“Some writers suggest that the bellowing serves as a call to the female; but the experienced observers above quoted inform me that female deer do not search for the male, though the males search eagerly for the females, as indeed might be expected from what we know of the habits of other male quadrupeds. The voice of the female, on the other hand, quickly brings to her one or more stags, as is well known to the hunters who in wild countries imitate her cry.
“As the case stands, the loud voice of the stag during the breeding season does not seem to be of any special service to him, either during his courtship or battles, or in any other way. But may we not believe that the frequent use of the voice, under the strong excitement of love, jealousy, and rage, continued during many generations, may at last have produced an inherited effect on the vocal organs of the stag, as well as of other male animals? This appears to me, in our present state of knowledge, the most probable view.”
Here once more we find that Darwin makes use, as a sort of last resort, of the principle of the inheritance of acquired characters. As long as the theory of selection, in any of its forms, appears to offer a satisfactory solution, we find the facts used in support of this theory, but as soon as a difficulty arises the Lamarckian theory is brought to the front. It is this shifting, as we have already more than once pointed out, that shows how little real basis there is for the theory of sexual selection.
The male gorilla has a tremendous voice, and he has, as has also the orang, a laryngeal sac. One species of gibbon has the power of producing a correct octave of musical notes.
“The vocal organs of the American Mycetes caraya are one-third larger in the male than in the female, and are wonderfully powerful. These monkeys in warm weather make the forests resound at morning and evening with their overwhelming voices. The males begin the dreadful concert, and often continue it during many hours, the females sometimes joining in with their less powerful voices. An excellent observer, Rengger, could not perceive that they were excited to begin by any special cause; he thinks that, like many birds, they delight in their own music, and try to excel each other. Whether most of the foregoing monkeys have acquired their powerful voices in order to beat their rivals and charm the females—or whether the vocal organs have been strengthened and enlarged through the inherited effects of long-continued use without any particular good being thus gained—I will not pretend to say; but the former view, at least in the case of the Hylobates agilis, seems the most probable.”
The odor of some mammals is confined to, or more developed, in the males; but in some forms, as in the skunk, it is present in both sexes. In the shrew mice, abdominal scent glands are present, but since these mice are rejected by birds of prey, their glands probably serve to protect them; “nevertheless the glands become enlarged in the males during the breeding season.” In many other quadrupeds the scent glands are of the same size in both sexes, and their function is unknown.
“In other species the glands are confined to the males, or are more developed than in the females; and they almost always become more active during the rutting season. At this period the glands on the sides of the face of the male elephant enlarge, and emit a secretion having a strong musky odor. The males, and rarely the females, of many kinds of bats have glands and protrudable sacs situated in various parts; and it is believed that these are odoriferous.
“The rank effluvium of the male goat is well known, and that of certain male deer is wonderfully strong and persistent. Besides the general odor, permeating the whole body of certain ruminants (for instance, Bos moschatus) in the breeding season, many deer, antelopes, sheep, and goats, possess odoriferous glands in various situations, more especially on their faces. The so-called tear-sacs, or suborbital pits, come under this head. These glands secrete a semifluid fetid matter which is sometimes so copious as to stain the whole face, as I have myself seen in an antelope. They are ‘usually larger in the male than in the female, and their development is checked by castration.’ According to Desmarest they are altogether absent in the female of Antilope subgutturosa. Hence, there can be no doubt that they stand in close relation with the reproductive functions. They are also sometimes present, and sometimes absent, in nearly allied forms. In the adult male musk-deer (Moschus moschiferus), a naked space round the tail is bedewed with an odoriferous fluid, whilst in the adult female and in the male until two years old, this space is covered with hair, and is not odoriferous.” Darwin believes in these cases that the odor serves to attract the females. He admits that here, “active and long-continued use cannot have come into play as in the case of the vocal organs.” He concludes, therefore, that “the odor emitted must be of considerable importance to the male, inasmuch as large and complex glands, furnished with muscles for everting the sac, and for closing or opening the orifice, have in some cases been developed. The development of these organs is intelligible through sexual selection, if the most odoriferous males are the most successful in winning the females, and in leaving offspring to inherit their gradually perfected glands and colors.”
