essential element in all this group of manifestations, furnishing
the key also to the dancing and other antics. Originally the song
consists only of call-cries and recognition-notes. Under the
parallel influence of natural selection and sexual selection they
become at the pairing season reflexes of excitement and thus
develop into methods of producing excitement, in the male by the
muscular energy required, and in the female through the ear;
finally they become play, though here also it is probable that
use is not excluded. Thus, so far as the male bird is concerned,
bird-song possesses a primary prenuptial significance in
attracting the female, a secondary nuptial significance in
producing 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).
Häcker 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