enera 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 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.

CHAPTER 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.

CHAPTER 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