THEORY OF THE EARTH. . 73 



proved ; and, by a careful investigation of them, 

 we may hope to ascertain the number and the 

 epochs of those irruptions of the sea. 



Secondly, the nature of the revolutions which 

 have changed the surface of our earth, must have 

 exerted a more powerful action upon terrestrial 

 quadrupeds than upon marine animals. As these 

 revolutions have consisted chiefly in changes of 

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

 destroyed all the quadrupeds which they reached, 

 if their irruption over the land was general, they 

 must have destroyed the entire class, or, if con- 

 fined only to certain continents at one time, they 

 must have destroyed at least all the species in- 

 habiting these continents, without having the same 

 effect upon the marine animals. On the other 

 hand, millions of aquatic animals may have been 

 left quite dry, or buried in newly-formed strata, or 

 thrown violently on the coasts, while their races 

 may have been still preserved in more peaceful 

 parts of the sea, whence they might again propa- 

 gate and spread after the agitation of the water 

 had ceased. 



Thirdly, this more complete action is also more 

 easily ascertained and demonstrated; because, 

 as the number of terrestrial quadrupeds is limited, 

 and as most of their species, at least the large ones, 

 are well known, we can more easily determine 

 whether fossil bones belong to a species which still 



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