140 THE BEGINNINGS OF LIFE. 



c On the other hand, this difference in the com- 

 position of the atmosphere, so favourable to the de- 

 velopment., growth, and preservation of vegetable 

 matter, must have proved a bar to the existence of 

 animals, particularly of warm-blooded animals, whose 

 respiration, as it is more active, also requires a purer 

 air : during this first period, consequently, not a single 

 animal breathing air appears to have existed. 



c During this period the atmosphere must have been 

 purged of some portion of the excess of carbonic acid 

 which it contained, by the vegetables which then existed ; 

 these assimilated it first, and subsequently buried it in 

 the state of coal in the bowels of the earth. It is after 

 this first period, in the course of our second and third 

 periods, that this immense variety of monstrous reptiles 

 makes its appearance, animals which, by the nature of 

 their respiration, are capable of living in an atmosphere 

 of much less purity than that which warm-blooded 

 animals require, and were the heralds and precursors 

 of these. 



c Vegetables continued incessantly to abstract a 

 portion of the carbon of the air, and thus rendered 



and their transformation into soil; for this kind of decomposition is 

 owing essentially to the abstraction of a portion of the carbon of the 

 wood by the oxygen of the air : and if the atmosphere contained less 

 oxygen and more carbonic acid, the decomposition in question must 

 have been without doubt both more difficult and slower. Hence the 

 accumulation of vegetable debris in extensive beds, even in circumstances 

 and from vegetables which, in the actual state of the atmosphere, would 



give rise to no such layers of combustible material. 1 



* J 



