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.