THEORY OF THE EARTH. 35 



ing ridges, different changes took place in everj 

 separate basin. 



Amidst these changes of the general fluid, it 

 must have been almost impossible for the same 

 kind of animals to continue to live : nor did they 

 do so in fact. Their species, and even their ge- 

 nera, change with the strata; and although the 

 same species occasionally recur at small distances, 

 it is generally the case that the shells of the an- 

 cient strata have forms peculiar to themselves ; that 

 they gradually disappear, till they are not to be 

 seen at all in the recent strata, still less in the ex- 

 isting seas, in which, indeed, we never discover 

 their corresponding species, and where several, 

 even of their genera, are not to be found ; that, on 

 the contrary, the shells of the recent strata resem- 

 ble, as it respects the genus, those which still ex- 

 ist in the sea; and that in the last-formed and 

 loosest of these strata, there are some species 

 which the eye of the most expert naturalists can- 

 not distinguish from those which at present inhabit 

 the ocean. 



In animal nature, therefore, there has been a 

 succession of changes corresponding to those which 

 have taken place in the chemical nature of the 

 fluid ; and when the sea last receded from our con- 

 tinent, its inhabitants were not very different from 

 those which it still continues to support. 



