d all that favors
tumescence in the sexual process. The so-called esthetic element
in sexual selection is only indirectly of importance. The male's
beauty is really a symbol of his force.

It will be seen that this attitude toward the facts of tumescence
among birds and other animals includes the recognition of dances,
songs, etc., as expressions of "gladness." As such they are
closely comparable to the art manifestations among human races.
Here, as Weismann in his _Gedanken ueber Musik_ has remarked, we
may regard the artistic faculty as a by-product: "This [musical]
faculty is, as it were, the mental hand with which we play on our
own emotional nature, a hand not shaped for this purpose, not due
to the necessity for the enjoyment of music, but owing its origin
to entirely different requirements."

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 s