CHAPTER VIII.

THE EVIDENCE FROM SECONDARY SEXUAL CHARACTERS.

The Nature of this Sort of Evidence.

I HAVE already given many reasons for believing that the male reproductive organ is especially adapted for gathering up the gemmules which are thrown off by the cells of the body; and for transmitting them to the next generation by impregnation, thus giving rise to variation; while the transmission of the gemmules which are formed in the body of the female is not thus provided for.

If this supposition is correct, we should expect to find that a variation which first appears in a male should have more tendency to become hereditary than one which first appears in a female. Any slight change in either the male or the female body will, as we have already seen, cause all the cells which are either directly or indirectly influenced by the change to throw off gemmules. This will happen in a female body as well as in a male body, but the gemmules are, in the latter case, much more likely to be transmitted to descendants, and thus to give rise to more extended modification.

We should also expect to find that an organ which is confined to males is much more likely than one which is confined to females to undergo hereditary changes, for even if the parts of the female body give rise to gemmules as frequently as the parts of the male body, the chance of transmission is much less.

.

We should also expect to find that parts which aro

confined to males are more variable than parts confined to females; for variation in any part is due to inheritance of a gemmule from the corresponding part of one parent or the other, but when the part is found in only one parent the gemmule must come from that parent.

As transmission of gemmules by the mother is more rare than transmission by the father, it is plain that parts which are confined to the male should be expected to vary more than parts found in the female alone.

Finally we should expect the male body as a whole to be more variable than the female body, for the same

reason.

In most cases it is impossible to trace any particular variation back to its first appearance. This is almost out of the question with wild animals, and most domesticated races have been formed so slowly that it is impossible to say whether the successive steps appeared in males or in females, nor can we be sure that a variation is new when it first attracts attention. Still it is interesting to note that the sudden variation which resulted in the ancon breed of sheep was first noticed in a male, although it is, of course, impossible to say whether it was due to inheritance of gemmules from the father rather than from the mother. Certain hereditary diseases and montrosities, such as albinism or polydactylism, are fully as often traceable to a male origin as they are to a female origin, but as we know that peculiarities of this kind frequently skip a generation or two, we can never be sure that we have traced them to their origin.

In the secondary sexual characters of animals we have a class of phenomena which are not rare and exceptional, for they are numbered by hundreds of thousands, and they can be observed and studied by every one.

A secondary sexual character is a peculiarity which is not directly concerned in the reproductive process, although it is normally either confined to one sex, or else is much more developed in one sex than it is in the other. The presence of a beard is a secondary sexual character of man; the comb, wattles, spurs and brilliant plumage of the domestic cock, the horns of a stag, the tusks of an elephant, the mane of a lion, or the brilliant plumage of the peacock or of the drake, are all of them examples of this sort of organs, for they are either confined to one sex, or else they are much more conspicuous and important in one sex than they are in the other.

They furnish, like hybrids, a means of disentangling or analyzing to some extent the influence of the two sexes in heredity, and I hope to show in this and the following chapters that they furnish evidence to prove—

1. That in most animals with separate sexes the males of allied species differ more than the females from the ancestral type.

2. That organs which are confined to males or are of more importance or are more perfectly developed in them than in the females, are much more likely to give rise to hereditary modifications than parts which are confined to or are most developed in females.

3. That a part which is confined to or is most developed in males is more likely than a similar female part to vary.

4. That males are, as a rule, more variable than females.

5. That the male leads and the female follows in the evolution of new races.

There are two criteria which are of great use in the attempt to trace the path which a species has followed in its evolution. One of these is by comparison of a

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 a 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.

2

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 discover 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 Separate 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.

ROTIFERA. In 1849, Dalrymple (Description of an Infusory Animalcule allied to the Genus Notommata, Phil. Trans. 1849) made the interesting and remarkable discovery that, in one species of the Rotifera, Notom