It will be conceded, I think, that such pleading does not help the argument, but exposes rather its inherent weaknesses. There is, however, a line of defense that is permissible. If monogamy is not the rule, if the male captures or attracts several females and keeps a harem, as do the fur seals and walruses, or rules a herd as does the bull, or has a flock as does the cock, or mates more frequently with random females than do some other males, then the advantage of his more developed weapon might lead to more offspring. If it could be shown that such intraspecific weapons prevail more frequently within polygamous species, a fair argument for natural selection might be made. I do not know whether such a census has been taken as yet, but it is true, I think, that in most polygamous groups we find weapons of offense very highly developed. The fur seal has a harem and the male is greater in size, in strength, and in the development of his tusks than is the female. Similarly for the walrus. The bull drives away rival bulls from the herd until through age or injury, or through the development of a better fighter, he is replaced. If the better endowment is due to a genetic factor, we should expect natural selection to keep the race at the highest possible level that variation supplies material for. If, then, we confine the application of natural selection to cases of this sort, the explanation is as valid as is the theory in other fields. Such a conclusion becomes weakened when an attempt is made to apply it to other groups of animals in which it appears improbable that the secondary sexual characters of the male have any obvious value as organs of offense. There are families of beetles, for example, in which the development of the horns of the male are as striking as are those of the ram or the stag. The males of these beetles are not known to fight with each other, nor are they polygamous. It may seem that we must look here for some other explanation, which, if found, might suffice to cover also the case of birds and mammals. In answer to this criticism it may be argued that it is also possible that the other explanation when found need not necessarily apply to the higher animals, where the laws of combat may still give the true explanation. On the whole, I think that, for our present purpose, it will suffice to state it is consistent with the theory of natural selection to accept provisionally this part of Darwin’s theory for those species in the higher groups in which polygamy holds, conceding, however, that even here it may have to be altered when fuller knowledge is gained.
We are more concerned with that special feature of Darwin’s theory of sexual selection that is applied to those cases where the characters are supposed to owe their special development to selection by the individuals of the opposite sex. It is assumed that the female chooses the better endowed males, because of the strong appeal he makes to her sense-organs. Here we must employ perforce or for brevity’s sake the terms used in human psychology, and run the risk at every turn of imputing to other animals the emotions and acquired associations which man himself utilizes. Even granting that other animals possess somewhat similar emotions to ours, there still remains always the danger, in the absence of real evidence, of imputing to them the particular emotion that we call “feeling for beauty”; and the greater danger of imputing an esthetic sense so highly developed that the choice falls in the long run on the suitor better ornamented than his rivals.
OTHER THEORIES TO ACCOUNT FOR SECONDARY SEXUAL CHARACTERS.
Wallace has always been an opponent of Darwin’s theory of sexual selection in so far as it is based on female choice. As already stated, he believes that the difference between the plumage of the male and female in birds is due to natural selection keeping down the ornamentation and high coloration in the female, because these would be expected to expose the female while sitting on the nest to the attacks of enemies, more especially of hawks. In support of this view he points to a long series of species which build exposed nests and in them the female is plainly and inconspicuously colored, while he also points out that in such birds as parrots, toucans, woodpeckers, hangnests, and starlings, which nest in holes or have covered nests, the female is often as highly colored as the male. It can not be denied that he makes out rather a strong case in support of this view, despite the fact that there are other birds, like the Baltimore oriole, that have covered nests and in which the sexes are very markedly different.
Wallace tries to meet cases like the last one by assuming that the covering keeps off the rain; but, if so, why are the sexes still so different? In the case of other highly colored birds, such as jays, magpies, hawks, and crows, Wallace believes that these birds are all aggressive, hence can protect their nests if attacked. As a further support of his view, Wallace points out that in the few cases where the female is more highly colored than the male (as the dotterel, species of phalarope, an Australian creeper) the male incubates the eggs.
