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, and 
