAPTER II.

  VARIATION UNDER NATURE.

  Variability--Individual Differences--Doubtful species--Wide ranging,
  much diffused, and common species, vary most--Species of the larger
  genera in each country vary more frequently than the species of the
  smaller genera--Many of the species of the larger genera resemble
  varieties in being very closely, but unequally, related to each other,
  and in having restricted ranges.


  CHAPTER III.

  STRUGGLE FOR EXISTENCE.

  Its bearing on natural selection--The term used in a wide
  sense--Geometrical ratio of increase--Rapid increase of naturalised
  animals and plants--Nature of the checks to increase--Competition
  universal--Effects of climate--Protection from the number of
  individuals--Complex relations of all animals and plants throughout
  nature--Struggle for life most severe between individuals and varieties
  of the same species; often severe between species of the same genus--The
  relation of organism to organism the most important of all relations.


  CHAPTER IV.

  NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.

  Natural Selection--its power compared with man's selection--its power
  on characters of trifling importance--its power at all ages and on
  both sexes--Sexual Selection--On the generality of intercrosses
  between individuals of the same species--Circumstances favourable and
  unfavourable to the results of Natural Selection, namely, intercrossing,
  isolation, number of individuals--Slow action--Extinction caused by
  Natural Selection--Divergence of Character, related to the diversity of
  inhabitants of any small area and to naturalisation--Action of Natural
  Selection, through Divergence of Character and Extinction, on the
  descendants from a common parent--Explains the Grouping of all organic
  beings--Advance in organisation--Low forms preserved--Convergence of
  character--Indefinite multiplication of species--Summary.


  CHAPTER V.

  LAWS OF VARIATION.

  Effects of changed conditions--Use and disuse, combined with natural
  selection; organs of flight and of vision--Acclimatisation--Correlated
  variation--Compensation and economy of growth--False
  correlations--Multiple, rudimentary, and lowly organised structures
  variable--Parts developed in an unusual manner are highly variable;
  specific characters more variable than generic; secondary sexual
  characters variable--Species of the same genus vary in an analogous
  manner--Reversions to long-lost characters--Summary.


  CHAPTER VI.

  DIFFICULTIES OF THE THEORY.

  Difficulties of the theory of descent with modification--Absence
  or rarity of transitional varieties--Transitions in habits of
  life--Diversified habits in the same species--Species with habits
  widely different from those of their allies--Organs of extreme
  perfection--Modes of transition--Cases of difficulty--Natura non facit
  saltum--Organs of small importance--Organs not in all cases absolutely
  perfect--The law of Unity of Type and of the Conditions of Existence
  embraced by the theory of Natural Selection.


  CHAPTER VII.

  MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.

  Longevity--Modifications not necessarily simultaneous--Modifications
  apparently of no direct service--Progressive development--Characters of
  small functional importance, the most constant--Supposed incompetence
  of natural selection to account for the incipient stages of useful
  structures--Causes which interfere with the acquisition through natural
  selection of useful structures--Gradations of structure with changed
  functions--Widely different organs in members of the same class,
  developed from one and the same source--Reasons for disbelieving in
  great and abrupt modifications.


  CHAPTER VIII.

  INSTINCT.

  Instincts comparable with habits, but different in their
  origin--Instincts graduated--Aphides and ants--Instincts
  variable--Domestic instincts, their origin--Natural instincts of
  the cuckoo, molothrus, ostrich, and parasitic bees--Slave-making
  ants--Hive-bee, its cell-making instinct--Changes of instinct and
  structure not necessarily simultaneous--Difficulties on the theory of
  the Natural Selection of instincts--Neuter or sterile insects--Summary.


  CHAPTER IX.

  HYBRIDISM.

  Distinction between the sterility of first crosses and of
  hybrids--Sterility various in degree, not universal, affected by close
  interbreeding, removed by domestication--Laws governing the sterility
  of hybrids--Sterility not a special endowment, but incidental on
  other differences, not accumulated by natural selection--Causes of
  the sterility of first crosses and of hybrids--Parallelism between the
  effects of changed conditions of life and of crossing--Dimorphism and
  Trimorphism--Fertility of varieties when crossed and of their mongrel
  offspring not universal--Hybrids and mongrels compared independently of
  their fertility--Summary.


  CHAPTER X.

  ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

  On the absence of intermediate varieties at the present day--On the
  nature of extinct intermediate varieties; on their number--On the lapse
  of time, as inferred from the rate of denudation and of deposition--On
  the lapse of time as estimated in years--On the poorness of our
  palaeontological collections--On the intermittence of geological
  formations--On the denudation of granitic areas--On the absence of
  intermediate varieties in any one formation--On the sudden appearance
  of groups of species--On their sudden appearance in the lowest known
  fossiliferous strata--Antiquity of the habitable earth.


  CHAPTER XI.

  ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.

  On the slow and successive appearance of new species--On their different
  rates of change--Species once lost do not reappear--Groups of species
  follow the same general rules in their appearance and disappearance as
  do single species--On extinction--On simultaneous changes in the forms
  of life throughout the world--On the affinities of extinct species to
  each other and to living species--On the state of development of
  ancient forms--On the succession of the same types within the same
  areas--Summary of preceding and present chapter.


  CHAPTER XII.

  GEOGRAPHICAL DISTRIBUTION.

  Present distribution cannot be accounted for by differences in physical
  conditions--Importance of barriers--Affinity of the productions of the
  same continent--Centres of creation--Means of dispersal by changes of
  climate and of the level of the land, and by occasional means--Dispersal
  during the Glacial period--Alternate Glacial periods in the north and
  south.


  CHAPTER XIII.

  GEOGRAPHICAL DISTRIBUTION--CONTINUED.

  Distribution of fresh-water productions--On the inhabitants of oceanic
  islands--Absence of Batrachians and of terrestrial Mammals--On
  the relation of the inhabitants of islands to those of the nearest
  mainland--On colonisation from the nearest source with subsequent
  modification--Summary of the last and present chapter.


  CHAPTER XIV.

  MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY--EMBRYOLOGY--RUDIMENTARY
  ORGANS.

  Classification, groups subordinate to groups--Natural system--Rules and
  difficulties in classification, explained on the theory of descent
  with modification--Classification of varieties--Descent always used in
  classification--Analogical or adaptive characters--Affinities,
  general, complex and radiating--Extinction separates and defines
  groups--Morphology, between members of the same class, between parts of
  the same individual--Embryology, laws of, explained by variations not
  supervening at an early age, and being inherited at a corresponding
  age--Rudimentary Organs; their origin explained--Summary.


  CHAPTER XV.

  RECAPITULATION AND CONCLUSION.

  Recapitulation of the objections to the theory of Natural
  Selection--Recapitulation of the general and special circumstances
  in its favour--Causes of the general belief in the immutability
  of species--How far the theory of Natural Selection may be
  extended--Effects of its adoption on the study of Natural
  history--Concluding remarks.


  GLOSSARY OF SCIENTIFIC TERMS.


  INDEX.



ORIGIN OF SPECIES.



INTRODUCTION.

When on board H.M.S. Beagle, as naturalist, I was much struck with
certain facts in the distribution of the organic beings inhabiting South
America, and in the geological relations of the present to the past
inhabitants of that continent. These facts, as will be seen in the
latter chapters of this volume, seemed to throw some light on the origin
of species--that mystery of mysteries, as it has been called by one
of our greatest philosophers. On my return home, it occurred to me,
in 1837, that something might perhaps be made out on this question by
patiently accumulating and reflecting on all sorts of facts which could
possibly have any bearing on it. After five years' work I allowed myself
to speculate on the subject, and drew up some short notes; these I
enlarged in 1844 into a sketch of the conclusions, which then seemed to
me probable: from that period to the present day I have steadily pursued
the same object. I hope that I may be excused for entering on these
personal details, as I give them to show that I have not been hasty in
coming to a decision.

My work is now (1859) nearly finished; but as it will take me many more
years to complete it, and as my health is far from strong, I have been
urged to publish this abstract. I have more especially been induced to
do this, as Mr. Wallace, who is now studying the natural history of
the Malay Archipelago, has arrived at almost exactly the same general
conclusions that I have on the origin of species. In 1858 he sent me a
memoir on this subject, with a request that I would forward it to Sir
Charles Lyell, who sent it to the Linnean Society, and it is published
in the third volume of the Journal of that Society. Sir C. Lyell and Dr.
Hooker, who both knew of my work--the latter having read my sketch
of 1844--honoured me by thinking it advisable to publish, with Mr.
Wallace's excellent memoir, some brief extracts from my manuscripts.

This abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements;
and I must trust to the reader reposing some confidence in my accuracy.
No doubt errors may have crept in, though I hope I have always been
cautious in trusting to good authorities alone. I can here give only
the general conclusions at which I have arrived, with a few facts in
illustration, but which, I hope, in most cases will suffice. No one can
feel more sensible than I do of the necessity of hereafter publishing in
detail all the facts, with references, on which my conclusions have been
grounded; and I hope in a future work to do this. For I am well aware
that scarcely a single point is discussed in this volume on which facts
cannot be adduced, often apparently leading to conclusions directly
opposite to those at which I have arrived. A fair result can be obtained
only by fully stating and balancing the facts and arguments on both
sides of each question; and this is here impossible.

I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very
many naturalists, some of them personally unknown to me. I cannot,
however, let this opportunity pass without expressing my deep
obligations to Dr. Hooker, who, for the last fifteen years, has aided me
in every possible way by his large stores of knowledge and his excellent
judgment.

In considering the origin of species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings,
on their embryological relations, their geographical distribution,
geological succession, and other such facts, might come to the
conclusion that species had not been independently created, but had
descended, like varieties, from other species. Nevertheless, such a
conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species, inhabiting this world
have been modified, so as to acquire that perfection of structure
and coadaptation which justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food, etc.,
as the only possible cause of variation. In one limited sense, as
we shall hereafter see, this may be true; but it is preposterous to
attribute to mere external conditions, the structure, for instance,
of the woodpecker, with its feet, tail, beak, and tongue, so admirably
adapted to catch insects under the bark of trees. In the case of the
mistletoe, which draws its nourishment from certain trees, which has
seeds that must be transported by certain birds, and which has flowers
with separate sexes absolutely requiring the agency of certain insects
to bring pollen from one flower to the other, it is equally preposterous
to account for the structure of this parasite, with its relations to
several distinct organic beings, by the effects of external conditions,
or of habit, or of the volition of the plant itself.

It is, therefore, of the highest importance to gain a clear insight into
the means of modification and coadaptation. At the commencement of
my observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best
chance of making out this obscure problem. Nor have I been disappointed;
in this and in all other perplexing cases I have invariably found that
our knowledge, imperfect though it be, of variation under domestication,
afforded the best and safest clue. I may venture to express my
conviction of the high value of such studies, although they have been
very commonly neglected by naturalists.

From these considerations, I shall devote the first chapter of this
abstract to variation under domestication. We shall thus see that a
large amount of hereditary modification is at least possible; and, what
is equally or more important, we shall see how great is the power of man
in accumulating by his selection successive slight variations. I will
then pass on to the variability of species in a state of nature; but
I shall, unfortunately, be compelled to treat this subject far too
briefly, as it can be treated properly only by giving long catalogues of
facts. We shall, however, be enabled to discuss what circumstances
are most favourable to variation. In the next chapter the struggle
for existence among all organic beings throughout the world, which
inevitably follows from the high geometrical ratio of their increase,
will be considered. This is the doctrine of Malthus, applied to the
whole animal and vegetable kingdoms. As many more individuals of each
species are born than can possibly survive; and as, consequently, there
is a frequently recurring struggle for existence, it follows that any
being, if it vary however slightly in any manner profitable to itself,
under the complex and sometimes varying conditions of life, will have
a better chance of surviving, and thus be NATURALLY SELECTED. From
the strong principle of inheritance, any selected variety will tend to
propagate its new and modified form.

This fundamental subject of natural selection will be treated at
some length in the fourth chapter; and we shall then see how natural
selection almost inevitably causes much extinction of the less improved
forms of life, and leads to what I have called divergence of character.
In the next chapter I shall discuss the complex and little known laws
of variation. In the five succeeding chapters, the most apparent and
gravest difficulties in accepting the theory will be given: namely,
first, the difficulties of transitions, or how a simple being or a
simple organ can be changed and perfected into a highly developed
being or into an elaborately constructed organ; secondly the subject of
instinct, or the mental powers of animals; thirdly, hybridism, or the
infertility of species and the fertility of varieties when intercrossed;
and fourthly, the imperfection of the geological record. In the next
chapter I shall consider the geological succession of organic beings
throughout time; in the twelfth and thirteenth, their geographical
distribution throughout space; in the fourteenth, their classification
or mutual affinities, both when mature and in an embryonic condition. In
the last chapter I shall give a brief recapitulation of the whole work,
and a few concluding remarks.

No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he make due allowance
for our profound ignorance in regard to the mutual relations of the
many beings which live around us. Who can explain why one species ranges
widely and is very numerous, and why another allied species has a narrow
range and is rare? Yet these relations are of the highest importance,
for they determine the present welfare and, as I believe, the future
success and modification of every inhabitant of this world. Still less
do we know of the mutual relations of the innumerable inhabitants of the
world during the many past geological epochs in its history. Although
much remains obscure, and will long remain obscure, I can entertain no
doubt, after the most deliberate study and dispassionate judgment of
which I am capable, that the view which most naturalists until recently
entertained, and which I formerly entertained--namely, that each species
has been independently created--is erroneous. I am fully convinced that
species are not immutable; but that those belonging to what are called
the same genera are lineal descendants of some other and generally
extinct species, in the same manner as the acknowledged varieties of
any one species are the descendants of that species. Furthermore, I am
convinced that natural selection has been the most important, but not
the exclusive, means of modification.



CHAPTER I. VARIATION UNDER DOMESTICATION.

 Causes of Variability--Effects of Habit and the use and disuse of
 Parts--Correlated Variation--Inheritance--Character of Domestic
 Varieties--Difficulty of distinguishing between Varieties and
 Species--Origin of Domestic Varieties from one or more Species--Domestic
 Pigeons, their Differences and Origin--Principles of Selection,
 anciently followed, their Effects--Methodical and Unconscious
 Selection--Unknown Origin of our Domestic Productions--Circumstances
 favourable to Man's power of Selection.


CAUSES OF VARIABILITY.

When we compare the individuals of the same variety or sub-variety of
our older cultivated plants and animals, one of the first points which
strikes us is, that they generally differ more from each other than do
the individuals of any one species or variety in a state of nature. And
if we reflect on the vast diversity of the plants and animals which have
been cultivated, and which have varied during all ages under the most
different climates and treatment, we are driven to conclude that this
great variability is due to our domestic productions having been raised
under conditions of life not so uniform as, and somewhat different from,
those to which the parent species had been exposed under nature. There
is, also, some probability in the view propounded by Andrew Knight, that
this variability may be partly connected with 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 organisation 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.

As far as I am able to judge, after long attending to the subject, the
conditions of life appear to act in two ways--directly on the whole
organisation or on certain parts alone and in directly by affecting the
reproductive system. With respect to the direct action, we must bear in
mind that in every case, as Professor Weismann has lately insisted,
and as I have incidently shown in my work on "Variation under
Domestication," there are two factors: namely, the nature of the
organism and the nature of the conditions. The former seems to be much
the more important; 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 effects on the offspring are either definite or in
definite. They may be considered as definite when all or nearly all the
offspring of individuals exposed to certain conditions during several
generations are modified in the same manner. It is extremely difficult
to come to any conclusion in regard to the extent of the changes which
have been thus definitely induced. There can, however, be little doubt
about many slight changes, such as size from the amount of food,
colour from the nature of the food, thickness of the skin and hair from
climate, etc. Each of the endless variations which we see in the plumage
of our fowls must have had some efficient cause; and if the same cause
were to act uniformly during a long series of generations on many
individuals, all probably would be modified in the same manner. Such
facts as the complex and extraordinary out growths which variably
follow from the insertion of a minute drop of poison by a gall-producing
insect, shows us what singular modifications might result in the case of
plants from a chemical change in the nature of the sap.

In definite 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 in
definite 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. All such changes of structure, whether extremely slight or
strongly marked, which appear among many individuals living together,
may be considered as the in definite effects of the conditions of life
on each individual organism, in nearly the same manner as the chill
effects different men in an in definite manner, according to their
state of body or constitution, causing coughs or colds, rheumatism, or
inflammation of various organs.

With respect to what I have called the in direct action of changed
conditions, namely, through the reproductive system of being affected,
we may infer that variability is thus induced, partly from the fact of
this system being extremely sensitive to any change in the conditions,
and partly from the similarity, as Kolreuter and others have remarked,
between the variability which follows from the crossing of distinct
species, and that which may be observed with plants and animals when
reared under new or unnatural conditions. Many facts clearly show how
eminently susceptible the reproductive system is to very slight changes
in the surrounding conditions. Nothing is more easy than to tame an
animal, and few things more difficult than to get it to breed freely
under confinement, even when the male and female unite. How many animals
there are which will not breed, though kept in an almost free state in
their native country! This is generally, but erroneously attributed to
vitiated instincts. Many cultivated plants display the utmost vigour,
and yet rarely or never seed! In some few cases it has been discovered
that a very trifling change, such as a little more or less water at some
particular period of growth, will determine whether or not a plant will
produce seeds. I cannot here give the details which I have collected and
elsewhere published on this curious subject; but to show how singular
the laws are which determine the reproduction of animals under
confinement, I may mention that carnivorous animals, even from the
tropics, breed in this country pretty freely under confinement, with
the exception of the plantigrades or bear family, which seldom produce
young; whereas, carnivorous birds, with the rarest exception, hardly
ever lay fertile eggs. Many exotic plants have pollen utterly worthless,
in the same condition as in the most sterile hybrids. When, on the one
hand, we see domesticated animals and plants, though often weak and
sickly, breeding freely under confinement; and when, on the other hand,
we see individuals, though taken young from a state of nature perfectly
tamed, long-lived, and healthy (of which I could give numerous
instances), yet having their reproductive system so seriously affected
by unperceived causes as to fail to act, we need not be surprised at
this system, when it does act under confinement, acting irregularly,
and producing offspring somewhat unlike their parents. I may add that
as some organisms breed freely under the most unnatural conditions--for
instance, rabbits and ferrets kept in hutches--showing that their
reproductive organs are not easily affected; so will some animals
and plants withstand domestication or cultivation, and vary very
slightly--perhaps hardly more than in a state of nature.

Some naturalists have maintained that all variations are connected with
the act of sexual reproduction; but this is certainly an error; for I
have given in another work a long list of "sporting plants;" as they are
called by gardeners; that is, of plants which have suddenly produced a
single bud with a new and sometimes widely different character from that
of the other buds on the same plant. These bud variations, as they may
be named, can be propagated by grafts, offsets, etc., and sometimes
by seed. They occur rarely under nature, but are far from rare under
culture. As a single bud out of many thousands produced year after year
on the same tree under uniform conditions, has been known suddenly to
assume a new character; and as buds on distinct trees, growing
under different conditions, have sometimes yielded nearly the same
variety--for instance, buds on peach-trees producing nectarines, and
buds on common roses producing moss-roses--we clearly see that the
nature of the conditions is of subordinate importance in comparison
with the nature of the organism in determining each particular form of
variation; perhaps of not more importance than the nature of the spark,
by which a mass of combustible matter is ignited, has in determining the
nature of the flames.

EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS; CORRELATED
VARIATION; INHERITANCE.

Changed habits produce an inherited effect as in the period of the
flowering of plants when transported from one climate to another. With
animals the increased use or disuse of parts has had a more marked
influence; thus 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 in cows and goats in countries where they are habitually milked,
in comparison with these organs in other countries, is probably another
instance of the effects of use. Not one of our domestic animals can be
named which has not in some country drooping ears; and the view which
has been suggested that the drooping is due to disuse of the muscles of
the ear, from the animals being seldom much alarmed, seems probable.

Many laws regulate variation, some few of which can be dimly seen, and
will hereafter be briefly discussed. I will here only allude to what may
be called correlated variation. Important changes in the embryo or larva
will probably entail changes in the mature animal. In monstrosities,
the correlations between quite distinct parts are very curious; and many
instances are given in Isidore Geoffroy St. Hilaire's great work on this
subject. Breeders believe that long limbs are almost always accompanied
by an elongated head. Some instances of correlation are quite whimsical;
thus cats which are entirely white and have blue eyes are generally
deaf; but it has been lately stated by Mr. Tait that this is confined to
the males. Colour and constitutional peculiarities go together, of which
many remarkable cases could be given among animals and plants. From
facts collected by Heusinger, it appears that white sheep and pigs
are injured by certain plants, while dark-coloured individuals escape:
Professor Wyman has recently communicated to me a good illustration of
this fact; on asking some farmers in Virginia how it was that all their
pigs were black, they informed him that the pigs ate the paint-root
(Lachnanthes), which coloured their bones pink, and which caused
the hoofs of all but the black varieties to drop off; and one of the
"crackers" (i.e. Virginia squatters) added, "we select the black members
of a litter for raising, as they alone have a good chance of living."
Hairless dogs have imperfect teeth; long-haired and coarse-haired
animals are apt to have, as is asserted, long or many horns; pigeons
with feathered feet have skin between their outer toes; pigeons with
short beaks have small feet, and those with long beaks large feet. Hence
if man goes on selecting, and thus augmenting, any peculiarity, he will
almost certainly modify unintentionally other parts of the structure,
owing to the mysterious laws of correlation.

The results of the various, unknown, or but dimly understood laws of
variation are infinitely complex and diversified. It is well worth while
carefully to study the several treatises on some of our old cultivated
plants, as on the hyacinth, potato, even the dahlia, etc.; and it
is really surprising to note the endless points of structure and
constitution in which the varieties and sub-varieties differ slightly
from each other. The whole organisation seems to have become plastic,
and departs in a slight degree from that of the parental type.

Any variation which is not inherited is unimportant for us. But the
number and diversity of inheritable deviations of structure, both
those of slight and those of considerable physiological importance,
are endless. Dr. Prosper Lucas' treatise, in two large volumes, is the
fullest and the best on this subject. No breeder doubts how strong is
the tendency to inheritance; that like produces like is his fundamental
belief: doubts have been thrown on this principle only by theoretical
writers. When any deviation of structure often appears, and we see it
in the father and child, we cannot tell whether it may not be due to the
same cause having acted on both; but when among individuals, apparently
exposed to the same conditions, any very rare deviation, due to some
extraordinary combination of circumstances, appears in the parent--say,
once among several million individuals--and it reappears in the
child, the mere doctrine of chances almost compels us to attribute
its reappearance to inheritance. Every one must have heard of cases of
albinism, prickly skin, hairy bodies, etc., appearing in several members
of the same family. If strange and rare deviations of structure are
truly inherited, less strange and commoner deviations may be freely
admitted to be inheritable. Perhaps the correct way of viewing the whole
subject would be, to look at the inheritance of every character whatever
as the rule, and non-inheritance as the anomaly.

The laws governing inheritance are for the most part unknown; no one
can say why the same peculiarity in different individuals of the same
species, or in different species, is sometimes inherited and sometimes
not so; why the child often reverts in certain characteristics to its
grandfather or grandmother or more remote ancestor; why a peculiarity is
often transmitted from one sex to both sexes, or to one sex alone,
more commonly but not exclusively to the like sex. It is a fact of
some importance to us, that peculiarities appearing in the males of our
domestic breeds are often transmitted, either exclusively or in a much
greater degree, to the males alone. A much more important rule, which I
think may be trusted, is that, at whatever period of life a peculiarity
first appears, it tends to reappear in the offspring at a corresponding
age, though sometimes earlier. In many cases this could not be
otherwise; thus the inherited peculiarities in the horns of cattle could
appear only in the offspring when nearly mature; peculiarities in the
silk-worm are known to appear at the corresponding caterpillar or cocoon
stage. But hereditary diseases and some other facts make me believe
that the rule has a wider extension, and that, when there is no apparent
reason why a peculiarity should appear at any particular age, yet that
it does tend to appear in the offspring at the same period at which it
first appeared in the parent. I believe this rule to be of the highest
importance in explaining the laws of embryology. These remarks are of
course confined to the first APPEARANCE of the peculiarity, and not
to the primary cause which may have acted on the ovules or on the male
element; in nearly the same manner as the increased length of the horns
in the offspring from a short-horned cow by a long-horned bull, though
appearing late in life, is clearly due to the male element.

Having alluded to the subject of reversion, I may here refer to
a statement often made by naturalists--namely, that our domestic
varieties, when run wild, gradually but invariably revert in character
to their aboriginal stocks. Hence it has been argued that no deductions
can be drawn from domestic races to species in a state of nature. I
have in vain endeavoured to discover on what decisive facts the above
statement has so often and so boldly been made. There would be great
difficulty in proving its truth: we may safely conclude that very many
of the most strongly marked domestic varieties could not possibly live
in a wild state. In many cases we do not know what the aboriginal stock
was, and so could not tell whether or not nearly perfect reversion
had ensued. It would be necessary, in order to prevent the effects of
intercrossing, that only a single variety should be turned loose in
its new home. Nevertheless, as our varieties certainly do occasionally
revert in some of their characters to ancestral forms, it seems to me
not improbable that if we could succeed in naturalising, or were to
cultivate, during many generations, the several races, for instance,
of the cabbage, in very poor soil--in which case, however, some
effect would have to be attributed to the DEFINITE action of the poor
soil--that they would, to a large extent, or even wholly, revert to the
wild aboriginal stock. Whether or not the experiment would succeed is
not of great importance for our line of argument; for by the experiment
itself the conditions of life are changed. If it could be shown that our
domestic varieties manifested a strong tendency to reversion--that is,
to lose their acquired characters, while kept under the same conditions
and while kept in a considerable body, so that free intercrossing might
check, by blending together, any slight deviations in their structure,
in such case, I grant that we could deduce nothing from domestic
varieties in regard to species. But there is not a shadow of evidence
in favour of this view: to assert that we could not breed our cart
and race-horses, long and short-horned cattle, and poultry of various
breeds, and esculent vegetables, for an unlimited number of generations,
would be opposed to all experience.

CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF DISTINGUISHING BETWEEN
VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES FROM ONE OR MORE
SPECIES.

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. It has
often been stated that domestic races do not differ from each other in
characters of generic value. It can be shown that this statement is not
correct; but naturalists differ much in determining what characters are
of generic value; all such valuations being at present empirical. When
it is explained how genera originate under nature, it will be seen that
we have no right to expect often to find a generic amount of difference
in our domesticated races.

In attempting to estimate the amount of structural difference between
allied domestic races, we are soon involved in doubt, from not knowing
whether they are descended from one or several parent species. This
point, if it could be cleared up, would be interesting; if, for
instance, it could be shown that the greyhound, bloodhound, terrier,
spaniel and bull-dog, which we all know propagate their kind truly, were
the offspring of any single species, then such facts would have great
weight in making us doubt about the immutability of the many closely
allied natural species--for instance, of the many foxes--inhabiting the
different quarters of the world. I do not believe, as we shall presently
see, that the whole amount of difference between the several breeds of
the dog has been produced under domestication; I believe that a small
part of the difference is due to their being descended from distinct
species. In the case of strongly marked races of some other domesticated
species, there is presumptive or even strong evidence that all are
descended from a single wild stock.

It has often been assumed that man has chosen for domestication animals
and plants having an extraordinary inherent tendency to vary, and
likewise to withstand diverse climates. I do not dispute that these
capacities have added largely to the value of most of our domesticated
productions; but how could a savage possibly know, when he first tamed
an animal, whether it would vary in succeeding generations, and whether
it would endure other climates? Has the little variability of the ass
and goose, or the small power of endurance of warmth by the reindeer,
or of cold by the common camel, prevented their domestication? I
cannot doubt that if other animals and plants, equal in number to our
domesticated productions, and belonging to equally diverse classes and
countries, were taken from a state of nature, and could be made to breed
for an equal number of generations under domestication, they would on
an average vary as largely as the parent species of our existing
domesticated productions have varied.

In the case of most of our anciently domesticated animals and plants,
it is not possible to come to any definite conclusion, whether they are
descended from one or several wild species. The argument mainly relied
on by those who believe in the multiple origin of our domestic animals
is, that we find in the most ancient times, on the monuments of Egypt,
and in the lake-habitations of Switzerland, much diversity in the
breeds; and that some of these ancient breeds closely resemble, or are
even identical with, those still existing. But this only throws far
backward the history of civilisation, and shows that animals were
domesticated at a much earlier period than has hitherto been supposed.
The lake-inhabitants of Switzerland cultivated several kinds of wheat
and barley, the pea, the poppy for oil and flax; and they possessed
several domesticated animals. They also carried on commerce with other
nations. All this clearly shows, as Heer has remarked, that they had at
this early age progressed considerably in civilisation; and this again
implies a long continued previous period of less advanced civilisation,
during which the domesticated animals, kept by different tribes in
different districts, might have varied and given rise to distinct races.
Since the discovery of flint tools in the superficial formations of many
parts of the world, all geologists believe that barbarian men existed at
an enormously remote period; and we know that at the present day there
is hardly a tribe so barbarous as not to have domesticated at least the
dog.

The origin of most of our domestic animals will probably forever remain
vague. But I may here state that, looking to the domestic dogs of the
whole world, I have, after a laborious collection of all known facts,
come to the conclusion that several wild species of Canidae have been
tamed, and that their blood, in some cases mingled together, flows in
the veins of our domestic breeds. In regard to sheep and goats I can
form no decided opinion. From facts communicated to me by Mr. Blyth,
on the habits, voice, constitution and structure of the humped Indian
cattle, it is almost certain that they are descended from a different
aboriginal stock from our European cattle; and some competent judges
believe that these latter have had two or three wild progenitors,
whether or not these deserve to be called species. This conclusion, as
well as that of the specific distinction between the humped and common
cattle, may, indeed, be looked upon as established by the admirable
researches of Professor Rutimeyer. With respect to horses, from reasons
which I cannot here give, I am doubtfully inclined to believe, in
opposition to several authors, that all the races belong to the same
species. Having kept nearly all the English breeds of the fowl alive,
having bred and crossed them, and examined their skeletons, it appears
to me almost certain that all are the descendants of the wild Indian
fowl, Gallus bankiva; and this is the conclusion of Mr. Blyth, and
of others who have studied this bird in India. In regard to ducks and
rabbits, some breeds of which differ much from each other, the evidence
is clear that they are all descended from the common duck and wild
rabbit.

The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some
authors. They believe that every race which breeds true, let the
distinctive characters be ever so slight, has had its wild prototype.
At this rate there must have existed at least a score of species of wild
cattle, as many sheep, and several goats, in Europe alone, and several
even within Great Britain. One author believes that there formerly
existed eleven wild species of sheep peculiar to Great Britain! When we
bear in mind that Britain has now not one peculiar mammal, and France
but few distinct from those of Germany, and so with Hungary, Spain,
etc., but that each of these kingdoms possesses several peculiar breeds
of cattle, sheep, etc., we must admit that many domestic breeds must
have originated in Europe; for whence otherwise could they have been
derived? So it is in India. Even in the case of the breeds of the
domestic dog throughout the world, which I admit are descended from
several wild species, it cannot be doubted that there has been an
immense amount of inherited variation; for who will believe that animals
closely resembling the Italian greyhound, the bloodhound, the bull-dog,
pug-dog, or Blenheim spaniel, etc.--so unlike all wild Canidae--ever
existed in a state of nature? It has often been loosely said that all
our races of dogs have been produced by the crossing of a few
aboriginal species; but by crossing we can only get forms in some degree
intermediate between their parents; and if we account for our several
domestic races by this process, we must admit the former existence of
the most extreme forms, as the Italian greyhound, bloodhound, bull-dog,
etc., in the wild state. Moreover, the possibility of making distinct
races by crossing has been greatly exaggerated. Many cases are on record
showing that a race may be modified by occasional crosses if aided
by the careful selection of the individuals which present the desired
character; but to obtain a race intermediate between two quite distinct
races would be very difficult. Sir J. Sebright expressly experimented
with this object and failed. The offspring from the first cross between
two pure breeds is tolerably and sometimes (as I have found with
pigeons) quite uniform in character, and every thing seems simple
enough; but when these mongrels are crossed one with another for several
generations, hardly two of them are alike, and then the difficulty of
the task becomes manifest.

BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN.

Believing that it is always best to study some special group, I have,
after deliberation, taken up domestic pigeons. I have kept every breed
which I could purchase or obtain, and have been most kindly favoured
with skins from several quarters of the world, more especially by the
Hon. W. Elliot from India, and by the Hon. C. Murray from Persia. Many
treatises in different languages have been published on pigeons, and
some of them are very important, as being of considerable antiquity. I
have associated with several eminent fanciers, and have been permitted
to join two of the London Pigeon Clubs. The diversity of the breeds is
something astonishing. Compare the English carrier and the short-faced
tumbler, and see the wonderful difference in their beaks, entailing
corresponding differences in their skulls. The carrier, more especially
the male bird, is also remarkable from the wonderful development of the
carunculated skin about the head, and this is accompanied by greatly
elongated eyelids, very large external orifices to the nostrils, and a
wide gape of mouth. The short-faced tumbler has a beak in outline almost
like that of a finch; and the common tumbler has the singular inherited
habit of flying at a great height in a compact flock, and tumbling in
the air head over heels. The runt is a bird of great size, with long,
massive beak and large feet; some of the sub-breeds of runts have very
long necks, others very long wings and tails, others singularly short
tails. The barb is allied to the carrier, but, instead of a long beak,
has a very short and broad one. The pouter has a much elongated body,
wings, and legs; and its enormously developed crop, which it glories in
inflating, may well excite astonishment and even laughter. The turbit
has a short and conical beak, with a line of reversed feathers down the
breast; and it has the habit of continually expanding, slightly, the
upper part of the oesophagus. The Jacobin has the feathers so much
reversed along the back of the neck that they form a hood, and it
has, proportionally to its size, elongated wing and tail feathers. The
trumpeter and laugher, as their names express, utter a very different
coo from the other breeds. The fantail has thirty or even forty
tail-feathers, instead of twelve or fourteen, the normal number in all
the members of the great pigeon family: these feathers are kept expanded
and are carried so erect that in good birds the head and tail touch: the
oil-gland is quite aborted. Several other less distinct breeds might be
specified.

In the skeletons of the several breeds, the development of the bones of
the face, in length and breadth and curvature, differs enormously. The
shape, as well as the breadth and length of the ramus of the lower jaw,
varies in a highly remarkable manner. The caudal and sacral vertebrae
vary in number; as does the number of the ribs, together with their
relative breadth and the presence of processes. The size and shape of
the apertures in the sternum are highly variable; so is the degree
of divergence and relative size of the two arms of the furcula. The
proportional width of the gape of mouth, the proportional length of the
eyelids, of the orifice of the nostrils, of the tongue (not always in
strict correlation with the length of beak), the size of the crop and
of the upper part of the oesophagus; the development and abortion of
the oil-gland; the number of the primary wing and caudal feathers; the
relative length of the wing and tail to each other and to the body;
the relative length of the leg and foot; the number of scutellae on
the toes, the development of skin between the toes, are all points of
structure which are variable. The period at which the perfect plumage is
acquired varies, as does the state of the down with which the nestling
birds are clothed when hatched. The shape and size of the eggs vary. The
manner of flight, and in some breeds the voice and disposition, differ
remarkably. Lastly, in certain breeds, the males and females have come
to differ in a slight degree from each other.

Altogether at least a score of pigeons might be chosen, which, if shown
to an ornithologist, and he were told that they were wild birds, would
certainly be ranked by him as well-defined species. Moreover, I do not
believe that any ornithologist would in this case place the English
carrier, the short-faced tumbler, the runt, the barb, pouter, and
fantail in the same genus; more especially as in each of these breeds
several truly-inherited sub-breeds, or species, as he would call them,
could be shown him.

Great as are the differences between the breeds of the pigeon, I am
fully convinced that the common opinion of naturalists is correct,
namely, that all are descended from the rock-pigeon (Columba livia),
including under this term several geographical races or sub-species,
which differ from each other in the most trifling respects. As several
of the reasons which have led me to this belief are in some degree
applicable in other cases, I will here briefly give them. If the several
breeds are not varieties, and have not proceeded from the rock-pigeon,
they must have descended from at least seven or eight aboriginal stocks;
for it is impossible to make the present domestic breeds by the crossing
of any lesser number: how, for instance, could a pouter be produced
by crossing two breeds unless one of the parent-stocks possessed the
characteristic enormous crop? The supposed aboriginal stocks must all
have been rock-pigeons, that is, they did not breed or willingly perch
on trees. But besides C. livia, with its geographical sub-species, only
two or three other species of rock-pigeons are known; and these have
not any of the characters of the domestic breeds. Hence the supposed
aboriginal stocks must either still exist in the countries where they
were originally domesticated, and yet be unknown to ornithologists; and
this, considering their size, habits and remarkable characters, seems
improbable; or they must have become extinct in the wild state. But
birds breeding on precipices, and good flyers, are unlikely to be
exterminated; and the common rock-pigeon, which has the same habits with
the domestic breeds, has not been exterminated even on several of the
smaller British islets, or on the shores of the Mediterranean. Hence the
supposed extermination of so many species having similar habits with
the rock-pigeon seems a very rash assumption. Moreover, the several
above-named domesticated breeds have been transported to all parts of
the world, and, therefore, some of them must have been carried back
again into their native country; but not one has become wild or feral,
though the dovecot-pigeon, which is the rock-pigeon in a very slightly
altered state, has become feral in several places. Again, all recent
experience shows that it is difficult to get wild animals to breed
freely under domestication; yet on the hypothesis of the multiple origin
of our pigeons, it must be assumed that at least seven or eight species
were so thoroughly domesticated in ancient times by half-civilized man,
as to be quite prolific under confinement.

An argument of great weight, and applicable in several other cases, is,
that the above-specified breeds, though agreeing generally with the wild
rock-pigeon in constitution, habits, voice, colouring, and in most parts
of their structure, yet are certainly highly abnormal in other parts; we
may look in vain through the whole great family of Columbidae for a beak
like that of the English carrier, or that of the short-faced tumbler, or
barb; for reversed feathers like those of the Jacobin; for a crop like
that of the pouter; for tail-feathers like those of the fantail. Hence
it must be assumed, not only that half-civilized man succeeded in
thoroughly domesticating several species, but that he intentionally or
by chance picked out extraordinarily abnormal species; and further, that
these very species have since all become extinct or unknown. So many
strange contingencies are improbable in the highest degree.

Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, with white loins;
but the Indian sub-species, C. intermedia of Strickland, has this
part bluish. The tail has a terminal dark bar, with the outer feathers
externally edged at the base with white. The wings have two black bars.
Some semi-domestic breeds, and some truly wild breeds, have, besides the
two black bars, the wings chequered with black. These several marks do
not occur together in any other species of the whole family. Now, in
every one of the domestic breeds, taking thoroughly well-bred birds, all
the above marks, even to the white edging of the outer tail-feathers,
sometimes concur perfectly developed. Moreover, when birds belonging to
two or more distinct breeds are crossed, none of which are blue or have
any of the above-specified marks, the mongrel offspring are very apt
suddenly to acquire these characters. To give one instance out of
several which I have observed: I crossed some white fantails, which
breed very true, with some black barbs--and it so happens that blue
varieties of barbs are so rare that I never heard of an instance in
England; and the mongrels were black, brown and mottled. I also crossed
a barb with a spot, which is a white bird with a red tail and red spot
on the forehead, and which notoriously breeds very true; the mongrels
were dusky and mottled. I then crossed one of the mongrel barb-fantails
with a mongrel barb-spot, and they produced a bird of as beautiful a
blue colour, with the white loins, double black wing-bar, and barred and
white-edged tail-feathers, as any wild rock-pigeon! We can understand
these facts, on the well-known principle of reversion to ancestral
characters, if all the domestic breeds are descended from the
rock-pigeon. But if we deny this, we must make one of the two following
highly improbable suppositions. Either, first, that all the several
imagined aboriginal stocks were coloured and marked like the
rock-pigeon, although no other existing species is thus coloured and
marked, so that in each separate breed there might be a tendency to
revert to the very same colours and markings. Or, secondly, that each
breed, even the purest, has within a dozen, or at most within a score,
of generations, been crossed by the rock-pigeon: I say within a dozen
or twenty generations, for no instance is known of crossed descendants
reverting to an ancestor of foreign blood, removed by a greater number
of generations. In a breed which has been crossed only once the tendency
to revert to any character derived from such a cross will naturally
become less and less, as in each succeeding generation there will be
less of the foreign blood; but when there has been no cross, and there
is a tendency in the breed to revert to a character which was lost
during some former generation, this tendency, for all that we can see to
the contrary, may be transmitted undiminished for an indefinite
number of generations. These two distinct cases of reversion are often
confounded together by those who have written on inheritance.

Lastly, the hybrids or mongrels from between all the breeds of the
pigeon are perfectly fertile, as I can state from my own observations,
purposely made, on the most distinct breeds. Now, hardly any cases
have been ascertained with certainty of hybrids from two quite distinct
species of animals being perfectly fertile. Some authors believe
that long-continued domestication eliminates this strong tendency to
sterility in species. From the history of the dog, and of some other
domestic animals, this conclusion is probably quite correct, if applied
to species closely related to each other. But to extend it so far as to
suppose that species, aboriginally as distinct as carriers, tumblers,
pouters, and fantails now are, should yield offspring perfectly fertile,
inter se, seems to me rash in the extreme.

From these several reasons, namely, the improbability of man having
formerly made seven or eight supposed species of pigeons to breed freely
under domestication--these supposed species being quite unknown in a
wild state, and their not having become anywhere feral--these species
presenting certain very abnormal characters, as compared with all other
Columbidae, though so like the rock-pigeon in most other respects--the
occasional reappearance of the blue colour and various black marks in
all the breeds, both when kept pure and when crossed--and lastly, the
mongrel offspring being perfectly fertile--from these several reasons,
taken together, we may safely conclude that all our domestic breeds are
descended from the rock-pigeon or Columba livia with its geographical
sub-species.

In favour of this view, I may add, firstly, that the wild C. livia has
been found capable of domestication in Europe and in India; and that it
agrees in habits and in a great number of points of structure with all
the domestic breeds. Secondly, that although an English carrier or a
short-faced tumbler differs immensely in certain characters from the
rock-pigeon, yet that by comparing the several sub-breeds of these two
races, more especially those brought from distant countries, we can
make, between them and the rock-pigeon, an almost perfect series; so
we can in some other cases, but not with all the breeds. Thirdly,
those characters which are mainly distinctive of each breed are in each
eminently variable, for instance, the wattle and length of beak of
the carrier, the shortness of that of the tumbler, and the number of
tail-feathers in the fantail; and the explanation of this fact will be
obvious when we treat of selection. Fourthly, pigeons have been watched
and tended with the utmost care, and loved by many people. They have
been domesticated for thousands of years in several quarters of the
world; the earliest known record of pigeons is in the fifth Aegyptian
dynasty, about 3000 B.C., as was pointed out to me by Professor Lepsius;
but Mr. Birch informs me that pigeons are given in a bill of fare in
the previous dynasty. In the time of the Romans, as we hear from Pliny,
immense prices were given for pigeons; "nay, they are come to this pass,
that they can reckon up their pedigree and race." Pigeons were much
valued by Akber Khan in India, about the year 1600; never less than
20,000 pigeons were taken with the court. "The monarchs of Iran and
Turan sent him some very rare birds;" and, continues the courtly
historian, "His Majesty, by crossing the breeds, which method was never
practised before, has improved them astonishingly." About this same
period the Dutch were as eager about pigeons as were the old Romans. The
paramount importance of these considerations in explaining the immense
amount of variation which pigeons have undergone, will likewise be
obvious when we treat of selection. We shall then, also, see how it is
that the several breeds so often have a somewhat monstrous character.
It is also a most favourable circumstance for the production of distinct
breeds, that male and female pigeons can be easily mated for life; and
thus different breeds can be kept together in the same aviary.

I have discussed the probable origin of domestic pigeons at some,
yet quite insufficient, length; because when I first kept pigeons and
watched the several kinds, well knowing how truly they breed, I
felt fully as much difficulty in believing that since they had been
domesticated they had all proceeded from a common parent, as any
naturalist could in coming to a similar conclusion in regard to the
many species of finches, or other groups of birds, in nature. One
circumstance has struck me much; namely, that nearly all the breeders of
the various domestic animals and the cultivators of plants, with whom
I have conversed, or whose treatises I have read, are firmly convinced
that the several breeds to which each has attended, are descended
from so many aboriginally distinct species. Ask, as I have asked, a
celebrated raiser of Hereford cattle, whether his cattle might not have
descended from Long-horns, or both from a common parent-stock, and he
will laugh you to scorn. I have never met a pigeon, or poultry, or duck,
or rabbit fancier, who was not fully convinced that each main breed was
descended from a distinct species. Van Mons, in his treatise on pears
and apples, shows how utterly he disbelieves that the several sorts,
for instance a Ribston-pippin or Codlin-apple, could ever have proceeded
from the seeds of the same tree. Innumerable other examples could be
given. The explanation, I think, is simple: from long-continued study
they are strongly impressed with the differences between the several
races; and though they well know that each race varies slightly, for
they win their prizes by selecting such slight differences, yet they
ignore all general arguments, and refuse to sum up in their minds slight
differences accumulated during many successive generations. May not
those naturalists who, knowing far less of the laws of inheritance than
does the breeder, and knowing no more than he does of the intermediate
links in the long lines of descent, yet admit that many of our domestic
races are descended from the same parents--may they not learn a lesson
of caution, when they deride the idea of species in a state of nature
being lineal descendants of other species?

PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND THEIR EFFECTS.

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 can not 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 can not 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.

The great power of this principle of selection is not hypothetical.
It is certain that several of our eminent breeders have, even within a
single lifetime, modified to a large extent their breeds of cattle
and sheep. In order fully to realise what they have done it is almost
necessary to read several of the many treatises devoted to this subject,
and to inspect the animals. Breeders habitually speak of an animal's
organisation as something plastic, which they can model almost as they
please. If I had space I could quote numerous passages to this effect
from highly competent authorities. Youatt, who was probably better
acquainted with the works of agriculturalists than almost any other
individual, and who was himself a very good judge of animals, speaks of
the principle of selection as "that which enables the agriculturist, not
only to modify the character of his flock, but to change it altogether.
It is the magician's wand, by means of which he may summon into life
whatever form and mould he pleases." Lord Somerville, speaking of
what breeders have done for sheep, says: "It would seem as if they had
chalked out upon a wall a form perfect in itself, and then had given it
existence." In Saxony the importance of the principle of selection in
regard to merino sheep is so fully recognised, that men follow it as a
trade: the sheep are placed on a table and are studied, like a picture
by a connoisseur; this is done three times at intervals of months, and
the sheep are each time marked and classed, so that the very best may
ultimately be selected for breeding.

What English breeders have actually effected is proved by the enormous
prices given for animals with a good pedigree; and these have been
exported to almost every quarter of the world. The improvement is by no
means generally due to crossing different breeds; all the best breeders
are strongly opposed to this practice, except sometimes among closely
allied sub-breeds. And when a cross has been made, the closest selection
is far more indispensable even than in ordinary cases. If selection
consisted merely in separating some very distinct variety and breeding
from it, the principle would be so obvious as hardly to be worth
notice; but its importance consists in the great effect produced by
the accumulation in one direction, during successive generations, of
differences absolutely inappreciable by an uneducated eye--differences
which I for one have vainly attempted to appreciate. Not one man in
a thousand has accuracy of eye and judgment sufficient to become an
eminent breeder. If gifted with these qualities, and he studies his
subject for years, and devotes his lifetime to it with indomitable
perseverance, he will succeed, and may make great improvements; if he
wants any of these qualities, he will assuredly fail. Few would readily
believe in the natural capacity and years of practice requisite to
become even a skilful pigeon-fancier.

The same principles are followed by horticulturists; but the variations
are here often more abrupt. No one supposes that our choicest
productions have been produced by a single variation from the aboriginal
stock. We have proofs that this is not so in several cases in which
exact records have been kept; thus, to give a very trifling instance,
the steadily increasing size of the common gooseberry may be quoted.
We see an astonishing improvement in many florists' flowers, when the
flowers of the present day are compared with drawings made only
twenty or thirty years ago. When a race of plants is once pretty well
established, the seed-raisers do not pick out the best plants, but
merely go over their seed-beds, and pull up the "rogues," as they call
the plants that deviate from the proper standard. With animals this kind
of selection is, in fact, likewise followed; for hardly any one is so
careless as to breed from his worst animals.

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, colour, 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 ensure 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.

It may be objected that the principle of selection has been reduced to
methodical practice for scarcely more than three-quarters of a century;
it has certainly been more attended to of late years, and many treatises
have been published on the subject; and the result has been, in a
corresponding degree, rapid and important. But it is very far from
true that the principle is a modern discovery. I could give several
references to works of high antiquity, in which the full importance of
the principle is acknowledged. In rude and barbarous periods of English
history choice animals were often imported, and laws were passed to
prevent their exportation: the destruction of horses under a certain
size was ordered, and this may be compared to the "roguing" of plants
by nurserymen. The principle of selection I find distinctly given in an
ancient Chinese encyclopaedia. Explicit rules are laid down by some of
the Roman classical writers. From passages in Genesis, it is clear that
the colour of domestic animals was at that early period attended to.
Savages now sometimes cross their dogs with wild canine animals, to
improve the breed, and they formerly did so, as is attested by passages
in Pliny. The savages in South Africa match their draught cattle by
colour, as do some of the Esquimaux their teams of dogs. Livingstone
states that good domestic breeds are highly valued by the negroes in the
interior of Africa who have not associated with Europeans. Some of these
facts do not show actual selection, but they show that the breeding of
domestic animals was carefully attended to in ancient times, and is now
attended to by the lowest savages. It would, indeed, have been a strange
fact, had attention not been paid to breeding, for the inheritance of
good and bad qualities is so obvious.

UNCONSCIOUS SELECTION.

At the present time, eminent breeders try by methodical selection, with
a distinct object in view, to make a new strain or sub-breed, superior
to anything of the kind in the country. But, for our purpose, a form of
selection, which may be called unconscious, and which results from every
one trying to possess and breed from the best individual animals, is
more important. 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, in the same way as
Bakewell, Collins, etc., by this very same process, only carried on more
methodically, did greatly modify, even during their lifetimes, the forms
and qualities of their cattle. Slow and insensible changes of this kind
could never be recognised unless actual measurements or careful drawings
of the breeds in question have been made long ago, which may serve for
comparison. In some cases, however, unchanged, or but little changed,
individuals of the same breed exist in less civilised districts, where
the breed has been less improved. There is reason to believe that King
Charles' spaniel has been unconsciously modified to a large extent
since the time of that monarch. Some highly competent authorities are
convinced that the setter is directly derived from the spaniel, and
has probably been slowly altered from it. It is known that the English
pointer has been greatly changed within the last century, and in this
case the change has, it is believed, been chiefly effected by crosses
with the foxhound; but what concerns us is, that the change has been
effected unconsciously and gradually, and yet so effectually that,
though the old Spanish pointer certainly came from Spain, Mr. Borrow
has not seen, as I am informed by him, any native dog in Spain like our
pointer.

By a similar process of selection, and by careful training, English
race-horses have come to surpass in fleetness and size the parent Arabs,
so that the latter, by the regulations for the Goodwood Races, are
favoured in the weights which they carry. Lord Spencer and others have
shown how the cattle of England have increased in weight and in early
maturity, compared with the stock formerly kept in this country. By
comparing the accounts given in various old treatises of the former
and present state of carrier and tumbler pigeons in Britain, India,
and Persia, we can trace the stages through which they have insensibly
passed, and come to differ so greatly from the rock-pigeon.

Youatt gives an excellent illustration of the effects of a course of
selection which may be considered as unconscious, in so far that the
breeders could never have expected, or even wished, to produce the
result which ensued--namely, the production of the distinct strains. The
two flocks of Leicester sheep kept by Mr. Buckley and Mr. Burgess, as
Mr. Youatt remarks, "Have been purely bred from the original stock
of Mr. Bakewell for upwards of fifty years. There is not a suspicion
existing in the mind of any one at all acquainted with the subject that
the owner of either of them has deviated in any one instance from the
pure blood of Mr. Bakewell's flock, and yet the difference between the
sheep possessed by these two gentlemen is so great that they have the
appearance of being quite different varieties."

If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one animal
particularly useful to them, for any special purpose, would be carefully
preserved during famines and other accidents, to which savages are
so liable, and such choice animals would thus generally leave more
offspring than the inferior ones; so that in this case there would be a
kind of unconscious selection going on. We see the value set on animals
even by the barbarians of Tierra del Fuego, by their killing and
devouring their old women, in times of dearth, as of less value than
their dogs.

In plants the same gradual process of improvement through the occasional
preservation of the best individuals, whether or not sufficiently
distinct to be ranked at their first appearance as distinct varieties,
and whether or not two or more species or races have become blended
together by crossing, may plainly be recognised in the increased size
and beauty which we now see in the varieties of the heartsease, rose,
pelargonium, dahlia, and other plants, when compared with the older
varieties or with their parent-stocks. No one would ever expect to get
a first-rate heartsease or dahlia from the seed of a wild plant. No one
would expect to raise a first-rate melting pear from the seed of a wild
pear, though he might succeed from a poor seedling growing wild, if it
had come from a garden-stock. The pear, though cultivated in classical
times, appears, from Pliny's description, to have been a fruit of very
inferior quality. I have seen great surprise expressed in horticultural
works at the wonderful skill of gardeners in having produced such
splendid results from such poor materials; but the art has been simple,
and, as far as the final result is concerned, has been followed almost
unconsciously. It has consisted in always cultivating the best known
variety, sowing its seeds, and, when a slightly better variety chanced
to appear, selecting it, and so onwards. But the gardeners of the
classical period, who cultivated the best pears which they could
procure, never thought what splendid fruit we should eat; though we
owe our excellent fruit in some small degree to their having naturally
chosen and preserved the best varieties they could anywhere find.

A large amount of change, thus slowly and unconsciously accumulated,
explains, as I believe, the well-known fact, that in a number of cases
we cannot recognise, and therefore do not know, the wild parent-stocks
of the plants which have been longest cultivated in our flower and
kitchen gardens. 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
uncivilised 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 civilised.

In regard to the domestic animals kept by uncivilised man, it should
not be overlooked that they almost always have to struggle for their
own food, at least during certain seasons. And in two countries very
differently circumstanced, individuals of the same species, having
slightly different constitutions or structure, would often succeed
better in the one country than in the other, and thus by a process of
"natural selection," as will hereafter be more fully explained, two
sub-breeds might be formed. This, perhaps, partly explains why the
varieties kept by savages, as has been remarked by some authors,
have more of the character of true species than the varieties kept in
civilised countries.

On the view here given of the important part which selection by man has
played, it becomes at once obvious, how it is that our domestic races
show adaptation in their structure or in their habits to man's wants
or fancies. We can, I think, further understand the frequently abnormal
character of our domestic races, and likewise their differences being so
great in external characters, and relatively so slight in internal parts
or organs. Man can hardly select, or only with much difficulty, any
deviation of structure excepting such as is externally visible; and
indeed he rarely cares for what is internal. He can never act by
selection, excepting on variations which are first given to him in some
slight degree by nature. No man would ever try to make a fantail till he
saw a pigeon with a tail developed in some slight degree in an unusual
manner, or a pouter till he saw a pigeon with a crop of somewhat unusual
size; and the more abnormal or unusual any character was when it first
appeared, the more likely it would be to catch his attention. But to use
such an expression as trying to make a fantail is, I have no doubt, in
most cases, utterly incorrect. The man who first selected a pigeon
with a slightly larger tail, never dreamed what the descendants of
that pigeon would become through long-continued, partly unconscious and
partly methodical, selection. Perhaps the parent bird of all fantails
had only fourteen tail-feathers somewhat expanded, like the present Java
fantail, or like individuals of other and distinct breeds, in which as
many as seventeen tail-feathers have been counted. Perhaps the first
pouter-pigeon did not inflate its crop much more than the turbit now
does the upper part of its oesophagus--a habit which is disregarded by
all fanciers, as it is not one of the points of the breed.

Nor let it be thought that some great deviation of structure would
be necessary to catch the fancier's eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one's own possession. Nor must the value which would formerly
have been set on any slight differences in the individuals of the same
species, be judged of by the value which is now set on them, after
several breeds have fairly been established. It is known that with
pigeons many slight variations now occasionally appear, but these are
rejected as faults or deviations from the standard of perfection in
each breed. The common goose has not given rise to any marked varieties;
hence the Toulouse and the common breed, which differ only in colour,
that most fleeting of characters, have lately been exhibited as distinct
at our poultry-shows.

These views appear to explain what has sometimes been noticed, namely,
that we know hardly anything about the origin or history of any of our
domestic breeds. But, in fact, a breed, like a dialect of a language,
can hardly be said to have a distinct origin. A man preserves and breeds
from an individual with some slight deviation of structure, or takes
more care than usual in matching his best animals, and thus improves
them, and the improved animals slowly spread in the immediate
neighbourhood. But they will as yet hardly have a distinct name,
and from being only slightly valued, their history will have been
disregarded. When further improved by the same slow and gradual process,
they will spread more widely, and will be recognised as something
distinct and valuable, and will then probably first receive a provincial
name. In semi-civilised countries, with little free communication, the
spreading of a new sub-breed will be a slow process. As soon as the
points of value are once acknowledged, the principle, as I have called
it, of unconscious selection will always tend--perhaps more at one
period than at another, as the breed rises or falls in fashion--perhaps
more in one district than in another, according to the state of
civilisation of the inhabitants--slowly to add to the characteristic
features of the breed, whatever they may be. But the chance will be
infinitely small of any record having been preserved of such slow,
varying, and insensible changes.

CIRCUMSTANCES FAVOURABLE TO MAN'S POWER OF SELECTION.

I will now say a few words on the circumstances, favourable or the
reverse, to man's power of selection. A high degree of variability is
obviously favourable, as freely giving the materials for selection to
work on; not that mere individual differences are not amply sufficient,
with extreme care, to allow of the accumulation of a large amount
of modification in almost any desired direction. But as variations
manifestly useful or pleasing to man appear only occasionally, the
chance of their appearance will be much increased by a large number of
individuals being kept. Hence number is of the highest importance for
success. On this principle Marshall formerly remarked, with respect
to the sheep of part of Yorkshire, "As they generally belong to poor
people, and are mostly IN SMALL LOTS, they never can be improved." On
the other hand, nurserymen, from keeping large stocks of the same plant,
are generally far more successful than amateurs in raising new and
valuable varieties. A large number of individuals of an animal or
plant can be reared only where the conditions for its propagation are
favourable. When the individuals are scanty all will be allowed to
breed, whatever their quality may be, and this will effectually prevent
selection. But probably the most important element is that the animal or
plant should be so highly valued by man, that the closest attention is
paid to even the slightest deviations in its qualities or structure.
Unless such attention be paid nothing can be effected. I have seen it
gravely remarked, that it was most fortunate that the strawberry began
to vary just when gardeners began to attend to this plant. No doubt the
strawberry had always varied since it was cultivated, but the slight
varieties had been neglected. As soon, however, as gardeners picked out
individual plants with slightly larger, earlier, or better fruit, and
raised seedlings from them, and again picked out the best seedlings and
bred from them, then (with some aid by crossing distinct species)
those many admirable varieties of the strawberry were raised which have
appeared during the last half-century.

With animals, facility in preventing crosses is an important element
in the formation of new races--at least, in a country which is already
stocked with other races. In this respect enclosure of the land plays a
part. Wandering savages or the inhabitants of open plains rarely possess
more than one breed of the same species. Pigeons can be mated for life,
and this is a great convenience to the fancier, for thus many races may
be improved and kept true, though mingled in the same aviary; and this
circumstance must have largely favoured the formation of new breeds.
Pigeons, I may add, can be propagated in great numbers and at a very
quick rate, and inferior birds may be freely rejected, as when killed
they serve for food. On the other hand, cats, from their nocturnal
rambling habits, can not be easily matched, and, although so much valued
by women and children, we rarely see a distinct breed long kept up; such
breeds as we do sometimes see are almost always imported from some other
country. Although I do not doubt that some domestic animals vary less
than others, yet the rarity or absence of distinct breeds of the cat,
the donkey, peacock, goose, etc., may be attributed in main part
to selection not having been brought into play: in cats, from the
difficulty in pairing them; in donkeys, from only a few being kept by
poor people, and little attention paid to their breeding; for recently
in certain parts of Spain and of the United States this animal has been
surprisingly modified and improved by careful selection; in peacocks,
from not being very easily reared and a large stock not kept; in geese,
from being valuable only for two purposes, food and feathers, and more
especially from no pleasure having been felt in the display of distinct
breeds; but the goose, under the conditions to which it is exposed when
domesticated, seems to have a singularly inflexible organisation, though
it has varied to a slight extent, as I have elsewhere described.

Some authors have maintained that the amount of variation in our
domestic productions is soon reached, and can never afterward be
exceeded. It would be somewhat rash to assert that the limit has been
attained in any one case; for almost all our animals and plants have
been greatly improved in many ways within a recent period; and this
implies variation. It would be equally rash to assert that characters
now increased to their utmost limit, could not, after remaining fixed
for many centuries, again vary under new conditions of life. No doubt,
as Mr. Wallace has remarked with much truth, a limit will be at last
reached. For instance, there must be a limit to the fleetness of any
terrestrial animal, as this will be determined by the friction to
be overcome, the weight of the body to be carried, and the power of
contraction in the muscular fibres. But what concerns us is that the
domestic varieties of the same species differ from each other in almost
every character, which man has attended to and selected, more than do
the distinct species of the same genera. Isidore Geoffroy St. Hilaire
has proved this in regard to size, and so it is with colour, and
probably with the length of hair. With respect to fleetness, which
depends on many bodily characters, Eclipse was far fleeter, and
a dray-horse is comparably stronger, than any two natural species
belonging to the same genus. So with plants, the seeds of the different
varieties of the bean or maize probably differ more in size than do the
seeds of the distinct species in any one genus in the same two families.
The same remark holds good in regard to the fruit of the several
varieties of the plum, and still more strongly with the melon, as well
as in many other analogous cases.

To sum up on the origin of our domestic races of animals and plants.
Changed conditions of life are of the highest importance in causing
variability, both by acting directly on the organisation, and
indirectly by affecting the reproductive system. It is not probable
that variability is an inherent and necessary contingent, under all
circumstances. The greater or less force of inheritance and reversion
determine whether variations shall endure. Variability is governed
by many unknown laws, of which correlated growth is probably the most
important. Something, but how much we do not know, may be attributed to
the definite action of the conditions of life. Some, perhaps a great,
effect may be attributed to the increased use or disuse of parts. The
final result is thus rendered infinitely complex. In some cases the
intercrossing of aboriginally distinct species appears to have played
an important part in the origin of our breeds. When several breeds have
once been formed in any country, their occasional intercrossing, with
the aid of selection, has, no doubt, largely aided in the formation
of new sub-breeds; but the importance of crossing has been much
exaggerated, both in regard to animals and to those plants which are
propagated by seed. With plants which are temporarily propagated by
cuttings, buds, etc., the importance of crossing is immense; for the
cultivator may here disregard the extreme variability both of hybrids
and of mongrels, and the sterility of hybrids; but plants not propagated
by seed are of little importance to us, for their endurance is only
temporary. 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.



CHAPTER II. VARIATION UNDER NATURE.

 Variability--Individual differences--Doubtful species--Wide ranging,
 much diffused, and common species, vary most--Species of the larger
 genera in each country vary more frequently than the species of the
 smaller genera--Many of the species of the larger genera resemble
 varieties in being very closely, but unequally, related to each other,
 and in having restricted ranges.


Before applying the principles arrived at in the last chapter to organic
beings in a state of nature, we must briefly discuss whether these
latter are subject to any variation. To treat this subject properly, a
long catalogue of dry facts ought to be given; but these I shall reserve
for a future work. Nor shall I here discuss the various definitions
which have been given of the term species. No one definition has
satisfied all naturalists; yet every naturalist knows vaguely what
he means when he speaks of a species. Generally the term includes the
unknown element of a distinct act of creation. The term "variety" is
almost equally difficult to define; but here community of descent is
almost universally implied, though it can rarely be proved. We have also
what are called monstrosities; but they graduate into varieties. By a
monstrosity I presume is meant some considerable deviation of structure,
generally injurious, or not useful to the species. Some authors use
the term "variation" in a technical sense, as implying a modification
directly due to the physical conditions of life; and "variations" in
this sense are supposed not to be inherited; but who can say that the
dwarfed condition of shells in the brackish waters of the Baltic, or
dwarfed plants on Alpine summits, or the thicker fur of an animal from
far northwards, would not in some cases be inherited for at least a few
generations? And in this case I presume that the form would be called a
variety.

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 favourable
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. But I shall have to return
in a future chapter to the preservation and perpetuation of single or
occasional variations.

INDIVIDUAL DIFFERENCES.

The many slight differences which appear in the offspring from the
same parents, or which it may be presumed have thus arisen, from being
observed in the individuals of the same species inhabiting the same
confined locality, may be called individual differences. No one supposes
that all the individuals of the same species are cast in the same actual
mould. These individual differences are of the highest importance for
us, for they are often inherited, as must be familiar to every one;
and they thus afford materials for natural 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 the 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. It should be remembered
that systematists are far from being pleased at finding variability
in important characters, and that there are not many men who will
laboriously examine internal and important organs, and compare them in
many specimens of the same species. It would never have been expected
that the branching of the main nerves close to the great central
ganglion of an insect would have been variable in the same species;
it might have been thought that changes of this nature could have been
effected only by slow degrees; yet Sir J. Lubbock has shown a degree of
variability in these main nerves in Coccus, which may almost be compared
to the irregular branching of the stem of a tree. This philosophical
naturalist, I may add, has also shown that the muscles in the larvae
of certain insects are far from uniform. Authors sometimes argue in a
circle when they state that important organs never vary; for these
same authors practically rank those parts as important (as some few
naturalists have honestly confessed) which do not vary; and, under
this point of view, no instance will ever be found of an important part
varying; but under any other point of view many instances assuredly can
be given.

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 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 among 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 and rendered definite by natural selection, as hereafter to be
explained.

Individuals of the same species often present, as is known to every one,
great differences of structure, independently of variation, as in the
two sexes of various animals, in the two or three castes of sterile
females or workers among insects, and in the immature and larval states
of many of the lower animals. There are, also, cases of dimorphism and
trimorphism, both with animals and plants. Thus, Mr. Wallace, who has
lately called attention to the subject, has shown that the females of
certain species of butterflies, in the Malayan Archipelago, regularly
appear under two or even three conspicuously distinct forms, not
connected by intermediate varieties. Fritz Muller has described
analogous but more extraordinary cases with the males of certain
Brazilian Crustaceans: thus, the male of a Tanais regularly occurs
under two distinct forms; one of these has strong and differently shaped
pincers, and the other has antennae much more abundantly furnished with
smelling-hairs. Although in most of these cases, the two or three forms,
both with animals and plants, are not now connected by intermediate
gradations, it is possible that they were once thus connected. Mr.
Wallace, for instance, describes a certain butterfly which presents in
the same island a great range of varieties connected by intermediate
links, and the extreme links of the chain closely resemble the two forms
of an allied dimorphic species inhabiting another part of the Malay
Archipelago. Thus also with ants, the several worker-castes are
generally quite distinct; but in some cases, as we shall hereafter see,
the castes are connected together by finely graduated varieties. So it
is, as I have myself observed, with some dimorphic plants. It certainly
at first appears a highly remarkable fact that the same female butterfly
should have the power of producing at the same time three distinct
female forms and a male; and that an hermaphrodite plant should produce
from the same seed-capsule three distinct hermaphrodite forms, bearing
three different kinds of females and three or even six different kinds
of males. Nevertheless these cases are only exaggerations of the common
fact that the female produces offspring of two sexes which sometimes
differ from each other in a wonderful manner.

DOUBTFUL SPECIES.

The forms which possess in some considerable degree the character of
species, but which are so closely similar to other forms, or are so
closely linked to them by intermediate gradations, that naturalists do
not like to rank them as distinct species, are in several respects the
most important for us. We have every reason to believe that many of
these doubtful and closely allied forms have permanently retained their
characters for a long time; for as long, as far as we know, as have good
and true species. Practically, when a naturalist can unite by means of
intermediate links any two forms, he treats the one as a variety of the
other, ranking the most common, but sometimes the one first described
as the species, and the other as the variety. But cases of great
difficulty, which I will not here enumerate, sometimes arise in deciding
whether or not to rank one form as a variety of another, even when
they are closely connected by intermediate links; nor will the commonly
assumed hybrid nature of the intermediate forms always remove the
difficulty. In very many cases, however, one form is ranked as a variety
of another, not because the intermediate links have actually been found,
but because analogy leads the observer to suppose either that they do
now somewhere exist, or may formerly have existed; and here a wide door
for the entry of doubt and conjecture is opened.

Hence, in determining whether a form should be ranked as a species or
a variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as species
by at least some competent judges.

That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France, or
of the United States, drawn up by different botanists, and see what
a surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H.C. Watson, to whom I
lie under deep obligation for assistance of all kinds, has marked for
me 182 British plants, which are generally considered as varieties, but
which have all been ranked by botanists as species; and in making this
list he has omitted many trifling varieties, but which nevertheless have
been ranked by some botanists as species, and he has entirely omitted
several highly polymorphic genera. Under genera, including the most
polymorphic forms, Mr. Babington gives 251 species, whereas Mr. Bentham
gives only 112--a difference of 139 doubtful forms! Among animals which
unite for each birth, and which are highly locomotive, doubtful forms,
ranked by one zoologist as a species and by another as a variety, can
rarely be found within the same country, but are common in separated
areas. How many of the birds and insects in North America and Europe,
which differ very slightly from each other, have been ranked by one
eminent naturalist as undoubted species, and by another as varieties,
or, as they are often called, geographical races! Mr. Wallace, in
several valuable papers on the various animals, especially on the
Lepidoptera, inhabiting the islands of the great Malayan Archipelago,
shows that they may be classed under four heads, namely, as variable
forms, as local forms, as geographical races or sub-species, and as true
representative species. The first or variable forms vary much within the
limits of the same island. The local forms are moderately constant and
distinct in each separate island; but when all from the several islands
are compared together, the differences are seen to be so slight and
graduated that it is impossible to define or describe them, though
at the same time the extreme forms are sufficiently distinct. The
geographical races or sub-species are local forms completely fixed and
isolated; but as they do not differ from each other by strongly marked
and important characters, "There is no possible test but individual
opinion to determine which of them shall be considered as species and
which as varieties." Lastly, representative species fill the same
place in the natural economy of each island as do the local forms and
sub-species; but as they are distinguished from each other by a greater
amount of difference than that between the local forms and sub-species,
they are almost universally ranked by naturalists as true species.
Nevertheless, no certain criterion can possibly be given by which
variable forms, local forms, sub species and representative species can
be recognised.

Many years ago, when comparing, and seeing others compare, the birds
from the closely neighbouring islands of the Galapagos Archipelago,
one with another, and with those from the American mainland, I was
much struck how entirely vague and arbitrary is the distinction between
species and varieties. On the islets of the little Madeira group there
are many insects which are characterized as varieties in Mr. Wollaston's
admirable work, but which would certainly be ranked as distinct species
by many entomologists. Even Ireland has a few animals, now generally
regarded as varieties, but which have been ranked as species by some
zoologists. Several experienced ornithologists consider our British red
grouse as only a strongly marked race of a Norwegian species, whereas
the greater number rank it as an undoubted species peculiar to Great
Britain. A wide distance between the homes of two doubtful forms leads
many naturalists to rank them as distinct species; but what distance, it
has been well asked, will suffice if that between America and Europe
is ample, will that between Europe and the Azores, or Madeira, or the
Canaries, or between the several islets of these small archipelagos, be
sufficient?

Mr. B.D. Walsh, a distinguished entomologist of the United States, has
described what he calls Phytophagic varieties and Phytophagic species.
Most vegetable-feeding insects live on one kind of plant or on one
group of plants; some feed indiscriminately on many kinds, but do not
in consequence vary. In several cases, however, insects found living on
different plants, have been observed by Mr. Walsh to present in their
larval or mature state, or in both states, slight, though constant
differences in colour, size, or in the nature of their secretions.
In some instances the males alone, in other instances, both males and
females, have been observed thus to differ in a slight degree. When the
differences are rather more strongly marked, and when both sexes and
all ages are affected, the forms are ranked by all entomologists as good
species. But no observer can determine for another, even if he can do so
for himself, which of these Phytophagic forms ought to be called species
and which varieties. Mr. Walsh ranks the forms which it may be supposed
would freely intercross, as varieties; and those which appear to have
lost this power, as species. As the differences depend on the insects
having long fed on distinct plants, it cannot be expected that
intermediate links connecting the several forms should now be found.
The naturalist thus loses his best guide in determining whether to rank
doubtful forms as varieties or species. This likewise necessarily occurs
with closely allied organisms, which inhabit distinct continents or
islands. When, on the other hand, an animal or plant ranges over the
same continent, or inhabits many islands in the same archipelago, and
presents different forms in the different areas, there is always a
good chance that intermediate forms will be discovered which will link
together the extreme states; and these are then degraded to the rank of
varieties.

Some few naturalists maintain that animals never present varieties; but
then these same naturalists rank the slightest difference as of specific
value; and when the same identical form is met with in two distant
countries, or in two geological formations, they believe that two
distinct species are hidden under the same dress. The term species thus
comes to be a mere useless abstraction, implying and assuming a separate
act of creation. It is certain that many forms, considered by highly
competent judges to be varieties, resemble species so completely in
character that they have been thus ranked by other highly competent
judges. But to discuss whether they ought to be called species or
varieties, before any definition of these terms has been generally
accepted, is vainly to beat the air.

Many of the cases of strongly marked varieties or doubtful species well
deserve consideration; for several interesting lines of argument, from
geographical distribution, analogical variation, hybridism, etc., have
been brought to bear in the attempt to determine their rank; but space
does not here permit me to discuss them. Close investigation, in many
cases, will no doubt bring naturalists to agree how to rank doubtful
forms. Yet it must be confessed that it is in the best known countries
that we find the greatest number of them. I have been struck with the
fact that if any animal or plant in a state of nature be highly useful
to man, or from any cause closely attracts his attention, varieties of
it will almost universally be found recorded. These varieties, moreover,
will often be ranked by some authors as species. Look at the common oak,
how closely it has been studied; yet a German author makes more than a
dozen species out of forms, which are almost universally considered
by other botanists to be varieties; and in this country the highest
botanical authorities and practical men can be quoted to show that the
sessile and pedunculated oaks are either good and distinct species or
mere varieties.

I may here allude to a remarkable memoir lately published by A. de
Candolle, on the oaks of the whole world. No one ever had more ample
materials for the discrimination of the species, or could have worked on
them with more zeal and sagacity. He first gives in detail all the many
points of structure which vary in the several species, and estimates
numerically the relative frequency of the variations. He specifies above
a dozen characters which may be found varying even on the same branch,
sometimes according to age or development, sometimes without any
assignable reason. Such characters are not of course of specific value,
but they are, as Asa Gray has remarked in commenting on this memoir,
such as generally enter into specific definitions. De Candolle then goes
on to say that he gives the rank of species to the forms that differ by
characters never varying on the same tree, and never found connected
by intermediate states. After this discussion, the result of so much
labour, he emphatically remarks: "They are mistaken, who repeat that the
greater part of our species are clearly limited, and that the doubtful
species are in a feeble minority. This seemed to be true, so long as
a genus was imperfectly known, and its species were founded upon a few
specimens, that is to say, were provisional. Just as we come to know
them better, intermediate forms flow in, and doubts as to specific
limits augment." He also adds that it is the best known species which
present the greatest number of spontaneous varieties and sub-varieties.
Thus Quercus robur has twenty-eight varieties, all of which, excepting
six, are clustered round three sub-species, namely Q. pedunculata,
sessiliflora and pubescens. The forms which connect these three
sub-species are comparatively rare; and, as Asa Gray again remarks, if
these connecting forms which are now rare were to become totally extinct
the three sub-species would hold exactly the same relation to each other
as do the four or five provisionally admitted species which closely
surround the typical Quercus robur. Finally, De Candolle admits that
out of the 300 species, which will be enumerated in his Prodromus
as belonging to the oak family, at least two-thirds are provisional
species, that is, are not known strictly to fulfil the definition above
given of a true species. It should be added that De Candolle no longer
believes that species are immutable creations, but concludes that the
derivative theory is the most natural one, "and the most accordant with
the known facts in palaeontology, geographical botany and zoology, of
anatomical structure and classification."

When a young naturalist commences the study of a group of organisms
quite unknown to him he is at first much perplexed in determining what
differences to consider as specific and what as varietal; for he knows
nothing of the amount and kind of variation to which the group is
subject; and this shows, at least, how very generally there is some
variation. But if he confine his attention to one class within one
country he will soon make up his mind how to rank most of the doubtful
forms. His general tendency will be to make many species, for he will
become impressed, just like the pigeon or poultry fancier before alluded
to, with the amount of difference in the forms which he is continually
studying; and he has little general knowledge of analogical variation
in other groups and in other countries by which to correct his first
impressions. As he extends the range of his observations he will meet
with more cases of difficulty; for he will encounter a greater number of
closely-allied forms. But if his observations be widely extended he
will in the end generally be able to make up his own mind; but he will
succeed in this at the expense of admitting much variation, and the
truth of this admission will often be disputed by other naturalists.
When he comes to study allied forms brought from countries not now
continuous, in which case he cannot hope to find intermediate links,
he will be compelled to trust almost entirely to analogy, and his
difficulties will rise to a climax.

Certainly no clear line of demarcation has as yet been drawn between
species and sub-species--that is, the forms which in the opinion of some
naturalists come very near to, but do not quite arrive at, the rank of
species; or, again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These differences
blend into each other by an insensible series; and a series impresses
the mind with the idea of an actual passage.

Hence I look at individual differences, though of small interest to the
systematist, as of the highest importance for us, as being the first
step towards 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 sub-species,
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.

It need not be supposed that all varieties or incipient species attain
the rank of species. They may become extinct, or they may endure as
varieties for very long periods, as has been shown to be the case by Mr.
Wollaston with the varieties of certain fossil land-shells in Madeira,
and with plants by Gaston de Saporta. If a variety were to flourish so
as to exceed in numbers the parent species, it would then rank as the
species, and the species as the variety; or it might come to supplant
and exterminate the parent species; or both might co-exist, and both
rank as independent species. But we shall hereafter return to this
subject.

From these remarks it will be seen that I look at the term species
as one arbitrarily given, for the sake of convenience, to a set
of individuals closely resembling each other, and that it does not
essentially differ from the term variety, which is given to less
distinct and more fluctuating forms. The term variety, again,
in comparison with mere individual differences, is also applied
arbitrarily, for convenience sake.

WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST.

Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H.C.
Watson, to whom I am much indebted for valuable advice and assistance
on this subject, soon convinced me that there were many difficulties, as
did subsequently Dr. Hooker, even in stronger terms. I shall reserve for
a future work the discussion of these difficulties, and the tables of
the proportional numbers of the varying species. Dr. Hooker permits me
to add that after having carefully read my manuscript, and examined
the tables, he thinks that the following statements are fairly well
established. The whole subject, however, treated as it necessarily here
is with much brevity, is rather perplexing, and allusions cannot be
avoided to the "struggle for existence," "divergence of character," and
other questions, hereafter to be discussed.

Alphonse de Candolle and others have shown that plants which have
very wide ranges generally present varieties; and this might have been
expected, as they are exposed to diverse physical conditions, and as
they come into competition (which, as we shall hereafter see, is a far
more important circumstance) with different sets of organic beings. But
my tables further show that, in any limited country, the species which
are the most common, that is abound most in individuals, and the species
which are most widely diffused within their own country (and this is a
different consideration from wide range, and to a certain extent from
commonness), oftenest give rise to varieties sufficiently well-marked to
have been recorded in botanical works. Hence it is the most flourishing,
or, as they may be called, the dominant species--those which range
widely, are the most diffused in their own country, and are the most
numerous in individuals--which oftenest produce well-marked varieties,
or, as I consider them, incipient species. And this, perhaps, might have
been anticipated; for, as varieties, in order to become in any degree
permanent, necessarily have to struggle with the other inhabitants of
the country, the species which are already dominant will be the most
likely to yield offspring, which, though in some slight degree modified,
still inherit those advantages that enabled their parents to become
dominant over their compatriots. In these remarks on predominence, it
should be understood that reference is made only to the forms which come
into competition with each other, and more especially to the members
of the same genus or class having nearly similar habits of life. With
respect to the number of individuals or commonness of species, the
comparison of course relates only to the members of the same group. One
of the higher plants may be said to be dominant if it be more numerous
in individuals and more widely diffused than the other plants of the
same country, which live under nearly the same conditions. A plant of
this kind is not the less dominant because some conferva inhabiting
the water or some parasitic fungus is infinitely more numerous in
individuals, and more widely diffused. But if the conferva or parasitic
fungus exceeds its allies in the above respects, it will then be
dominant within its own class.

SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY THAN
THE SPECIES OF THE SMALLER GENERA.

If the plants inhabiting a country as described in any Flora, be divided
into two equal masses, all those in the larger genera (i.e., those
including many species) being placed on one side, and all those in the
smaller genera on the other side, the former will be found to include a
somewhat larger number of the very common and much diffused or dominant
species. This might have been anticipated, for the mere fact of many
species of the same genus inhabiting any country, shows that there
is something in the organic or inorganic conditions of that country
favourable to the genus; and, consequently, we might have expected to
have found in the larger genera, or those including many species, a
larger proportional number of dominant species. But so many causes tend
to obscure this result, that I am surprised that my tables show even a
small majority on the side of the larger genera. I will here allude
to only two causes of obscurity. Fresh water and salt-loving plants
generally have very wide ranges and are much diffused, but this seems to
be connected with the nature of the stations inhabited by them, and has
little or no relation to the size of the genera to which the species
belong. Again, plants low in the scale of organisation are generally
much more widely diffused than plants higher in the scale; and here
again there is no close relation to the size of the genera. The cause of
lowly-organised plants ranging widely will be discussed in our chapter
on Geographical Distribution.

From looking at species as only strongly marked and well-defined
varieties, I was led to anticipate that the species of the larger genera
in each country would oftener present varieties, than the species of the
smaller genera; for wherever many closely related species (i.e., species
of the same genus) have been formed, many varieties or incipient species
ought, as a general rule, to be now forming. Where many large trees
grow, we expect to find saplings. Where many species of a genus have
been formed through variation, circumstances have been favourable
for variation; and hence we might expect that the circumstances would
generally still be favourable to variation. On the other hand, if we
look at each species as a special act of creation, there is no apparent
reason why more varieties should occur in a group having many species,
than in one having few.

To test the truth of this anticipation I have arranged the plants of
twelve countries, and the coleopterous insects of two districts, into
two nearly equal masses, the species of the larger genera on one side,
and those of the smaller genera on the other side, and it has invariably
proved to be the case that a larger proportion of the species on the
side of the larger genera presented varieties, than on the side of the
smaller genera. Moreover, the species of the large genera which present
any varieties, invariably present a larger average number of varieties
than do the species of the small genera. Both these results follow when
another division is made, and when all the least genera, with from only
one to four species, are altogether excluded from the tables. These
facts are of plain signification on the view that species are only
strongly marked and permanent varieties; for wherever many species of
the same genus have been formed, or where, if we may use the expression,
the manufactory of species has been active, we ought generally to find
the manufactory still in action, more especially as we have every reason
to believe the process of manufacturing new species to be a slow one.
And this certainly holds true if varieties be looked at as incipient
species; for my tables clearly show, as a general rule, that, wherever
many species of a genus have been formed, the species of that genus
present a number of varieties, that is, of incipient species, beyond the
average. It is not that all large genera are now varying much, and are
thus increasing in the number of their species, or that no small genera
are now varying and increasing; for if this had been so, it would have
been fatal to my theory; inasmuch as geology plainly tells us that small
genera have in the lapse of time often increased greatly in size;
and that large genera have often come to their maxima, declined, and
disappeared. All that we want to show is, that where many species of a
genus have been formed, on an average many are still forming; and this
certainly holds good.

MANY OF THE SPECIES INCLUDED WITHIN THE LARGER GENERA RESEMBLE VARIETIES
IN BEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH OTHER, AND IN
HAVING RESTRICTED RANGES.

There are other relations between the species of large genera and their
recorded varieties which deserve notice. We have seen that there is no
infallible criterion by which to distinguish species and well-marked
varieties; and when intermediate links have not been found between
doubtful forms, naturalists are compelled to come to a determination by
the amount of difference between them, judging by analogy whether or not
the amount suffices to raise one or both to the rank of species. Hence
the amount of difference is one very important criterion in settling
whether two forms should be ranked as species or varieties. Now Fries
has remarked in regard to plants, and Westwood in regard to insects,
that in large genera the amount of difference between the species is
often exceedingly small. I have endeavoured to test this numerically by
averages, and, as far as my imperfect results go, they confirm the view.
I have also consulted some sagacious and experienced observers,
and, after deliberation, they concur in this view. In this respect,
therefore, the species of the larger genera resemble varieties, more
than do the species of the smaller genera. Or the case may be put in
another way, and it may be said, that in the larger genera, in which a
number of varieties or incipient species greater than the average are
now manufacturing, many of the species already manufactured still to a
certain extent resemble varieties, for they differ from each other by a
less than the usual amount of difference.

