Appendix. 153 



regard the var. Umbrosa as the summer form, and the 

 var. Fabricii, which emerges in the autumn, hibernates in 

 the imago state, and lays eggs in the spring, as the 

 winter form. It would then be incomprehensible why 

 the var. Umbrosa (i. e. y the summer form) should be pro- 

 duced by cold. 



But it is quite as possible that the var. Umbrosa as 

 that the var. Fabricii is the winter form. We must not 

 forget that, in this species, not one of the four annual 

 generations is exposed to the cold of winter in the pupal 

 state. When, therefore, we have in such cases seasonal 

 dimorphism, to which complete certainty can only be 

 given by continued observations of this butterfly, which 

 does not occur very commonly in Virginia, this must 

 depend on the fact that the species formerly hibernated 

 in the pupal stage. This question now arises, which of 

 the existing generations was formerly the hibernating 

 one the first or the last ? 



Either may have done so d priori, according as the 

 summer was formerly shorter or longer than now for 

 this species. If the former were the case, the var. Fa- 

 bricii is the older winter form ; were the latter the case, 

 the var. Umbrosa is the original winter form, as shall 

 now be more closely established. 



Should the experiments which Mr. Edwards has per- 

 formed in the course of his interesting investigations be 

 repeated in future with always the same results, I should 

 be inclined to explain the case as follows : 



It is not the var. Fabricii, but Umbrosa, which is the 

 winter generation. By the northward migration of the 

 species and the relative shortening of the summer, this 

 winter generation would be pushed forward into the 

 summer, and would thereby lose only a portion of the 

 winter characters which it had till that time possessed. 

 The last of the four generations which occurs in Virginia 

 is extremely rare, so that it must be regarded either as 



