252 THE EVOLUTION OF SEX. 



abdominal region at this time, it has been argued that the bird 

 merely sits to cool itself ! This view has been supported by the 

 cruel experiment of singeing off the feathers from the same 

 region in a cock, which then sat to cool the irritated surface. 

 Yet the increased circulation may also be viewed as increased 

 by the sitting ; in any case, the patience and solicitude of the 

 brooding, and the subsequent diligence in feeding the hatched 

 young, are obviously the expression of genuine parental affec- 

 tion. 



The female Surinam Toad, \vith j-oung ones on its back. — From Leunis. 



Here too one must include the retention of the young in 

 skin pouches, exhibited by the great majority of marsupial 

 mammals and by the echidna. In the latter, the pouch is a 

 simple and possibly periodic structure, arising from an insinking 

 of the skin in the mammary region of the abdomen. Here the 

 eggs are somehow or other stowed away and the young 

 developed. The milk glands simply open on the surface of the 

 depression. In most marsupials, the young, which are born 



