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, by individual males having had, in successive generations, some
slight advantage over other males, in their weapons, means of defence, or
charms; which they have transmitted to their male offspring alone. Yet, I
would not wish to attribute all sexual differences to this agency: for we
see in our domestic animals peculiarities arising and becoming attached to
the male sex, which apparently have not been augmented through selection by
man. The tuft of hair on the breast of the wild turkey-cock cannot be of
any use, and it is doubtful whether it can be ornamental in the eyes of the
female bird; indeed, had the tuft appeared under domestication it would
have been called a monstrosity.
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THE
FITTEST.
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 was hardest pressed for food.
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