36 OLD WORLD AND NEW WORLD CUCKOOS. [CH. I 



or the voice organ has moved down the bronchus pro- 

 ducing that form of syrinx known as the " bronchial." 

 These two changes have in no case occurred simul- 

 taneously. We find either the one or the other. But 

 there is no correspondence between the change in 

 structure and range in space. In both the Old and 

 the New World we meet with cuckoos like Crotophaga 

 and Geococcyx (America) and Centropus (Old World) in 

 which the syrinx is bronchial and the muscles all present. 

 In both hemispheres are cuckoos like our common 

 Cuculus canorus and the American Piaya, in which the 

 trachea has retained the typical form, while the accessory 

 femoro-caudal muscle has completely vanished. If M. 

 Milne-Edwards is right in identifying a fossil cuckoo from 

 the Miocene of France as a Coucal (Centropus) the 

 problem is not rendered any easier. It shows, however, 

 that at this period the two main divisions of the family 

 were differentiated in Europe, whence they may have 

 spread over the world. It must be noted that the genera 

 of the New World are nearly all distinct from those of the 

 Old World, and that it is possible, so far as our present 

 knowledge goes, to distinguish into sub-families by the 

 differences in the arrangement of the feather tracts the 

 New World from the Old World representatives of the 

 two chief subdivisions of the group. This affords an 

 example of a frequently recurring series of facts. It is 

 not of any interest to point out that the American genera 

 are in every case distinct from the African or Indian 

 genera and then to leave the matter. We have to account 

 for, or to attempt to account for, the mutual relationships 



