emale--with violent emotion on one side and
interested curiosity on the other--the attitude of the former
"might gradually come to be a display made entirely for the
female, and of the latter a greater or less degree of pleasurable
excitement raised by it, with a choice in accordance." On this
view the interest of the female would first have been directed,
not to the plumage, but to the frenzied actions and antics of the
male. From these antics in undecorated birds would gradually
develop the interest in waving plumes and fluttering wings. Such
a dance might come to be of a quite formal and non-courting
nature.

Last, we owe to Professor Haecker what may fairly be regarded, in
all main outlines, as an almost final statement of the matter. In
his _Gesang der Voegel_ (1900) he gives a very clear account of
the evolution of bird-song, which he regards as the most
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).
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 an