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