In large 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