148 LAMARCK, HIS LIFE AND WORK 



We now come to the Cuvierian doctrine par ex- 

 cellence, one in which he radically differs from La- 

 marck's views as to the genetic relations between the 

 organisms of successive strata. 



" Amid 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 genera, change 

 with the strata, and although the same species occa- 

 sionally recur at small distances, it is generally the 

 case that the shells of the ancient 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 existing 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 

 resemble, as regards the genus, those which still exist 

 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 cannot distinguish from 

 those which at present inhabit the ocean. 



" In animal nature, therefore, there has been a suc- 

 cession of changes corresponding to those which have 

 taken place in the chemical nature of the fluid ; and 

 when the sea last receded from our continent its in- 

 habitants were not very different from those which it 

 still continues to support." 



He then refers to successive irruptions and retreats 

 of the sea, " the final result of which, however, has 

 been a universal depression of the level of the sea." 



" These repeated irruptions and retreats of the sea 

 have neither been slow nor gradual ; most of the ca- 

 tastrophes which have occasioned them have been 

 sudden." 



