CLASSES OE THE VERTEBRATED ANIMALS. 71 



branches of the aorta with the same efficiency as 

 upon those of the pulmonary artery. 



From all this result four different kinds of motion 

 for which the four classes of vertebrated animals 

 are severally and exclusively designed. Quadrupeds, 

 in which the quantity of respiration is moderate, 

 are formed for walking and running, and their pre- 

 dominant characteristic is vigour. Birds, whose 

 respiration is greater, possess the lightness and 

 strength of muscles necessary to support them in 

 their airy flight. Reptiles, which respire more feebly, 

 are doomed to creep upon the earth, and many of 

 them pass more or less of their allotted period of 

 existence in a kind of stupor. Finally, fishe&, 

 which move in a fluid almost as specifically heavy 

 as themselves, are enabled to execute their peculiar 

 motions by an arrangement altogether different 

 from the rest. 



Every peculiarity of organization proper to each 

 of these classes, and especially such as belong to 

 motion and external sensation, have a close and 

 necessary relation with the essential characters of 

 each just enumerated. 



In the mammalia, however, there are also certain 

 other peculiarities besides the above relative charac- 

 ters. These are their viviparous generation ; the 

 mode in which the foetus is nourished in the womb 

 by means of the placenta ; and the teats, by which 

 they afford nutriment to their young. 



The other three classes on the contrary are ovi- 

 parous, and when compared with the mammalia, 



