POLYMORPHISM IN SPECIES. 185 



two generations developed at different seasons of the 

 year. V. Levana is its winter form ; V. Prorsa the 

 summer form. The chrysalis of Levana remains 

 dormant during the winter. The butterfly emerges 

 in the spring, breeds immediately, and its progeny 

 go through their whole development in the summer ; 

 from their chrysalids emerge the V. Prorsa whose 

 progeny then pass the winter as chrysalids, and in 

 the spring produce the Levana. These two forms 

 of butterflies are differently coloured and marked, and 

 it is in complete agreement with many other exam- 

 ples of the effect of warmth on the formation of 

 pigment in the integument that the summer form 

 (exposed to warmth), Prorsa, is much more deeply 

 coloured than the winter form (exposed to cold), 

 Levana. The former is deep black, the latter 

 brown-yellow, in its ground colour." 



The caterpillars, pupae, and eggs are perfectly 

 alike in both summer and winter forms. Experi- 

 ments have shown that the chrysalids, which would 

 naturally produce the summer form, can be made to 

 produce the winter form if they be kept at an unu- 

 sually low temperature. The application of warmth 

 to these, however, ensures the production of the 

 summer form. On the other hand, every second or 

 third generation — according to the species — as- 

 sumes the winter form, no matter how much heat 



