2 3 2 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



did not exercise any choice, but fixed themselves in- 

 discriminately to colors which their pupa could re- 

 semble and those which they could not. In the nat- 

 ural conditions the latter would not exist, for the pupae 

 can imitate all the colors of their normal environ- 

 ments. 



"I began work with the common peacock butter- 

 fly ( Vanessa io), of which the chrysalis appears in two 

 forms, being commonly dark gray, but more rarely, 

 bright yellowish-green ; both forms are gilded, espe- 

 cially the latter. Only six caterpillars could be obtained, 

 and these were placed in glass cylinders surrounded by 

 yellowish-green tissue-paper. Five of them became 

 chrysalides of the corresponding color ; the sixth was 

 removed immediately after the caterpillar skin had been 

 thrown off, and was placed in a dark box lined with 

 black paper, but it subsequently deepened into a green 

 pupa exactly like the others. Obviously the surround- 

 ings had exercised their influence before the pupa was 

 removed. 



" Being unable to attain more larvae of the pea- 

 cock, I worked upon the allied small tortoise-shell 

 butterfly (Vanessa urtiaz), which can be obtained in 

 immense numbers. In the experiments conducted in 

 1886, over seven hundred chrysalides of this species 

 were obtained and their colors recorded. Green sur- 

 roundings were first employed in the hope that a green 

 form of pupa, unknown in the natural state, might be 

 obtained. The results were, however, highly irregular, 

 and there seemed to be no susceptibility to the color. 

 The pupae were, however, somewhat darker than usual, 

 and this result suggested a trial of black surroundings, 

 from which the strongest effects were at once wit- 

 nessed : the pupae were as a rule extremely dark, with 



