i8o CLUES AND SUGGESTIONS CHAP. 



ment, the Mammalia must have come, 

 and if so it is not wonderful, but quite 

 natural, that we should find one branch 

 of the Mammalian type to be organisms 

 pisciform in shape, and otherwise speci- 

 ally adapted to a marine life. One 

 fundamental difference between the 

 Fishes and the Mammalia is in the 

 method and machinery for breathing, or, 

 in other words, for the oxygenation of 

 the blood. But comparative anatomists 

 tell us that in Fishes the homologue of 

 the Mammalian lung is the membranous 

 sac which is called the air-bladder. If 

 ordinary generation, doing nothing ex- 

 cept what we always see it doing now, 

 has given birth to all creatures, it must 

 have done much greater marvels than 

 converting a mere bladder of air into a 

 vascular organ for mixing that air with 

 a circulating current of blood. The 

 existence of rudiments of legs, and of a 



