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.

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.