of colour in the male, but rarely a decided difference. The female of the great red kangaroo, however, is a delicate gray; while in the Lemur macaco of Madagascar the male is jet-black and the female brown. In many monkeys also there 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