Moreover, the species of the larger genera are related to each other, in
the same manner as the varieties of any one species are related to
each other. No naturalist pretends that all the species of a genus are
equally distinct from each other; they may generally be divided into
sub-genera, or sections, or lesser groups. As Fries has well remarked,
little groups of species are generally clustered like satellites around
other species. And what are varieties but groups of forms, unequally
related to each other, and clustered round certain forms--that is, round
their parent-species. Undoubtedly there is one most important point of
difference between varieties and species, namely, that the amount of
difference between varieties, when compared with each other or with
their parent-species, is much less than that between the species of the
same genus. But when we come to discuss the principle, as I call it, of
divergence of character, we shall see how this may be explained, and
how the lesser differences between varieties tend to increase into the
greater differences between species.

There is one other point which is worth notice. Varieties generally have
much restricted ranges. This statement is indeed scarcely more than a
truism, for if a variety were found to have a wider range than that of
its supposed parent-species, their denominations would be reversed.
But there is reason to believe that the species which are very closely
allied to other species, and in so far resemble varieties, often have
much restricted ranges. For instance, Mr. H.C. Watson has marked for me
in the well-sifted London catalogue of Plants (4th edition) sixty-three
plants which are therein ranked as species, but which he considers as
so closely allied to other species as to be of doubtful value: these
sixty-three reputed species range on an average over 6.9 of the
provinces into which Mr. Watson has divided Great Britain. Now, in this
same catalogue, fifty-three acknowledged varieties are recorded, and
these range over 7.7 provinces; whereas, the species to which these
varieties belong range over 14.3 provinces. So that the acknowledged
varieties have very nearly the same restricted average range, as have
the closely allied forms, marked for me by Mr. Watson as doubtful
species, but which are almost universally ranked by British botanists as
good and true species.

SUMMARY.

Finally, varieties cannot be distinguished from species--except, first,
by the discovery of intermediate linking forms; and, secondly, by a
certain indefinite amount of difference between them; for two forms,
if differing very little, are generally ranked as varieties,
notwithstanding that they cannot be closely connected; but the amount
of difference considered necessary to give to any two forms the rank of
species cannot be defined. In genera having more than the average number
of species in any country, the species of these genera have more than
the average number of varieties. In large genera the species are apt to
be closely but unequally allied together, forming little clusters round
other species. Species very closely allied to other species apparently
have restricted ranges. In all these respects the species of large
genera present a strong analogy with varieties. And we can clearly
understand these analogies, if species once existed as varieties, and
thus originated; whereas, these analogies are utterly inexplicable if
species are independent creations.

We have also seen that it is the most flourishing or dominant species
of the larger genera within each class which on an average yield the
greatest number of varieties, and varieties, as we shall hereafter see,
tend to become converted into new and distinct species. Thus the larger
genera tend to become larger; and throughout nature the forms of life
which are now dominant tend to become still more dominant by leaving
many modified and dominant descendants. But, by steps hereafter to be
explained, the larger genera also tend to break up into smaller genera.
And thus, the forms of life throughout the universe become divided into
groups subordinate to groups.



CHAPTER III. STRUGGLE FOR EXISTENCE.

 Its bearing on natural selection--The term used in a wide
 sense--Geometrical ratio of increase--Rapid increase of naturalised
 animals and plants--Nature of the checks to increase--Competition
 universal--Effects of climate--Protection from the number of
 individuals--Complex relations of all animals and plants throughout
 nature--Struggle for life most severe between individuals and varieties
 of the same species: often severe between species of the same genus--The
 relation of organism to organism the most important of all relations.

Before entering on the subject of this chapter I must make a few
preliminary remarks to show how the struggle for existence bears on
natural selection. It has been seen in the last chapter that
among 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 sub-species 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 adaptations of one part of the organisation to another part,
and to the conditions of life and of one organic being to another being,
been perfected? We see these beautiful co-adaptations 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 adaptations 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, 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.

We will now discuss in a little more detail the struggle for existence.
In my future work this subject will be treated, as it well deserves,
at greater length. The elder De Candolle and Lyell have largely and
philosophically shown that all organic beings are exposed to severe
competition. In regard to plants, no one has treated this subject with
more spirit and ability than W. Herbert, Dean of Manchester, evidently
the result of his great horticultural knowledge. Nothing is easier than
to admit in words the truth of the universal struggle for life, or
more difficult--at least I found it so--than constantly to bear this
conclusion in mind. Yet unless it be thoroughly engrained in the mind,
the whole economy of nature, with every fact on distribution, rarity,
abundance, extinction, and variation, will be dimly seen or quite
misunderstood. We behold the face of nature bright with gladness, we
often see superabundance of food; we do not see or we forget that the
birds which are idly singing round us mostly live on insects or seeds,
and are thus constantly destroying life; or we forget how largely these
songsters, or their eggs, or their nestlings, are destroyed by birds and
beasts of prey; we do not always bear in mind, that, though food may be
now superabundant, it is not so at all seasons of each recurring year.

THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE.

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 a 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 annually produces a thousand seeds, of which
only one of 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 of Struggle for
Existence.

GEOMETRICAL RATIO OF INCREASE.

A struggle for existence inevitably follows from the high rate at which
all organic beings tend to increase. Every being, which during its
natural lifetime produces several eggs or seeds, must suffer destruction
during some period of its life, and during some season or occasional
year, otherwise, on the principle of geometrical increase, its numbers
would quickly become so inordinately great that no country could support
the product. Hence, as more individuals are produced than can possibly
survive, there must in every case be a struggle for existence, either
one individual with another of the same species, or with the individuals
of distinct species, or with the physical conditions of life. It is the
doctrine of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there can be no artificial increase
of food, and no prudential restraint from marriage. Although some
species may be now increasing, more or less rapidly, in numbers, all
cannot do so, for the world would not hold them.

There is no exception to the rule that every organic being naturally
increases at so high a rate, that, if not destroyed, the earth would
soon be covered by the progeny of a single pair. Even slow-breeding
man has doubled in twenty-five years, and at this rate, in less than
a thousand years, there would literally not be standing room for his
progeny. Linnaeus has calculated that if an annual plant produced only
two seeds--and there is no plant so unproductive as this--and their
seedlings next year produced two, and so on, then in twenty years there
would be a million plants. The elephant is reckoned the slowest breeder
of all known animals, and I have taken some pains to estimate its
probable minimum rate of natural increase; it will be safest to assume
that it begins breeding when thirty years old, and goes on breeding
till ninety years old, bringing forth six young in the interval, and
surviving till one hundred years old; if this be so, after a period of
from 740 to 750 years there would be nearly nineteen million elephants
alive descended from the first pair.

But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three following
seasons. Still more striking is the evidence from our domestic animals
of many kinds which have run wild in several parts of the world; if the
statements of the rate of increase of slow-breeding cattle and horses
in South America, and latterly in Australia, had not been well
authenticated, they would have been incredible. So it is with plants;
cases could be given of introduced plants which have become common
throughout whole islands in a period of less than ten years. Several of
the plants, such as the cardoon and a tall thistle, which are now the
commonest over the wide plains of La Plata, clothing square leagues
of surface almost to the exclusion of every other plant, have been
introduced from Europe; and there are plants which now range in India,
as I hear from Dr. Falconer, from Cape Comorin to the Himalaya, which
have been imported from America since its discovery. In such cases, and
endless others could be given, no one supposes that the fertility of
the animals or plants has been suddenly and temporarily increased in any
sensible degree. The obvious explanation is that the conditions of life
have been highly favourable, and that there has consequently been less
destruction of the old and young and that nearly all the young have been
enabled to breed. Their geometrical ratio of increase, the result
of which never fails to be surprising, simply explains their
extraordinarily rapid increase and wide diffusion in their new homes.

In a state of nature almost every full-grown plant annually produces
seed, and among animals there are very few which do not annually pair.
Hence we may confidently assert that all plants and animals are tending
to increase at a geometrical ratio--that all would rapidly stock every
station in which they could any how exist, and that this geometrical
tendency to increase must be checked by destruction at some period of
life. Our familiarity with the larger domestic animals tends, I think,
to mislead us; we see no great destruction falling on them, and we do
not keep in mind that thousands are annually slaughtered for food,
and that in a state of nature an equal number would have somehow to be
disposed of.

The only difference between organisms which annually produce eggs or
seeds by the thousand, and those which produce extremely few, is,
that the slow breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever so large. The
condor lays a couple of eggs and the ostrich a score, and yet in the
same country the condor may be the more numerous of the two. The Fulmar
petrel lays but one egg, yet it is believed to be the most numerous bird
in the world. One fly deposits hundreds of eggs, and another, like the
hippobosca, a single one. But this difference does not determine how
many individuals of the two species can be supported in a district. A
large number of eggs is of some importance to those species which depend
on a fluctuating amount of food, for it allows them rapidly to increase
in number. But the real importance of a large number of eggs or seeds is
to make up for much destruction at some period of life; and this period
in the great majority of cases is an early one. If an animal can in any
way protect its own eggs or young, a small number may be produced, and
yet the average stock be fully kept up; but if many eggs or young are
destroyed, many must be produced or the species will become extinct. It
would suffice to keep up the full number of a tree, which lived on an
average for a thousand years, if a single seed were produced once in a
thousand years, supposing that this seed were never destroyed and could
be ensured to germinate in a fitting place; so that, in all cases, the
average number of any animal or plant depends only indirectly on the
number of its eggs or seeds.

In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind--never to forget that every single organic
being may be said to be striving to the utmost to increase in numbers;
that each lives by a struggle at some period of its life; that heavy
destruction inevitably falls either on the young or old during each
generation or at recurrent intervals. Lighten any check, mitigate the
destruction ever so little, and the number of the species will almost
instantaneously increase to any amount.

NATURE OF THE CHECKS TO INCREASE.

The causes which check the natural tendency of each species to increase
are most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will it tend to increase still further. We
know not exactly what the checks are even in a single instance. Nor will
this surprise any one who reflects how ignorant we are on this head,
even in regard to mankind, although so incomparably better known than
any other animal. This subject of the checks to increase has been ably
treated by several authors, and I hope in a future work to discuss it at
considerable length, more especially in regard to the feral animals of
South America. Here I will make only a few remarks, just to recall to
the reader's mind some of the chief points. Eggs or very young animals
seem generally to suffer most, but this is not invariably the case. With
plants there is a vast destruction of seeds, but from some observations
which I have made it appears that the seedlings suffer most from
germinating in ground already thickly stocked with other plants.
Seedlings, also, are destroyed in vast numbers by various enemies; for
instance, on a piece of ground three feet long and two wide, dug and
cleared, and where there could be no choking from other plants, I marked
all the seedlings of our native weeds as they came up, and out of 357
no less than 295 were destroyed, chiefly by slugs and insects. If turf
which has long been mown, and the case would be the same with turf
closely browsed by quadrupeds, be let to grow, the more vigorous plants
gradually kill the less vigorous, though fully grown plants; thus out of
twenty species grown on a little plot of mown turf (three feet by four)
nine species perished, from the other species being allowed to grow up
freely.

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 number of a species. Thus, there seems to be little doubt that
the stock of partridges, grouse, and hares on any large estate depends
chiefly on the destruction of vermin. If not one head of game were shot
during the next twenty years in England, and, at the same time, if no
vermin were destroyed, there would, in all probability, be less game
than at present, although hundreds of thousands of game animals are now
annually shot. 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 numbers of
a species, and periodical seasons of extreme cold or drought seem to be
the most effective of all checks. I estimated (chiefly from the greatly
reduced numbers of nests in the spring) that the winter of 1854-5
destroyed four-fifths of the birds in my own grounds; and this is
a tremendous destruction, when we remember that ten per cent. is an
extraordinarily severe mortality from epidemics with man. The action of
climate seems at first sight to be quite independent of the struggle for
existence; but in so far as climate chiefly 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.
Even when climate, for instance, extreme cold, acts directly, it will
be the least vigorous individuals, or those which have got least food
through the advancing winter, which will suffer the most. When we travel
from south to north, or from a damp region to a dry, we invariably
see some species gradually getting rarer and rarer, and finally
disappearing; and the change of climate being conspicuous, we are
tempted to attribute the whole effect to its direct action. But this is
a false view; we forget that each species, even where it most abounds,
is constantly suffering enormous destruction at some period of its life,
from enemies or from competitors for the same place and food; and if
these enemies or competitors be in the least degree favoured by any
slight change of climate, they will increase in numbers; and as each
area is already fully stocked with inhabitants, the other species must
decrease. When we travel southward and see a species decreasing in
numbers, we may feel sure that the cause lies quite as much in other
species being favoured, as in this one being hurt. So it is when we
travel northward, but in a somewhat lesser degree, for the number of
species of all kinds, and therefore of competitors, decreases northward;
hence in going northward, or in ascending a mountain, we far oftener
meet with stunted forms, due to the DIRECTLY injurious action of
climate, than we do in proceeding southward or in descending a mountain.
When we reach the Arctic regions, or snow-capped summits, or absolute
deserts, the struggle for life is almost exclusively with the elements.

That climate acts in main part indirectly by favouring other species we
clearly see in the prodigious number of plants which in our gardens can
perfectly well endure our climate, but which never become naturalised,
for they cannot compete with our native plants nor resist destruction by
our native animals.

When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics--at least, this
seems generally to occur with our game animals--often ensue; and here
we have a limiting check independent of the struggle for life. But even
some of these so-called epidemics appear to be due to parasitic worms,
which have from some cause, possibly in part through facility of
diffusion among the crowded animals, been disproportionally favoured:
and here comes in a sort of struggle between the parasite and its prey.

On the other hand, in many cases, a large stock of individuals of the
same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of
corn and rape-seed, etc., in our fields, because the seeds are in great
excess compared with the number of birds which feed on them; nor can
the birds, though having a superabundance of food at this one season,
increase in number proportionally to the supply of seed, as their
numbers are checked during the winter; but any one who has tried knows
how troublesome it is to get seed from a few wheat or other such plants
in a garden; I have in this case lost every single seed. This view of
the necessity of a large stock of the same species for its preservation,
explains, I believe, some singular facts in nature such as that of very
rare plants being sometimes extremely abundant, in the few spots where
they do exist; and that of some social plants being social, that is
abounding in individuals, even on the extreme verge of their range. For
in such cases, we may believe, that a plant could exist only where
the conditions of its life were so favourable that many could exist
together, and thus save the species from utter destruction. I should
add that the good effects of intercrossing, and the ill effects of close
interbreeding, no doubt come into play in many of these cases; but I
will not here enlarge on this subject.

COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE
STRUGGLE FOR EXISTENCE.

Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance, which,
though a simple one, interested me. In Staffordshire, on the estate of
a relation, where I had ample means of investigation, there was a large
and extremely barren heath, which had never been touched by the hand
of man; but several hundred acres of exactly the same nature had been
enclosed twenty-five years previously and planted with Scotch fir. The
change in the native vegetation of the planted part of the heath was
most remarkable, more than is generally seen in passing from one quite
different soil to another: not only the proportional numbers of the
heath-plants were wholly changed, but twelve species of plants (not
counting grasses and carices) flourished in the plantations, which could
not be found on the heath. The effect on the insects must have been
still greater, for six insectivorous birds were very common in the
plantations, which were not to be seen on the heath; and the heath was
frequented by two or three distinct insectivorous birds. Here we see how
potent has been the effect of the introduction of a single tree, nothing
whatever else having been done, with the exception of the land having
been enclosed, so that cattle could not enter. But how important an
element enclosure is, I plainly saw near Farnham, in Surrey. Here
there are extensive heaths, with a few clumps of old Scotch firs on
the distant hill-tops: within the last ten years large spaces have been
enclosed, and self-sown firs are now springing up in multitudes, so
close together that all cannot live. When I ascertained that these young
trees had not been sown or planted I was so much surprised at their
numbers that I went to several points of view, whence I could examine
hundreds of acres of the unenclosed heath, and literally I could not
see a single Scotch fir, except the old planted clumps. But on looking
closely between the stems of the heath, I found a multitude of seedlings
and little trees, which had been perpetually browsed down by the cattle.
In one square yard, at a point some hundred yards distant from one of
the old clumps, I counted thirty-two little trees; and one of them, with
twenty-six rings of growth, had, during many years tried to raise its
head above the stems of the heath, and had failed. No wonder that, as
soon as the land was enclosed, it became thickly clothed with vigorously
growing young firs. Yet the heath was so extremely barren and so
extensive that no one would ever have imagined that cattle would have so
closely and effectually searched it for food.

Here we see that cattle absolutely determine the existence of the Scotch
fir; but in several parts of the world insects determine the existence
of cattle. Perhaps Paraguay offers the most curious instance of this;
for here neither cattle nor horses nor dogs have ever run wild, though
they swarm southward and northward in a feral state; and Azara and
Rengger have shown that this is caused by the greater number in Paraguay
of a certain fly, which lays its eggs in the navels of these animals
when first born. The increase of these flies, numerous as they are,
must be habitually checked by some means, probably by other parasitic
insects. 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!

I am tempted to give one more instance showing how plants and animals,
remote in the scale of nature, are bound together by a web of complex
relations. I shall hereafter have occasion to show that the exotic
Lobelia fulgens is never visited in my garden by insects, and
consequently, from its peculiar structure, never sets a seed. Nearly
all our orchidaceous plants absolutely require the visits of insects
to remove their pollen-masses and thus to fertilise them. I find
from experiments that humble-bees are almost indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do
not visit this flower. I have also found that the visits of bees are
necessary for the fertilisation of some kinds of clover; for instance
twenty heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but
twenty other heads, protected from bees, produced not one. Again, 100
heads of red clover (T. pratense) produced 2,700 seeds, but the same
number of protected heads produced not a single seed. Humble bees alone
visit red clover, as other bees cannot reach the nectar. It has been
suggested that moths may fertilise the clovers; but I doubt whether they
could do so in the case of the red clover, from their weight not being
sufficient to depress the wing petals. Hence we may infer as highly
probable that, if the whole genus of humble-bees became extinct or very
rare in England, the heartsease and red clover would become very rare,
or wholly disappear. The number of humble-bees in any district depends
in a great measure upon the number of field-mice, which destroy their
combs and nests; and Colonel Newman, who has long attended to the habits
of humble-bees, believes that "more than two-thirds of them are
thus destroyed all over England." Now the number of mice is largely
dependent, as every one knows, on the number of cats; and Colonel
Newman says, "Near villages and small towns I have found the nests
of humble-bees more numerous than elsewhere, which I attribute to the
number of cats that destroy the mice." Hence it is quite credible that
the presence of a feline animal in large numbers in a district might
determine, through the intervention first of mice and then of bees, the
frequency of certain flowers in that district!

In the case of every species, many different checks, acting at different
periods of life, and during different seasons or years, probably come
into play; some one check or some few being generally the most potent,
but all will concur in determining the average number, or even
the existence of the species. In some cases it can be shown that
widely-different checks act on the same species in different districts.
When we look at the plants and bushes clothing an entangled bank, we
are tempted to attribute their proportional numbers and kinds to what we
call chance. But how false a view is this! Every one has heard that when
an American forest is cut down, a very different vegetation springs
up; but it has been observed that ancient Indian ruins in the Southern
United States, which must formerly have been cleared of trees, now
display the same beautiful diversity and proportion of kinds as in the
surrounding virgin forests. What a struggle must have gone on during
long centuries between the several kinds of trees, each annually
scattering its seeds by the thousand; what war between insect and
insect--between insects, snails, and other animals with birds and beasts
of prey--all striving to increase, all feeding on each other, or on the
trees, their seeds and seedlings, or on the other plants which first
clothed the ground and thus checked the growth of the trees. Throw up
a handful of feathers, and all fall to the ground according to definite
laws; but how simple is the problem where each shall fall compared to
that of the action and reaction of the innumerable plants and animals
which have determined, in the course of centuries, the proportional
numbers and kinds of trees now growing on the old Indian ruins!

The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This
is likewise sometimes the case with those which may strictly be said to
struggle with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle will almost invariably
be most severe between the individuals of the same species, for they
frequent the same districts, require the same food, and are exposed
to the same dangers. In the case of varieties of the same species, the
struggle will generally be almost equally severe, and we sometimes see
the contest soon decided: for instance, if several varieties of wheat be
sown together, and the mixed seed be resown, some of the varieties which
best suit the soil or climate, or are naturally the most fertile, will
beat the others and so yield more seed, and will consequently in a few
years supplant the other varieties. To keep up a mixed stock of even
such extremely close varieties as the variously coloured sweet-peas,
they must be each year harvested separately, and the seed then mixed
in due proportion, otherwise the weaker kinds will steadily decrease in
number and disappear. So again with the varieties of sheep: it has
been asserted that certain mountain-varieties will starve out other
mountain-varieties, so that they cannot be kept together. The same
result has followed from keeping together different varieties of the
medicinal leech. It may even be doubted whether the varieties of any
of our domestic plants or animals have so exactly the same strength,
habits, and constitution, that the original proportions of a mixed
stock (crossing being prevented) could be kept up for half-a-dozen
generations, if they were allowed to struggle together, in the same
manner as beings in a state of nature, and if the seed or young were not
annually preserved in due proportion.

STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF THE
SAME SPECIES.

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.

A corollary of the highest importance may be deduced from the foregoing
remarks, namely, that the structure of every organic being is related,
in the most essential yet often hidden manner, to that of all other
organic beings, with which it comes into competition for food or
residence, or from which it has to escape, or on which it preys. This
is obvious in the structure of the teeth and talons of the tiger; and in
that of the legs and claws of the parasite which clings to the hair on
the tiger's body. But in the beautifully plumed seed of the dandelion,
and in the flattened and fringed legs of the water-beetle, the relation
seems at first confined to the elements of air and water. Yet the
advantage of the plumed seeds no doubt stands in the closest relation
to the land being already thickly clothed with other plants; so that the
seeds may be widely distributed and fall on unoccupied ground. In the
water-beetle, the structure of its legs, so well adapted for diving,
allows it to compete with other aquatic insects, to hunt for its own
prey, and to escape serving as prey to other animals.

The store of nutriment laid up within the seeds of many plants seems at
first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds, as peas and
beans, when sown in the midst of long grass, it may be suspected that
the chief use of the nutriment in the seed is to favour the growth of
the seedlings, whilst struggling with other plants growing vigorously
all around.

Look at a plant in the midst of its range! Why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges
into slightly hotter or colder, damper or drier districts. In this case
we can clearly see that if we wish in imagination to give the plant the
power of increasing in numbers, we should have to give it some advantage
over its competitors, or over the animals which prey on it. On the
confines of its geographical range, a change of constitution with
respect to climate would clearly be an advantage to our plant; but we
have reason to believe that only a few plants or animals range so far,
that they are destroyed exclusively by the rigour of the climate. Not
until we reach the extreme confines of life, in the Arctic regions or on
the borders of an utter desert, will competition cease. The land may be
extremely cold or dry, yet there will be competition between some few
species, or between the individuals of the same species, for the warmest
or dampest spots.

Hence we can see that when a plant or animal is placed in a new country,
among new competitors, the conditions of its life will generally be
changed in an essential manner, although the climate may be exactly the
same as in its former home. If its average numbers are to increase in
its new home, we should have to modify it in a different way to what we
should have had to do in its native country; for we should have to give
it some advantage over a different set of competitors or enemies.

It is good thus to try in imagination to give 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.



CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.

 Natural Selection--its power compared with man's selection--its power
 on characters of trifling importance--its power at all ages and on
 both sexes--Sexual Selection--On the generality of intercrosses
 between individuals of the same species--Circumstances favourable and
 unfavourable to the results of Natural Selection, namely, intercrossing,
 isolation, number of individuals--Slow action--Extinction caused by
 Natural Selection--Divergence of Character, related to the diversity of
 inhabitants of any small area and to naturalisation--Action of Natural
 Selection, through Divergence of Character and Extinction, on the
 descendants from a common parent--Explains the Grouping of all organic
 beings--Advance in organisation--Low forms preserved--Convergence of
 character--Indefinite multiplication of species--Summary.


How will the struggle for existence, briefly discussed in the last
chapter, act in regard to variation? 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. Under
domestication, it may truly be said that the whole organisation becomes
in some degree plastic. But the variability, which we almost universally
meet with in our domestic productions is not directly produced, as
Hooker and Asa Gray have well remarked, by man; he can neither originate
varieties nor prevent their occurrence; he can only preserve and
accumulate such as do occur. Unintentionally he exposes organic beings
to new and changing conditions of life, and variability ensues; but
similar changes of conditions might and do occur under nature. Let it
also be borne in mind how infinitely complex and close-fitting are
the mutual relations of all organic beings to each other and to their
physical conditions of life; and consequently what infinitely varied
diversities of structure might be of use to each being under changing
conditions of life. 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 of life, should occur in the course of many successive
generations? If such do occur, can we doubt (remembering that 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 procreating their kind? On the other hand, we may feel
sure that any variation in the least degree injurious would be rigidly
destroyed. This preservation of favourable individual differences and
variations, and the destruction of those which are injurious, I have
called Natural Selection, or the Survival of the Fittest. 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.

Several writers have misapprehended or objected to the term Natural
Selection. Some have even imagined that natural selection induces
variability, whereas it implies only the preservation of such variations
as arise and are beneficial to the being under its conditions of life.
No one objects to agriculturists speaking of the potent effects of man's
selection; and in this case the individual differences given by nature,
which man for some object selects, must of necessity first occur. Others
have objected that the term selection implies conscious choice in the
animals which become modified; and it has even been urged that, as
plants have no volition, natural selection is not applicable to them!
In the literal sense of the word, no doubt, natural selection is a
false term; but who ever objected to chemists speaking of the elective
affinities of the various elements?--and yet an acid cannot strictly be
said to elect the base with which it in preference combines. It has been
said that I speak of natural selection as an active power or Deity; but
who objects to an author speaking of the attraction of gravity as ruling
the movements of the planets? Every one knows what is meant and is
implied by such metaphorical expressions; and they are almost necessary
for brevity. So again it is difficult to avoid personifying the word
Nature; but I mean by nature, only the aggregate action and product of
many natural laws, and by laws the sequence of events as ascertained
by us. With a little familiarity such superficial objections will be
forgotten.

We shall 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, independently of the change of climate itself, would
seriously affect the others. If the country were open on its borders,
new forms would certainly immigrate, and this would likewise seriously
disturb the relations of some of the former inhabitants. Let it be
remembered how powerful the influence of a single introduced tree or
mammal has been shown to be. But in the case of an island, or of a
country partly surrounded by barriers, into which new and better adapted
forms could not freely enter, we should then have places in the economy
of nature which would assuredly be better filled up if some of the
original inhabitants were in some manner modified; for, had the area
been open to immigration, these same places would have been seized on
by intruders. In such cases, slight modifications, which in any way
favoured 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.

We have good reason to believe, as shown in the first chapter,
that changes in the conditions of life give a tendency to increased
variability; and in the foregoing cases the conditions the changed, and
this would manifestly be favourable to natural selection, by affording
a better chance of the occurrence of profitable variations. Unless such
occur, natural selection can do nothing. Under the term of "variations,"
it must never be forgotten that mere individual differences are
included. As 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. Nor do I believe that any great physical change,
as of climate, or any unusual degree of isolation, to check immigration,
is necessary in order that new and unoccupied places should be left
for natural selection to fill up by improving some of the varying
inhabitants. For as all the inhabitants of each country are struggling
together with nicely balanced forces, extremely slight modifications in
the structure or habits of one species would often give it an advantage
over others; and still further modifications of the same kind would
often still further increase the advantage, as long as the species
continued under the same conditions of life and profited by similar
means of subsistence and defence. No country can be named in which all
the native inhabitants are now so perfectly adapted to each other and to
the physical conditions under which they live, that none of them could
be still better adapted or improved; for in all countries, the natives
have been so far conquered by naturalised productions that they have
allowed some foreigners to take firm possession of the land. And as
foreigners have thus in every country beaten some of the natives, we
may safely conclude that the natives might have been modified with
advantage, so as to have better resisted the intruders.

As man can produce, and certainly has produced, a great result by his
methodical and unconscious means of selection, what may not natural
selection effect? Man can act only on external and visible characters:
Nature, if I may be allowed to personify the natural preservation or
survival of the fittest, cares nothing for appearances, except in so far
as they are useful to any being. She can act on every internal organ,
on every shade of constitutional difference, on the whole machinery of
life. Man selects only for his own good; Nature only for that of the
being which she tends. Every selected character is fully exercised by
her, as is implied by the fact of their selection. Man keeps the natives
of many climates in the same country. He seldom exercises each selected
character in some peculiar and fitting manner; he feeds a long and a
short-beaked pigeon on the same food; he does not exercise a long-backed
or long-legged quadruped in any peculiar manner; he exposes sheep
with long and short wool to the same climate; does not allow the most
vigorous males to struggle for the females; he does not rigidly destroy
all inferior animals, but protects during each varying season, as far as
lies in his power, all his productions. He often begins his selection
by some half-monstrous form, or at least by some modification prominent
enough to catch the eye or to be plainly useful to him. Under nature,
the slightest differences of structure or constitution may well turn the
nicely-balanced scale in the struggle for life, and so be preserved.
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?

It may metaphorically be said that natural selection is daily and hourly
scrutinising, throughout the world, the slightest variations; rejecting
those that are bad, preserving and adding up all that are good; silently
and insensibly working, WHENEVER AND WHEREVER OPPORTUNITY OFFERS, at
the improvement of each organic being in relation to its organic and
inorganic conditions of life. We see nothing of these slow changes in
progress, until the hand of time has marked the long lapse of ages, and
then so imperfect is our view into long-past geological ages that we see
only that the forms of life are now different from what they formerly
were.

In order that any great amount of modification should be effected in a
species, a variety, when once formed must again, perhaps after a long
interval of time, vary or present individual differences of the same
favourable nature as before; and these must again be preserved, and so
onward, step by step. Seeing that individual differences of the
same kind perpetually recur, this can hardly be considered as an
unwarrantable assumption. But whether it is true, we can judge only
by seeing how far the hypothesis accords with and explains the general
phenomena of nature. On the other hand, the ordinary belief that the
amount of possible variation is a strictly limited quantity, is likewise
a simple assumption.

Although natural selection can act only through and for the good of each
being, yet characters and structures, which we are apt to consider as of
very trifling importance, may thus be acted on. When we see leaf-eating
insects green, and bark-feeders mottled-grey; the alpine ptarmigan white
in winter, the red-grouse the colour of heather, we must believe that
these tints are of service to these birds and insects in preserving them
from danger. Grouse, if not destroyed at some period of their lives,
would increase in countless numbers; they are known to suffer largely
from birds of prey; and hawks are guided by eyesight to their prey,--so
much so that on parts of the continent persons are warned not to keep
white pigeons, as being the most liable to destruction. Hence natural
selection might be effective in giving the proper colour to each kind
of grouse, and in keeping that colour, when once acquired, true and
constant. Nor ought we to think that the occasional destruction of an
animal of any particular colour would produce little effect; we should
remember how essential it is in a flock of white sheep to destroy a lamb
with the faintest trace of black. We have seen how the colour of hogs,
which feed on the "paint-root" in Virginia, determines whether they
shall live or die. In plants, the down on the fruit and the colour of
the flesh are considered by botanists as characters of the most trifling
importance; yet we hear from an excellent horticulturist, Downing,
that in the United States smooth-skinned fruits suffer far more from a
beetle, a Curculio, than those with down; that purple plums suffer far
more from a certain disease than yellow plums; whereas another disease
attacks yellow-fleshed peaches far more than those with other coloured
flesh. If, with all the aids of art, these slight differences make a
great difference in cultivating the several varieties, assuredly, in a
state of nature, where the trees would have to struggle with other trees
and with a host of enemies, such differences would effectually settle
which variety, whether a smooth or downy, a yellow or a purple-fleshed
fruit, should succeed.

In looking at many small points of difference between species, which,
as far as our ignorance permits us to judge, seem quite unimportant, we
must not forget that climate, food, etc., have no doubt produced some
direct effect. It is also necessary to bear in mind that, owing to
the law of correlation, when one part varies and the variations are
accumulated through natural selection, other modifications, often of the
most unexpected nature, will ensue.

As we see that those variations which, under domestication, appear at
any particular period of life, tend to reappear in the offspring at the
same period; for instance, in the shape, size and flavour of the seeds
of the many varieties of our culinary and agricultural plants; in the
caterpillar and cocoon stages of the varieties of the silkworm; in the
eggs of poultry, and in the colour of the down of their chickens; in the
horns of our sheep and cattle when nearly adult; so in a state of nature
natural selection will be enabled to act on and modify organic beings at
any age, by the accumulation of variations profitable at that age, and
by their inheritance at a corresponding age. If it profit a plant to
have its seeds more and more widely disseminated by the wind, I can see
no greater difficulty in this being effected through natural selection,
than in the cotton-planter increasing and improving by selection the
down in the pods on his cotton-trees. Natural selection may modify
and adapt the larva of an insect to a score of contingencies, wholly
different from those which concern the mature insect; and these
modifications may affect, through correlation, the structure of the
adult. So, conversely, modifications in the adult may affect the
structure of the larva; but in all cases natural selection will ensure
that they shall not be injurious: for if they were so, the species would
become extinct.

Natural selection will modify the structure of the young in relation to
the parent and of the parent in relation to the young. In social animals
it will adapt the structure of each individual for the benefit of the
whole community; if the community profits by the selected change. What
natural selection cannot do, is to modify the structure of one species,
without giving it any advantage, for the good of another species;
and though statements to this effect may be found in works of natural
history, I cannot find one case which will bear investigation. A
structure used only once in an animal's life, if of high importance to
it, might be modified to any extent by natural selection; for instance,
the great jaws possessed by certain insects, used exclusively for
opening the cocoon--or the hard tip to the beak of unhatched birds,
used for breaking the eggs. It has been asserted, that of the best
short-beaked tumbler-pigeons a greater number perish in the egg than are
able to get out of it; so that fanciers assist in the act of hatching.
Now, if nature had to make the beak of a full-grown pigeon very short
for the bird's own advantage, the process of modification would be very
slow, and there would be simultaneously the most rigorous selection
of all the young birds within the egg, which had the most powerful and
hardest beaks, for all with weak beaks would inevitably perish: or, more
delicate and more easily broken shells might be selected, the thickness
of the shell being known to vary like every other structure.

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 favourable 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.

SEXUAL SELECTION.

Inasmuch as peculiarities often appear under domestication in one sex
and become hereditarily attached to that sex, so no doubt it will be
under nature. Thus it is rendered possible for the two sexes to be
modified through natural selection in relation to different habits
of life, as is sometimes the case; or for one sex to be modified in
relation to the other sex, as commonly occurs. This leads me to say a
few words on what I have called sexual selection. 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 places in nature, will leave most progeny. But in
many cases victory depends not so much on general vigour, but 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 of spur, and strength to the wing to
strike in the spurred leg, in nearly the same manner as does the brutal
cockfighter by the careful selection of his best cocks. How low in the
scale of nature the law of battle descends I know not; male alligators
have been described as fighting, bellowing, and whirling round, like
Indians in a war-dance, for the possession of the females; male salmons
have been observed fighting all day long; male stag-beetles sometimes
bear wounds from the huge mandibles of other males; the males of certain
hymenopterous insects have been frequently seen by that inimitable
observer M. Fabre, fighting for a particular female who sits by, an
apparently unconcerned beholder of the struggle, and then retires
with the conqueror. The war is, perhaps, severest between the males
of polygamous animals, and these seem oftenest provided with special
weapons. The males of carnivorous animals are already well armed; though
to them and to others, special means of defence may be given through
means of sexual selection, as the mane of the lion, and the hooked jaw
to the male salmon; for the shield may be as important for victory as
the sword or spear.

Among birds, the contest is often of a more peaceful character. All
those who have attended to the subject, believe that there is the
severest rivalry between the males of many species to attract, by
singing, the females. The rock-thrush of Guiana, birds of paradise,
and some others, congregate, and successive males display with the most
elaborate care, and show off in the best manner, their gorgeous plumage;
they likewise perform strange antics before the females, which, standing
by as spectators, at last choose the most attractive partner. Those who
have closely attended to birds in confinement well know that they
often take individual preferences and dislikes: thus Sir R. Heron has
described how a pied peacock was eminently attractive to all his hen
birds. I cannot here enter on the necessary details; but if man can in a
short time give beauty and an elegant carriage to his bantams, according
to his standard of beauty, I can see no good reason to doubt that female
birds, by selecting, during thousands of generations, the most melodious
or beautiful males, according to their standard of beauty, might produce
a marked effect. Some well-known laws, with respect to the plumage of
male and female birds, in comparison with the plumage of the young, can
partly be explained through the action of sexual selection on variations
occurring at different ages, and transmitted to the males alone or to
both sexes at corresponding ages; but I have not space here to enter on
this subject.

Thus it is, as I believe, that when the males and females of any animal
have the same general habits of life, but differ in structure, colour,
or ornament, such differences have been mainly caused by sexual
selection: that is, by individual males having had, in successive
generations, some slight advantage over other males, in their weapons,
means of defence, or charms; which they have transmitted to their
male offspring alone. Yet, I would not wish to attribute all sexual
differences to this agency: for we see in our domestic animals
peculiarities arising and becoming attached to the male sex, which
apparently have not been augmented through selection by man. The tuft of
hair on the breast of the wild turkey-cock cannot be of any use, and it
is doubtful whether it can be ornamental in the eyes of the female bird;
indeed, had the tuft appeared under domestication it would have been
called a monstrosity.

ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THE
FITTEST.

In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some
by craft, some by strength, and some by fleetness; and let us suppose
that the fleetest prey, a deer for instance, had from any change in
the country increased in numbers, or that other prey had decreased
in numbers, during that season of the year when the wolf was hardest
pressed for food. Under such circumstances the swiftest and slimmest
wolves have the best chance of surviving, and so be preserved or
selected, provided always that they retained strength to master their
prey at this or some other period of the year, when they were compelled
to prey on other animals. I can see no more reason to doubt that
this would be the result, than that man should be able to improve the
fleetness of his greyhounds by careful and methodical selection, or by
that kind of unconscious selection which follows from each man trying to
keep the best dogs without any thought of modifying the breed. I may
add that, according to Mr. Pierce, there are two varieties of the wolf
inhabiting the Catskill Mountains, in the United States, one with a
light greyhound-like form, which pursues deer, and the other more bulky,
with shorter legs, which more frequently attacks the shepherd's flocks.

Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable;
for we often observe great differences in the natural tendencies of our
domestic animals; one cat, for instance, taking to catch rats, another
mice; one cat, according to Mr. St. John, bringing home winged game,
another hares or rabbits, and another hunting on marshy ground and
almost nightly catching woodcocks or snipes. The tendency to catch rats
rather than mice is known to be inherited. Now, if any slight innate
change of habit or of structure benefited an individual wolf, it would
have the best chance of surviving and of leaving offspring. Some of its
young would probably inherit the same habits or structure, and by the
repetition of this process, a new variety might be formed which would
either supplant or coexist with the parent-form of wolf. Or, again,
the wolves inhabiting a mountainous district, and those frequenting the
lowlands, would naturally be forced to hunt different prey; and from the
continued preservation of the individuals best fitted for the two sites,
two varieties might slowly be formed. These varieties would cross and
blend where they met; but to this subject of intercrossing we shall soon
have to return. I may add, that, according to Mr. Pierce, there are two
varieties of the wolf inhabiting the Catskill Mountains in the United
States, one with a light greyhound-like form, which pursues deer, and
the other more bulky, with shorter legs, which more frequently attacks
the shepherd's flocks.

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 pro-create 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 onto
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.

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 organisation 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.

To the effects of intercrossing in eliminating variations of all kinds,
I shall have to recur; but it may be here remarked that most animals and
plants keep to their proper homes, and do not needlessly wander about;
we see this even with migratory birds, which almost always return to the
same spot. Consequently each newly-formed variety would generally be at
first local, as seems to be the common rule with varieties in a state
of nature; so that similarly modified individuals would soon exist in a
small body together, and would often breed together. If the new variety
were successful in its battle for life, it would slowly spread from
a central district, competing with and conquering the unchanged
individuals on the margins of an ever-increasing circle.

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 Leguminosae, 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; and 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 flowers, also, which had their stamens and pistils placed, in
relation to the size and habits of the particular insect which visited
them, so as to favour in any degree the transportal of the pollen, would
likewise be favoured. We might have taken the case of insects visiting
flowers for the sake of collecting pollen instead of nectar; and as
pollen is formed for the sole purpose of fertilisation, its destruction
appears to be a simple loss to the plant; yet if a little pollen
were carried, at first occasionally and then habitually, by the
pollen-devouring insects from flower to flower, and a cross thus
effected, although nine-tenths of the pollen were destroyed it
might still be a great gain to the plant to be thus robbed; and the
individuals which produced more and more pollen, and had larger anthers,
would be selected.

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. Some holly-trees bear only male flowers, which have four stamens
producing a rather small quantity of pollen, and a rudimentary pistil;
other holly-trees bear only female flowers; these have a full-sized
pistil, and four stamens with shrivelled anthers, in which not a grain
of pollen can be detected. Having found a female tree exactly sixty
yards from a male tree, I put the stigmas of twenty flowers, taken from
different branches, under the microscope, and on all, without exception,
there were a few pollen-grains, and on some a profusion. As the wind had
set for several days from the female to the male tree, the pollen could
not thus have been carried. The weather had been cold and boisterous and
therefore not favourable to bees, nevertheless every female flower which
I examined had been effectually fertilised by the bees, which had flown
from tree to tree in search of nectar. But to return to our imaginary
case; 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
dioeciously polygamous.

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. The tubes of the corolla of the common red or incarnate
clovers (Trifolium pratense and incarnatum) do not on a hasty glance
appear to differ in length; yet the hive-bee can easily suck the nectar
out of the incarnate clover, but not out of the common red clover, which
is visited by humble-bees alone; so that whole fields of the red clover
offer in vain an abundant supply of precious nectar to the hive-bee.
That this nectar is much liked by the hive-bee is certain; for I have
repeatedly seen, but only in the autumn, many hive-bees sucking the
flowers through holes bitten in the base of the tube by humble bees.
The difference in the length of the corolla in the two kinds of clover,
which determines the visits of the hive-bee, must be very trifling; for
I have been assured that when red clover has been mown, the flowers of
the second crop are somewhat smaller, and that these are visited by many
hive-bees. I do not know whether this statement is accurate; nor whether
another published statement can be trusted, namely, that the Ligurian
bee, which is generally considered a mere variety of the common
hive-bee, and which freely crosses with it, is able to reach and suck
the nectar of the red clover. Thus, in a country where this kind of
clover abounded, it might be a great advantage to the hive-bee to have a
slightly longer or differently constructed proboscis. On the other hand,
as the fertility of this clover absolutely depends on bees visiting the
flowers, if humble-bees were to become rare in any country, it might be
a great advantage to the plant to have a shorter or more deeply divided
corolla, so that the hive-bees should be enabled to suck its flowers.
Thus I can understand how a flower and a bee might slowly become, either
simultaneously or one after the other, modified and adapted to each
other in the most perfect manner, by the continued preservation of all
the individuals which presented slight deviations of structure mutually
favourable to each other.

I am well aware that this doctrine of natural selection, exemplified in
the above imaginary instances, is open to the same objections which
were first urged against Sir Charles Lyell's noble views on "the modern
changes of the earth, as illustrative of geology;" but we now seldom
hear the agencies which we see still at work, spoken of as trifling and
insignificant, when used in explaining the excavation of the deepest
valleys or the formation of long lines of inland cliffs. Natural
selection acts only by the preservation and accumulation of small
inherited modifications, each profitable to the preserved being; and
as modern geology has almost banished such views as the excavation of a
great valley by a single diluvial wave, so will natural selection banish
the belief of the continued creation of new organic beings, or of any
great and sudden modification in their structure.

ON THE INTERCROSSING OF INDIVIDUALS.

I must here introduce a short digression. In the case of animals
and plants with separated sexes, it is of course obvious that two
individuals must always (with the exception of the curious and not well
understood cases of parthenogenesis) unite for each birth; but in the
case of hermaphrodites this is far from obvious. Nevertheless there is
reason to believe that with all hermaphrodites two individuals, either
occasionally or habitually, concur for the reproduction of their kind.
This view was long ago doubtfully suggested by Sprengel, Knight and
Kolreuter. We shall presently see its importance; but I must here treat
the subject with extreme brevity, though I have the materials prepared
for an ample discussion. All vertebrate animals, all insects and some
other large groups of animals, pair for each birth. Modern research
has much diminished the number of supposed hermaphrodites and of real
hermaphrodites a large number pair; that is, two individuals regularly
unite for reproduction, which is all that concerns us. But still there
are many hermaphrodite animals which certainly do not habitually pair,
and a vast majority of plants are hermaphrodites. What reason, it may be
asked, is there for supposing in these cases that two individuals ever
concur in reproduction? As it is impossible here to enter on details, I
must trust to some general considerations alone.

In the first place, I have collected so large a body of facts, and made
so many experiments, showing, in accordance with the almost universal
belief of breeders, that with animals and plants a cross between
different varieties, or between individuals of the same variety but of
another strain, gives vigour and fertility to the offspring; and on the
other hand, that CLOSE interbreeding diminishes vigour and fertility;
that these facts alone incline me to believe that it is a general law
of nature that no organic being fertilises itself for a perpetuity
of generations; but that a cross with another individual is
occasionally--perhaps at long intervals of time--indispensable.

On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any
other view are inexplicable. Every hybridizer knows how unfavourable
exposure to wet is to the fertilisation of a flower, yet what a
multitude of flowers have their anthers and stigmas fully exposed to the
weather! If an occasional cross be indispensable, notwithstanding that
the plant's own anthers and pistil stand so near each other as almost
to ensure self-fertilisation, the fullest freedom for the entrance of
pollen from another individual will explain the above state of exposure
of the organs. Many flowers, on the other hand, have their organs of
fructification closely enclosed, as in the great papilionaceous or
pea-family; but these almost invariably present beautiful and curious
adaptations in relation to the visits of insects. So necessary are the
visits of bees to many papilionaceous flowers, that their fertility is
greatly diminished if these visits be prevented. Now, it is scarcely
possible for insects to fly from flower to flower, and not to carry
pollen from one to the other, to the great good of the plant.
Insects act like a camel-hair pencil, and it is sufficient, to ensure
fertilisation, just to touch with the same brush the anthers of one
flower and then the stigma of another; but it must not be supposed that
bees would thus produce a multitude of hybrids between distinct species;
for if a plant's own pollen and that from another species are placed
on the same stigma, the former is so prepotent that it invariably and
completely destroys, as has been shown by Gartner, the influence of the
foreign pollen.

When the stamens of a flower suddenly spring towards the pistil, or
slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but the agency of insects is often required to cause the
stamens to spring forward, as Kolreuter has shown to be the case with
the barberry; and in this very genus, which seems to have a special
contrivance for self-fertilisation, it is well known that, if
closely-allied forms or varieties are planted near each other, it is
hardly possible to raise pure seedlings, so largely do they naturally
cross. In numerous other cases, far from self-fertilisation being
favoured, there are special contrivances which effectually prevent the
stigma receiving pollen from its own flower, as I could show from the
works of Sprengel and others, as well as from my own observations: for
instance, in Lobelia fulgens, there is a really beautiful and elaborate
contrivance by which all the infinitely numerous pollen-granules are
swept out of the conjoined anthers of each flower, before the stigma of
that individual flower is ready to receive them; and as this flower is
never visited, at least in my garden, by insects, it never sets a seed,
though by placing pollen from one flower on the stigma of another, I
raise plenty of seedlings. Another species of Lobelia, which is visited
by bees, seeds freely in my garden. In very many other cases, though
there is no special mechanical contrivance to prevent the stigma
receiving pollen from the same flower, yet, as Sprengel, and more
recently Hildebrand and others have shown, and as I can confirm, either
the anthers burst before the stigma is ready for fertilisation, or the
stigma is ready before the pollen of that flower is ready, so that
these so-named dichogamous plants have in fact separated sexes, and
must habitually be crossed. So it is with the reciprocally dimorphic and
trimorphic plants previously alluded to. How strange are these facts!
How strange that the pollen and stigmatic surface of the same flower,
though placed so close together, as if for the very purpose of
self-fertilisation, should be in so many cases mutually useless to each
other! How simply are these facts explained on the view of an occasional
cross with a distinct individual being advantageous or indispensable!

If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority of the
seedlings thus raised turn out, as I found, mongrels: for instance, I
raised 233 seedling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true to their kind,
and some even of these were not perfectly true. Yet the pistil of each
cabbage-flower is surrounded not only by its own six stamens but by
those of the many other flowers on the same plant; and the pollen of
each flower readily gets on its stigma without insect agency; for I
have found that plants carefully protected from insects produce the
full number of pods. How, then, comes it that such a vast number of the
seedlings are mongrelized? It must arise from the pollen of a distinct
VARIETY having a prepotent effect over the flower's own pollen; and
that this is part of the general law of good being derived from the
intercrossing of distinct individuals of the same species. When distinct
SPECIES are crossed the case is reversed, for a plant's own pollen
is always prepotent over foreign pollen; but to this subject we shall
return in a future chapter.

In the case of a large tree covered with innumerable flowers, it may be
objected that pollen could seldom be carried from tree to tree, and at
most only from flower to flower on the same tree; and flowers on the
same tree can be considered as distinct individuals only in a limited
sense. I believe this objection to be valid, but that nature has largely
provided against it by giving to trees a strong tendency to bear flowers
with separated sexes. When the sexes are separated, although the male
and female flowers may be produced on the same tree, pollen must be
regularly carried from flower to flower; and this will give a better
chance of pollen being occasionally carried from tree to tree. That
trees belonging to all orders have their sexes more often separated than
other plants, I find to be the case in this country; and at my request
Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those
of the United States, and the result was as I anticipated. On the
other hand, Dr. Hooker informs me that the rule does not hold good in
Australia: but if most of the Australian trees are dichogamous, the same
result would follow as if they bore flowers with separated sexes. I have
made these few remarks on trees simply to call attention to the subject.

Turning for a brief space to animals: various terrestrial species are
hermaphrodites, such as the land-mollusca and earth-worms; but these
all pair. As yet I have not found a single terrestrial animal which
can fertilise itself. This remarkable fact, which offers so strong a
contrast with terrestrial plants, is intelligible on the view of an
occasional cross being indispensable; for owing to the nature of the
fertilising element there are no means, analogous to the action of
insects and of the wind with plants, by which an occasional cross could
be effected with terrestrial animals without the concurrence of two
individuals. Of aquatic animals, there are many self-fertilising
hermaphrodites; but here the currents of water offer an obvious means
for an occasional cross. As in the case of flowers, I have as yet
failed, after consultation with one of the highest authorities, namely,
Professor Huxley, to discover a single hermaphrodite animal with the
organs of reproduction so perfectly enclosed that access from without,
and the occasional influence of a distinct individual, can be shown to
be physically impossible. Cirripedes long appeared to me to present,
under this point of view, a case of great difficulty; but I have been
enabled, by a fortunate chance, to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.

It must have struck most naturalists as a strange anomaly that, both
with animals and plants, some species of the same family and even of
the same genus, though agreeing closely with each other in their whole
organisation, are hermaphrodites, and some unisexual. But if, in fact,
all hermaphrodites do occasionally intercross, the difference between
them and unisexual species is, as far as function is concerned, very
small.

From these several considerations and from the many special facts which
I have collected, but which I am unable here to give, it appears that
with animals and plants an occasional intercross between distinct
individuals is a very general, if not universal, law of nature.

CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW FORMS THROUGH NATURAL
SELECTION.

This is an extremely intricate subject. A great amount of variability,
under which term individual differences are always included, will
evidently be favourable. A large number of individuals, by giving a
better chance within any given period for the appearance of profitable
variations, will compensate for a lesser amount of variability in each
individual, and is, I believe, a highly important element of success.
Though nature grants long periods of time for the work of natural
selection, she does not grant an indefinite period; for as all organic
beings are striving to seize on each place in the economy of nature, if
any one species does not become modified and improved in a corresponding
degree with its competitors it will be exterminated. Unless favourable
variations be inherited by some at least of the offspring, nothing can
be effected by natural selection. The tendency to reversion may often
check or prevent the work; but as this tendency has not prevented man
from forming by selection numerous domestic races, why should it prevail
against natural selection?

In the case of methodical selection, a breeder selects for some definite
object, and if the individuals be allowed freely to intercross, his work
will completely fail. But when many men, without intending to alter
the breed, have a nearly common standard of perfection, and all try to
procure and breed from the best animals, improvement surely but slowly
follows from this unconscious process of selection, notwithstanding that
there is no separation of selected individuals. Thus it will be under
nature; for within a confined area, with some place in the natural
polity not perfectly occupied, all the individuals varying in the right
direction, though in different degrees, will tend to be preserved.
But if the area be large, its several districts will almost certainly
present different conditions of life; and then, if the same species
undergoes modification in different districts, the newly formed
varieties will intercross on the confines of each. But we shall see in
the sixth chapter that intermediate varieties, inhabiting intermediate
districts, will in the long run generally be supplanted by one of the
adjoining varieties. Intercrossing will chiefly affect those animals
which unite for each birth and wander much, and which do not breed at a
very quick rate. Hence with animals of this nature, for instance birds,
varieties will generally be confined to separated countries; and this
I find to be the case. With hermaphrodite organisms which cross only
occasionally, and likewise with animals which unite for each birth, but
which wander little and can increase at a rapid rate, a new and improved
variety might be quickly formed on any one spot, and might there
maintain itself in a body and afterward spread, so that the individuals
of the new variety would chiefly cross together. On this principle
nurserymen always prefer saving seed from a large body of plants, as the
chance of intercrossing is thus lessened.

Even with animals which unite for each birth, and which do not propagate
rapidly, we must not assume that free intercrossing would always
eliminate the effects of natural selection; for I can bring forward
a considerable body of facts showing that within the same area two
varieties of the same animal may long remain distinct, from haunting
different stations, from breeding at slightly different seasons, or from
the individuals of each variety preferring to pair together.

Intercrossing plays a very important part in nature by keeping the
individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals which unite for each birth; but, as already stated,
we have reason to believe that occasional intercrosses take place with
all animals and plants. Even if these take place only at long intervals
of time, the young thus produced will gain so much in vigour and
fertility over the offspring from long-continued self-fertilisation,
that they will have a better chance of surviving and propagating their
kind; and thus in the long run the influence of crosses, even at rare
intervals, will be great. With respect to organic beings extremely low
in the scale, which do not propagate sexually, nor conjugate, and which
cannot possibly intercross, uniformity of character can be retained by
them under the same conditions of life, only through the principle
of inheritance, and through natural selection which will destroy any
individuals departing from the proper type. If the conditions of life
change and the form undergoes modification, uniformity of character
can be given to the modified offspring, solely by natural selection
preserving similar favourable variations.

Isolation also is an important element in the modification of species
through natural selection. In a confined or isolated area, if not very
large, the organic and inorganic conditions of life will generally be
almost uniform; so that natural selection will tend to modify all
the varying individuals of the same species in the same manner.
Intercrossing with the inhabitants of the surrounding districts,
will also be thus prevented. Moritz Wagner has lately published an
interesting essay on this subject, and has shown that the service
rendered by isolation in preventing crosses between newly-formed
varieties is probably greater even than I supposed. But from reasons
already assigned I can by no means agree with this naturalist, that
migration and isolation are necessary elements for the formation of new
species. The importance of isolation is likewise great in preventing,
after any physical change in the conditions, such as of climate,
elevation of the land, etc., the immigration of better adapted
organisms; and thus new places in the natural economy of the district
will be left open to be filled up by the modification of the old
inhabitants. Lastly, isolation will give time for a new variety to be
improved at a slow rate; and this may sometimes be of much importance.
If, however, an isolated area be very small, either from being
surrounded by barriers, or from having very peculiar physical
conditions, the total number of the inhabitants will be small; and this
will retard the production of new species through natural selection, by
decreasing the chances of favourable variations arising.

The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously asserted
that the element of time has been assumed by me to play an all-important
part in modifying species, as if all the forms of life were necessarily
undergoing change through some innate law. Lapse of time is only so far
important, and its importance in this respect is great, that it gives
a better chance of beneficial variations arising and of their being
selected, accumulated, and fixed. It likewise tends to increase the
direct action of the physical conditions of life, in relation to the
constitution of each organism.

If we turn to nature to test the truth of these remarks, and look at any
small isolated area, such as an oceanic island, although the number of
the species inhabiting it is small, as we shall see in our chapter on
Geographical Distribution; yet of these species a very large proportion
are endemic,--that is, have been produced there and nowhere else in the
world. Hence an oceanic island at first sight seems to have been highly
favourable for the production of new species. But we may thus deceive
ourselves, for to ascertain whether a small isolated area, or a large
open area like a continent, has been most favourable for the production
of new organic forms, we ought to make the comparison within equal
times; and this we are incapable of doing.

Although isolation is of great importance in the production of new
species, on the whole I am inclined to believe that largeness of area
is still more important, especially for the production of species which
shall prove capable of enduring for a long period, and of spreading
widely. Throughout a great and open area, not only will there be a
better chance of favourable variations, arising from the large number of
individuals of the same species there supported, but the conditions of
life are much more complex from the large number of already existing
species; and if some of these many species become modified and improved,
others will have to be improved in a corresponding degree, or they
will be exterminated. Each new form, also, as soon as it has been much
improved, will be able to spread over the open and continuous area, and
will thus come into competition with many other forms. Moreover, great
areas, though now continuous, will often, owing to former oscillations
of level, have existed in a broken condition, so that the good effects
of isolation will generally, to a certain extent, have concurred.
Finally, I conclude that, although small isolated areas have been in
some respects highly favourable for the production of new species, yet
that the course of modification will generally have been more rapid on
large areas; and what is more important, that the new forms produced on
large areas, which already have been victorious over many competitors,
will be those that will spread most widely, and will give rise to the
greatest number of new varieties and species. They will thus play a more
important part in the changing history of the organic world.

In accordance with this view, we can, perhaps, understand some
facts which will be again alluded to in our chapter on Geographical
Distribution; for instance, the fact of the productions of the
smaller continent of Australia now yielding before those of the larger
Europaeo-Asiatic area. Thus, also, it is that continental productions
have everywhere become so largely naturalised on islands. On a small
island, the race for life will have been less severe, and there will
have been less modification and less extermination. Hence, we can
understand how it is that the flora of Madeira, according to Oswald
Heer, resembles to a certain extent the extinct tertiary flora of
Europe. All fresh water basins, taken together, make a small area
compared with that of the sea or of the land. Consequently, the
competition between fresh water productions will have been less severe
than elsewhere; new forms will have been more slowly produced, and old
forms more slowly exterminated. And it is in fresh water basins that
we find seven genera of Ganoid fishes, remnants of a once preponderant
order: and in fresh water we find some of the most anomalous forms now
known in the world, as the Ornithorhynchus and Lepidosiren, which, like
fossils, connect to a certain extent orders at present widely separated
in the natural scale. These anomalous forms may be called living
fossils; they have endured to the present day, from having inhabited
a confined area, and from having been exposed to less varied, and
therefore less severe, competition.

To sum up, as far as the extreme intricacy of the subject permits, the
circumstances favourable and unfavourable for the production of new
species through natural selection. I conclude that for terrestrial
productions a large continental area, which has undergone many
oscillations of level, will have been the most favourable for the
production of many new forms of life, fitted to endure for a long
time and to spread widely. While the area existed as a continent the
inhabitants will have been numerous in individuals and kinds, and will
have been subjected to severe competition. When converted by subsidence
into large separate islands there will still have existed many
individuals of the same species on each island: intercrossing on the
confines of the range of each new species will have been checked: after
physical changes of any kind immigration will have been prevented, so
that new places in the polity of each island will have had to be filled
up by the modification of the old inhabitants; and time will have been
allowed for the varieties in each to become well modified and perfected.
When, by renewed elevation, the islands were reconverted into a
continental area, there will again have been very severe competition;
the most favoured or improved varieties will have been enabled to
spread; there will have been much extinction of the less improved forms,
and the relative proportional numbers of the various inhabitants of the
reunited continent will again have been changed; and again there will
have been a fair field for natural selection to improve still further
the inhabitants, and thus to produce new species.

That natural selection generally act with extreme slowness I fully
admit. It can act only when there are places in the natural polity of a
district which can be better occupied by the modification of some of its
existing inhabitants. The occurrence of such places will often depend
on physical changes, which generally take place very slowly, and on the
immigration of better adapted forms being prevented. As some few of
the old inhabitants become modified the mutual relations of others will
often be disturbed; and this will create new places, ready to be filled
up by better adapted forms; but all this will take place very slowly.
Although all the individuals of the same species differ in some slight
degree from each other, it would often be long before differences of
the right nature in various parts of the organisation might occur. The
result would often be greatly retarded by free intercrossing. Many will
exclaim that these several causes are amply sufficient to neutralise the
power of natural selection. I do not believe so. But I do believe that
natural selection will generally act very slowly, only at long intervals
of time, and only on a few of the inhabitants of the same region. I
further believe that these slow, intermittent results accord well with
what geology tells us of the rate and manner at which the inhabitants of
the world have changed.

Slow though the process of selection may be, if feeble man can do much
by artificial selection, I can see no limit to the amount of change,
to the beauty and complexity of the coadaptations between all organic
beings, one with another and with their physical conditions of life,
which may have been effected in the long course of time through nature's
power of selection, that is by the survival of the fittest.

EXTINCTION CAUSED BY NATURAL SELECTION.

This subject will be more fully discussed in our chapter on Geology; but
it must here be alluded to from being intimately connected with natural
selection. Natural selection acts solely through the preservation of
variations in some way advantageous, which consequently endure. Owing to
the high geometrical rate of increase of all organic beings, each area
is already fully stocked with inhabitants, and it follows from this,
that as the favoured forms increase in number, so, generally, will the
less favoured decrease and become rare. Rarity, as geology tells us,
is the precursor to extinction. We can see that any form which
is represented by few individuals will run a good chance of utter
extinction, during great fluctuations in the nature or the seasons, or
from a temporary increase in the number of its enemies. But we may go
further than this; for as new forms are produced, unless we admit that
specific forms can go on indefinitely increasing in number, many old
forms must become extinct. That the number of specific forms has not
indefinitely increased, geology plainly tells us; and we shall presently
attempt to show why it is that the number of species throughout the
world has not become immeasurably great.

We have seen that the species which are most numerous in individuals
have the best chance of producing favourable variations within any given
period. We have evidence of this, in the facts stated in the second
chapter, showing that it is the common and diffused or dominant species
which offer the greatest number of recorded varieties. Hence, rare
species will be less quickly modified or improved within any given
period; they will consequently be beaten in the race for life by the
modified and improved descendants of the commoner species.

From these several considerations I think it inevitably follows,
that as new species in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct.
The forms which stand in closest competition with those undergoing
modification and improvement, will naturally suffer most. And we have
seen in the chapter on the Struggle for Existence that it is the most
closely-allied forms,--varieties of the same species, and species of
the same genus or related genera,--which, from having nearly the same
structure, constitution and habits, generally come into the severest
competition with each other. Consequently, each new variety or species,
during the progress of its formation, will generally press hardest
on its nearest kindred, and tend to exterminate them. We see the same
process of extermination among our domesticated productions, through
the selection of improved forms by man. Many curious instances could be
given showing how quickly new breeds of cattle, sheep and other animals,
and varieties of flowers, take the place of older and inferior kinds. In
Yorkshire, it is historically known that the ancient black cattle were
displaced by the long-horns, and that these "were swept away by the
short-horns" (I quote the words of an agricultural writer) "as if by
some murderous pestilence."

DIVERGENCE OF CHARACTER.

The principle, which I have designated by this term, is of high
importance, and explains, as I believe, several important facts. In
the first place, varieties, even strongly-marked ones, though having
somewhat of the character of species--as is shown by the hopeless doubts
in many cases how to rank them--yet certainly differ far less from each
other than do good and distinct species. Nevertheless according to my
view, varieties are species in the process of formation, or are, as
I have called them, incipient species. How, then, does the lesser
difference between varieties become augmented into the greater
difference between species? That this does habitually happen, we must
infer from most of the innumerable species throughout nature presenting
well-marked differences; whereas varieties, the supposed prototypes and
parents of future well-marked species, present slight and ill-defined
differences. 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. We shall here find something analogous. It
will be admitted that the production of races so different as short-horn
and Hereford cattle, race and cart horses, the several breeds of
pigeons, etc., could never have been effected by the mere chance
accumulation of similar variations during many successive generations.
In practice, a fancier is, for instance, struck by a pigeon having a
slightly shorter beak; another fancier is struck by a pigeon having a
rather longer beak; and on the acknowledged principle that "fanciers do
not and will not admire a medium standard, but like extremes," they
both go on (as has actually occurred with the sub-breeds of the
tumbler-pigeon) choosing and breeding from birds with longer and longer
beaks, or with shorter and shorter beaks. Again, we may suppose that at
an early period of history, the men of one nation or district required
swifter horses, while those of another required stronger and bulkier
horses. The early differences would be very slight; but, in the course
of time, from the continued selection of swifter horses in the one case,
and of stronger ones in the other, the differences would become greater,
and would be noted as forming two sub-breeds. Ultimately after the
lapse of centuries, these sub-breeds would become converted into two
well-established and distinct breeds. As the differences became greater,
the inferior animals with intermediate characters, being neither very
swift nor very strong, would not have been used for breeding, and will
thus have tended to disappear. Here, then, we see in man's productions
the action of what may be called the principle of divergence, causing
differences, at first barely appreciable, steadily to increase, and
the breeds to diverge in character, both from each other and from their
common parent.

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.

We can clearly discern this in the case of animals with simple habits.
Take the case of a carnivorous quadruped, of which the number that can
be supported in any country has long ago arrived at its full average.
If its natural power of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in conditions) only by
its varying descendants seizing on places at present occupied by other
animals: some of them, for instance, being enabled to feed on new kinds
of prey, either dead or alive; some inhabiting new stations, climbing
trees, frequenting water, and some perhaps becoming less carnivorous.
The more diversified in habits and structure the descendants of our
carnivorous animals become, the more places they will be enabled to
occupy. What applies to one animal will apply throughout all time to
all animals--that is, if they vary--for otherwise natural selection can
effect nothing. So it will be with plants. It has been experimentally
proved, that if a plot of ground be sown with one species of grass,
and a similar plot be sown with several distinct genera of grasses,
a greater number of plants and a greater weight of dry herbage can be
raised in the latter than in the former case. The same has been found
to hold good when one variety and several mixed varieties of wheat have
been sown on equal spaces of ground. Hence, if any one species of grass
were to go on varying, and the varieties were continually selected which
differed from each other in the same manner, though in a very slight
degree, as do the distinct species and genera of grasses, a greater
number of individual plants of this species, including its modified
descendants, would succeed in living on the same piece of ground. And
we know that each species and each variety of grass is annually sowing
almost countless seeds; and is thus striving, as it may be said, to
the utmost to increase in number. Consequently, in the course of many
thousand generations, the most distinct varieties of any one species
of grass would have the best chance of succeeding and of increasing
in numbers, and thus of supplanting the less distinct varieties; and
varieties, when rendered very distinct from each other, take the rank of
species.

The truth of the principle that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if freely
open to immigration, and where the contest between individual and
individual must be very severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly the same
conditions, supported twenty species of plants, and these belonged to
eighteen genera and to eight orders, which shows how much these plants
differed from each other. So it is with the plants and insects on small
and uniform islets: also in small ponds of fresh water. Farmers find
that they can raise more food by a rotation of plants belonging to the
most different orders: nature follows what may be called a simultaneous
rotation. Most of the animals and plants which live close round any
small piece of ground, could live on it (supposing its nature not to be
in any way peculiar), and may be said to be striving to the utmost
to live there; but, it is seen, that where they come into the closest
competition, the advantages of diversification of structure, with the
accompanying differences of habit and constitution, determine that the
inhabitants, which thus jostle each other most closely, shall, as a
general rule, belong to what we call different genera and orders.

The same principle is seen in the naturalisation of plants through man's
agency in foreign lands. It might have been expected that the plants
which would succeed in becoming naturalised in any land would generally
have been closely allied to the indigenes; for these are commonly looked
at as specially created and adapted for their own country. It might
also, perhaps, have been expected that naturalised plants would have
belonged to a few groups more especially adapted to certain stations in
their new homes. But the case is very different; and Alph. de Candolle
has well remarked, in his great and admirable work, that floras gain by
naturalisation, proportionally with the number of the native genera and
species, far more in new genera than in new species. To give a single
instance: in the last edition of Dr. Asa Gray's "Manual of the Flora of
the Northern United States," 260 naturalised plants are enumerated, and
these belong to 162 genera. We thus see that these naturalised plants
are of a highly diversified nature. They differ, moreover, to a large
extent, from the indigenes, for out of the 162 naturalised genera,
no less than 100 genera are not there indigenous, and thus a large
proportional addition is made to the genera now living in the United
States.

By considering the nature of the plants or animals which have in any
country struggled successfully with the indigenes, and have there become
naturalised, we may gain some crude idea in what manner some of the
natives would have had to be modified in order to gain an advantage over
their compatriots; and we may at least infer that diversification of
structure, amounting to new generic differences, would be profitable to
them.

The advantage of diversification of structure in the inhabitants of the
same region is, in fact, the same as that of the physiological division
of labour in the organs of the same individual body--a subject so well
elucidated by Milne Edwards. No physiologist doubts that a stomach by
being adapted to digest vegetable matter alone, or flesh alone, draws
most nutriment from these substances. So in the general economy of
any land, the more widely and perfectly the animals and plants are
diversified for different habits of life, so will a greater number of
individuals be capable of there supporting themselves. A set of animals,
with their organisation but little diversified, could hardly compete
with a set more perfectly diversified in structure. It may be doubted,
for instance, whether the Australian marsupials, which are divided into
groups differing but little from each other, and feebly representing, as
Mr. Waterhouse and others have remarked, our carnivorous, ruminant, and
rodent mammals, could successfully compete with these well-developed
orders. In the Australian mammals, we see the process of diversification
in an early and incomplete stage of development.

THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH
DIVERGENCE OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF A COMMON
ANCESTOR.

After the foregoing discussion, which has been much compressed, we may
assume that the modified descendants of any one species will succeed so
much the better as they become more diversified in structure, and are
thus enabled to encroach on places occupied by other beings. Now let
us see how this principle of benefit being derived from divergence of
character, combined with the principles of natural selection and of
extinction, tends to act.

The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large
in its own country; these species are supposed to resemble each other
in unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal distances.
I have said a large genus, because as we saw in the second chapter, on
an average more species vary in large genera than in small genera; and
the varying species of the large genera present a greater number of
varieties. We have, also, seen that the species, which are the commonest
and most widely-diffused, vary more than do the rare and restricted
species. Let (A) be a common, widely-diffused, and varying species,
belonging to a genus large in its own country. The branching and
diverging dotted lines of unequal lengths proceeding from (A), may
represent its varying offspring. The variations are supposed to be
extremely slight, but of the most diversified nature; they are not
supposed all to appear simultaneously, but often after long intervals of
time; nor are they all supposed to endure for equal periods. Only
those variations which are in some way profitable will be preserved or
naturally selected. And here the importance of the principle of benefit
derived from divergence of character comes in; for this will generally
lead to the most different or divergent variations (represented by
the outer dotted lines) being preserved and accumulated by natural
selection. When a dotted line reaches one of the horizontal lines,
and is there marked by a small numbered letter, a sufficient amount of
variation is supposed to have been accumulated to form it into a fairly
well-marked variety, such as would be thought worthy of record in a
systematic work.

The intervals between the horizontal lines in the diagram, may represent
each a thousand or more generations. After a thousand generations,
species (A) is supposed to have produced two fairly well-marked
varieties, namely a1 and m1. These two varieties will generally still
be exposed to the same conditions which made their parents variable, and
the tendency to variability is in itself hereditary; consequently they
will likewise tend to vary, and commonly in nearly the same manner as
did their parents. Moreover, these two varieties, being only slightly
modified forms, will tend to inherit those advantages which made their
parent (A) more numerous than most of the other inhabitants of the same
country; they will also partake of those more general advantages which
made the genus to which the parent-species belonged, a large genus
in its own country. And all these circumstances are favourable to the
production of new varieties.

If, then, these two varieties be variable, the most divergent of
their variations will generally be preserved during the next thousand
generations. And after this interval, variety a1 is supposed in the
diagram to have produced variety a2, which will, owing to the principle
of divergence, differ more from (A) than did variety a1. Variety m1 is
supposed to have produced two varieties, namely m2 and s2, differing
from each other, and more considerably from their common parent (A). We
may continue the process by similar steps for any length of time; some
of the varieties, after each thousand generations, producing only
a single variety, but in a more and more modified condition, some
producing two or three varieties, and some failing to produce any. Thus
the varieties or modified descendants of the common parent (A), will
generally go on increasing in number and diverging in character. In the
diagram the process is represented up to the ten-thousandth generation,
and under a condensed and simplified form up to the fourteen-thousandth
generation.

But I must here remark that I do not suppose that the process ever goes
on so regularly as is represented in the diagram, though in itself made
somewhat irregular, nor that it goes on continuously; it is far more
probable that each form remains for long periods unaltered, and then
again undergoes modification. Nor do I suppose that the most divergent
varieties are invariably preserved: a medium form may often long endure,
and may or may not produce more than one modified descendant; for
natural selection will always act according to the nature of the places
which are either unoccupied or not perfectly occupied by other beings;
and this will depend on infinitely complex relations. But as a general
rule, the more diversified in structure the descendants from any one
species can be rendered, the more places they will be enabled to seize
on, and the more their modified progeny will increase. In our diagram
the line of succession is broken at regular intervals by small numbered
letters marking the successive forms which have become sufficiently
distinct to be recorded as varieties. But these breaks are imaginary,
and might have been inserted anywhere, after intervals long enough to
allow the accumulation of a considerable amount of divergent variation.

As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the
same advantages which made their parent successful in life, they will
generally go on multiplying in number as well as diverging in character:
this is represented in the diagram by the several divergent branches
proceeding from (A). The modified offspring from the later and more
highly improved branches in the lines of descent, will, it is probable,
often take the place of, and so destroy, the earlier and less improved
branches: this is represented in the diagram by some of the lower
branches not reaching to the upper horizontal lines. In some cases no
doubt the process of modification will be confined to a single line of
descent, and the number of modified descendants will not be increased;
although the amount of divergent modification may have been augmented.
This case would be represented in the diagram, if all the lines
proceeding from (A) were removed, excepting that from a1 to a10. In the
same way the English racehorse and English pointer have apparently
both gone on slowly diverging in character from their original stocks,
without either having given off any fresh branches or races.

After ten thousand generations, species (A) is supposed to have produced
three forms, a10, f10, and m10, which, from having diverged in character
during the successive generations, will have come to differ largely, but
perhaps unequally, from each other and from their common parent. If we
suppose the amount of change between each horizontal line in our diagram
to be excessively small, these three forms may still be only well-marked
varieties; but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to convert these
three forms into doubtful or at least into well-defined species:
thus the diagram illustrates the steps by which the small differences
distinguishing varieties are increased into the larger differences
distinguishing species. By continuing the same process for a greater
number of generations (as shown in the diagram in a condensed and
simplified manner), we get eight species, marked by the letters between
a14 and m14, all descended from (A). Thus, as I believe, species are
multiplied and genera are formed.

In a large genus it is probable that more than one species would vary.
In the diagram I have assumed that a second species (I) has produced, by
analogous steps, after ten thousand generations, either two well-marked
varieties (w10 and z10) or two species, according to the amount of
change supposed to be represented between the horizontal lines. After
fourteen thousand generations, six new species, marked by the letters
n14 to z14, are supposed to have been produced. In any genus, the
species which are already very different in character from each
other, will generally tend to produce the greatest number of modified
descendants; for these will have the best chance of seizing on new and
widely different places in the polity of nature: hence in the diagram I
have chosen the extreme species (A), and the nearly extreme species (I),
as those which have largely varied, and have given rise to new varieties
and species. The other nine species (marked by capital letters) of our
original genus, may for long but unequal periods continue to transmit
unaltered descendants; and this is shown in the diagram by the dotted
lines unequally prolonged upwards.

But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played
an important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency in
the improved descendants of any one species to supplant and exterminate
in each stage of descent their predecessors and their original
progenitor. For it should be remembered that the competition will
generally be most severe between those forms which are most nearly
related to each other in habits, constitution and structure. Hence all
the intermediate forms between the earlier and later states, that is
between the less and more improved states of a the same species, as well
as the original parent-species itself, will generally tend to become
extinct. So it probably will be with many whole collateral lines of
descent, which will be conquered by later and improved lines. If,
however, the modified offspring of a species get into some distinct
country, or become quickly adapted to some quite new station, in which
offspring and progenitor do not come into competition, both may continue
to exist.

If, then, our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have become
extinct, being replaced by eight new species (a14 to m14); and species
(I) will be replaced by six (n14 to z14) new species.

But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally
the case in nature; species (A) being more nearly related to B, C, and
D than to the other species; and species (I) more to G, H, K, L, than to
the others. These two species (A and I), were also supposed to be very
common and widely diffused species, so that they must originally have
had some advantage over most of the other species of the genus. Their
modified descendants, fourteen in number at the fourteen-thousandth
generation, will probably have inherited some of the same advantages:
they have also been modified and improved in a diversified manner at
each stage of descent, so as to have become adapted to many related
places in the natural economy of their country. It seems, therefore,
extremely probable that they will have taken the places of, and thus
exterminated, not only their parents (A) and (I), but likewise some of
the original species which were most nearly related to their parents.
Hence very few of the original species will have transmitted offspring
to the fourteen-thousandth generation. We may suppose that only one (F)
of the two species (E and F) which were least closely related to the
other nine original species, has transmitted descendants to this late
stage of descent.

The new species in our diagram, descended from the original eleven
species, will now be fifteen in number. Owing to the divergent tendency
of natural selection, the extreme amount of difference in character
between species a14 and z14 will be much greater than that between the
most distinct of the original eleven species. The new species, moreover,
will be allied to each other in a widely different manner. Of the eight
descendants from (A) the three marked a14, q14, p14, will be nearly
related from having recently branched off from a10; b14 and f14, from
having diverged at an earlier period from a5, will be in some degree
distinct from the three first-named species; and lastly, o14, e14, and
m14, will be nearly related one to the other, but, from having diverged
at the first commencement of the process of modification, will be widely
different from the other five species, and may constitute a sub-genus or
a distinct genus.

The six descendants from (I) will form two sub-genera or genera. But as
the original species (I) differed largely from (A), standing nearly
at the extreme end of the original genus, the six descendants from (I)
will, owing to inheritance alone, differ considerably from the eight
descendants from (A); the two groups, moreover, are supposed to have
gone on diverging in different directions. The intermediate species,
also (and this is a very important consideration), which connected the
original species (A) and (I), have all become, except (F), extinct, and
have left no descendants. Hence the six new species descended from
(I), and the eight descendants from (A), will have to be ranked as very
distinct genera, or even as distinct sub-families.

Thus it is, as I believe, that two or more genera are produced by
descent with modification, from two or more species of the same genus.
And the two or more parent-species are supposed to be descended from
some one species of an earlier genus. In our diagram this is indicated
by the broken lines beneath the capital letters, converging in
sub-branches downwards towards a single point; this point represents
a species, the supposed progenitor of our several new sub-genera and
genera.

It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not to have diverged much in character,
but to have retained the form of (F), either unaltered or altered only
in a slight degree. In this case its affinities to the other fourteen
new species will be of a curious and circuitous nature. Being descended
from a form that stood between the parent-species (A) and (I),
now supposed to be extinct and unknown, it will be in some degree
intermediate in character between the two groups descended from these
two species. But as these two groups have gone on diverging in character
from the type of their parents, the new species (F14) will not be
directly intermediate between them, but rather between types of the two
groups; and every naturalist will be able to call such cases before his
mind.

In the diagram each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or
more generations; it may also represent a section of the successive
strata of the earth's crust including extinct remains. We shall, when we
come to our chapter on geology, have to refer again to this subject,
and I think we shall then see that the diagram throws light on the
affinities of extinct beings, which, though generally belonging to the
same orders, families, or genera, with those now living, yet are often,
in some degree, intermediate in character between existing groups; and
we can understand this fact, for the extinct species lived at various
remote epochs when the branching lines of descent had diverged less.

I see no reason to limit the process of modification, as now explained,
to the formation of genera alone. If, in the diagram, we suppose the
amount of change represented by each successive group of diverging
dotted lines to be great, the forms marked a14 to p14, those marked
b14 and f14, and those marked o14 to m14, will form three very distinct
genera. We shall also have two very distinct genera descended from (I),
differing widely from the descendants of (A). These two groups of genera
will thus form two distinct families, or orders, according to the amount
of divergent modification supposed to be represented in the diagram. And
the two new families, or orders, are descended from two species of the
original genus; and these are supposed to be descended from some still
more ancient and unknown form.

We have seen that in each country it is the species belonging to the
larger genera which oftenest present varieties or incipient species.
This, indeed, might have been expected; for as natural selection acts
through one form having some advantage over other forms in the struggle
for existence, it will chiefly act on those which already have some
advantage; and the largeness of any group shows that its species have
inherited from a common ancestor some advantage in common. Hence, the
struggle for the production of new and modified descendants will mainly
lie between the larger groups, which are all trying to increase in
number. One large group will slowly conquer another large group,
reduce its number, and thus lessen its chance of further variation and
improvement. Within the same large group, the later and more highly
perfected sub-groups, from branching out and seizing on many new places
in the polity of nature, will constantly tend to supplant and destroy
the earlier and less improved sub-groups. Small and broken groups and
sub-groups will finally disappear. Looking to the future, we can predict
that the groups of organic beings which are now large and triumphant,
and which are least broken up, that is, which have as yet suffered least
extinction, will, for a long period, continue to increase. But which
groups will ultimately prevail, no man can predict; for we know that
many groups, formerly most extensively developed, have now become
extinct. Looking still more remotely to the future, we may predict
that, owing to the continued and steady increase of the larger groups,
a multitude of smaller groups will become utterly extinct, and leave no
modified descendants; and consequently that, of the species living at
any one period, extremely few will transmit descendants to a remote
futurity. I shall have to return to this subject in the chapter on
classification, but I may add that as, according to this view, extremely
few of the more ancient species have transmitted descendants to the
present day, and, as all the descendants of the same species form a
class, we can understand how it is that there exist so few classes in
each main division of the animal and vegetable kingdoms. Although few of
the most ancient species have left modified descendants, yet, at remote
geological periods, the earth may have been almost as well peopled with
species of many genera, families, orders and classes, as at the present
day.

ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE.

Natural selection acts exclusively by the preservation and accumulation
of variations, which are beneficial under the organic and inorganic
conditions to which each creature is exposed at all periods of life.
The ultimate result is that each creature tends to become more and more
improved in relation to its conditions. This improvement inevitably
leads to the gradual advancement of the organisation of the greater
number of living beings throughout the world. But here we enter on a
very intricate subject, for naturalists have not defined to each other's
satisfaction what is meant by an advance in organisation. Among the
vertebrata the degree of intellect and an approach in structure to man
clearly come into play. It might be thought that the amount of change
which the various parts and organs pass through in their development
from embryo to maturity would suffice as a standard of comparison; but
there are cases, as with certain parasitic crustaceans, in which several
parts of the structure become less perfect, so that the mature animal
cannot be called higher than its larva. Von Baer's standard seems
the most widely applicable and the best, namely, the amount of
differentiation of the parts of the same organic being, in the adult
state, as I should be inclined to add, and their specialisation
for different functions; or, as Milne Edwards would express it, the
completeness of the division of physiological labour. But we shall see
how obscure this subject is if we look, for instance, to fishes, among
which some naturalists rank those as highest which, like the sharks,
approach nearest to amphibians; while other naturalists rank the common
bony or teleostean fishes as the highest, inasmuch as they are most
strictly fish-like, and differ most from the other vertebrate classes.
We see still more plainly the obscurity of the subject by turning
to plants, among which the standard of intellect is of course quite
excluded; and here some botanists rank those plants as highest which
have every organ, as sepals, petals, stamens and pistils, fully
developed in each flower; whereas other botanists, probably with more
truth, look at the plants which have their several organs much modified
and reduced in number as the highest.

If we take as the standard of high organisation, the amount of
differentiation and specialisation of the several organs in each being
when adult (and this will include the advancement of the brain for
intellectual purposes), natural selection clearly leads towards this
standard: for all physiologists admit that the specialisation of organs,
inasmuch as in this state they perform their functions better, is
an advantage to each being; and hence the accumulation of variations
tending towards specialisation is within the scope of natural selection.
On the other hand, we can see, bearing in mind that all organic
beings are striving to increase at a high ratio and to seize on every
unoccupied or less well occupied place in the economy of nature, that
it is quite possible for natural selection gradually to fit a being to
a situation in which several organs would be superfluous or useless: in
such cases there would be retrogression in the scale of organisation.
Whether organisation on the whole has actually advanced from the
remotest geological periods to the present day will be more conveniently
discussed in our chapter on Geological Succession.

