70 ON A PIECE OF CHALK 



ourselves how these changes have occurred. And when 

 we have explained them as they must be explained 

 by the alternate slow movements of elevation and 

 depression which have affected the crust of the earth, 

 we go still further back, and ask, Why these move- 

 ments ? 



I am not certain that any one can give you a satis- 

 factory answer to that question. Assuredly I cannot. 

 All that can be said, for certain, is, that such move- 

 ments are part of the ordinary course of nature, inas- 

 much 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 in- 

 direct, but perfectly satisfactory, proof, that an enor- 

 mous 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 believ- 

 ing 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 

 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 

 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 



