MELANIC VARIATION IN LEPIDOPTERA, 1 25 



the black edge of Colias which did not lie very flat upon the 

 surface had also made a slight impression, the others had made 

 none. At 12.30 the same three continued to increase their 

 impression, the others still made none. 



I then placed a piece of black and a piece of white cloth 

 in the same position. At i p.m. the mercury stood at 52°, and 

 the effect produced by the darker insects was still more decided. 

 But although the pale ones remained upon the surface they did 

 not appear to protect the underlying snow from the heat rays to 

 the same degree as did the white cloth, which remained on a 

 slight eminence as the sun thawed the snow around it. The 

 nearest approach to this protection was evidently accorded by 

 the white Acidalia immutata. At 1.30, ChcErophyllata and the 

 black cloth continued to sink, and the male of Thecla quercus 

 was apparently giving off as much heat below it as the female, 

 but not being set with the wings equally flat had not shown the 

 effect so soon. The Lithosise had by this time also had a very 

 slight effect upon the snow, but not so much as any of the 

 darker insects, and Chczrophyllata had decidedly won the down- 

 ward race among them. 



I then removed them; and though the limited time during 

 which the experiment lasted was amply sufficient to solve the 

 proposition, I much regret that I did not leave them during the 

 night, for on the following morning the contrast between the 

 positions of the black and white cloths was far more strongly 

 marked, and quite in accordance with the result obtained by 

 Benjamin Franklin in his similar but original experiment. 



I regret this the more, bearing in mind the rule : that, as 

 the intensity of heat diminishes absorption increases ; for I 

 should have wished to determine whether the insects would 

 have maintained their relative positions when no longer exposed 

 to the sun. This is a point to which further investigation 

 should be directed. It must necessarily occur to you that 



