It
has been experimentally proved, that if a plot of ground be sown with one
species of grass, and a similar plot be sown with several distinct genera
of grasses, a greater number of plants and a greater weight of dry herbage
can thus be raised. The same has been found to hold good when first one
variety and then several mixed varieties of wheat have been sown on equal
spaces of ground. Hence, if any one species of grass were to go on varying,
and those varieties were continually selected which differed from each
other in at all the same manner as distinct species and genera of grasses
differ from each other, a greater number of individual plants of this
species of grass, including its modified descendants, would succeed in
living on the same piece of ground. And we well know that each species and
each variety of grass is annually sowing almost countless seeds; and thus,
as it may be said, is striving its utmost to increase its numbers. {114}
Consequently, I cannot doubt that in the course of many thousands of
generations, the most distinct varieties of any one species of grass would
always have the best chance of succeeding and of increasing in numbers, and
thus of supplanting the less distinct varieties; and varieties, when
rendered very distinct from each other, take the rank of species.

The truth of the principle, that the greatest amount of life can be
supported by great diversification of structure, is seen under many natural
circumstances. In an extremely small area, especially if freely open to
immigration, and where the contest between individual and individual must
be severe, we always find great diversity in its inhabitants. For instance,
I found that a piece of turf, three feet by four in size, which had been
exposed for many years to exactly the same conditions, supported twenty
species of plants, and these belonged to eighteen genera and to eight
orders, which shows how much these plants differed from each other. So it
is with the plants and insects on small and uniform islets; and so in small
ponds of fresh water. Farmers find that they can raise most food by a
rotation of plants belonging to the most different orders: nature follows
what may be called a simultaneous rotation. Most of the animals and plants
which live close round any small piece of ground, could live on it
(supposing it not to be in any way peculiar in its nature), and may be said
to be striving to the utmost to live there; but, it is see