THEORY OF THE EARTH. 55 



selves have proved ; and it is by a careful investi- 

 gation of them, that we may hope to ascertain the 

 nnmber and the epochs of these irruptions. 



Secondly, The nature of the revolutions which 

 have altered the surface of the globe, must have 

 exerted a more powerful action upon terrestrial 

 quadrupeds, than upon marine animals. As these 

 revolutions have consisted chiefly of changes in 

 the bed of the sea, and as the waters must have 

 destroyed all the quadrupeds which they reached, 

 if their irruption was general, it would necessa- 

 rily have destroyed the entire class ; or if it only 

 overwhelmed certain continents at one time, it 

 would at least have destroyed the species pecu- 

 liar to those continents, without having the same 

 effect upon the marine animals. On the other 

 hand, millions of aquatic animals would have been 

 left dry, or buried under newly-formed strata, or 

 thrown violently on the coasts ; while their races 

 would still have been preserved in some more 

 peaceful parts of the sea, whence they might again 

 be propagated after the agitation of the waters 

 had ceased. 



Thirdly, This more complete action is also 

 more easily ascertained. It is more easy to de- 

 monstrate its effects, because, the number of qua- 

 drupeds being limited, and the greater part of 

 their species, at least the large ones, being known, 

 we have more means of determining whether fos- 