But it may be objected that if all organic beings thus tend to rise in
the scale, how is it that throughout the world a multitude of the lowest
forms still exist; and how is it that in each great class some forms
are far more highly developed than others? Why have not the more highly
developed forms every where supplanted and exterminated the lower?
Lamarck, who believed in an innate and inevitable tendency towards
perfection in all organic beings, seems to have felt this difficulty
so strongly that he was led to suppose that new and simple forms are
continually being produced by spontaneous generation. Science has not as
yet proved the truth of this belief, whatever the future may reveal.
On our theory the continued existence of lowly organisms offers no
difficulty; for natural selection, or the survival of the fittest, does
not necessarily include progressive development--it only takes advantage
of such variations as arise and are beneficial to each creature under
its complex relations of life. And it may be asked what advantage,
as far as we can see, would it be to an infusorian animalcule--to an
intestinal worm--or even to an earth-worm, to be highly organised. If
it were no advantage, these forms would be left, by natural selection,
unimproved or but little improved, and might remain for indefinite ages
in their present lowly condition. And geology tells us that some of
the lowest forms, as the infusoria and rhizopods, have remained for an
enormous period in nearly their present state. But to suppose that most
of the many now existing low forms have not in the least advanced since
the first dawn of life would be extremely rash; for every naturalist who
has dissected some of the beings now ranked as very low in the
scale, must have been struck with their really wondrous and beautiful
organisation.

Nearly the same remarks are applicable, if we look to the different
grades of organisation within the same great group; for instance, in the
vertebrata, to the co-existence of mammals and fish--among mammalia, to
the co-existence of man and the ornithorhynchus--among fishes, to the
co-existence of the shark and the lancelet (Amphioxus), which latter
fish in the extreme simplicity of its structure approaches the
invertebrate classes. But mammals and fish hardly come into competition
with each other; the advancement of the whole class of mammals, or of
certain members in this class, to the highest grade would not lead to
their taking the place of fishes. Physiologists believe that the brain
must be bathed by warm blood to be highly active, and this requires
aerial respiration; so that warm-blooded mammals when inhabiting the
water lie under a disadvantage in having to come continually to the
surface to breathe. With fishes, members of the shark family would not
tend to supplant the lancelet; for the lancelet, as I hear from Fritz
Muller, has as sole companion and competitor on the barren sandy shore
of South Brazil, an anomalous annelid. The three lowest orders of
mammals, namely, marsupials, edentata, and rodents, co-exist in South
America in the same region with numerous monkeys, and probably interfere
little with each other. Although organisation, on the whole, may have
advanced and be still advancing throughout the world, yet the scale will
always present many degrees of perfection; for the high advancement of
certain whole classes, or of certain members of each class, does not at
all necessarily lead to the extinction of those groups with which
they do not enter into close competition. In some cases, as we shall
hereafter see, lowly organised forms appear to have been preserved to
the present day, from inhabiting confined or peculiar stations, where
they have been subjected to less severe competition, and where their
scanty numbers have retarded the chance of favourable variations
arising.

Finally, I believe that many lowly organised forms now exist throughout
the world, from various causes. In some cases variations or individual
differences of a favourable nature may never have arisen for natural
selection to act on and accumulate. In no case, probably, has time
sufficed for the utmost possible amount of development. In some few
cases there has been what we must call retrogression or organisation.
But the main cause lies in the fact that under very simple conditions
of life a high organisation would be of no service--possibly would be of
actual disservice, as being of a more delicate nature, and more liable
to be put out of order and injured.

Looking to the first dawn of life, when all organic beings, as we may
believe, presented the simplest structure, how, it has been asked,
could the first step in the advancement or differentiation of parts
have arisen? Mr. Herbert Spencer would probably answer that, as soon as
simple unicellular organisms came by growth or division to be compounded
of several cells, or became attached to any supporting surface, his law
"that homologous units of any order become differentiated in proportion
as their relations to incident forces become different" would come into
action. But as we have no facts to guide us, speculation on the subject
is almost useless. It is, however, an error to suppose that there would
be no struggle for existence, and, consequently, no natural selection,
until many forms had been produced: variations in a single species
inhabiting an isolated station might be beneficial, and thus the whole
mass of individuals might be modified, or two distinct forms might
arise. But, as I remarked towards the close of the introduction, no
one ought to feel surprise at much remaining as yet unexplained on the
origin of species, if we make due allowance for our profound ignorance
on the mutual relations of the inhabitants of the world at the present
time, and still more so during past ages.

CONVERGENCE OF CHARACTER.

Mr. H.C. Watson thinks that I have overrated the importance of
divergence of character (in which, however, he apparently believes), and
that convergence, as it may be called, has likewise played a part. If
two species belonging to two distinct though allied genera, had both
produced a large number of new and divergent forms, it is conceivable
that these might approach each other so closely that they would have
all to be classed under the same genus; and thus the descendants of two
distinct genera would converge into one. But it would in most cases
be extremely rash to attribute to convergence a close and general
similarity of structure in the modified descendants of widely distinct
forms. The shape of a crystal is determined solely by the molecular
forces, and it is not surprising that dissimilar substances should
sometimes assume the same form; but with organic beings we should
bear in mind that the form of each depends on an infinitude of complex
relations, namely on the variations which have arisen, these being due
to causes far too intricate to be followed out--on the nature of the
variations which have been preserved or selected, and this depends on
the surrounding physical conditions, and in a still higher degree on
the surrounding organisms with which each being has come into
competition--and lastly, on inheritance (in itself a fluctuating
element) from innumerable progenitors, all of which have had their forms
determined through equally complex relations. It is incredible that the
descendants of two organisms, which had originally differed in a marked
manner, should ever afterwards converge so closely as to lead to a near
approach to identity throughout their whole organisation. If this had
occurred, we should meet with the same form, independently of genetic
connection, recurring in widely separated geological formations; and the
balance of evidence is opposed to any such an admission.

Mr. Watson has also objected that the continued action of natural
selection, together with divergence of character, would tend to make an
indefinite number of specific forms. As far as mere inorganic conditions
are concerned, it seems probable that a sufficient number of species
would soon become adapted to all considerable diversities of heat,
moisture, etc.; but I fully admit that the mutual relations of organic
beings are more important; and as the number of species in any country
goes on increasing, the organic conditions of life must become more and
more complex. Consequently there seems at first no limit to the amount
of profitable diversification of structure, and therefore no limit to
the number of species which might be produced. We do not know that even
the most prolific area is fully stocked with specific forms: at the Cape
of Good Hope and in Australia, which support such an astonishing number
of species, many European plants have become naturalised. But geology
shows us, that from an early part of the tertiary period the number of
species of shells, and that from the middle part of this same period,
the number of mammals has not greatly or at all increased. What then
checks an indefinite increase in the number of species? The amount of
life (I do not mean the number of specific forms) supported on an
area must have a limit, depending so largely as it does on physical
conditions; therefore, if an area be inhabited by very many species,
each or nearly each species will be represented by few individuals;
and such species will be liable to extermination from accidental
fluctuations in the nature of the seasons or in the number of their
enemies. The process of extermination in such cases would be rapid,
whereas the production of new species must always be slow. Imagine the
extreme case of as many species as individuals in England, and the
first severe winter or very dry summer would exterminate thousands on
thousands of species. Rare species, and each species will become rare
if the number of species in any country becomes indefinitely increased,
will, on the principal often explained, present within a given period
few favourable variations; consequently, the process of giving birth to
new specific forms would thus be retarded. When any species becomes
very rare, close interbreeding will help to exterminate it; authors have
thought that this comes into play in accounting for the deterioration
of the aurochs in Lithuania, of red deer in Scotland and of bears
in Norway, etc. Lastly, and this I am inclined to think is the most
important element, a dominant species, which has already beaten many
competitors in its own home, will tend to spread and supplant many
others. Alph. de Candolle has shown that those species which spread
widely tend generally to spread VERY widely, consequently they will tend
to supplant and exterminate several species in several areas, and thus
check the inordinate increase of specific forms throughout the world.
Dr. Hooker has recently shown that in the southeast corner of Australia,
where, apparently, there are many invaders from different quarters of
the globe, the endemic Australian species have been greatly reduced in
number. How much weight to attribute to these several considerations I
will not pretend to say; but conjointly they must limit in each country
the tendency to an indefinite augmentation of specific forms.

SUMMARY OF CHAPTER.

If under changing conditions of life organic beings present individual
differences in almost every part of their structure, and this cannot be
disputed; if there be, owing to their geometrical rate of increase, a
severe struggle for life at some age, season or year, and this certainly
cannot be disputed; then, considering the infinite complexity of the
relations of all organic beings to each other and to their conditions
of life, causing an infinite diversity in structure, constitution, and
habits, to be advantageous to them, it would be a most extraordinary
fact if no variations had ever occurred useful to each being's own
welfare, in the same manner as so many variations have occurred useful
to man. But if variations useful to any organic being ever do occur,
assuredly individuals thus characterised will have the best chance of
being preserved in the struggle for life; and from the strong principle
of inheritance, these will tend to produce offspring similarly
characterised. This principle of preservation, or the survival of the
fittest, I have called natural selection. It leads to the improvement
of each creature in relation to its organic and inorganic conditions of
life; and consequently, in most cases, to what must be regarded as an
advance in organisation. Nevertheless, low and simple forms will long
endure if well fitted for their simple conditions of life.

Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young as easily as the
adult. Among many animals sexual selection will have given its aid to
ordinary selection by assuring to the most vigorous and best adapted
males the greatest number of offspring. Sexual selection will also give
characters useful to the males alone in their struggles or rivalry with
other males; and these characters will be transmitted to one sex or to
both sexes, according to the form of inheritance which prevails.

Whether natural selection has really thus acted in adapting the various
forms of life to their several conditions and stations, must be judged
by the general tenour and balance of evidence given in the following
chapters. But we have already seen how it entails extinction; and how
largely extinction has acted in the world's history, geology plainly
declares. Natural selection, also, leads to divergence of character; for
the more organic beings diverge in structure, habits and constitution,
by so much the more can a large number be supported on the area, of
which we see proof by looking to the inhabitants of any small spot, and
to the productions naturalised in foreign lands. Therefore, during
the modification of the descendants of any one species, and during
the incessant struggle of all species to increase in numbers, the more
diversified the descendants become, the better will be their chance
of success in the battle for life. Thus the small differences
distinguishing varieties of the same species, steadily tend to increase,
till they equal the greater differences between species of the same
genus, or even of distinct genera.

We have seen that it is the common, the widely diffused, and widely
ranging species, belonging to the larger genera within each class, which
vary most; and these tend to transmit to their modified offspring
that superiority which now makes them dominant in their own countries.
Natural selection, as has just been remarked, leads to divergence of
character and to much extinction of the less improved and intermediate
forms of life. On these principles, the nature of the affinities, and
the generally well defined distinctions between the innumerable organic
beings in each class throughout the world, may be explained. It is a
truly wonderful fact--the wonder of which we are apt to overlook from
familiarity--that all animals and all plants throughout all time and
space should be related to each other in groups, subordinate to groups,
in the manner which we everywhere behold--namely, varieties of the same
species most closely related, species of the same genus less closely and
unequally related, forming sections and sub-genera, species of distinct
genera much less closely related, and genera related in different
degrees, forming sub-families, families, orders, sub-classes, and
classes. The several subordinate groups in any class cannot be ranked
in a single file, but seem clustered round points, and these round
other points, and so on in almost endless cycles. If species had been
independently created, no explanation would have been possible of this
kind of classification; but it is explained through inheritance and the
complex action of natural selection, entailing extinction and divergence
of character, as we have seen illustrated in the diagram.

The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during former years may represent the long succession
of extinct species. At each period of growth all the growing twigs
have tried to branch out on all sides, and to overtop and kill the
surrounding twigs and branches, in the same manner as species and groups
of species have at all times overmastered other species in the great
battle for life. The limbs divided into great branches, and these into
lesser and lesser branches, were themselves once, when the tree was
young, budding twigs; and this connexion of the former and present
buds by ramifying branches may well represent the classification of all
extinct and living species in groups subordinate to groups. Of the many
twigs which flourished when the tree was a mere bush, only two or three,
now grown into great branches, yet survive and bear the other branches;
so with the species which lived during long-past geological periods,
very few have left living and modified descendants. From the first
growth of the tree, many a limb and branch has decayed and dropped off;
and these fallen branches of various sizes may represent those whole
orders, families, and genera which have now no living representatives,
and which are known to us only in a fossil state. As we here and there
see a thin, straggling branch springing from a fork low down in a tree,
and which by some chance has been favoured and is still alive on its
summit, so we occasionally see an animal like the Ornithorhynchus or
Lepidosiren, which in some small degree connects by its affinities two
large branches of life, and which has apparently been saved from fatal
competition by having inhabited a protected station. As buds give rise
by growth to fresh buds, and these, if vigorous, branch out and overtop
on all sides many a feebler branch, so by generation I believe it has
been with the great Tree of Life, which fills with its dead and broken
branches the crust of the earth, and covers the surface with its
ever-branching and beautiful ramifications.



CHAPTER V. LAWS OF VARIATION.

 Effects of changed conditions--Use and disuse, combined with natural
 selection; organs of flight and of vision--Acclimatisation--Correlated
 variation--Compensation and economy of growth--False
 correlations--Multiple, rudimentary, and lowly organised structures
 variable--Parts developed in an unusual manner are highly variable:
 specific characters more variable than generic: secondary sexual
 characters variable--Species of the same genus vary in an analogous
 manner--Reversions to long-lost characters--Summary.


I have hitherto sometimes spoken as if the variations--so common and
multiform with organic beings under domestication, and in a lesser
degree with those under nature--were due to chance. This, of course is
a wholly incorrect expression, but it serves to acknowledge plainly
our ignorance of the cause of each particular variation. Some authors
believe it to be as much the function of the reproductive system to
produce individual differences, or slight deviations of structure, as
to make the child like its parents. But the fact of variations and
monstrosities occurring much more frequently under domestication than
under nature, and the greater variability of species having wide ranges
than of those with restricted ranges, lead to the conclusion that
variability is generally related to the conditions of life to which each
species has been exposed during several successive generations. In the
first chapter I attempted to show that changed conditions act in two
ways, directly on the whole organisation or on certain parts alone, and
indirectly through the reproductive system. In all cases there are two
factors, the nature of the organism, which is much the most important of
the two, and the nature of the conditions. The direct action of changed
conditions leads to definite or indefinite results. In the latter case
the organisation seems to become plastic, and we have much fluctuating
variability. In the former case the nature of the organism is such that
it yields readily, when subjected to certain conditions, and all, or
nearly all, the individuals become modified in the same way.

It is very difficult to decide how far changed conditions, such as of
climate, food, etc., have acted in a definite manner. There is reason
to believe that in the course of time the effects have been greater than
can be proved by clear evidence. But we may safely conclude that the
innumerable complex co-adaptations of structure, which we see throughout
nature between various organic beings, cannot be attributed simply to
such action. In the following cases the conditions seem to have produced
some slight definite effect: E. Forbes asserts that shells at their
southern limit, and when living in shallow water, are more brightly
coloured than those of the same species from further north or from a
greater depth; but this certainly does not always hold good. Mr. Gould
believes that birds of the same species are more brightly coloured under
a clear atmosphere, than when living near the coast or on islands; and
Wollaston is convinced that residence near the sea affects the colours
of insects. Moquin-Tandon gives a list of plants which, when growing
near the sea-shore, have their leaves in some degree fleshy, though not
elsewhere fleshy. These slightly varying organisms are interesting in
as far as they present characters analogous to those possessed by the
species which are confined to similar conditions.

When a variation is of the slightest use to any being, we cannot tell
how much to attribute to the accumulative action of natural selection,
and how much to the definite action of the conditions of life. Thus, it
is well known to furriers that animals of the same species have thicker
and better fur the further north they live; but who can tell how much of
this difference may be due to the warmest-clad individuals having been
favoured and preserved during many generations, and how much to the
action of the severe climate? For it would appear that climate has some
direct action on the hair of our domestic quadrupeds.

Instances could be given of similar varieties being produced from the
same species under external conditions of life as different as can well
be conceived; and, on the other hand, of dissimilar varieties being
produced under apparently the same external conditions. Again,
innumerable instances are known to every naturalist, of species keeping
true, or not varying at all, although living under the most opposite
climates. Such considerations as these incline me to lay less weight on
the direct action of the surrounding conditions, than on a tendency to
vary, due to causes of which we are quite ignorant.

In one sense the conditions of life may be said, not only to cause
variability, either directly or indirectly, but likewise to include
natural selection, for the conditions determine whether this or that
variety shall survive. But when man is the selecting agent, we clearly
see that the two elements of change are distinct; variability is in
some manner excited, but it is the will of man which accumulates the
variations in certain direction; and it is this latter agency which
answers to the survival of the fittest under nature.

EFFECTS OF THE INCREASED USE AND DISUSE OF PARTS, AS CONTROLLED BY
NATURAL SELECTION.

From the facts alluded to in the first chapter, I think there can be
no doubt that use in our domestic animals has strengthened and enlarged
certain parts, and disuse diminished them; and that such modifications
are inherited. Under free nature we have no standard of comparison by
which to judge of the effects of long-continued use or disuse, for we
know not the parent-forms; but many animals possess structures which
can be best explained by the effects of disuse. As Professor Owen has
remarked, there is no greater anomaly in nature than a bird that cannot
fly; yet there are several in this state. The logger-headed duck of
South America can only flap along the surface of the water, and has its
wings in nearly the same condition as the domestic Aylesbury duck: it is
a remarkable fact that the young birds, according to Mr. Cunningham, can
fly, while the adults have lost this power. As the larger ground-feeding
birds seldom take flight except to escape danger, it is probable that
the nearly wingless condition of several birds, now inhabiting or which
lately inhabited several oceanic islands, tenanted by no beasts of prey,
has been caused by disuse. The ostrich indeed inhabits continents, and
is exposed to danger from which it cannot escape by flight, but it
can defend itself, by kicking its enemies, as efficiently as many
quadrupeds. We may believe that the progenitor of the ostrich genus had
habits like those of the bustard, and that, as the size and weight of
its body were increased during successive generations, its legs were
used more and its wings less, until they became incapable of flight.

Kirby has remarked (and I have observed the same fact) that the anterior
tarsi, or feet, of many male dung-feeding beetles are often broken off;
he examined seventeen specimens in his own collection, and not one had
even a relic left. In the Onites apelles the tarsi are so habitually
lost that the insect has been described as not having them. In some
other genera they are present, but in a rudimentary condition. In the
Ateuchus or sacred beetle of the Egyptians, they are totally deficient.
The evidence that accidental mutilations can be inherited is at present
not decisive; but the remarkable cases observed by Brown-Sequard in
guinea-pigs, of the inherited effects of operations, should make us
cautious in denying this tendency. Hence, it will perhaps be safest to
look at the entire absence of the anterior tarsi in Ateuchus, and their
rudimentary condition in some other genera, not as cases of inherited
mutilations, but as due to the effects of long-continued disuse; for
as many dung-feeding beetles are generally found with their tarsi lost,
this must happen early in life; therefore the tarsi cannot be of much
importance or be much used by these insects.

In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of
the 550 species (but more are now known) inhabiting Madeira, are so far
deficient in wings that they cannot fly; and that, of the twenty-nine
endemic genera, no less than twenty-three have all their species in
this condition! Several facts, namely, that beetles in many parts of the
world are very frequently blown to sea and perish; that the beetles in
Madeira, as observed by Mr. Wollaston, lie much concealed, until the
wind lulls and the sun shines; that the proportion of wingless beetles
is larger on the exposed Desertas than in Madeira itself; and especially
the extraordinary fact, so strongly insisted on by Mr. Wollaston, that
certain large groups of beetles, elsewhere excessively numerous, which
absolutely require the use of their wings, are here almost entirely
absent. These several considerations make me believe that the wingless
condition of so many Madeira beetles is mainly due to the action of
natural selection, combined probably with disuse. For during many
successive generations each individual beetle which flew least, either
from its wings having been ever so little less perfectly developed or
from indolent habit, will have had the best chance of surviving from not
being blown out to sea; and, on the other hand, those beetles which most
readily took to flight would oftenest have been blown to sea, and thus
destroyed.

The insects in Madeira which are not ground-feeders, and which, as
certain flower-feeding coleoptera and lepidoptera, must habitually use
their wings to gain their subsistence, have, as Mr. Wollaston suspects,
their wings not at all reduced, but even enlarged. This is quite
compatible with the action of natural selection. For when a new insect
first arrived on the island, the tendency of natural selection to
enlarge or to reduce the wings, would depend on whether a greater number
of individuals were saved by successfully battling with the winds, or
by giving up the attempt and rarely or never flying. As with mariners
shipwrecked near a coast, it would have been better for the good
swimmers if they had been able to swim still further, whereas it would
have been better for the bad swimmers if they had not been able to swim
at all and had stuck to the wreck.

The eyes of moles and of some burrowing rodents are rudimentary in size,
and in some cases are quite covered by skin and fur. This state of the
eyes is probably due to gradual reduction from disuse, but aided
perhaps by natural selection. In South America, a burrowing rodent, the
tuco-tuco, or Ctenomys, is even more subterranean in its habits than the
mole; and I was assured by a Spaniard, who had often caught them, that
they were frequently blind. One which I kept alive was certainly in
this condition, the cause, as appeared on dissection, having been
inflammation of the nictitating membrane. As frequent inflammation of
the eyes must be injurious to any animal, and as eyes are certainly not
necessary to animals having subterranean habits, a reduction in their
size, with the adhesion of the eyelids and growth of fur over them,
might in such case be an advantage; and if so, natural selection would
aid the effects of disuse.

It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Carniola and Kentucky, are blind. In
some of the crabs the foot-stalk for the eye remains, though the eye is
gone; the stand for the telescope is there, though the telescope with
its glasses has been lost. As it is difficult to imagine that eyes,
though useless, could be in any way injurious to animals living in
darkness, their loss may be attributed to disuse. In one of the blind
animals, namely, the cave-rat (Neotoma), two of which were captured by
Professor Silliman at above half a mile distance from the mouth of
the cave, and therefore not in the profoundest depths, the eyes were
lustrous and of large size; and these animals, as I am informed by
Professor Silliman, after having been exposed for about a month to a
graduated light, acquired a dim perception of objects.

It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that, in accordance
with the old view of the blind animals having been separately created
for the American and European caverns, very close similarity in their
organisation and affinities might have been expected. This is certainly
not the case if we look at the two whole faunas; with respect to the
insects alone, Schiodte has remarked: "We are accordingly prevented
from considering the entire phenomenon in any other light than something
purely local, and the similarity which is exhibited in a few forms
between the Mammoth Cave (in Kentucky) and the caves in Carniola,
otherwise than as a very plain expression of that analogy which subsists
generally between the fauna of Europe and of North America." On my view
we must suppose that American animals, having in most cases ordinary
powers of vision, slowly migrated by successive generations from the
outer world into the deeper and deeper recesses of the Kentucky caves,
as did European animals into the caves of Europe. We have some evidence
of this gradation of habit; for, as Schiodte remarks: "We accordingly
look upon the subterranean faunas as small ramifications which have
penetrated into the earth from the geographically limited faunas of the
adjacent tracts, and which, as they extended themselves into darkness,
have been accommodated to surrounding circumstances. Animals not
far remote from ordinary forms, prepare the transition from light to
darkness. Next follow those that are constructed for twilight; and, last
of all, those destined for total darkness, and whose formation is quite
peculiar." These remarks of Schiodte's it should be understood, apply
not to the same, but to distinct species. By the time that an animal had
reached, after numberless generations, the deepest recesses, disuse
will on this view have more or less perfectly obliterated its eyes, and
natural selection will often have effected other changes, such as an
increase in the length of the antennae or palpi, as a compensation for
blindness. Notwithstanding such modifications, we might expect still to
see in the cave-animals of America, affinities to the other inhabitants
of that continent, and in those of Europe to the inhabitants of the
European continent. And this is the case with some of the American
cave-animals, as I hear from Professor Dana; and some of the European
cave-insects are very closely allied to those of the surrounding
country. It would be difficult to give any rational explanation of the
affinities of the blind cave-animals to the other inhabitants of the
two continents on the ordinary view of their independent creation. That
several of the inhabitants of the caves of the Old and New Worlds should
be closely related, we might expect from the well-known relationship
of most of their other productions. As a blind species of Bathyscia is
found in abundance on shady rocks far from caves, the loss of vision in
the cave species of this one genus has probably had no relation to its
dark habitation; for it is natural that an insect already deprived of
vision should readily become adapted to dark caverns. Another blind
genus (Anophthalmus) offers this remarkable peculiarity, that the
species, as Mr. Murray observes, have not as yet been found anywhere
except in caves; yet those which inhabit the several caves of Europe and
America are distinct; but it is possible that the progenitors of these
several species, while they were furnished with eyes, may formerly have
ranged over both continents, and then have become extinct, excepting in
their present secluded abodes. Far from feeling surprise that some of
the cave-animals should be very anomalous, as Agassiz has remarked in
regard to the blind fish, the Amblyopsis, and as is the case with the
blind Proteus, with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved,
owing to the less severe competition to which the scanty inhabitants of
these dark abodes will have been exposed.

ACCLIMATISATION.

Habit is hereditary with plants, as in the period of flowering, in the
time of sleep, in the amount of rain requisite for seeds to germinate,
etc., and this leads me to say a few words on acclimatisation. As it
is extremely common for distinct species belonging to the same genus to
inhabit hot and cold countries, if it be true that all the species of
the same genus are descended from a single parent-form, acclimatisation
must be readily effected during a long course of descent. It is
notorious that each species is adapted to the climate of its own home:
species from an arctic or even from a temperate region cannot endure a
tropical climate, or conversely. So again, many succulent plants cannot
endure a damp climate. But the degree of adaptation of species to the
climates under which they live is often overrated. We may infer this
from our frequent inability to predict whether or not an imported plant
will endure our climate, and from the number of plants and animals
brought from different countries which are here perfectly healthy. We
have reason to believe that species in a state of nature are closely
limited in their ranges by the competition of other organic beings quite
as much as, or more than, by adaptation to particular climates. But
whether or not this adaptation is in most cases very close, we have
evidence with some few plants, of their becoming, to a certain extent,
naturally habituated to different temperatures; that is, they become
acclimatised: thus the pines and rhododendrons, raised from seed
collected by Dr. Hooker from the same species growing at different
heights on the Himalayas, were found to possess in this country
different constitutional powers of resisting cold. Mr. Thwaites informs
me that he has observed similar facts in Ceylon; analogous observations
have been made by Mr. H.C. Watson on European species of plants brought
from the Azores to England; and I could give other cases. In regard to
animals, several authentic instances could be adduced of species having
largely extended, within historical times, their range from warmer to
colder latitudes, and conversely; but we do not positively know that
these animals were strictly adapted to their native climate, though in
all ordinary cases we assume such to be the case; nor do we know that
they have subsequently become specially acclimatised to their new homes,
so as to be better fitted for them than they were at first.

As we may infer that our domestic animals were originally chosen by
uncivilised man because they were useful, and because they bred readily
under confinement, and not because they were subsequently found capable
of far-extended transportation, the common and extraordinary capacity
in our domestic animals of not only withstanding the most different
climates, but of being perfectly fertile (a far severer test) under
them, may be used as an argument that a large proportion of other
animals now in a state of nature could easily be brought to bear widely
different climates. We must not, however, push the foregoing argument
too far, on account of the probable origin of some of our domestic
animals from several wild stocks: the blood, for instance, of a tropical
and arctic wolf may perhaps be mingled in our domestic breeds. The rat
and mouse cannot be considered as domestic animals, but they have been
transported by man to many parts of the world, and now have a far wider
range than any other rodent; for they live under the cold climate of
Faroe in the north and of the Falklands in the south, and on many an
island in the torrid zones. Hence adaptation to any special climate may
be looked at as a quality readily grafted on an innate wide flexibility
of constitution, common to most animals. On this view, the capacity of
enduring the most different climates by man himself and by his domestic
animals, and the fact of the extinct elephant and rhinoceros having
formerly endured a glacial climate, whereas the living species are now
all tropical or sub-tropical in their habits, ought not to be looked
at as anomalies, but as examples of a very common flexibility of
constitution, brought, under peculiar circumstances, into action.

How much of the acclimatisation of species to any peculiar climate is
due to mere habit, and how much to the natural selection of varieties
having different innate constitutions, and how much to both means
combined, is an obscure question. That habit or custom has some
influence, I must believe, both from analogy and from the incessant
advice given in agricultural works, even in the ancient Encyclopaedias
of China, to be very cautious in transporting animals from one district
to another. And as it is not likely that man should have succeeded in
selecting so many breeds and sub-breeds with constitutions specially
fitted for their own districts, the result must, I think, be due to
habit. On the other hand, natural selection would inevitably tend to
preserve those individuals which were born with constitutions best
adapted to any country which they inhabited. In treatises on many kinds
of cultivated plants, certain varieties are said to withstand certain
climates better than others; this is strikingly shown in works on
fruit-trees published in the United States, in which certain varieties
are habitually recommended for the northern and others for the southern
states; and as most of these varieties are of recent origin, they cannot
owe their constitutional differences to habit. The case of the Jerusalem
artichoke, which is never propagated in England by seed, and of which,
consequently, new varieties have not been produced, has even been
advanced, as proving that acclimatisation cannot be effected, for it
is now as tender as ever it was! The case, also, of the kidney-bean has
been often cited for a similar purpose, and with much greater weight;
but until some one will sow, during a score of generations, his
kidney-beans so early that a very large proportion are destroyed by
frost, and then collect seed from the few survivors, with care
to prevent accidental crosses, and then again get seed from these
seedlings, with the same precautions, the experiment cannot be said to
have been even tried. Nor let it be supposed that differences in the
constitution of seedling kidney-beans never appear, for an account has
been published how much more hardy some seedlings are than others; and
of this fact I have myself observed striking instances.

On the whole, we may conclude that habit, or use and disuse, have,
in some cases, played a considerable part in the modification of the
constitution and structure; but that the effects have often been largely
combined with, and sometimes overmastered by, the natural selection of
innate variations.

CORRELATED VARIATION.

I mean by this expression that the whole organisation is so tied
together, during its growth and development, that when slight variations
in any one part occur and are accumulated through natural selection,
other parts become modified. This is a very important subject, most
imperfectly understood, and no doubt wholly different classes of facts
may be here easily confounded together. We shall presently see that
simple inheritance often gives the false appearance of correlation. One
of the most obvious real cases is, that variations of structure arising
in the young or larvae naturally tend to affect the structure of the
mature animal. The several parts which are homologous, and which, at
an early embryonic period, are identical in structure, and which are
necessarily exposed to similar conditions, seem eminently liable to vary
in a like manner: we see this in the right and left sides of the body
varying in the same manner; in the front and hind legs, and even in the
jaws and limbs, varying together, for the lower jaw is believed by some
anatomists to be homologous with the limbs. These tendencies, I do not
doubt, may be mastered more or less completely by natural selection:
thus a family of stags once existed with an antler only on one side; and
if this had been of any great use to the breed, it might probably have
been rendered permanent by natural selection.

Homologous parts, as has been remarked by some authors, tend to cohere;
this is often seen in monstrous plants: and nothing is more common than
the union of homologous parts in normal structures, as in the union of
the petals into a tube. Hard parts seem to affect the form of adjoining
soft parts; it is believed by some authors that with birds the diversity
in the shape of the pelvis causes the remarkable diversity in the shape
of the kidneys. Others believe that the shape of the pelvis in the human
mother influences by pressure the shape of the head of the child. In
snakes, according to Schlegel, the shape of the body and the manner
of swallowing determine the position and form of several of the most
important viscera.

The nature of the bond is frequently quite obscure. M. Is. Geoffroy St.
Hilaire has forcibly remarked that certain malconformations frequently,
and that others rarely, coexist without our being able to assign any
reason. What can be more singular than the relation in cats between
complete whiteness and blue eyes with deafness, or between the
tortoise-shell colour and the female sex; or in pigeons, between their
feathered feet and skin betwixt the outer toes, or between the presence
of more or less down on the young pigeon when first hatched, with the
future colour of its plumage; or, again, the relation between the hair
and the teeth in the naked Turkish dog, though here no doubt homology
comes into play? With respect to this latter case of correlation, I
think it can hardly be accidental that the two orders of mammals which
are most abnormal in their dermal covering, viz., Cetacea (whales) and
Edentata (armadilloes, scaly ant-eaters, etc.), are likewise on the
whole the most abnormal in their teeth, but there are so many exceptions
to this rule, as Mr. Mivart has remarked, that it has little value.

I know of no case better adapted to show the importance of the laws of
correlation and variation, independently of utility, and therefore of
natural selection, than that of the difference between the outer and
inner flowers in some Compositous and Umbelliferous plants. Everyone is
familiar with the difference between the ray and central florets of, for
instance, the daisy, and this difference is often accompanied with the
partial or complete abortion of the reproductive organs. But in some
of these plants the seeds also differ in shape and sculpture. These
differences have sometimes been attributed to the pressure of the
involucra on the florets, or to their mutual pressure, and the shape of
the seeds in the ray-florets of some Compositae countenances this idea;
but with the Umbelliferae it is by no means, as Dr. Hooker informs me,
the species with the densest heads which most frequently differ in their
inner and outer flowers. It might have been thought that the development
of the ray-petals, by drawing nourishment from the reproductive organs
causes their abortion; but this can hardly be the sole case, for in some
Compositae the seeds of the outer and inner florets differ, without any
difference in the corolla. Possibly these several differences may be
connected with the different flow of nutriment towards the central and
external flowers. We know, at least, that with irregular flowers those
nearest to the axis are most subject to peloria, that is to become
abnormally symmetrical. I may add, as an instance of this fact, and as
a striking case of correlation, that in many pelargoniums the two upper
petals in the central flower of the truss often lose their patches
of darker colour; and when this occurs, the adherent nectary is quite
aborted, the central flower thus becoming peloric or regular. When the
colour is absent from only one of the two upper petals, the nectary is
not quite aborted but is much shortened.

With respect to the development of the corolla, Sprengel's idea that
the ray-florets serve to attract insects, whose agency is highly
advantageous, or necessary for the fertilisation of these plants, is
highly probable; and if so, natural selection may have come into
play. But with respect to the seeds, it seems impossible that their
differences in shape, which are not always correlated with any
difference in the corolla, can be in any way beneficial; yet in the
Umbelliferae these differences are of such apparent importance--the
seeds being sometimes orthospermous in the exterior flowers and
coelospermous in the central flowers--that the elder De Candolle founded
his main divisions in the order on such characters. Hence modifications
of structure, viewed by systematists as of high value, may be wholly due
to the laws of variation and correlation, without being, as far as we
can judge, of the slightest service to the species.

We may often falsely attribute to correlated variation structures which
are common to whole groups of species, and which in truth are simply
due to inheritance; for an ancient progenitor may have acquired through
natural selection some one modification in structure, and, after
thousands of generations, some other and independent modification; and
these two modifications, having been transmitted to a whole group of
descendants with diverse habits, would naturally be thought to be in
some necessary manner correlated. Some other correlations are apparently
due to the manner in which natural selection can alone act. For
instance, Alph. De Candolle has remarked that winged seeds are never
found in fruits which do not open; I should explain this rule by
the impossibility of seeds gradually becoming winged through natural
selection, unless the capsules were open; for in this case alone could
the seeds, which were a little better adapted to be wafted by the wind,
gain an advantage over others less well fitted for wide dispersal.

COMPENSATION AND ECONOMY OF GROWTH.

The elder Geoffroy and Goethe propounded, at about the same time, their
law of compensation or balancement of growth; or, as Goethe expressed
it, "in order to spend on one side, nature is forced to economise on
the other side." I think this holds true to a certain extent with our
domestic productions: if nourishment flows to one part or organ in
excess, it rarely flows, at least in excess, to another part; thus it is
difficult to get a cow to give much milk and to fatten readily. The same
varieties of the cabbage do not yield abundant and nutritious foliage
and a copious supply of oil-bearing seeds. When the seeds in our fruits
become atrophied, the fruit itself gains largely in size and quality.
In our poultry, a large tuft of feathers on the head is generally
accompanied by a diminished comb, and a large beard by diminished
wattles. With species in a state of nature it can hardly be maintained
that the law is of universal application; but many good observers, more
especially botanists, believe in its truth. I will not, however, here
give any instances, for I see hardly any way of distinguishing between
the effects, on the one hand, of a part being largely developed through
natural selection and another and adjoining part being reduced by the
same process or by disuse, and, on the other hand, the actual withdrawal
of nutriment from one part owing to the excess of growth in another and
adjoining part.

I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more
general principle, namely, that natural selection is continually
trying to economise in every part of the organisation. If under changed
conditions of life a structure, before useful, becomes less useful, its
diminution will be favoured, for it will profit the individual not to
have its nutriment wasted in building up a useless structure. I can
thus only understand a fact with which I was much struck when examining
cirripedes, and of which many other instances could be given: namely,
that when a cirripede is parasitic within another cirripede and is thus
protected, it loses more or less completely its own shell or carapace.
This is the case with the male Ibla, and in a truly extraordinary manner
with the Proteolepas: for the carapace in all other cirripedes consists
of the three highly important anterior segments of the head enormously
developed, and furnished with great nerves and muscles; but in the
parasitic and protected Proteolepas, the whole anterior part of the
head is reduced to the merest rudiment attached to the bases of the
prehensile antennae. Now the saving of a large and complex structure,
when rendered superfluous, would be a decided advantage to each
successive individual of the species; for in the struggle for life
to which every animal is exposed, each would have a better chance of
supporting itself, by less nutriment being wasted.

Thus, as I believe, natural selection will tend in the long run to
reduce any part of the organisation, as soon as it becomes, through
changed habits, superfluous, without by any means causing some other
part to be largely developed in a corresponding degree. And conversely,
that natural selection may perfectly well succeed in largely developing
an organ without requiring as a necessary compensation the reduction of
some adjoining part.

MULTIPLE, RUDIMENTARY, AND LOWLY-ORGANISED STRUCTURES ARE VARIABLE.

It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both
with varieties and species, that when any part or organ is repeated
many times in the same individual (as the vertebrae in snakes, and the
stamens in polyandrous flowers) the number is variable; whereas the
number of the same part or organ, when it occurs in lesser numbers,
is constant. The same author as well as some botanists, have further
remarked that multiple parts are extremely liable to vary in structure.
As "vegetative repetition," to use Professor Owen's expression, is
a sign of low organisation; the foregoing statements accord with the
common opinion of naturalists, that beings which stand low in the scale
of nature are more variable than those which are higher. I presume that
lowness here means that the several parts of the organisation have been
but little specialised for particular functions; and as long as the same
part has to perform diversified work, we can perhaps see why it
should remain variable, that is, why natural selection should not have
preserved or rejected each little deviation of form so carefully as when
the part has to serve for some one special purpose. In the same way that
a knife which has to cut all sorts of things may be of almost any shape;
whilst a tool for some particular purpose must be of some particular
shape. Natural selection, it should never be forgotten, can act solely
through and for the advantage of each being.

Rudimentary parts, as is generally admitted, are apt to be highly
variable. We shall have to recur to this subject; and I will here only
add that their variability seems to result from their uselessness,
and consequently from natural selection having had no power to check
deviations in their structure.

A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER,
IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY
VARIABLE.

Several years ago I was much struck by a remark to the above effect made
by Mr. Waterhouse. Professor Owen, also, seems to have come to a nearly
similar conclusion. It is hopeless to attempt to convince any one of the
truth of the above proposition without giving the long array of facts
which I have collected, and which cannot possibly be here introduced. I
can only state my conviction that it is a rule of high generality. I
am aware of several causes of error, but I hope that I have made due
allowances for them. It should be understood that the rule by no means
applies to any part, however unusually developed, unless it be unusually
developed in one species or in a few species in comparison with the same
part in many closely allied species. Thus, the wing of the bat is a most
abnormal structure in the class of mammals; but the rule would not apply
here, because the whole group of bats possesses wings; it would apply
only if some one species had wings developed in a remarkable manner in
comparison with the other species of the same genus. The rule applies
very strongly in the case of secondary sexual characters, when displayed
in any unusual manner. The term, secondary sexual characters, used by
Hunter, relates to characters which are attached to one sex, but are
not directly connected with the act of reproduction. The rule applies
to males and females; but more rarely to females, as they seldom offer
remarkable secondary sexual characters. The rule being so plainly
applicable in the case of secondary sexual characters, may be due to the
great variability of these characters, whether or not displayed in any
unusual manner--of which fact I think there can be little doubt. But
that our rule is not confined to secondary sexual characters is clearly
shown in the case of hermaphrodite cirripedes; I particularly attended
to Mr. Waterhouse's remark, whilst investigating this order, and I am
fully convinced that the rule almost always holds good. I shall, in a
future work, give a list of all the more remarkable cases. I will here
give only one, as it illustrates the rule in its largest application.
The opercular valves of sessile cirripedes (rock barnacles) are, in
every sense of the word, very important structures, and they differ
extremely little even in distinct genera; but in the several species
of one genus, Pyrgoma, these valves present a marvellous amount of
diversification; the homologous valves in the different species being
sometimes wholly unlike in shape; and the amount of variation in the
individuals of the same species is so great that it is no exaggeration
to state that the varieties of the same species differ more from each
other in the characters derived from these important organs, than do the
species belonging to other distinct genera.

As with birds the individuals of the same species, inhabiting the same
country, vary extremely little, I have particularly attended to them;
and the rule certainly seems to hold good in this class. I cannot make
out that it applies to plants, and this would have seriously shaken my
belief in its truth, had not the great variability in plants made it
particularly difficult to compare their relative degrees of variability.

When we see any part or organ developed in a remarkable degree or manner
in a species, the fair presumption is that it is of high importance
to that species: nevertheless it is in this case eminently liable to
variation. Why should this be so? On the view that each species has been
independently created, with all its parts as we now see them, I can see
no explanation. But on the view that groups of species are descended
from some other species, and have been modified through natural
selection, I think we can obtain some light. First let me make some
preliminary remarks. If, in our domestic animals, any part or the
whole animal be neglected, and no selection be applied, that part (for
instance, the comb in the Dorking fowl) or the whole breed will cease to
have a uniform character: and the breed may be said to be degenerating.
In rudimentary organs, and in those which have been but little
specialised for any particular purpose, and perhaps in polymorphic
groups, we see a nearly parallel case; for in such cases natural
selection either has not or cannot come into full play, and thus the
organisation is left in a fluctuating condition. But what here more
particularly concerns us is, that those points in our domestic animals,
which at the present time are undergoing rapid change by continued
selection, are also eminently liable to variation. Look at the
individuals of the same breed of the pigeon; and see what a prodigious
amount of difference there is in the beak of tumblers, in the beak and
wattle of carriers, in the carriage and tail of fantails, etc., these
being the points now mainly attended to by English fanciers. Even in the
same sub-breed, as in that of the short-faced tumbler, it is notoriously
difficult to breed nearly perfect birds, many departing widely from the
standard. There may truly be said to be a constant struggle going on
between, on the one hand, the tendency to reversion to a less perfect
state, as well as an innate tendency to new variations, and, on the
other hand, the power of steady selection to keep the breed true. In
the long run selection gains the day, and we do not expect to fail so
completely as to breed a bird as coarse as a common tumbler pigeon from
a good short-faced strain. But as long as selection is rapidly going
on, much variability in the parts undergoing modification may always be
expected.

Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other species
of the same genus, we may conclude that this part has undergone an
extraordinary amount of modification since the period when the several
species branched off from the common progenitor of the genus. This
period will seldom be remote in any extreme degree, as species rarely
endure for more than one geological period. An extraordinary amount of
modification implies an unusually large and long-continued amount of
variability, which has continually been accumulated by natural
selection for the benefit of the species. But as the variability of
the extraordinarily developed part or organ has been so great and
long-continued within a period not excessively remote, we might, as a
general rule, still expect to find more variability in such parts than
in other parts of the organisation which have remained for a much longer
period nearly constant. And this, I am convinced, is the case. That the
struggle between natural selection on the one hand, and the tendency to
reversion and variability on the other hand, will in the course of
time cease; and that the most abnormally developed organs may be made
constant, I see no reason to doubt. Hence, when an organ, however
abnormal it may be, has been transmitted in approximately the same
condition to many modified descendants, as in the case of the wing of
the bat, it must have existed, according to our theory, for an immense
period in nearly the same state; and thus it has come not to be more
variable than any other structure. It is only in those cases in which
the modification has been comparatively recent and extraordinarily great
that we ought to find the GENERATIVE VARIABILITY, as it may be called,
still present in a high degree. For in this case the variability
will seldom as yet have been fixed by the continued selection of the
individuals varying in the required manner and degree, and by the
continued rejection of those tending to revert to a former and less
modified condition.

SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC CHARACTERS.

The principle discussed under the last heading may be applied to our
present subject. It is notorious that specific characters are more
variable than generic. To explain by a simple example what is meant: if
in a large genus of plants some species had blue flowers and some had
red, the colour would be only a specific character, and no one would be
surprised at one of the blue species varying into red, or conversely;
but if all the species had blue flowers, the colour would become
a generic character, and its variation would be a more unusual
circumstance. I have chosen this example because the explanation which
most naturalists would advance is not here applicable, namely, that
specific characters are more variable than generic, because they are
taken from parts of less physiological importance than those commonly
used for classing genera. I believe this explanation is partly, yet only
indirectly, true; I shall, however, have to return to this point in
the chapter on Classification. It would be almost superfluous to adduce
evidence in support of the statement, that ordinary specific characters
are more variable than generic; but with respect to important
characters, I have repeatedly noticed in works on natural history, that
when an author remarks with surprise that some important organ or part,
which is generally very constant throughout a large group of species,
DIFFERS considerably in closely-allied species, it is often VARIABLE
in the individuals of the same species. And this fact shows that a
character, which is generally of generic value, when it sinks in value
and becomes only of specific value, often becomes variable, though its
physiological importance may remain the same. Something of the same kind
applies to monstrosities: at least Is. Geoffroy St. Hilaire apparently
entertains no doubt, that the more an organ normally differs in the
different species of the same group, the more subject it is to anomalies
in the individuals.

On the ordinary view of each species having been independently created,
why should that part of the structure, which differs from the same
part in other independently created species of the same genus, be
more variable than those parts which are closely alike in the several
species? I do not see that any explanation can be given. But on the
view that species are only strongly marked and fixed varieties, we might
expect often to find them still continuing to vary in those parts of
their structure which have varied within a moderately recent period, and
which have thus come to differ. Or to state the case in another manner:
the points in which all the species of a genus resemble each other, and
in which they differ from allied genera, are called generic characters;
and these characters may be attributed to inheritance from a common
progenitor, for it can rarely have happened that natural selection will
have modified several distinct species, fitted to more or less widely
different habits, in exactly the same manner: and as these so-called
generic characters have been inherited from before the period when the
several species first branched off from their common progenitor, and
subsequently have not varied or come to differ in any degree, or only in
a slight degree, it is not probable that they should vary at the present
day. On the other hand, the points in which species differ from other
species of the same genus are called specific characters; and as these
specific characters have varied and come to differ since the period when
the species branched off from a common progenitor, it is probable
that they should still often be in some degree variable--at least more
variable than those parts of the organisation which have for a very long
period remained constant.

SECONDARY SEXUAL CHARACTERS VARIABLE.

I think it will be admitted by naturalists, without my entering on
details, that secondary sexual characters are highly variable. It will
also be admitted that species of the same group differ from each other
more widely in their secondary sexual characters, than in other parts
of their organisation; compare, for instance, the amount of difference
between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference between
the females. The cause of the original variability of these characters
is not manifest; but we can see why they should not have been rendered
as constant and uniform as others, for they are accumulated by sexual
selection, which is less rigid in its action than ordinary selection,
as it does not entail death, but only gives fewer offspring to the
less favoured males. Whatever the cause may be of the variability
of secondary sexual characters, as they are highly variable, sexual
selection will have had a wide scope for action, and may thus have
succeeded in giving to the species of the same group a greater amount of
difference in these than in other respects.

It is a remarkable fact, that the secondary differences between the two
sexes of the same species are generally displayed in the very same parts
of the organisation in which the species of the same genus differ from
each other. Of this fact I will give in illustration the first two
instances which happen to stand on my list; and as the differences in
these cases are of a very unusual nature, the relation can hardly be
accidental. The same number of joints in the tarsi is a character common
to very large groups of beetles, but in the Engidae, as Westwood has
remarked, the number varies greatly and the number likewise differs in
the two sexes of the same species. Again in the fossorial hymenoptera,
the neuration of the wings is a character of the highest importance,
because common to large groups; but in certain genera the neuration
differs in the different species, and likewise in the two sexes of the
same species. Sir J. Lubbock has recently remarked, that several minute
crustaceans offer excellent illustrations of this law. "In Pontella,
for instance, the sexual characters are afforded mainly by the anterior
antennae and by the fifth pair of legs: the specific differences also
are principally given by these organs." This relation has a clear
meaning on my view: I look at all the species of the same genus as
having as certainly descended from the same progenitor, as have the two
sexes of any one species. Consequently, whatever part of the structure
of the common progenitor, or of its early descendants, became variable;
variations of this part would, it is highly probable, be taken advantage
of by natural and sexual selection, in order to fit the several places
in the economy of nature, and likewise to fit the two sexes of the same
species to each other, or to fit the males to struggle with other males
for the possession of the females.

Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which are possessed by all the species;
that the frequent extreme variability of any part which is developed in
a species in an extraordinary manner in comparison with the same part in
its congeners; and the slight degree of variability in a part, however
extraordinarily it may be developed, if it be common to a whole group of
species; that the great variability of secondary sexual characters and
their great difference in closely allied species; that secondary sexual
and ordinary specific differences are generally displayed in the
same parts of the organisation, are all principles closely connected
together. All being mainly due to the species of the same group being
the descendants of a common progenitor, from whom they have inherited
much in common, to parts which have recently and largely varied being
more likely still to go on varying than parts which have long been
inherited and have not varied, to natural selection having more or less
completely, according to the lapse of time, overmastered the tendency
to reversion and to further variability, to sexual selection being
less rigid than ordinary selection, and to variations in the same parts
having been accumulated by natural and sexual selection, and thus having
been adapted for secondary sexual, and for ordinary purposes.

DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS, SO THAT A VARIETY OF
ONE SPECIES OFTEN ASSUMES A CHARACTER PROPER TO AN ALLIED SPECIES, OR
REVERTS TO SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.

These propositions will be most readily understood by looking to our
domestic races. The most distinct breeds of the pigeon, in countries
widely apart, present sub-varieties with reversed feathers on the
head, and with feathers on the feet, characters not possessed by the
aboriginal rock-pigeon; these then are analogous variations in two or
more distinct races. The frequent presence of fourteen or even
sixteen tail-feathers in the pouter may be considered as a variation
representing the normal structure of another race, the fantail. I
presume that no one will doubt that all such analogous variations are
due to the several races of the pigeon having inherited from a common
parent the same constitution and tendency to variation, when acted on by
similar unknown influences. In the vegetable kingdom we have a case of
analogous variation, in the enlarged stems, or as commonly called roots,
of the Swedish turnip and ruta-baga, plants which several botanists rank
as varieties produced by cultivation from a common parent: if this
be not so, the case will then be one of analogous variation in two
so-called distinct species; and to these a third may be added, namely,
the common turnip. According to the ordinary view of each species having
been independently created, we should have to attribute this similarity
in the enlarged stems of these three plants, not to the vera causa
of community of descent, and a consequent tendency to vary in a like
manner, but to three separate yet closely related acts of creation. Many
similar cases of analogous variation have been observed by Naudin in the
great gourd family, and by various authors in our cereals. Similar
cases occurring with insects under natural conditions have lately been
discussed with much ability by Mr. Walsh, who has grouped them under his
law of equable variability.

With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars
on the wings, white loins, a bar at the end of the tail, with the outer
feathers externally edged near their bases with white. As all these
marks are characteristic of the parent rock-pigeon, I presume that no
one will doubt that this is a case of reversion, and not of a new yet
analogous variation appearing in the several breeds. We may, I think,
confidently come to this conclusion, because, as we have seen, these
coloured marks are eminently liable to appear in the crossed offspring
of two distinct and differently coloured breeds; and in this case there
is nothing in the external conditions of life to cause the reappearance
of the slaty-blue, with the several marks, beyond the influence of the
mere act of crossing on the laws of inheritance.

No doubt it is a very surprising fact that characters should reappear
after having been lost for many, probably for hundreds of generations.
But when a breed has been crossed only once by some other breed, the
offspring occasionally show for many generations a tendency to revert in
character to the foreign breed--some say, for a dozen or even a score of
generations. After twelve generations, the proportion of blood, to use a
common expression, from one ancestor, is only 1 in 2048; and yet, as we
see, it is generally believed that a tendency to reversion is retained
by this remnant of foreign blood. In a breed which has not been
crossed, but in which BOTH parents have lost some character which their
progenitor possessed, the tendency, whether strong or weak, to reproduce
the lost character might, as was formerly remarked, for all that we
can see to the contrary, be transmitted for almost any number of
generations. When a character which has been lost in a breed, reappears
after a great number of generations, the most probable hypothesis is,
not that one individual suddenly takes after an ancestor removed by
some hundred generations, but that in each successive generation the
character in question has been lying latent, and at last, under unknown
favourable conditions, is developed. With the barb-pigeon, for instance,
which very rarely produces a blue bird, it is probable that there is a
latent tendency in each generation to produce blue plumage. The abstract
improbability of such a tendency being transmitted through a vast number
of generations, is not greater than that of quite useless or rudimentary
organs being similarly transmitted. A mere tendency to produce a
rudiment is indeed sometimes thus inherited.

As all the species of the same genus are supposed to be descended from
a common progenitor, it might be expected that they would occasionally
vary in an analogous manner; so that the varieties of two or more
species would resemble each other, or that a variety of one species
would resemble in certain characters another and distinct species,
this other species being, according to our view, only a well-marked and
permanent variety. But characters exclusively due to analogous variation
would probably be of an unimportant nature, for the preservation of
all functionally important characters will have been determined through
natural selection, in accordance with the different habits of the
species. It might further be expected that the species of the same
genus would occasionally exhibit reversions to long-lost characters.
As, however, we do not know the common ancestor of any natural group,
we cannot distinguish between reversionary and analogous characters.
If, for instance, we did not know that the parent rock-pigeon was not
feather-footed or turn-crowned, we could not have told, whether such
characters in our domestic breeds were reversions or only analogous
variations; but we might have inferred that the blue colour was a case
of reversion from the number of the markings, which are correlated with
this tint, and which would not probably have all appeared together from
simple variation. More especially we might have inferred this from the
blue colour and the several marks so often appearing when differently
coloured breeds are crossed. Hence, although under nature it must
generally be left doubtful, what cases are reversions to formerly
existing characters, and what are new but analogous variations, yet
we ought, on our theory, sometimes to find the varying offspring of a
species assuming characters which are already present in other members
of the same group. And this undoubtedly is the case.

The difficulty in distinguishing variable species is largely due to
the varieties mocking, as it were, other species of the same genus.
A considerable catalogue, also, could be given of forms intermediate
between two other forms, which themselves can only doubtfully be ranked
as species; and this shows, unless all these closely allied forms be
considered as independently created species, that they have in varying
assumed some of the characters of the others. But the best evidence of
analogous variations is afforded by parts or organs which are generally
constant in character, but which occasionally vary so as to resemble,
in some degree, the same part or organ in an allied species. I have
collected a long list of such cases; but here, as before, I lie under
the great disadvantage of not being able to give them. I can only repeat
that such cases certainly occur, and seem to me very remarkable.

I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several species
of the same genus, partly under domestication and partly under nature.
It is a case almost certainly of reversion. The ass sometimes has very
distinct transverse bars on its legs, like those on the legs of a zebra.
It has been asserted that these are plainest in the foal, and from
inquiries which I have made, I believe this to be true. The stripe on
the shoulder is sometimes double, and is very variable in length and
outline. A white ass, but NOT an albino, has been described without
either spinal or shoulder stripe; and these stripes are sometimes very
obscure, or actually quite lost, in dark-coloured asses. The koulan
of Pallas is said to have been seen with a double shoulder-stripe.
Mr. Blyth has seen a specimen of the hemionus with a distinct
shoulder-stripe, though it properly has none; and I have been informed
by Colonel Poole that foals of this species are generally striped on the
legs and faintly on the shoulder. The quagga, though so plainly barred
like a zebra over the body, is without bars on the legs; but Dr. Gray
has figured one specimen with very distinct zebra-like bars on the
hocks.

With respect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of ALL colours;
transverse bars on the legs are not rare in duns, mouse-duns, and in one
instance in a chestnut; a faint shoulder-stripe may sometimes be seen
in duns, and I have seen a trace in a bay horse. My son made a careful
examination and sketch for me of a dun Belgian cart-horse with a double
stripe on each shoulder and with leg-stripes. I have myself seen a dun
Devonshire pony, and a small dun Welsh pony has been carefully described
to me, both with THREE parallel stripes on each shoulder.

In the northwest part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined
this breed for the Indian Government, a horse without stripes is not
considered as purely bred. The spine is always striped; the legs are
generally barred; and the shoulder-stripe, which is sometimes double
and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are often plainest in the foal; and
sometimes quite disappear in old horses. Colonel Poole has seen both
gray and bay Kattywar horses striped when first foaled. I have also
reason to suspect, from information given me by Mr. W.W. Edwards, that
with the English race-horse the spinal stripe is much commoner in the
foal than in the full-grown animal. I have myself recently bred a foal
from a bay mare (offspring of a Turkoman horse and a Flemish mare) by
a bay English race-horse. This foal, when a week old, was marked on its
hinder quarters and on its forehead with numerous very narrow, dark,
zebra-like bars, and its legs were feebly striped. All the stripes soon
disappeared completely. Without here entering on further details I may
state that I have collected cases of leg and shoulder stripes in horses
of very different breeds in various countries from Britain to Eastern
China; and from Norway in the north to the Malay Archipelago in the
south. In all parts of the world these stripes occur far oftenest
in duns and mouse-duns; by the term dun a large range of colour is
included, from one between brown and black to a close approach to cream
colour.

I am aware that Colonel Hamilton Smith, who has written on this subject,
believes that the several breeds of the horse are descended from several
aboriginal species, one of which, the dun, was striped; and that the
above-described appearances are all due to ancient crosses with the dun
stock. But this view may be safely rejected, for it is highly improbable
that the heavy Belgian cart-horse, Welsh ponies, Norwegian cobs, the
lanky Kattywar race, etc., inhabiting the most distant parts of the
world, should have all have been crossed with one supposed aboriginal
stock.

Now let us turn to the effects of crossing the several species of the
horse genus. Rollin asserts that the common mule from the ass and horse
is particularly apt to have bars on its legs; according to Mr. Gosse,
in certain parts of the United States, about nine out of ten mules have
striped legs. I once saw a mule with its legs so much striped that any
one might have thought that it was a hybrid zebra; and Mr. W.C. Martin,
in his excellent treatise on the horse, has given a figure of a similar
mule. In four coloured drawings, which I have seen, of hybrids between
the ass and zebra, the legs were much more plainly barred than the rest
of the body; and in one of them there was a double shoulder-stripe. In
Lord Morton's famous hybrid, from a chestnut mare and male quagga, the
hybrid and even the pure offspring subsequently produced from the same
mare by a black Arabian sire, were much more plainly barred across the
legs than is even the pure quagga. Lastly, and this is another most
remarkable case, a hybrid has been figured by Dr. Gray (and he informs
me that he knows of a second case) from the ass and the hemionus; and
this hybrid, though the ass only occasionally has stripes on his
legs and the hemionus has none and has not even a shoulder-stripe,
nevertheless had all four legs barred, and had three short
shoulder-stripes, like those on the dun Devonshire and Welsh ponies, and
even had some zebra-like stripes on the sides of its face. With respect
to this last fact, I was so convinced that not even a stripe of colour
appears from what is commonly called chance, that I was led solely
from the occurrence of the face-stripes on this hybrid from the ass and
hemionus to ask Colonel Poole whether such face-stripes ever occurred
in the eminently striped Kattywar breed of horses, and was, as we have
seen, answered in the affirmative.

What now are we to say to these several facts? We see several distinct
species of the horse genus becoming, by simple variation, striped on the
legs like a zebra, or striped on the shoulders like an ass. In the horse
we see this tendency strong whenever a dun tint appears--a tint which
approaches to that of the general colouring of the other species of the
genus. The appearance of the stripes is not accompanied by any change
of form, or by any other new character. We see this tendency to become
striped most strongly displayed in hybrids from between several of the
most distinct species. Now observe the case of the several breeds
of pigeons: they are descended from a pigeon (including two or three
sub-species or geographical races) of a bluish colour, with certain bars
and other marks; and when any breed assumes by simple variation a bluish
tint, these bars and other marks invariably reappear; but without any
other change of form or character. When the oldest and truest breeds of
various colours are crossed, we see a strong tendency for the blue tint
and bars and marks to reappear in the mongrels. I have stated that the
most probable hypothesis to account for the reappearance of very ancient
characters, is--that there is a TENDENCY in the young of each successive
generation to produce the long-lost character, and that this tendency,
from unknown causes, sometimes prevails. And we have just seen that in
several species of the horse genus the stripes are either plainer or
appear more commonly in the young than in the old. Call the breeds of
pigeons, some of which have bred true for centuries, species; and how
exactly parallel is the case with that of the species of the horse
genus! For myself, I venture confidently to look back thousands on
thousands of generations, and I see an animal striped like a zebra, but
perhaps otherwise very differently constructed, the common parent of
our domestic horse (whether or not it be descended from one or more wild
stocks) of the ass, the hemionus, quagga, and zebra.

He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a
tendency to vary, both under nature and under domestication, in this
particular manner, so as often to become striped like the other species
of the genus; and that each has been created with a strong tendency,
when crossed with species inhabiting distant quarters of the world, to
produce hybrids resembling in their stripes, not their own parents, but
other species of the genus. To admit this view is, as it seems to me, to
reject a real for an unreal, or at least for an unknown cause. It makes
the works of God a mere mockery and deception; I would almost as soon
believe with the old and ignorant cosmogonists, that fossil shells had
never lived, but had been created in stone so as to mock the shells now
living on the sea-shore.

SUMMARY.

Our ignorance of the laws of variation is profound. Not in one case out
of a hundred can we pretend to assign any reason why this or that part
has varied. But whenever we have the means of instituting a comparison,
the same laws appear to have acted in producing the lesser differences
between varieties of the same species, and the greater differences
between species of the same genus. Changed conditions generally induce
mere fluctuating variability, but sometimes they cause direct and
definite effects; and these may become strongly marked in the course
of time, though we have not sufficient evidence on this head. Habit in
producing constitutional peculiarities, and use in strengthening, and
disuse in weakening and diminishing organs, appear in many cases to have
been potent in their effects. Homologous parts tend to vary in the same
manner, and homologous parts tend to cohere. Modifications in hard parts
and in external parts sometimes affect softer and internal parts. When
one part is largely developed, perhaps it tends to draw nourishment from
the adjoining parts; and every part of the structure which can be saved
without detriment will be saved. Changes of structure at an early age
may affect parts subsequently developed; and many cases of correlated
variation, the nature of which we are unable to understand, undoubtedly
occur. Multiple parts are variable in number and in structure, perhaps
arising from such parts not having been closely specialised for any
particular function, so that their modifications have not been closely
checked by natural selection. It follows probably from this same cause,
that organic beings low in the scale are more variable than those
standing higher in the scale, and which have their whole organisation
more specialised. Rudimentary organs, from being useless, are not
regulated by natural selection, and hence are variable. Specific
characters--that is, the characters which have come to differ since the
several species of the same genus branched off from a common parent--are
more variable than generic characters, or those which have long been
inherited, and have not differed within this same period. In these
remarks we have referred to special parts or organs being still
variable, because they have recently varied and thus come to differ; but
we have also seen in the second chapter that the same principle applies
to the whole individual; for in a district where many species of a
genus are found--that is, where there has been much former variation and
differentiation, or where the manufactory of new specific forms has been
actively at work--in that district and among these species, we now find,
on an average, most varieties. Secondary sexual characters are highly
variable, and such characters differ much in the species of the same
group. Variability in the same parts of the organisation has generally
been taken advantage of in giving secondary sexual differences to the
two sexes of the same species, and specific differences to the
several species of the same genus. Any part or organ developed to an
extraordinary size or in an extraordinary manner, in comparison with
the same part or organ in the allied species, must have gone through an
extraordinary amount of modification since the genus arose; and thus we
can understand why it should often still be variable in a much higher
degree than other parts; for variation is a long-continued and slow
process, and natural selection will in such cases not as yet have had
time to overcome the tendency to further variability and to
reversion to a less modified state. But when a species with an
extraordinarily developed organ has become the parent of many modified
descendants--which on our view must be a very slow process, requiring a
long lapse of time--in this case, natural selection has succeeded in
giving a fixed character to the organ, in however extraordinary a manner
it may have been developed. Species inheriting nearly the same
constitution from a common parent, and exposed to similar influences,
naturally tend to present analogous variations, or these same species
may occasionally revert to some of the characters of their ancient
progenitors. Although new and important modifications may not arise from
reversion and analogous variation, such modifications will add to the
beautiful and harmonious diversity of nature.

Whatever the cause may be of each slight difference between the
offspring and their parents--and a cause for each must exist--we have
reason to believe that it is the steady accumulation of beneficial
differences which has given rise to all the more important modifications
of structure in relation to the habits of each species.



CHAPTER VI. DIFFICULTIES OF THE THEORY.

 Difficulties of the theory of descent with modification--Absence
 or rarity of transitional varieties--Transitions in habits of
 life--Diversified habits in the same species--Species with habits
 widely different from those of their allies--Organs of extreme
 perfection--Modes of transition--Cases of difficulty--Natura non facit
 saltum--Organs of small importance--Organs not in all cases absolutely
 perfect--The law of Unity of Type and of the Conditions of Existence
 embraced by the theory of Natural Selection.


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.

These difficulties and objections may be classed under the following
heads: First, 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?

Secondly, is it possible that an animal having, for instance,
the structure and habits of a bat, could have been formed by the
modification of some other animal with widely different habits and
structure? Can we believe that natural selection could produce, on
the one hand, an organ of trifling importance, such as the tail of a
giraffe, which serves as a fly-flapper, and, on the other hand, an organ
so wonderful as the eye?

Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to the instinct which leads the bee to make
cells, and which has practically anticipated the discoveries of profound
mathematicians?

Fourthly, how can we account for species, when crossed, being sterile
and producing sterile offspring, whereas, when varieties are crossed,
their fertility is unimpaired?

The two first heads will be here discussed; some miscellaneous
objections in the following chapter; Instinct and Hybridism in the two
succeeding chapters.

ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.

As natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to
take the place of, and finally to exterminate, its own less improved
parent-form and other less-favoured forms with which it comes into
competition. Thus extinction and natural selection go hand in hand.
Hence, if we look at each species as descended from some unknown form,
both the parent and all the transitional varieties will generally have
been exterminated by the very process of the formation and perfection of
the new form.

But, as by this theory innumerable transitional forms must have existed,
why do we not find them embedded in countless numbers in the crust of
the earth? It will be more convenient to discuss this question in the
chapter on the imperfection of the geological record; and I will here
only state that I believe the answer mainly lies in the record being
incomparably less perfect than is generally supposed. The crust of
the earth is a vast museum; but the natural collections have been
imperfectly made, and only at long intervals of time.

But it may be urged that when several closely allied species inhabit
the same territory, we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from north
to south over a continent, we generally meet at successive intervals
with closely allied or representative species, evidently filling nearly
the same place in the natural economy of the land. These representative
species often meet and interlock; and as the one becomes rarer and
rarer, the other becomes more and more frequent, till the one replaces
the other. But if we compare these species where they intermingle, they
are generally as absolutely distinct from each other in every detail of
structure as are specimens taken from the metropolis inhabited by each.
By my theory these allied species are descended from a common parent;
and during the process of modification, each has become adapted to
the conditions of life of its own region, and has supplanted and
exterminated its original parent-form and all the transitional varieties
between its past and present states. Hence we ought not to expect at
the present time to meet with numerous transitional varieties in each
region, though they must have existed there, and may be embedded
there in a fossil condition. But in the intermediate region, having
intermediate conditions of life, why do we not now find closely-linking
intermediate varieties? This difficulty for a long time quite confounded
me. But I think it can be in large part explained.

In the first place we should be extremely cautious in inferring, because
an area is now continuous, that it has been continuous during a long
period. Geology would lead us to believe that most continents have been
broken up into islands even during the later tertiary periods; and in
such islands distinct species might have been separately formed without
the possibility of intermediate varieties existing in the intermediate
zones. By changes in the form of the land and of climate, marine areas
now continuous must often have existed within recent times in a far less
continuous and uniform condition than at present. But I will pass
over this way of escaping from the difficulty; for I believe that many
perfectly defined species have been formed on strictly continuous areas;
though I do not doubt that the formerly broken condition of areas
now continuous, has played an important part in the formation of new
species, more especially with freely-crossing and wandering animals.

In looking at species as they are now distributed over a wide area,
we generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes it
is quite remarkable how abruptly, as Alph. De Candolle has observed, a
common alpine species disappears. The same fact has been noticed by E.
Forbes in sounding the depths of the sea with the dredge. 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 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. Moreover, each species
on the confines of its range, where it exists in lessened numbers, will,
during fluctuations in the number of its enemies or of its prey, or in
the nature of the seasons, be extremely liable to utter extermination;
and thus its geographical range will come to be still more sharply
defined.

As allied or representative species, when inhabiting a continuous area,
are generally distributed in such a manner that each has a wide range,
with a comparatively narrow neutral territory between them, in which
they become rather suddenly rarer and rarer; then, as varieties do not
essentially differ from species, the same rule will probably apply to
both; and if we take a varying species inhabiting a very large area,
we shall have to adapt two varieties to two large areas, and a third
variety to a narrow intermediate zone. The intermediate variety,
consequently, will exist in lesser numbers from inhabiting a narrow and
lesser area; and practically, as far as I can make out, this rule holds
good with varieties in a state of nature. I have met with striking
instances of the rule in the case of varieties intermediate between
well-marked varieties in the genus Balanus. And it would appear from
information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston,
that generally, when varieties intermediate between two other forms
occur, they are much rarer numerically than the forms which they
connect. Now, if we may trust these facts and inferences, and conclude
that varieties linking two other varieties together generally have
existed in lesser numbers than the forms which they connect, then we can
understand why intermediate varieties should not endure for very
long periods: why, as a general rule, they should be exterminated and
disappear, sooner than the forms which they originally linked together.

For any form existing in lesser numbers would, as already remarked,
run a greater chance of being exterminated than one existing in large
numbers; and in this particular case the intermediate form would be
eminently liable to the inroads of closely allied forms existing on both
sides of it. But it is a far more important consideration, that during
the process of further modification, by which two varieties are supposed
to be converted and perfected into two distinct species, the two which
exist in larger numbers, from inhabiting larger areas, will have a great
advantage over the intermediate variety, which exists in smaller numbers
in a narrow and intermediate zone. For forms existing in larger numbers
will have a better chance, within any given period, of presenting
further favourable variations for natural selection to seize on, than
will the rarer forms which exist in lesser numbers. Hence, the more
common forms, in the race for life, will tend to beat and supplant the
less common forms, for these will be more slowly modified and improved.
It is the same principle which, as I believe, accounts for the common
species in each country, as shown in the second chapter, presenting on
an average a greater number of well-marked varieties than do the rarer
species. I may illustrate what I mean by supposing three varieties of
sheep to be kept, one adapted to an extensive mountainous region; a
second to a comparatively narrow, hilly tract; and a third to the wide
plains at the base; and that the inhabitants are all trying with equal
steadiness and skill to improve their stocks by selection; the chances
in this case will be strongly in favour of the great holders on the
mountains or on the plains improving their breeds more quickly than the
small holders on the intermediate narrow, hilly tract; and consequently
the improved mountain or plain breed will soon take the place of the
less improved hill breed; and thus the two breeds, which originally
existed in greater numbers, will come into close contact with each
other, without the interposition of the supplanted, intermediate hill
variety.

To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: first, because new varieties are very
slowly formed, for variation is a slow process, and natural selection
can do nothing until favourable individual differences or variations
occur, and until a place in the natural polity of the country can
be better filled by some modification of some one or more of its
inhabitants. And such new places will depend on slow changes of climate,
or on the occasional immigration of new inhabitants, and, probably, in
a still more important degree, on some of the old inhabitants becoming
slowly modified, with the new forms thus produced and the old ones
acting and reacting on each other. So that, in any one region and at
any one time, we ought to see only a few species presenting slight
modifications of structure in some degree permanent; and this assuredly
we do see.

Secondly, areas now continuous must often have existed within the recent
period as isolated portions, in which many forms, more especially
among the classes which unite for each birth and wander much, may have
separately been rendered sufficiently distinct to rank as representative
species. In this case, intermediate varieties between the several
representative species and their common parent, must formerly have
existed within each isolated portion of the land, but these links
during the process of natural selection will have been supplanted and
exterminated, so that they will no longer be found in a living state.

Thirdly, when two or more varieties have been formed in different
portions of a strictly continuous area, intermediate varieties will, it
is probable, at first have been formed in the intermediate zones, but
they will generally have had a short duration. For these intermediate
varieties will, from reasons already assigned (namely from what we know
of the actual distribution of closely allied or representative species,
and likewise of acknowledged varieties), exist in the intermediate zones
in lesser numbers than the varieties which they tend to connect. From
this cause alone the intermediate varieties will be liable to accidental
extermination; and during the process of further modification through
natural selection, they will almost certainly be beaten and supplanted
by the forms which they connect; for these, from existing in greater
numbers will, in the aggregate, present more varieties, and thus be
further improved through natural selection and gain further advantages.

Lastly, looking not to any one time, but at all time, if my theory be
true, numberless intermediate varieties, linking closely together all
the species of the same group, must assuredly have existed; but the
very process of natural selection constantly tends, as has been so often
remarked, to exterminate the parent forms and the intermediate links.
Consequently evidence of their former existence could be found only
among fossil remains, which are preserved, as we shall attempt to show
in a future chapter, in an extremely imperfect and intermittent record.

ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH PECULIAR HABITS AND
STRUCTURE.

It has been asked by the opponents of such views as I hold, how, for
instance, could a land carnivorous animal have been converted into one
with aquatic habits; for how could the animal in its transitional
state have subsisted? It would be easy to show that there now exist
carnivorous animals presenting close intermediate grades from strictly
terrestrial to aquatic habits; and as each exists by a struggle for
life, it is clear that each must be well adapted to its place in nature.
Look at the Mustela vison of North America, which has webbed feet,
and which resembles an otter in its fur, short legs, and form of tail;
during summer this animal dives for and preys on fish, but during the
long winter it leaves the frozen waters, and preys, like other polecats
on mice and land animals. If a different case had been taken, and it
had been asked how an insectivorous quadruped could possibly have been
converted into a flying bat, the question would have been far more
difficult to answer. Yet I think such difficulties have little weight.

Here, as on other occasions, I lie under a heavy disadvantage, for, out
of the many striking cases which I have collected, I can give only
one or two instances of transitional habits and structures in allied
species; and of diversified habits, either constant or occasional, in
the same species. And it seems to me that nothing less than a long list
of such cases is sufficient to lessen the difficulty in any particular
case like that of the bat.

Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as
Sir J. Richardson has remarked, with the posterior part of their bodies
rather wide and with the skin on their flanks rather full, to the
so-called flying squirrels; and flying squirrels have their limbs and
even the base of the tail united by a broad expanse of skin, which
serves as a parachute and allows them to glide through the air to
an astonishing distance from tree to tree. We cannot doubt that each
structure is of use to each kind of squirrel in its own country, by
enabling it to escape birds or beasts of prey, or to collect food more
quickly, or, as there is reason to believe, to lessen the danger
from occasional falls. But it does not follow from this fact that the
structure of each squirrel is the best that it is possible to conceive
under all possible conditions. Let the climate and vegetation change,
let other competing rodents or new beasts of prey immigrate, or old ones
become modified, and all analogy would lead us to believe that some,
at least, of the squirrels would decrease in numbers or become
exterminated, unless they also become modified and improved in structure
in a corresponding manner. Therefore, I can see no difficulty,
more especially under changing conditions of life, in the continued
preservation of individuals with fuller and fuller flank-membranes,
each modification being useful, each being propagated, until, by the
accumulated effects of this process of natural selection, a perfect
so-called flying squirrel was produced.

Now look at the Galeopithecus or so-called flying lemur, which was
formerly ranked among bats, but is now believed to belong to the
Insectivora. An extremely wide flank-membrane stretches from the corners
of the jaw to the tail, and includes the limbs with the elongated
fingers. This flank-membrane is furnished with an extensor muscle.
Although no graduated links of structure, fitted for gliding through the
air, now connect the Galeopithecus with the other Insectivora, yet there
is no difficulty in supposing that such links formerly existed, and that
each was developed in the same manner as with the less perfectly gliding
squirrels; each grade of structure having been useful to its possessor.
Nor can I see any insuperable difficulty in further believing it
possible that the membrane-connected fingers and fore-arm of the
Galeopithecus might have been greatly lengthened by natural selection;
and this, as far as the organs of flight are concerned, would
have converted the animal into a bat. In certain bats in which the
wing-membrane extends from the top of the shoulder to the tail and
includes the hind-legs, we perhaps see traces of an apparatus originally
fitted for gliding through the air rather than for flight.

If about a dozen genera of birds were to become extinct, who would have
ventured to surmise that birds might have existed which used their wings
solely as flappers, like the logger headed duck (Micropterus of Eyton);
as fins in the water and as front legs on the land, like the penguin;
as sails, like the ostrich; and functionally for no purpose, like the
apteryx? Yet the structure of each of these birds is good for it, under
the conditions of life to which it is exposed, for each has to live by a
struggle: but it is not necessarily the best possible under all possible
conditions. It must not be inferred from these remarks that any of the
grades of wing-structure here alluded to, which perhaps may all be the
result of disuse, indicate the steps by which birds actually acquired
their perfect power of flight; but they serve to show what diversified
means of transition are at least possible.

Seeing that a few members of such water-breathing classes as the
Crustacea and Mollusca are adapted to live on the land; and seeing that
we have flying birds and mammals, flying insects of the most diversified
types, and formerly had flying reptiles, it is conceivable that
flying-fish, which now glide far through the air, slightly rising and
turning by the aid of their fluttering fins, might have been modified
into perfectly winged animals. If this had been effected, who would
have ever imagined that in an early transitional state they had been
inhabitants of the open ocean, and had used their incipient organs of
flight exclusively, so far as we know, to escape being devoured by other
fish?

When we see any structure highly perfected for any particular habit,
as the wings of a bird for flight, we should bear in mind that animals
displaying early transitional grades of the structure will seldom have
survived to the present day, for they will have been supplanted by their
successors, which were gradually rendered more perfect through natural
selection. Furthermore, we may conclude that transitional states between
structures fitted for very different habits of life will rarely have
been developed at an early period in great numbers and under many
subordinate forms. Thus, to return to our imaginary illustration of
the flying-fish, it does not seem probable that fishes capable of true
flight would have been developed under many subordinate forms, for
taking prey of many kinds in many ways, on the land and in the water,
until their organs of flight had come to a high stage of perfection,
so as to have given them a decided advantage over other animals in
the battle for life. Hence the chance of discovering species with
transitional grades of structure in a fossil condition will always be
less, from their having existed in lesser numbers, than in the case of
species with fully developed structures.

I will now give two or three instances, both of diversified and of
changed habits, in the individuals of the same species. In either case
it would be easy for natural selection to adapt the structure of the
animal to its changed habits, or exclusively to one of its several
habits. It is, however, difficult to decide and immaterial for us,
whether habits generally change first and structure afterwards; or
whether slight modifications of structure lead to changed habits; both
probably often occurring almost simultaneously. Of cases of changed
habits it will suffice merely to allude to that of the many British
insects which now feed on exotic plants, or exclusively on artificial
substances. Of diversified habits innumerable instances could be given:
I have often watched a tyrant flycatcher (Saurophagus sulphuratus) in
South America, hovering over one spot and then proceeding to another,
like a kestrel, and at other times standing stationary on the margin of
water, and then dashing into it like a kingfisher at a fish. In our own
country the larger titmouse (Parus major) may be seen climbing branches,
almost like a creeper; it sometimes, like a shrike, kills small birds by
blows on the head; and I have many times seen and heard it hammering the
seeds of the yew on a branch, and thus breaking them like a nuthatch. In
North America the black bear was seen by Hearne swimming for hours with
widely open mouth, thus catching, almost like a whale, insects in the
water.

As we sometimes see individuals following habits different from those
proper to their species and to the other species of the same genus, we
might expect that such individuals would occasionally give rise to
new species, having anomalous habits, and with their structure either
slightly or considerably modified from that of their type. And such
instances occur in nature. Can a more striking instance of adaptation be
given than that of a woodpecker for climbing trees and seizing insects
in the chinks of the bark? Yet in North America there are woodpeckers
which feed largely on fruit, and others with elongated wings which chase
insects on the wing. On the plains of La Plata, where hardly a tree
grows, there is a woodpecker (Colaptes campestris) which has two toes
before and two behind, a long-pointed tongue, pointed tail-feathers,
sufficiently stiff to support the bird in a vertical position on a post,
but not so stiff as in the typical wood-peckers, and a straight, strong
beak. The beak, however, is not so straight or so strong as in the
typical woodpeckers but it is strong enough to bore into wood. Hence
this Colaptes, in all the essential parts of its structure, is a
woodpecker. Even in such trifling characters as the colouring, the harsh
tone of the voice, and undulatory flight, its close blood-relationship
to our common woodpecker is plainly declared; yet, as I can assert, not
only from my own observations, but from those of the accurate Azara, in
certain large districts it does not climb trees, and it makes its
nest in holes in banks! In certain other districts, however, this same
woodpecker, as Mr. Hudson states, frequents trees, and bores holes in
the trunk for its nest. I may mention as another illustration of the
varied habits of this genus, that a Mexican Colaptes has been described
by De Saussure as boring holes into hard wood in order to lay up a store
of acorns.

Petrels are the most aerial and oceanic of birds, but, in the quiet
sounds of Tierra del Fuego, the Puffinuria berardi, in its general
habits, in its astonishing power of diving, in its manner of swimming
and of flying when made to take flight, would be mistaken by any one for
an auk or a grebe; nevertheless, it is essentially a petrel, but with
many parts of its organisation profoundly modified in relation to its
new habits of life; whereas the woodpecker of La Plata has had its
structure only slightly modified. In the case of the water-ouzel, the
acutest observer, by examining its dead body, would never have suspected
its sub-aquatic habits; yet this bird, which is allied to the thrush
family, subsists by diving,--using its wings under water and
grasping stones with its feet. All the members of the great order of
Hymenopterous insects are terrestrial, excepting the genus Proctotrupes,
which Sir John Lubbock has discovered to be aquatic in its habits; it
often enters the water and dives about by the use not of its legs but of
its wings, and remains as long as four hours beneath the surface; yet
it exhibits no modification in structure in accordance with its abnormal
habits.

He who believes that each being has been created as we now see it, must
occasionally have felt surprise when he has met with an animal having
habits and structure not in agreement. What can be plainer than that the
webbed feet of ducks and geese are formed for swimming? Yet there are
upland geese with webbed feet which rarely go near the water; and no one
except Audubon, has seen the frigate-bird, which has all its four toes
webbed, alight on the surface of the ocean. On the other hand, grebes
and coots are eminently aquatic, although their toes are only bordered
by membrane. What seems plainer than that the long toes, not furnished
with membrane, of the Grallatores, are formed for walking over swamps
and floating plants. The water-hen and landrail are members of this
order, yet the first is nearly as aquatic as the coot, and the second is
nearly as terrestrial as the quail or partridge. In such cases, and
many others could be given, habits have changed without a corresponding
change of structure. The webbed feet of the upland goose may be said to
have become almost rudimentary in function, though not in structure.
In the frigate-bird, the deeply scooped membrane between the toes shows
that structure has begun to change.

He who believes in separate and innumerable acts of creation may say,
that in these cases it has pleased the Creator to cause a being of one
type to take the place of one belonging to another type; but this seems
to me only restating the fact in dignified language. He who believes in
the struggle for existence and in the principle of natural selection,
will acknowledge that every organic being is constantly endeavouring to
increase in numbers; and that if any one being varies ever so little,
either in habits or structure, and thus gains an advantage over some
other inhabitant of the same country, it will seize on the place of that
inhabitant, however different that may be from its own place. Hence it
will cause him no surprise that there should be geese and frigate-birds
with webbed feet, living on the dry land and rarely alighting on the
water, that there should be long-toed corncrakes, living in meadows
instead of in swamps; that there should be woodpeckers where hardly a
tree grows; that there should be diving thrushes and diving Hymenoptera,
and petrels with the habits of auks.