There is sometimes a difference in the mammals in the hair of the two sexes both in amount and in color. In some species of goats the males have a beard, in others it is present in both sexes. The bull, but not the cow, has curly hair on the forehead. In some monkeys the beard is confined to the male, as in the orang; in other species it is only larger in the males.
“The males of various members of the ox family (Bovidæ), and of certain antelopes, are furnished with a dewlap, or great fold of skin on the neck, which is much less developed in the female.
“Now, what must we conclude with respect to such sexual differences as these? No one will pretend that the beards of certain male goats, or the dewlap of the bull, or the crests of hair along the backs of certain male antelopes, are of any use to them in their ordinary habits.
“Must we attribute all these appendages of hair or skin to mere purposeless variability in the male? It cannot be denied that this is possible; for in many domesticated quadrupeds, certain characters, apparently not derived through reversion from any wild parent form, are confined to the males, or are more developed in them than in the females—for instance, the hump on the male zebu cattle of India, the tail of fat-tailed rams, the arched outline of the forehead in the males of several breeds of sheep, and, lastly, the mane, the long hairs on the hind-legs, and the dewlap of the male of the Berbura goat.”
In these cases and in others that Darwin cites, which seem clearly to indicate that some of these secondary sexual characters are not the result of sexual selection, he concludes, “that they must be due to simple variability, together with sexually limited inheritance.
“Hence it appears reasonable to extend this same view to all analogous cases with animals in a state of nature. Nevertheless I cannot persuade myself that it generally holds good, as in the case of the extraordinary development of hair on the throat and fore-legs of the male Ammotragus, or in that of the immense beard of the male Pithecia. Such study as I have been able to give to nature makes me believe that parts or organs which are highly developed, were acquired at some period for a special purpose. With those antelopes in which the adult male is more strongly colored than the female, and with those monkeys in which the hair on the face is elegantly arranged and colored in a diversified manner, it seems probable that the crests and tufts of hair were gained as ornaments; and this I know is the opinion of some naturalists. If this be correct, there can be little doubt that they were gained, or at least modified through sexual selection; but how far the same view may be extended to other mammals is doubtful.”
The astonishing colors in some of the monkeys cannot be passed over without comment.
“In the beautiful Cercopithecus diana, the head of the adult male is of an intense black, whilst that of the female is dark gray; in the former the fur between the thighs is of an elegant fawn-color, in the latter it is paler.
“In the Cercopithecus cynosurus and griseoviridis one part of the body, which is confined to the male sex, is of the most brilliant blue or green, and contrasts strikingly with the naked skin on the hinder part of the body, which is vivid red.
“Lastly, in the baboon family, the adult male of Cynocephalus hamadryas differs from the female not only by his immense mane, but slightly in the color of the hair and of the naked callosities. In the drill (C. leucophæus) the females and young are much paler-colored, with less green, than the adult males. No other member in the whole class of mammals is colored in so extraordinary a manner as the adult male mandrill (C. mormon). The face at this age becomes of a fine blue, with the ridge and tip of the nose of the most brilliant red. According to some authors, the face is also marked with whitish stripes, and is shaded in parts with black, but the colors appear to be variable. On the forehead there is a crest of hair, and on the chin a yellow beard. ‘Toutes les parties supérieures de leurs cuisses et le grand espace nu de leurs fesses sont également colorés du rouge le plus vif, avec un mélange de bleu qui ne manque réellement pas d’élégance.’ When the animal is excited all the naked parts become much more vividly tinted.”
Darwin sums up the evidence in regard to the differences in color between the male and female in the following statement:—
“I have now given all the cases known to me of a difference in color between the sexes of mammals. Some of these may be the result of variations confined to one sex and transmitted to the same sex, without any good being gained, and therefore without the aid of selection. We have instances of this with our domesticated animals, as in the males of certain cats being rusty-red, whilst the females are tortoise-shell colored. Analogous cases occur in nature: Mr. Bartlett has seen many black varieties of the jaguar, leopard, vulpine phalanger, and wombat; and he is certain that all or nearly all these animals, were males. On the other hand, with wolves, foxes, and apparently American squirrels, both sexes are occasionally born black. Hence it is quite possible that with some mammals a difference in color between the sexes, especially when this is congenital, may simply be the result, without the aid of selection, of the occurrence of one or more variations, which from the first were sexually limited in their transmission. Nevertheless it is improbable that the diversified, vivid, and contrasted colors of certain quadrupeds, for instance, of the above monkeys and antelopes, can thus be accounted for.”
