PHYSIOGENESIS. 233 



only the smallest trace, and often no trace at all, of the 

 golden spots which are so conspicuous in the lighter 

 forms. These results suggested the use of white sur- 

 roundings, which appeared likely to produce the most 

 opposite effects. The colors of nearly one hundred 

 and fifty chrysalides obtained under such conditions 

 were very surprising. Not only was the black color- 

 ing matter as a rule absent, so that the pupa? were 

 light-colored, but there was often an immense devel- 

 opment of the golden spots, so that in many cases the 

 whole surface of the pupae glittered with an apparent 

 metallic lustre. So remarkable was the appearance 

 that a physicist, to whom I showed the chrysalides, 

 suggested that I had played him a trick and had cov- 

 ered them with gold-leaf. 



"These remarkable results led to the use of a gilt 

 back-ground as even more likely to produce and in- 

 tensify the glittering appearance. By this reasoning 

 I was led to make the experiment which had been sug- 

 gested by Mr. Wood nineteen years before. The re- 

 sults quite justified the reasoning, for a much higher 

 percentage of gilded chrysalides, and still more re- 

 markable individual instances, were obtained among 

 the pupae which were treated in this way. 



"These observations and experiments had been 

 made when I began to work at the subject in 1886: 

 they appeared to prove that the power certainly exists, 

 but nothing was really known as to the manner in 

 which the adjustment is effected. Mr. S. W. Wood's 

 original suggestion, that the ' skin of the pupa is pho- 

 tographically sensitive for a few hours only after the 

 caterpillar's skin has been shed,' was accepted by most 

 of those who had worked at the subject. And yet the 

 suggestion rested upon no shadow of proof ; it de- 



