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.


CHAPTER III.

STRUGGLE FOR EXISTENCE.

Its bearing on natural selection -- The term used in a wide sense --
Geometrical ratio of increase -- Rapid increase of naturalised animals and
plants -- Nature of the checks to increase -- Competition universal --
Effects of climate -- Protection from the number of individuals -- Complex
relations of all animals and plants throughout nature -- Struggle for life
most severe between individuals and varieties of the same species; often
severe between species of the same genus -- The relation of organism to
organism the most important of all relations.


CHAPTER 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