ctually occurred with
tumbler-pigeons) choosing and breeding from birds with longer and longer
beaks, or with shorter and shorter beaks. Again, we may suppose that at an
early period one man preferred swifter horses; another stronger and more
bulky horses. The early differences would be very slight; in the course of
time, from the continued selection of swifter horses by some breeders, and
of stronger ones by others, the differences would become greater, and would
be noted as forming two sub-breeds; finally, after the lapse of centuries,
the sub-breeds would become converted into two well-established and
distinct breeds. As the differences slowly become greater, the inferior
animals with intermediate characters, being neither very swift nor very
strong, will have been neglected, and will 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, 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. {113}

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.