56 THE METHOD OF DARWIN. 



made it a normal intellectual process to look 

 upon exceptional and isolated phenomena as 

 merely extreme instances of much larger 

 groups. One of the marked characteristics of 

 Darwin's work is that he selected such extreme 

 instances, and sought to connect them with the 

 more common facts to which they were related, 

 by proving, or at least suggesting, their deri- 

 vation from the latter. Wherever it was pos- 

 sible in his experiments, he varied the amount 

 of a cause in order to note the proportionate 

 variation in the amount of the effect; and 

 where he had to depend upon observation alone, 

 he made strenuous efforts to connect extreme 

 instances by gradations of character. 



Thus, he and his son Francis, by continuous 

 attention to the sleeping movements of plants, 

 were able to show that it is not true, as is gen- 

 erally supposed, that the leaves move only in the 

 evening when going to sleep, and in the morn- 

 ing when awaking ; for they found no exception 

 to the rule that leaves which sleep continue to 

 move during the whole twenty-four hours, only 

 moving more quickly when going to sleep and 

 awaking than at other times. 1 They were able 

 to show that sleeping movements are only 

 liighly specialized and exaggerated modifica- 



1 Power of Movement in Plants, p. 403. 



