On the Seasonal Dimorphism of Butterflies. 2 i 



the summer lasted long enough for the interpolation 

 of a second brood. The pupae of Levana, which 

 had hitherto hibernated through the long winter 

 to appear as butterflies in the following summer, 

 were now able to appear on the wing as butterflies 

 during the same summer as that in which they left 

 their eggs as larvae, and eggs deposited by the last 

 brood produced larvae which fed up and hibernated 

 as pupae. A state of things was thus established 

 in which the first brood was developed under very 

 different climatic conditions from the second. So 

 considerable a difference in colour and marking 

 between the two forms as we now witness could 

 not have arisen suddenly, but must have done so 

 gradually. It is evident from the foregoing ex 

 periments that the Prorsa form did not originate 

 suddenly. Had this been the case it would simply 

 signify that every individual of this species pos- 

 sessed the faculty of assuming two different forms 

 according as it was acted on by warmth or cold, 

 just in the same manner as litmus-paper becomes 

 red in acids and blue in alkalies. The experiments 



of its area of distribution, in the same condition as that in 

 which I conceive it to have been in mid Europe during the 

 glacial period. It would be of the greatest interest to make 

 experiments in breeding with this single-brooded Levana from 

 the Yenisei, i. e., to attempt to change its offspring into the 

 Prorsa form by the action of a high temperature. If this 

 could not be accomplished it would furnish a confirmation of 

 my hypothesis than which nothing more rigorous could be 

 desired.] 



