DARWINISM
AN EXPOSITION OF THE
THEORY OF NATURAL SELECTION
WITH SOME OF ITS APPLICATIONS
BY
ALFRED RUSSEL WALLACE
LL.D., F.L.S., ETC.
WITH A PORTRAIT OF THE AUTHOR, MAP AND ILLUSTRATIONS
MACMILLAN AND CO.
LONDON AND NEW YORK
[Second Edition] 1889
* * * * *
[Illustration: Alfred R. Wallace]
* * * * *
PREFACE TO SECOND EDITION
The present edition is a reprint of the first, with a few verbal
corrections and the alteration of some erroneous or doubtful statements.
Of these latter the following are the most important:--
_P._ 30. The statement as to the fulmar petrel, which Professor A.
Newton assures me is erroneous, has been modified.
_P._ 34. A note is added as to Darwin's statement about the missel and
song-thrushes in Scotland.
_P._ 172. An error as to the differently-coloured herds of cattle in the
Falkland Islands, is corrected.
PARKSTONE, DORSET
_August, 1889_.
PREFACE TO FIRST EDITION
The present work treats the problem of the Origin of Species on the same
general lines as were adopted by Darwin; but from the standpoint reached
after nearly thirty years of discussion, with an abundance of new facts
and the advocacy of many new or old theories.
While not attempting to deal, even in outline, with the vast subject of
evolution in general, an endeavour has been made to give such an account
of the theory of Natural Selection as may enable any intelligent reader
to obtain a clear conception of Darwin's work, and to understand
something of the power and range of his great principle.
Darwin wrote for a generation which had not accepted evolution, and
which poured contempt on those who upheld the derivation of species from
species by any natural law of descent. He did his work so well that
"descent with modification" is now universally accepted as the order of
nature in the organic world; and the rising generation of naturalists
can hardly realise the novelty of this idea, or that their fathers
considered it a scientific heresy to be condemned rather than seriously
discussed.
The objections now made to Darwin's theory apply, solely, to the
particular means by which the change of species has been brought about,
not to the fact of that change. The objectors seek to minimise the
agency of natural selection and to subordinate it to laws of variation,
of use and disuse, of intelligence, and of heredity. These views and
objections are urged with much force and more confidence, and for the
most part by the modern school of laboratory naturalists, to whom the
peculiarities and distinctions of species, as such, their distribution
and their affinities, have little interest as compared with the problems
of histology and embryology, of physiology and morphology. Their work in
these departments is of the greatest interest and of the highest
importance, but it is not the kind of work which, by itself, enables one
to form a sound judgment on the questions involved in the action of the
law of natural selection. These rest mainly on the external and vital
relations of species to species in a state of nature--on what has been
well termed by Semper the "physiology of organisms," rather than on the
anatomy or physiology of organs.
* * * * *
It has always been considered a weakness in Darwin's work that he based
his theory, primarily, on the evidence of variation in domesticated
animals and cultivated plants. I have endeavoured to secure a firm
foundation for the theory in the variations of organisms in a state of
nature; and as the exact amount and precise character of these
variations is of paramount importance in the numerous problems that
arise when we apply the theory to explain the facts of nature, I have
endeavoured, by means of a series of diagrams, to exhibit to the eye the
actual variations as they are found to exist in a sufficient number of
species. By doing this, not only does the reader obtain a better and
more precise idea of variation than can be given by any number of
tabular statements or cases of extreme individual variation, but we
obtain a basis of fact by which to test the statements and objections
usually put forth on the subject of specific variability; and it will be
found that, throughout the work, I have frequently to appeal to these
diagrams and the facts they illustrate, just as Darwin was accustomed to
appeal to the facts of variation among dogs and pigeons.
I have also made what appears to me an important change in the
arrangement of the subject. Instead of treating first the comparatively
difficult and unfamiliar details of variation, I commence with the
Struggle for Existence, which is really the fundamental phenomenon on
which natural selection depends, while the particular facts which
illustrate it are comparatively familiar and very interesting. It has
the further advantage that, after discussing variation and the effects
of artificial selection, we proceed at once to explain how natural
selection acts.
Among the subjects of novelty or interest discussed in this volume, and
which have important bearings on the theory of natural selection, are:
(1) A proof that all _specific_ characters are (or once have been)
either useful in themselves or correlated with useful characters (Chap.
VI); (2) a proof that natural selection can, in certain cases, increase
the sterility of crosses (Chap. VII); (3) a fuller discussion of the
colour relations of animals, with additional facts and arguments on the
origin of sexual differences of colour (Chaps. VIII-X); (4) an attempted
solution of the difficulty presented by the occurrence of both very
simple and very complex modes of securing the cross-fertilisation of
plants (Chap. XI); (5) some fresh facts and arguments on the
wind-carriage of seeds, and its bearing on the wide dispersal of many
arctic and alpine plants (Chap. XII); (6) some new illustrations of the
non-heredity of acquired characters, and a proof that the effects of use
and disuse, even if inherited, must be overpowered by natural selection
(Chap. XIV); and (7) a new argument as to the nature and origin of the
moral and intellectual faculties of man (Chap. XV).
