The Origin of Species - Charles Darwin
- 6th Edition -
Inhaltsverzeichnis
Preface...5
Introduction...16
Chapter 1 - Variation Under Domestication...21
Chapter 2 - Variation Under Nature...52
Chapter 3 - Struggle For Existence...69
Chapter 4 - Natural Selection...85
Chapter 5 - Laws Of Variation...136
Chapter 6 - Difficulties Of The Theory...168
Chapter 7 - Miscellaneous Objections To The Theory Of Natural Selection...208
Chapter 8 - Instinct...251
Chapter 9 - Hybridism...286
Chapter 10 - On The Imperfection Of The Geological Record...320
Chapter 11 - On The Geological Succession Of Organic Beings...351
Chapter 12 - Geographical Distribution...381
Chapter 13 - Geographical Distribution--Continued...412
Chapter 14 - Mutual Affinities Of Organic Beings: Morphology -- Embryology --
Rudimentary Organs...434
Chapter 15 - Recapitulation And Conclusion...481
Glossary Of The Principal Scientific Terms Used In The Present Volume...511
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Preface
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.
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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.
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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
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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
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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
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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
19
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.
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Chapter 1 - Variation Under Domestication
CAUSES OF VARIABILITY -- EFFECTS OF HABIT AND OF 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 -- BREEDS OF THE
DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN -- PRINCIPLES OF
SELECTION ANCIENTLY FOLLOWED, AND THEIR EFFECTS -- UNCONSCIOUS
SELECTION -- 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
21
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
22
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
23
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
24
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
25
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.
26
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
27
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
28
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.
29
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
30
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
31
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.
32
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
33
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
34
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;
35
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
36
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,
37
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
38
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
39
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
40
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
41
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
42
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.
43
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
44
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
45
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
46
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
47
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
48
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
49
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
50
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.
51
Chapter 2 - 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 INCLUDED WITHIN
THE LARGER GENERA RESEMBLE VARIETIES IN BEING VERY CLOSELY, BUT
UNEQUALLY, RELATED TO EACH OTHER, AND IN HAVING RESTRICTED RANGES.
-- SUMMARY
VARIABILITY
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
52
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
53
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
54
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.
55
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
56
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.
57
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
58
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;
59
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,
60
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
61
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.
62
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
63
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
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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
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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
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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
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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.
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Chapter 3 - Struggle For Existence
ITS BEARING ON NATURAL SELECTION -- THE TERM, STRUGGLE FOR EXISTENCE,
USED IN A LARGE SENSE -- GEOMETRICAL RATIO OF INCREASE -- NATURE OF
THE CHECKS TO INCREASE -- COMPLEX RELATIONS OF ALL ANIMALS AND
PLANTS TO EACH OTHER IN THE STRUGGLE FOR EXISTENCE -- STRUGGLE FOR
LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF THE SAME
SPECIES
ITS BEARING ON NATURAL SELECTION,
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
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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
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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,
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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.
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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
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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.
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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
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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
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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
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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
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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!
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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
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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
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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.
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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
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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.
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Chapter 4 - Natural Selection
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 -- ILLUSTRATIONS OF THE ACTION
OF NATURAL SELECTION, OR THE SURVIVAL OF THE FITTEST -- ON THE
INTERCROSSING OF INDIVIDUALS -- CIRCUMSTANCES FAVOURABLE FOR THE
PRODUCTION OF NEW FORMS THROUGH NATURAL SELECTION -- EXTINCTION
CAUSED BY NATURAL SELECTION -- DIVERGENCE OF CHARACTER -- THE
PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH
DIVERGENCE OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF A
COMMON ANCESTOR -- ON THE DEGREE TO WHICH ORGANISATION TENDS TO
ADVANCE -- CONVERGENCE OF CHARACTER -- SUMMARY OF CHAPTER
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
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
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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
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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
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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
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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.
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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
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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
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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.
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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