bird-dances are not nuptial, but that some birds--the
stone-curlew (or great plover), for example--have different kinds
of dances. Among these birds he has made the observation, very
significant from our present point of view, that the nuptial
dances, taken part in by both of the pair, are immediately
followed by intercourse. In spring "all such runnings and
chasings are, at this time, but a part of the business of
pairing, and one divines at once that such attitudes are of a
sexual character.... Here we have a bird with distinct nuptial
(sexual) and social (non-sexual) forms of display or antics, and
the former as well as the latter are equally indulged in by both
sexes." (E. Selous, _Bird Watching_, pp. 15-20.)

The same author (ibid., pp. 79, 94) argues that in the fights of
two males for one female--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 Häcker what may fairly be regarded, in
all main outlines, as an almost final statement of the matter. In
his _Gesang der Vögel_ (1900) he gives a very clear account of
the evolution of bird-song, which he regards as the most