On the Seasonal Dimorphism of Butterflies. 25 



in the intermediate ones ; it would not be improb- 

 able, according to previous experiences, that the 

 peculiarity should exceptionally, i. e., from a cause 

 unknown to us, appear in single individuals of the 

 second or fourth generation. But this completely 

 agrees with those cases in which " exceptional" 

 individuals of the winter brood took the Prorsa 

 form, with the difference only that a cause (warmth) 

 was here apparent which occasioned the develop- 

 ment of the latent characters, although we are not 

 in a position to say in what manner heat produces 

 this action. These exceptions to the rule are 

 therefore no objection to the theory. On the con- 

 trary, they give us a hint that after one Prorsa 

 generation had been produced, the gradual inter- 

 polation of a second Prorsa generation may have 

 been facilitated by the existence of the first. I do 

 not doubt that even in the natural state single 

 individuals of Prorsa sometimes emerge in Sep- 

 tember or October ; and if our summer were 

 lengthened by only one or two months this might 

 give rise to a third summer brood (just as a second 

 is now an accomplished fact), under which circum- 

 stances they would not only emerge, but would 

 also have time for copulation and for depositing 

 eggs, the larvae from which would have time to 

 grow up. 



A sharp distinction must be made between the 

 first establishment of a new climatic form and the 

 transference of the latter to newly interpolated 



