ere are some differences of colour, especially on the face. The
sexual weapons and ornaments of male mammalia, as horns, crests, manes,
and dewlaps, are well known, and are very numerous and remarkable.
Having thus briefly reviewed the facts, we will now consider the
theories to which they have given rise.

_Sexual Selection by the Struggles of Males._

Among the higher animals it is a very general fact that the males fight
together for the possession of the females. This leads, in polygamous
animals especially, to the stronger or better armed males becoming the
parents of the next generation, which inherits the peculiarities of the
parents; and thus vigour and offensive weapons are continually increased
in the males, resulting in the strength and horns of the bull, the tusks
of the boar, the antlers of the stag, and the spurs and fighting
instinct of the gamecock. But almost all male animals fight together,
though not specially armed; even hares, moles, squirrels, and beavers
fight to the death, and are often found to be scarred and wounded. The
same rule applies to almost all male birds; and these battles have been
observed in such different groups as humming-birds, finches,
goatsuckers, woodpeckers, ducks, and waders. Among reptiles, battles of
the males are known to occur in the cases of crocodiles, lizards, and
tortoises; among fishes, in those of salmon and sticklebats. Even among
insects the same law prevails; and male spiders, beetles of many groups,
crickets, and butterflies often fight together.

From this very general phenomenon there necessarily results a form of
natural selection which increases the vigour and fighting power of the
male animal, since, in every case, the weaker are either killed,
wounded, or driven away. This selection would be more powerful if males
were always in excess of females, but after much research Mr. Darwin
could not obtain any satisfactory evidence that this was the case. The
same effect, however, is produced in some cases by constitution or
habits; thus male insects usually emerge first from the pupa, and among
migrating birds the males arrive first both in this country and in North
America. The struggle is thus intensified, and the most vigorous males
are the first to have offspring. This in all probability is a great
advantage, as the early breeders have the start in securing food, and
the young are strong enough to protect themselves while the later broods
are being produ