In a charming volume entitled The Naturalist in La Plata (1892) Mr. W. H. Hudson included a remarkable chapter on "Music and Dancing in Nature." In this chapter he described many of the dances, songs, and love-antics of birds, but regarded all such phenomena as merely "periodical fits of gladness." While, however, we may quite well agree with Mr. Hudson that conscious sexual gratification on the part of the female is not the cause of music and dancing performances in birds, nor of the brighter colors and ornaments that distinguish the male, such an opinion by no means excludes the conclusion that these phenomena are primarily sexual and intimately connected with the process of tumescence in both sexes. It is noteworthy that, according to H. E. Howard ("On Sexual Selection in Birds," Zoölogist, Nov., 1903), color is most developed just before pairing, rapidly becoming less beautiful—even within a few hours—after this, and the most beautiful male is most successful in getting paired. The fact that, as Mr. Hudson himself points out, it is at the season of love that these manifestations mainly, if not exclusively, appear, and that it is the more brilliant and highly endowed males which play the chief part in them, only serves to confirm such a conclusion. To argue, with Mr. Hudson, that they cannot be sexual because they sometimes occur before the arrival of the females, is much the same as to argue that the antics of a kitten with a feather or a reel have no relationship whatever to mice. The birds that began earliest to practise their accomplishments would probably have most chance of success when the females arrived. Darwin himself said that nothing is commoner than for animals to take pleasure in practising whatever instinct they follow at other times for some real good. These manifestations are primarily for the sake of producing sexual tumescence, and could not well have been developed to the height they have reached unless they were connected closely with propagation. That they may incidentally serve to express "gladness" one need not feel called upon to question.
Another observer of birds, Mr. E. Selous, has made observations which are of interest in this connection. He finds that all bird-dances are not nuptial, but that some birds—the stone-curlew (or great plover), for example—have different kinds of dances. Among these birds he has made the observation, very significant from our present point of view, that the nuptial dances, taken part in by both of the pair, are immediately followed by intercourse. In spring "all such runnings and chasings are, at this time, but a part of the business of pairing, and one divines at once that such attitudes are of a sexual character.... Here we have a bird with distinct nuptial (sexual) and social (non-sexual) forms of display or antics, and the former as well as the latter are equally indulged in by both sexes." (E. Selous, Bird Watching, pp. 15-20.)
The same author (ibid., pp. 79, 94) argues that in the fights of two males for one female—with violent emotion on one side and interested curiosity on the other—the attitude of the former "might gradually come to be a display made entirely for the female, and of the latter a greater or less degree of pleasurable excitement raised by it, with a choice in accordance." On this view the interest of the female would first have been directed, not to the plumage, but to the frenzied actions and antics of the male. From these antics in undecorated birds would gradually develop the interest in waving plumes and fluttering wings. Such a dance might come to be of a quite formal and non-courting nature.
Last, we owe to Professor Häcker what may fairly be regarded, in all main outlines, as an almost final statement of the matter. In his Gesang der Vögel (1900) he gives a very clear account of the evolution of bird-song, which he regards as the most essential element in all this group of manifestations, furnishing the key also to the dancing and other antics. Originally the song consists only of call-cries and recognition-notes. Under the parallel influence of natural selection and sexual selection they become at the pairing season reflexes of excitement and thus develop into methods of producing excitement, in the male by the muscular energy required, and in the female through the ear; finally they become play, though here also it is probable that use is not excluded. Thus, so far as the male bird is concerned, bird-song possesses a primary prenuptial significance in attracting the female, a secondary nuptial significance in producing excitement (p. 48). He holds also that the less-developed voices of the females aid in attaining the same end (p. 51). Finally, bird-song possesses a tertiary extranuptial significance (including exercise play, expression of gladness). Häcker points out, at the same time, that the maintenance of some degree of sexual excitement beyond pairing time may be of value for the preservation of the species, in case of disturbance during breeding and consequent necessity for commencing breeding over again.
Such a theory as this fairly coincides with the views brought forward in the preceding pages,—views which are believed to be in harmony with the general trend of thought today,—since it emphasizes the importance of tumescence and all that favors tumescence in the sexual process. The so-called esthetic element in sexual selection is only indirectly of importance. The male's beauty is really a symbol of his force.
