THE BASIS OF CLASSIFICATION. 123 



another group. The last he calls vertebrate, or backboned 

 animals. 



But the vertebrates are not all planned alike. Let us sort 

 these into classes as we did the peas and beans : 



Vertebrates, creatures with a backbone 



Without a skull With a skull 



I. Lancelets | 



r ~i 



Skull imperfect Skull perfect 



II. Lampreys 



Breathing by gills all their life Not breathing by gills 

 III. Fishes all their life 



1 I 



Having a metamorphosis Not having a meta- 

 IV. Batrachians (Frogs, morphosis 



toads, etc.) 



VII. Mammals 



Thus we get seven classes of vertebrates. In some such 

 manner as this the scientist would sort out his living crea- 

 tures, and then he would test his work to see whether he had 

 made any error. He would ask questions about each group, 

 and then would make out a little table of answers. Are 

 mammals warm or cold blooded ? Do birds have lungs ? 

 How do fishes breathe? How are reptiles clothed? How 

 are mammals protected ? His table will look like this : 