Finally, the case of man must be considered from the point of view of sexual selection, for Darwin claims that man has acquired a number of his secondary sexual characters in this way. For instance, the beard is an excellent case of a secondary sexual character. Darwin’s interpretation is that the beard has been retained, or even developed, through the selection by the females of those males that had this outgrowth best developed. Conversely, the absence of hair on the face of the female is supposed by Darwin to have been brought about by men selecting those women having less hair on their faces. The greater intellect, energy, courage, pugnacity, and size of man are the outcome of the competition of the males with each other, since the individual excelling in these qualities will be able to select the most desirable wife, or wives, and it is assumed will, therefore, leave more descendants. The standard of beauty has been kept up by men selecting the most beautiful women in each generation (the fate of the other married women is ignored), and this beauty is supposed to have been transmitted primarily to their daughters, but also to their sons.
Although all these forms of selection are imagined to be acting in man, either alternately or simultaneously, yet Darwin recognizes in man a number of checks to the action of sexual selection: amongst savages, the so-called communal marriages; second, infanticide, generally of the young females, which appears in some races to be practised to an astonishing degree; third, early betrothals; fourth, the holding of women as slaves.
When we recall that selection to be effective can only be carried out under very exacting conditions, we cannot but be appalled at the demands made here on our credulity. The choice of the women has produced the beard of man, the choice of man the absence of a beard in women; the competition of the males with each other is leading at the same time to the development of at least half a dozen qualities that are supposed to be male specialities, and while all this is going on the results are being checked sometimes by one means, sometimes by another. Moreover, even this is not all that we are asked to accept, for there are several other qualities of the male that are put down as secondary sexual characters. For example, let us examine what Darwin has to say in regard to the development of the voice, and of singing in man.
In man the vocal cords are about a third longer than in woman and his voice deeper. Emasculation arrests the development of the vocal apparatus, and the voice remains like that of a woman. This difference between the sexes, Darwin thinks, is due probably to long-continued use by the male “under the excitement of love, rage, and jealousy.” In other words, an appeal is again made to the Lamarckian theory, and in this case to explain the origin of an organ that conforms to all the requirements of the secondary sexual characters.
“The capacity and love for singing, or music, though not a sexual character in man,” in the sense of being confined to one sex, yet is supposed to have arisen through sexual selection in the following way: “Human song is generally admitted to be the basis or origin of instrumental music. As neither the enjoyment nor the capacity of producing musical notes are faculties of the least use to man in reference to his daily habits of life, they must be ranked amongst the most mysterious with which he is endowed.”
Man is supposed to have possessed this faculty of song from a very remote time, and even the most savage races make musical sounds, although we do not enjoy their music, or they ours.
“We see that the musical faculties, which are not wholly deficient in any race, are capable of prompt and high development, for Hottentots and Negroes have become excellent musicians, although in their native countries they rarely practise anything that we should consider music. Hence the capacity for high musical development, which the savage races of man possess, may be due either to the practice by our semi-human progenitors of some rude form of music, or simply to their having acquired the proper vocal organs for a different purpose. But in this latter case we must assume, as in the above instance of parrots, and as seems to occur with many animals, that they already possessed some sense of melody.”