It will be seen that this attitude toward the facts of tumescence among birds and other animals includes the recognition of dances, songs, etc., as expressions of "gladness." As such they are closely comparable to the art manifestations among human races. Here, as Weismann in his Gedanken über Musik has remarked, we may regard the artistic faculty as a by-product: "This [musical] faculty is, as it were, the mental hand with which we play on our own emotional nature, a hand not shaped for this purpose, not due to the necessity for the enjoyment of music, but owing its origin to entirely different requirements."
The psychological significance of these facts has been carefully studied and admirably developed by Groos in his classic works on the play instinct in animals and in men.[[27]] Going beyond Wallace, Groos denies conscious sexual selection, but, as he points out, this by no means involves the denial of unconscious selection in the sense that "the female is most easily won by the male who most strongly excites her sexual instincts." Groos further quotes a pregnant generalization of Ziegler: "In all animals a high degree of excitement of the nervous system is necessary to procreation, and thus we find an excited prelude to procreation widely spread."[[28]] Such a stage, indeed, as Groos points out, is usually necessary before any markedly passionate discharge of motor energy, as may be observed in angry dogs and the Homeric heroes. While, however, in other motor explosions the prelude may be reduced to a minimum, in courtship it is found in a highly marked degree. The primary object of courtship, Groos insists, is to produce sexual excitement.
It is true that Groos's main propositions were by no means novel. Thus, as I have pointed out, he was at most points anticipated by Tillier. But Groos developed the argument in so masterly a manner, and with so many wide-ranging illustrations, that he has carried conviction where the mere insight of others had passed unperceived. Since Darwin wrote the Descent of Man the chief step in the development of the theory of sexual selection has been taken by Groos, who has at the same time made it clear that sexual selection is largely a special case of natural selection.[[29]] The conjunction of the sexes is seen to be an end only to be obtained with much struggle; the difficulty of achieving sexual erethism in both sexes, the difficulty of so stimulating such erethism in the female that her instinctive coyness is overcome, these difficulties the best and most vigorous males,[[30]] those most adapted in other respects to carry on the race, may most easily overcome. In this connection we may note what Marro has said in another connection, when attempting to answer the question why it is that among savages courtship becomes so often a matter in which persuasion takes the form of force. The explanation, he remarks, is yet very simple. Force is the foundation of virility, and its psychic manifestation is courage. In the struggle for life violence is the first virtue. The modesty of women—in its primordial form consisting in physical resistance, active or passive, to the assaults of the male—aided selection by putting to the test man's most important quality, force. Thus it is that when choosing among rivals for her favors a woman attributes value to violence.[[31]] Marro thus independently confirms the result reached by Groos.
The debate which has for so many years been proceeding concerning the validity of the theory of sexual selection may now be said to be brought to an end. Those who supported Darwin and those who opposed him were, both alike, in part right and in part wrong, and it is now possible to combine the elements of truth on either side into a coherent whole. This is now beginning to be widely recognized; Lloyd Morgan,[[32]] for instance, has readjusted his position as regards the "pairing instinct" in the light of Groos's contribution to the subject. "The hypothesis of sexual selection," he concludes, "suggests that the accepted male is the one which adequately evokes the pairing impulse.... Courtship may thus be regarded from the physiological point of view as a means of producing the requisite amount of pairing hunger; of stimulating the whole system and facilitating general and special vascular changes; of creating that state of profound and explosive irritability which has for its psychological concomitant or antecedent an imperious and irresistible craving.... Courtship is thus the strong and steady bending of the bow that the arrow may find its mark in a biological end of the highest importance in the survival of a healthy and vigorous race."
Having thus viewed the matter broadly, we may consider in detail a few examples of the process of tumescence among the lower animals and man, for, as will be seen, the process in both is identical. As regards animal courtship, the best treasury of facts is Brehm's Thierleben, while Büchner's Liebe und Liebes-Leben in der Thierwelt is a useful summary; the admirable discussion of bird-dancing and other forms of courtship in Häcker's Gesang der Vögel, chapter iv, may also be consulted. As regards man, Wallaschek's Primitive Music, chapter vii, brings together much scattered material, and is all the more valuable since the author rejects any form of sexual selection; Hirn's Origins of Art, chapter xvii, is well worth reading, and Finck's Primitive Love and Love-stories contains a large amount of miscellaneous information. I have preferred not to draw on any of these easily accessible sources (except that in one or two cases I have utilized references they supplied), but here simply furnish illustrations met with in the course of my own reading.