ON A PIECE OF CHALK 71 



epoch and those which lived before the chalk; but, in 

 the cretaceous epoch, as I have already mentioned, the 

 crocodiles had assumed the modern type of structure. 

 Notwithstanding this, the crocodiles of the chalk are 

 not identically the same as those which lived in the 

 times called "older tertiary," which succeeded the 

 cretaceous epoch; and the crocodiles of the older 

 tertiaries are not identical with those of the newer 

 tertiaries, nor are these identical with existing forms. 

 I leave open the question whether particular species 

 may have lived on from epoch to epoch. But each 

 epoch has had its peculiar crocodiles; though all, 

 since the chalk, have belonged to the modern type, and 

 differ simply in their proportions, and in such struc- 

 tural particulars as are discernible only to trained eyes. 



How is the existence of this long succession of dif- 

 ferent species of crocodiles to be accounted for? 



Only two suppositions seem to be open to us 

 Either each species of crocodile has been specially 

 created, or it has arisen out of some pre-existing form 

 by the operation of natural causes. 



Choose your hypothesis; I have chosen mine. I 

 can find no warranty for believing in the distinct crea- 

 tion of a score of successive species of crocodiles in the 

 course of countless ages of time. Science gives no coun- 

 tenance to such a wild fancy; nor can even the per- 

 verse ingenuity of a commentator pretend to discover 

 this sense, in the simple words in which the writer of 

 Genesis records the proceedings of the fifth and sixth 

 days of the Creation. 



On the other hand, I see no good reason for doubt- 

 ing the necessary alternative, that all these varied 

 species have been evolved from pre-existing crocodilian 

 forms, by the operation of causes as completely a part 