ORGANS OF EXTREME PERFECTION AND COMPLICATION.

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. When it was first said that the
sun stood still and the world turned round, the common sense of mankind
declared the doctrine false; but the old saying of Vox populi, vox Dei,
as every philosopher knows, cannot be trusted in science. Reason tells
me, that if numerous gradations from a simple and imperfect eye to one
complex and perfect can be shown to exist, each grade being useful to
its possessor, as is certainly the case; if further, the eye ever varies
and the variations be inherited, as is likewise certainly the case;
and if such variations should be useful to any animal under changing
conditions of life, then the difficulty of believing that a perfect and
complex eye could be formed by natural selection, though insuperable by
our imagination, should not be considered as subversive of the theory.
How a nerve comes to be sensitive to light, hardly concerns us more than
how life itself originated; but I may remark that, as some of the lowest
organisms in which nerves cannot be detected, are capable of perceiving
light, it does not seem impossible that certain sensitive elements
in their sarcode should become aggregated and developed into nerves,
endowed with this special sensibility.

In searching for the gradations through which an organ in any species
has been perfected, we ought to look exclusively to its lineal
progenitors; but this is scarcely ever possible, and we are forced
to look to other species and genera of the same group, that is to the
collateral descendants from the same parent-form, in order to see what
gradations are possible, and for the chance of some gradations having
been transmitted in an unaltered or little altered condition. But the
state of the same organ in distinct classes may incidentally throw light
on the steps by which it has been perfected.

The simplest organ which can be called an eye consists of an optic
nerve, surrounded by pigment-cells and covered by translucent skin, but
without any lens or other refractive body. We may, however, according
to M. Jourdain, descend even a step lower and find aggregates of
pigment-cells, apparently serving as organs of vision, without any
nerves, and resting merely on sarcodic tissue. Eyes of the above simple
nature are not capable of distinct vision, and serve only to distinguish
light from darkness. In certain star-fishes, small depressions in the
layer of pigment which surrounds the nerve are filled, as described by
the author just quoted, with transparent gelatinous matter, projecting
with a convex surface, like the cornea in the higher animals. He
suggests that this serves not to form an image, but only to concentrate
the luminous rays and render their perception more easy. In this
concentration of the rays we gain the first and by far the most
important step towards the formation of a true, picture-forming eye; for
we have only to place the naked extremity of the optic nerve, which in
some of the lower animals lies deeply buried in the body, and in
some near the surface, at the right distance from the concentrating
apparatus, and an image will be formed on it.

In the great class of the Articulata, we may start from an optic nerve
simply coated with pigment, the latter sometimes forming a sort of
pupil, but destitute of lens or other optical contrivance. With insects
it is now known that the numerous facets on the cornea of their great
compound eyes form true lenses, and that the cones include curiously
modified nervous filaments. But these organs in the Articulata are so
much diversified that Muller formerly made three main classes with seven
subdivisions, besides a fourth main class of aggregated simple eyes.

When we reflect on these facts, here given much too briefly, with
respect to the wide, diversified, and graduated range of structure in
the eyes of the lower animals; and when we bear in mind how small the
number of all living forms must be in comparison with those which have
become extinct, the difficulty ceases to be very great in believing that
natural selection may have converted the simple apparatus of an optic
nerve, coated with pigment and invested by transparent membrane, into
an optical instrument as perfect as is possessed by any member of the
Articulata class.

He who will go thus far, ought not to hesitate to go one step further,
if he finds on finishing this volume that large bodies of facts,
otherwise inexplicable, can be explained by the theory of modification
through natural selection; he ought to admit that a structure even as
perfect as an eagle's eye might thus be formed, although in this case
he does not know the transitional states. It has been objected that in
order to modify the eye and still preserve it as a perfect instrument,
many changes would have to be effected simultaneously, which, it is
assumed, could not be done through natural selection; but as I have
attempted to show in my work on the variation of domestic animals, it is
not necessary to suppose that the modifications were all simultaneous,
if they were extremely slight and gradual. Different kinds of
modification would, also, serve for the same general purpose: as Mr.
Wallace has remarked, "If a lens has too short or too long a focus, it
may be amended either by an alteration of curvature, or an alteration of
density; if the curvature be irregular, and the rays do not converge
to a point, then any increased regularity of curvature will be an
improvement. So the contraction of the iris and the muscular
movements of the eye are neither of them essential to vision, but only
improvements which might have been added and perfected at any stage of
the construction of the instrument." Within the highest division of
the animal kingdom, namely, the Vertebrata, we can start from an eye
so simple, that it consists, as in the lancelet, of a little sack of
transparent skin, furnished with a nerve and lined with pigment, but
destitute of any other apparatus. In fishes and reptiles, as Owen has
remarked, "The range of gradation of dioptric structures is very
great." It is a significant fact that even in man, according to the high
authority of Virchow, the beautiful crystalline lens is formed in the
embryo by an accumulation of epidermic cells, lying in a sack-like fold
of the skin; and the vitreous body is formed from embryonic subcutaneous
tissue. 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 to keenly to be
surprised at others hesitating to extend the principle of natural
selection to so startling a length.

It is scarcely possible to avoid comparing the eye with a telescope.
We know that this instrument has been perfected by the long-continued
efforts of the highest human intellects; and we naturally infer that the
eye has been formed by a somewhat analogous process. But may not this
inference be presumptuous? Have we any right to assume that the Creator
works by intellectual powers like those of man? 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 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 is produced, and then the old ones to be all
destroyed. In living bodies, variation will cause the slight alteration,
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?

MODES Of TRANSITION.

If it could be demonstrated that any complex organ existed, which
could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find
out no such case. No doubt many organs exist of which we do not know
the transitional grades, more especially if we look to much-isolated
species, around which, according to the theory, there has been much
extinction. Or again, if we take an organ common to all the members of
a class, for in this latter case the organ must have been originally
formed at a remote period, since which all the many members of the class
have been developed; and in order to discover the early transitional
grades through which the organ has passed, we should have to look to
very ancient ancestral forms, long since become extinct.

We should be extremely cautious in concluding that an organ could not
have been formed by transitional gradations of some kind. Numerous cases
could be given among the lower animals of the same organ performing
at the same time wholly distinct functions; thus in the larva of the
dragon-fly and in the fish Cobites the alimentary canal respires,
digests, and excretes. In the Hydra, the animal may be turned inside
out, and the exterior surface will then digest and the stomach respire.
In such cases natural selection might specialise, if any advantage
were thus gained, the whole or part of an organ, which had previously
performed two functions, for one function alone, and thus by insensible
steps greatly change its nature. Many plants are known which regularly
produce at the same time differently constructed flowers; and if such
plants were to produce one kind alone, a great change would be effected
with comparative suddenness in the character of the species. It is,
however, probable that the two sorts of flowers borne by the same plant
were originally differentiated by finely graduated steps, which may
still be followed in some few cases.

Again, two distinct organs, or the same organ under two very different
forms, may simultaneously perform in the same individual the same
function, and this is an extremely important means of transition: to
give one instance--there are fish with gills or branchiae that breathe
the air dissolved in the water, at the same time that they breathe free
air in their swim-bladders, this latter organ being divided by highly
vascular partitions and having a ductus pneumaticus for the supply of
air. To give another instance from the vegetable kingdom: plants climb
by three distinct means, by spirally twining, by clasping a support with
their sensitive tendrils, and by the emission of aerial rootlets; these
three means are usually found in distinct groups, but some few species
exhibit two of the means, or even all three, combined in the same
individual. In all such cases one of the two organs might readily be
modified and perfected so as to perform all the work, being aided during
the progress of modification by the other organ; and then this other
organ might be modified for some other and quite distinct purpose, or be
wholly obliterated.

The illustration of the swim-bladder in fishes is a good one, because
it shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into
one for a widely different purpose, namely respiration. The swim-bladder
has, also, been worked in as an accessory to the auditory organs
of certain fishes. All physiologists admit that the swim-bladder is
homologous, or "ideally similar" in position and structure with the
lungs of the higher vertebrate animals: hence there is no reason to
doubt that the swim-bladder has actually been converted into lungs, or
an organ used exclusively for respiration.

According to this view it may be inferred that all vertebrate animals
with true lungs are descended by ordinary generation from an ancient
and unknown prototype which was furnished with a floating apparatus or
swim-bladder. We can thus, as I infer from Professor Owen's interesting
description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the
orifice of the trachea, with some risk of falling into the lungs,
notwithstanding the beautiful contrivance by which the glottis
is closed. In the higher Vertebrata the branchiae have wholly
disappeared--but in the embryo the slits on the sides of the neck and
the loop-like course of the arteries still mark their former position.
But it is conceivable that the now utterly lost branchiae might have
been gradually worked in by natural selection for some distinct purpose:
for instance, Landois has shown that the wings of insects are developed
from the trachea; it is therefore highly probable that in this great
class organs which once served for respiration have been actually
converted into organs for flight.

In considering transitions of organs, it is so important to bear in mind
the probability of conversion from one function to another, that I will
give another instance. Pedunculated cirripedes have two minute folds of
skin, called by me the ovigerous frena, which serve, through the means
of a sticky secretion, to retain the eggs until they are hatched within
the sack. These cirripedes have no branchiae, the whole surface of
the body and of the sack, together with the small frena, serving for
respiration. The Balanidae or sessile cirripedes, on the other hand,
have no ovigerous frena, the eggs lying loose at the bottom of the sack,
within the well-enclosed shell; but they have, in the same relative
position with the frena, large, much-folded membranes, which freely
communicate with the circulatory lacunae of the sack and body, and which
have been considered by all naturalists to act as branchiae. Now I
think no one will dispute that the ovigerous frena in the one family are
strictly homologous with the branchiae of the other family; indeed, they
graduate into each other. Therefore it need not be doubted that the two
little folds of skin, which originally served as ovigerous frena, but
which, likewise, very slightly aided in the act of respiration, have
been gradually converted by natural selection into branchiae, simply
through an increase in their size and the obliteration of their adhesive
glands. If all pedunculated cirripedes had become extinct, and they have
suffered far more extinction than have sessile cirripedes, who would
ever have imagined that the branchiae in this latter family had
originally existed as organs for preventing the ova from being washed
out of the sack?

There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has
lately been insisted on by Professor Cope and others in the United
States. It is now known that some animals are capable of reproduction
at a very early age, before they have acquired their perfect characters;
and if this power became thoroughly well developed in a species, it
seems probable that the adult stage of development would sooner or later
be lost; and in this case, especially if the larva differed much from
the mature form, the character of the species would be greatly changed
and degraded. Again, not a few animals, after arriving at maturity, go
on changing in character during nearly their whole lives. With mammals,
for instance, the form of the skull is often much altered with age, of
which Dr. Murie has given some striking instances with seals. Every
one knows how the horns of stags become more and more branched, and the
plumes of some birds become more finely developed, as they grow older.
Professor Cope states that the teeth of certain lizards change much in
shape with advancing years. With crustaceans not only many trivial,
but some important parts assume a new character, as recorded by Fritz
Muller, after maturity. In all such cases--and many could be given--if
the age for reproduction were retarded, the character of the species, at
least in its adult state, would be modified; nor is it improbable that
the previous and earlier stages of development would in some cases be
hurried through and finally lost. Whether species have often or ever
been modified through this comparatively sudden mode of transition, I
can form no opinion; but if this has occurred, it is probable that the
differences between the young and the mature, and between the mature and
the old, were primordially acquired by graduated steps.

SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION.

Although we must be extremely cautious in concluding that any organ
could not have been produced by successive, small, transitional
gradations, yet undoubtedly serious cases of difficulty occur.

One of the most serious is that of neuter insects, which are often
differently constructed from either the males or fertile females; but
this case will be treated of in the next chapter. The electric organs of
fishes offer another case of special difficulty; for it is impossible
to conceive by what steps these wondrous organs have been produced. But
this is not surprising, for we do not even know of what use they are.
In the gymnotus and torpedo they no doubt serve as powerful means of
defence, and perhaps for securing prey; yet in the ray, as observed
by Matteucci, an analogous organ in the tail manifests but little
electricity, even when the animal is greatly irritated; so little that
it can hardly be of any use for the above purposes. Moreover, in the
ray, besides the organ just referred to, there is, as Dr. R. McDonnell
has shown, another organ near the head, not known to be electrical, but
which appears to be the real homologue of the electric battery in the
torpedo. It is generally admitted that there exists between these organs
and ordinary muscle a close analogy, in intimate structure, in the
distribution of the nerves, and in the manner in which they are acted
on by various reagents. It should, also, be especially observed that
muscular contraction is accompanied by an electrical discharge; and,
as Dr. Radcliffe insists, "in the electrical apparatus of the torpedo
during rest, there would seem to be a charge in every respect like that
which is met with in muscle and nerve during the rest, and the discharge
of the torpedo, instead of being peculiar, may be only another form of
the discharge which attends upon the action of muscle and motor nerve."
Beyond this we cannot at present go in the way of explanation; but as
we know so little about the uses of these organs, and as we know nothing
about the habits and structure of the progenitors of the existing
electric fishes, it would be extremely bold to maintain that no
serviceable transitions are possible by which these organs might have
been gradually developed.

These organs appear at first to offer another and far more serious
difficulty; for they occur in about a dozen kinds of fish, of which
several are widely remote in their affinities. When the same organ is
found in several members of the same class, especially if in members
having very different habits of life, we may generally attribute its
presence to inheritance from a common ancestor; and its absence in some
of the members to loss through disuse or natural selection. So that, if
the electric organs had been inherited from some one ancient progenitor,
we might have expected that all electric fishes would have been
specially related to each other; but this is far from the case. Nor does
geology at all lead to the belief that most fishes formerly possessed
electric organs, which their modified descendants have now lost. But
when we look at the subject more closely, we find in the several fishes
provided with electric organs, that these are situated in different
parts of the body, that they differ in construction, as in the
arrangement of the plates, and, according to Pacini, in the process or
means by which the electricity is excited--and lastly, in being supplied
with nerves proceeding from different sources, and this is perhaps
the most important of all the differences. Hence in the several
fishes furnished with electric organs, these cannot be considered as
homologous, but only as analogous in function. Consequently there is
no reason to suppose that they have been inherited from a common
progenitor; for had this been the case they would have closely resembled
each other in all respects. Thus the difficulty of an organ, apparently
the same, arising in several remotely allied species, disappears,
leaving only the lesser yet still great difficulty: namely, by what
graduated steps these organs have been developed in each separate group
of fishes.

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. Other similar cases
could be given; for instance in plants, the very curious contrivance of
a mass of pollen-grains, borne on a foot-stalk with an adhesive gland,
is apparently the same in Orchis and Asclepias, genera almost as remote
as is possible among flowering plants; but here again the parts are not
homologous. In all cases of beings, far removed from each other in the
scale of organisation, which are furnished with similar and peculiar
organs, it will be found that although the general appearance and
function of the organs may be the same, yet fundamental differences
between them can always be detected. For instance, the eyes of
Cephalopods or cuttle-fish and of vertebrate animals appear wonderfully
alike; and in such widely sundered groups no part of this resemblance
can be due to inheritance from a common progenitor. Mr. Mivart has
advanced this case as one of special difficulty, but I am unable to
see the force of his argument. An organ for vision must be formed of
transparent tissue, and must include some sort of lens for throwing
an image at the back of a darkened chamber. Beyond this superficial
resemblance, there is hardly any real similarity between the eyes of
cuttle-fish and vertebrates, as may be seen by consulting Hensen's
admirable memoir on these organs in the Cephalopoda. It is impossible
for me here to enter on details, but I may specify a few of the points
of difference. The crystalline lens in the higher cuttle-fish consists
of two parts, placed one behind the other like two lenses, both having
a very different structure and disposition to what occurs in the
vertebrata. The retina is wholly different, with an actual inversion of
the elemental parts, and with a large nervous ganglion included within
the membranes of the eye. The relations of the muscles are as different
as it is possible to conceive, and so in other points. Hence it is not
a little difficult to decide how far even the same terms ought to be
employed in describing the eyes of the Cephalopoda and Vertebrata. It
is, of course, open to any one to deny that the eye in either case could
have been developed through the natural selection of successive slight
variations; but if this be admitted in the one case it is clearly
possible in the other; and fundamental differences of structure in the
visual organs of two groups might have been anticipated, in accordance
with this view of their manner of formation. As two men have sometimes
independently hit on the same invention, so in the several foregoing
cases it appears that natural selection, working for the good of each
being, and taking advantage of all favourable variations, has produced
similar organs, as far as function is concerned, in distinct organic
beings, which owe none of their structure in common to inheritance from
a common progenitor.

Fritz Muller, in order to test the conclusions arrived at in this
volume, has followed out with much care a nearly similar line of
argument. Several families of crustaceans include a few species,
possessing an air-breathing apparatus and fitted to live out of the
water. In two of these families, which were more especially examined by
Muller, and which are nearly related to each other, the species agree
most closely in all important characters: namely in their sense organs,
circulating systems, in the position of the tufts of hair within
their complex stomachs, and lastly in the whole structure of the
water-breathing branchiae, even to the microscopical hooks by which they
are cleansed. Hence it might have been expected that in the few species
belonging to both families which live on the land, the equally important
air-breathing apparatus would have been the same; for why should this
one apparatus, given for the same purpose, have been made to differ,
while all the other important organs were closely similar, or rather,
identical.

Fritz Muller argues that this close similarity in so many points
of structure must, in accordance with the views advanced by me, be
accounted for by inheritance from a common progenitor. But as the vast
majority of the species in the above two families, as well as most
other crustaceans, are aquatic in their habits, it is improbable in the
highest degree that their common progenitor should have been adapted for
breathing air. Muller was thus led carefully to examine the apparatus in
the air-breathing species; and he found it to differ in each in several
important points, as in the position of the orifices, in the manner in
which they are opened and closed, and in some accessory details. Now
such differences are intelligible, and might even have been expected, on
the supposition that species belonging to distinct families had slowly
become adapted to live more and more out of water, and to breathe the
air. For these species, from belonging to distinct families, would have
differed to a certain extent, and in accordance with the principle that
the nature of each variation depends on two factors, viz., the nature of
the organism and that of the surrounding conditions, their variability
assuredly would not have been exactly the same. Consequently natural
selection would have had different materials or variations to work on,
in order to arrive at the same functional result; and the structures
thus acquired would almost necessarily have differed. On the hypothesis
of separate acts of creation the whole case remains unintelligible. This
line of argument seems to have had great weight in leading Fritz Muller
to accept the views maintained by me in this volume.

Another distinguished zoologist, the late Professor Claparede, has
argued in the same manner, and has arrived at the same result. He
shows that there are parasitic mites (Acaridae), belonging to distinct
sub-families and families, which are furnished with hair-claspers. These
organs must have been independently developed, as they could not have
been inherited from a common progenitor; and in the several groups they
are formed by the modification of the fore legs, of the hind legs, of
the maxillae or lips, and of appendages on the under side of the hind
part of the body.

In the foregoing cases, we see the same end gained and the same function
performed, in beings not at all or only remotely allied, by organs in
appearance, though not in development, closely similar. On the other
hand, it is a common rule throughout nature that the same end should
be gained, even sometimes in the case of closely related beings, by the
most diversified means. How differently constructed is the feathered
wing of a bird and the membrane-covered wing of a bat; and still more so
the four wings of a butterfly, the two wings of a fly, and the two wings
with the elytra of a beetle. Bivalve shells are made to open and shut,
but on what a number of patterns is the hinge constructed, from the long
row of neatly interlocking teeth in a Nucula to the simple ligament of
a Mussel! Seeds are disseminated by their minuteness, by their capsule
being converted into a light balloon-like envelope, by being embedded
in pulp or flesh, formed of the most diverse parts, and rendered
nutritious, as well as conspicuously coloured, so as to attract and
be devoured by birds, by having hooks and grapnels of many kinds and
serrated awns, so as to adhere to the fur of quadrupeds, and by being
furnished with wings and plumes, as different in shape as they are
elegant in structure, so as to be wafted by every breeze. I will give
one other instance: for this subject of the same end being gained by the
most diversified means well deserves attention. Some authors maintain
that organic beings have been formed in many ways for the sake of
mere variety, almost like toys in a shop, but such a view of nature is
incredible. With plants having separated sexes, and with those in which,
though hermaphrodites, the pollen does not spontaneously fall on the
stigma, some aid is necessary for their fertilisation. With several
kinds this is effected by the pollen-grains, which are light and
incoherent, being blown by the wind through mere chance on to the
stigma; and this is the simplest plan which can well be conceived. An
almost equally simple, though very different plan occurs in many plants
in which a symmetrical flower secretes a few drops of nectar, and is
consequently visited by insects; and these carry the pollen from the
anthers to the stigma.

From this simple stage we may pass through an inexhaustible number of
contrivances, all for the same purpose and effected in essentially the
same manner, but entailing changes in every part of the flower. The
nectar may be stored in variously shaped receptacles, with the
stamens and pistils modified in many ways, sometimes forming trap-like
contrivances, and sometimes capable of neatly adapted movements through
irritability or elasticity. From such structures we may advance till we
come to such a case of extraordinary adaptation as that lately described
by Dr. Cruger in the Coryanthes. This orchid has part of its labellum or
lower lip hollowed out into a great bucket, into which drops of almost
pure water continually fall from two secreting horns which stand above
it; and when the bucket is half-full, the water overflows by a spout on
one side. The basal part of the labellum stands over the bucket, and is
itself hollowed out into a sort of chamber with two lateral entrances;
within this chamber there are curious fleshy ridges. The most ingenious
man, if he had not witnessed what takes place, could never have imagined
what purpose all these parts serve. But Dr. Cruger saw crowds of large
humble-bees visiting the gigantic flowers of this orchid, not in order
to suck nectar, but to gnaw off the ridges within the chamber above the
bucket; in doing this they frequently pushed each other into the bucket,
and their wings being thus wetted they could not fly away, but were
compelled to crawl out through the passage formed by the spout or
overflow. Dr. Cruger saw a "continual procession" of bees thus crawling
out of their involuntary bath. The passage is narrow, and is roofed over
by the column, so that a bee, in forcing its way out, first rubs its
back against the viscid stigma and then against the viscid glands of the
pollen-masses. The pollen-masses are thus glued to the back of the
bee which first happens to crawl out through the passage of a lately
expanded flower, and are thus carried away. Dr. Cruger sent me a flower
in spirits of wine, with a bee which he had killed before it had quite
crawled out, with a pollen-mass still fastened to its back. When the
bee, thus provided, flies to another flower, or to the same flower a
second time, and is pushed by its comrades into the bucket and then
crawls out by the passage, the pollen-mass necessarily comes first into
contact with the viscid stigma, and adheres to it, and the flower is
fertilised. Now at last we see the full use of every part of the flower,
of the water-secreting horns of the bucket half-full of water, which
prevents the bees from flying away, and forces them to crawl out through
the spout, and rub against the properly placed viscid pollen-masses and
the viscid stigma.

The construction of the flower in another closely allied orchid, namely,
the Catasetum, is widely different, though serving the same end; and is
equally curious. Bees visit these flowers, like those of the Coryanthes,
in order to gnaw the labellum; in doing this they inevitably touch
a long, tapering, sensitive projection, or, as I have called it, the
antenna. This antenna, when touched, transmits a sensation or vibration
to a certain membrane which is instantly ruptured; this sets free a
spring by which the pollen-mass is shot forth, like an arrow, in the
right direction, and adheres by its viscid extremity to the back of the
bee. The pollen-mass of the male plant (for the sexes are separate in
this orchid) is thus carried to the flower of the female plant, where it
is brought into contact with the stigma, which is viscid enough to
break certain elastic threads, and retain the pollen, thus effecting
fertilisation.

How, it may be asked, in the foregoing and in innumerable other
instances, can we understand the graduated scale of complexity and the
multifarious means for gaining the same end. The answer no doubt is, as
already remarked, that when two forms vary, which already differ from
each other in some slight degree, the variability will not be of the
same exact nature, and consequently the results obtained through natural
selection for the same general purpose will not be the same. We should
also bear in mind that every highly developed organism has passed
through many changes; and that each modified structure tends to be
inherited, so that each modification will not readily be quite lost,
but may be again and again further altered. Hence, the structure of each
part of each species, for whatever purpose it may serve, is the sum of
many inherited changes, through which the species has passed during its
successive adaptations to changed habits and conditions of life.

Finally, then, although in many cases it is most difficult even to
conjecture by what transitions organs could have arrived at their
present state; yet, considering how small the proportion of living and
known forms is to the extinct and unknown, I have been astonished how
rarely an organ can be named, towards which no transitional grade is
known to lead. It is certainly true, that new organs appearing as if
created for some special purpose rarely or never appear in any being; as
indeed is shown by that old, but somewhat exaggerated, canon in natural
history of "Natura non facit saltum." We meet with this admission in the
writings of almost every experienced naturalist; or, as Milne Edwards
has well expressed it, "Nature is prodigal in variety, but niggard in
innovation." Why, on the theory of Creation, should there be so much
variety and so little real novelty? Why should all the parts and organs
of many independent beings, each supposed to have been separately
created for its own proper place in nature, be so commonly linked
together by graduated steps? Why should not Nature take a sudden leap
from structure to structure? On the theory of natural selection, we can
clearly understand why she should not; for natural selection acts only
by taking advantage of slight successive variations; she can never take
a great and sudden leap, but must advance by the short and sure, though
slow steps.

ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED BY NATURAL SELECTION.

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.

In the first place, we are much too ignorant in regard to the whole
economy of any one organic being to say what slight modifications would
be of importance or not. In a former chapter I have given instances of
very trifling characters, such as the down on fruit and the colour of
its flesh, the colour of the skin and hair of quadrupeds, which, from
being correlated with constitutional differences, or from determining
the attacks of insects, might assuredly be acted on by natural
selection. The tail of the giraffe looks like an artificially
constructed fly-flapper; and it seems at first incredible that this
could have been adapted for its present purpose by successive slight
modifications, each better and better fitted, for so trifling an object
as to drive away flies; yet we should pause before being too positive
even in this case, for we know that the distribution and existence of
cattle and other animals in South America absolutely depend on their
power of resisting the attacks of insects: so that individuals which
could by any means defend themselves from these small enemies, would be
able to range into new pastures and thus gain a great advantage. It is
not that the larger quadrupeds are actually destroyed (except in some
rare cases) by flies, but they are incessantly harassed and their
strength reduced, so that they are more subject to disease, or not so
well enabled in a coming dearth to search for food, or to escape from
beasts of prey.

Organs now of trifling importance have probably in some cases been of
high importance to an early progenitor, and, after having been slowly
perfected at a former period, have been transmitted to existing species
in nearly the same state, although now of very slight use; but any
actually injurious deviations in their structure would of course have
been checked by natural selection. Seeing how important an organ of
locomotion the tail is in most aquatic animals, its general presence and
use for many purposes in so many land animals, which in their lungs or
modified swim-bladders betray their aquatic origin, may perhaps be thus
accounted for. A well-developed tail having been formed in an aquatic
animal, it might subsequently come to be worked in for all sorts of
purposes, as a fly-flapper, an organ of prehension, or as an aid in
turning, as in the case of the dog, though the aid in this latter
respect must be slight, for the hare, with hardly any tail, can double
still more quickly.

In the second place, we may easily err in attributing importance to
characters, and in believing that they have been developed through
natural selection. We must by no means overlook the effects of the
definite action of changed conditions of life, of so-called spontaneous
variations, which seem to depend in a quite subordinate degree on the
nature of the conditions, of the tendency to reversion to long-lost
characters, of the complex laws of growth, such as of correlation,
comprehension, of the pressure of one part on another, etc., and finally
of sexual selection, by which characters of use to one sex are often
gained and then transmitted more or less perfectly to the other sex,
though of no use to the sex. But structures thus indirectly gained,
although at first of no advantage to a species, may subsequently
have been taken advantage of by its modified descendants, under new
conditions of life and newly acquired habits.

If green woodpeckers alone had existed, and we did not know that there
were many black and pied kinds, I dare say that we should have thought
that the green colour was a beautiful adaptation to conceal this
tree-frequenting bird from its enemies; and consequently that it was
a character of importance, and had been acquired through natural
selection; as it is, the colour is probably in chief part due to sexual
selection. A trailing palm in the Malay Archipelago climbs the loftiest
trees by the aid of exquisitely constructed hooks clustered around the
ends of the branches, and this contrivance, no doubt, is of the highest
service to the plant; but as we see nearly similar hooks on many trees
which are not climbers, and which, as there is reason to believe from
the distribution of the thorn-bearing species in Africa and South
America, serve as a defence against browsing quadrupeds, so the spikes
on the palm may at first have been developed for this object, and
subsequently have been improved and taken advantage of by the plant, as
it underwent further modification and became a climber. The naked skin
on the head of a vulture is generally considered as a direct adaptation
for wallowing in putridity; and so it may be, or it may possibly be due
to the direct action of putrid matter; but we should be very cautious in
drawing any such inference, when we see that the skin on the head of the
clean-feeding male turkey is likewise naked. The sutures in the skulls
of young mammals have been advanced as a beautiful adaptation for aiding
parturition, and no doubt they facilitate, or may be indispensable
for this act; but as sutures occur in the skulls of young birds and
reptiles, which have only to escape from a broken egg, we may infer that
this structure has arisen from the laws of growth, and has been taken
advantage of in the parturition of the higher animals.

We are profoundly ignorant of the cause of each slight variation or
individual difference; and we are immediately made conscious of this
by reflecting on the differences between the breeds of our domesticated
animals in different countries, more especially in the less civilized
countries, where there has been but little methodical selection. Animals
kept by savages in different countries often have to struggle for
their own subsistence, and are exposed to a certain extent to natural
selection, and individuals with slightly different constitutions would
succeed best under different climates. With cattle susceptibility to the
attacks of flies is correlated with colour, as is the liability to be
poisoned by certain plants; so that even colour would be thus subjected
to the action of natural selection. Some observers are convinced that a
damp climate affects the growth of the hair, and that with the hair the
horns are correlated. Mountain breeds always differ from lowland breeds;
and a mountainous country would probably affect the hind limbs from
exercising them more, and possibly even the form of the pelvis; and then
by the law of homologous variation, the front limbs and the head would
probably be affected. The shape, also, of the pelvis might affect
by pressure the shape of certain parts of the young in the womb. The
laborious breathing necessary in high regions tends, as we have
good reason to believe, to increase the size of the chest; and again
correlation would come into play. The effects of lessened exercise,
together with abundant food, on the whole organisation is probably still
more important, and this, as H. von Nathusius has lately shown in
his excellent Treatise, is apparently one chief cause of the great
modification which the breeds of swine have undergone. But we are far
too ignorant to speculate on the relative importance of the several
known and unknown causes of variation; and I have made these remarks
only to show that, if we are unable to account for the characteristic
differences of our several domestic breeds, which nevertheless are
generally admitted to have arisen through ordinary generation from
one or a few parent-stocks, we ought not to lay too much stress on
our ignorance of the precise cause of the slight analogous differences
between true species.

UTILITARIAN DOCTRINE, HOW FAR TRUE: BEAUTY, HOW ACQUIRED.

The foregoing remarks lead me to say a few words on the protest lately
made by some naturalists against the utilitarian doctrine that every
detail of structure has been produced for the good of its possessor.
They believe that many structures have been created for the sake of
beauty, to delight man or the Creator (but this latter point is beyond
the scope of scientific discussion), or for the sake of mere variety,
a view already discussed. Such doctrines, if true, would be absolutely
fatal to my theory. I fully admit that many structures are now of no
direct use to their possessors, and may never have been of any use to
their progenitors; but this does not prove that they were formed
solely for beauty or variety. No doubt the definite action of changed
conditions, and the various causes of modifications, lately specified,
have all produced an effect, probably a great effect, independently of
any advantage thus gained. But a still more important consideration is
that the chief part of the organisation of every living creature is due
to inheritance; and consequently, though each being assuredly is well
fitted for its place in nature, many structures have now no very close
and direct relation to present habits of life. Thus, we can
hardly believe that the webbed feet of the upland goose, or of the
frigate-bird, are of special use to these birds; we cannot believe
that the similar bones in the arm of the monkey, in the fore leg of the
horse, in the wing of the bat, and in the flipper of the seal, are of
special use to these animals. We may safely attribute these structures
to inheritance. But webbed feet no doubt were as useful to the
progenitor of the upland goose and of the frigate-bird, as they now
are to the most aquatic of living birds. So we may believe that the
progenitor of the seal did not possess a flipper, but a foot with five
toes fitted for walking or grasping; and we may further venture to
believe that the several bones in the limbs of the monkey, horse and
bat, were originally developed, on the principle of utility, probably
through the reduction of more numerous bones in the fin of some ancient
fish-like progenitor of the whole class. It is scarcely possible to
decide how much allowance ought to be made for such causes of change,
as the definite action of external conditions, so-called spontaneous
variations, and the complex laws of growth; but with these important
exceptions, we may conclude that the structure of every living creature
either now is, or was formerly, of some direct or indirect use to its
possessor.

With respect to the belief that organic beings have been created
beautiful for the delight of man--a belief which it has been pronounced
is subversive of my whole theory--I may first remark that the sense of
beauty obviously depends on the nature of the mind, irrespective of
any real quality in the admired object; and that the idea of what is
beautiful, is not innate or unalterable. We see this, for instance, in
the men of different races admiring an entirely different standard of
beauty in their women. If beautiful objects had been created solely for
man's gratification, it ought to be shown that before man appeared
there was less beauty on the face of the earth than since he came on the
stage. Were the beautiful volute and cone shells of the Eocene epoch,
and the gracefully sculptured ammonites of the Secondary period, created
that man might ages afterwards admire them in his cabinet? Few objects
are more beautiful than the minute siliceous cases of the diatomaceae:
were these created that they might be examined and admired under the
higher powers of the microscope? The beauty in this latter case, and
in many others, is apparently wholly due to symmetry of growth. Flowers
rank among the most beautiful productions of nature; but they have
been rendered conspicuous in contrast with the green leaves, and in
consequence at the same time beautiful, so that they may be easily
observed by insects. I have come to this conclusion from finding it an
invariable rule that when a flower is fertilised by the wind it never
has a gaily-coloured corolla. Several plants habitually produce two
kinds of flowers; one kind open and coloured so as to attract insects;
the other closed, not coloured, destitute of nectar, and never visited
by insects. Hence, we may conclude that, if insects had not been
developed on the face of the earth, our plants would not have been
decked with beautiful flowers, but would have produced only such poor
flowers as we see on our fir, oak, nut and ash trees, on grasses,
spinach, docks and nettles, which are all fertilised through the agency
of the wind. A similar line of argument holds good with fruits; that
a ripe strawberry or cherry is as pleasing to the eye as to the
palate--that the gaily-coloured fruit of the spindle-wood tree and the
scarlet berries of the holly are beautiful objects--will be admitted by
everyone. But this beauty serves merely as a guide to birds and
beasts, in order that the fruit may be devoured and the matured seeds
disseminated. I infer that this is the case from having as yet found
no exception to the rule that seeds are always thus disseminated when
embedded within a fruit of any kind (that is within a fleshy or
pulpy envelope), if it be coloured of any brilliant tint, or rendered
conspicuous by being white or black.

On the other hand, I willingly admit that a great number of male
animals, as all our most gorgeous birds, some fishes, reptiles, and
mammals, and a host of magnificently coloured butterflies, have been
rendered beautiful for beauty's sake. But this has been effected through
sexual selection, that is, by the more beautiful males having been
continually preferred by the females, and not for the delight of man. So
it is with the music of birds. We may infer from all this that a nearly
similar taste for beautiful colours and for musical sounds runs through
a large part of the animal kingdom. When the female is as beautifully
coloured as the male, which is not rarely the case with birds and
butterflies, the cause apparently lies in the colours acquired through
sexual selection having been transmitted to both sexes, instead of to
the males alone. How the sense of beauty in its simplest form--that is,
the reception of a peculiar kind of pleasure from certain colours, forms
and sounds--was first developed in the mind of man and of the lower
animals, is a very obscure subject. The same sort of difficulty is
presented if we enquire how it is that certain flavours and odours give
pleasure, and others displeasure. Habit in all these cases appears
to have come to a certain extent into play; but there must be some
fundamental cause in the constitution of the nervous system in each
species.

Natural selection cannot possibly produce any modification in a species
exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by the
structures of others. But natural selection can and does often produce
structures for the direct injury of other animals, as we see in the fang
of the adder, and in the ovipositor of the ichneumon, by which its eggs
are deposited in the living bodies of other insects. If it could be
proved that any part of the structure of any one species had been
formed for the exclusive good of another species, it would annihilate my
theory, for such could not have been produced through natural selection.
Although many statements may be found in works on natural history to
this effect, I cannot find even one which seems to me of any weight. It
is admitted that the rattlesnake has a poison-fang for its own defence
and for the destruction of its prey; but some authors suppose that at
the same time it is furnished with a rattle for its own injury, namely,
to warn its prey. I would almost as soon believe that the cat curls the
end of its tail when preparing to spring, in order to warn the doomed
mouse. It is a much more probable view that the rattlesnake uses its
rattle, the cobra expands its frill and the puff-adder swells while
hissing so loudly and harshly, in order to alarm the many birds and
beasts which are known to attack even the most venomous species. Snakes
act on the same principle which makes the hen ruffle her feathers and
expand her wings when a dog approaches her chickens. But I have not
space here to enlarge on the many ways by which animals endeavour to
frighten away their enemies.

Natural selection will never produce in a being any structure more
injurious than beneficial to that being, for natural selection acts
solely by and for the good of each. No organ will be formed, as Paley
has remarked, for the purpose of causing pain or for doing an injury
to its possessor. If a fair balance be struck between the good and evil
caused by each part, each will be found on the whole advantageous. After
the lapse of time, under changing conditions of life, if any part comes
to be injurious, it will be modified; or if it be not so, the being will
become extinct, as myriads have become extinct.

Natural selection tends only to make each organic being as perfect as,
or slightly more perfect than the other inhabitants of the same country
with which it comes into competition. And we see that this is the
standard of perfection attained under nature. The endemic productions of
New Zealand, for instance, are perfect, one compared with another; but
they are now rapidly yielding before the advancing legions of plants
and animals introduced from Europe. Natural selection will not produce
absolute perfection, nor do we always meet, as far as we can judge, with
this high standard under nature. The correction for the aberration of
light is said by Muller not to be perfect even in that most perfect
organ, the human eye. Helmholtz, whose judgment no one will dispute,
after describing in the strongest terms the wonderful powers of the
human eye, adds these remarkable words: "That which we have discovered
in the way of inexactness and imperfection in the optical machine and
in the image on the retina, is as nothing in comparison with the
incongruities which we have just come across in the domain of the
sensations. One might say that nature has taken delight in accumulating
contradictions in order to remove all foundation from the theory of a
pre-existing harmony between the external and internal worlds." If our
reason leads us to admire with enthusiasm a multitude of inimitable
contrivances in nature, this same reason tells us, though we may easily
err on both sides, that some other contrivances are less perfect. Can we
consider the sting of the bee as perfect, which, when used against many
kinds of enemies, cannot be withdrawn, owing to the backward serratures,
and thus inevitably causes the death of the insect by tearing out its
viscera?

If we look at the sting of the bee, as having existed in a remote
progenitor, as a boring and serrated instrument, like that in so many
members of the same great order, and that it has since been modified
but not perfected for its present purpose, with the poison originally
adapted for some other object, such as to produce galls, since
intensified, we can perhaps understand how it is that the use of the
sting should so often cause the insect's own death: for if on the whole
the power of stinging be useful to the social community, it will fulfil
all the requirements of natural selection, though it may cause the death
of some few members. If we admire the truly wonderful power of scent by
which the males of many insects find their females, can we admire the
production for this single purpose of thousands of drones, which are
utterly useless to the community for any other purpose, and which are
ultimately slaughtered by their industrious and sterile sisters? It may
be difficult, but we ought to admire the savage instinctive hatred
of the queen-bee, which urges her to destroy the young queens, her
daughters, as soon as they are born, or to perish herself in the combat;
for undoubtedly this is for the good of the community; and maternal love
or maternal hatred, though the latter fortunately is most rare, is all
the same to the inexorable principles of natural selection. If we admire
the several ingenious contrivances by which orchids and many other
plants are fertilised through insect agency, can we consider as equally
perfect the elaboration of dense clouds of pollen by our fir-trees, so
that a few granules may be wafted by chance on to the ovules?

SUMMARY: THE LAW OF UNITY OF TYPE AND OF THE CONDITIONS OF EXISTENCE
EMBRACED BY THE THEORY OF NATURAL SELECTION.

We have in this chapter discussed some of the difficulties and
objections which may be urged against the theory. Many of them are
serious; but I think that in the discussion light has been thrown on
several facts, which on the belief of independent acts of creation are
utterly obscure. We have seen that species at any one period are not
indefinitely variable, and are not linked together by a multitude of
intermediate gradations, partly because the process of natural selection
is always very slow, and at any one time acts only on a few forms;
and partly because the very process of natural selection implies the
continual supplanting and extinction of preceding and intermediate
gradations. Closely allied species, now living on a continuous area,
must often have been formed when the area was not continuous, and when
the conditions of life did not insensibly graduate away from one part to
another. When two varieties are formed in two districts of a continuous
area, an intermediate variety will often be formed, fitted for an
intermediate zone; but from reasons assigned, the intermediate variety
will usually exist in lesser numbers than the two forms which it
connects; consequently the two latter, during the course of further
modification, from existing in greater numbers, will have a great
advantage over the less numerous intermediate variety, and will thus
generally succeed in supplanting and exterminating it.

We have seen in this chapter how cautious we should be in concluding
that the most different habits of life could not graduate into each
other; that a bat, for instance, could not have been formed by natural
selection from an animal which at first only glided through the air.

We have seen that a species under new conditions of life may change its
habits, or it may have diversified habits, with some very unlike those
of its nearest congeners. Hence we can understand, bearing in mind that
each organic being is trying to live wherever it can live, how it has
arisen that there are upland geese with webbed feet, ground woodpeckers,
diving thrushes, and petrels with the habits of auks.

Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is enough to stagger any one; yet in
the case of any organ, if we know of a long series of gradations in
complexity, each good for its possessor, then under changing conditions
of life, there is no logical impossibility in the acquirement of any
conceivable degree of perfection through natural selection. In the cases
in which we know of no intermediate or transitional states, we should
be extremely cautious in concluding that none can have existed, for the
metamorphoses of many organs show what wonderful changes in function
are at least possible. For instance, a swim-bladder has apparently been
converted into an air-breathing lung. The same organ having performed
simultaneously very different functions, and then having been in part
or in whole specialised for one function; and two distinct organs
having performed at the same time the same function, the one having been
perfected whilst aided by the other, must often have largely facilitated
transitions.

We have seen that in two beings widely remote from each other in the
natural scale, organs serving for the same purpose and in external
appearance closely similar may have been separately and independently
formed; but when such organs are closely examined, essential differences
in their structure can almost always be detected; and this naturally
follows from the principle of natural selection. On the other hand, the
common rule throughout nature is infinite diversity of structure for
gaining the same end; and this again naturally follows from the same
great principle.

In many cases we are far too ignorant to be enabled to assert that
a part or organ is so unimportant for the welfare of a species, that
modifications in its structure could not have been slowly accumulated
by means of natural selection. In many other cases, modifications
are probably the direct result of the laws of variation or of growth,
independently of any good having been thus gained. But even such
structures have often, as we may feel assured, been subsequently taken
advantage of, and still further modified, for the good of species under
new conditions of life. We may, also, believe that a part formerly of
high importance has frequently been retained (as the tail of an aquatic
animal by its terrestrial descendants), though it has become of such
small importance that it could not, in its present state, have been
acquired by means of natural selection.

Natural selection can produce nothing in one species for the exclusive
good or injury of another; though it may well produce parts, organs, and
excretions highly useful or even indispensable, or highly injurious
to another species, but in all cases at the same time useful to the
possessor. In each well-stocked country natural selection acts through
the competition of the inhabitants and consequently leads to success
in the battle for life, only in accordance with the standard of that
particular country. Hence the inhabitants of one country, generally the
smaller one, often yield to the inhabitants of another and generally the
larger country. For in the larger country there will have existed more
individuals, and more diversified forms, and the competition will
have been severer, and thus the standard of perfection will have been
rendered higher. Natural selection will not necessarily lead to absolute
perfection; nor, as far as we can judge by our limited faculties, can
absolute perfection be everywhere predicated.

On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, "Natura non facit saltum."
This canon, if we look to the present inhabitants alone of the world, is
not strictly correct; but if we include all those of past times, whether
known or unknown, it must on this theory be strictly true.

It is generally acknowledged that all organic beings have been formed on
two great laws--Unity of Type, and the Conditions of Existence. By unity
of type is meant that fundamental agreement in structure which we see
in organic beings of the same class, and which is quite independent of
their habits of life. On my theory, unity of type is explained by unity
of descent. The expression of conditions of existence, so often insisted
on by the illustrious Cuvier, is fully embraced by the principle of
natural selection. For natural selection acts by either now adapting the
varying parts of each being to its organic and inorganic conditions
of life; or by having adapted them during past periods of time: the
adaptations being aided in many cases by the increased use or disuse
of parts, being affected by the direct action of external conditions
of life, and subjected in all cases to the several laws of growth and
variation. Hence, in fact, the law of the Conditions of Existence is the
higher law; as it includes, through the inheritance of former variations
and adaptations, that of Unity of Type.



CHAPTER VII. MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.

 Longevity--Modifications not necessarily simultaneous--Modifications
 apparently of no direct service--Progressive development--Characters of
 small functional importance, the most constant--Supposed incompetence
 of natural selection to account for the incipient stages of useful
 structures--Causes which interfere with the acquisition through natural
 selection of useful structures--Gradations of structure with changed
 functions--Widely different organs in members of the same class,
 developed from one and the same source--Reasons for disbelieving in
 great and abrupt modifications.


I will devote this chapter to the consideration of various miscellaneous
objections which have been advanced against my views, as some of the
previous discussions may thus be made clearer; but it would be useless
to discuss all of them, as many have been made by writers who have not
taken the trouble to understand the subject. Thus a distinguished German
naturalist has asserted that the weakest part of my theory is, that I
consider all organic beings as imperfect: what I have really said is,
that all are not as perfect as they might have been in relation to their
conditions; and this is shown to be the case by so many native forms
in many quarters of the world having yielded their places to intruding
foreigners. Nor can organic beings, even if they were at any one time
perfectly adapted to their conditions of life, have remained so, when
their conditions changed, unless they themselves likewise changed; and
no one will dispute that the physical conditions of each country, as
well as the number and kinds of its inhabitants, have undergone many
mutations.

A critic has lately insisted, with some parade of mathematical accuracy,
that longevity is a great advantage to all species, so that he who
believes in natural selection "must arrange his genealogical tree" in
such a manner that all the descendants have longer lives than their
progenitors! Cannot our critics conceive that a biennial plant or one of
the lower animals might range into a cold climate and perish there every
winter; and yet, owing to advantages gained through natural selection,
survive from year to year by means of its seeds or ova? Mr. E. Ray
Lankester has recently discussed this subject, and he concludes, as far
as its extreme complexity allows him to form a judgment, that longevity
is generally related to the standard of each species in the scale of
organisation, as well as to the amount of expenditure in reproduction
and in general activity. And these conditions have, it is probable, been
largely determined through natural selection.

It has been argued that, as none of the animals and plants of Egypt,
of which we know anything, have changed during the last three or four
thousand years, so probably have none in any part of the world. But, as
Mr. G.H. Lewes has remarked, this line of argument proves too much,
for the ancient domestic races figured on the Egyptian monuments, or
embalmed, are closely similar or even identical with those now living;
yet all naturalists admit that such races have been produced through
the modification of their original types. The many animals which have
remained unchanged since the commencement of the glacial period, would
have been an incomparably stronger case, for these have been exposed
to great changes of climate and have migrated over great distances;
whereas, in Egypt, during the last several thousand years, the
conditions of life, as far as we know, have remained absolutely uniform.
The fact of little or no modification having been effected since the
glacial period, would have been of some avail against those who believe
in an innate and necessary law of development, but is powerless against
the doctrine of natural selection or the survival of the fittest, which
implies that when variations or individual differences of a beneficial
nature happen to arise, these will be preserved; but this will be
effected only under certain favourable circumstances.

The celebrated palaeontologist, Bronn, at the close of his German
translation of this work, asks how, on the principle of natural
selection, can a variety live side by side with the parent species?
If both have become fitted for slightly different habits of life
or conditions, they might live together; and if we lay on one side
polymorphic species, in which the variability seems to be of a peculiar
nature, and all mere temporary variations, such as size, albinism,
etc., the more permanent varieties are generally found, as far as I can
discover, inhabiting distinct stations, such as high land or low land,
dry or moist districts. Moreover, in the case of animals which wander
much about and cross freely, their varieties seem to be generally
confined to distinct regions.

Bronn also insists that distinct species never differ from each other in
single characters, but in many parts; and he asks, how it always comes
that many parts of the organisation should have been modified at the
same time through variation and natural selection? But there is no
necessity for supposing that all the parts of any being have been
simultaneously modified. The most striking modifications, excellently
adapted for some purpose, might, as was formerly remarked, be acquired
by successive variations, if slight, first in one part and then in
another; and as they would be transmitted all together, they would
appear to us as if they had been simultaneously developed. The best
answer, however, to the above objection is afforded by those domestic
races which have been modified, chiefly through man's power of
selection, for some special purpose. Look at the race and dray-horse, or
at the greyhound and mastiff. Their whole frames, and even their mental
characteristics, have been modified; but if we could trace each step
in the history of their transformation--and the latter steps can be
traced--we should not see great and simultaneous changes, but first
one part and then another slightly modified and improved. Even when
selection has been applied by man to some one character alone--of which
our cultivated plants offer the best instances--it will invariably be
found that although this one part, whether it be the flower, fruit, or
leaves, has been greatly changed, almost all the other parts have been
slightly modified. This may be attributed partly to the principle of
correlated growth, and partly to so-called spontaneous variation.

A much more serious objection has been urged by Bronn, and recently by
Broca, namely, that many characters appear to be of no service whatever
to their possessors, and therefore cannot have been influenced through
natural selection. Bronn adduces the length of the ears and tails in the
different species of hares and mice--the complex folds of enamel in the
teeth of many animals, and a multitude of analogous cases. With respect
to plants, this subject has been discussed by Nageli in an admirable
essay. He admits that natural selection has effected much, but he
insists that the families of plants differ chiefly from each other in
morphological characters, which appear to be quite unimportant for the
welfare of the species. He consequently believes in an innate tendency
towards progressive and more perfect development. He specifies the
arrangement of the cells in the tissues, and of the leaves on the axis,
as cases in which natural selection could not have acted. To these
may be added the numerical divisions in the parts of the flower, the
position of the ovules, the shape of the seed, when not of any use for
dissemination, etc.

There is much force in the above objection. Nevertheless, we ought, in
the first place, to be extremely cautious in pretending to decide what
structures now are, or have formerly been, of use to each species. In
the second place, it should always be borne in mind that when one part
is modified, so will be other parts, through certain dimly seen causes,
such as an increased or diminished flow of nutriment to a part, mutual
pressure, an early developed part affecting one subsequently developed,
and so forth--as well as through other causes which lead to the
many mysterious cases of correlation, which we do not in the least
understand. These agencies may be all grouped together, for the sake of
brevity, under the expression of the laws of growth. In the third
place, we have to allow for the direct and definite action of changed
conditions of life, and for so-called spontaneous variations, in which
the nature of the conditions apparently plays a quite subordinate part.
Bud-variations, such as the appearance of a moss-rose on a common rose,
or of a nectarine on a peach-tree, offer good instances of spontaneous
variations; but even in these cases, if we bear in mind the power of a
minute drop of poison in producing complex galls, we ought not to feel
too sure that the above variations are not the effect of some local
change in the nature of the sap, due to some change in the conditions.
There must be some efficient cause for each slight individual
difference, as well as for more strongly marked variations which
occasionally arise; and if the unknown cause were to act persistently,
it is almost certain that all the individuals of the species would be
similarly modified.

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 than 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.

It may be worth while to illustrate some of the foregoing remarks. With
respect to the assumed inutility of various parts and organs, it is
hardly necessary to observe that even in the higher and best-known
animals many structures exist, which are so highly developed that no one
doubts that they are of importance, yet their use has not been, or has
only recently been, ascertained. As Bronn gives the length of the ears
and tail in the several species of mice as instances, though trifling
ones, of differences in structure which can be of no special use, I may
mention that, according to Dr. Schobl, the external ears of the common
mouse are supplied in an extraordinary manner with nerves, so that
they no doubt serve as tactile organs; hence the length of the ears can
hardly be quite unimportant. We shall, also, presently see that the tail
is a highly useful prehensile organ to some of the species; and its use
would be much influence by its length.

With respect to plants, to which on account of Nageli's essay I shall
confine myself in the following remarks, it will be admitted that the
flowers of the orchids present a multitude of curious structures,
which a few years ago would have been considered as mere morphological
differences without any special function; but they are now known to be
of the highest importance for the fertilisation of the species through
the aid of insects, and have probably been gained through natural
selection. No one until lately would have imagined that in dimorphic and
trimorphic plants the different lengths of the stamens and pistils, and
their arrangement, could have been of any service, but now we know this
to be the case.

In certain whole groups of plants the ovules stand erect, and in others
they are suspended; and within the same ovarium of some few plants, one
ovule holds the former and a second ovule the latter position. These
positions seem at first purely morphological, or of no physiological
signification; but Dr. Hooker informs me that within the same ovarium
the upper ovules alone in some cases, and in others the lower ones
alone are fertilised; and he suggests that this probably depends on
the direction in which the pollen-tubes enter the ovarium. If so, the
position of the ovules, even when one is erect and the other suspended
within the same ovarium, would follow the selection of any slight
deviations in position which favoured their fertilisation, and the
production of seed.

Several plants belonging to distinct orders habitually produce flowers
of two kinds--the one open, of the ordinary structure, the other closed
and imperfect. These two kinds of flowers sometimes differ wonderfully
in structure, yet may be seen to graduate into each other on the same
plant. The ordinary and open flowers can be intercrossed; and the
benefits which certainly are derived from this process are thus secured.
The closed and imperfect flowers are, however, manifestly of high
importance, as they yield with the utmost safety a large stock of seed,
with the expenditure of wonderfully little pollen. The two kinds of
flowers often differ much, as just stated, in structure. The petals in
the imperfect flowers almost always consist of mere rudiments, and the
pollen-grains are reduced in diameter. In Ononis columnae five of the
alternate stamens are rudimentary; and in some species of Viola three
stamens are in this state, two retaining their proper function, but
being of very small size. In six out of thirty of the closed flowers in
an Indian violet (name unknown, for the plants have never produced with
me perfect flowers), the sepals are reduced from the normal number of
five to three. In one section of the Malpighiaceae the closed flowers,
according to A. de Jussieu, are still further modified, for the five
stamens which stand opposite to the sepals are all aborted, a sixth
stamen standing opposite to a petal being alone developed; and this
stamen is not present in the ordinary flowers of this species; the style
is aborted; and the ovaria are reduced from three to two. Now although
natural selection may well have had the power to prevent some of
the flowers from expanding, and to reduce the amount of pollen, when
rendered by the closure of the flowers superfluous, yet hardly any of
the above special modifications can have been thus determined, but
must have followed from the laws of growth, including the functional
inactivity of parts, during the progress of the reduction of the pollen
and the closure of the flowers.

It is so necessary to appreciate the important effects of the laws of
growth, that I will give some additional cases of another kind, namely
of differences in the same part or organ, due to differences in relative
position on the same plant. In the Spanish chestnut, and in certain
fir-trees, the angles of divergence of the leaves differ, according to
Schacht, in the nearly horizontal and in the upright branches. In the
common rue and some other plants, one flower, usually the central or
terminal one, opens first, and has five sepals and petals, and five
divisions to the ovarium; while all the other flowers on the plant are
tetramerous. In the British Adoxa the uppermost flower generally has
two calyx-lobes with the other organs tetramerous, while the surrounding
flowers generally have three calyx-lobes with the other organs
pentamerous. In many Compositae and Umbelliferae (and in some other
plants) the circumferential flowers have their corollas much more
developed than those of the centre; and this seems often connected with
the abortion of the reproductive organs. It is a more curious fact,
previously referred to, that the achenes or seeds of the circumference
and centre sometimes differ greatly in form, colour and other
characters. In Carthamus and some other Compositae the central achenes
alone are furnished with a pappus; and in Hyoseris the same head yields
achenes of three different forms. In certain Umbelliferae the exterior
seeds, according to Tausch, are orthospermous, and the central one
coelospermous, and this is a character which was considered by De
Candolle to be in other species of the highest systematic importance.
Professor Braun mentions a Fumariaceous genus, in which the flowers in
the lower part of the spike bear oval, ribbed, one-seeded nutlets; and
in the upper part of the spike, lanceolate, two-valved and two-seeded
siliques. 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 inter-action
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.

In numerous other cases we find modifications of structure, which are
considered by botanists to be generally of a highly important nature,
affecting only some of the flowers on the same plant, or occurring on
distinct plants, which grow close together under the same conditions. As
these variations seem of no special use to the plants, they cannot
have been influenced by natural selection. Of their cause we are quite
ignorant; we cannot even attribute them, as in the last class of cases,
to any proximate agency, such as relative position. I will give only
a few instances. It is so common to observe on the same plant, flowers
indifferently tetramerous, pentamerous, etc., that I need not give
examples; but as numerical variations are comparatively rare when the
parts are few, I may mention that, according to De Candolle, the flowers
of Papaver bracteatum offer either two sepals with four petals (which
is the common type with poppies), or three sepals with six petals. The
manner in which the petals are folded in the bud is in most groups a
very constant morphological character; but Professor Asa Gray states
that with some species of Mimulus, the aestivation is almost as
frequently that of the Rhinanthideae as of the Antirrhinideae, to which
latter tribe the genus belongs. Aug. St. Hilaire gives the following
cases: the genus Zanthoxylon belongs to a division of the Rutaceae with
a single ovary, but in some species flowers may be found on the same
plant, and even in the same panicle, with either one or two ovaries.
In Helianthemum the capsule has been described as unilocular or
tri-locular; and in H. mutabile, "Une lame PLUS OU MOINS LARGE,
s'etend entre le pericarpe et le placenta." In the flowers of Saponaria
officinalis Dr. Masters has observed instances of both marginal and free
central placentation. Lastly, St. Hilaire found towards the southern
extreme of the range of Gomphia oleaeformis two forms which he did
not at first doubt were distinct species, but he subsequently saw them
growing on the same bush; and he then adds, "Voila donc dans un meme
individu des loges et un style qui se rattachent tantot a un axe
verticale et tantot a un gynobase."

We thus see that with plants many morphological changes may be
attributed to the laws of growth and the inter-action of parts,
independently of natural selection. But with respect to Nageli's
doctrine of an innate tendency towards perfection or progressive
development, can it be said in the case of these strongly pronounced
variations, that the plants have been caught in the act of progressing
towards a higher state of development? On the contrary, I should infer
from the mere fact of the parts in question differing or varying greatly
on the same plant, that such modifications were of extremely small
importance to the plants themselves, of whatever importance they may
generally be to us for our classifications. The acquisition of a useless
part can hardly be said to raise an organism in the natural scale; and
in the case of the imperfect, closed flowers, above described, if any
new principle has to be invoked, it must be one of retrogression rather
than of progression; and so it must be with many parasitic and degraded
animals. We are ignorant of the exciting cause of the above specified
modifications; but if the unknown cause were to act almost uniformly for
a length of time, we may infer that the result would be almost uniform;
and in this case all the individuals of the species would be modified in
the same manner.

From the fact of the above characters being unimportant for the welfare
of the species, any slight variations which occurred in them would
not have been accumulated and augmented through natural selection. A
structure which has been developed through long-continued selection,
when it ceases to be of service to a species, generally becomes
variable, as we see with rudimentary organs; for it will no longer be
regulated by this same power of selection. 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.

Although we have no good evidence of the existence in organic beings of
an innate tendency towards progressive development, yet this necessarily
follows, as I have attempted to show in the fourth chapter, through the
continued action of natural selection. For the best definition which
has ever been given of a high standard of organisation, is the degree
to which the parts have been specialised or differentiated; and natural
selection tends towards this end, inasmuch as the parts are thus enabled
to perform their functions more efficiently.

A distinguished zoologist, Mr. St. George Mivart, has recently collected
all the objections which have ever been advanced by myself and others
against the theory of natural selection, as propounded by Mr. Wallace
and myself, and has illustrated them with admirable art and force. When
thus marshalled, they make a formidable array; and as it forms no
part of Mr. Mivart's plan to give the various facts and considerations
opposed to his conclusions, no slight effort of reason and memory is
left to the reader, who may wish to weigh the evidence on both sides.
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, independently of
natural selection, whereas in the work just referred to I have collected
a greater number of well-established cases than can be found in any
other work known to me. My judgment may not be trustworthy, but after
reading with care Mr. Mivart's book, and comparing each section with
what I have said on the same head, I never before felt so strongly
convinced of the general truth of the conclusions here arrived at,
subject, of course, in so intricate a subject, to much partial error.

All Mr. Mivart's objections will be, or have been, considered in the
present volume. The one new point which appears to have struck many
readers is, "That natural selection is incompetent to account for the
incipient stages of useful structures." This subject is intimately
connected with that of the gradation of the characters, often
accompanied by a change of function, for instance, the conversion of a
swim-bladder into lungs, points which were discussed in the last chapter
under two headings. Nevertheless, I will here consider in some detail
several of the cases advanced by Mr. Mivart, selecting those which are
the most illustrative, as want of space prevents me from considering
all.

The giraffe, by its lofty stature, much elongated neck, fore legs, head
and tongue, has its whole frame beautifully adapted for browsing on the
higher branches of trees. It can thus obtain food beyond the reach of
the other Ungulata or hoofed animals inhabiting the same country; and
this must be a great advantage to it during dearths. The Niata cattle
in South America show us how small a difference in structure may make,
during such periods, a great difference in preserving an animal's
life. These cattle can browse as well as others on grass, but from the
projection of the lower jaw they cannot, during the often recurrent
droughts, browse on the twigs of trees, reeds, etc., to which food the
common cattle and horses are then driven; so that at these times the
Niatas perish, if not fed by their owners. Before coming to Mr. Mivart's
objections, it may be well to explain once again how natural selection
will act in all ordinary cases. Man has modified some of his animals,
without necessarily having attended to special points of structure, by
simply preserving and breeding from the fleetest individuals, as with
the race-horse and greyhound, or as with the game-cock, by breeding
from the victorious birds. So under nature with the nascent giraffe, the
individuals which were the highest browsers and were able during dearths
to reach even an inch or two above the others, will often have been
preserved; for they will have roamed over the whole country in search of
food. That the individuals of the same species often differ slightly
in the relative lengths of all their parts may be seen in many works of
natural history, in which careful measurements are given. These slight
proportional differences, due to the laws of growth and variation, are
not of the slightest use or importance to most species. But it will have
been otherwise with the nascent giraffe, considering its probable habits
of life; for those individuals which had some one part or several parts
of their bodies rather more elongated than usual, would generally
have survived. These will have intercrossed and left offspring, either
inheriting the same bodily peculiarities, or with a tendency to vary
again in the same manner; while the individuals less favoured in the
same respects will have been the most liable to perish.

We here see that there is no need to separate single pairs, as man does,
when he methodically improves a breed: natural selection will preserve
and thus separate all the superior individuals, allowing them freely
to intercross, and will destroy all the inferior individuals. By this
process long-continued, which exactly corresponds with what I have
called unconscious selection by man, combined, no doubt, in a most
important manner with the inherited effects of the increased use of
parts, it seems to me almost certain that an ordinary hoofed quadruped
might be converted into a giraffe.

To this conclusion Mr. Mivart brings forward two objections. One is
that the increased size of the body would obviously require an increased
supply of food, and he considers it as "very problematical whether the
disadvantages thence arising would not, in times of scarcity, more than
counterbalance the advantages." But as the giraffe does actually exist
in large numbers in Africa, and as some of the largest antelopes in the
world, taller than an ox, abound there, why should we doubt that, as
far as size is concerned, intermediate gradations could formerly have
existed there, subjected as now to severe dearths. Assuredly the being
able to reach, at each stage of increased size, to a supply of food,
left untouched by the other hoofed quadrupeds of the country, would have
been of some advantage to the nascent giraffe. Nor must we overlook the
fact, that increased bulk would act as a protection against almost all
beasts of prey excepting the lion; and against this animal, its tall
neck--and the taller the better--would, as Mr. Chauncey Wright has
remarked, serve as a watch-tower. It is from this cause, as Sir S. Baker
remarks, that no animal is more difficult to stalk than the giraffe.
This animal also uses its long neck as a means of offence or defence,
by violently swinging its head armed with stump-like horns. The
preservation of each species can rarely be determined by any one
advantage, but by the union of all, great and small.

Mr. Mivart then asks (and this is his second objection), if natural
selection be so potent, and if high browsing be so great an advantage,
why has not any other hoofed quadruped acquired a long neck and lofty
stature, besides the giraffe, and, in a lesser degree, the camel,
guanaco and macrauchenia? Or, again, why has not any member of the
group acquired a long proboscis? With respect to South Africa, which was
formerly inhabited by numerous herds of the giraffe, the answer is not
difficult, and can best be given by an illustration. In every meadow
in England, in which trees grow, we see the lower branches trimmed or
planed to an exact level by the browsing of the horses or cattle; and
what advantage would it be, for instance, to sheep, if kept there, to
acquire slightly longer necks? In every district some one kind of animal
will almost certainly be able to browse higher than the others; and it
is almost equally certain that this one kind alone could have its neck
elongated for this purpose, through natural selection and the effects
of increased use. In South Africa the competition for browsing on the
higher branches of the acacias and other trees must be between giraffe
and giraffe, and not with the other ungulate animals.

Why, in other quarters of the world, various animals belonging to this
same order have not acquired either an elongated neck or a proboscis,
cannot be distinctly answered; but it is as unreasonable to expect a
distinct answer to such a question as why some event in the history of
mankind did not occur in one country while it did in another. We are
ignorant with respect to the conditions which determine the numbers and
range of each species, and we cannot even conjecture what changes of
structure would be favourable to its increase in some new country. We
can, however, see in a general manner that various causes might have
interfered with the development of a long neck or proboscis. To reach
the foliage at a considerable height (without climbing, for which hoofed
animals are singularly ill-constructed) implies greatly increased
bulk of body; and we know that some areas support singularly few large
quadrupeds, for instance South America, though it is so luxuriant,
while South Africa abounds with them to an unparalleled degree. Why this
should be so we do not know; nor why the later tertiary periods should
have been much more favourable for their existence than the present
time. Whatever the causes may have been, we can see that certain
districts and times would have been much more favourable than others for
the development of so large a quadruped as the giraffe.

In order that an animal should acquire some structure specially and
largely developed, it is almost indispensable that several other parts
should be modified and coadapted. Although every part of the body varies
slightly, it does not follow that the necessary parts should always
vary in the right direction and to the right degree. With the different
species of our domesticated animals we know that the parts vary in
a different manner and degree, and that some species are much more
variable than others. Even if the fitting variations did arise, it
does not follow that natural selection would be able to act on them and
produce a structure which apparently would be beneficial to the species.
For instance, if the number of individuals existing in a country is
determined chiefly through destruction by beasts of prey--by external
or internal parasites, etc.--as seems often to be the case, then natural
selection will be able to do little, or will be greatly retarded, in
modifying any particular structure for obtaining food. Lastly, natural
selection is a slow process, and the same favourable conditions must
long endure in order that any marked effect should thus be produced.
Except by assigning such general and vague reasons, we cannot explain
why, in many quarters of the world, hoofed quadrupeds have not acquired
much elongated necks or other means for browsing on the higher branches
of trees.

Objections of the same nature as the foregoing have been advanced by
many writers. In each case various causes, besides the general ones just
indicated, have probably interfered with the acquisition through natural
selection of structures, which it is thought would be beneficial to
certain species. One writer asks, why has not the ostrich acquired the
power of flight? But a moment's reflection will show what an enormous
supply of food would be necessary to give to this bird of the desert
force to move its huge body through the air. Oceanic islands are
inhabited by bats and seals, but by no terrestrial mammals; yet as some
of these bats are peculiar species, they must have long inhabited their
present homes. Therefore Sir C. Lyell asks, and assigns certain reasons
in answer, why have not seals and bats given birth on such islands to
forms fitted to live on the land? But seals would necessarily be first
converted into terrestrial carnivorous animals of considerable size, and
bats into terrestrial insectivorous animals; for the former there would
be no prey; for the bats ground-insects would serve as food, but these
would already be largely preyed on by the reptiles or birds, which first
colonise and abound on most oceanic islands. Gradations of structure,
with each stage beneficial to a changing species, will be favoured only
under certain peculiar conditions. A strictly terrestrial animal, by
occasionally hunting for food in shallow water, then in streams or
lakes, might at last be converted into an animal so thoroughly aquatic
as to brave the open ocean. But seals would not find on oceanic
islands the conditions favourable to their gradual reconversion into a
terrestrial form. Bats, as formerly shown, probably acquired their
wings by at first gliding through the air from tree to tree, like the
so-called flying squirrels, for the sake of escaping from their enemies,
or for avoiding falls; but when the power of true flight had once been
acquired, it would never be reconverted back, at least for the above
purposes, into the less efficient power of gliding through the air.
Bats, might, indeed, like many birds, have had their wings greatly
reduced in size, or completely lost, through disuse; but in this case
it would be necessary that they should first have acquired the power of
running quickly on the ground, by the aid of their hind legs alone, so
as to compete with birds or other ground animals; and for such a change
a bat seems singularly ill-fitted. These conjectural remarks have been
made merely to show that a transition of structure, with each step
beneficial, is a highly complex affair; and that there is nothing
strange in a transition not having occurred in any particular case.

Lastly, more than one writer has asked why have some animals had their
mental powers more highly developed than others, as such development
would be advantageous to all? Why have not apes acquired the
intellectual powers of man? Various causes could be assigned; but as
they are conjectural, and their relative probability cannot be weighed,
it would be useless to give them. A definite answer to the latter
question ought not to be expected, seeing that no one can solve the
simpler problem, why, of two races of savages, one has risen higher in
the scale of civilisation than the other; and this apparently implies
increased brain power.

We will return to Mr. Mivart's other objections. Insects often resemble
for the sake of protection various objects, such as green or decayed
leaves, dead twigs, bits of lichen, flowers, spines, excrement of birds,
and living insects; but to this latter point I shall hereafter recur.
The resemblance is often wonderfully close, and is not confined to
colour, but extends to form, and even to the manner in which the insects
hold themselves. The caterpillars which project motionless like dead
twigs from the bushes on which they feed, offer an excellent instance of
a resemblance of this kind. The cases of the imitation of such objects
as the excrement of birds, are rare and exceptional. On this head,
Mr. Mivart remarks, "As, according to Mr. Darwin's theory, there is a
constant tendency to indefinite variation, and as the minute incipient
variations will be in ALL DIRECTIONS, they must tend to neutralize
each other, and at first to form such unstable modifications that it is
difficult, if not impossible, to see how such indefinite oscillations
of infinitesimal beginnings can ever build up a sufficiently appreciable
resemblance to a leaf, bamboo, or other object, for natural selection to
seize upon and perpetuate."

But in all the foregoing cases the insects in their original state
no doubt presented some rude and accidental resemblance to an object
commonly found in the stations frequented by them. Nor is this at
all improbable, considering the almost infinite number of surrounding
objects and the diversity in form and colour of the hosts of insects
which exist. As some rude resemblance is necessary for the first start,
we can understand how it is that the larger and higher animals do not
(with the exception, as far as I know, of one fish) resemble for the
sake of protection special objects, but only the surface which commonly
surrounds them, and this chiefly in colour. Assuming that an insect
originally happened to resemble in some degree a dead twig or a decayed
leaf, and that it varied slightly in many ways, then all the variations
which rendered the insect at all more like any such object, and thus
favoured its escape, would be preserved, while other variations would
be neglected and ultimately lost; or, if they rendered the insect at
all less like the imitated object, they would be eliminated. There would
indeed be force in Mr. Mivart's objection, if we were to attempt to
account for the above resemblances, independently of natural selection,
through mere fluctuating variability; but as the case stands there is
none.

Nor can I see any force in Mr. Mivart's difficulty with respect to "the
last touches of perfection in the mimicry;" as in the case given by
Mr. Wallace, of a walking-stick insect (Ceroxylus laceratus), which
resembles "a stick grown over by a creeping moss or jungermannia."
So close was this resemblance, that a native Dyak maintained that the
foliaceous excrescences were really moss. Insects are preyed on by birds
and other enemies whose sight is probably sharper than ours, and
every grade in resemblance which aided an insect to escape notice or
detection, would tend towards its preservation; and the more perfect the
resemblance so much the better for the insect. Considering the nature
of the differences between the species in the group which includes
the above Ceroxylus, there is nothing improbable in this insect having
varied in the irregularities on its surface, and in these having become
more or less green-coloured; for in every group the characters which
differ in the several species are the most apt to vary, while the
generic characters, or those common to all the species, are the most
constant.

The Greenland whale is one of the most wonderful animals in the world,
and the baleen, or whalebone, one of its greatest peculiarities. The
baleen consists of a row, on each side of the upper jaw, of about 300
plates or laminae, which stand close together transversely to the longer
axis of the mouth. Within the main row there are some subsidiary rows.
The extremities and inner margins of all the plates are frayed into
stiff bristles, which clothe the whole gigantic palate, and serve to
strain or sift the water, and thus to secure the minute prey on which
these great animals subsist. The middle and longest lamina in the
Greenland whale is ten, twelve, or even fifteen feet in length; but in
the different species of Cetaceans there are gradations in length; the
middle lamina being in one species, according to Scoresby, four feet,
in another three, in another eighteen inches, and in the Balaenoptera
rostrata only about nine inches in length. The quality of the whalebone
also differs in the different species.

With respect to the baleen, Mr. Mivart remarks that if it "had once
attained such a size and development as to be at all useful, then
its preservation and augmentation within serviceable limits would be
promoted by natural selection alone. But how to obtain the beginning of
such useful development?" In answer, it may be asked, why should not
the early progenitors of the whales with baleen have possessed a mouth
constructed something like the lamellated beak of a duck? Ducks,
like whales, subsist by sifting the mud and water; and the family has
sometimes been called Criblatores, or sifters. I hope that I may not
be misconstrued into saying that the progenitors of whales did actually
possess mouths lamellated like the beak of a duck. I wish only to show
that this is not incredible, and that the immense plates of baleen in
the Greenland whale might have been developed from such lamellae by
finely graduated steps, each of service to its possessor.

The beak of a shoveller-duck (Spatula clypeata) is a more beautiful
and complex structure than the mouth of a whale. The upper mandible is
furnished on each side (in the specimen examined by me) with a row or
comb formed of 188 thin, elastic lamellae, obliquely bevelled so as to
be pointed, and placed transversely to the longer axis of the mouth.
They arise from the palate, and are attached by flexible membrane to
the sides of the mandible. Those standing towards the middle are the
longest, being about one-third of an inch in length, and they project
fourteen one-hundredths of an inch beneath the edge. At their bases
there is a short subsidiary row of obliquely transverse lamellae. In
these several respects they resemble the plates of baleen in the mouth
of a whale. But towards the extremity of the beak they differ much, as
they project inward, instead of straight downward. The entire head of
the shoveller, though incomparably less bulky, is about one-eighteenth
of the length of the head of a moderately large Balaenoptera rostrata,
in which species the baleen is only nine inches long; so that if we were
to make the head of the shoveller as long as that of the Balaenoptera,
the lamellae would be six inches in length, that is, two-thirds of the
length of the baleen in this species of whale. The lower mandible of
the shoveller-duck is furnished with lamellae of equal length with these
above, but finer; and in being thus furnished it differs conspicuously
from the lower jaw of a whale, which is destitute of baleen. On the
other hand, the extremities of these lower lamellae are frayed into
fine bristly points, so that they thus curiously resemble the plates
of baleen. In the genus Prion, a member of the distinct family of the
Petrels, the upper mandible alone is furnished with lamellae, which are
well developed and project beneath the margin; so that the beak of this
bird resembles in this respect the mouth of a whale.

From the highly developed structure of the shoveller's beak we may
proceed (as I have learned from information and specimens sent to me by
Mr. Salvin), without any great break, as far as fitness for sifting
is concerned, through the beak of the Merganetta armata, and in some
respects through that of the Aix sponsa, to the beak of the common
duck. In this latter species the lamellae are much coarser than in the
shoveller, and are firmly attached to the sides of the mandible; they
are only about fifty in number on each side, and do not project at
all beneath the margin. They are square-topped, and are edged with
translucent, hardish tissue, as if for crushing food. The edges of the
lower mandible are crossed by numerous fine ridges, which project very
little. Although the beak is thus very inferior as a sifter to that of
a shoveller, yet this bird, as every one knows, constantly uses it for
this purpose. There are other species, as I hear from Mr. Salvin, in
which the lamellae are considerably less developed than in the common
duck; but I do not know whether they use their beaks for sifting the
water.

Turning to another group of the same family. In the Egyptian goose
(Chenalopex) the beak closely resembles that of the common duck; but the
lamellae are not so numerous, nor so distinct from each other, nor do
they project so much inward; yet this goose, as I am informed by Mr. E.
Bartlett, "uses its bill like a duck by throwing the water out at the
corners." Its chief food, however, is grass, which it crops like the
common goose. In this latter bird the lamellae of the upper mandible
are much coarser than in the common duck, almost confluent, about
twenty-seven in number on each side, and terminating upward in
teeth-like knobs. The palate is also covered with hard rounded knobs.
The edges of the lower mandible are serrated with teeth much more
prominent, coarser and sharper than in the duck. The common goose does
not sift the water, but uses its beak exclusively for tearing or cutting
herbage, for which purpose it is so well fitted that it can crop grass
closer than almost any other animal. There are other species of geese,
as I hear from Mr. Bartlett, in which the lamellae are less developed
than in the common goose.

We thus see that a member of the duck family, with a beak constructed
like that of a common goose and adapted solely for grazing, or even
a member with a beak having less well-developed lamellae, might be
converted by small changes into a species like the Egyptian goose--this
into one like the common duck--and, lastly, into one like the shoveller,
provided with a beak almost exclusively adapted for sifting the water;
for this bird could hardly use any part of its beak, except the hooked
tip, for seizing or tearing solid food. The beak of a goose, as I may
add, might also be converted by small changes into one provided with
prominent, recurved teeth, like those of the Merganser (a member of the
same family), serving for the widely different purpose of securing live
fish.

Returning to the whales. The Hyperoodon bidens is destitute of true
teeth in an efficient condition, but its palate is roughened, according
to Lacepede, with small unequal, hard points of horn. There is,
therefore, nothing improbable in supposing that some early Cetacean form
was provided with similar points of horn on the palate, but rather more
regularly placed, and which, like the knobs on the beak of the goose,
aided it in seizing or tearing its food. If so, it will hardly be denied
that the points might have been converted through variation and natural
selection into lamellae as well-developed as those of the Egyptian
goose, in which case they would have been used both for seizing objects
and for sifting the water; then into lamellae like those of the domestic
duck; and so onward, until they became as well constructed as those of
the shoveller, in which case they would have served exclusively as
a sifting apparatus. From this stage, in which the lamellae would be
two-thirds of the length of the plates of baleen in the Balaenoptera
rostrata, gradations, which may be observed in still-existing Cetaceans,
lead us onward to the enormous plates of baleen in the Greenland whale.
Nor is there the least reason to doubt that each step in this scale
might have been as serviceable to certain ancient Cetaceans, with
the functions of the parts slowly changing during the progress of
development, as are the gradations in the beaks of the different
existing members of the duck-family. We should bear in mind that each
species of duck is subjected to a severe struggle for existence, and
that the structure of every part of its frame must be well adapted to
its conditions of life.

The Pleuronectidae, or Flat-fish, are remarkable for their asymmetrical
bodies. They rest on one side--in the greater number of species on the
left, but in some on the right side; and occasionally reversed adult
specimens occur. The lower, or resting-surface, resembles at first sight
the ventral surface of an ordinary fish; it is of a white colour, less
developed in many ways than the upper side, with the lateral fins often
of smaller size. But the eyes offer the most remarkable peculiarity; for
they are both placed on the upper side of the head. During early youth,
however, they stand opposite to each other, and the whole body is then
symmetrical, with both sides equally coloured. Soon the eye proper to
the lower side begins to glide slowly round the head to the upper side;
but does not pass right through the skull, as was formerly thought to be
the case. It is obvious that unless the lower eye did thus travel round,
it could not be used by the fish while lying in its habitual position on
one side. The lower eye would, also, have been liable to be abraded by
the sandy bottom. That the Pleuronectidae are admirably adapted by
their flattened and asymmetrical structure for their habits of life,
is manifest from several species, such as soles, flounders, etc., being
extremely common. The chief advantages thus gained seem to be protection
from their enemies, and facility for feeding on the ground. The
different members, however, of the family present, as Schiodte
remarks, "a long series of forms exhibiting a gradual transition from
Hippoglossus pinguis, which does not in any considerable degree alter
the shape in which it leaves the ovum, to the soles, which are entirely
thrown to one side."

Mr. Mivart has taken up this case, and remarks that a sudden spontaneous
transformation in the position of the eyes is hardly conceivable, in
which I quite agree with him. He then 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." But he might have found
an answer to this objection in the excellent observations published
in 1867 by Malm. The Pleuronectidae, while very young and still
symmetrical, with their eyes standing on opposite sides of the head,
cannot long retain a vertical position, owing to the excessive depth of
their bodies, the small size of their lateral fins, and to their being
destitute of a swim-bladder. Hence, soon growing tired, they fall to
the bottom on one side. While thus at rest they often twist, as Malm
observed, the lower eye upward, to see above them; and they do this so
vigorously that the eye is pressed hard against the upper part of the
orbit. The forehead between the eyes consequently becomes, as could be
plainly seen, temporarily contracted in breadth. On one occasion Malm
saw a young fish raise and depress the lower eye through an angular
distance of about seventy degrees.

We should remember that the skull at this early age is cartilaginous and
flexible, so that it readily yields to muscular action. It is also known
with the higher animals, even after early youth, that the skull
yields and is altered in shape, if the skin or muscles be permanently
contracted through disease or some accident. With long-eared rabbits,
if one ear flops forward and downward, its weight drags forward all the
bones of the skull on the same side, of which I have given a figure.
Malm states that the newly-hatched young of perches, salmon, and several
other symmetrical fishes, have the habit of occasionally resting on
one side at the bottom; and he has observed that they often then
strain their lower eyes so as to look upward; and their skulls are thus
rendered rather crooked. These fishes, however, are soon able to hold
themselves in a vertical position, and no permanent effect is thus
produced. With the Pleuronectidae, on the other hand, the older they
grow the more habitually they rest on one side, owing to the increasing
flatness of their bodies, and a permanent effect is thus produced on the
form of the head, and on the position of the eyes. Judging from analogy,
the tendency to distortion would no doubt be increased through the
principle of inheritance. Schiodte believes, in opposition to some other
naturalists, that the Pleuronectidae are not quite symmetrical even
in the embryo; and if this be so, we could understand how it is that
certain species, while young, habitually fall over and rest on the left
side, and other species on the right side. Malm adds, in confirmation
of the above view, that the adult Trachypterus arcticus, which is not a
member of the Pleuronectidae, rests on its left side at the bottom, and
swims diagonally through the water; and in this fish, the two sides
of the head are said to be somewhat dissimilar. Our great authority
on Fishes, Dr. Gunther, concludes his abstract of Malm's paper, by
remarking that "the author gives a very simple explanation of the
abnormal condition of the Pleuronectoids."

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 endeavouring to look upward with both
eyes, while 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 flat-fish 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, 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 Yarrel 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 colourless state of
the ventral surface of most fishes and of many other animals, we may
reasonably suppose that the absence of colour in flat-fish on the side,
whether it be the right or left, which is under-most, is due to the
exclusion of light. But it cannot be supposed that the peculiar speckled
appearance of the upper side of the sole, so like the sandy bed of the
sea, or the power in some species, as recently shown by Pouchet, of
changing their colour in accordance with the surrounding surface, or the
presence of bony tubercles on the upper side of the turbot, are due to
the action of the light. Here natural selection has probably come into
play, as well as in adapting the general shape of the body of these
fishes, and many other peculiarities, to their habits of life. We should
keep in mind, as I have before insisted, that the inherited effects
of the increased use of parts, and perhaps of their disuse, will be
strengthened by natural selection. For all spontaneous variations in the
right direction will thus be preserved; as will those individuals
which inherit in the highest degree the effects of the increased and
beneficial use of any part. How much to attribute in each particular
case to the effects of use, and how much to natural selection, it seems
impossible to decide.

