[22] This is the theory which by many has alone been seen in Darwin's _Descent of Man_. Thus even his friend Wallace states unconditionally (_Tropical Nature_, p. 193) that Darwin accepted a "voluntary or conscious sexual selection," and seems to repeat the same statement in _Darwinism_ (1889), p. 283. Lloyd Morgan, in his discussion of the pairing instinct in _Habit and Instinct_ (1896), seems also only to see this side of Darwin's statement. [23] In his _Variation of Animals and Plants under Domestication_, Darwin was puzzled by the fact that, in captivity, animals often copulate without conceiving and failed to connect that fact with the processes behind his own theory of sexual selection. [24] Beaunis, _Sensations Internes_, ch. v, "Besoins Sexuels," 1889. It may be noted that many years earlier Burdach (in his _Physiologie als Erfahrungswissenschaft_, 1826) had recognized that the activity of the male favored procreation, and that mental and physical excitement seemed to have the same effect in the female also. [25] It is scarcely necessary to point out that this is too extreme a position. As J.G. Millais remarks of ducks (_Natural History of British Ducks_, p. 45), in courtship "success in winning the admiration of the female is rather a matter of persistent and active attention than physical force," though the males occasionally fight over the female. The ruff (_Machetes pugnax_) is a pugnacious bird, as his name indicates. Yet, the reeve, the female of this species, is, as E. Selous shows ("Sexual Selection in Birds," _Zoölogist_, Feb. and May, 1907), completely mistress of the situation. "She seems the plain and unconcerned little mistress of a numerous and handsome seraglio, each member of which, however he flounce and bounce, can only wait to be chosen." Any fighting among the males is only incidental and is not a factor in selection. Moreover, as R. Müller points out (loc. cit., p. 290), fighting would not usually attain the end desired, for if the males expend their time and strength in a serious combat they merely afford a third less pugnacious male a better opportunity of running off with the prize. [26] L. Tillier, _L'Instinct Sexuel_, 1889, pp. 74, 118, 119, 124 et seq., 289. [27] K. Groos, _Die Spiele der Thiere_, 1896; _Die Spiele der Menschen_, 1899; both are translated into English. [28] Prof. H.E. Ziegler, in a private letter to Professor Groos, _Spiele der Thiere_, p. 202. [29] _Die Spiele der Thiere_, p. 244. This had been briefly pointed out by earlier writers. Thus, Haeckel (_Gen. Morph._, ii, p. 244) remarked that fighting for females is a special or modified kind of struggle for existence, and that it acts on both sexes.