THEORY OF THE EARTH. 65 



culty with one consisted in explaining the change 

 which had taken place on the level of the seas ; 

 with another it consisted in accounting for the so- 

 lution of all terrestrial substances in the same fluid ; 

 and with a third, it consisted in showing how ani- 

 mals that were natives of the torrid could live un- 

 der the frigid zone. Exhausting the whole of their 

 ingenuity on these questions, they conceived that 

 they had done every thing that was necessary, 

 when they had contrived some method of answer- 

 ing them ; and yet, while they neglected all the 

 other phenomena, they did not always think of 

 determining with precision the measure and ex- 

 tent of those which they attempted to explain. 

 This is peculiarly the case in regard to the secon- 

 dary stratifications, which constitute, however, the 

 most difficult and most important portion of the 

 problem. It has hardly ever been attempted care- 

 fully to ascertain the superpositions of their strata, 

 or the connexions of these strata with the species 

 of animals and of plants whose remains they en- 

 close. 



Are there certain animals and plants peculiar to 

 certain strata, and not found in others ? What are 

 the species that appear first in order, and those 

 which succeed ? Do these two kinds of species 

 ever accompany one another? Are there altera- 

 tions in their appearances ; or, in other words, does 

 the first species appear a second time, and does 

 the second species then disappear ? Have these ani- 



9 



