more fundamental and general than the great exposure of the male, through the intensity of the sexual passion, to the influence of selection; for the parental instinct is fully as important for the welfare of the race as the sexual instinct, and the former is, as a rule, most developed in the female, just as the latter is greatest in the male, and it might be expected to lead to the selection and modification of females, as the latter passion does to the modification of males.

The Theory of Heredity Furnishes the Only Adequate Explanation.

We must acknowledge that the great body of facts detailed in the beginning of this chapter have no adequate explanation, except on the hypothesis that a part which is present, or functional, or most important in the male alone, is very much more likely than a part which is limited to females in the same way, to give rise to hereditary variations. The facts receive a ready explanation on the hypothesis that there is an especial adaptation for the transmission to the egg of gemmules thrown off by the cells of the male body, while their transmission in the female is not thus provided for, but is due to accident. According to this view we must, in animals where the sexes have long been separated, look to the cells of the male body for the origin of a large proportion of the variations which have gradually been accumulated in the past to give species their present character; and we must regard secondary sexual characters as differing from ordinary specific characteristics, simply in being especially useful to one sex, usually the male, or in being disadvantageous to the other sex, so that natural selection has developed them to a greater degree in one sex. than in the other.

It will be seen that the evidence from this source is, as far as it goes, very similar to the evidence from hybrids. A reciprocal cross between two species furnishes a means of analyzing the influence of the two sexes, and of distinguishing, to some slight degree, the effect of each sexual element in heredity. The study of sexual character gives us another means of doing the same thing on a more limited scale.

As each cell of the body may throw off gemmules, there is no way of showing that a variation in a part which is alike in both sexes, is due to the transmission of gemmules from the cells of one parent rather than from those of the other, but the case is different with a part which is more developed in one sex than it is in the other. In this case we should, according to our theory of heredity, expect it to throw off gemmules most frequently in the sex in which it. is of most functional importance, and as we suppose that there is an especial arrangement for the transmission to the egg of those gemmules which originate in the male body, we can see that an organ which is most important in the body of the male is much more likely to give rise to hereditary modification than one which is most important, and therefore most prolific of gemmules, in the female body.

The history of secondary sexual characters is, therefore, what our theory of heredity would lead us to expect, and no other explanation which has ever been. proposed fully accounts for all the phenomena.

Instances of Female Modification.

We should not expect, however, to find secondary sexual characters exclusively confined to males, but simply more general than they are in females, and as a matter of fact we do meet with many cases where the female has been more modified than the male.

I will now give a few of those which seem to me to be most opposed to my general conclusion.

Female Modification.

In certain species of the amphipod crustacean, genus Melita, the females differ from all other amphipods by having the sexual lamellæ of the penultimate pair of feet produced into hook-like processes, of which the males lay hold with the hands of the first pair. In another amphipod, Boachyscelus, the male possesses, like all other amphipods, a pair of posterior antennæ, but they are absent in the female, so that the latter differs more than the male from allied forms. Darwin states that the females of certain water-beetles, as Dytiscus Acilius and Hydroporus, have their wing-covers grooved or thickly set with hairs or punctured, in order to enable the male to cling to the slippery surface of their hard and polished bodies.

The call duck is a domesticated breed which receives its name from its extraordinary and exceptional loquacity, and as this loquacity is confined to the female, while the male hisses like other ducks, we must regard this as a case of female modification. We know from the statements of Blumenbach and Bechstein that, previously to the year 1813, the great bony protuberances on the skull which characterize the Polish breed of fowls, were confined to the females, although they are now equally developed in both sexes. There can be no doubt that this peculiarity originated in the females, and was subsequently inherited by the males.

Among the Phasmidæ or spectre insects the females. alone, in some species, show a most striking resem-. blance to leaves, while the males show only a rude approximation, and Darwin has pointed out that, as we can

hardly believe that such a resemblance is disadvantageous to the males, we must conclude that the females alone have varied, and that these variations have been preserved and augmented by natural selection for the sake of protection, and have been transmitted to the female offspring alone.

In two species of Birds of Paradise, Paradisia apoda and Paradisia Papuana, the females differ from each other more than do their respective males; the female of the latter species having the under surface pure white, while the female of P. apoda is deep brown beneath.

The males of two species of shrikes (Oxynotus) in the islands of Mauritius and Bourbon, differ but little in color, while the females differ much, so that the female of the Bourbon species might at first sight be mistaken for the young of the Mauritius species. In this case there seems to be every reason for believing that the female of the Mauritius species has varied, while the male has remained unmodified.

Semper states (Animal Life) on the authority of Dr. Hagen that the females of many species of cave-beetles are blind, while the males have perfect eyes. As we may feel confident that these beetles are descended from ordinary forms, we must regard this as an instance of female modification.

The remarkable shell which is secreted by the large fan-like arms of the paper nautilus (Argonauta) occurs in the females alone, and it probably owes its origin to female modification, although it it not impossible that our recent species may be descended from a form in which the male had a shell.

The most remarkable cases of female modification are those which are presented by polymorphic insects.

Papilio turnus is one of our common yellow butter

flies, and it is found over almost the whole of temperate North America. In New England and New York the sexes are alike, but south of lat. 42° some of the females are black, and they are so different from the yellow male and the northern yellow female, that they were for a long time regarded as a distinct species, and have received a specific name, Papilio glaucus. Between lat. 42° and lat. 37° both forms are found, and Prof. Uhler of Baltimore, has reared the yellow female Papilio turnus, and the black one, P. glaucus, from the same lot of eggs, but further south only the black female is found, although the male is exactly like that which in New England is associated with the yellow female alone.

Wallace has recorded a number of similar cases among the Malayan Papilionidæ, of which Papilio Memnon is one of the most striking. In this species there are two kinds of females, one closely resembling the male, and the other differently colored, and furnished with long spatulate tail-like elongations of the hinder wings. These tails are not present on the wings of the male nor on those of the second female, although they are found in both sexes of other species of Papilio, and in some other less specialized genera of the Papilio family. The males, the tailed and the tailless females have all been reared from a single group of eggs, so there is no doubt that they all belong to the same species.

Wallace has given other cases in which the same male form is found associated, in different countries, with their three different female forms.

It is possible, and indeed probable, that in some of these cases certain females have resembled the male, while others have either remained unmodified or else have reverted back to an ancestral form.

Darwin refers to a case of sexual dimorphism which