This subject will be more fully discussed in our chapter on Geology; but it must here be alluded to from being intimately connected with natural selection. Natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure. Owing to the high geometrical rate of increase of all organic beings, each area is already fully stocked with inhabitants, and it follows from this, that as the favoured forms increase in number, so, generally, will the less favoured decrease and become rare. Rarity, as geology tells us, is the precursor to extinction. We can see that any form which is represented by few individuals will run a good chance of utter extinction, during great fluctuations in the nature or the seasons, or from a temporary increase in the number of its enemies. But we may go further than this; for as new forms are produced, unless we admit that specific forms can go on indefinitely increasing in number, many old forms must become extinct. That the number of specific forms has not indefinitely increased, geology plainly tells us; and we shall presently attempt to show why it is that the number of species throughout the world has not become immeasurably great.
       We have seen that the species which are most numerous in individuals have the best chance of producing favourable variations within any given period. We have evidence of this, in the facts stated in the second chapter, showing that it is the common and diffused or dominant species which offer the greatest number of recorded varieties. Hence, rare species will be less quickly modified or improved within any given period; they will consequently be beaten in the race for life by the modified and improved descendants of the commoner species.
       >From these several considerations I think it inevitably follows, that as new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct. The forms which stand in closest competition with those undergoing modification and improvement, will naturally suffer most. And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms,--varieties of the same species, and species of the same genus or related genera,--which, from having nearly the same structure, constitution and habits, generally come into the severest competition with each other. Consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them. We see the same process of extermination among our domesticated productions, through the selection of improved forms by man. Many curious instances could be given showing how quickly new breeds of cattle, sheep and other animals, and varieties of flowers, take the place of older and inferior kinds. In Yorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that these "were swept away by the short-horns" (I quote the words of an agricultural writer) "as if by some murderous pestilence."

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
   Breeds of the Domestic Pigeon, Their Differences and Origin
   Principles of Selection, anciently followed, their Effects
   Unconscious Selection
   Circumstances favourable 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.
   Summary
chapter iii. 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
chapter iv. natural selection; or the survival of the fittest
   Natural Selection
   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
chapter v. laws of variation
   Effects of changed conditions
   Effects of the increased use and disuse of parts, as controlled by Natural Selection
   Acclimatisation
   Correlated variation
   Compensation and economy of growth
   Multiple, rudimentary, and lowly organised structures are variable
   A part developed in any species in an extraordinary degree or manner, in comparison with the same part in allied species, tends to be highly variable
   Specific characters more variable than generic characters
   Secondary sexual characters variable
   Distinct species present analogous variations, so that a variety of one species often assumes a character proper to an allied species, or reverts to some of the characters of an early progenitor
   Summary
chapter vi. difficulties of the theory
   Difficulties of the theory of descent with modification
   Absence or rarity of transitional varieties
   On the origin and transition of organic beings with peculiar habits and structure
   Organs of extreme perfection and complication
   Modes of transition
   Special difficulties of the theory of Natural Selection
   Organs of little apparent importance, as affected by Natural Selection
   Utilitarian doctrine, how far true: Beauty, how acquired
   Summary
chapter vii
   Miscellaneous Objections to the Theory of Natural Selection
chapter viii. instinct
   Instincts comparable with habits, but different in their origin
   Inherited changes of habit or instinct in domesticated animals
   Special instincts; Instincts of the cuckoo
   Slave-making instinct
   Cell-making instinct of the hive-bee
   Objections to the theory of natural selection as applied to instincts: neuter and sterile insects
   Summary
chapter ix. hybridism
   Distinction between the sterility of first crosses and of hybrids
   Degrees of sterility
   Laws governing the sterility of first crosses and of hybrids
   Origin and causes of the sterility of first crosses and of hybrids
   Reciprocal dimorphism and trimorphism
   Fertility of varieties when crossed and of their mongrel offspring not universal
   Hybrids and mongrels compared independently of their fertility
   Summary of Chapter
chapter x. on the imperfection of the geological record
   On the absence of intermediate varieties at the present day
   On the lapse of time, as inferred from the rate of denudation and of deposition
   On the poorness of our palaeontological collections
   On the absence of numerous intermediate varieties in any single formation
   On the sudden appearance of whole groups of allied species
   On the sudden appearance of groups of allied species in the lowest known fossiliferous strata
chapter xi. on the geological succession of organic beings
   On the slow and successive appearance of new species
   On extinction
   On the forms of life changing almost simultaneously throughout the world
   On the affinities of extinct species to each other and to living species
   On the state of development of ancient compared with living forms
   On the succession of the same types within the same areas, during the later Tertiary Periods.
   Summary of preceding and present chapter
chapter xii. geographical distribution
   Present distribution cannot be accounted for by differences in physical conditions
   Single centres of supposed creation
   Means of dispersal
   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 terrestrial Mammals on oceanic islands
   On the relation of the inhabitants of islands to those of the nearest mainland
   Summary of the last and present chapter
chapter xiv. mutual affinities of organic beings: morphology -- embryology -- rudimentary organs
   Classification
   Analogical resemblances
   On the nature of the affinities connecting organic beings
   Morphology
   Development and embryology
   Rudimentary, atrophied, and aborted organs
   Summary
chapter xv
   Recapitulation and Conclusion
Glossary of Scientific Terms