22 THE WHENCE AND THE WHITHER OF MAN 



these with amphibia than to call a whale a fish, and 

 not a mammal, because of its form and aquatic life. 



When the comparative anatomy of fish, amphibia, 

 and reptiles had been carefully studied it was evident 

 that the amphibia stood far nearer the fish in general 

 structure, while the higher reptiles closely approached 

 birds. Then it was noticed that our common fish 

 formed a fairly well-defined group, but that the ganoids, 

 including the sturgeons, gar-pikes, and some others, 

 had at least traces of amphibian characteristics. Such 

 generaUzed forms, with the characteristics of the class 

 less sharply marked, were usually by common consent 

 placed at the bottom of the class. And this suited 

 well their general structure, while in particular char- 

 acteristics they were often more highly organized than 

 higher groups of the same class. 



The palaeontologist found that the oldest fossil forms 

 belonged to these generalized groups, and that more 

 highly specialized forms — that is, those in which the 

 special class distinctions were more sharply and uni- 

 versally marked — were of later geological origin. Thus 

 the oldest fish were most like our present ganoids and 

 sharks, though differing much from both. Our com- 

 mon teleost fish, like perch and cod, appeared much 

 later. The oldest bird, the archseopteryx, had a long 

 tail like that of a lizard, and teeth ; and thus stood in 

 many respects almost midway between birds and rep- 

 tiles. And most of the earliest forms were " compre- 

 hensive," uniting the characteristics of two or more 

 later groups. Thus as the classification became more 

 natural, based on a careful comparison of the whole 

 anatomy of the animals, its order was found to coincide 

 in general with that of geological succession. 



