40 CATEGORIES OF CLASSIFICATION. 
that class being founded upon a particular mode 
of execution of the plan characteristic of the Ver- 
tebrates, while the order to which the Whales 
belong depends upon their complication of struc- 
ture, as compared with other members: of the 
same class. 
We may therefore say that neither form nor 
complication of structure distinguishes classes, 
but simply the mode of execution of a plan. In 
Vertebrates, for instance, how do we distinguish 
the class of Mammalia from the other classes of 
the type? By the peculiar development of the 
brain, by their breathing through lungs, by their 
double circulation, by their bringing forth living 
young and nursing them with milk. In this 
class the beasts of prey form a distinct order, 
superior to the Whales or the herbivorous ani- 
mals, on account of the higher complication of 
their structure; and for the same reason we 
place the Monkeys above them all. But among 
the beasts of prey we distinguish the Bears, as a 
family, from the family of Dogs, Wolves, and 
Cats, on account of their different form, which 
does not imply a difference either in the compli- 
cation of their structure or in the mode of execu- 
tion of their plan. 
