n the view
of plumage having been chiefly modified by sexual selection, acting when
the birds have come to the breeding age or during the breeding season;
the modifications thus produced being inherited at corresponding ages or
seasons, either by the males alone, or by the males and females; but I
have not space here to enter on this subject.

Thus it is, as I believe, that when the males and females of any animal
have the same general habits of life, but differ in structure, colour,
or ornament, such differences have been mainly caused by sexual
selection; that is, individual males have had, in successive
generations, some slight advantage over other males, in their weapons,
means of defence, or charms; and have transmitted these advantages to
their male offspring. Yet, I would not wish to attribute all such
sexual differences to this agency: for we see peculiarities arising and
becoming attached to the male sex in our domestic animals (as the wattle
in male carriers, horn-like protuberances in the cocks of certain fowls,
etc.), which we cannot believe to be either useful to the males in
battle, or attractive to the females. We see analogous cases under
nature, for instance, the tuft of hair on the breast of the turkey-cock,
which can hardly be either useful or ornamental to this bird;--indeed,
had the tuft appeared under domestication, it would have been called a
monstrosity.

ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION.

In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some
by craft, some by strength, and some by fleetness; and let us suppose
that the fleetest prey, a deer for instance, had from any change in
the country increased in numbers, or that other prey had decreased in
numbers, during that season of the year when the wolf is hardest pressed
for food. I can under such circumstances see no reason to doubt that the
swiftest and slimmest wolves would have the best chance of surviving,
and so be preserved or selected,--provided always that they retained
strength to master their prey at this or at some other period of the
year, when they might be compelled to prey on other animals. I can see
no more reason to doubt this, than that man can improve the fleetness
of his greyhounds by careful and methodical selection, or by that
unconscious selection whi