40 ANIMALIA VERTEBRATA. 
of their respiratory organ, and to that of fishes, from the nature of their 
surrounding element. 
The quantity of respiration in birds is even superior to that of quadru- 
peds, not only because they have a double circulation and an aerial respi- 
ration, but also because they respire by many other cavities besides. the 
lungs, the air penetrating throughout their bodies, and bathing the 
branches of the aorta, as well as those of the pulmonary artery. 
Hence result the four different kinds of motion for which the four classes 
of vertebrated animals are more particularly designed: quadrupeds, in 
which the quantity of respiration is moderate, are generally formed to 
walk and run, both motions being characterized by precision and vigour; 
birds, which have more of it, possess the muscular strength and lightness 
requisite for flight; reptiles, where it is diminished, are condemned to 
creep, and many of them pass a portion of their lives in a kind of torpor; 
fishes, in fine, to execute their motions, require to be supported in a fluid 
whose specific gravity is nearly as great as their own. 
All the circumstances of organization peculiar to each of these four 
classes, and those especially which regard motion and the external sensa- 
tions, have a necessary relation with these essential characters. 
The mammalia, however, have particular characters in their viviparous 
mode of generation, in the manner by which the foetus is nourished in the 
uterus through the medium of the placenta, and in the mamme by which 
they suckle their young. 
The other classes, on the contrary, are oviparous, and if we compare 
them to the first, we shall find such numerous points of resemblance as 
announce a peculiar system of organization in the great general plan of 
the vertebrata, 
