CLASSIFICATION 105 



But it would seem that they are more alike, one genus 

 with another, in external build and internal con- 

 formation, than are either of two groups cited. There 

 are, for example, larger differences in the organs of 

 digestion among the Rodents and Ungulates than 

 are met with in the whales ; the variability of external 

 form it is hardly necessary to dwell upon. The teeth 

 differ much more in form from one Rodent genus 

 to another, or from one Ungulate genus to another, 

 than in the whales, generally speaking. 



Fishes, on the other hand, which are born and bred 

 to the aquatic life, show just as many (if not more) 

 divergences of structure as do the mammals. The 

 expression "fish -like" is, it is true, often used to 

 describe a certain shape ; but what could be more 

 utterly different in shape than a skate and an eel, 

 or a sunfish and a sole? Here we have the precise 

 converse of the case afforded by whales. The whole 

 organisation being fitted to the marine or fresh-water 

 life,, there is ample room for much variation without 

 affecting the necessary essentials. 



Bearing in mind then the profound influence which 

 the aquatic life seems to have had in moulding the 

 external as well as the internal form of whales, 

 it is not surprising that several naturalists have 

 arrived at the conclusion that those structural 

 differences which do exist argue the justice of 

 dividing the group into two great orders, the toothed 

 and the whalebone whales, which have arisen from 

 separate ancestors, and have only come to resemble 

 each other in various details owing- to " convergence," 



