A Century of Science 11 



mates have undergone most complicated vicissi- 

 tudes ; and in which the earth's vegetable products 

 and its animal populations have again and again 

 assumed new forms, while the old forms have 

 passed away. In order to account for such whole- 

 sale changes, geologists were at first disposed to 

 imagine violent catastrophes brought about by 

 strange agencies, agencies which were perhaps 

 not exactly supernatural, but were in some vague, 

 unspecified way different from those which are 

 now at work in the visible and familiar order of 

 nature. But Lyell proved that the very same 

 kind of physical processes which are now going on 

 about us would suffice, during a long period of 

 time, to produce the changes in the inorganic world 

 which distinguish one geological period from an- 

 other. Here, in Lyell's geological investigations, 

 there was for the first time due attention paid 

 to the immense importance of the prolonged and 

 cumulative action of slight and unobtrusive causes. 

 The continual dropping that wears away stones 

 might have served as a text for the whole series of 

 beautiful researches of which he first summed up 

 the results in 1830. As astronomy was steadily 

 advancing toward the proof that in the abysses 

 of space the physical forces at work are the same 

 as our terrestrial forces, so geology, in carrying us 



