6 EARLY 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 Linnzus, all of 
which led to a clearer insight into the true re- 
lations among animals. Linnzus himself, in his 
last edition of the “ Systema Nature,” 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 
