ctly 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. Now let
us see how this principle of benefit being derived from divergence of
character, combined with the principles of natural selection and of
extinction, tends 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 as we saw in the second chapter, on
an average more species vary in large genera than in 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 most widely-diffused, vary more than do the rare and restricted
species. Let (A) be a common, widely-diffused, and varying species,
belonging to a genus large in its own country. The branching and
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 en