THE CENTURY'S PROGRESS IN PALEONTOLOGY 



such breaks in the strata furnish the only suggestion 

 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 



