12 THEORY OF THE EARTH. 



species, and even their genera, changed with the 

 strata ; and, although the same species occasion- 

 ally recur at small distances, it may he announced 

 as a general truth, that the shells of the ancient 

 strata have forms peculiar to themselves; that 

 they gradually disappear, so as no longer to he 

 seen at all in the recent strata, and still less in 

 the presently existing ocean, in which their cor- 

 responding species are never discovered, and where 

 several, even of their genera, do not occur : that, 

 on the contrary, the shells of the recent strata are 

 similar, in respect to their genera, to those which 

 exist in our seas ; and that, in the latest and least 

 consolidated of these strata, and in certain recent 

 and limited deposits, there are some species which 

 the most experienced eye could not distinguish 

 from those which are found in the neighbouring 

 seas. 



There has, therefore, been a succession of va- 

 riations in the economy of organic nature, which 

 has been occasioned by those of the fluid in which 

 the animals lived, or which has at least corres- 

 ponded with them ; and these variations have 

 gradually conducted the classes of aquatic ani- 

 mals to their present state, till, at length, at the 

 time when the sea retired from our continents for 

 the last time, its inhabitants did not differ much 

 from those which are found in it at the present 

 day. 



