ADAPTATIONS 



405 



Blackcap, Skylark, Thrush and Mocking-bird. This being so, 

 it is somewhat remarkable that a very large proportion of what 

 are technically known as " song "-birds should have no song, yet 

 in them the syrinx and its muscles appear to be as perfectly 

 developed as in the master performers ! How is it that these 

 muscles have not degenerated from disuse? Why are they 

 maintained in a condition of useless perfection? The Crow 

 and the Jay are conspicuous examples of songless song-birds, 

 yet the organ of voice is, so far as we can see, as perfect as in 

 the Nightingale. Similarly, the females of the Nightingale, 



G.magnirostris 



G.strenua 



G. fort is 



G.FortfS 



G.Fulginosa 



G.fjBrvula 



Forms of the beak in the genus Geospiza 



III. 45 



Thrush and other performers have the syrinx apparently as 

 perfect as in their mates, yet they sing not. 



Finally, there remain two other cases of apparently mean- 

 ingless structures associated with the windpipe that demand 

 notice here. ^ 



In the Blackcock {Lyrurus tetrix) there is found, at the 

 lower end of the trachea in the males, a relatively enormous 

 pair of oval bodies — one on either side of the windpipe— having 

 a convoluted outer surface and an almost unique structure in 