* * * * *
Although I maintain, and even enforce, my differences from some of
Darwin's views, my whole work tends forcibly to illustrate the
overwhelming importance of Natural Selection over all other agencies in
the production of new species. I thus take up Darwin's earlier
position, from which he somewhat receded in the later editions of his
works, on account of criticisms and objections which I have endeavoured
to show are unsound. Even in rejecting that phase of sexual selection
depending on female choice, I insist on the greater efficacy of natural
selection. This is pre-eminently the Darwinian doctrine, and I therefore
claim for my book the position of being the advocate of pure Darwinism.
I wish to express my obligation to Mr. Francis Darwin for lending me
some of his father's unused notes, and to many other friends for facts
or information, which have, I believe, been acknowledged either in the
text or footnotes. Mr. James Sime has kindly read over the proofs and
given me many useful suggestions; and I have to thank Professor Meldola,
Mr. Hemsley, and Mr. E.B. Poulton for valuable notes or corrections in
the later chapters in which their special subjects are touched upon.
GODALMING, _March 1889_.
CONTENTS
CHAPTER I
WHAT ARE "SPECIES" AND WHAT IS MEANT BY THEIR "ORIGIN"
Definition of species--Special creation--The early
transmutationists--Scientific opinion before Darwin--The problem
before Darwin--The change of opinion effected by Darwin--The
Darwinian theory--Proposed mode of treatment of the subject
CHAPTER II
THE STRUGGLE FOR EXISTENCE
Its importance--The struggle among plants--Among
animals--Illustrative cases--Succession of trees in forests of
Denmark--The struggle for existence on the Pampas--Increase of
organisms in a geometrical ratio--Examples of rapid increase of
animals--Rapid increase and wide spread of plants--Great
fertility not essential to rapid increase--Struggle between
closely allied species most severe--The ethical aspect of the
struggle for existence
CHAPTER III
THE VARIABILITY OF SPECIES IN A STATE OF NATURE
Importance of variability--Popular ideas regarding
it--Variability of the lower animals--The variability of
insects--Variation among lizards--Variation among
birds--Diagrams of bird-variation--Number of varying
individuals--Variation in the mammalia--Variation in internal
organs--Variations in the skull--Variations in the habits of
animals--The variability of plants--Species which vary
little--Concluding remarks
CHAPTER IV
VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS
The facts of variation and artificial selection--Proofs of the
generality of variation--Variations of apples and
melons--Variations of flowers--Variations of domestic
animals--Domestic pigeons--Acclimatisation--Circumstances
favourable to selection by man--Conditions favourable to
variation--Concluding remarks
CHAPTER V
NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST
Effect of struggle for existence under unchanged conditions--The
effect under change of conditions--Divergence of character--In
insects--In birds--In mammalia--Divergence leads to a maximum of
life in each area--Closely allied species inhabit distinct
areas--Adaptation to conditions at various periods of life--The
continued existence of low forms of life--Extinction of low
types among the higher animals--Circumstances favourable to the
origin of new species--Probable origin of the dippers--The
importance of isolation--On the advance of organisation by
natural selection--Summary of the first five chapters
CHAPTER VI
DIFFICULTIES AND OBJECTIONS
Difficulty as to smallness of variations--As to the right
variations occurring when required--The beginnings of important
organs--The mammary glands--The eyes of flatfish--Origin of the
eye--Useless or non-adaptive characters--Recent extension of the
region of utility in plants--The same in animals--Uses of
tails--Of the horns of deer--Of the scale-ornamentation of
reptiles--Instability of non-adaptive characters--Delboeuf's
law--No "specific" character proved to be useless--The swamping
effects of intercrossing--Isolation as preventing
intercrossing--Gulick on the effects of isolation--Cases in
which isolation is ineffective
CHAPTER VII
ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL
STERILITY OF THEIR HYBRID OFFSPRING
Statement of the problem--Extreme susceptibility of the
reproductive functions--Reciprocal crosses--Individual
differences in respect to cross-fertilisation--Dimorphism and
trimorphism among plants--Cases of the fertility of hybrids and
of the infertility of mongrels--The effects of close
interbreeding--Mr. Huth's objections--Fertile hybrids among
animals--Fertility of hybrids among plants--Cases of sterility
of mongrels--Parallelism between crossing and change of
conditions--Remarks on the facts of hybridity--Sterility due to
changed conditions and usually correlated with other
