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 foundation for the work, helps us but little in understanding how species arise in nature.