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 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
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