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 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 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 über 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."