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 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.


CHAPTER 5. LAWS OF VARIATION.

Effects of external conditions.
Use and disuse, combined with natural selection; organs of flight and
of vision.
Acclimatisation.
Correlation of growth.
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.