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species with its nearest allies. The other is by comparison of the young with the adult.
If most of the species of a genus resemble each other in certain characters, while one species presents^, marked deviation, we may in most cases safely conclude that the latter species has undergone recent modification in this respect. Of course this rule does not hold good where the peculiarities of the exceptional species are features of resemblance to other genera of the family, for in this case we must conclude that it has remained comparatively stationary, while all the other species of the genus have been modified.
If in the second place we find that the adults of several related species differ greatly, while the young are much alike, we must attribute the difference in the adults to the fact that they have recently diverged from a common stock.
Now I hope to show that throughout the animal kingdom, wherever the sexes differ from each other, the general law holds good that the males of allied species differ from each other more than the females do, and that the adult male differs more than the adult female from the young. There are many marked exceptions to this law, but the existence of the law has long been recognized by all naturalists. Every one who has worked at the systematic zoology of insects or vertebrates knows how difficult it often is to decide upon the specific identity of an immature or a female specimen, even in cases where the mature males can be recognized and identified without difficulty.
Darwin's interesting essay on "Sexual Selection" is well known. It is almost entirely devoted to the study of secondary sexual characters, and to a masterly discussion of the subject in all its aspects and relations.
Darwin has gone over the whole field so thoroughly and exhaustively that little remains to be said upon the subject, and the reader who is familiar with the essay will discqver that almost all the facts in this chapter are borrowed from this source.
Darwin's aim, however, is simply to show the potency of sexual selection, while our present object is to show the frequency of hereditary male modification as compared with female modifications, and I have therefore rearranged the facts, so as to give especial prominence to this aspect of the subject. The critical reader will discover that in many cases I have borrowed the descriptive portion of one of Darwin's paragraphs, but have said nothing about the theoretical portion. As Darwin's conclusions are in many cases opposed to my own, this may convey to some the impression that I have made an unfair use of the weight of his authority, and have quoted him in support of conclusions which he in reality opposes. I will refer such readers to the chapter which follows this, where I have devoted a section to a statement of Darwin's view of the origin of secondary sexual characters, and have given my reasons for believing that it is only a partial explanation of the phenomena in question.
Examples from Various Groups of the Animal Kingdom to show that in all Groups where the Sexes are Sepai rate the Male is, as a Rule, more Modified than the • Female, and that the Adult Males of Allied Species differ more, as a Rule, than the Females or Young.
Eotifeba.—In 1849, Dalrymple (Description of an Infusory Animalcule allied to the Genus Eotommata, Phil. Trans. 1849) made the interesting and remarkable discovery that, in one species of the Rotifera, Notommata Anglica, the animals are not hermaphrodites, as earlier writers had supposed, but that the males, which are rarely met with, are very much smaller than the females. The latter sex is furnished with a digestive tract which is quite complicated in structure, and is armed at the mouth with a highly specialized masticating apparatus. The digestive organs of the male, on the other hand, are almost absent. The jaws, the oesophagus and the mouth are wanting, and the stomach and intestine are reduced to a f unctionless rudiment. The males receive no nourishment after they leave the egg, and they live only a short time. The presence of a digestive tract is characteristic of all groups of animals above the protozoa, so we are compelled to believe that the ancestral form from which the Rotifera are descended had, like the ordinary metazoa, a mouth, a stomach, and an intestine; and no one who is at all familiar with comparative anatomy can doubt that the male, in which it is absent,"rather than the female, in which it is present, is the sex which has been modified. The digestive tract is usually one of the first parts to be developed in the embryo, and its disappearance or absence in the adult male rotifer is therefore very different from the absence of the wings in certain female insects. Wings appear very late in life, and the failure of the female to acquire them is simply an arrest short of perfect development, while the absence of digestive organs shows active degeneration. In 1855 Leydig verified Dalryinple's observation (Zeit. f. Wiss. Zool. vi. p. 96) in the same species, and also in a second species of the same genus; and as he was able to distinguish the outline of the male inside the egg, while this was still contained within the body of the female, he removed all reason for doubting, that the two sexes belong to one species. In these two species the females were much alike, while the males were not only very different from the females, but also from each other.
Since the year 1855 the subject has been studied by many naturalists, and the males have been found in such a number of species that it is probable that the sexes are separate in all the Eotifera. In some forms' the males are even more simplified than in Notommata, while in others they are less so, and in a few they are like the females in size and structure, and have the digestive organs perfectly developed.
Annelids.—Among the marine polychaetous annelids there is often considerable difference between the sexes, and the points in which the male differs from the female are also points in which the males of various species differ from each other.
Abthkopoda.—Among the Arthropods, the Insects, Crustacea, etc., the female is often very greatly modified, and in some cases the females of allied species differ from each other much more than the males, and in other cases it is hardly possible to say whether the males or the females of allied species differ most, but, taking the group as a whole, the Anthropods seem to follow the law which prevails in other groups of animals, and male modifications are more numerous than female modifications.
In the Branchiopod Crustacea the males are smaller than the females, and are much less abundant. The male differs from the female in the possession of a number of secondary sexual characters. The second antennae of the male are more richly supplied with sensory hairs than those of the female, and various appendages of the male may be so modified as to form clasping organs for holding the female. In Branchippus the second antennae of the male are greatly modified for this purpose. Figure 3 shows the head of a female specimen of Branchippus Orubei, figure 4 the head of the male of the same species, and figures 5 and 6 the heads of the males in two closely allied species. These figures show how much the males of the various species