| INTRODUCTION | | 1 | |
| CHAPTER I. VARIATION UNDER DOMESTICATION. | | 5 | |
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| Character of Domestic Varieties |
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| Difficulty of distinguishing between Varieties and Species |
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| Origin of Domestic Varieties from one or more Species |
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| Domestic Pigeons, their Differences and Origin |
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| Principle of Selection anciently followed, its Effects |
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| Methodical and Unconscious Selection |
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| Unknown Origin of our Domestic Productions |
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| Circumstances favourable to Man's power of Selection |
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| CHAPTER II. VARIATION UNDER NATURE. | | 29 | |
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| Wide ranging, much diffused, and common species vary most |
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| Species of the larger genera in any country vary more than the species of the smaller genera |
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| Many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges |
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| CHAPTER III. STRUGGLE FOR EXISTENCE. | | 39 | |
| Bears on natural selection |
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| The term used in a wide sense |
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| Geometrical powers of increase |
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| Rapid increase of naturalised animals and plants |
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| Nature of the checks to increase |
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| Protection from the number of individuals |
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| Complex relations of all animals and plants throughout nature |
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| Struggle for life most severe between individuals and varieties of the same species; often severe between species of the same genus |
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| The relation of organism to organism the most important of all relations |
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| CHAPTER IV. NATURAL SELECTION. | | 51 | |
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| its power compared with man's selection |
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| its power on characters of trifling importance |
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| its power at all ages and on both sexes |
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| On the generality of intercrosses between individuals of the same species |
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| Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing, isolation, number of individuals |
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| Extinction caused by Natural Selection |
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| Divergence of Character, related to the diversity of inhabitants of any small area, and to naturalisation |
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| Action of Natural Selection, through Divergence of Character and Extinction, on the descendants from a common parent |
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| Explains the Grouping of all organic beings |
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| CHAPTER V. LAWS OF VARIATION. | | 83 | |
| Effects of external conditions |
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| Use and disuse, combined with natural selection; organs of flight and of vision |
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| Compensation and economy of growth |
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| Multiple, rudimentary, and lowly organised structures variable |
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| Parts developed in an unusual manner are highly variable: specific characters more variable than generic: secondary sexual characters variable |
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| Species of the same genus vary in an analogous manner |
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| Reversions to long-lost characters |
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| CHAPTER VI. DIFFICULTIES ON THEORY. | | 108 | |
| Difficulties on the theory of descent with modification |
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| Absence or rarity of transitional varieties |
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| Transitions in habits of life |
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| Diversified habits in the same species |
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| Species with habits widely different from those of their allies |
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| Organs of extreme perfection |
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| Organs of small importance |
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| Organs not in all cases absolutely perfect |
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| The law of Unity of Type and of the Conditions of Existence embraced by the theory of Natural Selection |
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| CHAPTER VII. INSTINCT. | | 131 | |
| Instincts comparable with habits, but different in their origin |
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| Domestic instincts, their origin |
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| Natural instincts of the cuckoo, ostrich, and parasitic bees |
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| Hive-bee, its cell-making instinct |
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| Difficulties on the theory of the Natural Selection of instincts |
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| Neuter or sterile insects |
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| CHAPTER VIII. HYBRIDISM. | | 155 | |
| Distinction between the sterility of first crosses and of hybrids |
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| Sterility various in degree, not universal, affected by close interbreeding, removed by domestication |
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| Laws governing the sterility of hybrids |
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| Sterility not a special endowment, but incidental on other differences |
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| Causes of the sterility of first crosses and of hybrids |
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| Parallelism between the effects of changed conditions of life and crossing |
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| Fertility of varieties when crossed and of their mongrel offspring not universal |
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| Hybrids and mongrels compared independently of their fertility |
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| CHAPTER IX. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. | | 176 | |
| On the absence of intermediate varieties at the present day |
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| On the nature of extinct intermediate varieties; on their number |
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| On the vast lapse of time, as inferred from the rate of deposition and of denudation |
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| On the poorness of our palaeontological collections |
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| On the intermittence of geological formations |
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| On the absence of intermediate varieties in any one formation |
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| On the sudden appearance of groups of species |
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| On their sudden appearance in the lowest known fossiliferous strata |
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| CHAPTER X. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. | | 196 | |
| On the slow and successive appearance of new species |
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| On their different rates of change |
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| Species once lost do not reappear |
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| Groups of species follow the same general rules in their appearance and disappearance as do single species |
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| On simultaneous changes in the forms of life throughout the world |
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| On the affinities of extinct species to each other and to living species |
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| On the state of development of ancient forms |
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| On the succession of the same types within the same areas |
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| Summary of preceding and present chapters |
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| CHAPTER XI. GEOGRAPHICAL DISTRIBUTION. | | 217 | |
| Present distribution cannot be accounted for by differences in physical conditions |
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| Affinity of the productions of the same continent |
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| Means of dispersal, by changes of climate and of the level of the land, and by occasional means |
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| Dispersal during the Glacial period co-extensive with the world |
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| CHAPTER XII. GEOGRAPHICAL DISTRIBUTION | | 240 | |
| Distribution of fresh-water productions |
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| On the inhabitants of oceanic islands |
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| Absence of Batrachians and of terrestrial Mammals |
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| On the relation of the inhabitants of islands to those of the nearest mainland |
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| On colonisation from the nearest source with subsequent modification |
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| Summary of the last and present chapters |
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| CHAPTER XIII. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS. | | 258 | |
| CLASSIFICATION, groups subordinate to groups |
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| Rules and difficulties in classification, explained on the theory of descent with modification |
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| Classification of varieties |
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| Descent always used in classification |
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| Analogical or adaptive characters |
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| Affinities, general, complex and radiating |
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| Extinction separates and defines groups |
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| MORPHOLOGY, between members of the same class, between parts of the same individual |
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| EMBRYOLOGY, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age |
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| RUDIMENTARY ORGANS; their origin explained |
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| CHAPTER XIV. RECAPITULATION AND CONCLUSION. | | 288 | |
| Recapitulation of the difficulties on the theory of Natural Selection |
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| Recapitulation of the general and special circumstances in its favour |
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| Causes of the general belief in the immutability of species |
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| How far the theory of natural selection may be extended |
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| Effects of its adoption on the study of Natural history |
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| INDEX | | 307 | |
