Cuvierian System qfZoology, 133 



Birds exceed mammiferous quadrupeds in the quantity of 

 their respiration, for they have not only a double circulation, 

 and an aerial respiration, but they respire also through other 

 cavities besides the lungs, the air penetrating through the 

 whole body, and bathing the branches of the aorta or great 

 artery of the body, as well as those of the pulmonary artery. 



Fishes have a double circulation, but their respiratory or- 

 gans (the gills) are only formed to respire by the intervention 

 of water, and their blood only receives the portion of oxygen 

 dissolved or mixed in the water, so that the quantity of re- 

 spiration is, perhaps, less than that of the next class. Reptiles. 



Reptiles. The organs of circulation are simple, and only 

 a portion of the blood brought back by the veins passes 

 through the organs of respiration. Their quantity of respir- 

 ation, and all the other qualities that depend on it, vary ac- 

 cording to the proportion of blood which enters the lungs at 

 each pulsation. 



In mammiferous quadrupeds the quantity of respiration is 

 less than that of birds ; but it is greater than that of reptiles, 

 on account of the structure of the respiratory organs ; and ex- 

 ceeds that of fishes, on account of the different elements in 

 which they live. Hence result the four kinds of movements, 

 which the four classes of vertebrated animals are particularly 

 destined to exert. 



Mammiferous animals, in which the quantity of respiration 

 is moderate, are generally formed to develope their strength in 

 walking or running. Birds, which have a larger quantity of 

 respiration, have the activity and strength of muscles neces- 

 sary for flying. Reptiles, in which respiration is more feeble, 

 are condemned to crawl ; and many of them pass a part of 

 their lives in a kind of torpor. Fishes require to be sup- 

 ported in an element nearly as heavy as themselves, in order 

 to exert their proper motions in swimming. 



All the circumstances of organisation proper to each of the 

 four classes, and particularly those which regard their move- 

 ments and exterior sensations, have a necessary relation with 

 their essential characters ; nevertheless, the class of mammi- 

 ferous animals has particular characters belonging to their 

 viviparous generation, the manner in which the foetus is 

 nourished in the womb by means of the placenta, and the 

 teats with which they suckle their young. On the contrary, 

 the other three classes are oviparous ; and if we contrast them 

 together with the first class, we shall find certain resem- 

 blances which indicate in the three classes, birds, fishes, and 

 reptiles, a special plan of organisation, comprised in the 

 general plan of all vertebrated animals. B. 



