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.” 
