Vol 'wi9 XVI ] Allen, Evolution of Bird-song. 531 



force at work in the universe to produce beauty, — and that, of 

 course, would be getting outside the realm of science, — how can 

 we escape imputing the origin and development of this beauty in 

 bird-song to an aesthetic sense in the birds themselves? And how 

 can we imagine an upward evolution in the beauty of the song 

 and the proficiency of the singer without postulating some form 

 of selection as the active principle? Finally, is any theory more 

 reasonable than that of sexual selection to account for the beauty 

 of bird-song? Is there, indeed, any other workable theory left 

 to us ? 



Mr. Hawkins has pretty thoroughly recapitulated the evidence 

 in favor of the hormone theory of the origin of bird-song, 1 and I 

 fail to find in his paper any argument that would apply against 

 this view of the action of sexual selection in producing and develop- 

 ing beauty in song, except the evidence he cites that display and 

 ardent singing serve the purpose of overcoming the coyness of the 

 female, and that in many cases there is no indisputable evidence 

 that the female exercises any choice between suitors (or possible 

 suitors). This is a strong argument but not an insuperable one. 

 For one thing, even though but one male may be seen with the 

 female at a given time, she may nevertheless have had opportuni- 

 ties to choose, — just as in the human species it frequently happens 

 that but one suitor is heard at a time ! More observation is needed 

 on this point. But many evolutionary questions must be decided 

 by a nice balancing of evidence and arguments, and the difficulties 

 of accounting for bird-song without admitting sexual selection as 

 an important factor seem to be far greater than those of reconciling 

 the latter with the theory of superabundant sexuality. 



If we agree that sexual selection has thus played its part, we have, 

 then, in addition to natural selection or whatever other evolution- 

 ary process may be the chief agency in the origin of species, these 

 three coordinate factors in the production of bird-song: the hor- 

 mones generated by the male sex glands originating the song- 

 impulse; invention and imitation producing the variety and fixing 



i He fails to mention a comparatively recent opinion on the other side of the question 

 in the case of Weismann, who says in his ' The Evolution Theory,' " From [the] simple 

 love-call the modern song of many species must have developed by means of sexual 

 selection." 



