the Earth at different Epochs^ ^c. 369 



coal in the bowels of the earth. It is after this epoch, during 

 our second and third periods, that the immense variety of mon- 

 strous reptiles begin to appear, animals which, by their mode of 

 respiration, are yet capable of living in a much less pure air than 

 that which the warm-blooded animals require, and which, in 

 fact, have preceded them at the earth's surface. 



The vegetables continued to withdraw a part of the carbon 

 from the air, and thus rendered our atmosphere daily more pure; 

 but it was only after the appearance of a quite new vegetation, 

 rich in large trees, and the origin of the numerous deposites of 

 lignite, a vegetation which appears to have covered the surface 

 of the earth with vast forests, that a great number of mammife- 

 rous animals, resembling, in the essential features of their orga- 

 nization, those which still exist upon the earth, for the iSrst time 

 made their appearance upon its surface. * 



May it not be supposed that our atmosphere has, in this man- 

 ner, arrived at the degree of purity which could alone suit the 

 more active respiration of the warm-blooded animals, and equally 

 favour the developement of vegetables and animals ; while the si- 

 multaneous existence of these two orders of beings, and the reci- 

 procating influence of their respiration, now keep our atmosphere 

 in a state of stability, which is one of the remarkable characters 

 of the present period.. 



• In this general indication, I overlook the solitary exception resulting 

 from the existence of the Stonesfield mammiferous animal in strata inferior to 

 the chalk. 



