40 ANIMALIA VERTEBRATA. 



ration, and all their qualities which depend on it, vary with the 

 amount of blood thrown into the lungs at each pulsation. 



Fishes have a double circulation, but their organ of respiration 

 is formed to execute its function through the medium of water; and 

 their blood is only acted on by the portion .of oxygen it contains, so 

 that the quantity of their respiration is perhaps less than that of 

 reptiles. 



In the Mammalia the circulation is double, and the aerial respi- 

 ration simple, that is, it is performed in the lungs only; their quan- 

 tity of respiration is, consequently, superior to that of Reptiles, on 

 account of the form 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 

 Quadrupeds, not only because they have a double circulation and an 

 aerial respiration, but also because they respire by many other cavi- 

 ties besides the lungs, the air penetrating throughout their bodies, 

 and bathing the branches of the aorta, as well as those of the pul- 

 monary artery. 



Hence result the four different kinds of motion for which the four 

 classes of verteb rated 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 gra- 

 vity 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 ex- 

 ternal sensations, have a necessary relation with these essential 

 characters. 



The Mammalia, however, have particular characters in their 

 viviparous mode of production, in the mammse by which they suckle 

 their young, &c. 



The other classes, on the contrary, are oviparous, and if we com- 

 pare them to the first, we shall find such numerous points of resem- 

 blance as announce a peculiar system of organization in the great 

 general plan of the Vertebrata. 



