INTRODUCTION

CHAPTER I - VARIATION UNDER DOMESTICATION.

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

CHAPTER II - VARIATION UNDER NATURE.

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

CHAPTER III - STRUGGLE FOR EXISTENCE.

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

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

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

CHAPTER V - LAWS OF VARIATION.

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

CHAPTER VI - DIFFICULTIES OF THE THEORY.

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

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

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

CHAPTER VIII - INSTINCT.

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

CHAPTER IX - HYBRIDISM.

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

CHAPTER X - ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

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

CHAPTER XI - ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.

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

CHAPTER XII - GEOGRAPHICAL DISTRIBUTION.

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

CHAPTER XIII - GEOGRAPHICAL DISTRIBUTION -- CONTINUED.

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

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

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

CHAPTER XV - RECAPITULATION AND CONCLUSION.

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

GLOSSARY OF SCIENTIFIC TERMS.