Wallace’s suggestion still leaves unexplained the ornamentation of the male, which he tries to account for as the direct result of the greater vitality of the male. He tries to show that excessive ornaments and high coloration develop especially in those parts of the body to which there is an unusual supply of blood or where nerves and blood-vessels emerge to go to the skin or to the muscles.
“If we have found a vera causa for the origin of ornamental appendages of birds and other animals in a surplus of vital energy, leading to abnormal growths in those parts of the integument where muscular and nervous action are greatest, the continuous development of these appendages will result from the ordinary action of natural selection in preserving the most healthy and vigorous individuals, and the still further selective agency of sexual struggle in giving to the very strongest and most energetic the parentage of the next generation. And, as all the evidence goes to show that, so far as female birds exercise any choice, it is for ‘the most vigorous, defiant, and mettlesome male,’ this form of sexual selection will act in the same direction, and help to carry on the process of plume development to its culmination. That culmination will be reached when the excessive length or abundance of the plumes begins to be injurious to the bearer of them; and it may be this check to the further lengthening of the peacock’s train that has led to the broadening of the feathers at the ends, and the consequent production of the magnificent eye-spots which now form its crowning ornament.
“The display of these plumes will result from the same causes which led to their production. Just in proportion as the feathers themselves increased in length and abundance, the skin-muscles which serve to elevate them would increase also; and the nervous development as well as the supply of blood to these parts being at a maximum, the erection of the plumes would become a habit at all periods of nervous or sexual excitement. The display of the plumes, like the existence of the plumes themselves, would be the chief external indication of the maturity and vigor of the male, and would, therefore, be necessarily attractive to the female. We have, thus, no reason for imputing to her any of those esthetic emotions which are excited in us, by the beauty of form, color, and pattern of these plumes; or the still more improbable esthetic tastes, which would cause her to choose her mate on account of minute differences in their forms, colors, or patterns.”
Wallace says, referring to the immense tuft of golden plumage in the best known birds of paradise (Paradisea apoda and P. minor) that springs from a very small area on the side of the breast, that Mr. Frank E. Beddard, who has kindly examined a specimen, says that “this area lies upon the pectoral muscles, and near to the point where the fibers of the muscle converge towards their attachment to the humerus. The plumes arise, therefore, close to the most powerful muscle of the body, and near to where the activities of that muscle would be at a maximum. Furthermore, the area of attachment of the plumes is just above the point where the arteries and nerves for the supply of the pectoral muscles, and neighboring regions, leave the interior of the body. The area of attachment of the plume is, also, as you say in your letter, just above the junction of the coracoid and sternum.” “Ornamental plumes of considerable size rise from the same part in many other species of paradise birds, sometimes extending laterally in front, so as to form breast shields. They also occur in many hummingbirds, and in some sun birds and honey-suckers; and in all these cases there is a wonderful amount of activity and rapid movement, indicating a surplus of vitality, which is able to manifest itself in the development of these accessory plumes.”[7]
There are two serious defects in such an attempt to explain the facts. In the first place, it has been shown in several cases that have been studied that it is not the lessened “vitality” of the female but the suppression caused by the ovary that keeps down the development of the full plumage in that sex. In the second place, the anatomical influences appealed to are imaginary rather than real, for it is by no means apparent that the local exits of blood-vessels and nerves to muscles are at all correlated with the location of the ornamental parts, in the skin. Even when larger blood-vessels run to the region of excessive development of feather ornaments it may well be that they go there because the ornaments in question use them for their nourishment; in other words, Wallace puts the cart before the horse. The top of the head, where crests so often develop, the throat coloration and throat shields of hummingbirds and birds of paradise, the two long tail feathers of several species of hummingbirds, etc., do not arise, so far as known, from regions which are conspicuous for a rich supply of blood and nerves. Wallace’s appeal to underlying organs such as muscles that supposedly influence the special development of the feathers in the skin above does not strike one as a fortunate appeal to physiological principles.