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.
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