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amount of difference between the sexes is also incom ^rably less than that which frequently occurs in thi last class ; so that the cause of the difference, whatever ii may have been, has acted upon the females in the present class either less energetically or less persistently than on the males in the last class. {Descent of Man, II. p. 198.)

Mammals.—Among the mammalia the sexes often differ in their weapons of offence and defence, as we see in the deer, when the horns are usually absent in the female; in their voices, as in the case with the cow and bull; in odor, as goats for example, and in the musk deer, where both the musk-producing organ and other organs of a similar character are confined to the male; in color, as in many antelopes, and in the character and distribution of the hair, as we may see by comparing the lion with the lioness, or the human male with the human female.

A little thought will show that among the mammals, as in other groups of the animal kingdom, the males are more modified than the females.

Thus man differs from woman by the possession of a beard, but the boy resembles the girl or the mature female, thus showing that the human race is influenced by the general law of which we have seen the evidence in so many groups of animals, and that the adult female is more like the young of both sexes than the adult male. So, too, the young stag, or the young male goat, resembles the adult female in the absence of horns.

The fact that different human races are characterized by the presence or absence of a beard in the males, and that the horns of different species of deer differ very greatly, shows that the males of allied species of mammals differ more than the females.

Among the mammalia we sometimes find that the male has been modified by the acquisition of new structures, while in other cases organs common to both sexes and to great groups have become changed in the male, but have remained comparatively unmodified in the fern-ale.

The spurs on the leg of the male Ornithorinchus may, perhaps, be regarded as a case of the first kind, as may also the horns of the rhinoceros, which are longer and more important in the male than they are in the female, while the great tusks of the boar are organs which must have been present in both sexes of the remote ancestors, although they have recently undergone great change in the male.

No one who will compare the hesid of the common boar with that of the male Babyrusa, the male wart-hog, and the male river hog, can doubt that the males of these allied species differ much more than the females.

In some cases certain teeth of the male are so greatly modified that they must be regarded as new organs. This is true of the narwhal, in which one of the teeth is greatly elongated, and forms a long, spirally-twisted spciir, nine or ten feet long, while the correspondirug tooth in the male, and both teeth in the female, are rudimentary.

The tusks of the male walrus, and those of the male elephant, are greatly modified teeth, but they differ so greatly from ordinary teeth that they are almost as truly new organs as the horns of ruminants.

It is interesting to note how greatly the various races of elephants differ in the development of the tusks. In Ceylon they arc never found in the females, and they occur in only about one per cent, of the males. In India they occur in all or nearly all the males, but in the males alone, while in Africa the female usually has small tusks.

The same thing is true of the horns of ruminants. In the hollow-horned species, as in cattle, they are not at all uncommon in the females, although they are usually much less important than they are in the males. Among the antelopes the females of some species have horns like the males; in other species they are somewhat smaller in the female than they are in the male; in others they are large in the male, but rudimentary in the female, while in others they are entirely absent in the female.

In female deer they are usually absent entirely, but in some they are rudimentary, and in the female reindeer they are fully developed. It is interesting to note that in females which normally lack them, they may be developed as the result of injury or disease of the reproductive organs, and that their development in the male may be arrested by castration.

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CHAPTER IX.

The Evidence From Secondary Sexual CharacTers CONTINUED. —THE CAUSE OF THE EXCESSIVE MODIFICATION OF MALE CHARACTERS.

The Explanation of Daines^Barrington and Wallace—Reasons for considering it inadequate—Darwin's explanation—History of domesticated races shows that this does not go to the root of the matter—The view that the male is more exposed than the female to the action of selection—A more fundamental explanation is needed—This is furnished by our theory of heredity—Special difficulties—Summary.

The sexual characteristics of animals have been made the subject of considerable discussion by various naturalists, and among birds especially there have been many attempts to explain why the female has not acquired the same ornaments as the male.

The Explanations of Daines^ Barrington and Wallace.

Wallace points out that conspicuous ornaments and brilliant plumage would render the female bird prominent while incubating, and would thus enable enemies I to detect the presence of the nest. He believes that "» 6inceincubating females are exposed to this danger, natural selection has acted, by the destruction of the most conspicuous females, to gradually produce races in which the females have nothing to render them conspicuous.

In 1773 the Hon. Daines Barrington called attention {Phil. Trails. 1773, p. 164) to the fact that singing birds arc all small, and he believes that thin arises from the difficulty larger birds would have in concealing themselves if they called the attention of their enemies by loud notes. He also says that he conceives it is for the same reason that no hen bird sings, because this' talent would be still more dangerous during incubation, and he suggests that the inferiority of the female bird in point of plumage may be due to the same cause.

This argument, that the dull color and lack of ornament in female birds is a dkect adaptation to their peculiar life, has been elaborated by Wallace. (On Natural Selection, p. 231.) He says that in the struggle for existence incessantly going on, protection or concealment is one of the most general and most effectual means of maintaining life, and it is by modifications of color that this protection can be most readily obtained, since no other character is subject to such numerous and rapid variations. He says that, as a general rule, the female butterfly is of dull and inconspicuous colors, even when the male is most gorgeously arrayed, and that in all these cases the difference can be traced to the greater need of protection for the female, on whose continued existence, while depositing her eggs, the safety of the race depends.

Since a male insect is, by its structure and habits, less exposed to danger, it does not need any special means of protection, as the female does, to balance the greater danger to which she is exposed, and Wallace believes that on account of this danger, and because of her greater importance to the existence of the species, tho female insect always acquires this protection in one waj or another through the action of natural selection.

He also says that "the female bird, while sitting 01 her eggs in an uncovered nest, is much exposed to tin, attacks of enemies, and any modification of color whict]

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