But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently, from the simple
circumstance that the more diversified the descendants from any one
species become in structure, constitution, and habits, by so much will
they be better enabled to seize on many and widely diversified places
in the polity of nature, and so be enabled to increase in numbers.
We can clearly see this in the case of animals with simple habits.
Take the case of a carnivorous quadruped, of which the number that can
be supported in any country has long ago arrived at its full average.
If its natural powers of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in its conditions)
only by its varying descendants seizing on places at present occupied
by other animals: some of them, for instance, being enabled to feed on
new kinds of prey, either dead or alive; some inhabiting new stations,
climbing trees, frequenting water, and some perhaps becoming less
carnivorous. The more diversified in habits and structure the
descendants of our carnivorous animal became, the more places they
would 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 do 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 thus be raised.
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