22 THE SCIENTIFIC EVIDENCES OF 



appearance was to be taken as a guide to classi- 

 fication, the question was, What features in the 

 internal anatomy are to take precedence over 

 the other features ? And this question it was 

 not hard to answer. A porpoise, for instance, 

 has a large number of teeth, and in this feature 

 resembles most fish, while it differs from all 

 mammals. But it also gives suck to its young, 

 and in this feature it differs from all fish, while 

 it resembles all mammals. Now, looking at 

 those two features alone, should we say that a 

 porpoise ought to be classed as a fish or as a 

 mammal ? Assuredly as a mammal, and for 

 this reason : The number of teeth is a very 

 variable feature both in fish and in mammals, 

 whereas the giving of suck is an invariable 

 feature among mammals, and occurs nowhere 

 else in the animal kingdom. This, of course, 

 is purposely chosen as a very simple illustration ; 

 but it exemplifies the general fact that the 



