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