58 THE METHOD OF DARWIN. 



dow, towards which they bent all day in a 

 strongly zigzag course. On the following day 

 they continued to bend in the same direction, 

 but zigzagged much less. The sky, however, 

 became, between 12 140 and 2 135 P.M., overcast 

 with extraordinarily dark thunder-clouds, and 

 it was interesting to note how plainly the 

 cotyledons circumnutated during this inter- 

 val." 1 These observations they considered of 

 some value from their having made them while 

 they were not attending to heliotropism ; and 

 they were led by them "to experiment on 

 several kinds of seedlings, by exposing them 

 to a dim lateral light, so as to observe the gra- 

 dations between ordinary circumnutation and 

 heliotropism." An accidental observation led 

 to variations in the experiments, which re- 

 sulted in demonstrating continuity between two 

 apparently distinct classes of movements. 



In some of his remarkable studies on grada- 

 tions of characters, where it was impossible to 

 make experiments, he sought out and observed 

 Nature's own variations. Perhaps the most 

 striking instance of the study of gradations of 

 character is that connected with the "ocellus " 

 on the tail coverts of the peacock. 2 This 



1 Power of Movement in Plants, p. 421. 



2 Descent of Man, Vol. II. pp. 132-145 



