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

How have all those exquisite adaptations of one part of the organisation to another part, and to the conditions of life and of one organic being to another being, been perfected?
We see these beautiful co-adaptations most plainly in the woodpecker and the mistletoe; and only a little less plainly in the humblest parasite which clings to the hairs of a quadruped or feathers of a bird; in the structure of the beetle which dives through the water; in the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful adaptations everywhere and in every part of the organic world.
Again, it may be asked, how is it that varieties, which I have called incipient species, become ultimately converted into good and distinct species, which in most cases obviously differ from each other far more than do the varieties of the same species?
How do those groups of species, which constitute what are called distinct genera and which differ from each other more than do the species of the same genus, arise?
All these results, as we shall more fully see in the next chapter, follow from the struggle for life.