Darwin sums up the evidence in the two following statements, the insufficiency of which to explain the phenomena is I think only too obvious: “All these facts in respect to music and impassioned speech become intelligible to a certain extent, if we assume that musical tones and rhythm were used by our half-human ancestors, during the season of courtship, when animals of all kinds are excited not only by love, but by the strong passions of jealousy, rivalry, and triumph. From the deeply laid principle of inherited associations, musical tones in this case would be likely to call up vaguely and indefinitely the strong emotions of a long past age.” Thus the difficulty is shifted to the shoulders of our long-lost savage ancestors; or even, in fact, to our simian forefathers, as the following paragraph indicates:—
“As the males of several quadrumanous animals have their vocal organs much more developed than in the females, and as a gibbon, one of the anthropomorphous apes, pours forth a whole octave of musical notes and may be said to sing, it appears probable that the progenitors of man, either the males or females or both sexes, before acquiring the power of expressing their mutual love in articulate language, endeavored to charm each other with musical notes and rhythm. So little is known about the use of the voice by the Quadrumana during the season of love, that we have no means of judging whether the habit of singing was first acquired by our male or female ancestors. Women are generally thought to possess sweeter voices than men, and as far as this serves as any guide, we may infer that they first acquired musical powers in order to attract the other sex. But if so, this must have occurred long ago, before our ancestors had become sufficiently human to treat and value their women merely as useful slaves. The impassioned orator, bard, or musician, when with his varied tones and cadences he excites the strongest emotions in his hearers, little suspects that he uses the same means by which his half-human ancestors long ago aroused each other’s ardent passions during their courtship and rivalry.”
We have now examined in some detail the evidence that Darwin has brought forward in support of his hypothesis of sexual selection. A running comment has been made while considering the individual cases, but it may be well to sum up the matter by briefly indicating the reasons why the hypothesis seems incompetent to explain the facts.
1. Some of the objections that apply to the theory of natural selection apply also with equal force to the theory of sexual selection in so far as the results in both cases are supposed to be the outcome of the selection of individual, or fluctuating, variations. If these variations appear in only a few individuals, their perpetuation is not possible, since they will soon disappear through crossing. It would be, of course, preposterous to suppose that at any one time only those few individuals pair and leave descendants that have the secondary sexual characters developed to the highest point, but if something of this sort does not occur, the extreme of fluctuating variations cannot be maintained. Even if half of the individuals are selected in each generation, the accumulation of a variation in a given direction could not go very far. The assumption, however, that only half of all the individuals that reach maturity breed, and that all of these are chosen on account of the special development of their secondary sexual characters, seems preposterous. Furthermore, if it is assumed that the high development of the new character appears in a large number of individuals, then it is not improbable that its continued appearance might be accounted for without bringing in, at all, the hypothesis of sexual selection.
2. But even supposing that the females select the most beautiful males, then, since in the vast majority of higher animals the males and the females are in equal numbers, the others will also be able to unite with each other in pairs after this first selection has taken place. Nothing will therefore be gained in the next generation. It is interesting to see how Darwin attempts to meet this argument. He tries to show in the case of birds, that there are always unpaired individuals, but since the few facts that he has been able to collect show that there are as many additional females as males, the argument proves too much. A few species are polygamous, one male having a number of female birds; but on this basis we can only account, at best, for the development through competition of the organs of offence and defence used to keep away the weaker males. Yet it is just amongst these birds that we often find the ornamental characters well developed. In fact, since all the females in such cases are selected, and since they will transmit the characters of all the males, it is evident that the secondary sexual characters could not be formed in the way imagined.
3. If the female fails to select only the more ornamental males, no result will follow. It has not been shown that she is capable of making such a choice, and in the lower forms particularly, it does not seem probable that this is done. The argument that Darwin often employs, namely, that unless she does select, the display of the males before her is meaningless, is not to the point. So far as we can detect the “cause” of the display of the male, it appears to be due to his own excitement; and even if we go so far as to admit that the “purpose” is to attract the other sex, it still does not in the least follow that the most ornamental male is selected, and unless this occurs the display has no bearing on the hypothesis of sexual selection.
4. The two forms of sexual selection, namely, competition of the males with one another (really one form of natural selection), and the selection of the most ornamental or gifted individuals, are both used by Darwin to explain secondary sexual characters, the one for organs of offence and defence, and the other for ornamental characters. If we fully appreciate the difficulties that any theory of selection meets with, we shall realize how extraordinarily complex the action must be, when two such processes are carried out at the same time, or even during alternating periods.
