such terms borrowed from human psychology. The female responds to the stimulation of the male at the right moment just as the tree responds to the stimulation of the warmest days in spring. We should but obscure this fact by stating that the tree "chooses" the most beautiful days on which to put forth its young sprouts. In explaining the correlation between responsive females and accomplished males the supposition of esthetic choice is equally unnecessary. It is, however, interesting to observe that, though Darwin failed to see that the love-combats, pursuits, dances, and parades of the males served as a method of stimulating the impulse of contrectation--or, as it would be better to term it, tumescence--in the male himself,[23] he to some extent realized the part thus played in exciting the equally necessary activity of tumescence in the female. The justification for using the term "tumescence," which I here propose, is to be found in the fact that vascular congestion, more especially of the parts related to generation, is an essential preliminary to acute sexual desire. This is clearly brought out in Heape's careful study of the "sexual season" in mammals. Heape distinguishes between the "pro-estrum," or preliminary period of congestion, in female animals and the immediately following "estrus," or period of desire. The latter period is the result of the former, and, among the lower animals at all events, intercourse only takes place during the estrus, not during the pro-estrum. Tumescence must thus be obtained before desire can become acute, and courtship runs _pari passu_ with physiological processes. "Normal estrus," Heape states, "occurs in conjunction with certain changes in the uterine tissue, and this is accompanied by congestion and stimulation or irritation of the copulatory organs.... Congestion is invariably present and is an essential condition.... The first sign of pro-estrum noticed in the lower mammals is a swollen and congested vulva and a general restlessness, excitement, or uneasiness. There are other signs familiar to breeders of various mammals, such as the congested conjunctiva of the rabbit's eye and the drooping ears of the pig. Many monkeys exhibit congestion of the face and nipples, as well as of the buttocks, thighs, and neighboring parts; sometimes they are congested to a very marked extent, and in some species a swelling, occasionally prodigious, of the soft tissues round the anal and generative openings, which is also at the time brilliantly congested, indicates the progress of the pro-estrum.... The growth of the stroma-tissue [in the uterus of monkeys during the pro-estrum] is rapidly followed by an increase in the number and size of the vessels of the stroma; the whole becomes richly supplied with blood, and the surface is flushed and highly vascular.