On the Seasonal Dimorphism of Butterflies. 39 



summer form (var. Napwe) y yet out of these 

 numerous pupae I did not get a single butterfly 

 during the year 1872. In winter I kept them in 

 a warm room, and the first butterflies emerged in 

 January, 1873, the remainder following in Feb- 

 ruary, March, and April, and two females not 

 until June. All appeared, however, as exquisite 

 winter forms. The whole course of development 

 was precisely as though cold had acted on the 

 pupae ; and in fact, I could find no other cause for 

 this quite exceptional deportment than the seven 

 hours' shaking to which the pupae were exposed 

 by the railway journey, immediately after or during 

 their transformation. 



It is obviously a fact of fundamental importance 

 to the theory of seasonal dimorphism, that the 

 summer form can be readily changed into the 

 winter form, whilst the latter cannot be changed 

 into the summer form. I have thus far only made 

 experiments on this subject with A. Levana, but 

 the same fact appears to me to obtain for P. Napi. 

 I did not, however, operate upon the ordinary 

 winter form of P. Napi, but chose for this experi- 

 ment the variety Bryonice, well known to all ento- 

 mologists. This is, to a certain extent, the poten- 

 tial winter form of P. Napi ; the male (Fig. 14, 

 Plate I.) exactly resembles the ordinary winter 

 form in the most minute detail, but the female is 

 distinguished from Napi by a sprinkling of greyish 

 brown scales over the whole of the upper side of 