5. It has been objected to Darwin’s theory of sexual selection, that he suddenly reverses its mode of action to explain those cases in which the female is the stronger and more ornamented sex; but if, as Darwin shows, the instincts of the male have also changed, and have become more like those of the female, I can see no inherent difficulty in this way of applying the theory. A much more serious objection, it seems to me, is that the male is supposed to select the female for one set of characteristics, and the female to select the male for another set. It sounds a little strange to suppose that women have caused the beard of man to develop by selecting the best-bearded individuals, and the compliment has been returned by the males selecting the females that have the least amount of beard. It is also assumed that the results of the selection are transmitted to one sex only. Unless, in fact, the character in question were from the beginning peculiar to only one sex as to its inheritance, the two sexes might go on forever selecting at cross-purposes, and the result would be nothing.
6. The development, or the presence, of the æsthetic feeling in the selecting sex is not accounted for on the theory. There is just as much need to explain why the females are gifted with an appreciation of the beautiful, as that the beautiful colors develop in the males. Shall we assume that still another process of selection is going on, as a result of which those females are selected by the males that appreciate their unusual beauty, or that those females whose taste has soared a little higher than that of the average (a variation of this sort having appeared) select males to correspond, and thus the two continue heaping up the ornaments on one side and the appreciation of these ornaments on the other? No doubt an interesting fiction could be built up along these lines, but would any one believe it, and, if he did, could he prove it?
Darwin assumes that the appreciation on the part of the female is always present, and he thus simplifies, in appearance, the problem, but he leaves half of it unexplained.
7. It has been pointed out, that it is important to distinguish between the possible excitement of the female by the display or antics of the male, and the selection of the more beautiful or agile performer. Darwin himself records a few cases, which plainly show that the more beautiful is not always the more successful. It has also been suggested that the battles of the males are sometimes sham performances, and even when they are real, if the less vigorous do not remain to be destroyed but run away, they live to find mates of their own. In fact, the conduct of the males at the breeding season appears to be much more the outcome of their own excitement than an attempt to attract the females.
8. There is another side to the question, the importance of which is so great, that it is surprising that Darwin has not taken any notice of it. If, in order to bring about, or even maintain, the results of sexual selection, such a tremendous elimination of individuals must take place, it is surprising that natural selection would not counteract this by destroying those species in which a process, so useless for the welfare of the species, is going on. It is curious that this has not been realized by those who believe in both of these two hypotheses.
9. What has just been said applies also with almost equal force to the development of such structures as the horns of deer, bison, antelopes, and the brilliant colors of many insects and birds. If in nature, competition between species takes place on the scale that the Darwinian theory of natural selection postulates, such forms, if they are much exposed, would be needlessly reduced in numbers in the process of acquiring these structures. So many individuals would have been at such a disadvantage in breeding, that if competition is as severe as the theory of natural selection postulates, these species could hardly be expected to compete successfully with other species in which sexual selection was not taking place.
10. Darwin admits that, in certain cases, external conditions may have acted directly to produce the colors in certain forms, and if these were not injurious he thinks they might have become constant. Such cases are left unexplained in the sense that they are not supposed to be adaptations to anything in particular. That colors produced in this way might afterward be found useful, irrespective of how they arose, is admitted as one of the ways in which sexual differences may have arisen.
11. It is baffling to find Darwin resorting to the Lamarckian explanation in those cases in which the improbability of the hypothesis of sexual selection is manifest. If either principle is true, we should expect it to apply to all phenomena of the same sort; yet Darwin makes use of the Lamarckian principle, in the hypothesis of sexual selection, only when difficulties arise.
12. In attempting to explain the development of the musical sense in man, it is clear that the hypothesis of sexual selection fails to give a satisfactory explanation. To suppose that the genius of a Beethoven or of a Mozart could have been the result of a process of sexual selection is too absurd to discuss. Neither the power of appreciation nor of expression in music could possibly have been the outcome of such a process, and it does not materially help the problem to refer it back to a troop of monkeys making the woods hideous with their cries.
We come now to some of the special cases to which Darwin’s hypothesis has been applied.
