The Origin of Species: By Means of Natural Selection of the Preservation of Favoured Races in the Struggle for Life. –
-- Summary by Susan Fleck

Summary of Chapter 5 – Laws of Variation: Our ignorance of the laws of variation is profound . . . we cannot assign to any reason why this or that part has varied. But when we can do comparisons, the same laws appear to have acted in producing the lesser differences between varieties of the same species, and the greater differences between species of the same genus.

1. Changed conditions generally induce mere fluctuating variability, but sometimes they cause direct and definite effects; and these may become strongly marked in the course of time

2. Habit in producing constitutional peculiarities and use in strengthening and disuse in weakening and diminishing organs, appear in many cases to have been potent in their effects

3. Homologous parts tend to vary in the same manner, and homologous parts tend to cohere.

4. Modifications in hard parts and in external parts sometimes affect softer and internal parts.

5. Changes of structure at an early age may affect parts subsequently developed; and many cases of correlated variation undoubtedly occur

6. Multiple parts are variable in number and in structure, perhaps arising from such parts not having been closely specialized for any particular function, so that their modifications have not been closely checked by natural selection.

7. Rudimentary organs, from being useless, are not regulated by natural selection, and hence are variable.

8. Specific characters—that is, the characters which have come to differ since the several species of the same genus has branched off from a common parent—are more variable than generic characters, or those which have long been inherited, and have not differed from this same period.

9. Secondary sexual characters are highly variable, and such characters differ much in the species or the same group. Variability in the same parts of the organization has generally been taken advantage of in giving secondary sexual differences to the several species of the same genus.

10. Any part or organ developed to an extraordinary size or in an extraordinary manner, in comparison with the same part in the allied species, must have gone through an extraordinary amount of modification since the genus arose; and thus we can understand why it should often still be variable in a much higher degree than other parts; for variation is a long-continued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state.

11. But when a species with any extraordinarily-developed organ has become the parent of many modified descendants—which on our view must be a very slow process, requiring a long lapse of time—in this case, natural selection has succeeded in giving a fixed character to the organ, in however extraordinary a manner it may have been developed.

12. Species inheriting nearly the same constitution from a common parent, and exposed to similar influences, naturally tend to present analogous variations, or these same species may occasionally revert to some of the characters of their ancient progenitors.

13. Although new and important modifications may not arise from reversion and analogous variation, such modifications will add to the beautiful and harmonious diversity of nature.

14. Whatever the cause may be of each slight difference between the offspring and their parents—and a cause for each must exist—it is the steady accumulation of beneficial differences which has given rise to all the more important modifications of structure in relation to the habits of each species.