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 prey, either dead or alive; some inhabiting new stations,
climbing trees, frequenting water, and some perhaps becoming less
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