But we have already seen how it entails extinction; and how largely
extinction has acted in the world's history, geology plainly declares.
Natural selection, also, leads to divergence of character; for the more
organic beings diverge in structure, habits and constitution, by so much
the more can a large number be supported on the area, of which we see proof
by looking to the inhabitants of any small spot, and to the productions
naturalised in foreign lands. Therefore, during the modification of the
descendants of any one species, and during the incessant struggle of all
species to increase in numbers, the more diversified the descendants
become, the better will be their chance of success in the battle for life.
Thus the small differences distinguishing varieties of the same species,
steadily tend to increase, till they equal the greater differences between
species of the same genus, or even of distinct genera.
We have seen that it is the common, the widely diffused, and widely ranging
species, belonging to the larger genera within each class, which vary most;
and these tend to transmit to their modified offspring that superiority
which now makes them dominant in their own countries. Natural selection,
as has just been remarked, leads to divergence of character and to much
extinction of the less improved and intermediate forms of life. On these
principles, the nature of the affinities, and the generally well defined
distinctions between the innumerable organic beings in each class
throughout the world, may be explained.
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