The more diversified in
habits and structure the descendants of our carnivorous animals become, the
more places they will be enabled to occupy. What applies to one animal
will apply throughout all time to all animals--that is, if they vary--for
otherwise natural selection can effect nothing. So it will be with plants.
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.
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