STABILITY OF THE GENE 305 



Diirken 's experiments consisted in rearing caterpillars 

 in orange (or red) light, where the pupae assumed a light 

 or green color. The butterflies that emerged were reared 

 in open cages and their eggs collected. Some of the young 

 from these eggs were reared again in colored light, others 

 in bright light or in darkness. The latter are the controls. 

 The results are summarized in the chart (Fig. 154). The 

 number of dark chrysalids is indicated by the length of 

 the black band, and the green or light ones by the light 

 band in the chart. As a matter of fact, the pupae were 

 classified in five color groups. Three of these were then 

 lumped together as dark and the other two as light. 



As shown in Fig. 154 at 1 (which gives the normal 

 coloration), nearly all pupae, collected at random or in 

 normal surroundings, are dark; only a few are light or 

 green. The caterpillars that came from these were reared 

 in an orange environment. When they transformed into 

 pupae there was a very high percentage of light-colored 

 types, 2. If the light-colored types only are now picked 

 out and reared, some in orange, some in the light, and 

 others in the dark, the results are shown in 3a and 3b. In 

 the former, there are more light pupae than before ; since 

 two generations have been in orange, the effect is aug- 

 mented. It is the other set, 3b, however, that is more sig- 

 nificant. As the bands show there were more light pupae 

 than in the wild pupae, 1, that were reared in the light or 

 dark. This increase Diirken attributes in part to the in- 

 herited effect of the orange light on the preceding genera- 

 tion, and in part to the new environment, whose effect is 

 in the opposite direction. 



Now this interpretation, from the point of view of 

 genetics, is not satisfactory. The experiment shows, in the 

 first place, that not all caterpillars respond to the orange 

 light. If those that do respond are genetically different, 

 then of course when they — the light pupae in the experi- 



