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 female.

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 head 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 spear, nine or ten feet long, while the corresponding 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 are 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.

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 to detect the presence of the nest. He believes that sinceincubating 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. Trans. 1773, p. 164) to the fact that singing birds are all small, and he believes that this 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 orna ment in female birds is a direct 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, the female insect always acquires this protection in one way or another through the action of natural selection.

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

rendered her more conspicuous would often lead to her destruction, and that of her offspring. All variations in this direction in the female would therefore, sooner or later, be eliminated, while such modifications as rendered her inconspicuous by assimilating her to surrounding objects, as the earth or the foliage, would, on the whole, survive the longest, and thus lead to the attainment of those brown or green and inconspicuous tints which form the coloring (of the upper surface at least) of the vast majority of female birds which sit upon open nests." As a proof that this is the true explanation of the dull plumage and lack of ornaments in so many female birds, he states that wherever the nest is domed or covered, or so placed as to conceal the sitting bird, the plumage is strikingly gay and conspicuously colored in both sexes; but that in those species where there is a strong contrast in colors, and the male is gay and conspicuous, while the female is dull and obscure, the nest is open, and the sitting bird is exposed to view.

Reasons for Holding that this Explanation is In

adequate.

The argument of Wallace, which is fully stated in the essay above quoted, is briefly, that the dull plumage of so many female birds, as contrasted with the gay colors of the males, has been directly acquired in the females by the destruction of the most conspicuous ones, and the natural selection of the inconspicuous varieties.

Darwin has discussed it at length in his essay on sexual selection, and has given many reasons for refusing to give it unqualified acceptance, but I will give here a few additional reasons for believing that the phenomena in question depend upon some more fundamental law. In the first place, we must bear in mind that,