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• Additions and Corrections to the Sixth Edition.
• An Historical Sketch of the Progress of Opinion On the Origin of Species.: Previously to the Publication of the First Edition of This Work.
• Origin of Species.
• Introduction.
• Chapter I.: Variation Under Domestication.
• Causes of Variability.
• Effects of Habit and of 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, and Their Effects.
• Unconscious Selection.
• Circumstances Favourable to Man’s Power of Selection.
• Chapter II.: Variation Under Nature.
• 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 Included Within 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.
• 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.
• 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 of Chapter.
• Chapter V.: Laws of Variation.
• 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.
• Chapter VI.: Difficulties of the Theory.
• 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: 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.
• Chapter VIII.: Instinct.
• Inherited Changes of Habit Or Instinct In Domesticated Animals.
• Special Instincts.
• Objections to the Theory of Natural Selection As Applied to Instincts: Neuter and Sterile Insects.
• Summary.

Multiple, Rudimentary, and Lowly-organised Structures are Variable.

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 vertebræ in snakes, and the stamens in polyandrous flowers) the number is variable; whereas 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 Prof. 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 it 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.