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.

The debate which has for so many years been proceeding concerning the validity of the theory of sexual selection may now be said to be brought to an end.  Those who supported Darwin and those who opposed him were, both alike, in part right and in part wrong, and it is now possible to combine the elements of truth on either side into a coherent whole.  This is now beginning to be widely recognized; Lloyd Morgan,[32] for instance, has readjusted his position as regards the “pairing instinct” in the light of Groos’s contribution to the subject.  “The hypothesis of sexual selection,” he concludes, “suggests that the accepted male is the one which adequately evokes the pairing impulse....  Courtship may thus be regarded from the physiological point of view as a means of producing the requisite amount of pairing hunger; of stimulating the whole system and facilitating general and special vascular changes; of creating that state of profound and explosive irritability which has for its psychological concomitant or antecedent an imperious and irresistible craving....  Courtship is thus the strong and steady bending of the bow that the arrow may find its mark in a biological end of the highest importance in the survival of a healthy and vigorous race.”