The advantage of diversification of structure 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-developed orders. In the Australian
mammals, we see the process of diversification in an early and incomplete
stage of development.
THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH DIVERGENCE
OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF A COMMON ANCESTOR.
After the foregoing discussion, which has been much compressed, we may
assume that the modified descendants of any one species will succeed so
much the better as they become more diversified in structure, and are thus
enabled to encroach on places occupied by other beings.
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