characters--Correlation of colour with constitutional
peculiarities--The isolation of varieties by selective
association--The influence of natural selection upon sterility
and fertility--Physiological selection--Summary and concluding
remarks
CHAPTER VIII
THE ORIGIN AND USES OF COLOUR IN ANIMALS
The Darwinian theory threw new light on organic colour--The
problem to be solved--The constancy of animal colour indicates
utility--Colour and environment--Arctic animals
white--Exceptions prove the rule--Desert, forest, nocturnal, and
oceanic animals--General theories of animal colour--Variable
protective colouring--Mr. Poulton's experiments--Special or
local colour adaptations--Imitation of particular objects--How
they have been produced--Special protective colouring of
butterflies--Protective resemblance among marine
animals--Protection by terrifying enemies--Alluring
coloration--The coloration of birds' eggs--Colour as a means of
recognition--Summary of the preceding exposition--Influence of
locality or of climate on colour--Concluding remarks
CHAPTER IX
WARNING COLORATION AND MIMICRY
The skunk as an example of warning coloration--Warning colours
among insects--Butterflies--Caterpillars--Mimicry--How mimicry
has been produced--Heliconidae--Perfection of the
imitation--Other cases of mimicry among Lepidoptera--Mimicry
among protected groups--Its explanation--Extension of the
principle--Mimicry in other orders of insects--Mimicry among the
vertebrata--Snakes--The rattlesnake and the cobra--Mimicry among
birds--Objections to the theory of mimicry--Concluding remarks
on warning colours and mimicry
CHAPTER X
COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX
Sex colours in the mollusca and crustacea--In insects--In
butterflies and moths--Probable causes of these colours--Sexual
selection as a supposed cause--Sexual coloration of birds--Cause
of dull colours of female birds--Relation of sex colour to
nesting habits--Sexual colours of other vertebrates--Sexual
selection by the struggles of males--Sexual characters due to
natural selection--Decorative plumage of males and its effect on
the females--Display of decorative plumage by the males--A
theory of animal coloration--The origin of accessory
plumes--Development of accessory plumes and their display--The
effect of female preference will be neutralised by natural
selection--General laws of animal coloration--Concluding remarks
CHAPTER XI
THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND PURPOSE
The general colour relations of plants--Colours of fruits--The
meaning of nuts--Edible or attractive fruits--The colours of
flowers--Modes of securing cross-fertilisation--The
interpretation of the facts--Summary of additional facts
bearing on insect fertilisation--Fertilisation of flowers by
birds--Self-fertilisation of flowers--Difficulties and
contradictions--Intercrossing not necessarily
advantageous--Supposed evil results of close interbreeding--How
the struggle for existence acts among flowers--Flowers the
product of insect agency--Concluding remarks on colour in nature
CHAPTER XII
THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS
The facts to be explained--The conditions which have determined
distribution--The permanence of oceans--Oceanic and continental
areas--Madagascar and New Zealand--The teachings of the
thousand-fathom line--The distribution of marsupials--The
distribution of tapirs--Powers of dispersal as illustrated by
insular organisms--Birds and insects at sea--Insects at great
altitudes--The dispersal of plants--Dispersal of seeds by the
wind--Mineral matter carried by the wind--Objections to the
theory of wind-dispersal answered--Explanation of north
temperate plants in the southern hemisphere--No proof of
glaciation in the tropics--Lower temperature not needed to
explain the facts--Concluding remarks
CHAPTER XIII
THE GEOLOGICAL EVIDENCES OF EVOLUTION
What we may expect--The number of known species of extinct
animals--Causes of the imperfection of the geological
record--Geological evidences of
evolution--Shells--Crocodiles--The rhinoceros tribe--The
pedigree of the horse tribe--Development of deer's horns--Brain
development--Local relations of fossil and living animals--Cause
of extinction of large animals--Indications of general progress
in plants and animals--The progressive development of
plants--Possible cause of sudden late appearance of
exogens--Geological distribution of insects--Geological
succession of vertebrata--Concluding remarks
CHAPTER XIV
FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY
Fundamental difficulties and objections--Mr. Herbert Spencer's
factors of organic evolution--Disuse and effects of withdrawal
of natural selection--Supposed effects of disuse among wild
animals--Difficulty as to co-adaptation of parts by variation
and selection--Direct action of the environment--The American