13. In one case at least, it is stated that a bird living on the ground might have acquired the color of the upper surface of the body through natural selection, while the under surface of the males of the same species might have become ornamented through the action of sexual selection. Thus in one and the same individual the two processes are supposed to have been at work, and it does not lessen the difficulty very much by supposing the two processes to have been carried out at different times, because it is evident that what had been gained at one time by one process might become lost while the color of certain parts was being acquired through the other process.
14. Darwin points out that “the plumage of certain birds goes on increasing in beauty during many years after they are fully mature,” as in the peacock, and in some of the birds of paradise, and with the plumes and crests of some herons. This is explained as possibly merely the result of “continued growth.” The improbability of selection is manifest in these cases, but if “continued growth” can accomplish this much, why may not the whole process be also the outcome of such growth? At any rate, whatever the explanation is, it is important to find a case of a secondary sexual character that the hypothesis obviously is insufficient to explain.
15. It is admitted in a number of cases, as in the stag for instance, that, although the larynx of the male is enlarged, this is not, in all probability, the outcome of sexual selection, but in other forms this same enlargement is ascribed to the selection process.
16. It is admitted that in none of the highly colored British moths is there much difference according to sex, although when a difference of color is found in butterflies this is put down to the action of sexual selection. If such wonderful colors as those of moths can arise without the action of selection, why make a special explanation for those cases in which this difference is associated with sex?
17. It is well known that birds sing at other times of the year than at the breeding season, and an attempt is made to account for this in that birds take pleasure in practising those instincts that they make use of at other times, as the cat plays with the captive mouse. Does not this suggest that, if they had certain instincts, they would be more likely to employ them at the times when their vitality or excitement is at its highest without regard to the way in which they have come by them?
18. The color of the iris of the eyes of many species of hornbills is said to be an intense crimson in the males, and white in the females. In the male condor the eye is yellowish brown, and in the female a bright red. Darwin admits that it is doubtful if this difference is the result of sexual selection, since in the latter case the lining of the mouth is black in the males, and flesh-colored in the females, which does not affect the external beauty. Yet if these colors were more extensive and on the exterior, there can be little doubt that they would have been explained as due to sexual selection.
19. When the females in certain species of birds differ more from each other than they do from their respective males, the case is compared to “those inexplicable ones, which occur independently of man’s selection in certain sub-breeds of the game-fowl, in which the females are very different, whilst the males can hardly be distinguished.” Here then is a case of difference in color associated with sex, but not the outcome of sexual selection.
20. The long hairs on the throat of the stag are said possibly to be of use to him when hunted, since the dogs generally seize him by the throat, “but it is not probable that the hairs were specially developed for this purpose; otherwise the young and the females would have been equally protected.” Here also is a sexual difference that can scarcely be ascribed to selection.
Some cases of differences in color between the sexes “may be the result of variations confined to one sex, and transmitted to the same sex without any good being gained, and, therefore, without the aid of selection. We have instances of this with our domesticated animals, as in the males of certain cats being rusty-red while the females are tortoise-shell colored. Analogous cases occur in nature: Mr. Bartlett has seen many black varieties of the jaguar, leopard, vulpine phalanger, and wombat; and he is certain that all or nearly all of these animals were males.” If changes of this sort occur, associated with one sex, why is there any need of a special explanation in other cases of difference?
In the light of the many difficulties that the theory of sexual selection meets with, I think we shall be justified in rejecting it as an explanation of the secondary sexual differences amongst animals. Other attempts to explain these differences have been equally unsuccessful. Thus Wallace accounts for them as due to the excessive vigor of the male, but Darwin’s reply to Wallace appears to show that this is not the cause of the difference. He points out that, while the hypothesis might appear plausible in the case of color, it is not so evident in the case of other secondary sexual characters, such, for instance, as the musical apparatus of the males of certain insects, and the difference in the size of the larynx of certain birds and mammals.
Darwin’s theory served to draw attention to a large number of most interesting differences between the sexes, and, even if it prove to be a fiction, it has done much good in bringing before us an array of important facts in regard to differences in secondary sexual characters. More than this I do not believe it has done. The theory meets with fatal objections at every turn.
In a later chapter the question will be more fully discussed as to the sense in which these secondary sexual differences may be looked upon as adaptations.