rter beaks. Again, we may suppose that at
an early period of history, the men of one nation or district required
swifter horses, while those of another required stronger and bulkier
horses. The early differences would be very slight; but, in the course
of time, from the continued selection of swifter horses in the one case,
and of stronger ones in the other, the differences would become greater,
and would be noted as forming two sub-breeds. Ultimately after the
lapse of centuries, these sub-breeds would become converted into two
well-established and distinct breeds. As the differences became greater,
the inferior animals with intermediate characters, being neither very
swift nor very strong, would not have been used for breeding, and will
thus have tended to disappear. Here, then, we see in man's productions
the action of what may be called the principle of divergence, causing
differences, at first barely appreciable, steadily to increase, and
the breeds to diverge in character, both from each other and from their
common parent.

But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently (though it was a long
time before I saw how), 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 discern 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 power of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in 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 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.