school of evolutionists--Origin of the feet of the
ungulates--Supposed action of animal intelligence--Semper on the
direct influence of the environment--Professor Geddes's theory
of variation in plants--Objections to the theory--On the origin
of spines--Variation and selection overpower the effects of use
and disuse--Supposed action of the environment in imitating
variations--Weismann's theory of heredity--The cause of
variation--The non-heredity of acquired characters--The theory
of instinct--Concluding remarks
CHAPTER XV
DARWINISM APPLIED TO MAN
General identity of human and animal structure--Rudiments and
variations showing relation of man to other mammals--The
embryonic development of man and other mammalia--Diseases common
to man and the lower animals--The animals most nearly allied to
man--The brains of man and apes--External differences of man and
apes--Summary of the animal characteristics of man--The
geological antiquity of man--The probable birthplace of man--The
origin of the moral and intellectual nature of man--The argument
from continuity--The origin of the mathematical faculty--The
origin of the musical and artistic faculties--Independent proof
that these faculties have not been developed by natural
selection--The interpretation of the facts--Concluding remarks
LIST OF ILLUSTRATIONS
PORTRAIT OF AUTHOR
MAP SHOWING THE 1000-FATHOM LINE
1. DIAGRAM OF VARIATIONS OF LACERTA MURALIS
2. " VARIATION OF LIZARDS
3. " VARIATION OF WINGS AND TAIL OF BIRDS
4. " VARIATION OF DOLICHONYX ORYZIVORUS
5. " VARIATION OF AGELAEUS PHOENICEUS
6. " VARIATION OF CARDINALIS VIRGINIANUS
7. " VARIATION OF TARSUS AND TOES
8. " VARIATION OF BIRDS IN LEYDEN MUSEUM
9. " VARIATION OF ICTERUS BALTIMORE
10. " VARIATION OF AGELAEUS PHOENICEUS
11. " CURVES OF VARIATION
12. " VARIATION OF CARDINALIS VIRGINIANUS
13. " VARIATION OF SCIURUS CAROLINENSIS
14. " VARIATION OF SKULLS OF WOLF
15. " VARIATION OF SKULLS OF URSUS LABIATUS
16. " VARIATION OF SKULLS OF SUS CRISTATUS
17. PRIMULA VERIS (Cowslip). From Darwin's _Forms of Flowers_
18. GAZELLA SOEMMERRINGI (to show recognition marks)
19. RECOGNITION MARKS OF AFRICAN PLOVERS
(from Seebohm's _Charadriadae_)
20. RECOGNITION OF OEDICNEMUS VERMICULATUS AND OE. SENEGALENSIS
(from Seebohm's _Charadriadae_)
21. RECOGNITION OF CURSORIUS CHALCOPTERUS AND C. GALLICUS
(from Seebohm's _Charadriadae_)
22. RECOGNITION OF SCOLOPAX MEGALA AND S. STENURA
(from Seebohm's _Charadriadae_)
23. METHONA PSIDII AND LEPTALIS ORISE
24. OPTHALMIS LINCEA AND ARTAXA SIMULANS
(from the Official _Narrative of the Voyage of the Challenger_)
25. WINGS OF ITUNA ILIONE AND THYRIDIA MEGISTO
(from _Proceedings of the Entomological Society_)
26. MYGNIMIA AVICULUS AND COLOBORHOMBUS FASCIATIPENNIS
27. MIMICKING INSECTS FROM THE PHILIPPINES
(from Semper's _Animal Life_)
28. MALVA SYLVESTRIS AND M. ROTUNDIFOLIA
(from Lubbock's _British Wild Flowers in Relation to Insects_)
29. LYTHRUM SALICARIA, THREE FORMS OF
(from Lubbock's _British Wild Flowers in Relation to Insects_)
30. ORCHIS PYRAMIDALIS (from Darwin's _Fertilisation of Orchids_)
31. HUMMING-BIRD FERTILISING MARCGRAVIA NEPENTHOIDES
32. DIAGRAM OF MEAN HEIGHT OF LAND AND DEPTH OF OCEANS
33. GEOLOGICAL DEVELOPMENT OF THE HORSE TRIBE
(from Huxley's _American Addresses_)
34. DIAGRAM ILLUSTRATING THE GEOLOGICAL DISTRIBUTION OF PLANTS
(from Ward's _Sketch of Palaeobotany_)
35. TRANSFORMATION OF ARTEMIA SALINA TO A. MILHAUSENII
(from Semper's _Animal Life_)
36. BRANCHIPUS STAGNALIS AND ARTEMIA SALINA
(from Semper's _Animal Life_)
37. CHIMPANZEE (TROGLODYTES NIGER)
CHAPTER I
WHAT ARE "SPECIES," AND WHAT IS MEANT BY THEIR "ORIGIN"
Definition of species--Special creation--The early
Transmutationists--Scientific opinion before Darwin--The problem
before Darwin--The change of opinion effected by Darwin--The
Darwinian theory--Proposed mode of treatment of the subject.
The title of Mr. Darwin's great work is--_On the Origin of Species by
means of Natural Selection and the Preservation of Favoured Races in the
Struggle for Life_. In order to appreciate fully the aim and object of
this work, and the change which it has effected not only in natural
history but in many other sciences, it is necessary to form a clear
conception of the meaning of the term "species," to know what was the
general belief regarding them at the time when Mr. Darwin's book first
appeared, and to understand what he meant, and what was generally meant,
by discovering their "origin." It is for want of this preliminary
knowledge that the majority of educated persons who are not naturalists
are so ready to accept the innumerable objections, criticisms, and
difficulties of its opponents as proofs that the Darwinian theory is
unsound, while it also renders them unable to appreciate, or even to
comprehend, the vast change which that theory has effected in the whole
mass of thought and opinion on the great question of evolution.
