K. Groos, Die Spiele der Thiere, 1896; Die Spiele der Menschen, 1899; both are translated into English.
Prof. H. E. Ziegler, in a private letter to Professor Groos, Spiele der Thiere, p. 202.
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.
It may be added that in the human species, as Bray remarks ("Le Beau dans la Nature," Revue Philosophique, October, 1901, p. 403), "the hymen would seem to tend to the same end, as if nature had wished to reinforce by a natural obstacle the moral restraint of modesty, so that only the vigorous male could insure his reproduction." There can be no doubt that among many animals pairing is delayed so far as possible until maturity is reached. "It is a strict rule amongst birds," remarks J. G. Millais (op. cit., p. 46), "that they do not breed until both sexes have attained the perfect adult plumage." Until that happens, it seems probable, the conditions for sexual excitation are not fully established. We know little, says Howard (Zoölogist, 1903, p. 407), of the age at which birds begin to breed, but it is known that "there are yearly great numbers of individuals who do not breed, and the evidence seems to show that such individuals are immature."