ANIMALIA VERTEBRATA. 37 
The quantity of respiration in birds is even superior to that 
of quadrupeds, not only because they have a double circula- 
tion and an aerial respiration, 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: guadrupeds, 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 con- 
demned to creep, and many of them pass a portion of ‘their 
lives in a kind of torpor; fishes, in fine, to execute their mo- 
tions, 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 sensations, 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 mammez 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 or ganization 
in the great general plan of the vertebrata. 
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