A set of
animals, with their organisation but little diversified, could hardly
compete with a set more perfectly diversified in structure. It may be
doubted, for instance, whether the Australian marsupials, which are
divided into groups differing but little from each other, and feebly
representing, as Mr. Waterhouse and others have remarked, our
carnivorous, ruminant, and rodent mammals, could successfully compete
with these well-pronounced orders. In the Australian mammals, we see
the process of diversification in an early and incomplete stage of
development. After the foregoing discussion, which ought to have been
much amplified, we may, I think, assume that the modified descendants
of any one species will succeed by so much the better as they become
more diversified in structure, and are thus enabled to encroach on
places occupied by other beings. Now let us see how this principle of
great benefit being derived from divergence of character, combined
with the principles of natural selection and of extinction, will tend
to act.
The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large
in its own country; these species are supposed to resemble each other
in unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal
distances. I have said a large genus, because we have seen in the
second chapter, that on an average more of the species of large genera
vary than of small genera; and the varying species of the large genera
present a greater number of varieties.
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