422 



THE DESCENT OF MAN. 



sexes. In the Tetrao cupido (fig 39) the male has two 

 bare, orange- colored sacks, one on each side of the neck ; 

 and these are largely inflated when the male, during the 

 breeding-season, makes his curious hollow sound, audible 

 at a great distance. Audubon proved that the sound was 



'^^^g^*'- 



intimately connected with this apparatus (which reminds 

 us of the air-sacks on each side of the mouth of cer- 

 tain male frogs), for he found that the sound was 

 much diminished when one of the sacks of a tame 

 bird was pricked, and when both were pricked it 



