excitement (p. 48).  He holds also that the less-developed voices of the females aid in attaining the same end (p. 51).  Finally, bird-song possesses a tertiary extranuptial significance (including exercise play, expression of gladness).  Haecker points out, at the same time, that the maintenance of some degree of sexual excitement beyond pairing time may be of value for the preservation of the species, in case of disturbance during breeding and consequent necessity for commencing breeding over again.

Such a theory as this fairly coincides with the views brought forward in the preceding pages,—­views which are believed to be in harmony with the general trend of thought today,—­since it emphasizes the importance of tumescence and 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.