36 Studies in the Theory of Descent. 



From this last observation it follows that it is 

 not the duration of development which, in indi- 

 vidual cases, determines the form of the butterfly, 

 and which consequently decides whether the winter 

 or summer form shall emerge, but that, on the 

 contrary, the duration of the pupal stage is depen- 

 dent on the tendency which the forthcoming 

 butterfly had taken in the chrysalis state. This 

 can be well understood when we consider that 

 the winter form must have had a long, and the 

 summer form a short pupal period, during innu- 

 merable generations. In the former the habit of 

 slow development must have been just as well 

 established as that of rapid development in the 

 latter ; and we cannot be at all surprised if we do 

 not see this habit abandoned by the winter form 

 when the opportunity presents itself. But that it 

 may be occasionally abandoned the more proves 

 that the duration of the pupal development less 

 determines the butterfly form than does the tem- 

 perature directly, in individual cases. 



Thus, for instance, Edwards explicitly states 

 that, whereas the two winter forms of P. Ajax, viz. 

 the vars. Walshii and Telamonides, generally 

 appear only after a pupal period of 150 to 270 

 days, yet individual cases occur in which the pupal 

 stage is no longer than in the summer form, viz. 

 fourteen days. 21 A similar thing occurs with 



21 Thus from eggs of Walshii, laid on April loth, Edwards 

 obtained, after a pupal period of fourteen days, from the ist 



