STABILITY OF THE GENE 



293 



Diirken worked with the chrysalids (or pupae) of the 

 common cabbage butterfly (Colias brassicae). Since 1890 

 it has been known that when the caterpillars of some 

 butterflies pupate (that is, when they transform into the 

 resting chrysalis) the color of the pupa is to some extent 

 influenced by the background, or by the color of the light 

 that falls on it. 



Fig. 153. 

 In the center four differently colored pupae of the cabbage butter- 

 fly. Around them is shown the characteristic arrangement of the 

 pigment cells in the epidermis in different color types. (After 

 Leonore Brecher.) 



For example, the pupae of the cabbage butterfly are 

 quite dark, if the caterpillars live and transform in day- 

 light, or even in a faint light ; but if the caterpillar lives 

 in yellow or red surroundings or behind a yellow or red 

 screen the pupae are green. The green color is due to the 

 absence of superficial black pigment. In its absence the 

 greenish yellow color of the interior shows through the 

 skin (Fig. 153). 



