Milne Edwards. No physiologist doubts that a stomach by being adapted
to digest vegetable matter alone, or flesh alone, draws most nutriment
from these substances. So in the general economy of any land, the more
widely and perfectly the animals and plants are diversified for
different habits of life, so will a greater number of individuals be
capable of there supporting themselves. 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. We have, also, seen that the species,
which are the commonest and the most widely-diffused, vary more than
rare species with restricted ranges. Let (A) be a common,
widely-diffused, and varying species, belonging to a genus large in its
own country. The little fan of diverging dotted lines of unequal
lengths proceeding from (A), may represent its varying offspring. The
variations are supposed to be extremely slight, but of the most
diversified nature; they are not supposed all to appear simultaneously,
but often after long intervals of time; nor are they all supposed to
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