Table of Contents 
- List of illustrations
- Chapter I - WHAT ARE "SPECIES" AND WHAT IS MEANT BY THEIR "ORIGIN"
- Chapter II - THE STRUGGLE FOR EXISTENCE
- Chapter III - THE VARIABILITY OF SPECIES IN A STATE OF NATURE
- Chapter IV - VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS
- Chapter V - NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST
- Chapter VI - DIFFICULTIES AND OBJECTIONS
- Chapter VII - ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL STERILITY OF THEIR HYBRID OFFSPRING
- Chapter VIII - THE ORIGIN AND USES OF COLOUR IN ANIMALS
- Chapter IX - WARNING COLOURATION AND MIMICRY
- Chapter X - COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX
- Chapter XI - THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND PURPOSE
- Chapter XII - THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS
- Chapter XIII - THE GEOLOGICAL EVIDENCES OF EVOLUTION
- Chapter XIV - FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY
- Chapter XV - DARWINISM APPLIED TO MAN
- View Copyright
AN EXPOSITION OF THE
THEORY OF NATURAL SELECTION
WITH SOME OF ITS APPLICATIONS
ALFRED RUSSEL WALLACE
LL.D., F.L.S., ETC.
WITH MAP AND ILLUSTRATIONS
MACMILLAN AND CO.
AND NEW YORK
All rights reserved
PRESSWORK BY JOHN WILSON AND SON,
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 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.
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 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
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
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
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
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—Delbœuf'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
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
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
WARNING COLORATION AND MIMICRY
The skunk as an example of warning coloration—Warning colours among insects—Butterflies—Caterpillars—Mimicry—How mimicry has been produced—Heliconidæ—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
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
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
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
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
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
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
|To face page 349|
Diagram of Variations of Lacerta muralis
Diag”am of Variation of Lizards
Diag”am of Variation of wings and tail of Birds
Diag”am of Variation of Dolichonyx oryzivorus
Diag”am of Variation of Agelæus phœniceus
Diag”am of Variation of Cardinalis virginianus
Diag”am of Variation of tarsus and toes
Diag”am of Variation of Birds in Leyden Museum
Diag”am of Variation of Icterus Baltimore
Diag”am of Variation of Agelæus phœniceus
Diag”am of Curves of Variation
Diag”am of Variation of Cardinalis virginianus
Diag”am of Variation of Sciurus carolinensis
Diag”am of Variation of skulls of Wolf
Diag”am of Variation of skulls of Ursus labiatus
Diag”am of Variation of skulls of Sus cristatus
Primula veris (Cowslip). From Darwin's Forms of Flowers
Gazella sœmmerringi (to show recognition marks)
Recognition marks of African Plovers (from Seebohm's Charadriadæ)
Recognition of Œdicnemus vermiculatus and Œ. senegalensis (from Seebohm's Charadriadæ)
Recognition of Cursorius chalcopterus and C. gallicus (from Seebohm's Charadriadæ)
Recognition of Scolopax megala and S. stenura (from Seebohm's Charadriadæ)
Methoa psidii and Leptalis orise
Opthalmis licea and Artaxa simulans (from the Official Narrative of the Voyage of the Challenger)
Wings of Ituna Ilione and Thyridia megisto (from Proceedings of the Entomological Society)
Mygnimia aviculus and Coloborhombus fasciatipennis
Mimicking Insects from the Philippines (from Semper's Animal Life)
Malva sylvestris and M. rotundifolia (from Lubbock's British Wild Flowers in Relation to Insects)
Lythrum salicaria, three forms of (from Lubbock's British Wild Flowers in Relation to Insects)
Orchis pyramidalis (from Darwin's Fertilisation of Orchids)
Humming-bird fertilising Marcgravia nepenthoides
Diagram of mean height of Land and depth of Oceans
Geological development of the Horse tribe (from Huxley's American Addresses)
Diagram illustrating the Geological Distribution of Plants (from Ward's Sketch of Palæobotany)
Transformation of Artemia salina to A. Milhausenii (from Semper's Animal Life)
Branchipus stagnalis and Artemia salina (from Semper's Animal Life)
Chimpanzee (Troglodytes niger)