The term "species" was thus defined by the celebrated botanist De
Candolle: "A species is a collection of all the individuals which
resemble each other more than they resemble anything else, which can by
mutual fecundation produce fertile individuals, and which reproduce
themselves by generation, in such a manner that we may from analogy
suppose them all to have sprung from one single individual." And the
zoologist Swainson gives a somewhat similar definition: "A species, in
the usual acceptation of the term, is an animal which, in a state of
nature, is distinguished by certain peculiarities of form, size, colour,
or other circumstances, from another animal. It propagates, 'after its
kind,' individuals perfectly resembling the parent; its peculiarities,
therefore, are permanent."[1]
To illustrate these definitions we will take two common English birds,
the rook (Corvus frugilegus) and the crow (Corvus corone). These are
distinct _species_, because, in the first place, they always differ from
each other in certain slight peculiarities of structure, form, and
habits, and, in the second place, because rooks always produce rooks,
and crows produce crows, and they do not interbreed. It was therefore
concluded that all the rooks in the world had descended from a single
pair of rooks, and the crows in like manner from a single pair of crows,
while it was considered impossible that crows could have descended from
rooks or _vice versâ_. The "origin" of the first pair of each kind was a
mystery. Similar remarks may be applied to our two common plants, the
sweet violet (Viola odorata) and the dog violet (Viola canina). These
also produce their like and never produce each other or intermingle, and
they were therefore each supposed to have sprung from a single
individual whose "origin" was unknown. But besides the crow and the rook
there are about thirty other kinds of birds in various parts of the
world, all so much like our species that they receive the common name of
crows; and some of them differ less from each other than does our crow
from our rook. These are all _species_ of the genus Corvus, and were
therefore believed to have been always as distinct as they are now,
neither more nor less, and to have each descended from one pair of
ancestral crows of the same identical species, which themselves had an
unknown "origin." Of violets there are more than a hundred different
kinds in various parts of the world, all differing very slightly from
each other and forming distinct _species_ of the genus Viola. But, as
these also each produce their like and do not intermingle, it was
believed that every one of them had always been as distinct from all the
others as it is now, that all the individuals of each kind had descended
from one ancestor, but that the "origin" of these hundred slightly
differing ancestors was unknown. In the words of Sir John Herschel,
quoted by Mr. Darwin, the origin of such species was "the mystery of
mysteries."
_The Early Transmutationists_.
A few great naturalists, struck by the very slight difference between
many of these species, and the numerous links that exist between the
most different forms of animals and plants, and also observing that a
great many species do vary considerably in their forms, colours, and
habits, conceived the idea that they might be all produced one from the
other. The most eminent of these writers was a great French naturalist,
Lamarck, who published an elaborate work, the _Philosophie Zoologique_,
in which he endeavoured to prove that all animals whatever are descended
from other species of animals. He attributed the change of species
chiefly to the effect of changes in the conditions of life--such as
climate, food, etc.--and especially to the desires and efforts of the
animals themselves to improve their condition, leading to a modification
of form or size in certain parts, owing to the well-known physiological
law that all organs are strengthened by constant use, while they are
weakened or even completely lost by disuse. The arguments of Lamarck did
not, however, satisfy naturalists, and though a few adopted the view
that closely allied species had descended from each other, the general
belief of the educated public was, that each species was a "special
creation" quite independent of all others; while the great body of
naturalists equally held, that the change from one species to another by
any known law or cause was impossible, and that the "origin of species"
was an unsolved and probably insoluble problem. The only other important
work dealing with the question was the celebrated _Vestiges of
Creation_, published anonymously, but now acknowledged to have been
written by the late Robert Chambers. In this work the action of general
laws was traced throughout the universe as a system of growth and
development, and it was argued that the various species of animals and
plants had been produced in orderly succession from each other by the
action of unknown laws of development aided by the action of external
conditions. Although this work had a considerable effect in influencing
public opinion as to the extreme improbability of the doctrine of the
independent "special creation" of each species, it had little effect
upon naturalists, because it made no attempt to grapple with the problem
in detail, or to show in any single case how the allied species of a
genus could have arisen, and have preserved their numerous slight and
apparently purposeless differences from each other. No clue whatever was
afforded to a law which should produce from any one species one or more
slightly differing but yet permanently distinct species, nor was any
reason given why such slight yet constant differences should exist at
all.
