THE CENTURY'S PROGRESS IN PALEONTOLOGY 

 such breaks in the strata furnish the only suggestion 



/ oo 



geology can offer of sudden and catastrophic changes of 

 wide extent. 



When evidence from widely separated regions is 

 gathered, said Lyell, it becomes clear that the number- 

 less species that have been exterminated in the past 



METAMYNODON, OR SWIMMING RHINOCEROS, FROM SOUTH DAKOTA 



have died out one by one, just as individuals of a species 

 die, not in vast shoals ; if whole populations have passed 

 away, it has been not by instantaneous extermination, 

 but by the elimination of a species now here, now there, 

 much as one generation succeeds another in the life his- 

 tory of any single species. The causes which have 

 brought about such gradual exterminations, and in the 

 long lapse of ages have resulted in rotations of popula- 

 tion, are the same natural causes that are still in opera- 

 tion. Species have died out in the past as they are 

 dying out in the present, under influence of changed 



101 



