On the Seasonal Dimorphism of Butterflies. 1 5 



senting the appearance of a perfect Levana, while 

 three were of the pure summer form (Prorsa). 



Thus with Levana it was not possible, by refri- 

 geration and retardation of development, to change 

 the summer completely into the winter form in all 

 specimens. It may, of course, be objected that 

 the period of refrigeration had been too short, 

 and that, instead of leaving the pupae in the refri- 

 gerator for two months, they should have remained 

 there six months, that is, about as long as the 

 winter brood remains under natural conditions in 

 the chrysalis state. The force of this last objec- 

 tion must be recognized, notwithstanding the im- 

 probability that the desired effect would be pro- 

 duced by a longer period of cold, since the doubling 

 of this period from four to eight weeks did not 

 produce 13 any decided increase in the strength of 

 the transformation. I should not have omitted 

 to repeat the experiment in this modified form, 

 but unfortunately, in spite of all trouble, I was 

 unable to collect during the summer of 1873 a 

 sufficient number of caterpillars. But the omission 

 thus caused is of quite minor importance from a 

 theoretical point of view. 



For let us assume that the omitted experiment 

 had been performed that pupae of the summer 

 brood were retarded in their development by cold 

 until the following spring, and that every specimen 



13 See Exps. 4, 9, and u, Appendix I 



