ge genera the species are apt to
be closely but unequally allied together, forming little clusters round
other species. Species very closely allied to other species apparently
have restricted ranges. In all these respects the species of large
genera present a strong analogy with varieties. And we can clearly
understand these analogies, if species once existed as varieties, and
thus originated; whereas, these analogies are utterly inexplicable if
species are independent creations.

We have also seen that it is the most flourishing or dominant species
of the larger genera within each class which on an average yield the
greatest number of varieties, and varieties, as we shall hereafter see,
tend to become converted into new and distinct species. Thus the larger
genera tend to become larger; and throughout nature the forms of life
which are now dominant tend to become still more dominant by leaving
many modified and dominant descendants. But, by steps hereafter to be
explained, the larger genera also tend to break up into smaller genera.
And thus, the forms of life throughout the universe become divided into
groups subordinate to groups.

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.

Before entering on the subject of this chapter I must make a few
preliminary remarks to show how the struggle for existence bears on
natural selection. It has been seen in the last chapter that
among organic beings in a state of nature there is some individual
variability: indeed I am not aware that this has ever been disputed.
It is immaterial for us whether a multitude of doubtful forms be called
species or sub-species or varieties; what rank, for instance, the two or
three hundred doubtful forms of British plants are entitled to hold,
if the existence of any well-marked varieties be admitted. But the
mere existence of individual variability and of some few well-marked
varieties, though necessary as the fo