ORGANIC E VOL UTION. 27 



evolution infers, from the whole structure of these 

 animals, that their progenitors must have been 

 terrestrial quadrupeds of some kind, which be- 

 came aquatic in their habits. Now the change in 

 the conditions of their life thus brought about 

 would render desirable great modifications of 

 structure. These changes would, in the first 

 instance, begin to affect the least typical — that is, 

 the least strongly inherited structures — such as 

 the skin, claws, and teeth, &c. But as time went 

 on, the adaptation would begin to extend to the 

 more typical structures, until the shape of the 

 body began to be affected by the bones and 

 muscles required for terrestrial locomotion be- 

 coming better adapted for aquatic locomotion, 

 and the whole outline of the animal more 

 fish-like in shape. This is the stage which 

 we actually observe in the seals, where the 

 hind legs, although retaining all their typical 

 bones, have become shortened up almost to 



