CHAPTER III. 



MORPHOLOGY. 



THE theory of evolution supposes that hereditary 

 characters admit of being slowly modified wherever 

 their modification will render an organism better 

 suited to a change in its conditions of life. Let 

 us, then, observe the evidence which we have of such 

 adaptive modifications of structure, in cases where 

 the need of such modification is apparent. We may 

 begin by again taking the case of the whales and 

 porpoises. The theory of evolution infers, from the 

 whole structure of these animals, that their pro- 

 genitors must have been terrestrial quadrupeds of 

 some kind, which gradually became more and more 

 aquatic in their habits. Now the change in the 

 conditions of their life thus brought about would 

 have rendered desirable great modifications of struc- 

 ture. These changes would have begun by affecting 

 the least typical that is, the least strongly inherited 

 structures, such as the skin, claws, and teeth. But, 

 as time went on, the adaptation would have ex- 

 tended to more typical structures, until the shape of 

 the body would have become affected by the bones 

 and muscles required for terrestrial locomotion be- 

 coming better adapted for aquatic locomotion, arid 