I may give another instance of a structure which apparently owes its
origin exclusively to use or habit. The extremity of the tail in
some American monkeys has been converted into a wonderfully perfect
prehensile organ, and serves as a fifth hand. A reviewer, who agrees
with Mr. Mivart in every detail, remarks on this structure: "It is
impossible to believe that in any number of ages the first slight
incipient tendency to grasp could preserve the lives of the individuals
possessing it, or favour their chance of having and of rearing
offspring." But there is no necessity for any such belief. Habit, and
this almost implies that some benefit great or small is thus derived,
would in all probability suffice for the work. Brehm saw the young of
an African monkey (Cercopithecus) clinging to the under surface of their
mother by their hands, and at the same time they hooked their little
tails round that of their mother. Professor Henslow kept in confinement
some harvest mice (Mus messorius) which do not possess a structurally
prehensive tail; but he frequently observed that they curled their
tails round the branches of a bush placed in the cage, and thus aided
themselves in climbing. I have received an analogous account from Dr.
Gunther, who has seen a mouse thus suspend itself. If the harvest mouse
had been more strictly arboreal, it would perhaps have had its tail
rendered structurally prehensile, as is the case with some members of
the same order. Why Cercopithecus, considering its habits while young,
has not become thus provided, it would be difficult to say. It is,
however, possible that the long tail of this monkey may be of more
service to it as a balancing organ in making its prodigious leaps, than
as a prehensile organ.

The mammary glands are common to the whole class of mammals, and are
indispensable for their existence; they must, therefore, have been
developed at an extremely remote period, and we can know nothing
positively about their manner of development. Mr. Mivart asks: "Is it
conceivable that the young of any animal was ever saved from destruction
by accidentally sucking a drop of scarcely nutritious fluid from an
accidentally hypertrophied cutaneous gland of its mother? And even
if one was so, what chance was there of the perpetuation of such a
variation?" But the case is not here put fairly. It is admitted by most
evolutionists that mammals are descended from a marsupial form; and
if so, the mammary glands will have been at first developed within
the marsupial sack. In the case of the fish (Hippocampus) the eggs are
hatched, and the young are reared for a time, within a sack of this
nature; and an American naturalist, Mr. Lockwood, believes from what he
has seen of the development of the young, that they are nourished by
a secretion from the cutaneous glands of the sack. Now, with the
early progenitors of mammals, almost before they deserved to be thus
designated, is it not at least possible that the young might have been
similarly nourished? And in this case, the individuals which secreted a
fluid, in some degree or manner the most nutritious, so as to partake of
the nature of milk, would in the long run have reared a larger number
of well-nourished offspring, than would the individuals which secreted a
poorer fluid; and thus the cutaneous glands, which are the homologues of
the mammary glands, would have been improved or rendered more effective.
It accords with the widely extended principle of specialisation, that
the glands over a certain space of the sack should have become more
highly developed than the remainder; and they would then have formed a
breast, but at first without a nipple, as we see in the Ornithorhyncus,
at the base of the mammalian series. Through what agency the glands over
a certain space became more highly specialised than the others, I will
not pretend to decide, whether in part through compensation of growth,
the effects of use, or of natural selection.

The development of the mammary glands would have been of no service, and
could not have been affected through natural selection, unless the young
at the same time were able to partake of the secretion. There is no
greater difficulty in understanding how young mammals have instinctively
learned to suck the breast, than in understanding how unhatched chickens
have learned to break the egg-shell by tapping against it with their
specially adapted beaks; or how a few hours after leaving the shell they
have learned to pick up grains of food. In such cases the most probable
solution seems to be, that the habit was at first acquired by practice
at a more advanced age, and afterwards transmitted to the offspring
at an earlier age. But the young kangaroo is said not to suck, only to
cling to the nipple of its mother, who has the power of injecting milk
into the mouth of her helpless, half-formed offspring. On this head
Mr. Mivart remarks: "Did no special provision exist, the young one must
infallibly be choked by the intrusion of the milk into the wind-pipe.
But there IS a special provision. The larynx is so elongated that
it rises up into the posterior end of the nasal passage, and is thus
enabled to give free entrance to the air for the lungs, while the milk
passes harmlessly on each side of this elongated larynx, and so safely
attains the gullet behind it." Mr. Mivart then asks how did natural
selection remove in the adult kangaroo (and in most other mammals, on
the assumption that they are descended from a marsupial form), "this at
least perfectly innocent and harmless structure?" It may be suggested
in answer that the voice, which is certainly of high importance to many
animals, could hardly have been used with full force as long as the
larynx entered the nasal passage; and Professor Flower has suggested
to me that this structure would have greatly interfered with an animal
swallowing solid food.

We will now turn for a short space to the lower divisions of the
animal kingdom. The Echinodermata (star-fishes, sea-urchins, etc.) are
furnished with remarkable organs, called pedicellariae, which consist,
when well developed, of a tridactyle forceps--that is, of one formed of
three serrated arms, neatly fitting together and placed on the summit of
a flexible stem, moved by muscles. These forceps can seize firmly hold
of any object; and Alexander Agassiz has seen an Echinus or sea-urchin
rapidly passing particles of excrement from forceps to forceps down
certain lines of its body, in order that its shell should not be fouled.
But there is no doubt that besides removing dirt of all kinds, they
subserve other functions; and one of these apparently is defence.

With respect to these organs, Mr. Mivart, as on so many previous
occasions, asks: "What would be the utility of the FIRST RUDIMENTARY
BEGINNINGS of such structures, and how could such insipient buddings
have ever preserved the life of a single Echinus?" He adds, "not even
the SUDDEN development of the snapping action would have been beneficial
without the freely movable stalk, nor could the latter have been
efficient without the snapping jaws, yet no minute, nearly indefinite
variations could simultaneously evolve these complex co-ordinations of
structure; to deny this seems to do no less than to affirm a startling
paradox." Paradoxical as this may appear to Mr. Mivart, tridactyle
forcepses, immovably fixed at the base, but capable of a snapping
action, certainly exist on some star-fishes; and this is intelligible
if they serve, at least in part, as a means of defence. Mr. Agassiz, to
whose great kindness I am indebted for much information on the subject,
informs me that there are other star-fishes, in which one of the three
arms of the forceps is reduced to a support for the other two; and
again, other genera in which the third arm is completely lost. In
Echinoneus, the shell is described by M. Perrier as bearing two kinds of
pedicellariae, one resembling those of Echinus, and the other those of
Spatangus; and such cases are always interesting as affording the means
of apparently sudden transitions, through the abortion of one of the two
states of an organ.

With respect to the steps by which these curious organs have been
evolved, Mr. Agassiz infers from his own researches and those of Mr.
Muller, that both in star-fishes and sea-urchins the pedicellariae must
undoubtedly be looked at as modified spines. This may be inferred from
their manner of development in the individual, as well as from a long
and perfect series of gradations in different species and genera, from
simple granules to ordinary spines, to perfect tridactyle pedicellariae.
The gradation extends even to the manner in which ordinary spines
and the pedicellariae, with their supporting calcareous rods, are
articulated to the shell. In certain genera of star-fishes, "the very
combinations needed to show that the pedicellariae are only modified
branching spines" may be found. Thus we have fixed spines, with three
equi-distant, serrated, movable branches, articulated to near their
bases; and higher up, on the same spine, three other movable branches.
Now when the latter arise from the summit of a spine they form, in fact,
a rude tridactyle pedicellariae, and such may be seen on the same spine
together with the three lower branches. In this case the identity in
nature between the arms of the pedicellariae and the movable branches
of a spine, is unmistakable. It is generally admitted that the ordinary
spines serve as a protection; and if so, there can be no reason to doubt
that those furnished with serrated and movable branches likewise serve
for the same purpose; and they would thus serve still more effectively
as soon as by meeting together they acted as a prehensile or snapping
apparatus. Thus every gradation, from an ordinary fixed spine to a fixed
pedicellariae, would be of service.

In certain genera of star-fishes these organs, instead of being fixed
or borne on an immovable support, are placed on the summit of a flexible
and muscular, though short, stem; and in this case they probably
subserve some additional function besides defence. In the sea-urchins
the steps can be followed by which a fixed spine becomes articulated to
the shell, and is thus rendered movable. I wish I had space here to
give a fuller abstract of Mr. Agassiz's interesting observations on the
development of the pedicellariae. All possible gradations, as he adds,
may likewise be found between the pedicellariae of the star-fishes and
the hooks of the Ophiurians, another group of the Echinodermata; and
again between the pedicellariae of sea-urchins and the anchors of the
Holothuriae, also belonging to the same great class.

Certain compound animals, or zoophytes, as they have been termed, namely
the Polyzoa, are provided with curious organs called avicularia. These
differ much in structure in the different species. In their most perfect
condition they curiously resemble the head and beak of a vulture in
miniature, seated on a neck and capable of movement, as is likewise the
lower jaw or mandible. In one species observed by me, all the avicularia
on the same branch often moved simultaneously backwards and forwards,
with the lower jaw widely open, through an angle of about 90 degrees,
in the course of five seconds; and their movement caused the whole
polyzoary to tremble. When the jaws are touched with a needle they seize
it so firmly that the branch can thus be shaken.

Mr. Mivart adduces this case, chiefly on account of the supposed
difficulty of organs, namely the avicularia of the Polyzoa and the
pedicellariae of the Echinodermata, which he considers as "essentially
similar," having been developed through natural selection in widely
distinct divisions of the animal kingdom. But, as far as structure is
concerned, I can see no similarity between tridactyle pedicellariae and
avicularia. The latter resembles somewhat more closely the chelae or
pincers of Crustaceans; and Mr. Mivart might have adduced with equal
appropriateness this resemblance as a special difficulty, or even their
resemblance to the head and beak of a bird. The avicularia are believed
by Mr. Busk, Dr. Smitt and Dr. Nitsche--naturalists who have carefully
studied this group--to be homologous with the zooids and their
cells which compose the zoophyte, the movable lip or lid of the cell
corresponding with the lower and movable mandible of the avicularium.
Mr. Busk, however, does not know of any gradations now existing between
a zooid and an avicularium. It is therefore impossible to conjecture by
what serviceable gradations the one could have been converted into the
other, but it by no means follows from this that such gradations have
not existed.

As the chelae of Crustaceans resemble in some degree the avicularia of
Polyzoa, both serving as pincers, it may be worth while to show that
with the former a long series of serviceable gradations still exists. In
the first and simplest stage, the terminal segment of a limb shuts down
either on the square summit of the broad penultimate segment, or against
one whole side, and is thus enabled to catch hold of an object, but the
limb still serves as an organ of locomotion. We next find one corner of
the broad penultimate segment slightly prominent, sometimes furnished
with irregular teeth, and against these the terminal segment shuts down.
By an increase in the size of this projection, with its shape, as well
as that of the terminal segment, slightly modified and improved, the
pincers are rendered more and more perfect, until we have at last
an instrument as efficient as the chelae of a lobster. And all these
gradations can be actually traced.

Besides the avicularia, the polyzoa possess curious organs called
vibracula. These generally consist of long bristles, capable of movement
and easily excited. In one species examined by me the vibracula were
slightly curved and serrated along the outer margin, and all of them on
the same polyzoary often moved simultaneously; so that, acting like
long oars, they swept a branch rapidly across the object-glass of my
microscope. When a branch was placed on its face, the vibracula became
entangled, and they made violent efforts to free themselves. They are
supposed to serve as a defence, and may be seen, as Mr. Busk remarks,
"to sweep slowly and carefully over the surface of the polyzoary,
removing what might be noxious to the delicate inhabitants of the cells
when their tentacula are protruded." The avicularia, like the vibracula,
probably serve for defence, but they also catch and kill small living
animals, which, it is believed, are afterwards swept by the currents
within reach of the tentacula of the zooids. Some species are provided
with avicularia and vibracula, some with avicularia alone and a few with
vibracula alone.

It is not easy to imagine two objects more widely different in
appearance than a bristle or vibraculum, and an avicularium like the
head of a bird; yet they are almost certainly homologous and have been
developed from the same common source, namely a zooid with its cell.
Hence, we can understand how it is that these organs graduate in some
cases, as I am informed by Mr. Busk, into each other. Thus, with the
avicularia of several species of Lepralia, the movable mandible is so
much produced and is so like a bristle that the presence of the upper
or fixed beak alone serves to determine its avicularian nature. The
vibracula may have been directly developed from the lips of the cells,
without having passed through the avicularian stage; but it seems more
probable that they have passed through this stage, as during the early
stages of the transformation, the other parts of the cell, with the
included zooid, could hardly have disappeared at once. In many cases the
vibracula have a grooved support at the base, which seems to represent
the fixed beak; though this support in some species is quite absent.
This view of the development of the vibracula, if trustworthy, is
interesting; for supposing that all the species provided with avicularia
had become extinct, no one with the most vivid imagination would ever
have thought that the vibracula had originally existed as part of an
organ, resembling a bird's head, or an irregular box or hood. It is
interesting to see two such widely different organs developed from a
common origin; and as the movable lip of the cell serves as a protection
to the zooid, there is no difficulty in believing that all the
gradations, by which the lip became converted first into the lower
mandible of an avicularium, and then into an elongated bristle,
likewise served as a protection in different ways and under different
circumstances.

In the vegetable kingdom Mr. Mivart only alludes to two cases, namely
the structure of the flowers of orchids, and the movements of climbing
plants. With respect to the former, he says: "The explanation of their
ORIGIN is deemed thoroughly unsatisfactory--utterly insufficient to
explain the incipient, infinitesimal beginnings of structures which are
of utility only when they are considerably developed." As I have fully
treated this subject in another work, I will here give only a few
details on one alone of the most striking peculiarities of the flowers
of orchids, namely, their pollinia. A pollinium, when highly developed,
consists of a mass of pollen-grains, affixed to an elastic foot-stalk
or caudicle, and this to a little mass of extremely viscid matter. The
pollinia are by this means transported by insects from one flower to
the stigma of another. In some orchids there is no caudicle to the
pollen-masses, and the grains are merely tied together by fine threads;
but as these are not confined to orchids, they need not here be
considered; yet I may mention that at the base of the orchidaceous
series, in Cypripedium, we can see how the threads were probably
first developed. In other orchids the threads cohere at one end of the
pollen-masses; and this forms the first or nascent trace of a caudicle.
That this is the origin of the caudicle, even when of considerable
length and highly developed, we have good evidence in the aborted
pollen-grains which can sometimes be detected embedded within the
central and solid parts.

With respect to the second chief peculiarity, namely, the little mass
of viscid matter attached to the end of the caudicle, a long series of
gradations can be specified, each of plain service to the plant. In most
flowers belonging to other orders the stigma secretes a little viscid
matter. Now, in certain orchids similar viscid matter is secreted, but
in much larger quantities by one alone of the three stigmas; and this
stigma, perhaps in consequence of the copious secretion, is rendered
sterile. When an insect visits a flower of this kind, it rubs off some
of the viscid matter, and thus at the same time drags away some of the
pollen-grains. From this simple condition, which differs but
little from that of a multitude of common flowers, there are endless
gradations--to species in which the pollen-mass terminates in a very
short, free caudicle--to others in which the caudicle becomes firmly
attached to the viscid matter, with the sterile stigma itself much
modified. In this latter case we have a pollinium in its most highly
developed and perfect condition. He who will carefully examine the
flowers of orchids for himself will not deny the existence of the above
series of gradations--from a mass of pollen-grains merely tied together
by threads, with the stigma differing but little from that of the
ordinary flowers, to a highly complex pollinium, admirably adapted for
transportal by insects; nor will he deny that all the gradations in
the several species are admirably adapted in relation to the general
structure of each flower for its fertilisation by different insects. In
this, and in almost every other case, the enquiry may be pushed further
backwards; and it may be asked how did the stigma of an ordinary flower
become viscid, but as we do not know the full history of any one
group of beings, it is as useless to ask, as it is hopeless to attempt
answering, such questions.

We will now turn to climbing plants. These can be arranged in a long
series, from those which simply twine round a support, to those which I
have called leaf-climbers, and to those provided with tendrils. In these
two latter classes the stems have generally, but not always, lost the
power of twining, though they retain the power of revolving, which the
tendrils likewise possess. The gradations from leaf-climbers to tendril
bearers are wonderfully close, and certain plants may be differently
placed in either class. But in ascending the series from simple twiners
to leaf-climbers, an important quality is added, namely sensitiveness
to a touch, by which means the foot-stalks of the leaves or flowers, or
these modified and converted into tendrils, are excited to bend round
and clasp the touching object. He who will read my memoir on these
plants will, I think, admit that all the many gradations in function and
structure between simple twiners and tendril-bearers are in each case
beneficial in a high degree to the species. For instance, it is clearly
a great advantage to a twining plant to become a leaf-climber; and it is
probable that every twiner which possessed leaves with long foot-stalks
would have been developed into a leaf-climber, if the foot-stalks had
possessed in any slight degree the requisite sensitiveness to a touch.

As twining is the simplest means of ascending a support, and forms the
basis of our series, it may naturally be asked how did plants acquire
this power in an incipient degree, afterwards to be improved and
increased through natural selection. The power of twining depends,
firstly, on the stems while young being extremely flexible (but this
is a character common to many plants which are not climbers); and,
secondly, on their continually bending to all points of the compass, one
after the other in succession, in the same order. By this movement the
stems are inclined to all sides, and are made to move round and round.
As soon as the lower part of a stem strikes against any object and is
stopped, the upper part still goes on bending and revolving, and thus
necessarily twines round and up the support. The revolving movement
ceases after the early growth of each shoot. As in many widely separated
families of plants, single species and single genera possess the power
of revolving, and have thus become twiners, they must have independently
acquired it, and cannot have inherited it from a common progenitor.
Hence, I was led to predict that some slight tendency to a movement of
this kind would be found to be far from uncommon with plants which did
not climb; and that this had afforded the basis for natural selection
to work on and improve. When I made this prediction, I knew of only one
imperfect case, namely, of the young flower-peduncles of a Maurandia
which revolved slightly and irregularly, like the stems of twining
plants, but without making any use of this habit. Soon afterwards
Fritz Muller discovered that the young stems of an Alisma and of a
Linum--plants which do not climb and are widely separated in the natural
system--revolved plainly, though irregularly, and he states that he has
reason to suspect that this occurs with some other plants. These slight
movements appear to be of no service to the plants in question; anyhow,
they are not of the least use in the way of climbing, which is the point
that concerns us. Nevertheless we can see that if the stems of these
plants had been flexible, and if under the conditions to which they are
exposed it had profited them to ascend to a height, then the habit
of slightly and irregularly revolving might have been increased and
utilised through natural selection, until they had become converted into
well-developed twining species.

With respect to the sensitiveness of the foot-stalks of the leaves and
flowers, and of tendrils, nearly the same remarks are applicable as in
the case of the revolving movements of twining plants. As a vast number
of species, belonging to widely distinct groups, are endowed with this
kind of sensitiveness, it ought to be found in a nascent condition in
many plants which have not become climbers. This is the case: I observed
that the young flower-peduncles of the above Maurandia curved themselves
a little towards the side which was touched. Morren found in several
species of Oxalis that the leaves and their foot-stalks moved,
especially after exposure to a hot sun, when they were gently and
repeatedly touched, or when the plant was shaken. I repeated these
observations on some other species of Oxalis with the same result; in
some of them the movement was distinct, but was best seen in the young
leaves; in others it was extremely slight. It is a more important fact
that according to the high authority of Hofmeister, the young shoots and
leaves of all plants move after being shaken; and with climbing plants
it is, as we know, only during the early stages of growth that the
foot-stalks and tendrils are sensitive.

It is scarcely possible that the above slight movements, due to a touch
or shake, in the young and growing organs of plants, can be of any
functional importance to them. But plants possess, in obedience to
various stimuli, powers of movement, which are of manifest importance
to them; for instance, towards and more rarely from the light--in
opposition to, and more rarely in the direction of, the attraction
of gravity. When the nerves and muscles of an animal are excited by
galvanism or by the absorption of strychnine, the consequent movements
may be called an incidental result, for the nerves and muscles have not
been rendered specially sensitive to these stimuli. So with plants it
appears that, from having the power of movement in obedience to certain
stimuli, they are excited in an incidental manner by a touch, or by
being shaken. Hence there is no great difficulty in admitting that in
the case of leaf-climbers and tendril-bearers, it is this tendency which
has been taken advantage of and increased through natural selection. It
is, however, probable, from reasons which I have assigned in my memoir,
that this will have occurred only with plants which had already acquired
the power of revolving, and had thus become twiners.

I have already endeavoured to explain how plants became twiners,
namely, by the increase of a tendency to slight and irregular revolving
movements, which were at first of no use to them; this movement, as
well as that due to a touch or shake, being the incidental result of
the power of moving, gained for other and beneficial purposes. Whether,
during the gradual development of climbing plants, natural selection
has been aided by the inherited effects of use, I will not pretend to
decide; but we know that certain periodical movements, for instance the
so-called sleep of plants, are governed by habit.

I have now considered enough, perhaps more than enough, of the cases,
selected with care by a skilful naturalist, to prove that natural
selection is incompetent to account for the incipient stages of
useful structures; and I have shown, as I hope, that there is no great
difficulty on this head. A good opportunity has thus been afforded for
enlarging a little on gradations of structure, often associated with
strange functions--an important subject, which was not treated at
sufficient length in the former editions of this work. I will now
briefly recapitulate the foregoing cases.

With the giraffe, the continued preservation of the individuals of some
extinct high-reaching ruminant, which had the longest necks, legs, etc.,
and could browse a little above the average height, and the continued
destruction of those which could not browse so high, would have sufficed
for the production of this remarkable quadruped; but the prolonged
use of all the parts, together with inheritance, will have aided in an
important manner in their co-ordination. With the many insects which
imitate various objects, there is no improbability in the belief that
an accidental resemblance to some common object was in each case the
foundation for the work of natural selection, since perfected through
the occasional preservation of slight variations which made the
resemblance at all closer; and this will have been carried on as long
as the insect continued to vary, and as long as a more and more perfect
resemblance led to its escape from sharp-sighted enemies. In certain
species of whales there is a tendency to the formation of irregular
little points of horn on the palate; and it seems to be quite within the
scope of natural selection to preserve all favourable variations, until
the points were converted, first into lamellated knobs or teeth, like
those on the beak of a goose--then into short lamellae, like those of
the domestic ducks--and then into lamellae, as perfect as those of the
shoveller-duck--and finally into the gigantic plates of baleen, as
in the mouth of the Greenland whale. In the family of the ducks, the
lamellae are first used as teeth, then partly as teeth and partly as
a sifting apparatus, and at last almost exclusively for this latter
purpose.

With such structures as the above lamellae of horn or whalebone, habit
or use can have done little or nothing, as far as we can judge, towards
their development. On the other hand, the transportal of the lower eye
of a flat-fish to the upper side of the head, and the formation of
a prehensile tail, may be attributed almost wholly to continued use,
together with inheritance. With respect to the mammae of the higher
animals, the most probable conjecture is that primordially the cutaneous
glands over the whole surface of a marsupial sack secreted a nutritious
fluid; and that these glands were improved in function through natural
selection, and concentrated into a confined area, in which case they
would have formed a mamma. There is no more difficulty in understanding
how the branched spines of some ancient Echinoderm, which served as
a defence, became developed through natural selection into tridactyle
pedicellariae, than in understanding the development of the pincers of
crustaceans, through slight, serviceable modifications in the ultimate
and penultimate segments of a limb, which was at first used solely
for locomotion. In the avicularia and vibracula of the Polyzoa we have
organs widely different in appearance developed from the same source;
and with the vibracula we can understand how the successive gradations
might have been of service. With the pollinia of orchids, the threads
which originally served to tie together the pollen-grains, can be traced
cohering into caudicles; and the steps can likewise be followed by which
viscid matter, such as that secreted by the stigmas of ordinary flowers,
and still subserving nearly but not quite the same purpose, became
attached to the free ends of the caudicles--all these gradations being
of manifest benefit to the plants in question. With respect to climbing
plants, I need not repeat what has been so lately said.

It has often been asked, if natural selection be so potent, why has not
this or that structure been gained by certain species, to which it would
apparently have been advantageous? But it is unreasonable to expect a
precise answer to such questions, considering our ignorance of the past
history of each species, and of the conditions which at the present day
determine its numbers and range. In most cases only general reasons,
but in some few cases special reasons, can be assigned. Thus to adapt
a species to new habits of life, many co-ordinated modifications are
almost indispensable, and it may often have happened that the requisite
parts did not vary in the right manner or to the right degree. Many
species must have been prevented from increasing in numbers through
destructive agencies, which stood in no relation to certain structures,
which we imagine would have been gained through natural selection
from appearing to us advantageous to the species. In this case, as the
struggle for life did not depend on such structures, they could not
have been acquired through natural selection. In many cases complex and
long-enduring conditions, often of a peculiar nature, are necessary for
the development of a structure; and the requisite conditions may seldom
have concurred. The belief that any given structure, which we think,
often erroneously, would have been beneficial to a species, would
have been gained under all circumstances through natural selection, is
opposed to what we can understand of its manner of action. Mr. Mivart
does not deny that natural selection has effected something; but he
considers it as "demonstrably insufficient" to account for the phenomena
which I explain by its agency. His chief arguments have now been
considered, and the others will hereafter be considered. They seem to me
to partake little of the character of demonstration, and to have little
weight in comparison with those in favour of the power of natural
selection, aided by the other agencies often specified. I am bound to
add, that some of the facts and arguments here used by me, have been
advanced for the same purpose in an able article lately published in the
"Medico-Chirurgical Review."

At the present day almost all naturalists admit evolution under some
form. Mr. Mivart believes that species change through "an internal force
or tendency," about which it is not pretended that anything is known.
That species have a capacity for change will be admitted by all
evolutionists; but there is no need, as it seems to me, to invoke
any internal force beyond the tendency to ordinary variability, which
through the aid of selection, by man has given rise to many well-adapted
domestic races, and which, through the aid of natural selection, would
equally well give rise by graduated steps to natural races or species.
The final result will generally have been, as already explained, an
advance, but in some few cases a retrogression, in organisation.

Mr. Mivart is further inclined to believe, and some naturalists agree
with him, that new species manifest themselves "with suddenness and by
modifications appearing at once." For instance, he supposes that the
differences between the extinct three-toed Hipparion and the horse arose
suddenly. He thinks it difficult to believe that the wing of a bird "was
developed in any other way than by a comparatively sudden modification
of a marked and important kind;" and apparently he would extend the
same view to the wings of bats and pterodactyles. This conclusion,
which implies great breaks or discontinuity in the series, appears to me
improbable in the highest degree.

Everyone who believes in slow and gradual evolution, will of course
admit that specific changes may have been as abrupt and as great as
any single variation which we meet with under nature, or even under
domestication. But as species are more variable when domesticated or
cultivated than under their natural conditions, it is not probable that
such great and abrupt variations have often occurred under nature, as
are known occasionally to arise under domestication. Of these latter
variations several may be attributed to reversion; and the characters
which thus reappear were, it is probable, in many cases at first
gained in a gradual manner. A still greater number must be called
monstrosities, such as six-fingered men, porcupine men, Ancon sheep,
Niata cattle, etc.; and as they are widely different in character from
natural species, they throw very little light on our subject. Excluding
such cases of abrupt variations, the few which remain would at best
constitute, if found in a state of nature, doubtful species, closely
related to their parental types.

My reasons for doubting whether natural species have changed as abruptly
as have occasionally domestic races, and for entirely disbelieving that
they have changed in the wonderful manner indicated by Mr. Mivart, are
as follows. According to our experience, abrupt and strongly marked
variations occur in our domesticated productions, singly and at rather
long intervals of time. If such occurred under nature, they would
be liable, as formerly explained, to be lost by accidental causes of
destruction and by subsequent intercrossing; and so it is known to be
under domestication, unless abrupt variations of this kind are specially
preserved and separated by the care of man. Hence, in order that a new
species should suddenly appear in the manner supposed by Mr. Mivart,
it is almost necessary to believe, in opposition to all analogy, that
several wonderfully changed individuals appeared simultaneously within
the same district. This difficulty, as in the case of unconscious
selection by man, is avoided on the theory of gradual evolution, through
the preservation of a large number of individuals, which varied more
or less in any favourable direction, and of the destruction of a large
number which varied in an opposite manner.

That many species have been evolved in an extremely gradual manner,
there can hardly be a doubt. The species and even the genera of many
large natural families are so closely allied together that it is
difficult to distinguish not a few of them. On every continent, in
proceeding from north to south, from lowland to upland, etc., we meet
with a host of closely related or representative species; as we likewise
do on certain distinct continents, which we have reason to believe were
formerly connected. But in making these and the following remarks, I am
compelled to allude to subjects hereafter to be discussed. Look at
the many outlying islands round a continent, and see how many of their
inhabitants can be raised only to the rank of doubtful species. So it is
if we look to past times, and compare the species which have just passed
away with those still living within the same areas; or if we compare
the fossil species embedded in the sub-stages of the same geological
formation. It is indeed manifest that multitudes of species are related
in the closest manner to other species that still exist, or have lately
existed; and it will hardly be maintained that such species have been
developed in an abrupt or sudden manner. Nor should it be forgotten,
when we look to the special parts of allied species, instead of to
distinct species, that numerous and wonderfully fine gradations can be
traced, connecting together widely different structures.

Many large groups of facts are intelligible only on the principle that
species have been evolved by very small steps. For instance, the fact
that the species included in the larger genera are more closely related
to each other, and present a greater number of varieties than do the
species in the smaller genera. The former are also grouped in little
clusters, like varieties round species; and they present other analogies
with varieties, as was shown in our second chapter. On this same
principle we can understand how it is that specific characters are more
variable than generic characters; and how the parts which are developed
in an extraordinary degree or manner are more variable than other parts
of the same species. Many analogous facts, all pointing in the same
direction, could be added.

Although very many species have almost certainly been produced by
steps not greater than those separating fine varieties; yet it may
be maintained that some have been developed in a different and abrupt
manner. Such an admission, however, ought not to be made without strong
evidence being assigned. The vague and in some respects false analogies,
as they have been shown to be by Mr. Chauncey Wright, which have been
advanced in favour of this view, such as the sudden crystallisation of
inorganic substances, or the falling of a facetted spheroid from one
facet to another, hardly deserve consideration. One class of facts,
however, namely, the sudden appearance of new and distinct forms of
life in our geological formations supports at first sight the belief in
abrupt development. But the value of this evidence depends entirely on
the perfection of the geological record, in relation to periods remote
in the history of the world. If the record is as fragmentary as many
geologists strenuously assert, there is nothing strange in new forms
appearing as if suddenly developed.

Unless we admit transformations as prodigious as those advocated by Mr.
Mivart, such as the sudden development of the wings of birds or bats, or
the sudden conversion of a Hipparion into a horse, hardly any light
is thrown by the belief in abrupt modifications on the deficiency of
connecting links in our geological formations. But against the belief in
such abrupt changes, embryology enters a strong protest. It is notorious
that the wings of birds and bats, and the legs of horses or other
quadrupeds, are undistinguishable at an early embryonic period, and
that they become differentiated by insensibly fine steps. Embryological
resemblances of all kinds can be accounted for, as we shall hereafter
see, by the progenitors of our existing species having varied after
early youth, and having transmitted their newly-acquired characters to
their offspring, at a corresponding age. The embryo is thus left almost
unaffected, and serves as a record of the past condition of the species.
Hence it is that existing species during the early stages of their
development so often resemble ancient and extinct forms belonging to the
same class. On this view of the meaning of embryological resemblances,
and indeed on any view, it is incredible that an animal should have
undergone such momentous and abrupt transformations as those above
indicated, and yet should not bear even a trace in its embryonic
condition of any sudden modification, every detail in its structure
being developed by insensibly fine steps.

He who believes that some ancient form was transformed suddenly through
an internal force or tendency into, for instance, one furnished with
wings, will be almost compelled to assume, in opposition to all analogy,
that many individuals varied simultaneously. It cannot be denied that
such abrupt and great changes of structure are widely different from
those which most species apparently have undergone. He will further be
compelled to believe that many structures beautifully adapted to all the
other parts of the same creature and to the surrounding conditions,
have been suddenly produced; and of such complex and wonderful
co-adaptations, he will not be able to assign a shadow of an
explanation. He will be forced to admit that these great and sudden
transformations have left no trace of their action on the embryo.
To admit all this is, as it seems to me, to enter into the realms of
miracle, and to leave those of science.



CHAPTER VIII. INSTINCT.

 Instincts comparable with habits, but different in their
 origin--Instincts graduated--Aphides and ants--Instincts
 variable--Domestic instincts, their origin--Natural instincts of
 the cuckoo, molothrus, ostrich, and parasitic bees--Slave-making
 ants--Hive-bee, its cell-making instinct--Changes of instinct and
 structure not necessarily simultaneous--Difficulties of the theory of
 the Natural Selection of instincts--Neuter or sterile insects--Summary.


Many instincts are so wonderful that their development will probably
appear to the reader a difficulty sufficient to overthrow my whole
theory. I may here premise, that I have nothing to do with the origin of
the mental powers, any more than I have with that of life itself. We are
concerned only with the diversities of instinct and of the other mental
faculties in animals of the same class.

I will not attempt any definition of instinct. It would be easy to show
that several distinct mental actions are commonly embraced by this term;
but every one understands what is meant, when it is said that instinct
impels the cuckoo to migrate and to lay her eggs in other birds'
nests. An action, which we ourselves require experience to enable us to
perform, when performed by an animal, more especially by a very young
one, without experience, and when performed by many individuals in the
same way, without their knowing for what purpose it is performed, is
usually said to be instinctive. But I could show that none of these
characters are universal. A little dose of judgment or reason, as Pierre
Huber expresses it, often comes into play, even with animals low in the
scale of nature.

Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, an accurate notion
of the frame of mind under which an instinctive action is performed, but
not necessarily of its origin. How unconsciously many habitual actions
are performed, indeed not rarely in direct opposition to our conscious
will! yet they may be modified by the will or reason. Habits easily
become associated with other habits, with certain periods of time and
states of the body. When once acquired, they often remain constant
throughout life. Several other points of resemblance between instincts
and habits could be pointed out. As in repeating a well-known song,
so in instincts, one action follows another by a sort of rhythm; if a
person be interrupted in a song, or in repeating anything by rote, he is
generally forced to go back to recover the habitual train of thought: so
P. Huber found it was with a caterpillar, which makes a very complicated
hammock; for if he took a caterpillar which had completed its hammock
up to, say, the sixth stage of construction, and put it into a
hammock completed up only to the third stage, the caterpillar simply
re-performed the fourth, fifth, and sixth stages of construction.
If, however, a caterpillar were taken out of a hammock made up, for
instance, to the third stage, and were put into one finished up to the
sixth stage, so that much of its work was already done for it, far from
deriving any benefit from this, it was much embarrassed, and, in order
to complete its hammock, seemed forced to start from the third stage,
where it had left off, and thus tried to complete the already finished
work.

If we suppose any habitual action to become inherited--and it can be
shown that this does sometimes happen--then the resemblance between what
originally was a habit and an instinct becomes so close as not to be
distinguished. If Mozart, instead of playing the pianoforte at three
years old with wonderfully little practice, had played a tune with no
practice at all, be might truly be said to have done so instinctively.
But it would be a serious error to suppose that the greater number
of instincts have been acquired by habit in one generation, and then
transmitted by inheritance to succeeding generations. It can be clearly
shown that the most wonderful instincts with which we are acquainted,
namely, those of the hive-bee and of many ants, could not possibly have
been acquired by habit.

It will be universally admitted that instincts are as important as
corporeal structures for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary ever so little,
then I can see no difficulty in natural selection preserving and
continually accumulating variations of instinct to any extent that was
profitable. It is thus, as I believe, that all the most complex and
wonderful instincts have originated. As modifications of corporeal
structure arise from, and are increased by, use or habit, and are
diminished or lost by disuse, so I do not doubt it has been with
instincts. But I believe that the effects of habit are in many cases of
subordinate importance to the effects of the natural selection of
what may be called spontaneous variations of instincts;--that is of
variations produced by the same unknown causes which produce slight
deviations of bodily structure.

No complex instinct can possibly be produced through natural selection,
except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures,
we ought to find in nature, not the actual transitional gradations by
which each complex instinct has been acquired--for these could be found
only in the lineal ancestors of each species--but we ought to find in
the collateral lines of descent some evidence of such gradations; or
we ought at least to be able to show that gradations of some kind are
possible; and this we certainly can do. I have been surprised to find,
making allowance for the instincts of animals having been but little
observed, except in Europe and North America, and for no instinct being
known among extinct species, how very generally gradations, leading to
the most complex instincts, can be discovered. Changes of instinct may
sometimes be facilitated by the same species having different instincts
at different periods of life, or at different seasons of the year, or
when placed under different circumstances, etc.; in which case either
the one or the other instinct might be preserved by natural selection.
And such instances of diversity of instinct in the same species can be
shown to occur in nature.

Again, as in the case of corporeal structure, and conformably to my
theory, the instinct of each species is good for itself, but has never,
as far as we can judge, been produced for the exclusive good of others.
One of the strongest instances of an animal apparently performing an
action for the sole good of another, with which I am acquainted, is that
of aphides voluntarily yielding, as was first observed by Huber, their
sweet excretion to ants: that they do so voluntarily, the following
facts show. I removed all the ants from a group of about a dozen aphides
on a dock-plant, and prevented their attendance during several hours.
After this interval, I felt sure that the aphides would want to excrete.
I watched them for some time through a lens, but not one excreted; I
then tickled and stroked them with a hair in the same manner, as well
as I could, as the ants do with their antennae; but not one excreted.
Afterwards, I allowed an ant to visit them, and it immediately seemed,
by its eager way of running about to be well aware what a rich flock it
had discovered; it then began to play with its antennae on the abdomen
first of one aphis and then of another; and each, as soon as it felt the
antennae, immediately lifted up its abdomen and excreted a limpid drop
of sweet juice, which was eagerly devoured by the ant. Even the quite
young aphides behaved in this manner, showing that the action was
instinctive, and not the result of experience. It is certain, from the
observations of Huber, that the aphides show no dislike to the ants:
if the latter be not present they are at last compelled to eject their
excretion. But as the excretion is extremely viscid, it is no doubt a
convenience to the aphides to have it removed; therefore probably they
do not excrete solely for the good of the ants. Although there is no
evidence that any animal performs an action for the exclusive good of
another species, yet each tries to take advantage of the instincts of
others, as each takes advantage of the weaker bodily structure of other
species. So again certain instincts cannot be considered as absolutely
perfect; but as details on this and other such points are not
indispensable, they may be here passed over.

As some degree of variation in instincts under a state of nature, and
the inheritance of such variations, are indispensable for the action of
natural selection, as many instances as possible ought to be given; but
want of space prevents me. I can only assert that instincts certainly
do vary--for instance, the migratory instinct, both in extent and
direction, and in its total loss. So it is with the nests of birds,
which vary partly in dependence on the situations chosen, and on the
nature and temperature of the country inhabited, but often from causes
wholly unknown to us. Audubon has given several remarkable cases
of differences in the nests of the same species in the northern and
southern United States. Why, it has been asked, if instinct be variable,
has it not granted to the bee "the ability to use some other material
when wax was deficient?" But what other natural material could bees
use? They will work, as I have seen, with wax hardened with vermilion
or softened with lard. Andrew Knight observed that his bees, instead of
laboriously collecting propolis, used a cement of wax and turpentine,
with which he had covered decorticated trees. It has lately been shown
that bees, instead of searching for pollen, will gladly use a very
different substance, namely, oatmeal. Fear of any particular enemy is
certainly an instinctive quality, as may be seen in nestling birds,
though it is strengthened by experience, and by the sight of fear of the
same enemy in other animals. The fear of man is slowly acquired, as
I have elsewhere shown, by the various animals which inhabit desert
islands; and we see an instance of this, even in England, in the greater
wildness of all our large birds in comparison with our small birds;
for the large birds have been most persecuted by man. We may safely
attribute the greater wildness of our large birds to this cause; for in
uninhabited islands large birds are not more fearful than small; and the
magpie, so wary in England, is tame in Norway, as is the hooded crow in
Egypt.

That the mental qualities of animals of the same kind, born in a state
of nature, vary much, could be shown by many facts. Several cases could
also be adduced of occasional and strange habits in wild animals, which,
if advantageous to the species, might have given rise, through natural
selection, to new instincts. But I am well aware that these general
statements, without the facts in detail, can produce but a feeble effect
on the reader's mind. I can only repeat my assurance, that I do not
speak without good evidence.

INHERITED CHANGES OF HABIT OR INSTINCT IN DOMESTICATED ANIMALS.

The possibility, or even probability, of inherited variations
of instinct in a state of nature will be strengthened by briefly
considering a few cases under domestication. We shall thus be enabled
to see the part which habit and the selection of so-called spontaneous
variations have played in modifying the mental qualities of our domestic
animals. It is notorious how much domestic animals vary in their mental
qualities. With cats, for instance, one naturally takes to catching
rats, and another mice, and these tendencies are known to be inherited.
One cat, according to Mr. St. John, always brought home game birds,
another hares or rabbits, and another hunted on marshy ground and almost
nightly caught woodcocks or snipes. A number of curious and authentic
instances could be given of various shades of disposition and taste, and
likewise of the oddest tricks, associated with certain frames of mind or
periods of time. But let us look to the familiar case of the breeds
of dogs: it cannot be doubted that young pointers (I have myself seen
striking instances) will sometimes point and even back other dogs the
very first time that they are taken out; retrieving is certainly in some
degree inherited by retrievers; and a tendency to run round, instead of
at, a flock of sheep, by shepherd-dogs. I cannot see that these actions,
performed without experience by the young, and in nearly the same manner
by each individual, performed with eager delight by each breed, and
without the end being known--for the young pointer can no more know that
he points to aid his master, than the white butterfly knows why she lays
her eggs on the leaf of the cabbage--I cannot see that these actions
differ essentially from true instincts. If we were to behold one kind
of wolf, when young and without any training, as soon as it scented its
prey, stand motionless like a statue, and then slowly crawl forward with
a peculiar gait; and another kind of wolf rushing round, instead of at,
a herd of deer, and driving them to a distant point, we should assuredly
call these actions instinctive. Domestic instincts, as they may be
called, are certainly far less fixed than natural instincts; but
they have been acted on by far less rigorous selection, and have
been transmitted for an incomparably shorter period, under less fixed
conditions of life.

How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown when
different breeds of dogs are crossed. Thus it is known that a cross with
a bull-dog has affected for many generations the courage and obstinacy
of greyhounds; and a cross with a greyhound has given to a whole family
of shepherd-dogs a tendency to hunt hares. These domestic instincts,
when thus tested by crossing, resemble natural instincts, which in a
like manner become curiously blended together, and for a long period
exhibit traces of the instincts of either parent: for example, Le Roy
describes a dog, whose great-grandfather was a wolf, and this dog
showed a trace of its wild parentage only in one way, by not coming in a
straight line to his master, when called.

Domestic instincts are sometimes spoken of as actions which have become
inherited solely from long-continued and compulsory habit, but this is
not true. No one would ever have thought of teaching, or probably could
have taught, the tumbler-pigeon to tumble--an action which, as I have
witnessed, is performed by young birds, that have never seen a pigeon
tumble. We may believe that some one pigeon showed a slight tendency to
this strange habit, and that the long-continued selection of the best
individuals in successive generations made tumblers what they now are;
and near Glasgow there are house-tumblers, as I hear from Mr. Brent,
which cannot fly eighteen inches high without going head over heels. It
may be doubted whether any one would have thought of training a dog to
point, had not some one dog naturally shown a tendency in this line; and
this is known occasionally to happen, as I once saw, in a pure terrier:
the act of pointing is probably, as many have thought, only the
exaggerated pause of an animal preparing to spring on its prey. When the
first tendency to point was once displayed, methodical selection and the
inherited effects of compulsory training in each successive generation
would soon complete the work; and unconscious selection is still in
progress, as each man tries to procure, without intending to improve the
breed, dogs which stand and hunt best. On the other hand, habit alone
in some cases has sufficed; hardly any animal is more difficult to tame
than the young of the wild rabbit; scarcely any animal is tamer than the
young of the tame rabbit; but I can hardly suppose that domestic rabbits
have often been selected for tameness alone; so that we must attribute
at least the greater part of the inherited change from extreme wildness
to extreme tameness, to habit and long-continued close confinement.

Natural instincts are lost under domestication: a remarkable instance of
this is seen in those breeds of fowls which very rarely or never become
"broody," that is, never wish to sit on their eggs. Familiarity alone
prevents our seeing how largely and how permanently the minds of our
domestic animals have been modified. It is scarcely possible to doubt
that the love of man has become instinctive in the dog. All wolves,
foxes, jackals and species of the cat genus, when kept tame, are most
eager to attack poultry, sheep and pigs; and this tendency has been
found incurable in dogs which have been brought home as puppies from
countries such as Tierra del Fuego and Australia, where the savages do
not keep these domestic animals. How rarely, on the other hand, do
our civilised dogs, even when quite young, require to be taught not to
attack poultry, sheep, and pigs! No doubt they occasionally do make an
attack, and are then beaten; and if not cured, they are destroyed;
so that habit and some degree of selection have probably concurred in
civilising by inheritance our dogs. On the other hand, young chickens
have lost wholly by habit, that fear of the dog and cat which no doubt
was originally instinctive in them, for I am informed by Captain Hutton
that the young chickens of the parent stock, the Gallus bankiva, when
reared in India under a hen, are at first excessively wild. So it is
with young pheasants reared in England under a hen. It is not that
chickens have lost all fear, but fear only of dogs and cats, for if
the hen gives the danger chuckle they will run (more especially young
turkeys) from under her and conceal themselves in the surrounding grass
or thickets; and this is evidently done for the instinctive purpose of
allowing, as we see in wild ground-birds, their mother to fly away.
But this instinct retained by our chickens has become useless under
domestication, for the mother-hen has almost lost by disuse the power of
flight.

Hence, we may conclude that under domestication instincts have been
acquired and natural instincts have been lost, partly by habit and
partly by man selecting and accumulating, during successive generations,
peculiar mental habits and actions, which at first appeared from what we
must in our ignorance call an accident. In some cases compulsory habit
alone has sufficed to produce inherited mental changes; in other cases
compulsory habit has done nothing, and all has been the result of
selection, pursued both methodically and unconsciously; but in most
cases habit and selection have probably concurred.

SPECIAL INSTINCTS.

We shall, perhaps, best understand how instincts in a state of nature
have become modified by selection by considering a few cases. I will
select only three, namely, the instinct which leads the cuckoo to lay
her eggs in other birds' nests; the slave-making instinct of certain
ants; and the cell-making power of the hive-bee: these two latter
instincts have generally and justly been ranked by naturalists as the
most wonderful of all known instincts.

INSTINCTS OF THE CUCKOO.

It is supposed by some naturalists that the more immediate cause of
the instinct of the cuckoo is that she lays her eggs, not daily, but
at intervals of two or three days; so that, if she were to make her own
nest and sit on her own eggs, those first laid would have to be left
for some time unincubated or there would be eggs and young birds of
different ages in the same nest. If this were the case the process of
laying and hatching might be inconveniently long, more especially as
she migrates at a very early period; and the first hatched young would
probably have to be fed by the male alone. But the American cuckoo is
in this predicament, for she makes her own nest and has eggs and young
successively hatched, all at the same time. It has been both asserted
and denied that the American cuckoo occasionally lays her eggs in other
birds' nests; but I have lately heard from Dr. Merrill, of Iowa, that he
once found in Illinois a young cuckoo, together with a young jay in the
nest of a blue jay (Garrulus cristatus); and as both were nearly full
feathered, there could be no mistake in their identification. I could
also give several instances of various birds which have been known
occasionally to lay their eggs in other birds' nests. Now let us suppose
that the ancient progenitor of our European cuckoo had the habits of the
American cuckoo, and that she occasionally laid an egg in another bird's
nest. If the old bird profited by this occasional habit through being
enabled to emigrate earlier or through any other cause; or if the
young were made more vigorous by advantage being taken of the mistaken
instinct of another species than when reared by their own mother,
encumbered as she could hardly fail to be by having eggs and young of
different ages at the same time, then the old birds or the fostered
young would gain an advantage. And analogy would lead us to believe
that the young thus reared would be apt to follow by inheritance the
occasional and aberrant habit of their mother, and in their turn
would be apt to lay their eggs in other birds' nests, and thus be
more successful in rearing their young. By a continued process of this
nature, I believe that the strange instinct of our cuckoo has been
generated. It has, also recently been ascertained on sufficient
evidence, by Adolf Muller, that the cuckoo occasionally lays her eggs
on the bare ground, sits on them and feeds her young. This rare event
is probably a case of reversion to the long-lost, aboriginal instinct of
nidification.

It has been objected that I have not noticed other related instincts
and adaptations of structure in the cuckoo, which are spoken of as
necessarily co-ordinated. But in all cases, speculation on an instinct
known to us only in a single species, is useless, for we have hitherto
had no facts to guide us. Until recently the instincts of the European
and of the non-parasitic American cuckoo alone were known; now, owing
to Mr. Ramsay's observations, we have learned something about three
Australian species, which lay their eggs in other birds' nests. The
chief points to be referred to are three: first, that the common cuckoo,
with rare exceptions, lays only one egg in a nest, so that the large and
voracious young bird receives ample food. Secondly, that the eggs are
remarkably small, not exceeding those of the skylark--a bird about
one-fourth as large as the cuckoo. That the small size of the egg is a
real case of adaptation we may infer from the fact of the mon-parasitic
American cuckoo laying full-sized eggs. Thirdly, that the young cuckoo,
soon after birth, has the instinct, the strength and a properly shaped
back for ejecting its foster-brothers, which then perish from cold and
hunger. This has been boldly called a beneficent arrangement, in
order that the young cuckoo may get sufficient food, and that its
foster-brothers may perish before they had acquired much feeling!

Turning now to the Australian species: though these birds generally lay
only one egg in a nest, it is not rare to find two and even three eggs
in the same nest. In the bronze cuckoo the eggs vary greatly in size,
from eight to ten lines in length. Now, if it had been of an advantage
to this species to have laid eggs even smaller than those now laid, so
as to have deceived certain foster-parents, or, as is more probable, to
have been hatched within a shorter period (for it is asserted that
there is a relation between the size of eggs and the period of their
incubation), then there is no difficulty in believing that a race or
species might have been formed which would have laid smaller and smaller
eggs; for these would have been more safely hatched and reared. Mr.
Ramsay remarks that two of the Australian cuckoos, when they lay their
eggs in an open nest, manifest a decided preference for nests containing
eggs similar in colour to their own. The European species apparently
manifests some tendency towards a similar instinct, but not rarely
departs from it, as is shown by her laying her dull and pale-coloured
eggs in the nest of the hedge-warbler with bright greenish-blue eggs.
Had our cuckoo invariably displayed the above instinct, it would
assuredly have been added to those which it is assumed must all have
been acquired together. The eggs of the Australian bronze cuckoo vary,
according to Mr. Ramsay, to an extraordinary degree in colour; so
that in this respect, as well as in size, natural selection might have
secured and fixed any advantageous variation.

In the case of the European cuckoo, the offspring of the foster-parents
are commonly ejected from the nest within three days after the cuckoo is
hatched; and as the latter at this age is in a most helpless condition,
Mr. Gould was formerly inclined to believe that the act of ejection was
performed by the foster-parents themselves. But he has now received a
trustworthy account of a young cuckoo which was actually seen, while
still blind and not able even to hold up its own head, in the act of
ejecting its foster-brothers. One of these was replaced in the nest by
the observer, and was again thrown out. With respect to the means by
which this strange and odious instinct was acquired, if it were of great
importance for the young cuckoo, as is probably the case, to receive as
much food as possible soon after birth, I can see no special difficulty
in its having gradually acquired, during successive generations, the
blind desire, the strength, and structure necessary for the work of
ejection; for those cuckoos which had such habits and structure best
developed would be the most securely reared. The first step towards the
acquisition of the proper instinct might have been mere unintentional
restlessness on the part of the young bird, when somewhat advanced
in age and strength; the habit having been afterwards improved, and
transmitted to an earlier age. I can see no more difficulty in this than
in the unhatched young of other birds acquiring the instinct to break
through their own shells; or than in young snakes acquiring in their
upper jaws, as Owen has remarked, a transitory sharp tooth for cutting
through the tough egg-shell. For if each part is liable to individual
variations at all ages, and the variations tend to be inherited at
a corresponding or earlier age--propositions which cannot be
disputed--then the instincts and structure of the young could be slowly
modified as surely as those of the adult; and both cases must stand or
fall together with the whole theory of natural selection.

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 clamouring 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 it is known, invariably lays its eggs in the nests of strangers;
but it is remarkable that several together sometimes commence to build
an irregular untidy nest of their own, placed in singular 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. A third species, the M. pecoris of North America, has acquired
instincts as perfect as those of the cuckoo, for it never lays more than
one egg in a foster-nest, so that the young bird is securely reared. Mr.
Hudson is a strong disbeliever in evolution, but he appears to have been
so much struck by the imperfect instincts of the Molothrus bonariensis
that he quotes my words, and asks, "Must we consider these habits, not
as especially endowed or created instincts, but as small consequences of
one general law, namely, transition?"

Various birds, as has already been remarked, occasionally lay their eggs
in the nests of other birds. This habit is not very uncommon with the
Gallinaceae, and throws some light on the singular instinct of the
ostrich. In this family several hen birds unite and lay first a few eggs
in one nest and then in another; and these are hatched by the males.
This instinct may probably be accounted for by the fact of the hens
laying a large number of eggs, but, as with the cuckoo, at intervals of
two or three days. The instinct, however, of the American ostrich, as
in the case of the Molothrus bonariensis, has not as yet been perfected;
for a surprising number of eggs lie strewed over the plains, so that in
one day's hunting I picked up no less than twenty lost and wasted eggs.

Many bees are parasitic, and regularly lay their eggs in the nests
of other kinds of bees. This case is more remarkable than that of the
cuckoo; for these bees have not only had their instincts but their
structure modified in accordance with their parasitic habits; for they
do not possess the pollen-collecting apparatus which would have been
indispensable if they had stored up food for their own young. Some
species of Sphegidae (wasp-like insects) are likewise parasitic; and
M. Fabre has lately shown good reason for believing that, although
the Tachytes nigra generally makes its own burrow and stores it with
paralysed prey for its own larvae, yet that, when this insect finds a
burrow already made and stored by another sphex, it takes advantage of
the prize, and becomes for the occasion parasitic. In this case, as
with that of the Molothrus or cuckoo, I can see no difficulty in natural
selection making an occasional habit permanent, if of advantage to the
species, and if the insect whose nest and stored food are feloniously
appropriated, be not thus exterminated.

SLAVE-MAKING INSTINCT.

This remarkable instinct was first discovered in the Formica (Polyerges)
rufescens by Pierre Huber, a better observer even than his celebrated
father. This ant is absolutely dependent on its slaves; without their
aid, the species would certainly become extinct in a single year. The
males and fertile females do no work of any kind, and the workers or
sterile females, though most energetic and courageous in capturing
slaves, do no other work. They are incapable of making their own nests,
or of feeding their own larvae. When the old nest is found inconvenient,
and they have to migrate, it is the slaves which determine the
migration, and actually carry their masters in their jaws. So utterly
helpless are the masters, that when Huber shut up thirty of them without
a slave, but with plenty of the food which they like best, and with
their larvae and pupae to stimulate them to work, they did nothing; they
could not even feed themselves, and many perished of hunger. Huber then
introduced a single slave (F. fusca), and she instantly set to work, fed
and saved the survivors; made some cells and tended the larvae, and
put all to rights. What can be more extraordinary than these
well-ascertained facts? If we had not known of any other slave-making
ant, it would have been hopeless to speculate how so wonderful an
instinct could have been perfected.

Another species, Formica sanguinea, was likewise first discovered by P.
Huber to be a slave-making ant. This species is found in the southern
parts of England, and its habits have been attended to by Mr. F. Smith,
of the British Museum, to whom I am much indebted for information on
this and other subjects. Although fully trusting to the statements of
Huber and Mr. Smith, I tried to approach the subject in a sceptical
frame of mind, as any one may well be excused for doubting the existence
of so extraordinary an instinct as that of making slaves. Hence, I
will give the observations which I made in some little detail. I opened
fourteen nests of F. sanguinea, and found a few slaves in all. Males and
fertile females of the slave-species (F. fusca) are found only in their
own proper communities, and have never been observed in the nests of F.
sanguinea. The slaves are black and not above half the size of their
red masters, so that the contrast in their appearance is great. When the
nest is slightly disturbed, the slaves occasionally come out, and like
their masters are much agitated and defend the nest: when the nest is
much disturbed, and the larvae and pupae are exposed, the slaves work
energetically together with their masters in carrying them away to a
place of safety. Hence, it is clear that the slaves feel quite at home.
During the months of June and July, on three successive years, I watched
for many hours several nests in Surrey and Sussex, and never saw a slave
either leave or enter a nest. As, during these months, the slaves are
very few in number, I thought that they might behave differently when
more numerous; but Mr. Smith informs me that he has watched the nests at
various hours during May, June and August, both in Surrey and Hampshire,
and has never seen the slaves, though present in large numbers in
August, either leave or enter the nest. Hence, he considers them as
strictly household slaves. The masters, on the other hand, may be
constantly seen bringing in materials for the nest, and food of all
kinds. During the year 1860, however, in the month of July, I came
across a community with an unusually large stock of slaves, and I
observed a few slaves mingled with their masters leaving the nest, and
marching along the same road to a tall Scotch-fir tree, twenty-five
yards distant, which they ascended together, probably in search of
aphides or cocci. According to Huber, who had ample opportunities
for observation, the slaves in Switzerland habitually work with their
masters in making the nest, and they alone open and close the doors in
the morning and evening; and, as Huber expressly states, their principal
office is to search for aphides. This difference in the usual habits of
the masters and slaves in the two countries, probably depends merely
on the slaves being captured in greater numbers in Switzerland than in
England.

One day I fortunately witnessed a migration of F. sanguinea from one
nest to another, and it was a most interesting spectacle to behold the
masters carefully carrying their slaves in their jaws instead of
being carried by them, as in the case of F. rufescens. Another day my
attention was struck by about a score of the slave-makers haunting the
same spot, and evidently not in search of food; they approached and were
vigorously repulsed by an independent community of the slave species (F.
fusca); sometimes as many as three of these ants clinging to the legs of
the slave-making F. sanguinea. The latter ruthlessly killed their
small opponents and carried their dead bodies as food to their nest,
twenty-nine yards distant; but they were prevented from getting any
pupae to rear as slaves. I then dug up a small parcel of the pupae of F.
fusca from another nest, and put them down on a bare spot near the place
of combat; they were eagerly seized and carried off by the tyrants, who
perhaps fancied that, after all, they had been victorious in their late
combat.

At the same time I laid on the same place a small parcel of the pupae of
another species, F. flava, with a few of these little yellow ants still
clinging to the fragments of their nest. This species is sometimes,
though rarely, made into slaves, as has been described by Mr. Smith.
Although so small a species, it is very courageous, and I have seen it
ferociously attack other ants. In one instance I found to my surprise
an independent community of F. flava under a stone beneath a nest of the
slave-making F. sanguinea; and when I had accidentally disturbed both
nests, the little ants attacked their big neighbours with surprising
courage. Now I was curious to ascertain whether F. sanguinea could
distinguish the pupae of F. fusca, which they habitually make into
slaves, from those of the little and furious F. flava, which they rarely
capture, and it was evident that they did at once distinguish them;
for we have seen that they eagerly and instantly seized the pupae of F.
fusca, whereas they were much terrified when they came across the pupae,
or even the earth from the nest, of F. flava, and quickly ran away; but
in about a quarter of an hour, shortly after all the little yellow ants
had crawled away, they took heart and carried off the pupae.

One evening I visited another community of F. sanguinea, and found a
number of these ants returning home and entering their nests, carrying
the dead bodies of F. fusca (showing that it was not a migration) and
numerous pupae. I traced a long file of ants burdened with booty, for
about forty yards back, to a very thick clump of heath, whence I saw the
last individual of F. sanguinea emerge, carrying a pupa; but I was not
able to find the desolated nest in the thick heath. The nest, however,
must have been close at hand, for two or three individuals of F. fusca
were rushing about in the greatest agitation, and one was perched
motionless with its own pupa in its mouth on the top of a spray of
heath, an image of despair over its ravaged home.

Such are the facts, though they did not need confirmation by me, in
regard to the wonderful instinct of making slaves. Let it be observed
what a contrast the instinctive habits of F. sanguinea present with
those of the continental F. rufescens. The latter does not build its own
nest, does not determine its own migrations, does not collect food
for itself or its young, and cannot even feed itself: it is absolutely
dependent on its numerous slaves. Formica sanguinea, on the other
hand, possesses much fewer slaves, and in the early part of the summer
extremely few. The masters determine when and where a new nest shall
be formed, and when they migrate, the masters carry the slaves. Both in
Switzerland and England the slaves seem to have the exclusive care of
the larvae, and the masters alone go on slave-making expeditions. In
Switzerland the slaves and masters work together, making and bringing
materials for the nest: both, but chiefly the slaves, tend and milk
as it may be called, their aphides; and thus both collect food for
the community. In England the masters alone usually leave the nest to
collect building materials and food for themselves, their slaves and
larvae. So that the masters in this country receive much less service
from their slaves than they do in Switzerland.

By what steps the instinct of F. sanguinea originated I will not pretend
to conjecture. But as ants which are not slave-makers, will, as I have
seen, carry off pupae of other species, if scattered near their nests,
it is possible that such pupae originally stored as food might become
developed; and the foreign ants thus unintentionally reared would then
follow their proper instincts, and do what work they could. If their
presence proved useful to the species which had seized them--if it were
more advantageous to this species, to capture workers than to procreate
them--the habit of collecting pupae, originally for food, might by
natural selection be strengthened and rendered permanent for the
very different purpose of raising slaves. When the instinct was once
acquired, if carried out to a much less extent even than in our British
F. sanguinea, which, as we have seen, is less aided by its slaves than
the same species in Switzerland, natural selection might increase and
modify the instinct--always supposing each modification to be of use to
the species--until an ant was formed as abjectly dependent on its slaves
as is the Formica rufescens.

CELL-MAKING INSTINCT OF THE HIVE-BEE.

I will not here enter on minute details on this subject, but will merely
give an outline of the conclusions at which I have arrived. He must be
a dull man who can examine the exquisite structure of a comb, so
beautifully adapted to its end, without enthusiastic admiration. We
hear from mathematicians that bees have practically solved a recondite
problem, and have made their cells of the proper shape to hold the
greatest possible amount of honey, with the least possible consumption
of precious wax in their construction. It has been remarked that a
skilful workman, with fitting tools and measures, would find it very
difficult to make cells of wax of the true form, though this is effected
by a crowd of bees working in a dark hive. Granting whatever instincts
you please, it seems at first quite inconceivable how they can make
all the necessary angles and planes, or even perceive when they are
correctly made. But the difficulty is not nearly so great as at first
appears: all this beautiful work can be shown, I think, to follow from a
few simple instincts.

I was led to investigate this subject by Mr. Waterhouse, who has shown
that the form of the cell stands in close relation to the presence of
adjoining cells; and the following view may, perhaps, be considered only
as a modification of his theory. Let us look to the great principle of
gradation, and see whether Nature does not reveal to us her method of
work. At one end of a short series we have humble-bees, which use their
old cocoons to hold honey, sometimes adding to them short tubes of wax,
and likewise making separate and very irregular rounded cells of wax. At
the other end of the series we have the cells of the hive-bee, placed in
a double layer: each cell, as is well known, is an hexagonal prism,
with the basal edges of its six sides bevelled so as to join an inverted
pyramid, of three rhombs. These rhombs have certain angles, and the
three which form the pyramidal base of a single cell on one side of the
comb, enter into the composition of the bases of three adjoining cells
on the opposite side. In the series between the extreme perfection of
the cells of the hive-bee and the simplicity of those of the humble-bee,
we have the cells of the Mexican Melipona domestica, carefully described
and figured by Pierre Huber. The Melipona itself is intermediate in
structure between the hive and humble bee, but more nearly related to
the latter: it forms a nearly regular waxen comb of cylindrical cells,
in which the young are hatched, and, in addition, some large cells of
wax for holding honey. These latter cells are nearly spherical and of
nearly equal sizes, and are aggregated into an irregular mass. But the
important point to notice is, that these cells are always made at that
degree of nearness to each other that they would have intersected or
broken into each other if the spheres had been completed; but this is
never permitted, the bees building perfectly flat walls of wax between
the spheres which thus tend to intersect. Hence, each cell consists of
an outer spherical portion, and of two, three, or more flat surfaces,
according as the cell adjoins two, three or more other cells. When one
cell rests on three other cells, which, from the spheres being nearly
of the same size, is very frequently and necessarily the case, the three
flat surfaces are united into a pyramid; and this pyramid, as Huber has
remarked, is manifestly a gross imitation of the three-sided pyramidal
base of the cell of the hive-bee. As in the cells of the hive-bee, so
here, the three plane surfaces in any one cell necessarily enter into
the construction of three adjoining cells. It is obvious that the
Melipona saves wax, and what is more important, labour, by this manner
of building; for the flat walls between the adjoining cells are not
double, but are of the same thickness as the outer spherical portions,
and yet each flat portion forms a part of two cells.

Reflecting on this case, it occurred to me that if the Melipona had made
its spheres at some given distance from each other, and had made them of
equal sizes and had arranged them symmetrically in a double layer,
the resulting structure would have been as perfect as the comb of the
hive-bee. Accordingly I wrote to Professor Miller, of Cambridge, and
this geometer has kindly read over the following statement, drawn up
from his information, and tells me that it is strictly correct:--

If a number of equal spheres be described with their centres placed in
two parallel layers; with the centre of each sphere at the distance of
radius x sqrt(2) or radius x 1.41421 (or at some lesser distance), from
the centres of the six surrounding spheres in the same layer; and at the
same distance from the centres of the adjoining spheres in the other and
parallel layer; then, if planes of intersection between the several
spheres in both layers be formed, there will result a double layer of
hexagonal prisms united together by pyramidal bases formed of three
rhombs; and the rhombs and the sides of the hexagonal prisms will have
every angle identically the same with the best measurements which have
been made of the cells of the hive-bee. But I hear from Professor Wyman,
who has made numerous careful measurements, that the accuracy of the
workmanship of the bee has been greatly exaggerated; so much so, that
whatever the typical form of the cell may be, it is rarely, if ever,
realised.

Hence we may safely conclude that, if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not very
wonderful, this bee would make a structure as wonderfully perfect as
that of the hive-bee. We must suppose the Melipona to have the power of
forming her cells truly spherical, and of equal sizes; and this would
not be very surprising, seeing that she already does so to a certain
extent, and seeing what perfectly cylindrical burrows many insects make
in wood, apparently by turning round on a fixed point. We must suppose
the Melipona to arrange her cells in level layers, as she already does
her cylindrical cells; and we must further suppose, and this is the
greatest difficulty, that she can somehow judge accurately at what
distance to stand from her fellow-labourers when several are making
their spheres; but she is already so far enabled to judge of distance,
that she always describes her spheres so as to intersect to a certain
extent; and then she unites the points of intersection by perfectly flat
surfaces. By such modifications of instincts which in themselves are not
very wonderful--hardly more wonderful than those which guide a bird to
make its nest--I believe that the hive-bee has acquired, through natural
selection, her inimitable architectural powers.

But this theory can be tested by experiment. Following the example of
Mr. Tegetmeier, I separated two combs, and put between them a long,
thick, rectangular strip of wax: the bees instantly began to excavate
minute circular pits in it; and as they deepened these little pits, they
made them wider and wider until they were converted into shallow basins,
appearing to the eye perfectly true or parts of a sphere, and of
about the diameter of a cell. It was most interesting to observe that,
wherever several bees had begun to excavate these basins near together,
they had begun their work at such a distance from each other that by
the time the basins had acquired the above stated width (i.e. about the
width of an ordinary cell), and were in depth about one sixth of the
diameter of the sphere of which they formed a part, the rims of the
basins intersected or broke into each other. As soon as this occurred,
the bees ceased to excavate, and began to build up flat walls of wax
on the lines of intersection between the basins, so that each hexagonal
prism was built upon the scalloped edge of a smooth basin, instead of on
the straight edges of a three-sided pyramid as in the case of ordinary
cells.

I then put into the hive, instead of a thick, rectangular piece of wax,
a thin and narrow, knife-edged ridge, coloured with vermilion. The bees
instantly began on both sides to excavate little basins near to each
other, in the same way as before; but the ridge of wax was so thin, that
the bottoms of the basins, if they had been excavated to the same depth
as in the former experiment, would have broken into each other from the
opposite sides. The bees, however, did not suffer this to happen, and
they stopped their excavations in due time; so that the basins, as soon
as they had been a little deepened, came to have flat bases; and these
flat bases, formed by thin little plates of the vermilion wax left
ungnawed, were situated, as far as the eye could judge, exactly along
the planes of imaginary intersection between the basins on the opposite
side of the ridge of wax. In some parts, only small portions, in other
parts, large portions of a rhombic plate were thus left between the
opposed basins, but the work, from the unnatural state of things, had
not been neatly performed. The bees must have worked at very nearly the
same rate in circularly gnawing away and deepening the basins on both
sides of the ridge of vermilion wax, in order to have thus succeeded in
leaving flat plates between the basins, by stopping work at the planes
of intersection.

Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip
of wax, perceiving when they have gnawed the wax away to the proper
thinness, and then stopping their work. In ordinary combs it has
appeared to me that the bees do not always succeed in working at exactly
the same rate from the opposite sides; for I have noticed half-completed
rhombs at the base of a just-commenced cell, which were slightly concave
on one side, where I suppose that the bees had excavated too quickly,
and convex on the opposed side where the bees had worked less quickly.
In one well-marked instance, I put the comb back into the hive, and
allowed the bees to go on working for a short time, and again examined
the cell, and I found that the rhombic plate had been completed, and had
become PERFECTLY FLAT: it was absolutely impossible, from the extreme
thinness of the little plate, that they could have effected this by
gnawing away the convex side; and I suspect that the bees in such cases
stand in the opposed cells and push and bend the ductile and warm wax
(which as I have tried is easily done) into its proper intermediate
plane, and thus flatten it.

From the experiment of the ridge of vermilion wax we can see that, if
the bees were to build for themselves a thin wall of wax, they could
make their cells of the proper shape, by standing at the proper distance
from each other, by excavating at the same rate, and by endeavouring to
make equal spherical hollows, but never allowing the spheres to break
into each other. Now bees, as may be clearly seen by examining the edge
of a growing comb, do make a rough, circumferential wall or rim all
round the comb; and they gnaw this away from the opposite sides, always
working circularly as they deepen each cell. They do not make the whole
three-sided pyramidal base of any one cell at the same time, but only
that one rhombic plate which stands on the extreme growing margin, or
the two plates, as the case may be; and they never complete the upper
edges of the rhombic plates, until the hexagonal walls are commenced.
Some of these statements differ from those made by the justly celebrated
elder Huber, but I am convinced of their accuracy; and if I had space, I
could show that they are conformable with my theory.

Huber's statement, that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly
correct; the first commencement having always been a little hood of
wax; but I will not here enter on details. We see how important a part
excavation plays in the construction of the cells; but it would be a
great error to suppose that the bees cannot build up a rough wall of wax
in the proper position--that is, along the plane of intersection between
two adjoining spheres. I have several specimens showing clearly that
they can do this. Even in the rude circumferential rim or wall of wax
round a growing comb, flexures may sometimes be observed, corresponding
in position to the planes of the rhombic basal plates of future cells.
But the rough wall of wax has in every case to be finished off, by being
largely gnawed away on both sides. The manner in which the bees build is
curious; they always make the first rough wall from ten to twenty times
thicker than the excessively thin finished wall of the cell, which will
ultimately be left. We shall understand how they work, by supposing
masons first to pile up a broad ridge of cement, and then to begin
cutting it away equally on both sides near the ground, till a smooth,
very thin wall is left in the middle; the masons always piling up the
cut-away cement, and adding fresh cement on the summit of the ridge. We
shall thus have a thin wall steadily growing upward but always crowned
by a gigantic coping. From all the cells, both those just commenced and
those completed, being thus crowned by a strong coping of wax, the
bees can cluster and crawl over the comb without injuring the delicate
hexagonal walls. These walls, as Professor Miller has kindly ascertained
for me, vary greatly in thickness; being, on an average of twelve
measurements made near the border of the comb, 1/352 of an inch in
thickness; whereas the basal rhomboidal plates are thicker, nearly in
the proportion of three to two, having a mean thickness, from twenty-one
measurements, of 1/229 of an inch. By the above singular manner of
building, strength is continually given to the comb, with the utmost
ultimate economy of wax.

It seems at first to add to the difficulty of understanding how the
cells are made, that a multitude of bees all work together; one bee
after working a short time at one cell going to another, so that, as
Huber has stated, a score of individuals work even at the commencement
of the first cell. I was able practically to show this fact, by covering
the edges of the hexagonal walls of a single cell, or the extreme margin
of the circumferential rim of a growing comb, with an extremely thin
layer of melted vermilion wax; and I invariably found that the colour
was most delicately diffused by the bees--as delicately as a painter
could have done it with his brush--by atoms of the coloured wax having
been taken from the spot on which it had been placed, and worked into
the growing edges of the cells all round. The work of construction seems
to be a sort of balance struck between many bees, all instinctively
standing at the same relative distance from each other, all trying to
sweep equal spheres, and then building up, or leaving ungnawed, the
planes of intersection between these spheres. It was really curious to
note in cases of difficulty, as when two pieces of comb met at an angle,
how often the bees would pull down and rebuild in different ways the
same cell, sometimes recurring to a shape which they had at first
rejected.

When bees have a place on which they can stand in their proper positions
for working--for instance, on a slip of wood, placed directly under the
middle of a comb growing downwards, so that the comb has to be built
over one face of the slip--in this case the bees can lay the foundations
of one wall of a new hexagon, in its strictly proper place, projecting
beyond the other completed cells. It suffices that the bees should be
enabled to stand at their proper relative distances from each other
and from the walls of the last completed cells, and then, by striking
imaginary spheres, they can build up a wall intermediate between two
adjoining spheres; but, as far as I have seen, they never gnaw away and
finish off the angles of a cell till a large part both of that cell and
of the adjoining cells has been built. This capacity in bees of laying
down under certain circumstances a rough wall in its proper place
between two just-commenced cells, is important, as it bears on a fact,
which seems at first subversive of the foregoing theory; namely, that
the cells on the extreme margin of wasp-combs are sometimes strictly
hexagonal; but I have not space here to enter on this subject. Nor does
there seem to me any great difficulty in a single insect (as in the case
of a queen-wasp) making hexagonal cells, if she were to work alternately
on the inside and outside of two or three cells commenced at the same
time, always standing at the proper relative distance from the parts
of the cells just begun, sweeping spheres or cylinders, and building up
intermediate planes.

As natural selection acts only by the accumulation of slight
modifications of structure or instinct, each profitable to the
individual under its conditions of life, it may reasonably be asked, how
a long and graduated succession of modified architectural instincts,
all tending towards the present perfect plan of construction, could
have profited the progenitors of the hive-bee? I think the answer is not
difficult: cells constructed like those of the bee or the wasp gain in
strength, and save much in labour and space, and in the materials of
which they are constructed. With respect to the formation of wax, it is
known that bees are often hard pressed to get sufficient nectar; and
I am informed by Mr. Tegetmeier that it has been experimentally proved
that from twelve to fifteen pounds of dry sugar are consumed by a
hive of bees for the secretion of a pound of wax; so that a prodigious
quantity of fluid nectar must be collected and consumed by the bees in
a hive for the secretion of the wax necessary for the construction
of their combs. Moreover, many bees have to remain idle for many days
during the process of secretion. A large store of honey is indispensable
to support a large stock of bees during the winter; and the security
of the hive is known mainly to depend on a large number of bees being
supported. Hence the saving of wax by largely saving honey, and the
time consumed in collecting the honey, must be an important element of
success any family of bees. Of course the success of the species may
be dependent on the number of its enemies, or parasites, or on quite
distinct causes, and so be altogether independent of the quantity of
honey which the bees can collect. But let us suppose that this latter
circumstance determined, as it probably often has determined, whether
a bee allied to our humble-bees could exist in large numbers in any
country; and let us further suppose that the community lived through the
winter, and consequently required a store of honey: there can in
this case be no doubt that it would be an advantage to our imaginary
humble-bee if a slight modification of her instincts led her to make her
waxen cells near together, so as to intersect a little; for a wall in
common even to two adjoining cells would save some little labour and
wax. Hence, it would continually be more and more advantageous to our
humble-bees, if they were to make their cells more and more regular,
nearer together, and aggregated into a mass, like the cells of the
Melipona; for in this case a large part of the bounding surface of each
cell would serve to bound the adjoining cells, and much labour and wax
would be saved. Again, from the same cause, it would be advantageous to
the Melipona, if she were to make her cells closer together, and more
regular in every way than at present; for then, as we have seen, the
spherical surfaces would wholly disappear and be replaced by plane
surfaces; and the Melipona would make a comb as perfect as that of
the hive-bee. Beyond this stage of perfection in architecture, natural
selection could not lead; for the comb of the hive-bee, as far as we can
see, is absolutely perfect in economising labour and wax.

Thus, as I believe, the most wonderful of all known instincts, that
of the hive-bee, can be explained by natural selection having taken
advantage of numerous, successive, slight modifications of simpler
instincts; natural selection having, by slow degrees, more and more
perfectly led the bees to sweep equal spheres at a given distance from
each other in a double layer, and to build up and excavate the wax along
the planes of intersection. The bees, of course, no more knowing that
they swept their spheres at one particular distance from each other,
than they know what are the several angles of the hexagonal prisms and
of the basal rhombic plates; the motive power of the process of natural
selection having been the construction of cells of due strength and of
the proper size and shape for the larvae, this being effected with the
greatest possible economy of labour and wax; that individual swarm which
thus made the best cells with least labour, and least waste of honey
in the secretion of wax, having succeeded best, and having transmitted
their newly-acquired economical instincts to new swarms, which in their
turn will have had the best chance of succeeding in the struggle for
existence.

OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED TO INSTINCTS:
NEUTER AND STERILE INSECTS.

It has been objected to the foregoing view of the origin of instincts
that "the variations of structure and of instinct must have been
simultaneous and accurately adjusted to each other, as a modification
in the one without an immediate corresponding change in the other would
have been fatal." The force of this objection rests entirely on the
assumption that the changes in the instincts and structure are abrupt.
To take as an illustration the case of the larger titmouse, (Parus
major) alluded to in a previous chapter; this bird often holds the seeds
of the yew between its feet on a branch, and hammers with its beak till
it gets at the kernel. Now what special difficulty would there be in
natural selection preserving all the slight individual variations in the
shape of the beak, which were better and better adapted to break open
the seeds, until a beak was formed, as well constructed for this purpose
as that of the nuthatch, at the same time that habit, or compulsion, or
spontaneous variations of taste, led the bird to become more and more of
a seed-eater? In this case the beak is supposed to be slowly modified
by natural selection, subsequently to, but in accordance with, slowly
changing habits or taste; but let the feet of the titmouse vary and grow
larger from correlation with the beak, or from any other unknown cause,
and it is not improbable that such larger feet would lead the bird to
climb more and more until it acquired the remarkable climbing instinct
and power of the nuthatch. In this case a gradual change of structure is
supposed to lead to changed instinctive habits. To take one more case:
few instincts are more remarkable than that which leads the swift of
the Eastern Islands to make its nest wholly of inspissated saliva. Some
birds build their nests of mud, believed to be moistened with saliva;
and one of the swifts of North America makes its nest (as I have
seen) of sticks agglutinated with saliva, and even with flakes of this
substance. Is it then very improbable that the natural selection of
individual swifts, which secreted more and more saliva, should at last
produce a species with instincts leading it to neglect other materials
and to make its nest exclusively of inspissated saliva? And so in other
cases. It must, however, be admitted that in many instances we cannot
conjecture whether it was instinct or structure which first varied.

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 it is possible to reconcile this case
with the theory of natural selection?

First, let it be remembered that we have innumerable instances, both in
our domestic productions and in those in a state of nature, of all sorts
of differences of inherited structure which are correlated with certain
ages and with either sex. We have differences correlated not only with
one sex, but with that short period when the reproductive system is
active, as in the nuptial plumage of many birds, and in the hooked jaws
of the male salmon. We have even slight differences in the horns of
different breeds of cattle in relation to an artificially imperfect
state of the male sex; for oxen of certain breeds have longer horns than
the oxen of other breeds, relatively to the length of the horns in
both the bulls and cows of these same breeds. Hence, I can see no
great difficulty in any character becoming correlated with the sterile
condition of certain members of 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 gain the
desired end. Breeders of cattle wish the flesh and fat to be well
marbled together. An animal thus characterized has been slaughtered,
but the breeder has gone with confidence to the same stock and has
succeeded. Such faith may be placed in the power of selection that a
breed of cattle, always yielding oxen with extraordinarily long horns,
could, it is probable, be formed by carefully watching which individual
bulls and cows, when matched, produced oxen with the longest horns; and
yet no one ox would ever have propagated its kind. Here is a better and
real illustration: According to M. Verlot, some varieties of the double
annual stock, from having been long and carefully selected to the right
degree, always produce a large proportion of seedlings bearing double
and quite sterile flowers, but they likewise yield some single and
fertile plants. These latter, by which alone the variety can be
propagated, may be compared with the fertile male and female ants, and
the double sterile plants with the neuters of the same community. As
with the varieties of the stock, so with social insects, selection has
been applied to the family, and not to the individual, for the sake
of gaining a serviceable end. Hence, we may conclude that slight
modifications of structure or of instinct, correlated with the sterile
condition of certain members of the community, have proved advantageous;
consequently the fertile males and females have flourished, and
transmitted to their fertile offspring a tendency to produce sterile
members with the same modifications. This process must have been
repeated many times, until that prodigious amount of difference between
the fertile and sterile females of the same species has been produced
which we see in many social insects.

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 generally 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. Mr. F. Smith
has shown that the neuters of several British ants differ surprisingly
from each other in size and sometimes in colour; and that the extreme
forms can be linked together by individuals taken out of the same nest:
I have myself compared perfect gradations of this kind. It sometimes
happens that the larger or the smaller sized workers are the most
numerous; or that both large and small are numerous, while those of an
intermediate size are scanty in numbers. Formica flava has larger
and smaller workers, with some few of intermediate size; and, in this
species, as Mr. F. Smith has observed, the larger workers have simple
eyes (ocelli), which, though small, can be plainly distinguished,
whereas the smaller workers have their ocelli rudimentary. Having
carefully dissected several specimens of these workers, I can affirm
that the eyes are far more rudimentary in the smaller workers than can
be accounted for merely by their proportionately lesser size; and I
fully believe, though I dare not assert so positively, that the workers
of intermediate size have their ocelli in an exactly intermediate
condition. So that here we have two bodies of sterile workers in the
same nest, differing not only in size, but in their organs of vision,
yet connected by some few members in an intermediate condition. I may
digress by adding, that if the smaller workers had been the most useful
to the community, and those males and females had been continually
selected, which produced more and more of the smaller workers, until all
the workers were in this condition; we should then have had a species of
ant with neuters in nearly the same condition as those of Myrmica. For
the workers of Myrmica have not even rudiments of ocelli, though the
male and female ants of this genus have well-developed ocelli.

I may give one other case: so confidently did I expect occasionally to
find gradations of important structures between the different castes
of neuters in the same species, that I gladly availed myself of Mr. F.
Smith's offer of numerous specimens from the same nest of the driver
ant (Anomma) of West Africa. The reader will perhaps best appreciate
the amount of difference in these workers by my giving, not the actual
measurements, but a strictly accurate illustration: the difference was
the same as if we were to see a set of workmen building a house, of whom
many were five feet four inches high, and many sixteen feet high; but we
must in addition suppose that the larger workmen had heads four instead
of three times as big as those of the smaller men, and jaws nearly five
times as big. The jaws, moreover, of the working ants of the several
sizes differed wonderfully in shape, and in the form and number of the
teeth. But the important fact for us is that, though the workers can
be grouped into castes of different sizes, yet they graduate insensibly
into each other, as does the widely-different structure of their jaws. I
speak confidently on this latter point, as Sir J. Lubbock made drawings
for me, with the camera lucida, of the jaws which I dissected from the
workers of the several sizes. Mr. Bates, in his interesting "Naturalist
on the Amazons," has described analogous cases.

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.

An analogous explanation has been given by Mr. Wallace, of the equally
complex case, of certain Malayan butterflies regularly appearing under
two or even three distinct female forms; and by Fritz Muller, of certain
Brazilian crustaceans likewise appearing under two widely distinct male
forms. But this subject need not here be discussed.

I have now explained how, 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
labour is useful to civilised man. Ants, however, work by inherited
instincts and by inherited organs or tools, while 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 of 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 advanced this demonstrative
case of neuter insects, against the well-known doctrine of inherited
habit, as advanced by Lamarck.

SUMMARY.

