THEORY OF THE EARTH. 47 



explaining the changes that had taken place in 

 the level of the sea ; with another, it consisted in 

 accounting for the solution of all terrestrial sub- 

 stances in one and the same menstruum ; and with 7 

 a third, in shewing how animals that were be- 

 lieved to be natives of the torrid zone could live 

 in the frigid zone. Exhausting all the powers 

 of the mind upon these questions, they conceived 

 that they had done every thing that was neces- 

 sary when they had contrived some method of 

 answering them ; and yet, while they neglected 

 all the other phenomena, they did not always 

 think of determining with precision the measure 

 and limits of those which they had endeavoured 

 to explain. 



This is peculiarly the case with regard to the 

 secondary formations, which constitute, however, 

 the most important and most difficult part of 

 the problem. During a long time, all that was 

 done with respect to these, consisted of feeble at- 

 tempts to determine the order of superposition of 

 their strata, and the connections of these strata 

 with the species of animals and plants whose re- 

 mains they contain. 



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 what 

 those which succeed ? Do these two kinds of 

 species sometimes accompany each other ? Are 



