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 be
raised in the latter than in the former case. The same has been found
to hold good when one variety and 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 the varieties were continually selected which
differed from each other in the same manner, though in a very slight
degree, as do the distinct species and genera of grasses, a greater
number of individual plants of this species, including its modified
descendants, would succeed in living on the same piece of ground. And
we know that each species and each variety of grass is annually sowing
almost countless seeds; and is thus striving, as it may be said, to
the utmost to increase in number. Consequently, in the course of many
thousand generations, the most distinct varieties of any one species
of grass would 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 very 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: also in small ponds of fresh water. Farmers find
that they can raise more 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 its nature not to be
in any way peculiar), and may be said to be striving to the utmost
to live there; but, it is seen, that where they com