I have endeavoured in this chapter briefly to show that the mental
qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that instincts
vary slightly in a state of nature. No one will dispute that instincts
are of the highest importance to each animal. Therefore, there is no
real difficulty, under changing conditions of life, in natural selection
accumulating to any extent slight modifications of instinct which are in
any way useful. In many cases habit or use and disuse have probably
come into play. I do not pretend that the facts given in this chapter
strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On the other
hand, the fact that instincts are not always absolutely perfect and are
liable to mistakes; that no instinct can be shown to have been produced
for the good of other animals, though animals take advantage of the
instincts of others; that the canon in natural history, of "Natura
non facit saltum," is applicable to instincts as well as to corporeal
structure, and is plainly explicable on the foregoing views, but is
otherwise inexplicable--all tend to corroborate the theory of natural
selection.

This theory is also strengthened by some few other facts in regard
to instincts; as by that common case of closely allied, but distinct,
species, when inhabiting distant parts of the world and living under
considerably different conditions of life, yet often retaining nearly
the same instincts. For instance, we can understand, on the principle of
inheritance, how it is that the thrush of tropical South America lines
its nest with mud, in the same peculiar manner as does our British
thrush; how it is that the Hornbills of Africa and India have the same
extraordinary instinct of plastering up and imprisoning the females in a
hole in a tree, with only a small hole left in the plaster through which
the males feed them and their young when hatched; how it is that the
male wrens (Troglodytes) of North America, build "cock-nests," to roost
in, like the males of our Kitty-wrens,--a habit wholly unlike that of
any other known bird. Finally, it may not be a logical deduction, but to
my imagination it is far more satisfactory to look at such instincts as
the young cuckoo ejecting its foster-brothers, ants making slaves, the
larvae of ichneumonidae feeding within the live bodies of caterpillars,
not as specially endowed or created instincts, but as small consequences
of one general law leading to the advancement of all organic
beings--namely, multiply, vary, let the strongest live and the weakest
die.



CHAPTER IX. HYBRIDISM.

 Distinction between the sterility of first crosses and of
 hybrids--Sterility various in degree, not universal, affected by close
 interbreeding, removed by domestication--Laws governing the sterility
 of hybrids--Sterility not a special endowment, but incidental on
 other differences, not accumulated by natural selection--Causes of
 the sterility of first crosses and of hybrids--Parallelism between the
 effects of changed conditions of life and of crossing--Dimorphism and
 trimorphism--Fertility of varieties when crossed and of their mongrel
 offspring not universal--Hybrids and mongrels compared independently of
 their fertility--Summary.


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 treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely, the
sterility of species when first crossed, and the sterility of the
hybrids produced from them.

Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no
offspring. Hybrids, on the other hand, have their reproductive organs
functionally impotent, as may be clearly seen in the state of the
male element in both plants and animals; though the formative organs
themselves are perfect in structure, as far as the microscope reveals.
In the first case the two sexual elements which go to form the embryo
are perfect; in the second case they are either not at all developed, or
are imperfectly developed. This distinction is important, when the
cause of the sterility, which is common to the two cases, has to be
considered. The distinction probably has been slurred over, owing to the
sterility in both cases being looked on as a special endowment, beyond
the province of our reasoning powers.

The fertility of varieties, that is of the forms known or believed to be
descended from common parents, when crossed, and likewise the fertility
of their mongrel offspring, is, with reference to my theory, of equal
importance with the sterility of species; for it seems to make a broad
and clear distinction between varieties and species.

DEGREES OF STERILITY.

First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of
those two conscientious and admirable observers, Kolreuter and Gartner,
who almost devoted their lives to this subject, without being deeply
impressed with the high generality of some degree of sterility.
Kolreuter makes the rule universal; but then he cuts the knot, for in
ten cases in which he found two forms, considered by most authors as
distinct species, quite fertile together, he unhesitatingly ranks them
as varieties. Gartner, also, makes the rule equally universal; and he
disputes the entire fertility of Kolreuter's ten cases. But in these and
in many other cases, Gartner is obliged carefully to count the seeds, in
order to show that there is any degree of sterility. He always compares
the maximum number of seeds produced by two species when first crossed,
and the maximum produced by their hybrid offspring, with the average
number produced by both pure parent-species in a state of nature. But
causes of serious error here intervene: a plant, to be hybridised, must
be castrated, and, what is often more important, must be secluded
in order to prevent pollen being brought to it by insects from other
plants. Nearly all the plants experimented on by Gartner were potted,
and were kept in a chamber in his house. That these processes are often
injurious to the fertility of a plant cannot be doubted; for Gartner
gives in his table about a score of cases of plants which he castrated,
and artificially fertilised with their own pollen, and (excluding
all cases such as the Leguminosae, in which there is an acknowledged
difficulty in the manipulation) half of these twenty plants had their
fertility in some degree impaired. Moreover, as Gartner repeatedly
crossed some forms, such as the common red and blue pimpernels
(Anagallis arvensis and coerulea), which the best botanists rank as
varieties, and found them absolutely sterile, we may doubt whether many
species are really so sterile, when intercrossed, as he believed.

It is certain, on the one hand, that the sterility of various species
when crossed is so different in degree and graduates away so insensibly,
and, on the other hand, that the fertility of pure species is so easily
affected by various circumstances, that for all practical purposes it is
most difficult to say where perfect fertility ends and sterility begins.
I think no better evidence of this can be required than that the two
most experienced observers who have ever lived, namely Kolreuter and
Gartner, arrived at diametrically opposite conclusions in regard to some
of the very same forms. It is also most instructive to compare--but I
have not space here to enter on details--the evidence advanced by our
best botanists on the question whether certain doubtful forms should be
ranked as species or varieties, with the evidence from fertility adduced
by different hybridisers, or by the same observer from experiments made
during different years. It can thus be shown that neither sterility nor
fertility affords any certain distinction between species and varieties.
The evidence from this source graduates away, and is doubtful in the
same degree as is the evidence derived from other constitutional and
structural differences.

In regard to the sterility of hybrids in successive generations; though
Gartner was enabled to rear some hybrids, carefully guarding them from a
cross with either pure parent, for six or seven, and in one case for
ten generations, yet he asserts positively that their fertility never
increases, but generally decreases greatly and suddenly. With respect
to this decrease, it may first be noticed that when any deviation in
structure or constitution is common to both parents, this is often
transmitted in an augmented degree to the offspring; and both sexual
elements in hybrid plants are already affected in some degree. But I
believe that their fertility has been diminished in nearly all these
cases by an independent cause, namely, by too close interbreeding. I
have made so many experiments and collected so many facts, showing on
the one hand that an occasional cross with a distinct individual or
variety increases the vigour and fertility of the offspring, and on
the other hand that very close interbreeding lessens their vigour and
fertility, that I cannot doubt the correctness of this conclusion.
Hybrids are seldom raised by experimentalists in great numbers; and as
the parent-species, or other allied hybrids, generally grow in the same
garden, the visits of insects must be carefully prevented during the
flowering season: hence hybrids, if left to themselves, will generally
be fertilised during each generation by pollen from the same flower; and
this would probably be injurious to their fertility, already lessened
by their hybrid origin. I am strengthened in this conviction by a
remarkable statement repeatedly made by Gartner, namely, that if even
the less fertile hybrids be artificially fertilised with hybrid pollen
of the same kind, their fertility, notwithstanding the frequent ill
effects from manipulation, sometimes decidedly increases, and goes on
increasing. Now, in the process of artificial fertilisation, pollen is
as often taken by chance (as I know from my own experience) from the
anthers of another flower, as from the anthers of the flower itself
which is to be fertilised; so that a cross between two flowers, though
probably often on the same plant, would be thus effected. Moreover,
whenever complicated experiments are in progress, so careful an observer
as Gartner would have castrated his hybrids, and this would have insured
in each generation a cross with pollen from a distinct flower, either
from the same plant or from another plant of the same hybrid nature.
And thus, the strange fact of an increase of fertility in the successive
generations of ARTIFICIALLY FERTILISED hybrids, in contrast with those
spontaneously self-fertilised, may, as I believe, be accounted for by
too close interbreeding having been avoided.

Now let us turn to the results arrived at by a third most experienced
hybridiser, namely, the Hon. and Rev. W. Herbert. He is as emphatic in
his conclusion that some hybrids are perfectly fertile--as fertile as
the pure parent-species--as are Kolreuter and Gartner that some degree
of sterility between distinct species is a universal law of nature.
He experimented on some of the very same species as did Gartner. The
difference in their results may, I think, be in part accounted for by
Herbert's great horticultural skill, and by his having hot-houses at his
command. Of his many important statements I will here give only a single
one as an example, namely, that "every ovule in a pod of Crinum capense
fertilised by C. revolutum produced a plant, which I never saw to occur
in a case of its natural fecundation." So that here we have perfect, or
even more than commonly perfect fertility, in a first cross between two
distinct species.

This case of the Crinum leads me to refer to a singular fact, namely,
that individual plants of certain species of Lobelia, Verbascum and
Passiflora, can easily be fertilised by the pollen from a distinct
species, but not by pollen from the same plant, though this pollen can
be proved to be perfectly sound by fertilising 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 Muller, 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 hybridised much more readily than they can be fertilised
by pollen from the same individual plant! To give one instance, a bulb
of Hippeastrum aulicum produced four flowers; three were fertilised
by Herbert with their own pollen, and the fourth was subsequently
fertilised by the pollen of a compound hybrid descended from three
distinct species: the result was that "the ovaries of the three first
flowers soon ceased to grow, and after a few days perished entirely,
whereas the pod impregnated by the pollen of the hybrid made vigorous
growth and rapid progress to maturity, and bore good seed, which
vegetated freely." Mr. Herbert tried similar experiments during many
years, and always with the same result. These cases serve to show on
what slight and mysterious causes the lesser or greater fertility of a
species sometimes depends.

The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, etc., have been crossed, yet many of these
hybrids seed freely. For instance, Herbert asserts that a hybrid from
Calceolaria integrifolia and plantaginea, species most widely dissimilar
in general habit, "reproduces itself as perfectly as if it had been a
natural species from the mountains of Chile." I have taken some pains
to ascertain the degree of fertility of some of the complex crosses of
Rhododendrons, and I am assured that many of them are perfectly fertile.
Mr. C. Noble, for instance, informs me that he raises stocks for
grafting from a hybrid between Rhod. ponticum and catawbiense, and that
this hybrid "seeds as freely as it is possible to imagine." Had hybrids,
when fairly treated, always gone on decreasing in fertility in each
successive generation, as Gartner believed to be the case, the fact
would have been notorious to nurserymen. Horticulturists raise large
beds of the same hybrid, and such alone are fairly treated, for by
insect agency the several individuals are allowed to cross freely with
each other, and the injurious influence of close interbreeding is thus
prevented. Any one may readily convince himself of the efficiency of
insect agency by examining the flowers of the more sterile kinds of
hybrid Rhododendrons, which produce no pollen, for he will find on their
stigmas plenty of pollen brought from other flowers.

In regard to animals, much fewer experiments have been carefully tried
than with plants. If our systematic arrangements can be trusted, that
is, 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. It
should, however, be borne in mind that, owing to few animals breeding
freely under confinement, few experiments have been fairly tried: for
instance, the canary-bird has been crossed with nine distinct species of
finches, but, as not one of these breeds freely in confinement, we have
no right to expect that the first crosses between them and the canary,
or that their hybrids, should be perfectly fertile. Again, with respect
to the fertility in successive generations of the more fertile hybrid
animals, I hardly know of an instance in which two families of the same
hybrid have been raised at the same time from different parents, so
as to avoid the ill effects of close interbreeding. On the contrary,
brothers and sisters have usually been crossed in each successive
generation, in opposition to the constantly repeated admonition of every
breeder. And in this case, it is not at all surprising that the inherent
sterility in the hybrids should have gone on increasing.

Although I know of hardly any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have reason to believe that the
hybrids from Cervulus vaginalis and Reevesii, and from Phasianus
colchicus with P. torquatus, are perfectly fertile. M. Quatrefages
states that the hybrids from two moths (Bombyx cynthia and arrindia)
were proved in Paris to be fertile inter se for eight generations. It
has lately been asserted that two such distinct species as the hare and
rabbit, when they can be got to breed together, produce offspring, which
are highly fertile when crossed with one of the parent-species. The
hybrids from the common and Chinese geese (A. cygnoides), species which
are so different that they are generally ranked in distinct genera, have
often bred in this country with either pure parent, and in one single
instance they have bred inter se. This was effected by Mr. Eyton, who
raised two hybrids from the same parents, but from different hatches;
and from these two birds he raised no less than eight hybrids
(grandchildren of the pure geese) from one nest. In India, however,
these cross-bred geese must be far more fertile; for I am assured by
two eminently capable judges, namely Mr. Blyth and Captain Hutton, that
whole flocks of these crossed geese are kept in various parts of
the country; and as they are kept for profit, where neither pure
parent-species exists, they must certainly be highly or perfectly
fertile.

With our domesticated animals, the various races when crossed together
are quite fertile; yet in many cases they are descended from two or more
wild species. From this fact we must conclude either that the aboriginal
parent-species at first produced perfectly fertile hybrids, or that the
hybrids subsequently reared under domestication became quite fertile.
This latter alternative, which was first propounded by Pallas, seems by
far the most probable, and can, indeed, hardly be doubted. It is, for
instance, almost certain that our dogs are descended from several wild
stocks; yet, with perhaps the exception of certain indigenous domestic
dogs of South America, all are quite fertile together; but analogy makes
me greatly doubt, whether the several aboriginal species would at first
have freely bred together and have produced quite fertile hybrids.
So again I have lately acquired decisive evidence that the crossed
offspring from the Indian humped and common cattle are inter se
perfectly fertile; and from the observations by Rutimeyer on their
important osteological differences, as well as from those by Mr. Blyth
on their differences in habits, voice, constitution, etc., these two
forms must be regarded as good and distinct species. The same remarks
may be extended to the two chief races of the pig. We must, therefore,
either give up the belief of the universal sterility of species when
crossed; or we must look at this sterility in animals, not as an
indelible characteristic, but as one capable of being removed by
domestication.

Finally, considering all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility,
both in first crosses and in hybrids, is an extremely general result;
but that it cannot, under our present state of knowledge, be considered
as absolutely universal.

LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

We will now consider a little more in detail the laws governing the
sterility of first crosses and of hybrids. Our chief object will be to
see whether or not these laws indicate that species have been specially
endowed with this quality, in order to prevent their crossing and
blending together in utter confusion. The following conclusions are
drawn up chiefly from Gartner's admirable work on the hybridisation
of plants. I have taken much pains to ascertain how far they apply
to animals, and, considering how scanty our knowledge is in regard to
hybrid animals, I have been surprised to find how generally the same
rules apply to both kingdoms.

It has been already remarked, that the degree of fertility, both of
first crosses and of hybrids, graduates from zero to perfect fertility.
It is surprising in how many curious ways this gradation can be shown;
but only the barest outline of the facts can here be given. When pollen
from a plant of one family is placed on the stigma of a plant of a
distinct family, it exerts no more influence than so much inorganic
dust. From this absolute zero of fertility, the pollen of different
species applied to the stigma of some one species of the same genus,
yields a perfect gradation in the number of seeds produced, up to nearly
complete or even quite complete fertility; and, as we have seen, in
certain abnormal cases, even to an excess of fertility, beyond that
which the plant's own pollen produces. So in hybrids themselves, there
are some which never have produced, and probably never would produce,
even with the pollen of the pure parents, a single fertile seed: but in
some of these cases a first trace of fertility may be detected, by
the pollen of one of the pure parent-species causing the flower of the
hybrid to wither earlier than it otherwise would have done; and the
early withering of the flower is well known to be a sign of incipient
fertilisation. From this extreme degree of sterility we have
self-fertilised hybrids producing a greater and greater number of seeds
up to perfect fertility.

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.

The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is that of pure species. But
the fertility of first crosses is likewise innately variable; for it
is not always the same in degree when the same two species are crossed
under the same circumstances; it depends in part upon the constitution
of the individuals which happen to have been chosen for the experiment.
So it is with hybrids, for their degree of fertility is often found to
differ greatly in the several individuals raised from seed out of the
same capsule and exposed to the same conditions.

By the term systematic affinity is meant, the general resemblance
between species in structure and constitution. Now the fertility
of first crosses, and of the hybrids produced from them, is largely
governed by their systematic affinity. This is clearly shown by hybrids
never having been raised between species ranked by systematists in
distinct families; and on the other hand, by very closely allied
species generally uniting with facility. But the correspondence between
systematic affinity and the facility of crossing is by no means strict.
A multitude of cases could be given of very closely allied species which
will not unite, or only with extreme difficulty; and on the other hand
of very distinct species which unite with the utmost facility. In
the same family there may be a genus, as Dianthus, in which very many
species can most readily be crossed; and another genus, as Silene,
in which the most persevering efforts have failed to produce between
extremely close species a single hybrid. Even within the limits of the
same genus, we meet with this same difference; for instance, the many
species of Nicotiana have been more largely crossed than the species of
almost any other genus; but Gartner found that N. acuminata, which is
not a particularly distinct species, obstinately failed to fertilise, or
to be fertilised, by no less than eight other species of Nicotiana. Many
analogous facts could be given.

No one has been able to point out what kind or what amount of
difference, in any recognisable character, is sufficient to prevent two
species crossing. It can be shown that plants most widely different in
habit and general appearance, and having strongly marked differences in
every part of the flower, even in the pollen, in the fruit, and in the
cotyledons, can be crossed. Annual and perennial plants, deciduous and
evergreen trees, plants inhabiting different stations and fitted for
extremely different climates, can often be crossed with ease.

By a reciprocal cross between two species, I mean the case, for
instance, of a female-ass being first crossed by a stallion, and then
a mare by a male-ass: these two species may then be said to have been
reciprocally crossed. There is often the widest possible difference
in the facility of making reciprocal crosses. Such cases are highly
important, for they prove that the capacity in any two species to cross
is often completely independent of their systematic affinity, that is
of any difference in their structure or constitution, excepting in their
reproductive systems. The diversity of the result in reciprocal crosses
between the same two species was long ago observed by Kolreuter. To give
an instance: Mirabilis jalapa can easily be fertilised by the pollen of
M. longiflora, and the hybrids thus produced are sufficiently fertile;
but Kolreuter tried more than two hundred times, during eight following
years, to fertilise reciprocally M. longiflora with the pollen of M.
jalapa, and utterly failed. Several other equally striking cases could
be given. Thuret has observed the same fact with certain sea-weeds
or Fuci. Gartner, moreover, found that this difference of facility in
making reciprocal crosses is extremely common in a lesser degree. He has
observed it even between closely related forms (as Matthiola annua
and glabra) which many botanists rank only as varieties. It is also a
remarkable fact that hybrids raised from reciprocal crosses, though of
course compounded of the very same two species, the one species having
first been used as the father and then as the mother, though they rarely
differ in external characters, yet generally differ in fertility in a
small, and occasionally in a high degree.

Several other singular rules could be given from Gartner: for instance,
some species have a remarkable power of crossing with other species;
other species of the same genus have a remarkable power of impressing
their likeness on their hybrid offspring; but these two powers do not at
all necessarily go together. There are certain hybrids which, instead
of having, as is usual, an intermediate character between their two
parents, always closely resemble one of them; and such hybrids, though
externally so like one of their pure parent-species, are with rare
exceptions extremely sterile. So again among hybrids which are usually
intermediate in structure between their parents, exceptional and
abnormal individuals sometimes are born, which closely resemble one of
their pure parents; and these hybrids are almost always utterly sterile,
even when the other hybrids raised from seed from the same capsule have
a considerable degree of fertility. These facts show how completely the
fertility of a hybrid may be independent of its external resemblance to
either pure parent.

Considering the several rules now given, which govern the fertility
of first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess; that their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is
innately variable; that it is by no means always the same in degree in
the first cross and in the hybrids produced from this cross; that the
fertility of hybrids is not related to the degree in which they resemble
in external appearance either parent; and lastly, that the facility of
making a first cross between any two species is not always governed by
their systematic affinity or degree of resemblance to each other. This
latter statement is clearly proved by the difference in the result of
reciprocal crosses between the same two species, for, according as the
one species or the other is used as the father or the mother, there
is generally some difference, and occasionally the widest possible
difference, in the facility of effecting an union. The hybrids,
moreover, produced from reciprocal crosses often differ in fertility.

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? Why should the degree of sterility be innately variable in
the individuals of the same species? Why should some species cross with
facility and yet produce very sterile hybrids; and other species cross
with extreme difficulty, and yet produce fairly fertile hybrids?
Why should there often be so great a difference in the result of a
reciprocal cross between the same two species? Why, it may even be
asked, has the production of hybrids been permitted? To grant to species
the special power of producing hybrids, and then to stop their further
propagation by different degrees of sterility, not strictly related to
the facility of the first union between their parents, seems a strange
arrangement.

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. It will be
advisable to explain a little more fully, by an example, what I mean
by sterility being incidental on other differences, and not a specially
endowed quality. 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
adaptation to widely different climates, does not always prevent the two
grafting together. As in hybridisation, 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 hybridisation, 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 distinct 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.

As Gartner found that there was sometimes an innate difference in
different INDIVIDUALS of the same two species in crossing; so Sagaret
believes this to be the case with different individuals of the same
two species in being grafted together. As in reciprocal crosses, the
facility of effecting an union is often very far from equal, so it
sometimes is in grafting. The common gooseberry, for instance, cannot
be grafted on the currant, whereas the currant will take, though with
difficulty, on the gooseberry.

We have seen that the sterility of hybrids which have their reproductive
organs in an imperfect condition, is a different case from the
difficulty of uniting two pure species, which have their reproductive
organs perfect; yet these two distinct classes of cases run to a large
extent parallel. Something analogous occurs in grafting; for Thouin
found that three species of Robinia, which seeded freely on their own
roots, and which could be grafted with no great difficulty on a fourth
species, when thus grafted were rendered barren. On the other hand,
certain species of Sorbus, when grafted on other species, yielded twice
as much fruit as when on their own roots. We are reminded by this latter
fact of the extraordinary cases of Hippeastrum, Passiflora, etc., which
seed much more freely when fertilised with the pollen of a distinct
species than when fertilised with pollen from the same plant.

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 distinct species.
And as 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. These
differences in both cases follow, to a certain extent, as might
have been expected, systematic affinity, by which term every kind of
resemblance and dissimilarity between organic beings is attempted to be
expressed. 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.

ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

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 the first place,
it may be remarked that species inhabiting distinct regions are often
sterile when crossed; now it could clearly have been of no advantage
to such separated species to have been rendered mutually sterile,
and consequently this could not have been effected through natural
selection; but it may perhaps be argued, that, if a species was rendered
sterile with some one compatriot, sterility with other species would
follow as a necessary contingency. In the second place, it is almost as
much opposed to the theory of natural selection as to that of special
creation, that in reciprocal crosses the male element of one form should
have been rendered utterly impotent on a second form, while at the same
time the male element of this second form is enabled freely to fertilise
the first form; for this peculiar state of the reproductive system could
hardly have been advantageous to either species.

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 bastardised 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 favour 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
towards 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. With
sterile neuter insects we have reason to believe that modifications in
their structure and fertility have been slowly accumulated by natural
selection, from an advantage having been thus indirectly given to the
community to which they belonged over other communities of the same
species; but an individual animal not belonging to a social community,
if rendered slightly sterile when crossed with some other variety, would
not thus itself gain any advantage or indirectly give any advantage
to the other individuals of the same variety, thus leading to their
preservation.

But it would be superfluous to discuss this question in detail: for with
plants we have conclusive evidence that the sterility of crossed species
must be due to some principle, quite independent of natural selection.
Both Gartner and Kolreuter have proved that in genera including numerous
species, a series can be formed from species which when crossed yield
fewer and fewer seeds, to species which never produce a single seed, but
yet are affected by the pollen of certain other species, for the germen
swells. It is here manifestly impossible to select the more sterile
individuals, which have already ceased to yield seeds; so that this acme
of sterility, when the germen alone is effected, cannot have been gained
through selection; and from the laws governing the various grades of
sterility being so uniform throughout the animal and vegetable kingdoms,
we may infer that the cause, whatever it may be, is the same or nearly
the same in all cases.

We will now look a little closer at the probable nature of the
differences between species which induce sterility in first crosses and
in hybrids. In the case of first crosses, the greater or less difficulty
in effecting a union and in obtaining offspring apparently depends
on several distinct causes. There must sometimes be a physical
impossibility in the male element reaching the ovule, as would be the
case with a plant having a pistil too long for the pollen-tubes to
reach the ovarium. It has also been observed that when the pollen of one
species is placed on the stigma of a distantly allied species, though
the pollen-tubes protrude, they do not penetrate the stigmatic surface.
Again, the male element may reach the female element, but be incapable
of causing an embryo to be developed, as seems to have been the case
with some of Thuret's experiments on Fuci. No explanation can be given
of these facts, any more than why certain trees cannot be grafted on
others. Lastly, an embryo may be developed, and then perish at an early
period. This latter alternative has not been sufficiently attended to;
but I believe, from observations communicated to me by Mr. Hewitt, who
has had great experience in hybridising pheasants and fowls, that the
early death of the embryo is a very frequent cause of sterility in first
crosses. Mr. Salter has recently given the results of an examination of
about 500 eggs produced from various crosses between three species
of Gallus and their hybrids; the majority of these eggs had been
fertilised; and in the majority of the fertilised eggs, the embryos had
either been partially developed and had then perished, or had become
nearly mature, but the young chickens had been unable to break through
the shell. Of the chickens which were born, more than four-fifths died
within the first few days, or at latest weeks, "without any obvious
cause, apparently from mere inability to live;" so that from the 500
eggs only twelve chickens were reared. With plants, hybridized embryos
probably often perish in a like manner; at least it is known that
hybrids raised from very distinct species are sometimes weak and
dwarfed, and perish at an early age; of which fact Max Wichura has
recently given some striking cases with hybrid willows. It may be here
worth noticing that in some cases of parthenogenesis, the embryos within
the eggs of silk moths which had not been fertilised, pass through their
early stages of development and then perish like the embryos produced by
a cross between distinct species. Until becoming acquainted with these
facts, I was unwilling to believe in the frequent early death of
hybrid embryos; for hybrids, when once born, are generally healthy and
long-lived, as we see in the case of the common mule. Hybrids, however,
are differently circumstanced before and after birth: when born and
living in a country where their two parents live, they are generally
placed under suitable conditions of life. But a hybrid partakes of only
half of the nature and constitution of its mother; it may therefore,
before birth, as long as it is nourished within its mother's womb, or
within the egg or seed produced by the mother, be exposed to conditions
in some degree unsuitable, and consequently be liable to perish at an
early period; more especially as all very young beings are eminently
sensitive to injurious or unnatural conditions of life. But after all,
the cause more probably lies in some imperfection in the original act
of impregnation, causing the embryo to be imperfectly developed, rather
than in the conditions to which it is subsequently exposed.

In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is somewhat different. I have more than
once alluded to a large body of facts showing that, when animals and
plants are removed from their natural conditions, they are extremely
liable to have their reproductive systems seriously affected. This,
in fact, is the great bar to the domestication of animals. Between the
sterility thus superinduced and that of hybrids, there are many points
of similarity. In both cases the sterility is independent of general
health, and is often accompanied by excess of size or great luxuriance.
In both cases the sterility occurs in various degrees; in both, the male
element is the most liable to be affected; but sometimes the female
more than the male. In both, the tendency goes to a certain extent with
systematic affinity, for whole groups of animals and plants are rendered
impotent by the same unnatural conditions; and whole groups of species
tend to produce sterile hybrids. On the other hand, one species in a
group will sometimes resist great changes of conditions with unimpaired
fertility; and certain species in a group will produce unusually fertile
hybrids. No one can tell till he tries, whether any particular animal
will breed under confinement, or any exotic plant seed freely under
culture; nor can he tell till he tries, whether any two species of a
genus will produce more or less sterile hybrids. Lastly, when organic
beings are placed during several generations under conditions not
natural to them, they are extremely liable to vary, which seems to be
partly due to their reproductive systems having been specially affected,
though in a lesser degree than when sterility ensues. So it is with
hybrids, for their offspring in successive generations are eminently
liable to vary, as every experimentalist has observed.

Thus we see that when organic beings are placed under new and unnatural
conditions, and when hybrids are produced by the unnatural crossing of
two species, the reproductive system, independently of the general state
of health, is affected in a very similar manner. In the one case, the
conditions of life have been disturbed, though often in so slight a
degree as to be inappreciable by us; in the other case, or that of
hybrids, the external conditions have remained the same, but the
organisation has been disturbed by two distinct structures and
constitutions, including of course the reproductive systems, having been
blended into one. For it is scarcely possible that two organisations
should be compounded into one, without some disturbance occurring in the
development, or periodical action, or mutual relations of the different
parts and organs one to another or to the conditions of life. When
hybrids are able to breed inter se, they transmit to their offspring
from generation to generation the same compounded organisation, and
hence we need not be surprised that their sterility, though in some
degree variable, does not diminish; it is even apt to increase,
this being generally the result, as before explained, of too close
interbreeding. The above view of the sterility of hybrids being caused
by two constitutions being compounded into one has been strongly
maintained by Max Wichura.

It must, however, be owned that we cannot understand, on the above or
any other view, several facts with respect to the sterility of hybrids;
for instance, the unequal fertility of hybrids produced from reciprocal
crosses; or the increased sterility in those hybrids which occasionally
and exceptionally resemble closely either pure parent. Nor do I pretend
that the foregoing remarks go to the root of the matter: no explanation
is offered why an organism, when placed under unnatural conditions,
is rendered sterile. All that I have attempted to show is, that in two
cases, in some respects allied, sterility is the common result--in the
one case from the conditions of life having been disturbed, in the other
case from the organisation having been disturbed by two organisations
being compounded into one.

A similar parallelism holds good with an allied yet very different
class of facts. It is an old and almost universal belief, founded on a
considerable body of evidence, which I have elsewhere given, that slight
changes in the conditions of life are beneficial to all living things.
We see this acted on by farmers and gardeners in their frequent
exchanges of seed, tubers, etc., from one soil or climate to another,
and back again. During the convalescence of animals, great benefit is
derived from almost any change in their habits of life. Again, both with
plants and animals, there is the clearest evidence that a cross between
individuals of the same species, which differ to a certain extent, gives
vigour and fertility to the offspring; and that close interbreeding
continued during several generations between the nearest relations, if
these be kept under the same conditions of life, almost always leads to
decreased size, weakness, or sterility.

Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is, crosses between the males and females of the same
species, which have been subjected to slightly different conditions, or
which have slightly varied, give vigour and fertility to the offspring.
But, as we have seen, organic beings long habituated to certain
uniform conditions under a state of nature, when subjected, as under
confinement, to a considerable change in their conditions, very
frequently are rendered more or less sterile; and we know that a cross
between two forms that have become widely or specifically different,
produce hybrids which are almost always in some degree sterile. I am
fully persuaded that this double parallelism is by no means an accident
or an illusion. He who is able to explain why the elephant, and a
multitude of other animals, are incapable of breeding when kept under
only partial confinement in their native country, will be able to
explain the primary cause of hybrids being so generally sterile. He will
at the same time be able to explain how it is that the races of some of
our domesticated animals, which have often been subjected to new and
not uniform conditions, are quite fertile together, although they are
descended from distinct species, which would probably have been sterile
if aboriginally crossed. The above two parallel series of facts seem
to be connected together by some common but unknown bond, which is
essentially related to the principle of life; this principle, according
to Mr. Herbert Spencer, being that life depends on, or consists in, the
incessant action and reaction of various forces, which, as throughout
nature, are always tending towards an equilibrium; and when this
tendency is slightly disturbed by any change, the vital forces gain in
power.

RECIPROCAL DIMORPHISM AND TRIMORPHISM.

This subject may be here briefly discussed, and will be found to throw
some light on hybridism. Several plants belonging to distinct orders
present two forms, which exist in about equal numbers and which differ
in no respect except in their reproductive organs; one form having
a long pistil with short stamens, the other a short pistil with long
stamens; the two having differently sized pollen-grains. With trimorphic
plants there are three forms likewise differing in the lengths of their
pistils and stamens, in the size and colour of the pollen-grains, and
in some other respects; and as in each of the three forms there are two
sets of stamens, the three forms possess altogether six sets of stamens
and three kinds of pistils. These organs are so proportioned in length
to each other that half the stamens in two of the forms stand on a level
with the stigma of the third form. Now I have shown, and the result
has been confirmed by other observers, that in order to obtain full
fertility with these plants, it is necessary that the stigma of the
one form should be fertilised by pollen taken from the stamens of
corresponding height in another form. So that with dimorphic species
two unions, which may be called legitimate, are fully fertile; and
two, which may be called illegitimate, are more or less infertile. With
trimorphic species six unions are legitimate, or fully fertile, and
twelve are illegitimate, or more or less infertile.

The infertility which may be observed in various dimorphic and
trimorphic plants, when they are illegitimately fertilised, that is by
pollen taken from stamens not corresponding in height with the pistil,
differs much in degree, up to absolute and utter sterility; just in the
same manner as occurs in crossing distinct species. As the degree
of sterility in the latter case depends in an eminent degree on the
conditions of life being more or less favourable, so I have found it
with illegitimate unions. It is well known that if pollen of a distinct
species be placed on the stigma of a flower, and its own pollen be
afterwards, even after a considerable interval of time, placed on the
same stigma, its action is so strongly prepotent that it generally
annihilates the effect of the foreign pollen; so it is with the pollen
of the several forms of the same species, for legitimate pollen is
strongly prepotent over illegitimate pollen, when both are placed on the
same stigma. I ascertained this by fertilising several flowers, first
illegitimately, and twenty-four hours afterwards legitimately, with
pollen taken from a peculiarly coloured variety, and all the seedlings
were similarly coloured; this shows that the legitimate pollen,
though applied twenty-four hours subsequently, had wholly destroyed
or prevented the action of the previously applied illegitimate pollen.
Again, as in making reciprocal crosses between the same two species,
there is occasionally a great difference in the result, so the same
thing occurs with trimorphic plants; for instance, the mid-styled form
of Lythrum salicaria was illegitimately fertilised with the greatest
ease by pollen from the longer stamens of the short-styled form, and
yielded many seeds; but the latter form did not yield a single seed when
fertilised by the longer stamens of the mid-styled form.

In all these respects, and in others which might be added, the forms
of the same undoubted species, when illegitimately united, behave in
exactly the same manner as do two distinct species when crossed. This
led me carefully to observe during four years many seedlings, raised
from several illegitimate unions. The chief result is that these
illegitimate plants, as they may be called, are not fully fertile. It
is possible to raise from dimorphic species, both long-styled and
short-styled illegitimate plants, and from trimorphic plants all three
illegitimate forms. These can then be properly united in a legitimate
manner. When this is done, there is no apparent reason why they
should not yield as many seeds as did their parents when legitimately
fertilised. But such is not the case. They are all infertile, in various
degrees; some being so utterly and incurably sterile that they did
not yield during four seasons a single seed or even seed-capsule. The
sterility of these illegitimate plants, when united with each other in
a legitimate manner, may be strictly compared with that of hybrids when
crossed inter se. If, on the other hand, a hybrid is crossed with either
pure parent-species, the sterility is usually much lessened: and so it
is when an illegitimate plant is fertilised by a legitimate plant. In
the same manner as the sterility of hybrids does not always run
parallel with the difficulty of making the first cross between the two
parent-species, so that sterility of certain illegitimate plants was
unusually great, while the sterility of the union from which they
were derived was by no means great. With hybrids raised from the same
seed-capsule the degree of sterility is innately variable, so it is in a
marked manner with illegitimate plants. Lastly, many hybrids are profuse
and persistent flowerers, while other and more sterile hybrids produce
few flowers, and are weak, miserable dwarfs; exactly similar cases occur
with the illegitimate offspring of various dimorphic and trimorphic
plants.

Altogether there is the closest identity in character and behaviour
between illegitimate plants and hybrids. It is hardly an exaggeration
to maintain that illegitimate plants are hybrids, produced within the
limits of the same species by the improper union of certain forms, while
ordinary hybrids are produced from an improper union between so-called
distinct species. We have also already seen that there is the closest
similarity in all respects between first illegitimate unions and first
crosses between distinct species. This will perhaps be made more fully
apparent by an illustration; we may suppose that a botanist found two
well-marked varieties (and such occur) of the long-styled form of the
trimorphic Lythrum salicaria, and that he determined to try by crossing
whether they were specifically distinct. He would find that they yielded
only about one-fifth of the proper number of seed, and that they behaved
in all the other above specified respects as if they had been two
distinct species. But to make the case sure, he would raise plants from
his supposed hybridised seed, and he would find that the seedlings were
miserably dwarfed and utterly sterile, and that they behaved in all
other respects like ordinary hybrids. He might then maintain that he
had actually proved, in accordance with the common view, that his two
varieties were as good and as distinct species as any in the world; but
he would be completely mistaken.

The facts now given on dimorphic and trimorphic plants are important,
because they show us, first, that the physiological test of lessened
fertility, both in first crosses and in hybrids, is no safe criterion
of specific distinction; secondly, because we may conclude that there is
some unknown bond which connects the infertility of illegitimate unions
with that of their illegitimate offspring, and we are led to extend the
same view to first crosses and hybrids; thirdly, because we find, and
this seems to me of especial importance, that two or three forms of the
same species may exist and may differ in no respect whatever, either in
structure or in constitution, relatively to external conditions, and yet
be sterile when united in certain ways. For we must remember that it is
the union of the sexual elements of individuals of the same form, for
instance, of two long-styled forms, which results in sterility; while it
is the union of the sexual elements proper to two distinct forms which
is fertile. Hence the case appears at first sight exactly the reverse
of what occurs, in the ordinary unions of the individuals of the same
species and with crosses between distinct species. It is, however,
doubtful whether this is really so; but I will not enlarge on this
obscure subject.

We may, however, infer as probable from the consideration of dimorphic
and trimorphic plants, that the sterility of distinct species when
crossed and of their hybrid progeny, depends exclusively on the nature
of their sexual elements, and not on any difference in their structure
or general constitution. We are also led to this same conclusion by
considering reciprocal crosses, in which the male of one species cannot
be united, or can be united with great difficulty, with the female of
a second species, while the converse cross can be effected with perfect
facility. That excellent observer, Gartner, likewise concluded that
species when crossed are sterile owing to differences confined to their
reproductive systems.

FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING, NOT
UNIVERSAL.

It may be urged as an overwhelming argument that there must be some
essential distinction between species and varieties inasmuch as the
latter, however much they may differ from each other in external
appearance, cross with perfect facility, and yield perfectly fertile
offspring. With some exceptions, presently to be given, I fully admit
that this is the rule. But the subject is surrounded by difficulties,
for, looking to varieties produced under nature, if two forms hitherto
reputed to be varieties be found in any degree sterile together, they
are at once ranked by most naturalists as species. For instance, the
blue and red pimpernel, which are considered by most botanists as
varieties, are said by Gartner to be quite sterile when crossed, and
he consequently ranks them as undoubted species. If we thus argue in
a circle, the fertility of all varieties produced under nature will
assuredly have to be granted.

If we turn to varieties, produced, or supposed to have been produced,
under domestication, we are still involved in some doubt. For when it
is stated, for instance, that certain South American indigenous domestic
dogs do not readily unite with European dogs, the explanation which will
occur to everyone, and probably the true one, is that they are descended
from aboriginally distinct species. Nevertheless the perfect fertility
of so many domestic races, differing widely from each other in
appearance, for instance, those of the pigeon, or of the cabbage, is a
remarkable fact; more especially when we reflect how many species there
are, which, though resembling each other most closely, are utterly
sterile when intercrossed. Several considerations, however, render the
fertility of domestic varieties less remarkable. In the first place,
it may be observed that the amount of external difference between two
species is no sure guide to their degree of mutual sterility, so that
similar differences in the case of varieties would be no sure guide. It
is certain that with species the cause lies exclusively in differences
in their sexual constitution. Now the varying conditions to which
domesticated animals and cultivated plants have been subjected, have had
so little tendency towards modifying the reproductive system in a manner
leading to mutual sterility, that we have good grounds for admitting
the directly opposite doctrine of Pallas, namely, that such conditions
generally eliminate this tendency; so that the domesticated descendants
of species, which in their natural state probably would have been in
some degree sterile when crossed, become perfectly fertile together.
With plants, so far is cultivation from giving a tendency towards
sterility between distinct species, that in several well-authenticated
cases already alluded to, certain plants have been affected in an
opposite manner, for they have become self-impotent, while still
retaining the capacity of fertilising, and being fertilised by, other
species. If the Pallasian doctrine of the elimination of sterility
through long-continued domestication be admitted, and it can hardly
be rejected, it becomes in the highest degree improbable that similar
conditions long-continued should likewise induce this tendency; though
in certain cases, with species having a peculiar constitution, sterility
might occasionally be thus caused. Thus, as I believe, we can understand
why, with domesticated animals, varieties have not been produced
which are mutually sterile; and why with plants only a few such cases,
immediately to be given, have been observed.

The real difficulty in our present subject is not, as it appears to me,
why domestic varieties have not become mutually infertile when crossed,
but why this has so generally occurred with natural varieties, as soon
as they have been permanently modified in a sufficient degree to take
rank as species. We are far from precisely knowing the cause; nor is
this surprising, seeing how profoundly ignorant we are in regard to the
normal and abnormal action of the reproductive system. But we can
see that species, owing to their struggle for existence with numerous
competitors, will have been exposed during long periods of time to more
uniform conditions, than have domestic varieties; and this may well make
a wide difference in the result. For we know how commonly wild animals
and plants, when taken from their natural conditions and subjected
to captivity, are rendered sterile; and the reproductive functions of
organic beings which have always lived under natural conditions would
probably in like manner be eminently sensitive to the influence of an
unnatural cross. Domesticated productions, on the other hand, which,
as shown by the mere fact of their domestication, were not originally
highly sensitive to changes in their conditions of life, and which can
now generally resist with undiminished fertility repeated changes of
conditions, might be expected to produce varieties, which would be
little liable to have their reproductive powers injuriously affected by
the act of crossing with other varieties which had originated in a like
manner.

I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it is impossible to resist the
evidence of the existence of a certain amount of sterility in the few
following cases, which I will briefly abstract. The evidence is at least
as good as that from which we believe in the sterility of a multitude of
species. The evidence is also derived from hostile witnesses, who in
all other cases consider fertility and sterility as safe criterions of
specific distinction. Gartner kept, during several years, a dwarf kind
of maize with yellow seeds, and a tall variety with red seeds growing
near each other in his garden; and although these plants have separated
sexes, they never naturally crossed. He then fertilised thirteen
flowers of the one kind with pollen of the other; but only a single
head produced any seed, and this one head produced only five grains.
Manipulation in this case could not have been injurious, as the plants
have separated sexes. No one, I believe, has suspected that these
varieties of maize are distinct species; and it is important to notice
that the hybrid plants thus raised were themselves PERFECTLY fertile;
so that even Gartner did not venture to consider the two varieties as
specifically distinct.

Girou de Buzareingues crossed three varieties of gourd, which like
the maize has separated sexes, and he asserts that their mutual
fertilisation is by so much the less easy as their differences are
greater. How far these experiments may be trusted, I know not; but
the forms experimented on are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties, and Naudin has
come to the same conclusion.

The following case is far more remarkable, and seems at first
incredible; but it is the result of an astonishing number of experiments
made during many years on nine species of Verbascum, by so good an
observer and so hostile a witness as Gartner: namely, that the yellow
and white varieties when crossed produce less seed than the similarly
coloured varieties of the same species. Moreover, he asserts that, when
yellow and white varieties of one species are crossed with yellow and
white varieties of a DISTINCT species, more seed is produced by the
crosses between the similarly coloured flowers, than between those which
are differently coloured. Mr. Scott also has experimented on the species
and varieties of Verbascum; and although unable to confirm Gartner's
results on the crossing of the distinct species, he finds that the
dissimilarly coloured varieties of the same species yield fewer seeds,
in the proportion of eighty-six to 100, than the similarly coloured
varieties. Yet these varieties differ in no respect, except in the
colour of their flowers; and one variety can sometimes be raised from
the seed of another.

Kolreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact that one particular variety
of the common tobacco was more fertile than the other varieties, when
crossed with a widely distinct species. He experimented on five forms
which are commonly reputed to be varieties, and which he tested by
the severest trial, namely, by reciprocal crosses, and he found their
mongrel offspring perfectly fertile. But one of these five varieties,
when used either as the father or mother, and crossed with the Nicotiana
glutinosa, always yielded hybrids not so sterile as those which were
produced from the four other varieties when crossed with N. glutinosa.
Hence the reproductive system of this one variety must have been in some
manner and in some degree modified.

From these facts it can no longer be maintained that varieties when
crossed are invariably quite fertile. From the great difficulty of
ascertaining the infertility of varieties in a state of nature, for a
supposed variety, if proved to be infertile in any degree, would almost
universally be ranked as a species; from man attending only to external
characters in his domestic varieties, and from such varieties not having
been exposed for very long periods to uniform conditions of life; from
these several considerations we may conclude that fertility does not
constitute a fundamental distinction between varieties and species when
crossed. The general sterility of crossed species may safely be looked
at, not as a special acquirement or endowment, but as incidental on
changes of an unknown nature in their sexual elements.

HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY.

Independently of the question of fertility, the offspring of species
and of varieties when crossed may be compared in several other respects.
Gartner, whose strong wish it was to draw a distinct line between
species and varieties, could find very few, and, as it seems to me,
quite unimportant differences between the so-called hybrid offspring of
species, and the so-called mongrel offspring of varieties. And, on the
other hand, they agree most closely in many important respects.

I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are
more variable than hybrids; but Gartner admits that hybrids from
species which have long been cultivated are often variable in the first
generation; and I have myself seen striking instances of this fact.
Gartner further admits that hybrids between very closely allied species
are more variable than those from very distinct species; and this shows
that the difference in the degree of variability graduates away.
When mongrels and the more fertile hybrids are propagated for several
generations, an extreme amount of variability in the offspring in both
cases is notorious; but some few instances of both hybrids and mongrels
long retaining a uniform character could be given. The variability,
however, in the successive generations of mongrels is, perhaps, greater
than in hybrids.

This greater variability in mongrels than in hybrids does not seem at
all surprising. For the parents of mongrels are varieties, and mostly
domestic varieties (very few experiments having been tried on natural
varieties), and this implies that there has been recent variability;
which would often continue and would augment that arising from the act
of crossing. The slight variability of hybrids in the first generation,
in contrast with that in the succeeding generations, is a curious fact
and deserves attention. For it bears on the view which I have taken of
one of the causes of ordinary variability; namely, that the reproductive
system, from being eminently sensitive to changed conditions of life,
fails under these circumstances to perform its proper function of
producing offspring closely similar in all respects to the parent-form.
Now, hybrids in the first generation are descended from species
(excluding those long cultivated) which have not had their reproductive
systems in any way affected, and they are not variable; but hybrids
themselves have their reproductive systems seriously affected, and their
descendants are highly variable.

But to return to our comparison of mongrels and hybrids: Gartner states
that mongrels are more liable than hybrids to revert to either parent
form; but this, if it be true, is certainly only a difference in degree.
Moreover, Gartner expressly states that the hybrids from long cultivated
plants are more subject to reversion than hybrids from species in their
natural state; and this probably explains the singular difference in
the results arrived at by different observers. Thus Max Wichura doubts
whether hybrids ever revert to their parent forms, and he experimented
on uncultivated species of willows, while Naudin, on the other hand,
insists in the strongest terms on the almost universal tendency to
reversion in hybrids, and he experimented chiefly on cultivated plants.
Gartner further states that when any two species, although most closely
allied to each other, are crossed with a third species, the hybrids are
widely different from each other; whereas if two very distinct varieties
of one species are crossed with another species, the hybrids do not
differ much. But this conclusion, as far as I can make out, is founded
on a single experiment; and seems directly opposed to the results of
several experiments made by Kolreuter.

Such alone are the unimportant differences which Gartner is able to
point out between hybrid and mongrel plants. On the other hand, the
degrees and kinds of resemblance in mongrels and in hybrids to their
respective parents, more especially in hybrids produced from nearly
related species, follow, according to Gartner the same laws. When two
species are crossed, one has sometimes a prepotent power of impressing
its likeness on the hybrid. So I believe it to be with varieties of
plants; and with animals, one variety certainly often has this prepotent
power over another variety. Hybrid plants produced from a reciprocal
cross generally resemble each other closely, and so it is with mongrel
plants from a reciprocal cross. Both hybrids and mongrels can be
reduced to either pure parent form, by repeated crosses in successive
generations with either parent.

These several remarks are apparently applicable to animals; but the
subject is here much complicated, partly owing to the existence of
secondary sexual characters; but more especially owing to prepotency
in transmitting likeness running more strongly in one sex than in
the other, both when one species is crossed with another and when one
variety is crossed with another variety. For instance, I think those
authors are right who maintain that the ass has a prepotent power over
the horse, so that both the mule and the hinny resemble more closely the
ass than the horse; but that the prepotency runs more strongly in the
male than in the female ass, so that the mule, which is an offspring of
the male ass and mare, is more like an ass than is the hinny, which is
the offspring of the female-ass and stallion.

Much stress has been laid by some authors on the supposed fact, that
it is only with mongrels that the offspring are not intermediate in
character, but closely resemble one of their parents; but this does
sometimes occur with hybrids, yet I grant much less frequently than
with mongrels. Looking to the cases which I have collected of cross-bred
animals closely resembling one parent, the resemblances seem chiefly
confined to characters almost monstrous in their nature, and which have
suddenly appeared--such as albinism, melanism, deficiency of tail or
horns, or additional fingers and toes; and do not relate to characters
which have been slowly acquired through selection. A tendency to sudden
reversions to the perfect character of either parent would, also,
be much more likely to occur with mongrels, which are descended from
varieties often suddenly produced and semi-monstrous in character, than
with hybrids, which are descended from species slowly and naturally
produced. On the whole, I entirely agree with Dr. Prosper Lucas, who,
after arranging an enormous body of facts with respect to animals,
comes to the conclusion that the laws of resemblance of the child to its
parents are the same, whether the two parents differ little or much from
each other, namely, in the union of individuals of the same variety, or
of different varieties, or of distinct species.

Independently of the question of fertility and sterility, in all
other respects there seems to be a general and close similarity in the
offspring of crossed species, and of crossed varieties. If we look at
species as having been specially created, and at varieties as having
been produced by secondary laws, this similarity would be an astonishing
fact. But it harmonises perfectly with the view that there is no
essential distinction between species and varieties.

SUMMARY OF CHAPTER.

First crosses between forms, sufficiently distinct to be ranked as
species, and their hybrids, are very generally, but not universally,
sterile. The sterility is of all degrees, and is often so slight that
the most careful experimentalists have arrived at diametrically opposite
conclusions in ranking forms by this test. The sterility is innately
variable in individuals of the same species, and is eminently
susceptible to action of favourable and unfavourable conditions. The
degree of sterility does not strictly follow systematic affinity, but is
governed by several curious and complex laws. It is generally different,
and sometimes widely different in reciprocal crosses between the same
two species. It is not always equal in degree in a first cross and in
the hybrids produced from this cross.

In the same manner as in grafting trees, the capacity in one species or
variety to take on another, is incidental on differences, generally
of an unknown nature, in their vegetative systems, so in crossing,
the greater or less facility of one species to unite with another is
incidental on unknown differences in their reproductive systems. There
is no more reason to think that species have been specially endowed with
various degrees of sterility to prevent their crossing and blending
in nature, than to think that trees have been specially endowed with
various and somewhat analogous degrees of difficulty in being grafted
together in order to prevent their inarching in our forests.

The sterility of first crosses and of their hybrid progeny has not been
acquired through natural selection. In the case of first crosses it
seems to depend on several circumstances; in some instances in chief
part on the early death of the embryo. In the case of hybrids, it
apparently depends on their whole organisation having been disturbed by
being compounded from two distinct forms; the sterility being closely
allied to that which so frequently affects pure species, when exposed to
new and unnatural conditions of life. He who will explain these latter
cases will be able to explain the sterility of hybrids. This view is
strongly supported by a parallelism of another kind: namely, that,
firstly, slight changes in the conditions of life add to the vigour
and fertility of all organic beings; and secondly, that the crossing of
forms, which have been exposed to slightly different conditions of life,
or which have varied, favours the size, vigour and fertility of their
offspring. The facts given on the sterility of the illegitimate unions
of dimorphic and trimorphic plants and of their illegitimate progeny,
perhaps render it probable that some unknown bond in all cases connects
the degree of fertility of first unions with that of their offspring.
The consideration of these facts on dimorphism, as well as of the
results of reciprocal crosses, clearly leads to the conclusion that
the primary cause of the sterility of crossed species is confined to
differences in their sexual elements. But why, in the case of distinct
species, the sexual elements should so generally have become more or
less modified, leading to their mutual infertility, we do not know; but
it seems to stand in some close relation to species having been exposed
for long periods of time to nearly uniform conditions of life.

It is not surprising that the difficulty in crossing any two species,
and the sterility of their hybrid offspring, should in most cases
correspond, even if due to distinct causes: for both depend on the
amount of difference between the species which are crossed. Nor is
it surprising that the facility of effecting a first cross, and the
fertility of the hybrids thus produced, and the capacity of being
grafted together--though this latter capacity evidently depends on
widely different circumstances--should all run, to a certain extent,
parallel with the systematic affinity of the forms subjected to
experiment; for systematic affinity includes resemblances of all kinds.

First crosses between forms known to be varieties, or sufficiently alike
to be considered as varieties, and their mongrel offspring, are very
generally, but not, as is so often stated, invariably fertile. Nor
is this almost universal and perfect fertility surprising, when it
is remembered how liable we are to argue in a circle with respect to
varieties in a state of nature; and when we remember that the greater
number of varieties have been produced under domestication by the
selection of mere external differences, and that they have not been long
exposed to uniform conditions of life. It should also be especially kept
in mind, that long-continued domestication tends to eliminate
sterility, and is therefore little likely to induce this same quality.
Independently of the question of fertility, in all other respects there
is the closest general resemblance between hybrids and mongrels, in
their variability, in their power of absorbing each other by repeated
crosses, and in their inheritance of characters from both parent-forms.
Finally, then, although we are as ignorant of the precise cause of the
sterility of first crosses and of hybrids as we are why animals and
plants removed from their natural conditions become sterile, yet the
facts given in this chapter do not seem to me opposed to the belief that
species aboriginally existed as varieties.



CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

 On the absence of intermediate varieties at the present day--On the
 nature of extinct intermediate varieties; on their number--On the lapse
 of time, as inferred from the rate of denudation and of deposition
 number--On the lapse of time as estimated by years--On the poorness of
 our palaeontological collections--On the intermittence of geological
 formations--On the denudation of granitic areas--On the absence of
 intermediate varieties in any one formation--On the sudden appearance
 of groups of species--On their sudden appearance in the lowest known
 fossiliferous strata--Antiquity of the habitable earth.


In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely, the distinctness of specific forms
and their not being blended together by innumerable transitional links,
is a very obvious difficulty. I assigned reasons why such links do not
commonly occur at the present day under the circumstances apparently
most favourable for their presence, namely, on an extensive and
continuous area with graduated physical conditions. I endeavoured to
show, that the life of each species depends in a more important manner
on the presence of other already defined organic forms, than on climate,
and, therefore, that the really governing conditions of life do not
graduate away quite insensibly like heat or moisture. I endeavoured,
also, to show that intermediate varieties, from existing in lesser
numbers than the forms which they connect, will generally be beaten
out and exterminated during the course of further modification and
improvement. The main cause, however, of innumerable intermediate links
not now occurring everywhere throughout nature depends, on the very
process of natural selection, through which new varieties continually
take the places of and supplant their parent-forms. But just in
proportion as this process of extermination has acted on an enormous
scale, so must the number of intermediate varieties, which have formerly
existed, be truly enormous. Why then is not every geological formation
and every stratum full of such intermediate links? Geology assuredly
does not reveal any such finely graduated organic chain; and this,
perhaps, is the most obvious and serious objection which can be urged
against my theory. The explanation lies, as I believe, in the extreme
imperfection of the geological record.

In the first place, it should always be borne in mind what sort of
intermediate forms must, on the theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid picturing
to myself forms DIRECTLY intermediate between them. But this is a wholly
false view; we should always look for forms intermediate between each
species and a common but unknown progenitor; and the progenitor
will generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter
pigeons are both descended from the rock-pigeon; if we possessed all
the intermediate varieties which have ever existed, we should have an
extremely close series between both and the rock-pigeon; but we should
have no varieties directly intermediate between the fantail and pouter;
none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds.
These two breeds, moreover, have become so much modified, that, if we
had no historical or indirect evidence regarding their origin, it would
not have been possible to have determined from a mere comparison of
their structure with that of the rock-pigeon, C. livia, whether they had
descended from this species or from some other allied species, such as
C. oenas.

So with natural species, if we look to forms very distinct, for instance
to the horse and tapir, we have no reason to suppose that links directly
intermediate between them ever existed, but between each and an unknown
common parent. The common parent will have had in its whole organisation
much general resemblance to the tapir and to the horse; but in some
points of structure may have differed considerably from both, even
perhaps more than they differ from each other. Hence, in all such cases,
we should be unable to recognise the parent-form of any two or more
species, even if we closely compared the structure of the parent with
that of its modified descendants, unless at the same time we had a
nearly perfect chain of the intermediate links.

It is just possible, by the theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and
in this case DIRECT intermediate links will have existed between them.
But such a case would imply that one form had remained for a very long
period unaltered, whilst its descendants had undergone a vast amount of
change; and the principle of competition between organism and organism,
between child and parent, will render this a very rare event; for in all
cases the new and improved forms of life tend to supplant the old and
unimproved forms.

By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences not
greater than we see between the natural and domestic varieties of the
same species at the present day; and these parent-species, now generally
extinct, have in their turn been similarly connected with more ancient
forms; and so on backwards, always converging to the common ancestor of
each great class. So that the number of intermediate and transitional
links, between all living and extinct species, must have been
inconceivably great. But assuredly, if this theory be true, such have
lived upon the earth.

ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF DEPOSITION AND EXTENT
OF DENUDATION.

Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected that time cannot have
sufficed for so great an amount of organic change, all changes having
been effected slowly. It is hardly possible for me to recall to the
reader who is not a practical geologist, the facts leading the mind
feebly to comprehend the lapse of time. He who can read Sir Charles
Lyell's grand work on the Principles of Geology, which the future
historian will recognise as having produced a revolution in natural
science, and yet does not admit how vast have been the past periods of
time, may at once close this volume. Not that it suffices to study
the Principles of Geology, or to read special treatises by different
observers on separate formations, and to mark how each author attempts
to give an inadequate idea of the duration of each formation, or even
of each stratum. We can best gain some idea of past time by knowing the
agencies at work; and learning how deeply the surface of the land has
been denuded, and how much sediment has been deposited. As Lyell has
well remarked, the extent and thickness of our sedimentary formations
are the result and the measure of the denudation which the earth's crust
has elsewhere undergone. Therefore a man should examine for himself the
great piles of superimposed strata, and watch the rivulets bringing down
mud, and the waves wearing away the sea-cliffs, in order to comprehend
something about the duration of past time, the monuments of which we see
all around us.

It is good to wander along the coast, when formed of moderately hard
rocks, and mark the process of degradation. The tides in most cases
reach the cliffs only for a short time twice a day, and the waves eat
into them only when they are charged with sand or pebbles; for there is
good evidence that pure water effects nothing in wearing away rock. At
last the base of the cliff is undermined, huge fragments fall down, and
these remaining fixed, have to be worn away atom by atom, until after
being reduced in size they can be rolled about by the waves, and then
they are more quickly ground into pebbles, sand, or mud. But how often
do we see along the bases of retreating cliffs rounded boulders, all
thickly clothed by marine productions, showing how little they are
abraded and how seldom they are rolled about! Moreover, if we follow for
a few miles any line of rocky cliff, which is undergoing degradation,
we find that it is only here and there, along a short length or round
a promontory, that the cliffs are at the present time suffering. The
appearance of the surface and the vegetation show that elsewhere years
have elapsed since the waters washed their base.

We have, however, recently learned from the observations of Ramsay, in
the van of many excellent observers--of Jukes, Geikie, Croll and
others, that subaerial degradation is a much more important agency than
coast-action, or the power of the waves. The whole surface of the land
is exposed to the chemical action of the air and of the rainwater,
with its dissolved carbonic acid, and in colder countries to frost; the
disintegrated matter is carried down even gentle slopes during heavy
rain, and to a greater extent than might be supposed, especially in
arid districts, by the wind; it is then transported by the streams
and rivers, which, when rapid deepen their channels, and triturate the
fragments. On a rainy day, even in a gently undulating country, we see
the effects of subaerial degradation in the muddy rills which flow down
every slope. Messrs. Ramsay and Whitaker have shown, and the observation
is a most striking one, that the great lines of escarpment in the
Wealden district and those ranging across England, which formerly were
looked at as ancient sea-coasts, cannot have been thus formed, for each
line is composed of one and the same formation, while our sea-cliffs are
everywhere formed by the intersection of various formations. This being
the case, we are compelled to admit that the escarpments owe their
origin in chief part to the rocks of which they are composed, having
resisted subaerial denudation better than the surrounding surface;
this surface consequently has been gradually lowered, with the lines of
harder rock left projecting. Nothing impresses the mind with the vast
duration of time, according to our ideas of time, more forcibly than the
conviction thus gained that subaerial agencies, which apparently have
so little power, and which seem to work so slowly, have produced great
results.

When thus impressed with the slow rate at which the land is worn
away through subaerial and littoral action, it is good, in order to
appreciate the past duration of time, to consider, on the one hand, the
masses of rock which have been removed over many extensive areas, and on
the other hand the thickness of our sedimentary formations. I remember
having been much struck when viewing volcanic islands, which have been
worn by the waves and pared all round into perpendicular cliffs of
one or two thousand feet in height; for the gentle slope of the
lava-streams, due to their formerly liquid state, showed at a glance how
far the hard, rocky beds had once extended into the open ocean. The same
story is told still more plainly by faults--those great cracks along
which the strata have been upheaved on one side, or thrown down on the
other, to the height or depth of thousands of feet; for since the crust
cracked, and it makes no great difference whether the upheaval was
sudden, or, as most geologists now believe, was slow and effected by
many starts, the surface of the land has been so completely planed down
that no trace of these vast dislocations is externally visible. The
Craven fault, for instance, extends for upward of thirty miles, and
along this line the vertical displacement of the strata varies from 600
to 3,000 feet. Professor Ramsay has published an account of a downthrow
in Anglesea of 2,300 feet; and he informs me that he fully believes that
there is one in Merionethshire of 12,000 feet; yet in these cases there
is nothing on the surface of the land to show such prodigious movements;
the pile of rocks on either side of the crack having been smoothly swept
away.

On the other hand, in all parts of the world the piles of sedimentary
strata are of wonderful thickness. In the Cordillera, I estimated one
mass of conglomerate at ten thousand feet; and although conglomerates
have probably been accumulated at a quicker rate than finer sediments,
yet from being formed of worn and rounded pebbles, each of which bears
the stamp of time, they are good to show how slowly the mass must
have been heaped together. Professor Ramsay has given me the maximum
thickness, from actual measurement in most cases, of the successive
formations in DIFFERENT parts of Great Britain; and this is the
result:--

                                                  Feet

 Palaeozoic strata (not including igneous beds)..57,154
 Secondary strata................................13,190
 Tertiary strata..................................2,240

 --making altogether 72,584 feet;


that is, very nearly thirteen and three-quarters British miles.
Some of these formations, which are represented in England by thin beds,
are thousands of feet in thickness on the Continent. Moreover, between
each successive formation we have, in the opinion of most geologists,
blank periods of enormous length. So that the lofty pile of sedimentary
rocks in Britain gives but an inadequate idea of the time which has
elapsed during their accumulation. The consideration of these various
facts impresses the mind almost in the same manner as does the vain
endeavour to grapple with the idea of eternity.

Nevertheless this impression is partly false. Mr. Croll, in an
interesting paper, remarks that we do not err "in forming too great a
conception of the length of geological periods," but in estimating them
by years. When geologists look at large and complicated phenomena, and
then at the figures representing several million years, the two produce
a totally different effect on the mind, and the figures are at once
pronounced too small. In regard to subaerial denudation, Mr. Croll
shows, by calculating the known amount of sediment annually brought down
by certain rivers, relatively to their areas of drainage, that 1,000
feet of solid rock, as it became gradually disintegrated, would thus
be removed from the mean level of the whole area in the course of six
million years. This seems an astonishing result, and some considerations
lead to the suspicion that it may be too large, but if halved or
quartered it is still very surprising. Few of us, however, know what a
million really means: Mr. Croll gives the following illustration: Take
a narrow strip of paper, eighty-three feet four inches in length, and
stretch it along the wall of a large hall; then mark off at one end
the tenth of an inch. This tenth of an inch will represent one hundred
years, and the entire strip a million years. But let it be borne in
mind, in relation to the subject of this work, what a hundred years
implies, represented as it is by a measure utterly insignificant in a
hall of the above dimensions. Several eminent breeders, during a single
lifetime, have so largely modified some of the higher animals, which
propagate their kind much more slowly than most of the lower animals,
that they have formed what well deserves to be called a new sub-breed.
Few men have attended with due care to any one strain for more than half
a century, so that a hundred years represents the work of two breeders
in succession. It is not to be supposed that species in a state of
nature ever change so quickly as domestic animals under the guidance of
methodical selection. The comparison would be in every way fairer
with the effects which follow from unconscious selection, that is, the
preservation of the most useful or beautiful animals, with no intention
of modifying the breed; but by this process of unconscious selection,
various breeds have been sensibly changed in the course of two or three
centuries.

Species, however, probably change much more slowly, and within the same
country only a few change at the same time. This slowness follows from
all the inhabitants of the same country being already so well adapted to
each other, that new places in the polity of nature do not occur until
after long intervals, due to the occurrence of physical changes of some
kind, or through the immigration of new forms. Moreover, variations
or individual differences of the right nature, by which some of the
inhabitants might be better fitted to their new places under the altered
circumstance, would not always occur at once. Unfortunately we have no
means of determining, according to the standard of years, how long a
period it takes to modify a species; but to the subject of time we must
return.

ON THE POORNESS OF PALAEONTOLOGICAL COLLECTIONS.

Now let us turn to our richest museums, and what a paltry display we
behold! That our collections are imperfect is admitted by every one. The
remark of that admirable palaeontologist, Edward Forbes, should never
be forgotten, namely, that very many fossil species are known and
named from single and often broken specimens, or from a few specimens
collected on some one spot. Only a small portion of the surface of the
earth has been geologically explored, and no part with sufficient
care, as the important discoveries made every year in Europe prove.
No organism wholly soft can be preserved. Shells and bones decay and
disappear when left on the bottom of the sea, where sediment is not
accumulating. We probably take a quite erroneous view, when we assume
that sediment is being deposited over nearly the whole bed of the sea,
at a rate sufficiently quick to embed and preserve fossil remains.
Throughout an enormously large proportion of the ocean, the bright blue
tint of the water bespeaks its purity. The many cases on record of a
formation conformably covered, after an immense interval of time, by
another and later formation, without the underlying bed having suffered
in the interval any wear and tear, seem explicable only on the view
of the bottom of the sea not rarely lying for ages in an unaltered
condition. The remains which do become embedded, if in sand or gravel,
will, when the beds are upraised, generally be dissolved by the
percolation of rain water charged with carbonic acid. Some of the many
kinds of animals which live on the beach between high and low water mark
seem to be rarely preserved. For instance, the several species of the
Chthamalinae (a sub-family of sessile cirripedes) coat the rocks all
over the world in infinite numbers: they are all strictly littoral, with
the exception of a single Mediterranean species, which inhabits deep
water and this has been found fossil in Sicily, whereas not one other
species has hitherto been found in any tertiary formation: yet it is
known that the genus Chthamalus existed during the Chalk period.
Lastly, many great deposits, requiring a vast length of time for their
accumulation, are entirely destitute of organic remains, without our
being able to assign any reason: one of the most striking instances is
that of the Flysch formation, which consists of shale and sandstone,
several thousand, occasionally even six thousand feet in thickness,
and extending for at least 300 miles from Vienna to Switzerland; and
although this great mass has been most carefully searched, no fossils,
except a few vegetable remains, have been found.

With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that
our evidence is fragmentary in an extreme degree. For instance, until
recently not a land-shell was known belonging to either of these vast
periods, with the exception of one species discovered by Sir C. Lyell
and Dr. Dawson in the carboniferous strata of North America; but now
land-shells have been found in the lias. In regard to mammiferous
remains, a glance at the historical table published in Lyell's
Manual, will bring home the truth, how accidental and rare is their
preservation, far better than pages of detail. Nor is their rarity
surprising, when we remember how large a proportion of the bones of
tertiary mammals have been discovered either in caves or in lacustrine
deposits; and that not a cave or true lacustrine bed is known belonging
to the age of our secondary or palaeozoic formations.

But the imperfection in the geological record largely results from
another and more important cause than any of the foregoing; namely, from
the several formations being separated from each other by wide intervals
of time. This doctrine has been emphatically admitted by many geologists
and palaeontologists, who, like E. Forbes, entirely disbelieve in the
change of species. When we see the formations tabulated in written
works, or when we follow them in nature, it is difficult to avoid
believing that they are closely consecutive. But we know, for instance,
from Sir R. Murchison's great work on Russia, what wide gaps there are
in that country between the superimposed formations; so it is in
North America, and in many other parts of the world. The most skilful
geologist, if his attention had been confined exclusively to these large
territories, would never have suspected that during the periods which
were blank and barren in his own country, great piles of sediment,
charged with new and peculiar forms of life, had elsewhere been
accumulated. And if, in every separate territory, hardly any idea can be
formed of the length of time which has elapsed between the consecutive
formations, we may infer that this could nowhere be ascertained.
The frequent and great changes in the mineralogical composition
of consecutive formations, generally implying great changes in the
geography of the surrounding lands, whence the sediment was derived,
accord with the belief of vast intervals of time having elapsed between
each formation.

We can, I think, see why the geological formations of each region are
almost invariably intermittent; that is, have not followed each other
in close sequence. Scarcely any fact struck me more when examining many
hundred miles of the South American coasts, which have been upraised
several hundred feet within the recent period, than the absence of
any recent deposits sufficiently extensive to last for even a short
geological period. Along the whole west coast, which is inhabited by
a peculiar marine fauna, tertiary beds are so poorly developed that no
record of several successive and peculiar marine faunas will probably be
preserved to a distant age. A little reflection will explain why, along
the rising coast of the western side of South America, no extensive
formations with recent or tertiary remains can anywhere be found, though
the supply of sediment must for ages have been great, from the enormous
degradation of the coast rocks and from the muddy streams entering the
sea. The explanation, no doubt, is that the littoral and sub-littoral
deposits are continually worn away, as soon as they are brought up by
the slow and gradual rising of the land within the grinding action of
the coast-waves.

We may, I think, conclude that sediment must be accumulated in extremely
thick, solid, or extensive masses, in order to withstand the incessant
action of the waves, when first upraised and during subsequent
oscillations of level, as well as the subsequent subaerial degradation.
Such thick and extensive accumulations of sediment may be formed in two
ways; either in profound depths of the sea, in which case the bottom
will not be inhabited by so many and such varied forms of life as the
more shallow seas; and the mass when upraised will give an imperfect
record of the organisms which existed in the neighbourhood during
the period of its accumulation. Or sediment may be deposited to any
thickness and extent over a shallow bottom, if it continue slowly to
subside. In this latter case, as long as the rate of subsidence and
supply of sediment nearly balance each other, the sea will remain
shallow and favourable for many and varied forms, and thus a rich
fossiliferous formation, thick enough, when upraised, to resist a large
amount of denudation, may be formed.

I am convinced that nearly all our ancient formations, which are
throughout the greater part of their thickness RICH IN FOSSILS, have
thus been formed during subsidence. Since publishing my views on this
subject in 1845, I have watched the progress of geology, and have been
surprised to note how author after author, in treating of this or that
great formation, has come to the conclusion that it was accumulated
during subsidence. I may add, that the only ancient tertiary formation
on the west coast of South America, which has been bulky enough to
resist such degradation as it has as yet suffered, but which will
hardly last to a distant geological age, was deposited during a downward
oscillation of level, and thus gained considerable thickness.

All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscillations
have affected wide spaces. Consequently, formations rich in fossils and
sufficiently thick and extensive to resist subsequent degradation, will
have been formed over wide spaces during periods of subsidence, but only
where the supply of sediment was sufficient to keep the sea shallow and
to embed and preserve the remains before they had time to decay. On the
other hand, as long as the bed of the sea remained stationary, THICK
deposits cannot have been accumulated in the shallow parts, which are
the most favourable to life. Still less can this have happened during
the alternate periods of elevation; or, to speak more accurately, the
beds which were then accumulated will generally have been destroyed by
being upraised and brought within the limits of the coast-action.

These remarks apply chiefly to littoral and sublittoral deposits. In the
case of an extensive and shallow sea, such as that within a large part
of the Malay Archipelago, where the depth varies from thirty or forty
to sixty fathoms, a widely extended formation might be formed during
a period of elevation, and yet not suffer excessively from denudation
during its slow upheaval; but the thickness of the formation could not
be great, for owing to the elevatory movement it would be less than
the depth in which it was formed; nor would the deposit be much
consolidated, nor be capped by overlying formations, so that it would
run a good chance of being worn away by atmospheric degradation and by
the action of the sea during subsequent oscillations of level. It has,
however, been suggested by Mr. Hopkins, that if one part of the area,
after rising and before being denuded, subsided, the deposit formed
during the rising movement, though not thick, might afterwards become
protected by fresh accumulations, and thus be preserved for a long
period.

Mr. Hopkins also expresses his belief that sedimentary beds of
considerable horizontal extent have rarely been completely destroyed.
But all geologists, excepting the few who believe that our present
metamorphic schists and plutonic rocks once formed the primordial
nucleus of the globe, will admit that these latter rocks have been
stripped of their covering to an enormous extent. For it is scarcely
possible that such rocks could have been solidified and crystallised
while uncovered; but if the metamorphic action occurred at profound
depths of the ocean, the former protecting mantle of rock may not have
been very thick. Admitting then that gneiss, mica-schist, granite,
diorite, etc., were once necessarily covered up, how can we account for
the naked and extensive areas of such rocks in many parts of the world,
except on the belief that they have subsequently been completely denuded
of all overlying strata? That such extensive areas do exist cannot be
doubted: the granitic region of Parime is described by Humboldt as being
at least nineteen times as large as Switzerland. South of the Amazon,
Boue colours an area composed of rocks of this nature as equal to that
of Spain, France, Italy, part of Germany, and the British Islands, all
conjoined. This region has not been carefully explored, but from the
concurrent testimony of travellers, the granitic area is very large:
thus Von Eschwege gives a detailed section of these rocks, stretching
from Rio de Janeiro for 260 geographical miles inland in a straight
line; and I travelled for 150 miles in another direction, and saw
nothing but granitic rocks. Numerous specimens, collected along the
whole coast, from near Rio de Janeiro to the mouth of the Plata, a
distance of 1,100 geographical miles, were examined by me, and they all
belonged to this class. Inland, along the whole northern bank of the
Plata, I saw, besides modern tertiary beds, only one small patch of
slightly metamorphosed rock, which alone could have formed a part of the
original capping of the granitic series. Turning to a well-known region,
namely, to the United States and Canada, as shown in Professor H.D.
Rogers' beautiful map, I have estimated the areas by cutting out and
weighing the paper, and I find that the metamorphic (excluding the
"semi-metamorphic") and granite rocks exceed, in the proportion of 19 to
12.5, the whole of the newer Palaeozoic formations. In many regions the
metamorphic and granite rocks would be found much more widely extended
than they appear to be, if all the sedimentary beds were removed which
rest unconformably on them, and which could not have formed part of
the original mantle under which they were crystallised. Hence, it is
probable that in some parts of the world whole formations have been
completely denuded, with not a wreck left behind.


One remark is here worth a passing notice. During periods of elevation
the area of the land and of the adjoining shoal parts of the sea will
be increased and new stations will often be formed--all circumstances
favourable, as previously explained, for the formation of new varieties
and species; but during such periods there will generally be a blank
in the geological record. On the other hand, during subsidence, the
inhabited area and number of inhabitants will decrease (excepting on
the shores of a continent when first broken up into an archipelago), and
consequently during subsidence, though there will be much extinction,
few new varieties or species will be formed; and it is during these very
periods of subsidence that the deposits which are richest in fossils
have been accumulated.

ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN ANY SINGLE
FORMATION.

From these several considerations it cannot be doubted that the
geological record, viewed as a whole, is extremely imperfect; but if
we confine our attention to any one formation, it becomes much more
difficult to understand why we do not therein find closely graduated
varieties between the allied species which lived at its commencement and
at its close. Several cases are on record of the same species presenting
varieties in the upper and lower parts of the same formation. Thus
Trautschold gives a number of instances with Ammonites, and Hilgendorf
has described a most curious case of ten graduated forms of Planorbis
multiformis in the successive beds of a fresh-water formation in
Switzerland. Although each formation has indisputably required a vast
number of years for its deposition, several reasons can be given why
each should not commonly include a graduated series of links between the
species which lived at its commencement and close, but I cannot assign
due proportional weight to the following considerations.

Although each formation may mark a very long lapse of years, each
probably is short compared with the period requisite to change one
species into another. I am aware that two palaeontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or thrice
as long as the average duration of specific forms. But insuperable
difficulties, as it seems to me, prevent us from coming to any just
conclusion on this head. When we see a species first appearing in the
middle of any formation, it would be rash in the extreme to infer
that it had not elsewhere previously existed. So again, when we find
a species disappearing before the last layers have been deposited, it
would be equally rash to suppose that it then became extinct. We forget
how small the area of Europe is compared with the rest of the world;
nor have the several stages of the same formation throughout Europe been
correlated with perfect accuracy.

We may safely infer that with marine animals of all kinds there has been
a large amount of migration due to climatal and other changes; and when
we see a species first appearing in any formation, the probability is
that it only then first immigrated into that area. It is well known, for
instance, that several species appear somewhat earlier in the palaeozoic
beds of North America than in those of Europe; time having apparently
been required for their migration from the American to the European
seas. In examining the latest deposits, in various quarters of the
world, it has everywhere been noted, that some few still existing
species are common in the deposit, but have become extinct in the
immediately surrounding sea; or, conversely, that some are now abundant
in the neighbouring sea, but are rare or absent in this particular
deposit. It is an excellent lesson to reflect on the ascertained amount
of migration of the inhabitants of Europe during the glacial epoch,
which forms only a part of one whole geological period; and likewise to
reflect on the changes of level, on the extreme change of climate,
and on the great lapse of time, all included within this same glacial
period. Yet it may be doubted whether, in any quarter of the
world, sedimentary deposits, INCLUDING FOSSIL REMAINS, have gone on
accumulating within the same area during the whole of this period. It is
not, for instance, probable that sediment was deposited during the whole
of the glacial period near the mouth of the Mississippi, within that
limit of depth at which marine animals can best flourish: for we know
that great geographical changes occurred in other parts of America
during this space of time. When such beds as were deposited in shallow
water near the mouth of the Mississippi during some part of the glacial
period shall have been upraised, organic remains will probably first
appear and disappear at different levels, owing to the migrations
of species and to geographical changes. And in the distant future, a
geologist, examining these beds, would be tempted to conclude that the
average duration of life of the embedded fossils had been less than that
of the glacial period, instead of having been really far greater, that
is, extending from before the glacial epoch to the present day.

In order to get a perfect gradation between two forms in the upper
and lower parts of the same formation, the deposit must have gone on
continuously accumulating during a long period, sufficient for the slow
process of modification; hence, the deposit must be a very thick one;
and the species undergoing change must have lived in the same district
throughout the whole time. But we have seen that a thick formation,
fossiliferous throughout its entire thickness, can accumulate only
during a period of subsidence; and to keep the depth approximately the
same, which is necessary that the same marine species may live on the
same space, the supply of sediment must nearly counterbalance the
amount of subsidence. But this same movement of subsidence will tend to
submerge the area whence the sediment is derived, and thus diminish the
supply, whilst the downward movement continues. In fact, this nearly
exact balancing between the supply of sediment and the amount of
subsidence is probably a rare contingency; for it has been observed
by more than one palaeontologist that very thick deposits are usually
barren of organic remains, except near their upper or lower limits.

It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation composed
of beds of widely different mineralogical composition, we may
reasonably suspect that the process of deposition has been more or less
interrupted. Nor will the closest inspection of a formation give us any
idea of the length of time which its deposition may have consumed.
Many instances could be given of beds, only a few feet in thickness,
representing formations which are elsewhere thousands of feet in
thickness, and which must have required an enormous period for their
accumulation; yet no one ignorant of this fact would have even suspected
the vast lapse of time represented by the thinner formation. Many cases
could be given of the lower beds of a formation having been upraised,
denuded, submerged, and then re-covered by the upper beds of the same
formation--facts, showing what wide, yet easily overlooked, intervals
have occurred in its accumulation. In other cases we have the plainest
evidence in great fossilised trees, still standing upright as they grew,
of many long intervals of time and changes of level during the process
of deposition, which would not have been suspected, had not the trees
been preserved: thus Sir C. Lyell and Dr. Dawson found carboniferous
beds 1,400 feet thick in Nova Scotia, with ancient root-bearing strata,
one above the other, at no less than sixty-eight different levels.
Hence, when the same species occurs at the bottom, middle, and top of
a formation, the probability is that it has not lived on the same
spot during the whole period of deposition, but has disappeared and
reappeared, perhaps many times, during the same geological period.
Consequently if it were to undergo a considerable amount of modification
during the deposition of any one geological formation, a section would
not include all the fine intermediate gradations which must on our
theory have existed, but abrupt, though perhaps slight, changes of form.

It is all-important to remember that naturalists have no golden rule
by which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species,
unless they are enabled to connect them together by the closest
intermediate gradations; and this, from the reasons just assigned, we
can seldom hope to effect in any one geological section. Supposing B
and C to be two species, and a third, A, to be found in an older and
underlying bed; even if A were strictly intermediate between B and C,
it would simply be ranked as a third and distinct species, unless at the
same time it could be closely connected by intermediate varieties
with either one or both forms. Nor should it be forgotten, as before
explained, that A might be the actual progenitor of B and C, and yet
would not necessarily be strictly intermediate between them in all
respects. So that we might obtain the parent-species and its several
modified descendants from the lower and upper beds of the same
formation, and unless we obtained numerous transitional gradations, we
should not recognise their blood-relationship, and should consequently
rank them as distinct species.

It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the
very fine species of D'Orbigny and others into the rank of varieties;
and on this view we do find the kind of evidence of change which on
the theory we ought to find. Look again at the later tertiary deposits,
which include many shells believed by the majority of naturalists to
be identical with existing species; but some excellent naturalists,
as Agassiz and Pictet, maintain that all these tertiary species are
specifically distinct, though the distinction is admitted to be very
slight; so that here, unless we believe that these eminent naturalists
have been misled by their imaginations, and that these late tertiary
species really present no difference whatever from their living
representatives, or unless we admit, in opposition to the judgment of
most naturalists, that these tertiary species are all truly distinct
from the recent, we have evidence of the frequent occurrence of slight
modifications of the kind required. If we look to rather wider intervals
of time, namely, to distinct but consecutive stages of the same great
formation, we find that the embedded fossils, though universally ranked
as specifically different, yet are far more closely related to each
other than are the species found in more widely separated formations;
so that here again we have undoubted evidence of change in the direction
required by the theory; but to this latter subject I shall return in the
following chapter.

With animals and plants that propagate rapidly and do not wander
much, there is reason to suspect, as we have formerly seen, that their
varieties are generally at first local; and that such local varieties
do not spread widely and supplant their parent-form until they have been
modified and perfected in some considerable degree. According to this
view, the chance of discovering in a formation in any one country all
the early stages of transition between any two forms, is small, for the
successive changes are supposed to have been local or confined to some
one spot. Most marine animals have a wide range; and we have seen that
with plants it is those which have the widest range, that oftenest
present varieties, so that, with shells and other marine animals, it is
probable that those which had the widest range, far exceeding the limits
of the known geological formations in Europe, have oftenest given rise,
first to local varieties and ultimately to new species; and this again
would greatly lessen the chance of our being able to trace the stages of
transition in any one geological formation.

It is a more important consideration, leading to the same result, as
lately insisted on by Dr. Falconer, namely, that the period during which
each species underwent modification, though long as measured by years,
was probably short in comparison with that during which it remained
without undergoing any change.

It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties, and thus proved to be the same species, until
many specimens are collected from many places; and with fossil
species this can rarely be done. We shall, perhaps, best perceive the
improbability of our being enabled to connect species by numerous, fine,
intermediate, fossil links, by asking ourselves whether, for instance,
geologists at some future period will be able to prove that our
different breeds of cattle, sheep, horses, and dogs are descended from
a single stock or from several aboriginal stocks; or, again, whether
certain sea-shells inhabiting the shores of North America, which are
ranked by some conchologists as distinct species from their European
representatives, and by other conchologists as only varieties, are
really varieties, or are, as it is called, specifically distinct. This
could be effected by the future geologist only by his discovering in
a fossil state numerous intermediate gradations; and such success is
improbable in the highest degree.

