200 3ajj Strmmw, (SssaBS, antr $kbfos. [ix. 



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 concomi- 

 tant 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 identically the same as those which lived in the 

 times called "older tertiary," which succeeded the cre- 

 taceous 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 structural 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 opera- 

 tion of natural causes. 



