On the Seasonal Dimorphism of Butterflies. 2 3 



the facts investigated appear to me of special im- 

 portance as furnishing this proof. I shall again 

 return to this view in considering climatic varieties, 

 and it will then appear that also the nature of the 

 transformation itself confirms the slow operation 

 of climate. 



During the transition from the glacial period to 

 the present climate A. Levana thus gradually 

 changed from a monogoneutic to a digoneutic 

 species, and at the same time became gradually 

 more distinctly dimorphic, this character origi- 

 nating only through the alteration of the summer 

 brood, the primary colouring and marking of the 

 species being retained unchanged by the winter 

 brood. As the summer became longer a third 

 generation could be interpolated the species 

 became polygoneutic ; and in this manner two 

 summer generations alternated with one winter 

 generation. 



We have now to inquire whether facts are 

 in complete accordance with this theory whether 

 they are never at variance with it and whether 

 they can all be explained by it. I will at once 

 state in anticipation, that this is the case to the 

 fullest extent. 



In the first place, the theory readily explains 

 why the summer but not the winter generations 

 are capable of being transformed ; the latter can- 

 not possibly revert to the Prorsa form, because 

 this is much the younger. When, however, it 