_Scientific Opinion before Darwin._
In order to show how little effect these writers had upon the public
mind, I will quote a few passages from the writings of Sir Charles
Lyell, as representing the opinions of the most advanced thinkers in the
period immediately preceding that of Darwin's work. When recapitulating
the facts and arguments in favour of the invariability and permanence of
species, he says: "The entire variation from the original type which any
given kind of change can produce may usually be effected in a brief
period of time, after which no further deviation can be obtained by
continuing to alter the circumstances, though ever so gradually,
indefinite divergence either in the way of improvement or deterioration
being prevented, and the least possible excess beyond the defined limits
being fatal to the existence of the individual." In another place he
maintains that "varieties of some species may differ more than other
species do from each other without shaking our confidence in the reality
of species." He further adduces certain facts in geology as being, in
his opinion, "fatal to the theory of progressive development," and he
explains the fact that there are so often distinct species in countries
of similar climate and vegetation by "special creations" in each
country; and these conclusions were arrived at after a careful study of
Lamarck's work, a full abstract of which is given in the earlier
editions of the _Principles of Geology_.[2]
Professor Agassiz, one of the greatest naturalists of the last
generation, went even further, and maintained not only that each species
was specially created, but that it was created in the proportions and in
the localities in which we now find it to exist. The following extract
from his very instructive book on Lake Superior explains this view:
"There are in animals peculiar adaptations which are characteristic of
their species, and which cannot be supposed to have arisen from
subordinate influences. Those which live in shoals cannot be supposed to
have been created in single pairs. Those which are made to be the food
of others cannot have been created in the same proportions as those
which live upon them. Those which are everywhere found in innumerable
specimens must have been introduced in numbers capable of maintaining
their normal proportions to those which live isolated and are
comparatively and constantly fewer. For we know that this harmony in the
numerical proportions between animals is one of the great laws of
nature. The circumstance that species occur within definite limits where
no obstacles prevent their wider distribution leads to the further
inference that these limits were assigned to them from the beginning,
and so we should come to the final conclusion that the order which
prevails throughout nature is intentional, that it is regulated by the
limits marked out on the first day of creation, and that it has been
maintained unchanged through ages with no other modifications than those
which the higher intellectual powers of man enable him to impose on some
few animals more closely connected with him."[3]
These opinions of some of the most eminent and influential writers of
the pre-Darwinian age seem to us, now, either altogether obsolete or
positively absurd; but they nevertheless exhibit the mental condition of
even the most advanced section of scientific men on the problem of the
nature and origin of species. They render it clear that,
notwithstanding the vast knowledge and ingenious reasoning of Lamarck,
and the more general exposition of the subject by the author of the
_Vestiges of Creation_, the first step had not been taken towards a
satisfactory explanation of the derivation of any one species from any
other. Such eminent naturalists as Geoffroy Saint Hilaire, Dean Herbert,
Professor Grant, Von Buch, and some others, had expressed their belief
that species arose as simple varieties, and that the species of each
genus were all descended from a common ancestor; but none of them gave a
clue as to the law or the method by which the change had been effected.
This was still "the great mystery." As to the further question--how far
this common descent could be carried; whether distinct families, such as
crows and thrushes, could possibly have descended from each other; or,
whether all birds, including such widely distinct types as wrens,
eagles, ostriches, and ducks, could all be the modified descendants of a
common ancestor; or, still further, whether mammalia, birds, reptiles,
and fishes, could all have had a common origin;--these questions had
hardly come up for discussion at all, for it was felt that, while the
very first step along the road of "transmutation of species" (as it was
then called) had not been made, it was quite useless to speculate as to
how far it might be possible to travel in the same direction, or where
the road would ultimately lead to.
_The Problem before Darwin_.
It is clear, then, that what was understood by the "origin" or the
"transmutation" of species before Darwin's work appeared, was the
comparatively simple question whether the allied species of each genus
had or had not been derived from one another and, remotely, from some
common ancestor, by the ordinary method of reproduction and by means of
laws and conditions still in action and capable of being thoroughly
investigated. If any naturalist had been asked at that day whether,
supposing it to be clearly shown that all the different species of each
genus had been derived from some one ancestral species, and that a full
and complete explanation were to be given of how each minute difference
in form, colour, or structure might have originated, and how the
several peculiarities of habit and of geographical distribution might
have been brought about--whether, if this were done, the "origin of
species" would be discovered, the great mystery solved, he would
undoubtedly have replied in the affirmative. He would probably have
added that he never expected any such marvellous discovery to be made in
his lifetime. But so much as this assuredly Mr. Darwin has done, not
only in the opinion of his disciples and admirers, but by the admissions
of those who doubt the completeness of his explanations. For almost all
their objections and difficulties apply to those larger differences
which separate genera, families, and orders from each other, not to
those which separate one species from the species to which it is most
nearly allied, and from the remaining species of the same genus. They
adduce such difficulties as the first development of the eye, or of the
milk-producing glands of the mammalia; the wonderful instincts of bees
and of ants; the complex arrangements for the fertilisation of orchids,
and numerous other points of structure or habit, as not being
satisfactorily explained. But it is evident that these peculiarities had
their origin at a very remote period of the earth's history, and no
theory, however complete, can do more than afford a probable conjecture
as to how they were produced. Our ignorance of the state of the earth's
surface and of the conditions of life at those remote periods is very
great; thousands of animals and plants must have existed of which we
have no record; while we are usually without any information as to the
habits and general life-history even of those of which we possess some
fragmentary remains; so that the truest and most complete theory would
not enable us to solve _all_ the difficult problems which the whole
course of the development of life upon our globe presents to us.
