Hudson, in his interesting book, “The Naturalist in La Plata,” has also criticized Darwin’s theory of sexual selection. He has brought together a considerable number of interesting observations that go to show that the displays—dancing, singing, and combats—of males and females have no relation to mating. Many of them involve birds already mated, sometimes several males participating, sometimes males and females together. Some of the tourneys he describes are more elaborate than the mating instincts themselves, yet are not concerned with mating. He attempts to explain them as overflow phenomena, i. e., as expressions of the high vitality of the males, especially at this time. If he is right, then elaborate exhibitions of these kinds have evolved that have no special connection with mating. Are we called upon for a different explanation for other differences that distinguish the sexes? One example will suffice to bring out a curious emotional (?) display that, elaborate as it is, has no apparent connection with mating (p. 269):
“The lapwing display, called by the natives its ‘dance’ or ‘serious dance’—by which they mean square dance—requires three birds for its performance, and is, so far as I know, unique in this respect. The birds are so fond of it that they indulge in it all the year round, and at frequent intervals during the day, also on moonlight nights. If a person watches any two birds for some time—for they live in pairs—he will see another lapwing, one of a neighboring couple, rise up and fly to them, leaving his own mate to guard their chosen ground; and instead of resenting this visit as an unwarranted intrusion on their domain, as they would certainly resent the approach of almost any other bird, they welcome it with notes and signs of pleasure. Advancing to the visitor, they place themselves behind it; then all three, keeping step, begin a rapid march, uttering resonant drumming notes in time with their movements; the notes of the pair behind being emitted in a stream, like a drumroll, while the leader utters loud single notes at regular intervals. The march ceases; the leader elevates his wings and stands erect and motionless, still uttering loud notes; while the other two, with puffed-out plumage and standing exactly abreast, stoop forward and downward until the tips of their beaks touch the ground, and sinking their rythmical voices to a murmur remain for some time in this posture. The performance is then over and the visitor goes back to his own ground and mate, to receive a visitor himself later on.”[8]
Cunningham, who has brought together many interesting cases of secondary sexual differences in his book on “Sexual Dimorphism in the Animal Kingdom,” attempts to show that the development of the secondary sexual characters of the males are due directly to the use of certain parts of the body during courtship—the use of the parts leading to the enlargement and excessive growth of the parts. The effects are believed by him to be inherited, and he tries, furthermore, to show the way in which such acquired characters could be inherited. He makes use of the modern idea of hormones—substances that are elaborated in many organs of the body, whose effects are often most conspicuously produced in other parts of the body. He imagines these hormones to be collected in the germ-cells and transmitted to the next generation, where their presence contributes to the further development of the special region (when it develops) that corresponds to the region in its parent in which the hormone was made. His speculation meets in the first place with the general objections inherent in Lamarck’s theory—objections so well recognized to-day that I need not go over them here. His special appeal to the hormone theory makes use of that theory in a way to which it was never intended to be put, by assuming that an internal secretion formed in one organ can be stored up in another organ, eggs and sperm—an assumption not only unsupported by any evidence, but, as I have stated, one quite foreign to the hormone theory. In fact, Cunningham’s suggestion is nothing more than Darwin’s old idea of pangens, which, being imaginary, could be endowed with all desirable properties. But one can not invoke a chemical substance, even a hormone, and then at the critical moment endow it with special virtues.
A rather unique explanation of the origin of secondary sexual characters is made by Stolzmann. His argument runs as follows: (1) There is a great excess of males in birds; (2) the males left over after mating are useless to the species, since they can not propagate and they consume food needed by the reproducing part of the population; (3) the conspicuous coloration of the male has been evolved in order that he could be seen more readily by birds of prey and the objectionable excess of males removed; the comb of the cock has developed in order that he may be the more easily killed by other cocks.
Stolzmann’s account of the origin of the plumes of the birds of paradise should be immortalized in the literature of the subject:
“Nous comprendrons aussi facilement la présence de longues plumes chez les males de nombreuses espèces, comme p. e. chez les oiseaux de paradis, chez les veuves (Vidua) et chez l’engoulevent africain (Cosmetornis). Telles plumes ont probablement pour but de relantir le vol des males. J’ai constate chez la Loddigesia mirabilis (oiseaumouche péruvien), que le vieux male posséde l’aile quelques millimetres plus courte que le jeune male ou la femelle. Cet avortement des remiges provient assurément a cause de développement extraordinaire de retrices externes chez le vieux male de cet oiseaumouche. Si donc d’une part les retrices allongees rendent le vol plus difficile et d’hautre les ailes plus petites diminuent sa vélocité, le vol du male doit ètre plus lent que celui de la femelle, le poids du corps restant la même. Le développement extraordinaire soit des remiges soit des rectrices, en relantissant le vol des males, rend leur rôle plus difficile, en facilitant en même temps celui des femelles. Nous pouvons prendre comme exemple le Cosmetornis, qui, comme tous les engoulevents, se nourrit d’insectes, qu’il attrape au vol. Chez cet oiseau quelques plumes des ailes se developpent extraordinairement pendant l’époque de reproduction, en retardant visiblement son vol. Il est donc facile a remarquer, qu’alors le male, ayant les mouvements plus lourds, n’est pas en êtat de se procurer la même quantité d’insectes qu’auparavant; ainsi donc la femelle a plus de chances de trouver une nourriture plus abondante.”[1]
Equally worthy of perpetuation is Stolzmann’s explanation of dancing and singing birds:
“Toutes les réunions des males, leurs danses bizarres, leur chant, enfin, ne servent pas probablement a séduire les femelles, mais pour distraire les males, ce qui rend plus faciles les besognes maternelles des femelles et au surplus les protege contre l’assiduite nuisible des célibataires. Darwin lui-mème constate le fait, qu’ordinairement pendant les réunions des males, quand ces derniers sont trop occupes par le combat ou la danse, la femelle s’echappe avec un d’eux pour copuler. Ainsi donc dans ce cas c’est bien la selection naturelle et non la selection sexuelle, qui agit pour la conservation d’équilibre sexuel.”[9]