ix.] ON A PIECE OF CHALK. 173 



tion and depression which have affected the crust of the earth, 

 we go still further back, and ask, Why these movements ? 



I am not certain that any one can give you a satisfactory 

 answer to that question. Assuredly I cannot. All that can be 

 said, for certain, is, that such movements are part of the ordinary 

 course 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 perfectly 

 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 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 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 identi 

 cally the same as those which lived in the times called &quot; older 

 tertiary,&quot; 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 



