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Capítol 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 favourable and unfavourable 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 naturalisation — 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 organisation — Low forms preserved — Convergence of character — Indefinite multiplication of species — Summary | 62-105 |
Capítol V. Laws of Variation. | |
| Effects of changed conditions — Use and disuse, combined with natural selection; organs of flight and of vision — Acclimatisation — Correlated variation — Compensation and economy of growth — False correlations — Multiple, rudimentary, and lowly organised 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 | 106-132 |
Capítol 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 fecit 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 df Natural Selection | 133-167 |
