It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both with varieties and species, that when any part or organ is repeated many times in the same individual (as the vertebrae in snakes, and the stamens in polyandrous flowers) the number is variable; whereas the number of the same part or organ, when it occurs in lesser numbers, is constant. The same author as well as some botanists, have further remarked that multiple parts are extremely liable to vary in structure. As "vegetative repetition," to use Professor Owen's expression, is a sign of low organisation; the foregoing statements accord with the common opinion of naturalists, that beings which stand low in the scale of nature are more variable than those which are higher. I presume that lowness here means that the several parts of the organisation have been but little specialised for particular functions; and as long as the same part has to perform diversified work, we can perhaps see why it should remain variable, that is, why natural selection should not have preserved or rejected each little deviation of form so carefully as when the part has to serve for some one special purpose. In the same way that a knife which has to cut all sorts of things may be of almost any shape; whilst a tool for some particular purpose must be of some particular shape. Natural selection, it should never be forgotten, can act solely through and for the advantage of each being.
       Rudimentary parts, as is generally admitted, are apt to be highly variable. We shall have to recur to this subject; and I will here only add that their variability seems to result from their uselessness, and consequently from natural selection having had no power to check deviations in their structure.

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