, ' ON A PIECE OF CHALK 87 



there is indirect, but perfectly satisfactory, proof, that an 

 enormous area now covered by the Pacific has been deep- 

 ened thousands of feet, since the present inhabitants of 

 that sea came into existence. Thus there is not a shadow 

 of a reason for believing that the physical changes of the 

 globe, in past times, have been affected by other than 

 natural causes. Is there any more reason for believing that 

 the concomitant modifications in the forms of the living 

 inhabitants of the globe have been brought about in other 

 ways ? 



Before attempting to answer this question, let us try to 

 form a distinct mental picture of what has happened in some 

 special case. The crocodiles are animals which, as a group, 

 have a very vast antiquity. They abounded ages before the 

 chalk was deposited; they throng the rivers in warm cli- 

 mates, at the present day. There is a difference in the form 

 of the joints of the back-bone, and in some minor partic- 

 ulars, between the crocodiles of the present 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 croco- 

 diles 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 tertia- 

 ries 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 croc- 

 odiles; though all, since the chalk, have belonged to the 

 modern type, and differ simply in their proportions, and in 

 such structural particulars as are discernible only to trained 

 eyes. 



How is the existence of this long succession of different 



