been profitable to them.

The advantage of diversification in the inhabitants of the same region
is, in fact, the same as that of the physiological division of labour in
the organs of the same individual body--a subject so well elucidated by
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, b