7 6 Studies in the Theory of Descent. 



have been a disturbance of the period of develop- 

 ment in the manner already indicated, the species 

 which formerly hibernated in the pupal stage 

 becoming subsequently disturbed in its course of 

 development by the interpolation of a summer 

 generation, and hibernating in consequence in the 

 caterpillar state. Under these circumstances we 

 must regard the present winter form (var. Poly- 

 sperchori) as having been established under the 

 influence of a winter climate, this form, since the 

 supposed disturbance in its development, having 

 had no reason to become changed, the spring tem- 

 perature under which its pupation now takes place 

 not being sufficiently high. The interpolated 

 second generation on the other hand, the pupal 

 period of which falls in the height of summer, may 

 easily have become formed into a summer variety. 

 This latter explanation agrees precisely with the 

 former, both starting with the assumption that in 

 the present case, as in that of A. Levana and the 

 Pierincz, the winter form is the primary one, so 

 that the dimorphism proceeds from the said win- 

 ter form and does not originate the winter but the 

 summer form, as will be explained. Whether the 

 winter form has been produced by the action 

 of the winter or spring temperature is immaterial 

 in judging single cases, inasmuch as we are not 

 in a position to state what temperature is neces- 

 sary to cause any particular species to become 

 transformed. 