What we may expect a true theory to do is to enable us to comprehend and
follow out in some detail those changes in the form, structure, and
relations of animals and plants which are effected in short periods of
time, geologically speaking, and which are now going on around us. We
may expect it to explain satisfactorily most of the lesser and
superficial differences which distinguish one species from another. We
may expect it to throw light on the mutual relations of the animals and
plants which live together in any one country, and to give some rational
account of the phenomena presented by their distribution in different
parts of the world. And, lastly, we may expect it to explain many
difficulties and to harmonise many incongruities in the excessively
complex affinities and relations of living things. All this the
Darwinian theory undoubtedly does. It shows us how, by means of some of
the most universal and ever-acting laws in nature, new species are
necessarily produced, while the old species become extinct; and it
enables us to understand how the continuous action of these laws during
the long periods with which geology makes us acquainted is calculated to
bring about those greater differences presented by the distinct genera,
families, and orders into which all living things are classified by
naturalists. The differences which these present are all of the same
_nature_ as those presented by the species of many large genera, but
much greater in _amount_; and they can all be explained by the action of
the same general laws and by the extinction of a larger or smaller
number of intermediate species. Whether the distinctions between the
higher groups termed Classes and Sub-kingdoms may be accounted for in
the same way is a much more difficult question. The differences which
separate the mammals, birds, reptiles, and fishes from each other,
though vast, yet seem of the same nature as those which distinguish a
mouse from an elephant or a swallow from a goose. But the vertebrate
animals, the mollusca, and the insects, are so radically distinct in
their whole organisation and in the very plan of their structure, that
objectors may not unreasonably doubt whether they can all have been
derived from a common ancestor by means of the very same laws as have
sufficed for the differentiation of the various species of birds or of
reptiles.
_The Change of Opinion effected by Darwin_.
The point I wish especially to urge is this. Before Darwin's work
appeared, the great majority of naturalists, and almost without
exception the whole literary and scientific world, held firmly to the
belief that _species_ were realities, and had not been derived from
other species by any process accessible to us; the different species of
crow and of violet they are now, and to have originated by some totally
unknown process so far removed from ordinary reproduction that it was
usually spoken of as "special creation." There was, then, no question of
the origin of families, orders, and classes, because the very first step
of all, the "origin of species," was believed to be an insoluble
problem. But now this is all changed. The whole scientific and literary
world, even the whole educated public, accepts, as a matter of common
knowledge, the origin of species from other allied species by the
ordinary process of natural birth. The idea of special creation or any
altogether exceptional mode of production is absolutely extinct! Yet
more: this is held also to apply to many higher groups as well as to the
species of a genus, and not even Mr. Darwin's severest critics venture
to suggest that the primeval bird, reptile, or fish must have been
"specially created." And this vast, this totally unprecedented change in
public opinion has been the result of the work of one man, and was
brought about in the short space of twenty years! This is the answer to
those who continue to maintain that the "origin of species" is not yet
discovered; that there are still doubts and difficulties; that there are
divergencies of structure so great that we cannot understand how they
had their beginning. We may admit all this, just as we may admit that
there are enormous difficulties in the way of a complete comprehension
of the origin and nature of all the parts of the solar system and of the
stellar universe. But we claim for Darwin that he is the Newton of
natural history, and that, just so surely as that the discovery and
demonstration by Newton of the law of gravitation established order in
place of chaos and laid a sure foundation for all future study of the
starry heavens, so surely has Darwin, by his discovery of the law of
natural selection and his demonstration of the great principle of the
preservation of useful variations in the struggle for life, not only
thrown a flood of light on the process of development of the whole
organic world, but also established a firm foundation for all future
study of nature.
In order to show the view Darwin took of his own work, and what it was
that he alone claimed to have done, the concluding passage of the
introduction to the _Origin of_ _Species_ should be carefully
considered. It is as follows: "Although much remains obscure, and will
long remain obscure, I can entertain no doubt, after the most deliberate
and dispassionate judgment of which I am capable, that the view which
most naturalists until recently entertained and which I formerly
entertained--namely, that each species has been independently
created--is erroneous. I am fully convinced that species are not
immutable; but that those belonging to what are called the same genera
are lineal descendants of some other and generally extinct species, in
the same manner as the acknowledged varieties of any one species are the
descendants of that species. Furthermore, I am convinced that Natural
Selection has been the most important, but not the exclusive, means of
modification."
It should be especially noted that all which is here claimed is now
almost universally admitted, while the criticisms of Darwin's works
refer almost exclusively to those numerous questions which, as he
himself says, "will long remain obscure."
_The Darwinian Theory_.
As it will be necessary, in the following chapters, to set forth a
considerable body of facts in almost every department of natural
history, in order to establish the fundamental propositions on which the
theory of natural selection rests, I propose to give a preliminary
statement of what the theory really is, in order that the reader may
better appreciate the necessity for discussing so many details, and may
thus feel a more enlightened interest in them. Many of the facts to be
adduced are so novel and so curious that they are sure to be appreciated
by every one who takes an interest in nature, but unless the need of
them is clearly seen it may be thought that time is being wasted on mere
curious details and strange facts which have little bearing on the
question at issue.
The theory of natural selection rests on two main classes of facts which
apply to all organised beings without exception, and which thus take
rank as fundamental principles or laws. The first is, the power of rapid
multiplication in a geometrical progression; the second, that the
offspring always vary slightly from the parents, though generally very
closely resembling them. From the first fact or law there follows,
necessarily, a constant struggle for existence; because, while the
offspring always exceed the parents in number, generally to an enormous
extent, yet the total number of living organisms in the world does not,
and cannot, increase year by year. Consequently every year, on the
average, as many die as are born, plants as well as animals; and the
majority die premature deaths. They kill each other in a thousand
different ways; they starve each other by some consuming the food that
others want; they are destroyed largely by the powers of nature--by cold
and heat, by rain and storm, by flood and fire. There is thus a
perpetual struggle among them which shall live and which shall die; and
this struggle is tremendously severe, because so few can possibly remain
alive--one in five, one in ten, often only one in a hundred or even one
in a thousand.
