same
species and species of the same genus.

As has always been my practice, let us seek light on this head from our
domestic productions. We shall here find something analogous. A fancier
is struck by a pigeon having a slightly shorter beak; another fancier is
struck by a pigeon having a rather longer beak; and on the acknowledged
principle that "fanciers do not and will not admire a medium standard,
but like extremes," they both go on (as has actually 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.

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 pre