34 ON A PIECE OF CHALK i 



of nature, inasmuch as they are going on at the 

 present time. Direct proof may be given, that 

 some parts of the land of the northern hemisphere 

 are at this moment insensibly rising and others 

 insensibly sinking ; and there is indirect, but per- 

 fectly satisfactory, proof, that an enormous area 

 now covered by the Pacific has been deepened 

 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 effected by other than natural causes. Is 

 there any more reason for believing that the con- 

 comitant 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 

 climates, at the present day. There is a difference 

 in the form of the joints of the back-bone, and in 

 some minor particulars, 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. Notwithstand- 

 ing this, the crocodiles of the chalk are not 



