[30] 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." [31] A. Marro, _La Puberté_, 1901, p. 464. [32] Lloyd Morgan, _Animal Behavior_, 1900, pp. 264-5. It may be added that, on the esthetic side, Hirn, in his study (_The Origins of Art_, 1900), reaches conclusions which likewise, in the main, concord with those of Groos. [33] It may be noted that the marriage ceremony itself is often of the nature of a courtship, a symbolic courtship, embodying a method of attaining tumescence. As Crawley, who has brought out this point, puts it, "Marriage-rites of union are essentially identical with love charms," and he refers in illustration to the custom of the Australian Arunta, among whom the man or woman by making music on the bull-roarer compels a person of the opposite sex to court him or her, the marriage being thus completed. (E. Crawley, _The Mystic Rose_, p. 318.)