4 CLASS REPTILIA. 



with the species, which may, at pleasure, be rendered 

 viviparous by retarding their laying.* 



The quantity of respiration in reptiles is not fixed, 

 like that of mammifera and birds, but varies with 

 the proportion which the diameter of the pulmonary 

 artery bears to that of the aorta. Thus tortoises,^ 

 and lizards, respire considerably more than frogs, &c. 

 From this proceed differences of energy and sensi- 

 bility, much greater than can exist between one 

 mammiferous animal and another, or one bird and 

 another. 



Accordingly, the reptiles exhibit forms, move- 

 ments, and properties, much more various than the 

 two preceding classes ; and it is more especially in 

 their production that nature seems to have sported 

 in the formation of fantastic shapes, and to have 

 modified, in all possible ways, the general plan which 

 she has followed for vertebrated animals, and espe- 

 cially for the viviparous classes. 



The comparison of their quantity of respiration, 

 and their organs of motion, has, however, given a 

 foundation for M. Brogniart, to divide them into 

 four orders, viz, : — 



I. The Chelonians, or Tortoises (Chelonia), 

 in which the heart has two auricles, and the body 

 supported on four legs, is enveloped by two 



* For example, colubri when deprived of water, according to the experi- 

 ment of M, Geoftroy. 



