nt of life that
can be supported there. Hence the continued action of the struggle for
existence will tend to bring about more and more diversity in each area,
which may be shown to be the case by several kinds of evidence. As an
example, a piece of turf, three feet by four in size, was found by Mr.
Darwin to contain twenty species of plants, and these twenty species
belonged to eighteen genera and to eight orders, showing how greatly
they differed from each other. Farmers find that a greater quantity of
hay is obtained from ground sown with a variety of genera of grasses,
clover, etc., than from similar land sown with one or two species only;
and the same principle applies to rotation of crops, plants differing
very widely from each other giving the best results. So, in small and
uniform islands, and in small ponds of fresh water, the plants and
insects, though few in number, are found to be wonderfully varied in
character.

The same principle is seen in the naturalisation of plants and animals
by man's agency in distant lands, for the species that thrive best and
establish themselves permanently are not only very varied among
themselves but differ greatly from the native inhabitants. Thus, in the
Northern United States there are, according to Dr. Asa Gray, 260
naturalised flowering plants which belong to no less than 162 genera;
and of these, 100 genera are not natives of the United States. So, in
Australia, the rabbit, though totally unlike any native animal, has
increased so much that it probably outnumbers in individuals all the
native mammals of the country; and in New Zealand the rabbit and the pig
have equally multiplied. Darwin remarks that this "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 body. No physiologist doubts that a stomach adapted
to digest vegetable matter alone, or flesh alone, draws more 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."[39]

_The most closely allied Species inhabit distinct Areas._

One of the curious results of the general action of this principle in
nature is, that the most closely allied species--those whose differences
though often real and