Then comes the question, Why do some live rather than others? If all the
individuals of each species were exactly alike in every respect, we
could only say it is a matter of chance. But they are not alike. We find
that they vary in many different ways. Some are stronger, some swifter,
some hardier in constitution, some more cunning. An obscure colour may
render concealment more easy for some, keener sight may enable others to
discover prey or escape from an enemy better than their fellows. Among
plants the smallest differences may be useful or the reverse. The
earliest and strongest shoots may escape the slug; their greater vigour
may enable them to flower and seed earlier in a wet autumn; plants best
armed with spines or hairs may escape being devoured; those whose
flowers are most conspicuous may be soonest fertilised by insects. We
cannot doubt that, on the whole, any beneficial variations will give the
possessors of it a greater probability of living through the tremendous
ordeal they have to undergo. There may be something left to chance, but
on the whole _the fittest will survive_.
Then we have another important fact to consider, the principle of
heredity or transmission of variations. If we grow plants from seed or
breed any kind of animals year after year, consuming or giving away all
the increase we do not wish to keep just as they come to hand, our
plants or animals will continue much the same; but if every year we
carefully save the best seed to sow and the finest or brightest
coloured animals to breed from, we shall soon find that an improvement
will take place, and that the average quality of our stock will be
raised. This is the way in which all our fine garden fruits and
vegetables and flowers have been produced, as well as all our splendid
breeds of domestic animals; and they have thus become in many cases so
different from the wild races from which they originally sprang as to be
hardly recognisable as the same. It is therefore proved that if any
particular kind of variation is preserved and bred from, the variation
itself goes on increasing in amount to an enormous extent; and the
bearing of this on the question of the origin of species is most
important. For if in each generation of a given animal or plant the
fittest survive to continue the breed, then whatever may be the special
peculiarity that causes "fitness" in the particular case, that
peculiarity will go on increasing and strengthening _so long as it is
useful to the species_. But the moment it has reached its maximum of
usefulness, and some other quality or modification would help in the
struggle, then the individuals which vary in the new direction will
survive; and thus a species may be gradually modified, first in one
direction, then in another, till it differs from the original parent
form as much as the greyhound differs from any wild dog or the
cauliflower from any wild plant. But animals or plants which thus differ
in a state of nature are always classed as distinct species, and thus we
see how, by the continuous survival of the fittest or the preservation
of favoured races in the struggle for life, new species may be
originated.
This self-acting process which, by means of a few easily demonstrated
groups of facts, brings about change in the organic world, and keeps
each species in harmony with the conditions of its existence, will
appear to some persons so clear and simple as to need no further
demonstration. But to the great majority of naturalists and men of
science endless difficulties and objections arise, owing to the
wonderful variety of animal and vegetable forms, and the intricate
relations of the different species and groups of species with each
other; and it was to answer as many of these objections as possible, and
to show that the more we know of nature the more we find it to
harmonise with the development hypothesis, that Darwin devoted the whole
of his life to collecting facts and making experiments, the record of a
portion of which he has given us in a series of twelve masterly volumes.
_Proposed Mode of Treatment of the Subject_.
It is evidently of the most vital importance to any theory that its
foundations should be absolutely secure. It is therefore necessary to
show, by a wide and comprehensive array of facts, that animals and
plants _do_ perpetually vary in the manner and to the amount requisite;
and that this takes place in wild animals as well as in those which are
domesticated. It is necessary also to prove that all organisms _do_ tend
to increase at the great rate alleged, and that this increase actually
occurs, under favourable conditions. We have to prove, further, that
variations of all kinds can be increased and accumulated by selection;
and that the struggle for existence to the extent here indicated
actually occurs in nature, and leads to the continued preservation of
favourable variations.
These matters will be discussed in the four succeeding chapters, though
in a somewhat different order--the struggle for existence and the power
of rapid multiplication, which is its cause, occupying the first place,
as comprising those facts which are the most fundamental and those which
can be perfectly explained without any reference to the less generally
understood facts of variation. These chapters will be followed by a
discussion of certain difficulties, and of the vexed question of
hybridity. Then will come a rather full account of the more important of
the complex relations of organisms to each other and to the earth
itself, which are either fully explained or greatly elucidated by the
theory. The concluding chapter will treat of the origin of man and his
relations to the lower animals.
FOOTNOTES:
[Footnote 1: _Geography and Classification of Animals_, p. 350.]
[Footnote 2: These expressions occur in Chapter IX. of the earlier
editions (to the ninth) of the _Principles of Geology_.]
[Footnote 3: L. Agassiz, _Lake Superior_, p. 377.]