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." Having thus viewed the matter broadly, we may consider in detail a few examples of the process of tumescence among the lower animals and man, for, as will be seen, the process in both is identical. As regards animal courtship, the best treasury of facts is Brehm's _Thierleben_, while Büchner's _Liebe und Liebes-Leben in der Thierwelt_ is a useful summary; the admirable discussion of bird-dancing and other forms of courtship in Häcker's _Gesang der Vögel_, chapter iv, may also be consulted. As regards man, Wallaschek's _Primitive Music_, chapter vii, brings together much scattered material, and is all the more valuable since the author rejects any form of sexual selection; Hirn's _Origins of Art_, chapter xvii, is well worth reading, and Finck's _Primitive Love and Love-stories_ contains a large amount of miscellaneous information. I have preferred not to draw on any of these easily accessible sources (except that in one or two cases I have utilized references they supplied), but here simply furnish illustrations met with in the course of my own reading.