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CHAPTER III.

STRUGGLE FOR EXISTENCE.

Bears on natural selection--The term used in a wide sense--Geometrical
powers 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 amongst
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
distinct organic being to another being, been perfected? We see these
beautiful co-adaptations most {61} plainly in the woodpecker and missletoe;
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 the