ORGANIC EVOLUTION. 29 



than swimming. Similarly, the head, although 

 it so closely resembles the head of a fish in 

 shape, still retains the bones of the mammalian 

 skull in their proper anatomical relation to one 

 another, but modified in form so as to offer 

 the least possible amount of resistance to the 

 water. In short it may be said that all the 

 modifications have been effected with the least 

 possible divergence from the typical mammalian 

 type, which is compatible with securing so perfect 

 an adaptation to a purely aquatic mode of life. 



Now I have chosen the case of the whale and 

 porpoise group because they offer so extreme an 

 example of profound modification of structure in 

 adaptation to changed conditions of life. But the 

 same thing may be seen in hundreds and hundreds 

 of other cases. For instance, to confine our 

 attention to the arm, not only is the limb 

 modified in the whale for swimming, but in 

 another mammal — the bat — it is modified for 



