cies has
been independently created.

We have, also, seen that it is the most flourishing and dominant species
of the larger genera which on an average vary most; and varieties, as
we shall hereafter see, tend to become converted into new and distinct
species. The larger genera thus tend to become larger; and throughout
nature the forms of life which are now dominant tend to become still
more dominant by leaving many modified and dominant descendants. 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.

3. 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 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