It has been asserted over and over again, by writers who believe in
the immutability of species, that geology yields no linking forms. This
assertion, as we shall see in the next chapter, is certainly erroneous.
As Sir J. Lubbock has remarked, "Every species is a link between other
allied forms." If we take a genus having a score of species, recent
and extinct, and destroy four-fifths of them, no one doubts that the
remainder will stand much more distinct from each other. If the extreme
forms in the genus happen to have been thus destroyed, the genus itself
will stand more distinct from other allied genera. What geological
research has not revealed, is the former existence of infinitely
numerous gradations, as fine as existing varieties, connecting together
nearly all existing and extinct species. But this ought not to be
expected; yet this has been repeatedly advanced as a most serious
objection against my views.

It may be worth while to sum up the foregoing remarks on the causes
of the imperfection of the geological record under an imaginary
illustration. The Malay Archipelago is about the size of Europe from
the North Cape to the Mediterranean, and from Britain to Russia; and
therefore equals all the geological formations which have been examined
with any accuracy, excepting those of the United States of America. I
fully agree with Mr. Godwin-Austen, that the present condition of the
Malay Archipelago, with its numerous large islands separated by wide and
shallow seas, probably represents the former state of Europe, while most
of our formations were accumulating. The Malay Archipelago is one of
the richest regions in organic beings; yet if all the species were to
be collected which have ever lived there, how imperfectly would they
represent the natural history of the world!

But we have every reason to believe that the terrestrial productions of
the archipelago would be preserved in an extremely imperfect manner in
the formations which we suppose to be there accumulating. Not many
of the strictly littoral animals, or of those which lived on naked
submarine rocks, would be embedded; and those embedded in gravel or
sand would not endure to a distant epoch. Wherever sediment did not
accumulate on the bed of the sea, or where it did not accumulate at a
sufficient rate to protect organic bodies from decay, no remains could
be preserved.

Formations rich in fossils of many kinds, and of thickness sufficient to
last to an age as distant in futurity as the secondary formations lie
in the past, would generally be formed in the archipelago only during
periods of subsidence. These periods of subsidence would be separated
from each other by immense intervals of time, during which the area
would be either stationary or rising; whilst rising, the fossiliferous
formations on the steeper shores would be destroyed, almost as soon as
accumulated, by the incessant coast-action, as we now see on the shores
of South America. Even throughout the extensive and shallow seas within
the archipelago, sedimentary beds could hardly be accumulated of
great thickness during the periods of elevation, or become capped
and protected by subsequent deposits, so as to have a good chance of
enduring to a very distant future. During the periods of subsidence,
there would probably be much extinction of life; during the periods
of elevation, there would be much variation, but the geological record
would then be less perfect.

It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with
a contemporaneous accumulation of sediment, would EXCEED the average
duration of the same specific forms; and these contingencies are
indispensable for the preservation of all the transitional gradations
between any two or more species. If such gradations were not all fully
preserved, transitional varieties would merely appear as so many new,
though closely allied species. It is also probable that each great
period of subsidence would be interrupted by oscillations of level,
and that slight climatical changes would intervene during such lengthy
periods; and in these cases the inhabitants of the archipelago would
migrate, and no closely consecutive record of their modifications could
be preserved in any one formation.

Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy plainly leads to the
belief that it would be chiefly these far-ranging species, though
only some of them, which would oftenest produce new varieties; and
the varieties would at first be local or confined to one place, but
if possessed of any decided advantage, or when further modified and
improved, they would slowly spread and supplant their parent-forms. When
such varieties returned to their ancient homes, as they would differ
from their former state in a nearly uniform, though perhaps extremely
slight degree, and as they would be found embedded in slightly
different sub-stages of the same formation, they would, according to
the principles followed by many palaeontologists, be ranked as new and
distinct species.

If then there be some degree of truth in these remarks, we have no right
to expect to find, in our geological formations, an infinite number of
those fine transitional forms, which, on our theory, have connected
all the past and present species of the same group into one long and
branching chain of life. We ought only to look for a few links, and such
assuredly we do find--some more distantly, some more closely, related
to each other; and these links, let them be ever so close, if found in
different stages of the same formation, would, by many palaeontologists,
be ranked as distinct species. But I do not pretend that I should ever
have suspected how poor was the record in the best preserved geological
sections, had not the absence of innumerable transitional links between
the species which lived at the commencement and close of each formation,
pressed so hardly on my theory.

ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES.

The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palaeontologists--for
instance, by Agassiz, Pictet, and Sedgwick, as a fatal objection to the
belief in the transmutation of species. If numerous species, belonging
to the same genera or families, have really started into life at once,
the fact would be fatal to the theory of evolution through natural
selection. For the development by this means of a group of forms, all
of which are descended from some one progenitor, must have been an
extremely slow process; and the progenitors must have lived long before
their modified descendants. But we continually overrate the perfection
of the geological record, and falsely infer, because certain genera or
families have not been found beneath a certain stage, that they did not
exist before that stage. In all cases positive palaeontological evidence
may be implicitly trusted; negative evidence is worthless, as experience
has so often shown. We continually forget how large the world is,
compared with the area over which our geological formations have been
carefully examined; we forget that groups of species may elsewhere
have long existed, and have slowly multiplied, before they invaded the
ancient archipelagoes of Europe and the United States. We do not make
due allowance for the enormous intervals of time which have elapsed
between our consecutive formations, longer perhaps in many cases
than the time required for the accumulation of each formation. These
intervals will have given time for the multiplication of species from
some one parent-form: and in the succeeding formation, such groups or
species will appear as if suddenly created.

I may here recall a remark formerly made, namely, that it might require
a long succession of ages to adapt an organism to some new and peculiar
line of life, for instance, to fly through the air; and consequently
that the transitional forms would often long remain confined to some one
region; but that, when this adaptation had once been effected, and a
few species had thus acquired a great advantage over other organisms,
a comparatively short time would be necessary to produce many divergent
forms, which would spread rapidly and widely throughout the world.
Professor Pictet, in his excellent Review of this work, in commenting
on early transitional forms, and taking birds as an illustration, cannot
see how the successive modifications of the anterior limbs of a supposed
prototype could possibly have been of any advantage. But look at the
penguins of the Southern Ocean; have not these birds their front limbs
in this precise intermediate state of "neither true arms nor true
wings?" Yet these birds hold their place victoriously in the battle for
life; for they exist in infinite numbers and of many kinds. I do not
suppose that we here see the real transitional grades through which
the wings of birds have passed; but what special difficulty is there in
believing that it might profit the modified descendants of the penguin,
first to become enabled to flap along the surface of the sea like the
logger-headed duck, and ultimately to rise from its surface and glide
through the air?

I will now give a few examples to illustrate the foregoing remarks, and
to show how liable we are to error in supposing that whole groups of
species have suddenly been produced. Even in so short an interval as
that between the first and second editions of Pictet's great work on
Palaeontology, published in 1844-46 and in 1853-57, the conclusions on
the first appearance and disappearance of several groups of animals
have been considerably modified; and a third edition would require still
further changes. I may recall the well-known fact that in geological
treatises, published not many years ago, mammals were always spoken of
as having abruptly come in at the commencement of the tertiary series.
And now one of the richest known accumulations of fossil mammals belongs
to the middle of the secondary series; and true mammals have been
discovered in the new red sandstone at nearly the commencement of
this great series. Cuvier used to urge that no monkey occurred in any
tertiary stratum; but now extinct species have been discovered in India,
South America and in Europe, as far back as the miocene stage. Had it
not been for the rare accident of the preservation of footsteps in
the new red sandstone of the United States, who would have ventured to
suppose that no less than at least thirty different bird-like animals,
some of gigantic size, existed during that period? Not a fragment of
bone has been discovered in these beds. Not long ago, palaeontologists
maintained that the whole class of birds came suddenly into existence
during the eocene period; but now we know, on the authority of Professor
Owen, that a bird certainly lived during the deposition of the upper
greensand; and still more recently, that strange bird, the Archeopteryx,
with a long lizard-like tail, bearing a pair of feathers on each joint,
and with its wings furnished with two free claws, has been discovered in
the oolitic slates of Solenhofen. Hardly any recent discovery shows more
forcibly than this how little we as yet know of the former inhabitants
of the world.

I may give another instance, which, from having passed under my own eyes
has much struck me. In a memoir on Fossil Sessile Cirripedes, I stated
that, from the large number of existing and extinct tertiary species;
from the extraordinary abundance of the individuals of many species
all over the world, from the Arctic regions to the equator, inhabiting
various zones of depths, from the upper tidal limits to fifty fathoms;
from the perfect manner in which specimens are preserved in the oldest
tertiary beds; from the ease with which even a fragment of a valve
can be recognised; from all these circumstances, I inferred that, had
sessile cirripedes existed during the secondary periods, they would
certainly have been preserved and discovered; and as not one species had
then been discovered in beds of this age, I concluded that this great
group had been suddenly developed at the commencement of the tertiary
series. This was a sore trouble to me, adding, as I then thought, one
more instance of the abrupt appearance of a great group of species. But
my work had hardly been published, when a skilful palaeontologist, M.
Bosquet, sent me a drawing of a perfect specimen of an unmistakable
sessile cirripede, which he had himself extracted from the chalk of
Belgium. And, as if to make the case as striking as possible, this
cirripede was a Chthamalus, a very common, large, and ubiquitous genus,
of which not one species has as yet been found even in any tertiary
stratum. Still more recently, a Pyrgoma, a member of a distinct
subfamily of sessile cirripedes, has been discovered by Mr. Woodward in
the upper chalk; so that we now have abundant evidence of the existence
of this group of animals during the secondary period.

The case most frequently insisted on by palaeontologists of the
apparently sudden appearance of a whole group of species, is that of the
teleostean fishes, low down, according to Agassiz, in the Chalk period.
This group includes the large majority of existing species. But certain
Jurassic and Triassic forms are now commonly admitted to be teleostean;
and even some palaeozoic forms have thus been classed by one high
authority. If the teleosteans had really appeared suddenly in the
northern hemisphere at the commencement of the chalk formation, the
fact would have been highly remarkable; but it would not have formed an
insuperable difficulty, unless it could likewise have been shown that at
the same period the species were suddenly and simultaneously developed
in other quarters of the world. It is almost superfluous to remark
that hardly any fossil-fish are known from south of the equator; and
by running through Pictet's Palaeontology it will be seen that very few
species are known from several formations in Europe. Some few families
of fish now have a confined range; the teleostean fishes might formerly
have had a similarly confined range, and after having been largely
developed in some one sea, have spread widely. Nor have we any right to
suppose that the seas of the world have always been so freely open from
south to north as they are at present. Even at this day, if the Malay
Archipelago were converted into land, the tropical parts of the Indian
Ocean would form a large and perfectly enclosed basin, in which any
great group of marine animals might be multiplied; and here they would
remain confined, until some of the species became adapted to a cooler
climate, and were enabled to double the southern capes of Africa or
Australia, and thus reach other and distant seas.

From these considerations, from our ignorance of the geology of other
countries beyond the confines of Europe and the United States, and
from the revolution in our palaeontological knowledge effected by the
discoveries of the last dozen years, it seems to me to be about as rash
to dogmatize on the succession of organic forms throughout the world, as
it would be for a naturalist to land for five minutes on a barren
point in Australia, and then to discuss the number and range of its
productions.

ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST KNOWN
FOSSILIFEROUS STRATA.

There is another and allied difficulty, which is much more serious. I
allude to the manner in which species belonging to several of the main
divisions of the animal kingdom suddenly appear in the lowest known
fossiliferous rocks. Most of the arguments which have convinced me that
all the existing species of the same group are descended from a single
progenitor, apply with equal force to the earliest known species.
For instance, it cannot be doubted that all the Cambrian and Silurian
trilobites are descended from some one crustacean, which must have lived
long before the Cambrian age, and which probably differed greatly from
any known animal. Some of the most ancient animals, as the Nautilus,
Lingula, etc., do not differ much from living species; and it cannot on
our theory be supposed, that these old species were the progenitors of
all the species belonging to the same groups which have subsequently
appeared, for they are not in any degree intermediate in character.

Consequently, if the theory be true, it is indisputable that before the
lowest Cambrian stratum was deposited long periods elapsed, as long as,
or probably far longer than, the whole interval from the Cambrian age
to the present day; and that during these vast periods the world swarmed
with living creatures. Here we encounter a formidable objection; for it
seems doubtful whether the earth, in a fit state for the habitation of
living creatures, has lasted long enough. Sir W. Thompson concludes that
the consolidation of the crust can hardly have occurred less than twenty
or more than four hundred million years ago, but probably not less than
ninety-eight or more than two hundred million years. These very wide
limits show how doubtful the data are; and other elements may have
hereafter to be introduced into the problem. Mr. Croll estimates that
about sixty million years have elapsed since the Cambrian period,
but this, judging from the small amount of organic change since the
commencement of the Glacial epoch, appears a very short time for the
many and great mutations of life, which have certainly occurred since
the Cambrian formation; and the previous one hundred and forty million
years can hardly be considered as sufficient for the development of the
varied forms of life which already existed during the Cambrian period.
It is, however, probable, as Sir William Thompson insists, that the
world at a very early period was subjected to more rapid and violent
changes in its physical conditions than those now occurring; and such
changes would have tended to induce changes at a corresponding rate in
the organisms which then existed.

To the question why we do not find rich fossiliferous deposits belonging
to these assumed earliest periods prior to the Cambrian system, I can
give no satisfactory answer. Several eminent geologists, with Sir R.
Murchison at their head, were until recently convinced that we beheld
in the organic remains of the lowest Silurian stratum the first dawn
of life. Other highly competent judges, as Lyell and E. Forbes, have
disputed this conclusion. We should not forget that only a small portion
of the world is known with accuracy. Not very long ago M. Barrande
added another and lower stage, abounding with new and peculiar species,
beneath the then known Silurian system; and now, still lower down in the
Lower Cambrian formation, Mr Hicks has found South Wales beds rich in
trilobites, and containing various molluscs and annelids. The presence
of phosphatic nodules and bituminous matter, even in some of the
lowest azotic rocks, probably indicates life at these periods; and
the existence of the Eozoon in the Laurentian formation of Canada is
generally admitted. There are three great series of strata beneath the
Silurian system in Canada, in the lowest of which the Eozoon is found.
Sir W. Logan states that their "united thickness may possibly far
surpass that of all the succeeding rocks, from the base of the
palaeozoic series to the present time. We are thus carried back to a
period so remote, that the appearance of the so-called primordial
fauna (of Barrande) may by some be considered as a comparatively modern
event." The Eozoon belongs to the most lowly organised of all classes of
animals, but is highly organised for its class; it existed in countless
numbers, and, as Dr. Dawson has remarked, certainly preyed on other
minute organic beings, which must have lived in great numbers. Thus the
words, which I wrote in 1859, about the existence of living beings long
before the Cambrian period, and which are almost the same with those
since used by Sir W. Logan, have proved true. Nevertheless, the
difficulty of assigning any good reason for the absence of vast piles
of strata rich in fossils beneath the Cambrian system is very great. It
does not seem probable that the most ancient beds have been quite worn
away by denudation, or that their fossils have been wholly obliterated
by metamorphic action, for if this had been the case we should have
found only small remnants of the formations next succeeding them in
age, and these would always have existed in a partially metamorphosed
condition. But the descriptions which we possess of the Silurian
deposits over immense territories in Russia and in North America, do not
support the view that the older a formation is the more invariably it
has suffered extreme denudation and metamorphism.

The case at present must remain inexplicable; and may be truly urged as
a valid argument against the views here entertained. To show that it
may hereafter receive some explanation, I will give the following
hypothesis. From the nature of the organic remains which do not appear
to have inhabited profound depths, in the several formations of Europe
and of the United States; and from the amount of sediment, miles in
thickness, of which the formations are composed, we may infer that from
first to last large islands or tracts of land, whence the sediment was
derived, occurred in the neighbourhood of the now existing continents
of Europe and North America. This same view has since been maintained by
Agassiz and others. But we do not know what was the state of things in
the intervals between the several successive formations; whether Europe
and the United States during these intervals existed as dry land, or as
a submarine surface near land, on which sediment was not deposited, or
as the bed of an open and unfathomable sea.

Looking to the existing oceans, which are thrice as extensive as the
land, we see them studded with many islands; but hardly one truly
oceanic island (with the exception of New Zealand, if this can be called
a truly oceanic island) is as yet known to afford even a remnant of any
palaeozoic or secondary formation. Hence, we may perhaps infer, that
during the palaeozoic and secondary periods, neither continents nor
continental islands existed where our oceans now extend; for had they
existed, palaeozoic and secondary formations would in all probability
have been accumulated from sediment derived from their wear and tear;
and would have been at least partially upheaved by the oscillations of
level, which must have intervened during these enormously long periods.
If, then, we may infer anything from these facts, we may infer that,
where our oceans now extend, oceans have extended from the remotest
period of which we have any record; and on the other hand, that where
continents now exist, large tracts of land have existed, subjected, no
doubt, to great oscillations of level, since the Cambrian period. The
coloured map appended to my volume on Coral Reefs, led me to conclude
that the great oceans are still mainly areas of subsidence, the great
archipelagoes still areas of oscillations of level, and the continents
areas of elevation. But we have no reason to assume that things have
thus remained from the beginning of the world. Our continents seem to
have been formed by a preponderance, during many oscillations of
level, of the force of elevation. But may not the areas of preponderant
movement have changed in the lapse of ages? At a period long antecedent
to the Cambrian epoch, continents may have existed where oceans are
now spread out, and clear and open oceans may have existed where our
continents now stand. Nor should we be justified in assuming that if,
for instance, the bed of the Pacific Ocean were now converted into a
continent we should there find sedimentary formations, in recognisable
condition, older than the Cambrian strata, supposing such to have been
formerly deposited; for it might well happen that strata which had
subsided some miles nearer to the centre of the earth, and which had
been pressed on by an enormous weight of superincumbent water, might
have undergone far more metamorphic action than strata which have always
remained nearer to the surface. The immense areas in some parts of the
world, for instance in South America, of naked metamorphic rocks, which
must have been heated under great pressure, have always seemed to me to
require some special explanation; and we may perhaps believe that we see
in these large areas the many formations long anterior to the Cambrian
epoch in a completely metamorphosed and denuded condition.

The several difficulties here discussed, namely, that, though we find in
our geological formations many links between the species which now exist
and which formerly existed, we do not find infinitely numerous fine
transitional forms closely joining them all together. The sudden
manner in which several groups of species first appear in our European
formations, the almost entire absence, as at present known, of
formations rich in fossils beneath the Cambrian strata, are all
undoubtedly of the most serious nature. We see this in the fact that
the most eminent palaeontologists, namely, Cuvier, Agassiz, Barrande,
Pictet, Falconer, E. Forbes, etc., and all our greatest geologists, as
Lyell, Murchison, Sedgwick, etc., have unanimously, often vehemently,
maintained the immutability of species. But Sir Charles Lyell now
gives the support of his high authority to the opposite side, and most
geologists and palaeontologists are much shaken in their former belief.
Those who believe that the geological record is in any degree perfect,
will undoubtedly at once reject my theory. For my part, following out
Lyell's metaphor, I look at the geological record as a history of
the world imperfectly kept and written in a changing dialect. Of this
history we possess the last volume alone, relating only to two or three
countries. Of this volume, only here and there a short chapter has been
preserved, and of each page, only here and there a few lines. Each
word of the slowly-changing language, more or less different in the
successive chapters, may represent the forms of life, which are entombed
in our consecutive formations, and which falsely appear to have been
abruptly introduced. On this view the difficulties above discussed are
greatly diminished or even disappear.



CHAPTER XI. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.

 On the slow and successive appearance of new species--On their different
 rates of change--Species once lost do not reappear--Groups of species
 follow the same general rules in their appearance and disappearance as
 do single species--On extinction--On simultaneous changes in the forms
 of life throughout the world--On the affinities of extinct species to
 each other and to living species--On the state of development of
 ancient forms--On the succession of the same types within the same
 areas--Summary of preceding and present chapters.


Let us now see whether the several facts and laws relating to the
geological succession of organic beings accord best with the common view
of the immutability of species, or with that of their slow and gradual
modification, through variation and natural selection.

New species have appeared very slowly, one after another, both on the
land and in the waters. Lyell has shown that it is hardly possible to
resist the evidence on this head in the case of the several tertiary
stages; and every year tends to fill up the blanks between the stages,
and to make the proportion between the lost and existing forms more
gradual. In some of the most recent beds, though undoubtedly of high
antiquity if measured by years, only one or two species are extinct,
and only one or two are new, having appeared there for the first time,
either locally, or, as far as we know, on the face of the earth. The
secondary formations are more broken; but, as Bronn has remarked,
neither the appearance nor disappearance of the many species embedded in
each formation has been simultaneous.

Species belonging to different genera and classes have not changed at
the same rate, or in the same degree. In the older tertiary beds a few
living shells may still be found in the midst of a multitude of extinct
forms. Falconer has given a striking instance of a similar fact, for an
existing crocodile is associated with many lost mammals and reptiles in
the sub-Himalayan deposits. The Silurian Lingula differs but little from
the living species of this genus; whereas most of the other Silurian
Molluscs and all the Crustaceans have changed greatly. The productions
of the land seem to have changed at a quicker rate than those of the
sea, of which a striking instance has been observed in Switzerland.
There is some reason to believe that organisms high in the scale, change
more quickly than those that are low: though there are exceptions to
this rule. The amount of organic change, as Pictet has remarked, is not
the same in each successive so-called formation. Yet if we compare any
but the most closely related formations, all the species will be found
to have undergone some change. When a species has once disappeared
from the face of the earth, we have no reason to believe that the same
identical form ever reappears. The strongest apparent exception to this
latter rule is that of the so-called "colonies" of M. Barrande, which
intrude for a period in the midst of an older formation, and then allow
the pre-existing fauna to reappear; but Lyell's explanation, namely,
that it is a case of temporary migration from a distinct geographical
province, seems satisfactory.

These several facts accord well with our theory, which includes no fixed
law of development, causing all the inhabitants of an area to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be slow, and will generally affect only a few species
at the same time; for the variability of each species is independent of
that of all others. Whether such variations or individual differences as
may arise will be accumulated through natural selection in a greater
or less degree, thus causing a greater or less amount of permanent
modification, will depend on many complex contingencies--on
the variations being of a beneficial nature, on the freedom of
intercrossing, on the slowly changing physical conditions of the
country, on the immigration of new colonists, and on the nature of the
other inhabitants with which the varying species come into competition.
Hence it is by no means surprising that one species should retain the
same identical form much longer than others; or, if changing, should
change in a less degree. We find similar relations between the existing
inhabitants of distinct countries; for instance, the land-shells and
coleopterous insects of Madeira have come to differ considerably from
their nearest allies on the continent of Europe, whereas the marine
shells and birds have remained unaltered. We can perhaps understand
the apparently quicker rate of change in terrestrial and in more highly
organised productions compared with marine and lower productions, by
the more complex relations of the higher beings to their organic and
inorganic conditions of life, as explained in a former chapter. When
many of the inhabitants of any area have become modified and improved,
we can understand, on the principle of competition, and from the
all-important relations of organism to organism in the struggle for
life, that any form which did not become in some degree modified and
improved, would be liable to extermination. Hence, we see why all
the species in the same region do at last, if we look to long enough
intervals of time, become modified; for otherwise they would become
extinct.

In members of the same class the average amount of change, during long
and equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of enduring formations, rich in fossils, depends on great
masses of sediment being deposited on subsiding areas, our formations
have been almost necessarily accumulated at wide and irregularly
intermittent intervals of time; consequently the amount of organic
change exhibited by the fossils embedded in consecutive formations
is not equal. Each formation, on this view, does not mark a new and
complete act of creation, but only an occasional scene, taken almost at
hazard, in an ever slowly changing drama.

We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and
inorganic, should recur. For though the offspring of one species might
be adapted (and no doubt this has occurred in innumerable instances)
to fill the place of another species in the economy of nature, and
thus supplant it; yet the two forms--the old and the new--would not be
identically the same; for both would almost certainly inherit different
characters from their distinct progenitors; and organisms already
differing would vary in a different manner. For instance, it is
possible, if all our fantail-pigeons were destroyed, that fanciers might
make a new breed hardly distinguishable from the present breed; but if
the parent rock-pigeon were likewise destroyed, and under nature we have
every reason to believe that parent forms are generally supplanted
and exterminated by their improved offspring, it is incredible that a
fantail, identical with the existing breed, could be raised from any
other species of pigeon, or even from any other well established race
of the domestic pigeon, for the successive variations would almost
certainly be in some degree different, and the newly-formed variety
would probably inherit from its progenitor some characteristic
differences.

Groups of species, that is, genera and families, follow the same general
rules in their appearance and disappearance as do single species,
changing more or less quickly, and in a greater or lesser degree. A
group, when it has once disappeared, never reappears; that is, its
existence, as long as it lasts, is continuous. I am aware that there
are some apparent exceptions to this rule, but the exceptions are
surprisingly few, so few that E. Forbes, Pictet, and Woodward (though
all strongly opposed to such views as I maintain) admit its truth; and
the rule strictly accords with the theory. For all the species of
the same group, however long it may have lasted, are the modified
descendants one from the other, and all from a common progenitor. In
the genus Lingula, for instance, the species which have successively
appeared at all ages must have been connected by an unbroken series of
generations, from the lowest Silurian stratum to the present day.

We have seen in the last chapter that whole groups of species sometimes
falsely appear to have been abruptly developed; and I have attempted
to give an explanation of this fact, which if true would be fatal to my
views. But such cases are certainly exceptional; the general rule being
a gradual increase in number, until the group reaches its maximum, and
then, sooner or later, a gradual decrease. If the number of the species
included within a genus, or the number of the genera within a family, be
represented by a vertical line of varying thickness, ascending through
the successive geological formations, in which the species are found,
the line will sometimes falsely appear to begin at its lower end, not in
a sharp point, but abruptly; it then gradually thickens upwards, often
keeping of equal thickness for a space, and ultimately thins out in the
upper beds, marking the decrease and final extinction of the species.
This gradual increase in number of the species of a group is strictly
conformable with the theory; for the species of the same genus, and the
genera of the same family, can increase only slowly and progressively;
the process of modification and the production of a number of allied
forms necessarily being a slow and gradual process, one species first
giving rise to two or three varieties, these being slowly converted
into species, which in their turn produce by equally slow steps other
varieties and species, and so on, like the branching of a great tree
from a single stem, till the group becomes large.

ON EXTINCTION.

We have as yet only spoken incidentally of the disappearance of species
and of groups of species. On the theory of natural selection, the
extinction of old forms and the production of new and improved forms are
intimately connected together. The old notion of all the inhabitants of
the earth having been swept away by catastrophes at successive periods
is very generally given up, even by those geologists, as Elie de
Beaumont, Murchison, Barrande, etc., whose general views would naturally
lead them to this conclusion. On the contrary, we have every reason to
believe, from the study of the tertiary formations, that species and
groups of species gradually disappear, one after another, first from one
spot, then from another, and finally from the world. In some few cases,
however, as by the breaking of an isthmus and the consequent irruption
of a multitude of new inhabitants into an adjoining sea, or by the final
subsidence of an island, the process of extinction may have been rapid.
Both single species and whole groups of species last for very unequal
periods; some groups, as we have seen, have endured from the earliest
known dawn of life to the present day; some have disappeared before
the close of the palaeozoic period. No fixed law seems to determine
the length of time during which any single species or any single genus
endures. There is reason to believe that the extinction of a whole group
of species is generally a slower process than their production: if their
appearance and disappearance be represented, as before, by a vertical
line of varying thickness the line is found to taper more gradually at
its upper end, which marks the progress of extermination, than at its
lower end, which marks the first appearance and the early increase in
number of the species. In some cases, however, the extermination of
whole groups, as of ammonites, towards the close of the secondary
period, has been wonderfully sudden.

The extinction of species has been involved in the most gratuitous
mystery. Some authors have even supposed that, as the individual has a
definite length of life, so have species a definite duration. No one can
have marvelled more than I have done at the extinction of species. When
I found in La Plata the tooth of a horse embedded with the remains of
Mastodon, Megatherium, Toxodon and other extinct monsters, which all
co-existed with still living shells at a very late geological period,
I was filled with astonishment; for, seeing that the horse, since its
introduction by the Spaniards into South America, has run wild over the
whole country and has increased in numbers at an unparalleled rate, I
asked myself what could so recently have exterminated the former horse
under conditions of life apparently so favourable. But my astonishment
was groundless. Professor Owen soon perceived that the tooth, though
so like that of the existing horse, belonged to an extinct species. Had
this horse been still living, but in some degree rare, no naturalist
would have felt the least surprise at its rarity; for rarity is the
attribute of a vast number of species of all classes, in all countries.
If we ask ourselves why this or that species is rare, we answer that
something is unfavourable in its conditions of life; but what that
something is, we can hardly ever tell. On the supposition of the fossil
horse still existing as a rare species, we might have felt certain, from
the analogy of all other mammals, even of the slow-breeding elephant,
and from the history of the naturalisation of the domestic horse in
South America, that under more favourable conditions it would in a very
few years have stocked the whole continent. But we could not have
told what the unfavourable conditions were which checked its increase,
whether some one or several contingencies, and at what period of the
horse's life, and in what degree they severally acted. If the conditions
had gone on, however slowly, becoming less and less favourable, we
assuredly should not have perceived the fact, yet the fossil horse would
certainly have become rarer and rarer, and finally extinct--its place
being seized on by some more successful competitor.

It is most difficult always to remember that the increase of every
living creature is constantly being checked by unperceived hostile
agencies; and that these same unperceived agencies are amply sufficient
to cause rarity, and finally extinction. So little is this subject
understood, that I have heard surprise repeatedly expressed at such
great monsters as the Mastodon and the more ancient Dinosaurians having
become extinct; as if mere bodily strength gave victory in the battle of
life. Mere size, on the contrary, would in some cases determine, as has
been remarked by Owen, quicker extermination, from the greater amount
of requisite food. Before man inhabited India or Africa, some cause must
have checked the continued increase of the existing elephant. A highly
capable judge, Dr. Falconer, believes that it is chiefly insects which,
from incessantly harassing and weakening the elephant in India, check
its increase; and this was Bruce's conclusion with respect to
the African elephant in Abyssinia. It is certain that insects and
blood-sucking bats determine the existence of the larger naturalised
quadrupeds in several parts of South America.

We see in many cases in the more recent tertiary formations that rarity
precedes extinction; and we know that this has been the progress of
events with those animals which have been exterminated, either locally
or wholly, through man's agency. I may repeat what I published in 1845,
namely, that to admit that species generally become rare before they
become extinct--to feel no surprise at the rarity of a species, and yet
to marvel greatly when the species ceases to exist, is much the same as
to admit that sickness in the individual is the forerunner of death--to
feel no surprise at sickness, but, when the sick man dies, to wonder and
to suspect that he died by some deed of violence.

The theory of natural selection is grounded on the belief that each new
variety and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition;
and the consequent extinction of less-favoured forms almost inevitably
follows. It is the same with our domestic productions: when a new and
slightly improved variety has been raised, it at first supplants the
less improved varieties in the same neighbourhood; when much improved it
is transported far and near, like our short-horn cattle, and takes the
place of other breeds in other countries. Thus the appearance of new
forms and the disappearance of old forms, both those naturally and
artificially produced, are bound together. In flourishing groups, the
number of new specific forms which have been produced within a given
time has at some periods probably been greater than the number of
the old specific forms which have been exterminated; but we know that
species have not gone on indefinitely increasing, at least during the
later geological epochs, so that, looking to later times, we may believe
that the production of new forms has caused the extinction of about the
same number of old forms.

The competition will generally be most severe, as formerly explained
and illustrated by examples, between the forms which are most like each
other in all respects. Hence the improved and modified descendants of
a species will generally cause the extermination of the parent-species;
and if many new forms have been developed from any one species, the
nearest allies of that species, i.e. the species of the same genus, will
be the most liable to extermination. Thus, as I believe, a number of
new species descended from one species, that is a new genus, comes to
supplant an old genus, belonging to the same family. But it must often
have happened that a new species belonging to some one group has seized
on the place occupied by a species belonging to a distinct group, and
thus have caused its extermination. If many allied forms be developed
from the successful intruder, many will have to yield their places;
and it will generally be the allied forms, which will suffer from some
inherited inferiority in common. But whether it be species belonging
to the same or to a distinct class, which have yielded their places to
other modified and improved species, a few of the sufferers may often
be preserved for a long time, from being fitted to some peculiar line of
life, or from inhabiting some distant and isolated station, where they
will have escaped severe competition. For instance, some species of
Trigonia, a great genus of shells in the secondary formations, survive
in the Australian seas; and a few members of the great and almost
extinct group of Ganoid fishes still inhabit our fresh waters.
Therefore, the utter extinction of a group is generally, as we have
seen, a slower process than its production.

With respect to the apparently sudden extermination of whole families or
orders, as of Trilobites at the close of the palaeozoic period, and of
Ammonites at the close of the secondary period, we must remember what
has been already said on the probable wide intervals of time between our
consecutive formations; and in these intervals there may have been
much slow extermination. Moreover, when, by sudden immigration or by
unusually rapid development, many species of a new group have taken
possession of an area, many of the older species will have been
exterminated in a correspondingly rapid manner; and the forms which thus
yield their places will commonly be allied, for they will partake of the
same inferiority in common.

Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct accords well with the theory of natural
selection. We need not marvel at extinction; if we must marvel, let it
be at our presumption in imagining for a moment that we understand
the many complex contingencies on which the existence of each species
depends. If we forget for an instant that each species tends to increase
inordinately, and that some check is always in action, yet seldom
perceived by us, the whole economy of nature will be utterly obscured.
Whenever we can precisely say why this species is more abundant
in individuals than that; why this species and not another can be
naturalised in a given country; then, and not until then, we may justly
feel surprise why we cannot account for the extinction of any particular
species or group of species.

ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE
WORLD.

Scarcely any palaeontological discovery is more striking than the fact
that the forms of life change almost simultaneously throughout the
world. Thus our European Chalk formation can be recognised in many
distant regions, under the most different climates, where not a fragment
of the mineral chalk itself can be found; namely, in North America, in
equatorial South America, in Tierra del Fuego, at the Cape of Good Hope,
and in the peninsula of India. For at these distant points, the organic
remains in certain beds present an unmistakable resemblance to those
of the Chalk. It is not that the same species are met with; for in some
cases not one species is identically the same, but they belong to
the same families, genera, and sections of genera, and sometimes are
similarly characterised in such trifling points as mere superficial
sculpture. Moreover, other forms, which are not found in the Chalk of
Europe, but which occur in the formations either above or below, occur
in the same order at these distant points of the world. In the several
successive palaeozoic formations of Russia, Western Europe and North
America, a similar parallelism in the forms of life has been observed
by several authors; so it is, according to Lyell, with the European and
North American tertiary deposits. Even if the few fossil species which
are common to the Old and New Worlds were kept wholly out of view, the
general parallelism in the successive forms of life, in the palaeozoic
and tertiary stages, would still be manifest, and the several formations
could be easily correlated.

These observations, however, relate to the marine inhabitants of the
world: we have not sufficient data to judge whether the productions
of the land and of fresh water at distant points change in the same
parallel manner. We may doubt whether they have thus changed: if the
Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought
to Europe from La Plata, without any information in regard to their
geological position, no one would have suspected that they had
co-existed with sea-shells all still living; but as these anomalous
monsters co-existed with the Mastodon and Horse, it might at least
have been inferred that they had lived during one of the later tertiary
stages.

When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same year, or even to the same century, or
even that it has a very strict geological sense; for if all the marine
animals now living in Europe, and all those that lived in Europe during
the pleistocene period (a very remote period as measured by years,
including the whole glacial epoch) were compared with those now existing
in South America or in Australia, the most skilful naturalist would
hardly be able to say whether the present or the pleistocene inhabitants
of Europe resembled most closely those of the southern hemisphere. So,
again, several highly competent observers maintain that the existing
productions of the United States are more closely related to those which
lived in Europe during certain late tertiary stages, than to the
present inhabitants of Europe; and if this be so, it is evident that
fossiliferous beds now deposited on the shores of North America would
hereafter be liable to be classed with somewhat older European beds.
Nevertheless, looking to a remotely future epoch, there can be little
doubt that all the more modern MARINE formations, namely, the upper
pliocene, the pleistocene and strictly modern beds of Europe, North and
South America, and Australia, from containing fossil remains in some
degree allied, and from not including those forms which are found
only in the older underlying deposits, would be correctly ranked as
simultaneous in a geological sense.

The fact of the forms of life changing simultaneously in the above large
sense, at distant parts of the world, has greatly struck those admirable
observers, MM. de Verneuil and d'Archiac. After referring to the
parallelism of the palaeozoic forms of life in various parts of Europe,
they add, "If struck by this strange sequence, we turn our attention to
North America, and there discover a series of analogous phenomena,
it will appear certain that all these modifications of species, their
extinction, and the introduction of new ones, cannot be owing to mere
changes in marine currents or other causes more or less local and
temporary, but depend on general laws which govern the whole animal
kingdom." M. Barrande has made forcible remarks to precisely the same
effect. It is, indeed, quite futile to look to changes of currents,
climate, or other physical conditions, as the cause of these great
mutations in the forms of life throughout the world, under the most
different climates. We must, as Barrande has remarked, look to some
special law. We shall see this more clearly when we treat of the present
distribution of organic beings, and find how slight is the relation
between the physical conditions of various countries and the nature of
their inhabitants.

This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural selection.
New species are formed by having some advantage over older forms; and
the forms, which are already dominant, or have some advantage over the
other forms in their own country, give birth to the greatest number of
new varieties or incipient species. We have distinct evidence on this
head, in the plants which are dominant, that is, which are commonest and
most widely diffused, producing the greatest number of new varieties.
It is also natural that the dominant, varying and far-spreading species,
which have already invaded, to a certain extent, the territories of
other species, should be those which would have the best chance of
spreading still further, and of giving rise in new countries to other
new varieties and species. The process of diffusion would often be
very slow, depending on climatal and geographical changes, on strange
accidents, and on the gradual acclimatization of new species to the
various climates through which they might have to pass, but in the
course of time the dominant forms would generally succeed in spreading
and would ultimately prevail. The diffusion would, it is probable, be
slower with the terrestrial inhabitants of distinct continents than with
the marine inhabitants of the continuous sea. We might therefore expect
to find, as we do find, a less strict degree of parallelism in the
succession of the productions of the land than with those of the sea.

Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the world,
accords well with the principle of new species having been formed by
dominant species spreading widely and varying; the new species thus
produced being themselves dominant, owing to their having had some
advantage over their already dominant parents, as well as over other
species; and again spreading, varying, and producing new forms. The
old forms which are beaten and which yield their places to the new and
victorious forms, will generally be allied in groups, from inheriting
some inferiority in common; and, therefore, as new and improved groups
spread throughout the world, old groups disappear from the world; and
the succession of forms everywhere tends to correspond both in their
first appearance and final disappearance.

There is one other remark connected with this subject worth making. I
have given my reasons for believing that most of our great formations,
rich in fossils, were deposited during periods of subsidence; and that
blank intervals of vast duration, as far as fossils are concerned,
occurred during the periods when the bed of the sea was either
stationary or rising, and likewise when sediment was not thrown down
quickly enough to embed and preserve organic remains. During these
long and blank intervals I suppose that the inhabitants of each region
underwent a considerable amount of modification and extinction, and
that there was much migration from other parts of the world. As we have
reason to believe that large areas are affected by the same movement,
it is probable that strictly contemporaneous formations have often been
accumulated over very wide spaces in the same quarter of the world;
but we are very far from having any right to conclude that this has
invariably been the case, and that large areas have invariably been
affected by the same movements. When two formations have been deposited
in two regions during nearly, but not exactly, the same period,
we should find in both, from the causes explained in the foregoing
paragraphs, the same general succession in the forms of life; but the
species would not exactly correspond; for there will have been a
little more time in the one region than in the other for modification,
extinction, and immigration.

I suspect that cases of this nature occur in Europe. Mr. Prestwich, in
his admirable Memoirs on the eocene deposits of England and France, is
able to draw a close general parallelism between the successive stages
in the two countries; but when he compares certain stages in England
with those in France, although he finds in both a curious accordance in
the numbers of the species belonging to the same genera, yet the species
themselves differ in a manner very difficult to account for considering
the proximity of the two areas, unless, indeed, it be assumed that an
isthmus separated two seas inhabited by distinct, but contemporaneous
faunas. Lyell has made similar observations on some of the later
tertiary formations. Barrande, also, shows that there is a striking
general parallelism in the successive Silurian deposits of Bohemia and
Scandinavia; nevertheless he finds a surprising amount of difference in
the species. If the several formations in these regions have not been
deposited during the same exact periods--a formation in one region often
corresponding with a blank interval in the other--and if in both regions
the species have gone on slowly changing during the accumulation of the
several formations and during the long intervals of time between them;
in this case the several formations in the two regions could be arranged
in the same order, in accordance with the general succession of the
forms of life, and the order would falsely appear to be strictly
parallel; nevertheless the species would not all be the same in the
apparently corresponding stages in the two regions.

ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVING FORMS.

Let us now look to the mutual affinities of extinct and living species.
All fall into a few grand classes; and this fact is at once explained on
the principle of descent. The more ancient any form is, the more, as a
general rule, it differs from living forms. But, as Buckland long ago
remarked, extinct species can all be classed either in still existing
groups, or between them. That the extinct forms of life help to fill
up the intervals between existing genera, families, and orders, is
certainly true; but as this statement has often been ignored or even
denied, it may be well to make some remarks on this subject, and to give
some instances. If we confine our attention either to the living or to
the extinct species of the same class, the series is far less perfect
than if we combine both into one general system. In the writings of
Professor Owen we continually meet with the expression of generalised
forms, as applied to extinct animals; and in the writings of Agassiz, of
prophetic or synthetic types; and these terms imply that such forms
are, in fact, intermediate or connecting links. Another distinguished
palaeontologist, M. Gaudry, has shown in the most striking manner that
many of the fossil mammals discovered by him in Attica serve to break
down the intervals between existing genera. Cuvier ranked the Ruminants
and Pachyderms as two of the most distinct orders of mammals; but so
many fossil links have been disentombed that Owen has had to alter the
whole classification, and has placed certain Pachyderms in the same
sub-order with ruminants; for example, he dissolves by gradations the
apparently wide interval between the pig and the camel. The Ungulata
or hoofed quadrupeds are now divided into the even-toed or odd-toed
divisions; but the Macrauchenia of South America connects to a certain
extent these two grand divisions. No one will deny that the Hipparion
is intermediate between the existing horse and certain other ungulate
forms. What a wonderful connecting link in the chain of mammals is the
Typotherium from South America, as the name given to it by Professor
Gervais expresses, and which cannot be placed in any existing order. The
Sirenia form a very distinct group of the mammals, and one of the most
remarkable peculiarities in existing dugong and lamentin is the entire
absence of hind limbs, without even a rudiment being left; but the
extinct Halitherium had, according to Professor Flower, an ossified
thigh-bone "articulated to a well-defined acetabulum in the pelvis," and
it thus makes some approach to ordinary hoofed quadrupeds, to which the
Sirenia are in other respects allied. The cetaceans or whales are
widely different from all other mammals, but the tertiary Zeuglodon and
Squalodon, which have been placed by some naturalists in an order
by themselves, are considered by Professor Huxley to be undoubtedly
cetaceans, "and to constitute connecting links with the aquatic
carnivora."

Even the wide interval between birds and reptiles has been shown by
the naturalist just quoted to be partially bridged over in the
most unexpected manner, on the one hand, by the ostrich and extinct
Archeopteryx, and on the other hand by the Compsognathus, one of
the Dinosaurians--that group which includes the most gigantic of all
terrestrial reptiles. Turning to the Invertebrata, Barrande asserts, a
higher authority could not be named, that he is every day taught that,
although palaeozoic animals can certainly be classed under existing
groups, yet that at this ancient period the groups were not so
distinctly separated from each other as they now are.

Some writers have objected to any extinct species, or group of species,
being considered as intermediate between any two living species, or
groups of species. If by this term it is meant that an extinct form is
directly intermediate in all its characters between two living forms or
groups, the objection is probably valid. But in a natural classification
many fossil species certainly stand between living species, and some
extinct genera between living genera, even between genera belonging to
distinct families. The most common case, especially with respect to very
distinct groups, such as fish and reptiles, seems to be that, supposing
them to be distinguished at the present day by a score of characters,
the ancient members are separated by a somewhat lesser number of
characters, so that the two groups formerly made a somewhat nearer
approach to each other than they now do.

It is a common belief that the more ancient a form is, by so much the
more it tends to connect by some of its characters groups now widely
separated from each other. This remark no doubt must be restricted
to those groups which have undergone much change in the course of
geological ages; and it would be difficult to prove the truth of
the proposition, for every now and then even a living animal, as the
Lepidosiren, is discovered having affinities directed towards very
distinct groups. Yet if we compare the older Reptiles and Batrachians,
the older Fish, the older Cephalopods, and the eocene Mammals, with the
recent members of the same classes, we must admit that there is truth in
the remark.

Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat complex,
I must request the reader to turn to the diagram in the fourth chapter.
We may suppose that the numbered letters in italics represent genera,
and the dotted lines diverging from them the species in each genus. The
diagram is much too simple, too few genera and too few species being
given, but this is unimportant for us. The horizontal lines may
represent successive geological formations, and all the forms beneath
the uppermost line may be considered as extinct. The three existing
genera, a14, q14, p14, will form a small family; b14 and f14, a closely
allied family or subfamily; and o14, i14, m14, a third family. These
three families, together with the many extinct genera on the several
lines of descent diverging from the parent form (A) will form an order;
for all will have inherited something in common from their ancient
progenitor. On the principle of the continued tendency to divergence
of character, which was formerly illustrated by this diagram, the more
recent any form is the more it will generally differ from its ancient
progenitor. Hence, we can understand the rule that the most ancient
fossils differ most from existing forms. We must not, however, assume
that divergence of character is a necessary contingency; it depends
solely on the descendants from a species being thus enabled to seize
on many and different places in the economy of nature. Therefore it is
quite possible, as we have seen in the case of some Silurian forms,
that a species might go on being slightly modified in relation to its
slightly altered conditions of life, and yet retain throughout a vast
period the same general characteristics. This is represented in the
diagram by the letter F14.

All the many forms, extinct and recent, descended from (A), make, as
before remarked, one order; and this order, from the continued effects
of extinction and divergence of character, has become divided into
several sub-families and families, some of which are supposed to have
perished at different periods, and some to have endured to the present
day.

By looking at the diagram we can see that if many of the extinct forms
supposed to be embedded in the successive formations, were discovered
at several points low down in the series, the three existing families on
the uppermost line would be rendered less distinct from each other. If,
for instance, the genera a1, a5, a10, f8, m3, m6, m9, were disinterred,
these three families would be so closely linked together that they
probably would have to be united into one great family, in nearly the
same manner as has occurred with ruminants and certain pachyderms. Yet
he who objected to consider as intermediate the extinct genera, which
thus link together the living genera of three families, would be partly
justified, for they are intermediate, not directly, but only by a long
and circuitous course through many widely different forms. If many
extinct forms were to be discovered above one of the middle horizontal
lines or geological formations--for instance, above No. VI.--but none
from beneath this line, then only two of the families (those on the left
hand a14, etc., and b14, etc.) would have to be united into one; and
there would remain two families which would be less distinct from each
other than they were before the discovery of the fossils. So again, if
the three families formed of eight genera (a14 to m14), on the uppermost
line, be supposed to differ from each other by half-a-dozen important
characters, then the families which existed at a period marked VI would
certainly have differed from each other by a less number of characters;
for they would at this early stage of descent have diverged in a less
degree from their common progenitor. Thus it comes that ancient and
extinct genera are often in a greater or less degree intermediate
in character between their modified descendants, or between their
collateral relations.

Under nature the process will be far more complicated than is
represented in the diagram; for the groups will have been more numerous;
they will have endured for extremely unequal lengths of time, and will
have been modified in various degrees. As we possess only the last
volume of the geological record, and that in a very broken condition,
we have no right to expect, except in rare cases, to fill up the wide
intervals in the natural system, and thus to unite distinct families or
orders. All that we have a right to expect is, that those groups which
have, within known geological periods, undergone much modification,
should in the older formations make some slight approach to each other;
so that the older members should differ less from each other in some of
their characters than do the existing members of the same groups;
and this by the concurrent evidence of our best palaeontologists is
frequently the case.

Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, are explained in a satisfactory manner. And
they are wholly inexplicable on any other view.

On this same theory, it is evident that the fauna during any one great
period in the earth's history will be intermediate in general character
between that which preceded and that which succeeded it. Thus the
species which lived at the sixth great stage of descent in the diagram
are the modified offspring of those which lived at the fifth stage, and
are the parents of those which became still more modified at the
seventh stage; hence they could hardly fail to be nearly intermediate in
character between the forms of life above and below. We must, however,
allow for the entire extinction of some preceding forms, and in any one
region for the immigration of new forms from other regions, and for a
large amount of modification during the long and blank intervals between
the successive formations. Subject to these allowances, the fauna of
each geological period undoubtedly is intermediate in character, between
the preceding and succeeding faunas. I need give only one instance,
namely, the manner in which the fossils of the Devonian system,
when this system was first discovered, were at once recognised by
palaeontologists as intermediate in character between those of the
overlying carboniferous and underlying Silurian systems. But each fauna
is not necessarily exactly intermediate, as unequal intervals of time
have elapsed between consecutive formations.

It is no real objection to the truth of the statement that the fauna of
each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions
to the rule. For instance, the species of mastodons and elephants, when
arranged by Dr. Falconer in two series--in the first place according
to their mutual affinities, and in the second place according to their
periods of existence--do not accord in arrangement. The species extreme
in character are not the oldest or the most recent; nor are those which
are intermediate in character, intermediate in age. But supposing for
an instant, in this and other such cases, that the record of the first
appearance and disappearance of the species was complete, which is far
from the case, we have no reason to believe that forms successively
produced necessarily endure for corresponding lengths of time. A very
ancient form may occasionally have lasted much longer than a form
elsewhere subsequently produced, especially in the case of terrestrial
productions inhabiting separated districts. To compare small things with
great; if the principal living and extinct races of the domestic pigeon
were arranged in serial affinity, this arrangement would not closely
accord with the order in time of their production, and even less with
the order of their disappearance; for the parent rock-pigeon still
lives; and many varieties between the rock-pigeon and the carrier
have become extinct; and carriers which are extreme in the important
character of length of beak originated earlier than short-beaked
tumblers, which are at the opposite end of the series in this respect.

Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character,
is the fact, insisted on by all palaeontologists, that fossils from two
consecutive formations are far more closely related to each other, than
are the fossils from two remote formations. Pictet gives as a well-known
instance, the general resemblance of the organic remains from the
several stages of the Chalk formation, though the species are distinct
in each stage. This fact alone, from its generality, seems to have
shaken Professor Pictet in his belief in the immutability of species.
He who is acquainted with the distribution of existing species over the
globe, will not attempt to account for the close resemblance of distinct
species in closely consecutive formations, by the physical conditions of
the ancient areas having remained nearly the same. Let it be remembered
that the forms of life, at least those inhabiting the sea, have changed
almost simultaneously throughout the world, and therefore under the most
different climates and conditions. Consider the prodigious vicissitudes
of climate during the pleistocene period, which includes the whole
glacial epoch, and note how little the specific forms of the inhabitants
of the sea have been affected.

On the theory of descent, the full meaning of the fossil remains from
closely consecutive formations, being closely related, though ranked as
distinct species, is obvious. As the accumulation of each formation
has often been interrupted, and as long blank intervals have intervened
between successive formations, we ought not to expect to find, as
I attempted to show in the last chapter, in any one or in any two
formations, all the intermediate varieties between the species which
appeared at the commencement and close of these periods: but we ought
to find after intervals, very long as measured by years, but only
moderately long as measured geologically, closely allied forms, or, as
they have been called by some authors, representative species; and these
assuredly we do find. We find, in short, such evidence of the slow and
scarcely sensible mutations of specific forms, as we have the right to
expect.

ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED WITH LIVING FORMS.

We have seen in the fourth chapter that the degree of differentiation
and specialisation of the parts in organic beings, when arrived at
maturity, is the best standard, as yet suggested, of their degree of
perfection or highness. We have also seen that, as the specialisation of
parts is an advantage to each being, so natural selection will tend to
render the organisation of each being more specialised and perfect,
and in this sense higher; not but that it may leave many creatures with
simple and unimproved structures fitted for simple conditions of life,
and in some cases will even degrade or simplify the organisation, yet
leaving such degraded beings better fitted for their new walks of life.
In another and more general manner, new species become superior to their
predecessors; for they have to beat in the struggle for life all
the older forms, with which they come into close competition. We may
therefore conclude that if under a nearly similar climate the eocene
inhabitants of the world could be put into competition with the existing
inhabitants, the former would be beaten and exterminated by the
latter, as would the secondary by the eocene, and the palaeozoic by
the secondary forms. So that by this fundamental test of victory in the
battle for life, as well as by the standard of the specialisation of
organs, modern forms ought, on the theory of natural selection, to
stand higher than ancient forms. Is this the case? A large majority of
palaeontologists would answer in the affirmative; and it seems that this
answer must be admitted as true, though difficult of proof.

It is no valid objection to this conclusion, that certain Brachiopods
have been but slightly modified from an extremely remote geological
epoch; and that certain land and fresh-water shells have remained nearly
the same, from the time when, as far as is known, they first appeared.
It is not an insuperable difficulty that Foraminifera have not, as
insisted on by Dr. Carpenter, progressed in organisation since even the
Laurentian epoch; for some organisms would have to remain fitted for
simple conditions of life, and what could be better fitted for this end
than these lowly organised Protozoa? Such objections as the above
would be fatal to my view, if it included advance in organisation as
a necessary contingent. They would likewise be fatal, if the above
Foraminifera, for instance, could be proved to have first come into
existence during the Laurentian epoch, or the above Brachiopods during
the Cambrian formation; for in this case, there would not have been time
sufficient for the development of these organisms up to the standard
which they had then reached. When advanced up to any given point, there
is no necessity, on the theory of natural selection, for their further
continued process; though they will, during each successive age, have to
be slightly modified, so as to hold their places in relation to slight
changes in their conditions. The foregoing objections hinge on the
question whether we really know how old the world is, and at what period
the various forms of life first appeared; and this may well be disputed.

The problem whether organisation on the whole has advanced is in
many ways excessively intricate. The geological record, at all times
imperfect, does not extend far enough back to show with unmistakable
clearness that within the known history of the world organisation has
largely advanced. Even at the present day, looking to members of the
same class, naturalists are not unanimous which forms ought to be ranked
as highest: thus, some look at the selaceans or sharks, from their
approach in some important points of structure to reptiles, as the
highest fish; others look at the teleosteans as the highest. The ganoids
stand intermediate between the selaceans and teleosteans; the latter
at the present day are largely preponderant in number; but formerly
selaceans and ganoids alone existed; and in this case, according to
the standard of highness chosen, so will it be said that fishes have
advanced or retrograded in organisation. To attempt to compare members
of distinct types in the scale of highness seems hopeless; who will
decide whether a cuttle-fish be higher than a bee--that insect which
the great Von Baer believed to be "in fact more highly organised than a
fish, although upon another type?" In the complex struggle for life it
is quite credible that crustaceans, not very high in their own class,
might beat cephalopods, the highest molluscs; and such crustaceans,
though not highly developed, would stand very high in the scale of
invertebrate animals, if judged by the most decisive of all trials--the
law of battle. Beside these inherent difficulties in deciding which
forms are the most advanced in organisation, we ought not solely to
compare the highest members of a class at any two periods--though
undoubtedly this is one and perhaps the most important element in
striking a balance--but we ought to compare all the members, high
and low, at two periods. At an ancient epoch the highest and lowest
molluscoidal animals, namely, cephalopods and brachiopods, swarmed in
numbers; at the present time both groups are greatly reduced,
while others, intermediate in organisation, have largely increased;
consequently some naturalists maintain that molluscs were formerly more
highly developed than at present; but a stronger case can be made out on
the opposite side, by considering the vast reduction of brachiopods, and
the fact that our existing cephalopods, though few in number, are more
highly organised than their ancient representatives. We ought also to
compare the relative proportional numbers, at any two periods, of the
high and low classes throughout the world: if, for instance, at the
present day fifty thousand kinds of vertebrate animals exist, and if
we knew that at some former period only ten thousand kinds existed, we
ought to look at this increase in number in the highest class, which
implies a great displacement of lower forms, as a decided advance in the
organisation of the world. We thus see how hopelessly difficult it is to
compare with perfect fairness, under such extremely complex relations,
the standard of organisation of the imperfectly-known faunas of
successive periods.

We shall appreciate this difficulty more clearly by looking to certain
existing faunas and floras. From the extraordinary manner in which
European productions have recently spread over New Zealand, and have
seized on places which must have been previously occupied by the
indigenes, we must believe, that if all the animals and plants of Great
Britain were set free in New Zealand, a multitude of British forms would
in the course of time become thoroughly naturalized there, and would
exterminate many of the natives. On the other hand, from the fact that
hardly a single inhabitant of the southern hemisphere has become wild in
any part of Europe, we may well doubt whether, if all the productions
of New Zealand were set free in Great Britain, any considerable number
would be enabled to seize on places now occupied by our native plants
and animals. Under this point of view, the productions of Great Britain
stand much higher in the scale than those of New Zealand. Yet the
most skilful naturalist, from an examination of the species of the two
countries, could not have foreseen this result.

Agassiz and several other highly competent judges insist that ancient
animals resemble to a certain extent the embryos of recent animals
belonging to the same classes; and that the geological succession of
extinct forms is nearly parallel with the embryological development of
existing forms. This view accords admirably well with our theory. In a
future chapter I shall attempt to show that the adult differs from its
embryo, owing to variations having supervened at a not early age, and
having been inherited at a corresponding age. This process, whilst it
leaves the embryo almost unaltered, continually adds, in the course of
successive generations, more and more difference to the adult. Thus the
embryo comes to be left as a sort of picture, preserved by nature, of
the former and less modified condition of the species. This view may be
true, and yet may never be capable of proof. Seeing, for instance, that
the oldest known mammals, reptiles, and fishes strictly belong to their
proper classes, though some of these old forms are in a slight degree
less distinct from each other than are the typical members of the same
groups at the present day, it would be vain to look for animals having
the common embryological character of the Vertebrata, until beds rich
in fossils are discovered far beneath the lowest Cambrian strata--a
discovery of which the chance is small.

ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE
LATER TERTIARY PERIODS.

Mr. Clift many years ago showed that the fossil mammals from the
Australian caves were closely allied to the living marsupials of that
continent. In South America, a similar relationship is manifest, even to
an uneducated eye, in the gigantic pieces of armour, like those of the
armadillo, found in several parts of La Plata; and Professor Owen has
shown in the most striking manner that most of the fossil mammals,
buried there in such numbers, are related to South American types. This
relationship is even more clearly seen in the wonderful collection of
fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was
so much impressed with these facts that I strongly insisted, in 1839
and 1845, on this "law of the succession of types,"--on "this wonderful
relationship in the same continent between the dead and the living."
Professor Owen has subsequently extended the same generalisation to
the mammals of the Old World. We see the same law in this author's
restorations of the extinct and gigantic birds of New Zealand. We see
it also in the birds of the caves of Brazil. Mr. Woodward has shown that
the same law holds good with sea-shells, but, from the wide distribution
of most molluscs, it is not well displayed by them. Other cases could
be added, as the relation between the extinct and living land-shells of
Madeira; and between the extinct and living brackish water-shells of the
Aralo-Caspian Sea.

Now, what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man who, after comparing
the present climate of Australia and of parts of South America, under
the same latitude, would attempt to account, on the one hand through
dissimilar physical conditions, for the dissimilarity of the inhabitants
of these two continents; and, on the other hand through similarity
of conditions, for the uniformity of the same types in each continent
during the later tertiary periods. Nor can it be pretended that it is
an immutable law that marsupials should have been chiefly or solely
produced in Australia; or that Edentata and other American types should
have been solely produced in South America. For we know that Europe in
ancient times was peopled by numerous marsupials; and I have shown
in the publications above alluded to, that in America the law of
distribution of terrestrial mammals was formerly different from what it
now is. North America formerly partook strongly of the present character
of the southern half of the continent; and the southern half was
formerly more closely allied, than it is at present, to the northern
half. In a similar manner we know, from Falconer and Cautley's
discoveries, that Northern India was formerly more closely related in
its mammals to Africa than it is at the present time. Analogous facts
could be given in relation to the distribution of marine animals.

On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the
same areas, is at once explained; for the inhabitants of each quarter of
the world will obviously tend to leave in that quarter, during the next
succeeding period of time, closely allied though in some degree modified
descendants. If the inhabitants of one continent formerly differed
greatly from those of another continent, so will their modified
descendants still differ in nearly the same manner and degree. But
after very long intervals of time, and after great geographical changes,
permitting much intermigration, the feebler will yield to the more
dominant forms, and there will be nothing immutable in the distribution
of organic beings.

It may be asked in ridicule whether I suppose that the megatherium and
other allied huge monsters, which formerly lived in South America, have
left behind them the sloth, armadillo, and anteater, as their degenerate
descendants. This cannot for an instant be admitted. These huge animals
have become wholly extinct, and have left no progeny. But in the caves
of Brazil there are many extinct species which are closely allied in
size and in all other characters to the species still living in South
America; and some of these fossils may have been the actual progenitors
of the living species. It must not be forgotten that, on our theory, all
the species of the same genus are the descendants of some one species;
so that, if six genera, each having eight species, be found in one
geological formation, and in a succeeding formation there be six other
allied or representative genera, each with the same number of species,
then we may conclude that generally only one species of each of the
older genera has left modified descendants, which constitute the new
genera containing the several species; the other seven species of each
old genus having died out and left no progeny. Or, and this will be a
far commoner case, two or three species in two or three alone of the six
older genera will be the parents of the new genera: the other species
and the other old genera having become utterly extinct. In failing
orders, with the genera and species decreasing in numbers as is the case
with the Edentata of South America, still fewer genera and species will
leave modified blood-descendants.

SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.

I have attempted to show that the geological record is extremely
imperfect; that only a small portion of the globe has been geologically
explored with care; that only certain classes of organic beings have
been largely preserved in a fossil state; that the number both of
specimens and of species, preserved in our museums, is absolutely as
nothing compared with the number of generations which must have passed
away even during a single formation; that, owing to subsidence being
almost necessary for the accumulation of deposits rich in fossil species
of many kinds, and thick enough to outlast future degradation, great
intervals of time must have elapsed between most of our successive
formations; that there has probably been more extinction during
the periods of subsidence, and more variation during the periods
of elevation, and during the latter the record will have been least
perfectly kept; that each single formation has not been continuously
deposited; that the duration of each formation is probably short
compared with the average duration of specific forms; that migration has
played an important part in the first appearance of new forms in any
one area and formation; that widely ranging species are those which have
varied most frequently, and have oftenest given rise to new species;
that varieties have at first been local; and lastly, although each
species must have passed through numerous transitional stages, it is
probable that the periods, during which each underwent modification,
though many and long as measured by years, have been short in comparison
with the periods during which each remained in an unchanged condition.
These causes, taken conjointly, will to a large extent explain
why--though we do find many links--we do not find interminable
varieties, connecting together all extinct and existing forms by the
finest graduated steps. It should also be constantly borne in mind that
any linking variety between two forms, which might be found, would be
ranked, unless the whole chain could be perfectly restored, as a new
and distinct species; for it is not pretended that we have any sure
criterion by which species and varieties can be discriminated.

He who rejects this view of the imperfection of the geological record,
will rightly reject the whole theory. For he may ask in vain where are
the numberless transitional links which must formerly have connected the
closely allied or representative species, found in the successive stages
of the same great formation? He may disbelieve in the immense intervals
of time which must have elapsed between our consecutive formations;
he may overlook how important a part migration has played, when the
formations of any one great region, as those of Europe, are considered;
he may urge the apparent, but often falsely apparent, sudden coming in
of whole groups of species. He may ask where are the remains of those
infinitely numerous organisms which must have existed long before the
Cambrian system was deposited? We now know that at least one animal did
then exist; but I can answer this last question only by supposing that
where our oceans now extend they have extended for an enormous period,
and where our oscillating continents now stand they have stood since the
commencement of the Cambrian system; but that, long before that epoch,
the world presented a widely different aspect; and that the older
continents, formed of formations older than any known to us, exist now
only as remnants in a metamorphosed condition, or lie still buried under
the ocean.

Passing from these difficulties, the other great leading facts
in palaeontology agree admirably with the theory of descent with
modification through variation and natural selection. We can thus
understand how it is that new species come in slowly and successively;
how species of different classes do not necessarily change together, or
at the same rate, or in the same degree; yet in the long run that all
undergo modification to some extent. The extinction of old forms is the
almost inevitable consequence of the production of new forms. We can
understand why, when a species has once disappeared, it never reappears.
Groups of species increase in numbers slowly, and endure for unequal
periods of time; for the process of modification is necessarily
slow, and depends on many complex contingencies. The dominant species
belonging to large and dominant groups tend to leave many modified
descendants, which form new sub-groups and groups. As these are
formed, the species of the less vigorous groups, from their inferiority
inherited from a common progenitor, tend to become extinct together, and
to leave no modified offspring on the face of the earth. But the
utter extinction of a whole group of species has sometimes been a slow
process, from the survival of a few descendants, lingering in protected
and isolated situations. When a group has once wholly disappeared, it
does not reappear; for the link of generation has been broken.

We can understand how it is that dominant forms which spread widely and
yield the greatest number of varieties tend to people the world with
allied, but modified, descendants; and these will generally succeed
in displacing the groups which are their inferiors in the struggle for
existence. Hence, after long intervals of time, the productions of the
world appear to have changed simultaneously.

We can understand how it is that all the forms of life, ancient and
recent, make together a few grand classes. We can understand, from the
continued tendency to divergence of character, why the more ancient
a form is, the more it generally differs from those now living. Why
ancient and extinct forms often tend to fill up gaps between existing
forms, sometimes blending two groups, previously classed as distinct
into one; but more commonly bringing them only a little closer together.
The more ancient a form is, the more often it stands in some degree
intermediate between groups now distinct; for the more ancient a form
is, the more nearly it will be related to, and consequently resemble,
the common progenitor of groups, since become widely divergent. Extinct
forms are seldom directly intermediate between existing forms; but are
intermediate only by a long and circuitous course through other extinct
and different forms. We can clearly see why the organic remains of
closely consecutive formations are closely allied; for they are closely
linked together by generation. We can clearly see why the remains of an
intermediate formation are intermediate in character.

The inhabitants of the world at each successive period in its history
have beaten their predecessors in the race for life, and are, in so
far, higher in the scale, and their structure has generally become more
specialised; and this may account for the common belief held by so many
palaeontologists, that organisation on the whole has progressed. Extinct
and ancient animals resemble to a certain extent the embryos of the more
recent animals belonging to the same classes, and this wonderful fact
receives a simple explanation according to our views. The succession
of the same types of structure within the same areas during the later
geological periods ceases to be mysterious, and is intelligible on the
principle of inheritance.

If, then, the geological record be as imperfect as many believe, and
it may at least be asserted that the record cannot be proved to be much
more perfect, the main objections to the theory of natural selection are
greatly diminished or disappear. On the other hand, all the chief laws
of palaeontology plainly proclaim, as it seems to me, that species have
been produced by ordinary generation: old forms having been supplanted
by new and improved forms of life, the products of variation and the
survival of the fittest.



CHAPTER XII. GEOGRAPHICAL DISTRIBUTION.

 Present distribution cannot be accounted for by differences in physical
 conditions--Importance of barriers--Affinity of the productions of the
 same continent--Centres of creation--Means of dispersal by changes of
 climate and of the level of the land, and by occasional means--Dispersal
 during the Glacial period--Alternate Glacial periods in the North and
 South.


In considering the distribution of organic beings over the face of
the globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions
can be wholly accounted for by climatal and other physical conditions.
Of late, almost every author who has studied the subject has come to
this conclusion. The case of America alone would almost suffice to prove
its truth; for if we exclude the arctic and northern temperate parts,
all authors agree that one of the most fundamental divisions in
geographical distribution is that between the New and Old Worlds; yet
if we travel over the vast American continent, from the central parts of
the United States to its extreme southern point, we meet with the most
diversified conditions; humid districts, arid deserts, lofty mountains,
grassy plains, forests, marshes, lakes and great rivers, under almost
every temperature. There is hardly a climate or condition in the Old
World which cannot be paralleled in the New--at least so closely as the
same species generally require. No doubt small areas can be pointed out
in the Old World hotter than any in the New World; but these are not
inhabited by a fauna different from that of the surrounding districts;
for it is rare to find a group of organisms confined to a small area,
of which the conditions are peculiar in only a slight degree.
Notwithstanding this general parallelism in the conditions of Old and
New Worlds, how widely different are their living productions!

In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes
25 and 35 degrees, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas
and floras more utterly dissimilar. Or, again, we may compare the
productions of South America south of latitude 35 degrees with those
north of 25 degrees, which consequently are separated by a space of
ten degrees of latitude, and are exposed to considerably different
conditions; yet they are incomparably more closely related to each other
than they are to the productions of Australia or Africa under nearly
the same climate. Analogous facts could be given with respect to the
inhabitants of the sea.

A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions of
various regions. We see this in the great difference in nearly all the
terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a slightly
different climate, there might have been free migration for the northern
temperate forms, as there now is for the strictly arctic productions.
We see the same fact in the great difference between the inhabitants of
Australia, Africa, and South America under the same latitude; for these
countries are almost as much isolated from each other as is possible. On
each continent, also, we see the same fact; for on the opposite sides of
lofty and continuous mountain-ranges, and of great deserts and even of
large rivers, we find different productions; though as mountain chains,
deserts, etc., are not as impassable, or likely to have endured so long,
as the oceans separating continents, the differences are very inferior
in degree to those characteristic of distinct continents.

Turning to the sea, we find the same law. The marine inhabitants of
the eastern and western shores of South America are very distinct, with
extremely few shells, crustacea, or echinodermata in common; but Dr.
Gunther has recently shown that about thirty per cent of the fishes are
the same on the opposite sides of the isthmus of Panama; and this fact
has led naturalists to believe that the isthmus was formerly open.
Westward of the shores of America, a wide space of open ocean extends,
with not an island as a halting-place for emigrants; here we have a
barrier of another kind, and as soon as this is passed we meet in the
eastern islands of the Pacific with another and totally distinct fauna.
So that three marine faunas range northward and southward in parallel
lines not far from each other, under corresponding climate; but from
being separated from each other by impassable barriers, either of
land or open sea, they are almost wholly distinct. On the other hand,
proceeding still further westward from the eastern islands of the
tropical parts of the Pacific, we encounter no impassable barriers, and
we have innumerable islands as halting-places, or continuous coasts,
until, after travelling over a hemisphere, we come to the shores of
Africa; and over this vast space we meet with no well-defined and
distinct marine faunas. Although so few marine animals are common to the
above-named three approximate faunas of Eastern and Western America and
the eastern Pacific islands, yet many fishes range from the Pacific into
the Indian Ocean, and many shells are common to the eastern islands of
the Pacific and the eastern shores of Africa on almost exactly opposite
meridians of longitude.

A third great fact, partly included in the foregoing statement, is the
affinity of the productions of the same continent or of the same sea,
though the species themselves are distinct at different points and
stations. It is a law of the widest generality, and every continent
offers innumerable instances. Nevertheless, the naturalist, in
travelling, for instance, from north to south, never fails to be
struck by the manner in which successive groups of beings, specifically
distinct, though nearly related, replace each other. He hears from
closely allied, yet distinct kinds of birds, notes nearly similar, and
sees their nests similarly constructed, but not quite alike, with eggs
coloured in nearly the same manner. The plains near the Straits of
Magellan are inhabited by one species of Rhea (American ostrich), and
northward the plains of La Plata by another species of the same genus;
and not by a true ostrich or emu, like those inhabiting Africa and
Australia under the same latitude. On these same plains of La Plata we
see the agouti and bizcacha, animals having nearly the same habits as
our hares and rabbits, and belonging to the same order of Rodents, but
they plainly display an American type of structure. We ascend the lofty
peaks of the Cordillera, and we find an alpine species of bizcacha; we
look to the waters, and we do not find the beaver or muskrat, but the
coypu and capybara, rodents of the South American type. Innumerable
other instances could be given. If we look to the islands off the
American shore, however much they may differ in geological structure,
the inhabitants are essentially American, though they may be all
peculiar species. We may look back to past ages, as shown in the last
chapter, and we find American types then prevailing on the American
continent and in the American seas. We see in these facts some deep
organic bond, throughout space and time, over the same areas of land and
water, independently of physical conditions. The naturalist must be dull
who is not led to inquire what this bond is.

The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we
see in the case of varieties, nearly alike. The dissimilarity of the
inhabitants of different regions may be attributed to modification
through variation and natural selection, and probably in a subordinate
degree to the definite influence of different physical conditions.
The degrees of dissimilarity will depend on the migration of the more
dominant forms of life from one region into another having been more
or less effectually prevented, at periods more or less remote--on the
nature and number of the former immigrants--and on the action of the
inhabitants on each other in leading to the preservation of different
modifications; the relation of organism to organism in the struggle for
life being, as I have already often remarked, the most important of
all relations. Thus the high importance of barriers comes into play by
checking migration; as does time for the slow process of modification
through natural selection. Widely-ranging species, abounding in
individuals, which have already triumphed over many competitors in their
own widely-extended homes, will have the best chance of seizing on new
places, when they spread out into new countries. In their new homes they
will be exposed to new conditions, and will frequently undergo further
modification and improvement; and thus they will become still further
victorious, and will produce groups of modified descendants. On this
principle of inheritance with modification we can understand how it is
that sections of genera, whole genera, and even families, are confined
to the same areas, as is so commonly and notoriously the case.

There is no evidence, as was remarked in the last chapter, of the
existence of any law of necessary development. As the variability of
each species is an independent property, and will be taken advantage of
by natural selection, only so far as it profits each individual in its
complex struggle for life, so the amount of modification in different
species will be no uniform quantity. If a number of species, after
having long competed with each other in their old home, were to migrate
in a body into a new and afterwards isolated country, they would be
little liable to modification; for neither migration nor isolation in
themselves effect anything. These principles come into play only by
bringing organisms into new relations with each other and in a lesser
degree with the surrounding physical conditions. As we have seen in the
last chapter that some forms have retained nearly the same character
from an enormously remote geological period, so certain species have
migrated over vast spaces, and have not become greatly or at all
modified.

According to these views, it is obvious that the several species of the
same genus, though inhabiting the most distant quarters of the world,
must originally have proceeded from the same source, as they are
descended from the same progenitor. In the case of those species which
have undergone, during whole geological periods, little modification,
there is not much difficulty in believing that they have migrated from
the same region; for during the vast geographical and climatical
changes which have supervened since ancient times, almost any amount of
migration is possible. But in many other cases, in which we have
reason to believe that the species of a genus have been produced within
comparatively recent times, there is great difficulty on this head. It
is also obvious that the individuals of the same species, though now
inhabiting distant and isolated regions, must have proceeded from
one spot, where their parents were first produced: for, as has been
explained, it is incredible that individuals identically the same should
have been produced from parents specifically distinct.

SINGLE CENTRES OF SUPPOSED CREATION.

We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more
points of the earth's surface. Undoubtedly there are many cases of
extreme difficulty in understanding how the same species could possibly
have migrated from some one point to the several distant and isolated
points, where now found. Nevertheless the simplicity of the view that
each species was first produced within a single region captivates the
mind. He who rejects it, rejects the vera causa of ordinary generation
with subsequent migration, and calls in the agency of a miracle. It is
universally admitted, that in most cases the area inhabited by a species
is continuous; and that when a plant or animal inhabits two points so
distant from each other, or with an interval of such a nature, that the
space could not have been easily passed over by migration, the fact
is given as something remarkable and exceptional. The incapacity of
migrating across a wide sea is more clear in the case of terrestrial
mammals than perhaps with any other organic beings; and, accordingly,
we find no inexplicable instances of the same mammals inhabiting distant
points of the world. No geologist feels any difficulty in Great Britain
possessing the same quadrupeds with the rest of Europe, for they were
no doubt once united. But if the same species can be produced at two
separate points, why do we not find a single mammal common to Europe and
Australia or South America? The conditions of life are nearly the
same, so that a multitude of European animals and plants have become
naturalised in America and Australia; and some of the aboriginal plants
are identically the same at these distant points of the northern and
southern hemispheres? The answer, as I believe, is, that mammals have
not been able to migrate, whereas some plants, from their varied means
of dispersal, have migrated across the wide and broken interspaces. The
great and striking influence of barriers of all kinds, is intelligible
only on the view that the great majority of species have been produced
on one side, and have not been able to migrate to the opposite side.
Some few families, many subfamilies, very many genera, a still greater
number of sections of genera, are confined to a single region; and it
has been observed by several naturalists that the most natural genera,
or those genera in which the species are most closely related to each
other, are generally confined to the same country, or if they have a
wide range that their range is continuous. What a strange anomaly it
would be if a directly opposite rule were to prevail when we go down one
step lower in the series, namely to the individuals of the same species,
and these had not been, at least at first, confined to some one region!

Hence, it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration
and subsistence under past and present conditions permitted, is the most
probable. Undoubtedly many cases occur in which we cannot explain how
the same species could have passed from one point to the other. But the
geographical and climatical changes which have certainly occurred within
recent geological times, must have rendered discontinuous the formerly
continuous range of many species. So that we are reduced to consider
whether the exceptions to continuity of range are so numerous, and of so
grave a nature, that we ought to give up the belief, rendered probable
by general considerations, that each species has been produced within
one area, and has migrated thence as far as it could. It would be
hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points; nor do I for a
moment pretend that any explanation could be offered of many instances.
But, after some preliminary remarks, I will discuss a few of the most
striking classes of facts, namely, the existence of the same species
on the summits of distant mountain ranges, and at distant points in the
Arctic and Antarctic regions; and secondly (in the following chapter),
the wide distribution of fresh water productions; and thirdly, the
occurrence of the same terrestrial species on islands and on the nearest
mainland, though separated by hundreds of miles of open sea. If the
existence of the same species at distant and isolated points of the
earth's surface can in many instances be explained on the view of each
species having migrated from a single birthplace; then, considering our
ignorance with respect to former climatical and geographical changes,
and to the various occasional means of transport, the belief that a
single birthplace is the law seems to me incomparably the safest.

In discussing this subject we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
species of a genus which must on our theory all be descended from a
common progenitor, can have migrated, undergoing modification during
their migration from some one area. If, when most of the species
inhabiting one region are different from those of another region, though
closely allied to them, it can be shown that migration from the one
region to the other has probably occurred at some former period, our
general view will be much strengthened; for the explanation is obvious
on the principle of descent with modification. A volcanic island, for
instance, upheaved and formed at the distance of a few hundreds of miles
from a continent, would probably receive from it in the course of time
a few colonists, and their descendants, though modified, would still be
related by inheritance to the inhabitants of that continent. Cases of
this nature are common, and are, as we shall hereafter see, inexplicable
on the theory of independent creation. This view of the relation of the
species of one region to those of another, does not differ much from
that advanced by Mr. Wallace, who concludes that "every species has come
into existence coincident both in space and time with a pre-existing
closely allied species." And it is now well known that he attributes
this coincidence to descent with modification.

The question of single or multiple centres of creation differs from
another though allied question, namely, whether all the individuals
of the same species are descended from a single pair, or single
hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With organic beings which never
intercross, if such exist, each species, must be descended from a
succession of modified varieties, that have supplanted each other,
but have never blended with other individuals or varieties of the same
species, so that, at each successive stage of modification, all the
individuals of the same form will be descended from a single parent.
But in the great majority of cases, namely, with all organisms which
habitually unite for each birth, or which occasionally intercross, the
individuals of the same species inhabiting the same area will be kept
nearly uniform by intercrossing; so that many individuals will go on
simultaneously changing, and the whole amount of modification at each
stage will not be due to descent from a single parent. To illustrate
what I mean: our English race-horses differ from the horses of every
other breed; but they do not owe their difference and superiority to
descent from any single pair, but to continued care in the selecting and
training of many individuals during each generation.

Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of dispersal.

MEANS OF DISPERSAL.

Sir C. Lyell and other authors have ably treated this subject. I can
give here only the briefest abstract of the more important facts. Change
of climate must have had a powerful influence on migration. A region now
impassable to certain organisms from the nature of its climate, might
have been a high road for migration, when the climate was different. I
shall, however, presently have to discuss this branch of the subject
in some detail. Changes of level in the land must also have been highly
influential: a narrow isthmus now separates two marine faunas; submerge
it, or let it formerly have been submerged, and the two faunas will now
blend together, or may formerly have blended. Where the sea now extends,
land may at a former period have connected islands or possibly even
continents together, and thus have allowed terrestrial productions to
pass from one to the other. No geologist disputes that great mutations
of level have occurred within the period of existing organisms. Edward
Forbes insisted that all the islands in the Atlantic must have been
recently connected with Europe or Africa, and Europe likewise with
America. Other authors have thus hypothetically bridged over every
ocean, and united almost every island with some mainland. If, indeed,
the arguments used by Forbes are to be trusted, it must be admitted that
scarcely a single island exists which has not recently been united to
some continent. This view cuts the Gordian knot of the dispersal of the
same species to the most distant points, and removes many a difficulty;
but to the best of my judgment we are not authorized in admitting such
enormous geographical changes within the period of existing species. It
seems to me that we have abundant evidence of great oscillations in the
level of the land or sea; but not of such vast changes in the position
and extension of our continents, as to have united them within the
recent period to each other and to the several intervening oceanic
islands. I freely admit the former existence of many islands, now buried
beneath the sea, which may have served as halting places for plants and
for many animals during their migration. In the coral-producing oceans
such sunken islands are now marked by rings of coral or atolls standing
over them. Whenever it is fully admitted, as it will some day be, that
each species has proceeded from a single birthplace, and when in
the course of time we know something definite about the means of
distribution, we shall be enabled to speculate with security on the
former extension of the land. But I do not believe that it will ever be
proved that within the recent period most of our continents which now
stand quite separate, have been continuously, or almost continuously
united with each other, and with the many existing oceanic islands.
Several facts in distribution--such as the great difference in the
marine faunas on the opposite sides of almost every continent--the close
relation of the tertiary inhabitants of several lands and even seas to
their present inhabitants--the degree of affinity between the mammals
inhabiting islands with those of the nearest continent, being in part
determined (as we shall hereafter see) by the depth of the intervening
ocean--these and other such facts are opposed to the admission of such
prodigious geographical revolutions within the recent period, as are
necessary on the view advanced by Forbes and admitted by his followers.
The nature and relative proportions of the inhabitants of oceanic
islands are likewise opposed to the belief of their former continuity of
continents. Nor does the almost universally volcanic composition of
such islands favour the admission that they are the wrecks of sunken
continents; if they had originally existed as continental mountain
ranges, some at least of the islands would have been formed, like other
mountain summits, of granite, metamorphic schists, old fossiliferous and
other rocks, instead of consisting of mere piles of volcanic matter.

I must now say a few words on what are called accidental means, but
which more properly should be called occasional means of distribution.
I shall here confine myself to plants. In botanical works, this or that
plant is often stated to be ill adapted for wide dissemination; but the
greater or less facilities for transport across the sea may be said to
be almost wholly unknown. Until I tried, with Mr. Berkeley's aid, a
few experiments, it was not even known how far seeds could resist
the injurious action of sea-water. To my surprise I found that out
of eighty-seven kinds, sixty-four germinated after an immersion of
twenty-eight days, and a few survived an immersion of 137 days. It
deserves notice that certain orders were far more injured than others:
nine Leguminosae were tried, and, with one exception, they resisted the
salt-water badly; seven species of the allied orders, Hydrophyllaceae
and Polemoniaceae, were all killed by a month's immersion. For
convenience sake I chiefly tried small seeds without the capsules or
fruit; and as all of these sank in a few days, they could not have been
floated across wide spaces of the sea, whether or not they were injured
by salt water. Afterwards I tried some larger fruits, capsules, etc.,
and some of these floated for a long time. It is well known what a
difference there is in the buoyancy of green and seasoned timber; and it
occurred to me that floods would often wash into the sea dried plants or
branches with seed-capsules or fruit attached to them. Hence I was led
to dry the stems and branches of ninety-four plants with ripe fruit, and
to place them on sea-water. The majority sank quickly, but some which,
whilst green, floated for a very short time, when dried floated much
longer; for instance, ripe hazel-nuts sank immediately, but when dried
they floated for ninety days, and afterwards when planted germinated;
an asparagus plant with ripe berries floated for twenty-three days,
when dried it floated for eighty-five days, and the seeds afterwards
germinated: the ripe seeds of Helosciadium sank in two days, when
dried they floated for above ninety days, and afterwards germinated.
Altogether, out of the ninety-four dried plants, eighteen floated for
above twenty-eight days; and some of the eighteen floated for a very
much longer period. So that as 64/87 kinds of seeds germinated after an
immersion of twenty-eight days; and as 18/94 distinct species with ripe
fruit (but not all the same species as in the foregoing experiment)
floated, after being dried, for above twenty-eight days, we may
conclude, as far as anything can be inferred from these scanty facts,
that the seeds of 14/100 kinds of plants of any country might be floated
by sea-currents during twenty-eight days, and would retain their power
of germination. In Johnston's Physical Atlas, the average rate of the
several Atlantic currents is thirty-three miles per diem (some currents
running at the rate of sixty miles per diem); on this average, the seeds
of 14/100 plants belonging to one country might be floated across 924
miles of sea to another country; and when stranded, if blown by an
inland gale to a favourable spot, would germinate.

Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea,
so that they were alternately wet and exposed to the air like really
floating plants. He tried ninety-eight seeds, mostly different from
mine, but he chose many large fruits, and likewise seeds, from plants
which live near the sea; and this would have favoured both the average
length of their flotation and their resistance to the injurious action
of the salt-water. On the other hand, he did not previously dry the
plants or branches with the fruit; and this, as we have seen, would have
caused some of them to have floated much longer. The result was that
18/98 of his seeds of different kinds floated for forty-two days, and
were then capable of germination. But I do not doubt that plants exposed
to the waves would float for a less time than those protected from
violent movement as in our experiments. Therefore, it would perhaps be
safer to assume that the seeds of about 10/100 plants of a flora, after
having been dried, could be floated across a space of sea 900 miles in
width, and would then germinate. The fact of the larger fruits often
floating longer than the small, is interesting; as plants with large
seeds or fruit which, as Alph. de Candolle has shown, generally have
restricted ranges, could hardly be transported by any other means.

Seeds may be occasionally transported in another manner. Drift timber
is thrown up on most islands, even on those in the midst of the widest
oceans; and the natives of the coral islands in the Pacific procure
stones for their tools, solely from the roots of drifted trees, these
stones being a valuable royal tax. I find that when irregularly shaped
stones are embedded in the roots of trees, small parcels of earth
are very frequently enclosed in their interstices and behind them, so
perfectly that not a particle could be washed away during the longest
transport: out of one small portion of earth thus COMPLETELY enclosed by
the roots of an oak about fifty years old, three dicotyledonous plants
germinated: I am certain of the accuracy of this observation. Again,
I can show that the carcasses of birds, when floating on the sea,
sometimes escape being immediately devoured; and many kinds of seeds
in the crops of floating birds long retain their vitality: peas and
vetches, for instance, are killed by even a few days' immersion in
sea-water; but some taken out of the crop of a pigeon, which had floated
on artificial sea-water for thirty days, to my surprise nearly all
germinated.

Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the
ocean. We may safely assume that under such circumstances their rate of
flight would often be thirty-five miles an hour; and some authors have
given a far higher estimate. I have never seen an instance of nutritious
seeds passing through the intestines of a bird; but hard seeds of fruit
pass uninjured through even the digestive organs of a turkey. In the
course of two months, I picked up in my garden twelve kinds of seeds,
out of the excrement of small birds, and these seemed perfect, and some
of them, which were tried, germinated. But the following fact is more
important: the crops of birds do not secrete gastric juice, and do not,
as I know by trial, injure in the least the germination of seeds;
now, after a bird has found and devoured a large supply of food, it is
positively asserted that all the grains do not pass into the gizzard for
twelve or even eighteen hours. A bird in this interval might easily be
blown to the distance of five hundred miles, and hawks are known to look
out for tired birds, and the contents of their torn crops might thus
readily get scattered. Some hawks and owls bolt their prey whole, and
after an interval of from twelve to twenty hours, disgorge pellets,
which, as I know from experiments made in the Zoological Gardens,
include seeds capable of germination. Some seeds of the oat, wheat,
millet, canary, hemp, clover, and beet germinated after having been from
twelve to twenty-one hours in the stomachs of different birds of prey;
and two seeds of beet grew after having been thus retained for two days
and fourteen hours. Fresh-water fish, I find, eat seeds of many land and
water plants; fish are frequently devoured by birds, and thus the seeds
might be transported from place to place. I forced many kinds of
seeds into the stomachs of dead fish, and then gave their bodies to
fishing-eagles, storks, and pelicans; these birds, after an interval of
many hours, either rejected the seeds in pellets or passed them in their
excrement; and several of these seeds retained the power of germination.
Certain seeds, however, were always killed by this process.

Locusts are sometimes blown to great distances from the land. I myself
caught one 370 miles from the coast of Africa, and have heard of others
caught at greater distances. The Rev. R.T. Lowe informed Sir C. Lyell
that in November, 1844, swarms of locusts visited the island of Madeira.
They were in countless numbers, as thick as the flakes of snow in the
heaviest snowstorm, and extended upward as far as could be seen with a
telescope. During two or three days they slowly careered round and round
in an immense ellipse, at least five or six miles in diameter, and at
night alighted on the taller trees, which were completely coated with
them. They then disappeared over the sea, as suddenly as they had
appeared, and have not since visited the island. Now, in parts of Natal
it is believed by some farmers, though on insufficient evidence, that
injurious seeds are introduced into their grass-land in the dung left
by the great flights of locusts which often visit that country. In
consequence of this belief Mr. Weale sent me in a letter a small packet
of the dried pellets, out of which I extracted under the microscope
several seeds, and raised from them seven grass plants, belonging to
two species, of two genera. Hence a swarm of locusts, such as that which
visited Madeira, might readily be the means of introducing several kinds
of plants into an island lying far from the mainland.

Although the beaks and feet of birds are generally clean, earth
sometimes adheres to them: in one case I removed sixty-one grains, and
in another case twenty-two grains of dry argillaceous earth from the
foot of a partridge, and in the earth there was a pebble as large as the
seed of a vetch. Here is a better case: the leg of a woodcock was sent
to me by a friend, with a little cake of dry earth attached to the
shank, weighing only nine grains; and this contained a seed of
the toad-rush (Juncus bufonius) which germinated and flowered. Mr.
Swaysland, of Brighton, who during the last forty years has paid close
attention to our migratory birds, informs me that he has often shot
wagtails (Motacillae), wheatears, and whinchats (Saxicolae), on their
first arrival on our shores, before they had alighted; and he has
several times noticed little cakes of earth attached to their feet. Many
facts could be given showing how generally soil is charged with seeds.
For instance, Professor Newton sent me the leg of a red-legged partridge
(Caccabis rufa) which had been wounded and could not fly, with a ball of
hard earth adhering to it, and weighing six and a half ounces. The earth
had been kept for three years, but when broken, watered and placed
under a bell glass, no less than eighty-two plants sprung from it: these
consisted of twelve monocotyledons, including the common oat, and at
least one kind of grass, and of seventy dicotyledons, which consisted,
judging from the young leaves, of at least three distinct species.
With such facts before us, can we doubt that the many birds which are
annually blown by gales across great spaces of ocean, and which
annually migrate--for instance, the millions of quails across the
Mediterranean--must occasionally transport a few seeds embedded in
dirt adhering to their feet or beaks? But I shall have to recur to this
subject.

As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, it can
hardly be doubted that they must occasionally, as suggested by Lyell,
have transported seeds from one part to another of the arctic and
antarctic regions; and during the Glacial period from one part of the
now temperate regions to another. In the Azores, from the large number
of plants common to Europe, in comparison with the species on the other
islands of the Atlantic, which stand nearer to the mainland, and (as
remarked by Mr. H.C. Watson) from their somewhat northern character, in
comparison with the latitude, I suspected that these islands had been
partly stocked by ice-borne seeds during the Glacial epoch. At my
request Sir C. Lyell wrote to M. Hartung to inquire whether he had
observed erratic boulders on these islands, and he answered that he had
found large fragments of granite and other rocks, which do not occur in
the archipelago. Hence we may safely infer that icebergs formerly landed
their rocky burdens on the shores of these mid-ocean islands, and it
is at least possible that they may have brought thither the seeds of
northern plants.

Considering that these several means of transport, and that other means,
which without doubt remain to be discovered, have been in action year
after year for tens of thousands of years, it would, I think, be a
marvellous fact if many plants had not thus become widely transported.
These means of transport are sometimes called accidental, but this is
not strictly correct: the currents of the sea are not accidental, nor
is the direction of prevalent gales of wind. It should be observed
that scarcely any means of transport would carry seeds for very great
distances; for seeds do not retain their vitality when exposed for a
great length of time to the action of sea water; nor could they be long
carried in the crops or intestines of birds. These means, however, would
suffice for occasional transport across tracts of sea some hundred
miles in breadth, or from island to island, or from a continent to a
neighbouring island, but not from one distant continent to another. The
floras of distant continents would not by such means become mingled;
but would remain as distinct as they now are. The currents, from their
course, would never bring seeds from North America to Britain, though
they might and do bring seeds from the West Indies to our western
shores, where, if not killed by their very long immersion in salt
water, they could not endure our climate. Almost every year, one or two
land-birds are blown across the whole Atlantic Ocean, from North
America to the western shores of Ireland and England; but seeds could be
transported by these rare wanderers only by one means, namely, by dirt
adhering to their feet or beaks, which is in itself a rare accident.
Even in this case, how small would be the chance of a seed falling on
favourable soil, and coming to maturity! But it would be a great error
to argue that because a well-stocked island, like Great Britain, has
not, as far as is known (and it would be very difficult to prove this),
received within the last few centuries, through occasional means
of transport, immigrants from Europe or any other continent, that a
poorly-stocked island, though standing more remote from the mainland,
would not receive colonists by similar means. Out of a hundred kinds of
seeds or animals transported to an island, even if far less well-stocked
than Britain, perhaps not more than one would be so well fitted to
its new home, as to become naturalised. But this is no valid argument
against what would be effected by occasional means of transport, during
the long lapse of geological time, whilst the island was being upheaved,
and before it had become fully stocked with inhabitants. On almost bare
land, with few or no destructive insects or birds living there, nearly
every seed which chanced to arrive, if fitted for the climate, would
germinate and survive.

DISPERSAL DURING THE GLACIAL PERIOD.

The identity of many plants and animals, on mountain-summits, separated
from each other by hundreds of miles of lowlands, where Alpine species
could not possibly exist, is one of the most striking cases known of the
same species living at distant points, without the apparent possibility
of their having migrated from one point to the other. It is indeed a
remarkable fact to see so many plants of the same species living on the
snowy regions of the Alps or Pyrenees, and in the extreme northern parts
of Europe; but it is far more remarkable, that the plants on the White
Mountains, in the United States of America, are all the same with those
of Labrador, and nearly all the same, as we hear from Asa Gray, with
those on the loftiest mountains of Europe. Even as long ago as 1747,
such facts led Gmelin to conclude that the same species must have
been independently created at many distinct points; and we might have
remained in this same belief, had not Agassiz and others called vivid
attention to the Glacial period, which, as we shall immediately see,
affords a simple explanation of these facts. We have evidence of almost
every conceivable kind, organic and inorganic, that, within a very
recent geological period, central Europe and North America suffered
under an Arctic climate. The ruins of a house burnt by fire do not tell
their tale more plainly than do the mountains of Scotland and Wales,
with their scored flanks, polished surfaces, and perched boulders, of
the icy streams with which their valleys were lately filled. So greatly
has the climate of Europe changed, that in Northern Italy, gigantic
moraines, left by old glaciers, are now clothed by the vine and maize.
Throughout a large part of the United States, erratic boulders and
scored rocks plainly reveal a former cold period.

The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained by Edward Forbes, is substantially
as follows. But we shall follow the changes more readily, by supposing
a new glacial period slowly to come on, and then pass away, as formerly
occurred. As the cold came on, and as each more southern zone became
fitted for the inhabitants of the north, these would take the places of
the former inhabitants of the temperate regions. The latter, at the
same time would travel further and further southward, unless they were
stopped by barriers, in which case they would perish. The mountains
would become covered with snow and ice, and their former Alpine
inhabitants would descend to the plains. By the time that the cold had
reached its maximum, we should have an arctic fauna and flora, covering
the central parts of Europe, as far south as the Alps and Pyrenees,
and even stretching into Spain. The now temperate regions of the United
States would likewise be covered by arctic plants and animals and
these would be nearly the same with those of Europe; for the present
circumpolar inhabitants, which we suppose to have everywhere travelled
southward, are remarkably uniform round the world.

As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more
temperate regions. And as the snow melted from the bases of the
mountains, the arctic forms would seize on the cleared and thawed
ground, always ascending, as the warmth increased and the snow still
further disappeared, higher and higher, whilst their brethren were
pursuing their northern journey. Hence, when the warmth had fully
returned, the same species, which had lately lived together on the
European and North American lowlands, would again be found in the arctic
regions of the Old and New Worlds, and on many isolated mountain-summits
far distant from each other.

Thus we can understand the identity of many plants at points so
immensely remote as the mountains of the United States and those of
Europe. We can thus also understand the fact that the Alpine plants
of each mountain-range are more especially related to the arctic forms
living due north or nearly due north of them: for the first migration
when the cold came on, and the re-migration on the returning warmth,
would generally have been due south and north. The Alpine plants, for
example, of Scotland, as remarked by Mr. H.C. Watson, and those of
the Pyrenees, as remarked by Ramond, are more especially allied to the
plants of northern Scandinavia; those of the United States to Labrador;
those of the mountains of Siberia to the arctic regions of that country.
These views, grounded as they are on the perfectly well-ascertained
occurrence of a former Glacial period, seem to me to explain in so
satisfactory a manner the present distribution of the Alpine and Arctic
productions of Europe and America, that when in other regions we find
the same species on distant mountain-summits, we may almost conclude,
without other evidence, that a colder climate formerly permitted their
migration across the intervening lowlands, now become too warm for their
existence.

As the arctic forms moved first southward and afterwards backward to
the north, in unison with the changing climate, they will not have
been exposed during their long migrations to any great diversity of
temperature; and as they all migrated in a body together, their mutual
relations will not have been much disturbed. Hence, in accordance with
the principles inculcated in this volume, these forms will not have
been liable to much modification. But with the Alpine productions, left
isolated from the moment of the returning warmth, first at the bases
and ultimately on the summits of the mountains, the case will have
been somewhat different; for it is not likely that all the same arctic
species will have been left on mountain ranges far distant from each
other, and have survived there ever since; they will also, in all
probability, have become mingled with ancient Alpine species, which must
have existed on the mountains before the commencement of the Glacial
epoch, and which during the coldest period will have been temporarily
driven down to the plains; they will, also, have been subsequently
exposed to somewhat different climatical influences. Their mutual
relations will thus have been in some degree disturbed; consequently
they will have been liable to modification; and they have been modified;
for if we compare the present Alpine plants and animals of the several
great European mountain ranges, one with another, though many of the
species remain identically the same, some exist as varieties, some as
doubtful forms or sub-species and some as distinct yet closely allied
species representing each other on the several ranges.

In the foregoing illustration, I have assumed that at the commencement
of our imaginary Glacial period, the arctic productions were as uniform
round the polar regions as they are at the present day. But it is also
necessary to assume that many sub-arctic and some few temperate forms
were the same round the world, for some of the species which now exist
on the lower mountain slopes and on the plains of North America and
Europe are the same; and it may be asked how I account for this degree
of uniformity of the sub-arctic and temperate forms round the world,
at the commencement of the real Glacial period. At the present day, the
sub-arctic and northern temperate productions of the Old and New Worlds
are separated from each other by the whole Atlantic Ocean and by the
northern part of the Pacific. During the Glacial period, when the
inhabitants of the Old and New Worlds lived further southwards than they
do at present, they must have been still more completely separated from
each other by wider spaces of ocean; so that it may well be asked
how the same species could then or previously have entered the two
continents. The explanation, I believe, lies in the nature of the
climate before the commencement of the Glacial period. At this, the
newer Pliocene period, the majority of the inhabitants of the world were
specifically the same as now, and we have good reason to believe that
the climate was warmer than at the present day. Hence, we may suppose
that the organisms which now live under latitude 60 degrees, lived
during the Pliocene period further north, under the Polar Circle, in
latitude 66-67 degrees; and that the present arctic productions then
lived on the broken land still nearer to the pole. Now, if we look at
a terrestrial globe, we see under the Polar Circle that there is almost
continuous land from western Europe through Siberia, to eastern America.
And this continuity of the circumpolar land, with the consequent freedom
under a more favourable climate for intermigration, will account for the
supposed uniformity of the sub-arctic and temperate productions of the
Old and New Worlds, at a period anterior to the Glacial epoch.

Believing, from reasons before alluded to, that our continents have long
remained in nearly the same relative position, though subjected to great
oscillations of level, I am strongly inclined to extend the above view,
and to infer that during some earlier and still warmer period, such as
the older Pliocene period, a large number of the same plants and animals
inhabited the almost continuous circumpolar land; and that these plants
and animals, both in the Old and New Worlds, began slowly to migrate
southwards as the climate became less warm, long before the commencement
of the Glacial period. We now see, as I believe, their descendants,
mostly in a modified condition, in the central parts of Europe and the
United States. On this view we can understand the relationship with very
little identity, between the productions of North America and Europe--a
relationship which is highly remarkable, considering the distance of
the two areas, and their separation by the whole Atlantic Ocean. We can
further understand the singular fact remarked on by several observers
that the productions of Europe and America during the later tertiary
stages were more closely related to each other than they are at the
present time; for during these warmer periods the northern parts of the
Old and New Worlds will have been almost continuously united by
land, serving as a bridge, since rendered impassable by cold, for the
intermigration of their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as
the species in common, which inhabited the New and Old Worlds, migrated
south of the Polar Circle, they will have been completely cut off from
each other. This separation, as far as the more temperate productions
are concerned, must have taken place long ages ago. As the plants and
animals migrated southward, they will have become mingled in the one
great region with the native American productions, and would have had to
compete with them; and in the other great region, with those of the
Old World. Consequently we have here everything favourable for
much modification--for far more modification than with the Alpine
productions, left isolated, within a much more recent period, on the
several mountain ranges and on the arctic lands of Europe and North
America. Hence, it has come, that when we compare the now living
productions of the temperate regions of the New and Old Worlds, we find
very few identical species (though Asa Gray has lately shown that more
plants are identical than was formerly supposed), but we find in every
great class many forms, which some naturalists rank as geographical
races, and others as distinct species; and a host of closely allied or
representative forms which are ranked by all naturalists as specifically
distinct.

As on the land, so in the waters of the sea, a slow southern migration
of a marine fauna, which, during the Pliocene or even a somewhat earlier
period, was nearly uniform along the continuous shores of the Polar
Circle, will account, on the theory of modification, for many closely
allied forms now living in marine areas completely sundered. Thus, I
think, we can understand the presence of some closely allied, still
existing and extinct tertiary forms, on the eastern and western shores
of temperate North America; and the still more striking fact of many
closely allied crustaceans (as described in Dana's admirable work), some
fish and other marine animals, inhabiting the Mediterranean and the seas
of Japan--these two areas being now completely separated by the breadth
of a whole continent and by wide spaces of ocean.

These cases of close relationship in species either now or formerly
inhabiting the seas on the eastern and western shores of North America,
the Mediterranean and Japan, and the temperate lands of North America
and Europe, are inexplicable on the theory of creation. We cannot
maintain that such species have been created alike, in correspondence
with the nearly similar physical conditions of the areas; for if we
compare, for instance, certain parts of South America with parts of
South Africa or Australia, we see countries closely similar in all their
physical conditions, with their inhabitants utterly dissimilar.

ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH.

But we must return to our more immediate subject. I am convinced that
Forbes's view may be largely extended. In Europe we meet with the
plainest evidence of the Glacial period, from the western shores of
Britain to the Ural range, and southward to the Pyrenees. We may infer
from the frozen mammals and nature of the mountain vegetation, that
Siberia was similarly affected. In the Lebanon, according to Dr. Hooker,
perpetual snow formerly covered the central axis, and fed glaciers which
rolled 4,000 feet down the valleys. The same observer has recently found
great moraines at a low level on the Atlas range in North Africa. Along
the Himalaya, at points 900 miles apart, glaciers have left the marks of
their former low descent; and in Sikkim, Dr. Hooker saw maize growing
on ancient and gigantic moraines. Southward of the Asiatic continent, on
the opposite side of the equator, we know, from the excellent researches
of Dr. J. Haast and Dr. Hector, that in New Zealand immense glaciers
formerly descended to a low level; and the same plants, found by Dr.
Hooker on widely separated mountains in this island tell the same story
of a former cold period. From facts communicated to me by the Rev. W.B.
Clarke, it appears also that there are traces of former glacial action
on the mountains of the south-eastern corner of Australia.

Looking to America: in the northern half, ice-borne fragments of rock
have been observed on the eastern side of the continent, as far south as
latitude 36 and 37 degrees, and on the shores of the Pacific, where
the climate is now so different, as far south as latitude 46 degrees.
Erratic boulders have, also, been noticed on the Rocky Mountains. In
the Cordillera of South America, nearly under the equator, glaciers once
extended far below their present level. In central Chile I examined
a vast mound of detritus with great boulders, crossing the Portillo
valley, which, there can hardly be a doubt, once formed a huge moraine;
and Mr. D. Forbes informs me that he found in various parts of the
Cordillera, from latitude 13 to 30 degrees south, at about the height of
12,000 feet, deeply-furrowed rocks, resembling those with which he was
familiar in Norway, and likewise great masses of detritus, including
grooved pebbles. Along this whole space of the Cordillera true glaciers
do not now exist even at much more considerable heights. Further
south, on both sides of the continent, from latitude 41 degrees to the
southernmost extremity, we have the clearest evidence of former glacial
action, in numerous immense boulders transported far from their parent
source.

From these several facts, namely, from the glacial action having
extended all round the northern and southern hemispheres--from
the period having been in a geological sense recent in both
hemispheres--from its having lasted in both during a great length of
time, as may be inferred from the amount of work effected--and lastly,
from glaciers having recently descended to a low level along the whole
line of the Cordillera, it at one time appeared to me that we could not
avoid the conclusion that the temperature of the whole world had been
simultaneously lowered during the Glacial period. But now, Mr. Croll,
in a series of admirable memoirs, has attempted to show that a glacial
condition of climate is the result of various physical causes, brought
into operation by an increase in the eccentricity of the earth's orbit.
All these causes tend towards the same end; but the most powerful
appears to be the indirect influence of the eccentricity of the orbit
upon oceanic currents. According to Mr. Croll, cold periods regularly
recur every ten or fifteen thousand years; and these at long intervals
are extremely severe, owing to certain contingencies, of which the most
important, as Sir C. Lyell has shown, is the relative position of the
land and water. Mr. Croll believes that the last great glacial period
occurred about 240,000 years ago, and endured, with slight alterations
of climate, for about 160,000 years. With respect to more ancient
glacial periods, several geologists are convinced, from direct evidence,
that such occurred during the miocene and eocene formations, not to
mention still more ancient formations. But the most important result for
us, arrived at by Mr. Croll, is that whenever the northern hemisphere
passes through a cold period the temperature of the southern hemisphere
is actually raised, with the winters rendered much milder, chiefly
through changes in the direction of the ocean currents. So conversely it
will be with the northern hemisphere, while the southern passes through
a glacial period. This conclusion throws so much light on geographical
distribution that I am strongly inclined to trust in it; but I will
first give the facts which demand an explanation.

In South America, Dr. Hooker has shown that besides many closely allied
species, between forty and fifty of the flowering plants of Tierra del
Fuego, forming no inconsiderable part of its scanty flora, are common to
North America and Europe, enormously remote as these areas in opposite
hemispheres are from each other. On the lofty mountains of equatorial
America a host of peculiar species belonging to European genera occur.
On the Organ Mountains of Brazil some few temperate European, some
Antarctic and some Andean genera were found by Gardner which do not
exist in the low intervening hot countries. On the Silla of Caraccas
the illustrious Humboldt long ago found species belonging to genera
characteristic of the Cordillera.

In Africa, several forms characteristic of Europe, and some few
representatives of the flora of the Cape of Good Hope, occur on the
mountains of Abyssinia. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the
mountains several representative European forms are found which have
not been discovered in the intertropical parts of Africa. Dr. Hooker has
also lately shown that several of the plants living on the upper parts
of the lofty island of Fernando Po, and on the neighbouring Cameroon
Mountains, in the Gulf of Guinea, are closely related to those on the
mountains of Abyssinia, and likewise to those of temperate Europe. It
now also appears, as I hear from Dr. Hooker, that some of these same
temperate plants have been discovered by the Rev. R.T. Lowe on the
mountains of the Cape Verde Islands. This extension of the same
temperate forms, almost under the equator, across the whole continent of
Africa and to the mountains of the Cape Verde archipelago, is one of the
most astonishing facts ever recorded in the distribution of plants.

On the Himalaya, and on the isolated mountain ranges of the peninsula of
India, on the heights of Ceylon, and on the volcanic cones of Java, many
plants occur either identically the same or representing each other,
and at the same time representing plants of Europe not found in the
intervening hot lowlands. A list of the genera of plants collected on
the loftier peaks of Java, raises a picture of a collection made on
a hillock in Europe. Still more striking is the fact that peculiar
Australian forms are represented by certain plants growing on the
summits of the mountains of Borneo. Some of these Australian forms, as
I hear from Dr. Hooker, extend along the heights of the peninsula of
Malacca, and are thinly scattered on the one hand over India, and on the
other hand as far north as Japan.

On the southern mountains of Australia, Dr. F. Muller has discovered
several European species; other species, not introduced by man, occur
on the lowlands; and a long list can be given, as I am informed by
Dr. Hooker, of European genera, found in Australia, but not in the
intermediate torrid regions. In the admirable "Introduction to the Flora
of New Zealand," by Dr. Hooker, analogous and striking facts are given
in regard to the plants of that large island. Hence, we see that certain
plants growing on the more lofty mountains of the tropics in all parts
of the world, and on the temperate plains of the north and south, are
either the same species or varieties of the same species. It should,
however, be observed that these plants are not strictly arctic forms;
for, as Mr. H.C. Watson has remarked, "in receding from polar toward
equatorial latitudes, the Alpine or mountain flora really become less
and less Arctic." Besides these identical and closely allied forms, many
species inhabiting the same widely sundered areas, belong to genera not
now found in the intermediate tropical lowlands.

These brief remarks apply to plants alone; but some few analogous facts
could be given in regard to terrestrial animals. In marine productions,
similar cases likewise occur; as an example, I may quote a statement
by the highest authority, Prof. Dana, that "it is certainly a wonderful
fact that New Zealand should have a closer resemblance in its crustacea
to Great Britain, its antipode, than to any other part of the world."
Sir J. Richardson, also, speaks of the reappearance on the shores of New
Zealand, Tasmania, etc., of northern forms of fish. Dr. Hooker informs
me that twenty-five species of Algae are common to New Zealand and to
Europe, but have not been found in the intermediate tropical seas.

From the foregoing facts, namely, the presence of temperate forms
on the highlands across the whole of equatorial Africa, and along the
Peninsula of India, to Ceylon and the Malay Archipelago, and in a less
well-marked manner across the wide expanse of tropical South America, it
appears almost certain that at some former period, no doubt during
the most severe part of a Glacial period, the lowlands of these great
continents were everywhere tenanted under the equator by a considerable
number of temperate forms. At this period the equatorial climate at the
level of the sea was probably about the same with that now experienced
at the height of from five to six thousand feet under the same latitude,
or perhaps even rather cooler. During this, the coldest period, the
lowlands under the equator must have been clothed with a mingled
tropical and temperate vegetation, like that described by Hooker as
growing luxuriantly at the height of from four to five thousand feet
on the lower slopes of the Himalaya, but with perhaps a still greater
preponderance of temperate forms. So again in the mountainous island of
Fernando Po, in the Gulf of Guinea, Mr. Mann found temperate European
forms beginning to appear at the height of about five thousand feet. On
the mountains of Panama, at the height of only two thousand feet, Dr.
Seemann found the vegetation like that of Mexico, "with forms of the
torrid zone harmoniously blended with those of the temperate."

Now let us see whether Mr. Croll's conclusion that when the northern
hemisphere suffered from the extreme cold of the great Glacial period,
the southern hemisphere was actually warmer, throws any clear light on
the present apparently inexplicable distribution of various organisms
in the temperate parts of both hemispheres, and on the mountains of the
tropics. The Glacial period, as measured by years, must have been very
long; and when we remember over what vast spaces some naturalised plants
and animals have spread within a few centuries, this period will have
been ample for any amount of migration. As the cold became more and more
intense, we know that Arctic forms invaded the temperate regions; and
from the facts just given, there can hardly be a doubt that some of the
more vigorous, dominant and widest-spreading temperate forms invaded the
equatorial lowlands. The inhabitants of these hot lowlands would at the
same time have migrated to the tropical and subtropical regions of the
south, for the southern hemisphere was at this period warmer. On the
decline of the Glacial period, as both hemispheres gradually recovered
their former temperature, the northern temperate forms living on the
lowlands under the equator, would have been driven to their former homes
or have been destroyed, being replaced by the equatorial forms returning
from the south. Some, however, of the northern temperate forms would
almost certainly have ascended any adjoining high land, where, if
sufficiently lofty, they would have long survived like the Arctic
forms on the mountains of Europe. They might have survived, even if the
climate was not perfectly fitted for them, for the change of temperature
must have been very slow, and plants undoubtedly possess a certain
capacity for acclimatisation, as shown by their transmitting to their
offspring different constitutional powers of resisting heat and cold.

In the regular course of events the southern hemisphere would in
its turn be subjected to a severe Glacial period, with the northern
hemisphere rendered warmer; and then the southern temperate forms would
invade the equatorial lowlands. The northern forms which had before been
left on the mountains would now descend and mingle with the southern
forms. These latter, when the warmth returned, would return to their
former homes, leaving some few species on the mountains, and carrying
southward with them some of the northern temperate forms which had
descended from their mountain fastnesses. Thus, we should have some
few species identically the same in the northern and southern temperate
zones and on the mountains of the intermediate tropical regions. But
the species left during a long time on these mountains, or in opposite
hemispheres, would have to compete with many new forms and would be
exposed to somewhat different physical conditions; hence, they would
be eminently liable to modification, and would generally now exist as
varieties or as representative species; and this is the case. We must,
also, bear in mind the occurrence in both hemispheres of former Glacial
periods; for these will account, in accordance with the same principles,
for the many quite distinct species inhabiting the same widely separated
areas, and belonging to genera not now found in the intermediate torrid
zones.

It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many more
identical or slightly modified species have migrated from the north to
the south, than in a reversed direction. We see, however, a few southern
forms on the mountains of Borneo and Abyssinia. I suspect that this
preponderant migration from the north to the south is due to the greater
extent of land in the north, and to the northern forms having existed
in their own homes in greater numbers, and having consequently been
advanced through natural selection and competition to a higher stage of
perfection, or dominating power, than the southern forms. And thus, when
the two sets became commingled in the equatorial regions, during the
alternations of the Glacial periods, the northern forms were the more
powerful and were able to hold their places on the mountains, and
afterwards migrate southward with the southern forms; but not so the
southern in regard to the northern forms. In the same manner, at the
present day, we see that very many European productions cover the ground
in La Plata, New Zealand, and to a lesser degree in Australia, and have
beaten the natives; whereas extremely few southern forms have become
naturalised in any part of the northern hemisphere, though hides, wool,
and other objects likely to carry seeds have been largely imported into
Europe during the last two or three centuries from La Plata and during
the last forty or fifty years from Australia. The Neilgherrie Mountains
in India, however, offer a partial exception; for here, as I hear from
Dr. Hooker, Australian forms are rapidly sowing themselves and becoming
naturalised. Before the last great Glacial period, no doubt the
intertropical mountains were stocked with endemic Alpine forms;
but these have almost everywhere yielded to the more dominant forms
generated in the larger areas and more efficient workshops of the north.
In many islands the native productions are nearly equalled, or even
outnumbered, by those which have become naturalised; and this is the
first stage towards their extinction. Mountains are islands on the land;
and their inhabitants have yielded to those produced within the larger
areas of the north, just in the same way as the inhabitants of real
islands have everywhere yielded and are still yielding to continental
forms naturalised through man's agency.

The same principles apply to the distribution of terrestrial animals and
of marine productions, in the northern and southern temperate zones, and
on the intertropical mountains. When, during the height of the Glacial
period, the ocean-currents were widely different to what they now are,
some of the inhabitants of the temperate seas might have reached
the equator; of these a few would perhaps at once be able to migrate
southwards, by keeping to the cooler currents, while others might remain
and survive in the colder depths until the southern hemisphere was in
its turn subjected to a glacial climate and permitted their further
progress; in nearly the same manner as, according to Forbes, isolated
spaces inhabited by Arctic productions exist to the present day in the
deeper parts of the northern temperate seas.

I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which
now live so widely separated in the north and south, and sometimes on
the intermediate mountain ranges, are removed on the views above given.
The exact lines of migration cannot be indicated. We cannot say why
certain species and not others have migrated; why certain species
have been modified and have given rise to new forms, while others have
remained unaltered. We cannot hope to explain such facts, until we can
say why one species and not another becomes naturalised by man's agency
in a foreign land; why one species ranges twice or thrice as far, and is
twice or thrice as common, as another species within their own homes.

Various special difficulties also remain to be solved; for instance,
the occurrence, as shown by Dr. Hooker, of the same plants at points
so enormously remote as Kerguelen Land, New Zealand, and Fuegia; but
icebergs, as suggested by Lyell, may have been concerned in their
dispersal. The existence at these and other distant points of the
southern hemisphere, of species, which, though distinct, belong to
genera exclusively confined to the south, is a more remarkable case.
Some of these species are so distinct, that we cannot suppose that there
has been time since the commencement of the last Glacial period for
their migration and subsequent modification to the necessary degree.
The facts seem to indicate that distinct species belonging to the same
genera have migrated in radiating lines from a common centre; and I am
inclined to look in the southern, as in the northern hemisphere, to a
former and warmer period, before the commencement of the last Glacial
period, when the Antarctic lands, now covered with ice, supported a
highly peculiar and isolated flora. It may be suspected that before this
flora was exterminated during the last Glacial epoch, a few forms
had been already widely dispersed to various points of the southern
hemisphere by occasional means of transport, and by the aid, as
halting-places, of now sunken islands. Thus the southern shores of
America, Australia, and New Zealand may have become slightly tinted by
the same peculiar forms of life.

Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
throughout the world on geographical distribution. And we have now seen
that Mr. Croll's conclusion that successive Glacial periods in the one
hemisphere coincide with warmer periods in the opposite hemisphere,
together with the admission of the slow modification of species,
explains a multitude of facts in the distribution of the same and of the
allied forms of life in all parts of the globe. The living waters have
flowed during one period from the north and during another from the
south, and in both cases have reached the equator; but the stream of
life has flowed with greater force from the north than in the opposite
direction, and has consequently more freely inundated the south. As the
tide leaves its drift in horizontal lines, rising higher on the shores
where the tide rises highest, so have the living waters left their
living drift on our mountain summits, in a line gently rising from
the Arctic lowlands to a great latitude under the equator. The various
beings thus left stranded may be compared with savage races of man,
driven up and surviving in the mountain fastnesses of almost every
land, which serves as a record, full of interest to us, of the former
inhabitants of the surrounding lowlands.



CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION--continued.

 Distribution of fresh-water productions--On the inhabitants of oceanic
 islands--Absence of Batrachians and of terrestrial Mammals--On
 the relation of the inhabitants of islands to those of the nearest
 mainland--On colonisation from the nearest source with subsequent
 modification--Summary of the last and present chapters.


FRESH-WATER PRODUCTIONS.

As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently
a still more formidable barrier, that they would never have extended to
distant countries. But the case is exactly the reverse. Not only have
many fresh-water species, belonging to different classes, an enormous
range, but allied species prevail in a remarkable manner throughout
the world. When first collecting in the fresh waters of Brazil, I well
remember feeling much surprise at the similarity of the fresh-water
insects, shells, etc., and at the dissimilarity of the surrounding
terrestrial beings, compared with those of Britain.

But the wide ranging power of fresh-water productions can, I think,
in most cases be explained by their having become fitted, in a manner
highly useful to them, for short and frequent migrations from pond
to pond, or from stream to stream, within their own countries; and
liability to wide dispersal would follow from this capacity as an almost
necessary consequence. We can here consider only a few cases; of these,
some of the most difficult to explain are presented by fish. It was
formerly believed that the same fresh-water species never existed on
two continents distant from each other. But Dr. Gunther has lately
shown that the Galaxias attenuatus inhabits Tasmania, New Zealand, the
Falkland Islands and the mainland of South America. This is a wonderful
case, and probably indicates dispersal from an Antarctic centre during a
former warm period. This case, however, is rendered in some degree less
surprising by the species of this genus having the power of crossing by
some unknown means considerable spaces of open ocean: thus there is
one species common to New Zealand and to the Auckland Islands, though
separated by a distance of about 230 miles. On the same continent
fresh-water fish often range widely, and as if capriciously; for in two
adjoining river systems some of the species may be the same and some
wholly different.

It is probable that they are occasionally transported by what may be
called accidental means. Thus fishes still alive are not very rarely
dropped at distant points by whirlwinds; and it is known that the ova
retain their vitality for a considerable time after removal from the
water. Their dispersal may, however, be mainly attributed to changes in
the level of the land within the recent period, causing rivers to flow
into each other. Instances, also, could be given of this having occurred
during floods, without any change of level. The wide differences of the
fish on the opposite sides of most mountain-ranges, which are continuous
and consequently must, from an early period, have completely prevented
the inosculation of the river systems on the two sides, leads to the
same conclusion. Some fresh-water fish belong to very ancient forms,
and in such cases there will have been ample time for great geographical
changes, and consequently time and means for much migration. Moreover,
Dr. Gunther has recently been led by several considerations to infer
that with fishes the same forms have a long endurance. Salt-water
fish can with care be slowly accustomed to live in fresh water; and,
according to Valenciennes, there is hardly a single group of which
all the members are confined to fresh water, so that a marine species
belonging to a fresh-water group might travel far along the shores
of the sea, and could, it is probable, become adapted without much
difficulty to the fresh waters of a distant land.

Some species of fresh-water shells have very wide ranges, and allied
species which, on our theory, are descended from a common parent, and
must have proceeded from a single source, prevail throughout the world.
Their distribution at first perplexed me much, as their ova are not
likely to be transported by birds; and the ova, as well as the adults,
are immediately killed by sea-water. I could not even understand
how some naturalised species have spread rapidly throughout the same
country. But two facts, which I have observed--and many others no
doubt will be discovered--throw some light on this subject. When ducks
suddenly emerge from a pond covered with duck-weed, I have twice seen
these little plants adhering to their backs; and it has happened to me,
in removing a little duck-weed from one aquarium to another, that I have
unintentionally stocked the one with fresh-water shells from the other.
But another agency is perhaps more effectual: I suspended the feet of a
duck in an aquarium, where many ova of fresh-water shells were hatching;
and I found that numbers of the extremely minute and just-hatched shells
crawled on the feet, and clung to them so firmly that when taken out
of the water they could not be jarred off, though at a somewhat more
advanced age they would voluntarily drop off. These just-hatched
molluscs, though aquatic in their nature, survived on the duck's feet,
in damp air, from twelve to twenty hours; and in this length of time
a duck or heron might fly at least six or seven hundred miles, and
if blown across the sea to an oceanic island, or to any other distant
point, would be sure to alight on a pool or rivulet. Sir Charles Lyell
informs me that a Dyticus has been caught with an Ancylus (a fresh-water
shell like a limpet) firmly adhering to it; and a water-beetle of
the same family, a Colymbetes, once flew on board the "Beagle," when
forty-five miles distant from the nearest land: how much farther it
might have been blown by a favouring gale no one can tell.

With respect to plants, it has long been known what enormous ranges many
fresh-water, and even marsh-species, have, both over continents and
to the most remote oceanic islands. This is strikingly illustrated,
according to Alph. de Candolle, in those large groups of terrestrial
plants, which have very few aquatic members; for the latter seem
immediately to acquire, as if in consequence, a wide range. I think
favourable means of dispersal explain this fact. I have before mentioned
that earth occasionally adheres in some quantity to the feet and beaks
of birds. Wading birds, which frequent the muddy edges of ponds, if
suddenly flushed, would be the most likely to have muddy feet. Birds
of this order wander more than those of any other; and are occasionally
found on the most remote and barren islands of the open ocean; they
would not be likely to alight on the surface of the sea, so that any
dirt on their feet would not be washed off; and when gaining the land,
they would be sure to fly to their natural fresh-water haunts. I do not
believe that botanists are aware how charged the mud of ponds is with
seeds: I have tried several little experiments, but will here give only
the most striking case: I took in February three tablespoonfuls of mud
from three different points, beneath water, on the edge of a little
pond; this mud when dry weighed only 6 and 3/4 ounces; I kept it covered
up in my study for six months, pulling up and counting each plant as it
grew; the plants were of many kinds, and were altogether 537 in number;
and yet the viscid mud was all contained in a breakfast cup! Considering
these facts, I think it would be an inexplicable circumstance if
water-birds did not transport the seeds of fresh-water plants to
unstocked ponds and streams, situated at very distant points. The same
agency may have come into play with the eggs of some of the smaller
fresh-water animals.

Other and unknown agencies probably have also played a part. I have
stated that fresh-water fish eat some kinds of seeds, though they reject
many other kinds after having swallowed them; even small fish swallow
seeds of moderate size, as of the yellow water-lily and Potamogeton.
Herons and other birds, century after century, have gone on daily
devouring fish; they then take flight and go to other waters, or are
blown across the sea; and we have seen that seeds retain their power of
germination, when rejected many hours afterwards in pellets or in
the excrement. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle's remarks
on the distribution of this plant, I thought that the means of its
dispersal must remain inexplicable; but Audubon states that he found
the seeds of the great southern water-lily (probably according to Dr.
Hooker, the Nelumbium luteum) in a heron's stomach. Now this bird must
often have flown with its stomach thus well stocked to distant ponds,
and, then getting a hearty meal of fish, analogy makes me believe
that it would have rejected the seeds in the pellet in a fit state for
germination.

In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for instance on a
rising islet, it will be unoccupied; and a single seed or egg will have
a good chance of succeeding. Although there will always be a struggle
for life between the inhabitants of the same pond, however few in kind,
yet as the number even in a well-stocked pond is small in comparison
with the number of species inhabiting an equal area of land, the
competition between them will probably be less severe than between
terrestrial species; consequently an intruder from the waters of a
foreign country would have a better chance of seizing on a new place,
than in the case of terrestrial colonists. We should also remember that
many fresh-water productions are low in the scale of nature, and we have
reason to believe that such beings become modified more slowly than the
high; and this will give time for the migration of aquatic species.
We should not forget the probability of many fresh-water forms having
formerly ranged continuously over immense areas, and then having become
extinct at intermediate points. But the wide distribution of fresh-water
plants, and of the lower animals, whether retaining the same identical
form, or in some degree modified, apparently depends in main part on the
wide dispersal of their seeds and eggs by animals, more especially by
fresh-water birds, which have great powers of flight, and naturally
travel from one piece of water to another.

ON THE INHABITANTS OF OCEANIC ISLANDS.

We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty with respect to
distribution, on the view that not only all the individuals of the
same species have migrated from some one area, but that allied species,
although now inhabiting the most distant points, have proceeded from a
single area, the birthplace of their early progenitors. I have already
given my reasons for disbelieving in continental extensions within the
period of existing species on so enormous a scale that all the many
islands of the several oceans were thus stocked with their present
terrestrial inhabitants. This view removes many difficulties, but it
does not accord with all the facts in regard to the productions of
islands. In the following remarks I shall not confine myself to the mere
question of dispersal, but shall consider some other cases bearing on
the truth of the two theories of independent creation and of descent
with modification.

The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. New Zealand, for instance,
with its lofty mountains and diversified stations, extending over 780
miles of latitude, together with the outlying islands of Auckland,
Campbell and Chatham, contain altogether only 960 kinds of flowering
plants; if we compare this moderate number with the species which swarm
over equal areas in Southwestern Australia or at the Cape of Good Hope,
we must admit that some cause, independently of different physical
conditions, has given rise to so great a difference in number. Even the
uniform county of Cambridge has 847 plants, and the little island of
Anglesea 764, but a few ferns and a few introduced plants are included
in these numbers, and the comparison in some other respects is not quite
fair. We have evidence that the barren island of Ascension aboriginally
possessed less than half-a-dozen flowering plants; yet many species have
now become naturalised on it, as they have in New Zealand and on every
other oceanic island which can be named. In St. Helena there is reason
to believe that the naturalised plants and animals have nearly or quite
exterminated many native productions. He who admits the doctrine of the
creation of each separate species, will have to admit that a sufficient
number of the best adapted plants and animals were not created for
oceanic islands; for man has unintentionally stocked them far more fully
and perfectly than did nature.

Although in oceanic islands the species are few in number, the
proportion of endemic kinds (i.e. those found nowhere else in the world)
is often extremely large. If we compare, for instance, the number of
endemic land-shells in Madeira, or of endemic birds in the Galapagos
Archipelago, with the number found on any continent, and then compare
the area of the island with that of the continent, we shall see that
this is true. This fact might have been theoretically expected, for, as
already explained, species occasionally arriving, after long intervals
of time in the new and isolated district, and having to compete with new
associates, would be eminently liable to modification, and would often
produce groups of modified descendants. But it by no means follows that,
because in an island nearly all the species of one class are peculiar,
those of another class, or of another section of the same class, are
peculiar; and this difference seems to depend partly on the species
which are not modified having immigrated in a body, so that their mutual
relations have not been much disturbed; and partly on the frequent
arrival of unmodified immigrants from the mother-country, with which
the insular forms have intercrossed. It should be borne in mind that the
offspring of such crosses would certainly gain in vigour; so that even
an occasional cross would produce more effect than might have been
anticipated. I will give a few illustrations of the foregoing remarks:
in the Galapagos Islands there are twenty-six land birds; of these
twenty-one (or perhaps twenty-three) are peculiar; whereas of the eleven
marine birds only two are peculiar; and it is obvious that marine birds
could arrive at these islands much more easily and frequently than
land-birds. Bermuda, on the other hand, which lies at about the same
distance from North America as the Galapagos Islands do from South
America, and which has a very peculiar soil, does not possess a single
endemic land bird; and we know from Mr. J.M. Jones's admirable account
of Bermuda, that very many North American birds occasionally or even
frequently visit this island. Almost every year, as I am informed by
Mr. E.V. Harcourt, many European and African birds are blown to Madeira;
this island is inhabited by ninety-nine kinds, of which one alone is
peculiar, though very closely related to a European form; and three or
four other species are confined to this island and to the Canaries.
So that the islands of Bermuda and Madeira have been stocked from the
neighbouring continents with birds, which for long ages have there
struggled together, and have become mutually co-adapted. Hence, when
settled in their new homes, each kind will have been kept by the others
to its proper place and habits, and will consequently have been but
little liable to modification. Any tendency to modification will also
have been checked by intercrossing with the unmodified immigrants,
often arriving from the mother-country. Madeira again is inhabited by
a wonderful number of peculiar land-shells, whereas not one species
of sea-shell is peculiar to its shores: now, though we do not know how
sea-shells are dispersed, yet we can see that their eggs or larvae,
perhaps attached to seaweed or floating timber, or to the feet of wading
birds, might be transported across three or four hundred miles of open
sea far more easily than land-shells. The different orders of insects
inhabiting Madeira present nearly parallel cases.

Oceanic islands are sometimes deficient in animals of certain whole
classes, and their places are occupied by other classes; thus in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless birds,
take, or recently took, the place of mammals. Although New Zealand
is here spoken of as an oceanic island, it is in some degree doubtful
whether it should be so ranked; it is of large size, and is not
separated from Australia by a profoundly deep sea; from its geological
character and the direction of its mountain ranges, the Rev. W.B. Clarke
has lately maintained that this island, as well as New Caledonia, should
be considered as appurtenances of Australia. Turning to plants, Dr.
Hooker has shown that in the Galapagos Islands the proportional numbers
of the different orders are very different from what they are elsewhere.
All such differences in number, and the absence of certain whole
groups of animals and plants, are generally accounted for by supposed
differences in the physical conditions of the islands; but this
explanation is not a little doubtful. Facility of immigration seems to
have been fully as important as the nature of the conditions.

Many remarkable little facts could be given with respect to the
inhabitants of oceanic islands. For instance, in certain islands not
tenanted by a single mammal, some of the endemic plants have beautifully
hooked seeds; yet few relations are more manifest than that hooks serve
for the transportal of seeds in the wool or fur of quadrupeds. But a
hooked seed might be carried to an island by other means; and the plant
then becoming modified would form an endemic species, still retaining
its hooks, which would form a useless appendage, like the shrivelled
wings under the soldered wing-covers of many insular beetles. Again,
islands often possess trees or bushes belonging to orders which
elsewhere include only herbaceous species; now trees, as Alph. de
Candolle has shown, generally have, whatever the cause may be, confined
ranges. Hence trees would be little likely to reach distant oceanic
islands; and an herbaceous plant, which had no chance of successfully
competing with the many fully developed trees growing on a continent,
might, when established on an island, gain an advantage over other
herbaceous plants by growing taller and taller and overtopping them.
In this case, natural selection would tend to add to the stature of the
plant, to whatever order it belonged, and thus first convert it into a
bush and then into a tree.

ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON OCEANIC ISLANDS.

With respect to the absence of whole orders of animals on oceanic
islands, Bory St. Vincent long ago remarked that Batrachians (frogs,
toads, newts) are never found on any of the many islands with which the
great oceans are studded. I have taken pains to verify this assertion,
and have found it true, with the exception of New Zealand, New
Caledonia, the Andaman Islands, and perhaps the Solomon Islands and the
Seychelles. But I have already remarked that it is doubtful whether New
Zealand and New Caledonia ought to be classed as oceanic islands; and
this is still more doubtful with respect to the Andaman and Solomon
groups and the Seychelles. This general absence of frogs, toads and
newts on so many true oceanic islands cannot be accounted for by their
physical conditions; indeed it seems that islands are peculiarly fitted
for these animals; for frogs have been introduced into Madeira, the
Azores, and Mauritius, and have multiplied so as to become a nuisance.
But as these animals and their spawn are immediately killed (with the
exception, as far as known, of one Indian species) by sea-water, there
would be great difficulty in their transportal across the sea, and
therefore we can see why they do not exist on strictly oceanic islands.
But why, on the theory of creation, they should not have been created
there, it would be very difficult to explain.

Mammals offer another and similar case. I have carefully searched the
oldest voyages, and have not found a single instance, free from doubt,
of a terrestrial mammal (excluding domesticated animals kept by the
natives) inhabiting an island situated above 300 miles from a continent
or great continental island; and many islands situated at a much less
distance are equally barren. The Falkland Islands, which are inhabited
by a wolf-like fox, come nearest to an exception; but this group cannot
be considered as oceanic, as it lies on a bank in connection with the
mainland at a distance of about 280 miles; moreover, icebergs formerly
brought boulders to its western shores, and they may have formerly
transported foxes, as now frequently happens in the arctic regions. Yet
it cannot be said that small islands will not support at least small
mammals, for they occur in many parts of the world on very small
islands, when lying close to a continent; and hardly an island can be
named on which our smaller quadrupeds have not become naturalised and
greatly multiplied. It cannot be said, on the ordinary view of creation,
that there has not been time for the creation of mammals; many volcanic
islands are sufficiently ancient, as shown by the stupendous degradation
which they have suffered, and by their tertiary strata: there has also
been time for the production of endemic species belonging to other
classes; and on continents it is known that new species of mammals
appear and disappear at a quicker rate than other and lower animals.
Although terrestrial mammals do not occur on oceanic islands, aerial
mammals do occur on almost every island. New Zealand possesses two bats
found nowhere else in the world: Norfolk Island, the Viti Archipelago,
the Bonin Islands, the Caroline and Marianne Archipelagoes, and
Mauritius, all possess their peculiar bats. Why, it may be asked, has
the supposed creative force produced bats and no other mammals on
remote islands? On my view this question can easily be answered; for no
terrestrial mammal can be transported across a wide space of sea, but
bats can fly across. Bats have been seen wandering by day far over the
Atlantic Ocean; and two North American species, either regularly or
occasionally, visit Bermuda, at the distance of 600 miles from the
mainland. I hear from Mr. Tomes, who has specially studied this family,
that many species have enormous ranges, and are found on continents
and on far distant islands. Hence, we have only to suppose that such
wandering species have been modified in their new homes in relation to
their new position, and we can understand the presence of endemic bats
on oceanic islands, with the absence of all other terrestrial mammals.

Another interesting relation exists, namely, between the depth of the
sea separating islands from each other, or from the nearest continent,
and the degree of affinity of their mammalian inhabitants. Mr. Windsor
Earl has made some striking observations on this head, since greatly
extended by Mr. Wallace's admirable researches, in regard to the great
Malay Archipelago, which is traversed near Celebes by a space of deep
ocean, and this separates two widely distinct mammalian faunas. On
either side, the islands stand on a moderately shallow submarine
bank, and these islands are inhabited by the same or by closely allied
quadrupeds. I have not as yet had time to follow up this subject in all
quarters of the world; but as far as I have gone, the relation holds
good. For instance, Britain is separated by a shallow channel from
Europe, and the mammals are the same on both sides; and so it is with
all the islands near the shores of Australia. The West Indian Islands,
on the other hand, stand on a deeply submerged bank, nearly one thousand
fathoms in depth, and here we find American forms, but the species and
even the genera are quite distinct. As the amount of modification which
animals of all kinds undergo partly depends on the lapse of time, and as
the islands which are separated from each other, or from the mainland,
by shallow channels, are more likely to have been continuously united
within a recent period than the islands separated by deeper channels, we
can understand how it is that a relation exists between the depth of the
sea separating two mammalian faunas, and the degree of their affinity,
a relation which is quite inexplicable on the theory of independent acts
of creation.

The foregoing statements in regard to the inhabitants of oceanic
islands, namely, the fewness of the species, with a large proportion
consisting of endemic forms--the members of certain groups, but not
those of other groups in the same class, having been modified--the
absence of certain whole orders, as of batrachians and of terrestrial
mammals, notwithstanding the presence of aerial bats, the singular
proportions of certain orders of plants, herbaceous forms having been
developed into trees, etc., seem to me to accord better with the belief
in the efficiency of occasional means of transport, carried on during
a long course of time, than with the belief in the former connection of
all oceanic islands with the nearest continent; for on this latter
view it is probable that the various classes would have immigrated more
uniformly, and from the species having entered in a body, their mutual
relations would not have been much disturbed, and consequently, they
would either have not been modified, or all the species in a more
equable manner.

I do not deny that there are many and serious difficulties in
understanding how many of the inhabitants of the more remote islands,
whether still retaining the same specific form or subsequently modified,
have reached their present homes. But the probability of other islands
having once existed as halting-places, of which not a wreck now remains,
must not be overlooked. I will specify one difficult case. Almost all
oceanic islands, even the most isolated and smallest, are inhabited
by land-shells, generally by endemic species, but sometimes by species
found elsewhere striking instances of which have been given by Dr. A.A.
Gould in relation to the Pacific. Now it is notorious that land-shells
are easily killed by sea-water; their eggs, at least such as I have
tried, sink in it and are killed. Yet there must be some unknown,
but occasionally efficient means for their transportal. Would the
just-hatched young sometimes adhere to the feet of birds roosting on
the ground and thus get transported? It occurred to me that land-shells,
when hybernating and having a membranous diaphragm over the mouth of the
shell, might be floated in chinks of drifted timber across moderately
wide arms of the sea. And I find that several species in this state
withstand uninjured an immersion in sea-water during seven days. One
shell, the Helix pomatia, after having been thus treated, and again
hybernating, was put into sea-water for twenty days and perfectly
recovered. During this length of time the shell might have been
carried by a marine country of average swiftness to a distance of 660
geographical miles. As this Helix has a thick calcareous operculum
I removed it, and when it had formed a new membranous one, I again
immersed it for fourteen days in sea-water, and again it recovered and
crawled away. Baron Aucapitaine has since tried similar experiments. He
placed 100 land-shells, belonging to ten species, in a box pierced with
holes, and immersed it for a fortnight in the sea. Out of the hundred
shells twenty-seven recovered. The presence of an operculum seems
to have been of importance, as out of twelve specimens of Cyclostoma
elegans, which is thus furnished, eleven revived. It is remarkable,
seeing how well the Helix pomatia resisted with me the salt-water, that
not one of fifty-four specimens belonging to four other species of Helix
tried by Aucapitaine recovered. It is, however, not at all probable that
land-shells have often been thus transported; the feet of birds offer a
more probable method.

ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO THOSE OF THE NEAREST
MAINLAND.

The most striking and important fact for us is the affinity of the
species which inhabit islands to those of the nearest mainland,
without being actually the same. Numerous instances could be given. The
Galapagos Archipelago, situated under the equator, lies at a distance of
between 500 and 600 miles from the shores of South America. Here almost
every product of the land and of the water bears the unmistakable stamp
of the American continent. There are twenty-six land birds. Of these
twenty-one, or perhaps twenty-three, are ranked as distinct species,
and would commonly be assumed to have been here created; yet the close
affinity of most of these birds to American species is manifest in every
character in their habits, gestures, and tones of voice. So it is with
the other animals, and with a large proportion of the plants, as
shown by Dr. Hooker in his admirable Flora of this archipelago. The
naturalist, looking at the inhabitants of these volcanic islands in the
Pacific, distant several hundred miles from the continent, feels that
he is standing on American land. Why should this be so? Why should
the species which are supposed to have been created in the Galapagos
Archipelago, and nowhere else, bear so plainly the stamp of affinity to
those created in America? There is nothing in the conditions of life, in
the geological nature of the islands, in their height or climate, or in
the proportions in which the several classes are associated together,
which closely resembles the conditions of the South American coast. In
fact, there is a considerable dissimilarity in all these respects. On
the other hand, there is a considerable degree of resemblance in the
volcanic nature of the soil, in the climate, height, and size of the
islands, between the Galapagos and Cape Verde Archipelagos: but what an
entire and absolute difference in their inhabitants! The inhabitants of
the Cape Verde Islands are related to those of Africa, like those of
the Galapagos to America. Facts, such as these, admit of no sort of
explanation on the ordinary view of independent creation; whereas, on
the view here maintained, it is obvious that the Galapagos Islands would
be likely to receive colonists from America, whether by occasional means
of transport or (though I do not believe in this doctrine) by formerly
continuous land, and the Cape Verde Islands from Africa; such colonists
would be liable to modification--the principle of inheritance still
betraying their original birthplace.

Many analogous facts could be given: indeed it is an almost universal
rule that the endemic productions of islands are related to those of the
nearest continent, or of the nearest large island. The exceptions are
few, and most of them can be explained. Thus, although Kerguelen Land
stands nearer to Africa than to America, the plants are related, and
that very closely, as we know from Dr. Hooker's account, to those of
America: but on the view that this island has been mainly stocked
by seeds brought with earth and stones on icebergs, drifted by the
prevailing currents, this anomaly disappears. New Zealand in its endemic
plants is much more closely related to Australia, the nearest mainland,
than to any other region: and this is what might have been expected; but
it is also plainly related to South America, which, although the next
nearest continent, is so enormously remote, that the fact becomes an
anomaly. But this difficulty partially disappears on the view that New
Zealand, South America, and the other southern lands, have been stocked
in part from a nearly intermediate though distant point, namely, from
the antarctic islands, when they were clothed with vegetation, during
a warmer tertiary period, before the commencement of the last Glacial
period. The affinity, which, though feeble, I am assured by Dr. Hooker
is real, between the flora of the south-western corner of Australia
and of the Cape of Good Hope, is a far more remarkable case; but this
affinity is confined to the plants, and will, no doubt, some day be
explained.

The same law which has determined the relationship between the
inhabitants of islands and the nearest mainland, is sometimes displayed
on a small scale, but in a most interesting manner, within the limits
of the same archipelago. Thus each separate island of the Galapagos
Archipelago is tenanted, and the fact is a marvellous one, by many
distinct species; but these species are related to each other in a very
much closer manner than to the inhabitants of the American continent,
or of any other quarter of the world. This is what might have been
expected, for islands situated so near to each other would almost
necessarily receive immigrants from the same original source, and
from each other. But how is it that many of the immigrants have been
differently modified, though only in a small degree, in islands situated
within sight of each other, having the same geological nature, the same
height, climate, etc? This long appeared to me a great difficulty: but
it arises in chief part from the deeply-seated error of considering
the physical conditions of a country as the most important; whereas it
cannot be disputed that the nature of the other species with which
each has to compete, is at least as important, and generally a far
more important element of success. Now if we look to the species which
inhabit the Galapagos Archipelago, and are likewise found in other
parts of the world, we find that they differ considerably in the several
islands. This difference might indeed have been expected if the
islands have been stocked by occasional means of transport--a seed, for
instance, of one plant having been brought to one island, and that of
another plant to another island, though all proceeding from the same
general source. Hence, when in former times an immigrant first settled
on one of the islands, or when it subsequently spread from one to
another, it would undoubtedly be exposed to different conditions in the
different islands, for it would have to compete with a different set of
organisms; a plant, for instance, would find the ground best-fitted for
it occupied by somewhat different species in the different islands, and
would be exposed to the attacks of somewhat different enemies. If, then,
it varied, natural selection would probably favour different varieties
in the different islands. Some species, however, might spread and yet
retain the same character throughout the group, just as we see some
species spreading widely throughout a continent and remaining the same.

The really surprising fact in this case of the Galapagos Archipelago,
and in a lesser degree in some analogous cases, is that each new species
after being formed in any one island, did not spread quickly to the
other islands. But the islands, though in sight of each other, are
separated by deep arms of the sea, in most cases wider than the British
Channel, and there is no reason to suppose that they have at any former
period been continuously united. The currents of the sea are rapid and
deep between the islands, and gales of wind are extraordinarily rare; so
that the islands are far more effectually separated from each other than
they appear on a map. Nevertheless, some of the species, both of
those found in other parts of the world and of those confined to the
archipelago, are common to the several islands; and we may infer from
the present manner of distribution that they have spread from one island
to the others. But we often take, I think, an erroneous view of the
probability of closely allied species invading each other's territory,
when put into free intercommunication. Undoubtedly, if one species has
any advantage over another, it will in a very brief time wholly or in
part supplant it; but if both are equally well fitted for their own
places, both will probably hold their separate places for almost
any length of time. Being familiar with the fact that many species,
naturalised through man's agency, have spread with astonishing rapidity
over wide areas, we are apt to infer that most species would thus
spread; but we should remember that the species which become naturalised
in new countries are not generally closely allied to the aboriginal
inhabitants, but are very distinct forms, belonging in a large
proportion of cases, as shown by Alph. de Candolle, to distinct genera.
In the Galapagos Archipelago, many even of the birds, though so well
adapted for flying from island to island, differ on the different
islands; thus there are three closely allied species of mocking-thrush,
each confined to its own island. Now let us suppose the mocking-thrush
of Chatham Island to be blown to Charles Island, which has its own
mocking-thrush; why should it succeed in establishing itself there?
We may safely infer that Charles Island is well stocked with its own
species, for annually more eggs are laid and young birds hatched
than can possibly be reared; and we may infer that the mocking-thrush
peculiar to Charles Island is at least as well fitted for its home as is
the species peculiar to Chatham Island. Sir C. Lyell and Mr. Wollaston
have communicated to me a remarkable fact bearing on this subject;
namely, that Madeira and the adjoining islet of Porto Santo possess many
distinct but representative species of land-shells, some of which
live in crevices of stone; and although large quantities of stone are
annually transported from Porto Santo to Madeira, yet this latter island
has not become colonised by the Porto Santo species: nevertheless, both
islands have been colonised by some European land-shells, which no
doubt had some advantage over the indigenous species. From these
considerations I think we need not greatly marvel at the endemic species
which inhabit the several islands of the Galapagos Archipelago not
having all spread from island to island. On the same continent, also,
pre-occupation has probably played an important part in checking the
commingling of the species which inhabit different districts with
nearly the same physical conditions. Thus, the south-east and south-west
corners of Australia have nearly the same physical conditions, and are
united by continuous land, yet they are inhabited by a vast number of
distinct mammals, birds, and plants; so it is, according to Mr. Bates,
with the butterflies and other animals inhabiting the great, open, and
continuous valley of the Amazons.

The same principle which governs the general character of the
inhabitants of oceanic islands, namely, the relation to the source
whence colonists could have been most easily derived, together with
their subsequent modification, is of the widest application throughout
nature. We see this on every mountain-summit, in every lake and marsh.
For Alpine species, excepting in as far as the same species have become
widely spread during the Glacial epoch, are related to those of
the surrounding lowlands; thus we have in South America, Alpine
humming-birds, Alpine rodents, Alpine plants, etc., all strictly
belonging to American forms; and it is obvious that a mountain, as
it became slowly upheaved, would be colonised from the surrounding
lowlands. So it is with the inhabitants of lakes and marshes, excepting
in so far as great facility of transport has allowed the same forms
to prevail throughout large portions of the world. We see the same
principle in the character of most of the blind animals inhabiting the
caves of America and of Europe. Other analogous facts could be given.
It will, I believe, be found universally true, that wherever in
two regions, let them be ever so distant, many closely allied or
representative species occur, there will likewise be found some
identical species; and wherever many closely-allied species occur,
there will be found many forms which some naturalists rank as distinct
species, and others as mere varieties; these doubtful forms showing us
the steps in the process of modification.

The relation between the power and extent of migration in certain
species, either at the present or at some former period, and the
existence at remote points of the world of closely allied species, is
shown in another and more general way. Mr. Gould remarked to me long
ago, that in those genera of birds which range over the world, many of
the species have very wide ranges. I can hardly doubt that this rule
is generally true, though difficult of proof. Among mammals, we see it
strikingly displayed in Bats, and in a lesser degree in the Felidae and
Canidae. We see the same rule in the distribution of butterflies and
beetles. So it is with most of the inhabitants of fresh water, for many
of the genera in the most distinct classes range over the world, and
many of the species have enormous ranges. It is not meant that all, but
that some of the species have very wide ranges in the genera which range
very widely. Nor is it meant that the species in such genera have, on an
average, a very wide range; for this will largely depend on how far the
process of modification has gone; for instance, two varieties of the
same species inhabit America and Europe, and thus the species has an
immense range; but, if variation were to be carried a little further,
the two varieties would be ranked as distinct species, and their range
would be greatly reduced. Still less is it meant, that species which
have the capacity of crossing barriers and ranging widely, as in the
case of certain powerfully-winged birds, will necessarily range widely;
for we should never forget that to range widely implies not only the
power of crossing barriers, but the more important power of being
victorious in distant lands in the struggle for life with foreign
associates. But according to the view that all the species of a genus,
though distributed to the most remote points of the world, are descended
from a single progenitor, we ought to find, and I believe as a general
rule we do find, that some at least of the species range very widely.

We should bear in mind that many genera in all classes are of ancient
origin, and the species in this case will have had ample time for
dispersal and subsequent modification. There is also reason to believe,
from geological evidence, that within each great class the lower
organisms change at a slower rate than the higher; consequently they
will have had a better chance of ranging widely and of still retaining
the same specific character. This fact, together with that of the seeds
and eggs of most lowly organised forms being very minute and better
fitted for distant transportal, probably accounts for a law which has
long been observed, and which has lately been discussed by Alph. de
Candolle in regard to plants, namely, that the lower any group of
organisms stands the more widely it ranges.

The relations just discussed--namely, lower organisms ranging more
widely than the higher--some of the species of widely-ranging genera
themselves ranging widely--such facts, as alpine, lacustrine, and
marsh productions being generally related to those which live on the
surrounding low lands and dry lands--the striking relationship between
the inhabitants of islands and those of the nearest mainland--the still
closer relationship of the distinct inhabitants of the islands of
the same archipelago--are inexplicable on the ordinary view of the
independent creation of each species, but are explicable if we admit
colonisation from the nearest or readiest source, together with the
subsequent adaptation of the colonists to their new homes.

SUMMARY OF THE LAST AND PRESENT CHAPTERS.

In these chapters I have endeavoured to show that if we make due
allowance for our ignorance of the full effects of changes of climate
and of the level of the land, which have certainly occurred within the
recent period, and of other changes which have probably occurred--if we
remember how ignorant we are with respect to the many curious means of
occasional transport--if we bear in mind, and this is a very important
consideration, how often a species may have ranged continuously over a
wide area, and then have become extinct in the intermediate tracts--the
difficulty is not insuperable in believing that all the individuals of
the same species, wherever found, are descended from common parents.
And we are led to this conclusion, which has been arrived at by many
naturalists under the designation of single centres of creation, by
various general considerations, more especially from the importance
of barriers of all kinds, and from the analogical distribution of
subgenera, genera, and families.

With respect to distinct species belonging to the same genus, which
on our theory have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms of
life have changed very slowly, enormous periods of time having been thus
granted for their migration, the difficulties are far from insuperable;
though in this case, as in that of the individuals of the same species,
they are often great.

As exemplifying the effects of climatical changes on distribution, I
have attempted to show how important a part the last Glacial period has
played, which affected even the equatorial regions, and which, during
the alternations of the cold in the north and the south, allowed the
productions of opposite hemispheres to mingle, and left some of them
stranded on the mountain-summits in all parts of the world. As showing
how diversified are the means of occasional transport, I have discussed
at some little length the means of dispersal of fresh-water productions.

If the difficulties be not insuperable in admitting that in the long
course of time all the individuals of the same species, and likewise
of the several species belonging to the same genus, have proceeded
from some one source; then all the grand leading facts of geographical
distribution are explicable on the theory of migration, together with
subsequent modification and the multiplication of new forms. We can thus
understand the high importance of barriers, whether of land or water, in
not only separating but in apparently forming the several zoological and
botanical provinces. We can thus understand the concentration of related
species within the same areas; and how it is that under different
latitudes, for instance, in South America, the inhabitants of the plains
and mountains, of the forests, marshes, and deserts, are linked together
in so mysterious a manner, and are likewise linked to the extinct beings
which formerly inhabited the same continent. Bearing in mind that the
mutual relation of organism to organism is of the highest importance,
we can see why two areas, having nearly the same physical conditions,
should often be inhabited by very different forms of life; for according
to the length of time which has elapsed since the colonists entered one
of the regions, or both; according to the nature of the communication
which allowed certain forms and not others to enter, either in greater
or lesser numbers; according or not as those which entered happened to
come into more or less direct competition with each other and with the
aborigines; and according as the immigrants were capable of varying
more or less rapidly, there would ensue in the to or more regions,
independently of their physical conditions, infinitely diversified
conditions of life; there would be an almost endless amount of organic
action and reaction, and we should find some groups of beings greatly,
and some only slightly modified; some developed in great force, some
existing in scanty numbers--and this we do find in the several great
geographical provinces of the world.

On these same principles we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but that of
these, a large proportion should be endemic or peculiar; and why, in
relation to the means of migration, one group of beings should have all
its species peculiar, and another group, even within the same class,
should have all its species the same with those in an adjoining quarter
of the world. We can see why whole groups of organisms, as batrachians
and terrestrial mammals, should be absent from oceanic islands, whilst
the most isolated islands should possess their own peculiar species of
aerial mammals or bats. We can see why, in islands, there should be some
relation between the presence of mammals, in a more or less modified
condition, and the depth of the sea between such islands and the
mainland. We can clearly see why all the inhabitants of an archipelago,
though specifically distinct on the several islets, should be closely
related to each other, and should likewise be related, but less closely,
to those of the nearest continent, or other source whence immigrants
might have been derived. We can see why, if there exist very closely
allied or representative species in two areas, however distant from each
other, some identical species will almost always there be found.

As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space; the laws
governing the succession of forms in past times being nearly the same
with those governing at the present time the differences in different
areas. We see this in many facts. The endurance of each species and
group of species is continuous in time; for the apparent exceptions to
the rule are so few that they may fairly be attributed to our not having
as yet discovered in an intermediate deposit certain forms which are
absent in it, but which occur above and below: so in space, it certainly
is the general rule that the area inhabited by a single species, or by a
group of species, is continuous, and the exceptions, which are not rare,
may, as I have attempted to show, be accounted for by former migrations
under different circumstances, or through occasional means of transport,
or by the species having become extinct in the intermediate tracts. Both
in time and space species and groups of species have their points of
maximum development. Groups of species, living during the same period
of time, or living within the same area, are often characterised by
trifling features in common, as of sculpture or colour. In looking to
the long succession of past ages, as in looking to distant provinces
throughout the world, we find that species in certain classes differ
little from each other, whilst those in another class, or only in a
different section of the same order, differ greatly from each other. In
both time and space the lowly organised members of each class generally
change less than the highly organised; but there are in both cases
marked exceptions to the rule. According to our theory, these several
relations throughout time and space are intelligible; for whether we
look to the allied forms of life which have changed during successive
ages, or to those which have changed after having migrated into distant
quarters, in both cases they are connected by the same bond of ordinary
generation; in both cases the laws of variation have been the same,
and modifications have been accumulated by the same means of natural
selection.



CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS:

MORPHOLOGY--EMBRYOLOGY--RUDIMENTARY ORGANS.

 Classification, groups subordinate to groups--Natural system--Rules and
 difficulties in classification, explained on the theory of descent
 with modification--Classification of varieties--Descent always used in
 classification--Analogical or adaptive characters--Affinities,
 general, complex and radiating--Extinction separates and defines
 groups--Morphology, between members of the same class, between parts of
 the same individual--Embryology, laws of, explained by variations not
 supervening at an early age, and being inherited at a corresponding
 age--Rudimentary organs; their origin explained--Summary.


CLASSIFICATION.

From the most remote period in the history of the world organic beings
have been found to resemble each other in descending degrees, so that
they can be classed in groups under groups. This classification is
not arbitrary like the grouping of the stars in constellations. The
existence of groups would have been of simple significance, if one group
had been exclusively fitted to inhabit the land, and another the water;
one to feed on flesh, another on vegetable matter, and so on; but the
case is widely different, for it is notorious how commonly members of
even the same subgroup have different habits. In the second and fourth
chapters, on Variation and on Natural Selection, I have attempted
to show that within each country it is the widely ranging, the much
diffused and common, that is the dominant species, belonging to
the larger genera in each class, which vary most. The varieties, or
incipient species, thus produced, ultimately become converted into new
and distinct species; and these, on the principle of inheritance, tend
to produce other new and dominant species. Consequently the groups which
are now large, and which generally include many dominant species, tend
to go on increasing in size. I further attempted to show that from the
varying descendants of each species trying to occupy as many and as
different places as possible in the economy of nature, they constantly
tend to diverge in character. This latter conclusion is supported by
observing the great diversity of forms, which, in any small area, come
into the closest competition, and by certain facts in naturalisation.

I attempted also to show that there is a steady tendency in the forms
which are increasing in number and diverging in character, to supplant
and exterminate the preceding, less divergent and less improved forms.
I request the reader to turn to the diagram illustrating the action, as
formerly explained, of these several principles; and he will see that
the inevitable result is, that the modified descendants proceeding from
one progenitor become broken up into groups subordinate to groups. In
the diagram each letter on the uppermost line may represent a genus
including several species; and the whole of the genera along this upper
line form together one class, for all are descended from one ancient
parent, and, consequently, have inherited something in common. But the
three genera on the left hand have, on this same principle, much in
common, and form a subfamily, distinct from that containing the next
two genera on the right hand, which diverged from a common parent at
the fifth stage of descent. These five genera have also much in common,
though less than when grouped in subfamilies; and they form a family
distinct from that containing the three genera still further to the
right hand, which diverged at an earlier period. And all these genera,
descended from (A), form an order distinct from the genera descended
from (I). So that we here have many species descended from a single
progenitor grouped into genera; and the genera into subfamilies,
families and orders, all under one great class. The grand fact of the
natural subordination of organic beings in groups under groups, which,
from its familiarity, does not always sufficiently strike us, is in
my judgment thus explained. No doubt organic beings, like all other
objects, can be classed in many ways, either artificially by single
characters, or more naturally by a number of characters. We know,
for instance, that minerals and the elemental substances can be thus
arranged. In this case there is of course no relation to genealogical
succession, and no cause can at present be assigned for their falling
into groups. But with organic beings the case is different, and the view
above given accords with their natural arrangement in group under group;
and no other explanation has ever been attempted.

Naturalists, as we have seen, try to arrange the species, genera and
families in each class, on what is called the Natural System. But what
is meant by this system? Some authors look at it merely as a scheme for
arranging together those living objects which are most alike, and for
separating those which are most unlike; or as an artificial method of
enunciating, as briefly as possible, general propositions--that is,
by one sentence to give the characters common, for instance, to all
mammals, by another those common to all carnivora, by another those
common to the dog-genus, and then, by adding a single sentence, a full
description is given of each kind of dog. The ingenuity and utility of
this system are indisputable. But many naturalists think that something
more is meant by the Natural System; they believe that it reveals the
plan of the Creator; but unless it be specified whether order in time
or space, or both, or what else is meant by the plan of the Creator,
it seems to me that nothing is thus added to our knowledge. Expressions
such as that famous one by Linnaeus, which we often meet with in a more
or less concealed form, namely, that the characters do not make the
genus, but that the genus gives the characters, seem to imply that some
deeper bond is included in our classifications than mere resemblance.
I believe that this is the case, and that community of descent--the one
known cause of close similarity in organic beings--is the bond, which,
though observed by various degrees of modification, is partially
revealed to us by our classifications.

Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme for
enunciating general propositions and of placing together the forms most
like each other. It might have been thought (and was in ancient times
thought) that those parts of the structure which determined the habits
of life, and the general place of each being in the economy of nature,
would be of very high importance in classification. Nothing can be more
false. No one regards the external similarity of a mouse to a shrew, of
a dugong to a whale, of a whale to a fish, as of any importance. These
resemblances, though so intimately connected with the whole life of the
being, are ranked as merely "adaptive or analogical characters;" but to
the consideration of these resemblances we shall recur. It may even be
given as a general rule, that the less any part of the organisation
is concerned with special habits, the more important it becomes for
classification. As an instance: Owen, in speaking of the dugong, says,
"The generative organs, being those which are most remotely related to
the habits and food of an animal, I have always regarded as affording
very clear indications of its true affinities. We are least likely in
the modifications of these organs to mistake a merely adaptive for an
essential character." With plants how remarkable it is that the organs
of vegetation, on which their nutrition and life depend, are of little
signification; whereas the organs of reproduction, with their product
the seed and embryo, are of paramount importance! So again, in formerly
discussing certain morphological characters which are not functionally
important, we have seen that they are often of the highest service in
classification. This depends on their constancy throughout many allied
groups; and their constancy chiefly depends on any slight deviations not
having been preserved and accumulated by natural selection, which acts
only on serviceable characters.

That the mere physiological importance of an organ does not determine
its classificatory value, is almost proved by the fact, that in allied
groups, in which the same organ, as we have every reason to suppose, has
nearly the same physiological value, its classificatory value is widely
different. No naturalist can have worked at any group without being
struck with this fact; and it has been fully acknowledged in the
writings of almost every author. It will suffice to quote the highest
authority, Robert Brown, who, in speaking of certain organs in the
Proteaceae, says their generic importance, "like that of all their
parts, not only in this, but, as I apprehend in every natural family,
is very unequal, and in some cases seems to be entirely lost." Again,
in another work he says, the genera of the Connaraceae "differ in having
one or more ovaria, in the existence or absence of albumen, in the
imbricate or valvular aestivation. Any one of these characters singly is
frequently of more than generic importance, though here even, when
all taken together, they appear insufficient to separate Cnestis from
Connarus." To give an example among insects: in one great division
of the Hymenoptera, the antennae, as Westwood has remarked, are most
constant in structure; in another division they differ much, and the
differences are of quite subordinate value in classification; yet no one
will say that the antennae in these two divisions of the same order are
of unequal physiological importance. Any number of instances could be
given of the varying importance for classification of the same important
organ within the same group of beings.

Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of much value in classification. No one will dispute
that the rudimentary teeth in the upper jaws of young ruminants,
and certain rudimentary bones of the leg, are highly serviceable in
exhibiting the close affinity between Ruminants and Pachyderms. Robert
Brown has strongly insisted on the fact that the position of the
rudimentary florets is of the highest importance in the classification
of the Grasses.

Numerous instances could be given of characters derived from parts which
must be considered of very trifling physiological importance, but which
are universally admitted as highly serviceable in the definition of
whole groups. For instance, whether or not there is an open passage from
the nostrils to the mouth, the only character, according to Owen, which
absolutely distinguishes fishes and reptiles--the inflection of the
angle of the lower jaw in Marsupials--the manner in which the wings of
insects are folded--mere colour in certain Algae--mere pubescence on
parts of the flower in grasses--the nature of the dermal covering, as
hair or feathers, in the Vertebrata. If the Ornithorhynchus had been
covered with feathers instead of hair, this external and trifling
character would have been considered by naturalists as an important aid
in determining the degree of affinity of this strange creature to birds.

The importance, for classification, of trifling characters, mainly
depends on their being correlated with many other characters of more or
less importance. The value indeed of an aggregate of characters is very
evident in natural history. Hence, as has often been remarked, a
species may depart from its allies in several characters, both of high
physiological importance, and of almost universal prevalence, and yet
leave us in no doubt where it should be ranked. Hence, also, it has been
found that a classification founded on any single character,
however important that may be, has always failed; for no part of the
organisation is invariably constant. The importance of an aggregate of
characters, even when none are important, alone explains the aphorism
enunciated by Linnaeus, namely, that the characters do not give the
genus, but the genus gives the character; for this seems founded on the
appreciation of many trifling points of resemblance, too slight to be
defined. Certain plants, belonging to the Malpighiaceae, bear perfect
and degraded flowers; in the latter, as A. de Jussieu has remarked, "The
greater number of the characters proper to the species, to the genus,
to the family, to the class, disappear, and thus laugh at our
classification." When Aspicarpa produced in France, during several
years, only these degraded flowers, departing so wonderfully in a number
of the most important points of structure from the proper type of the
order, yet M. Richard sagaciously saw, as Jussieu observes, that this
genus should still be retained among the Malpighiaceae. This case well
illustrates the spirit of our classifications.

Practically, when naturalists are at work, they do not trouble
themselves about the physiological value of the characters which they
use in defining a group or in allocating any particular species. If they
find a character nearly uniform, and common to a great number of forms,
and not common to others, they use it as one of high value; if common to
some lesser number, they use it as of subordinate value. This principle
has been broadly confessed by some naturalists to be the true one; and
by none more clearly than by that excellent botanist, Aug. St. Hilaire.
If several trifling characters are always found in combination, though
no apparent bond of connexion can be discovered between them, especial
value is set on them. As in most groups of animals, important organs,
such as those for propelling the blood, or for aerating it, or those for
propagating the race, are found nearly uniform, they are considered as
highly serviceable in classification; but in some groups all these,
the most important vital organs, are found to offer characters of quite
subordinate value. Thus, as Fritz Muller has lately remarked, in the
same group of crustaceans, Cypridina is furnished with a heart, while in
two closely allied genera, namely Cypris and Cytherea, there is no such
organ; one species of Cypridina has well-developed branchiae, while
another species is destitute of them.

We can see why characters derived from the embryo should be of
equal importance with those derived from the adult, for a natural
classification of course includes all ages. But it is by no means
obvious, on the ordinary view, why the structure of the embryo should
be more important for this purpose than that of the adult, which alone
plays its full part in the economy of nature. Yet it has been strongly
urged by those great naturalists, Milne Edwards and Agassiz, that
embryological characters are the most important of all; and this
doctrine has very generally been admitted as true. Nevertheless,
their importance has sometimes been exaggerated, owing to the adaptive
characters of larvae not having been excluded; in order to show this,
Fritz Muller arranged, by the aid of such characters alone, the great
class of crustaceans, and the arrangement did not prove a natural one.
But there can be no doubt that embryonic, excluding larval characters,
are of the highest value for classification, not only with animals but
with plants. Thus the main divisions of flowering plants are founded on
differences in the embryo--on the number and position of the cotyledons,
and on the mode of development of the plumule and radicle. We shall
immediately see why these characters possess so high a value in
classification, namely, from the natural system being genealogical in
its arrangement.

Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of characters
common to all birds; but with crustaceans, any such definition has
hitherto been found impossible. There are crustaceans at the opposite
ends of the series, which have hardly a character in common; yet the
species at both ends, from being plainly allied to others, and these to
others, and so onwards, can be recognised as unequivocally belonging to
this, and to no other class of the Articulata.

Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity
of this practice in certain groups of birds; and it has been followed by
several entomologists and botanists.

Finally, with respect to the comparative value of the various groups of
species, such as orders, suborders, families, subfamilies, and genera,
they seem to be, at least at present, almost arbitrary. Several of the
best botanists, such as Mr. Bentham and others, have strongly insisted
on their arbitrary value. Instances could be given among plants and
insects, of a group first ranked by practised naturalists as only a
genus, and then raised to the rank of a subfamily or family; and this
has been done, not because further research has detected important
structural differences, at first overlooked, but because numerous
allied species, with slightly different grades of difference, have been
subsequently discovered.

All the foregoing rules and aids and difficulties in classification may
be explained, if I do not greatly deceive myself, on the v