ANIMALTA 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: 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 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 

 fcetus is nourished in the uterus through the medium of the 

 placenta, and in the mammae 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. 



