6 EABLY PROGRESS 



mode of reproduction, as well as by their res- 

 piration and circulation, belong to this class as 

 much as the Quadrupeds, - as, for instance, all 

 the Cetaceans (Whales, Porpoises, and the like), 

 which, though they have not legs, nor are their 

 bodies covered with hair or fur, yet bring forth 

 living young, nurse them with milk, are warm- 

 blooded and air-breathing. As more was learned 

 of these animals, there arose serious discussion 

 and criticism among contemporary naturalists 

 respecting the classification of Linnaeus, all of 

 which led to a clearer insight into the true re- 

 lations among animals. Linnaeus himself, in his 

 last edition of the " Systema Naturae," shows us 

 what important progress he had made since he 

 first announced his views ; for he there substi- 

 tutes for the name of Quadrupedia that of Mam- 

 malia, including among them the Whales, which 

 he characterizes as air-breathing, warm-blooded, 

 and bringing forth living young which they nurse 

 with milk. Thus the very deficiencies of his 

 classification stimulated naturalists to new criti- 

 cism and investigation into the true limits of 

 classes, and led to the recognition of one most 

 important principle, that such groups are 

 founded, not on external appearance, but on 

 internal structure, and that internal structure, 

 therefore, is the thing to be studied. The group 

 of Quadrupeds was not the only defective one 



