THE CANADIAN ENTOMOLOGIST. 231 



may have spread a true polar climate over our temperate zone, or perhaps 

 a lesser degree of cold may have prevailed, with increased deposition of 

 rain and snow. At all events, the summer was then short and com- 

 paratively cool, and the existing butterflies could only produce one 

 generation in a year. They were all monogoneutic ; levana had but the 

 form of levana. When the climate gradually became warmer, a period 

 must have come on in which the summer lasted so long that a second 

 generation could be interpolated. The pupae of the levana brood, which 

 had hitherto slept through the long winter, could now during the same 

 summer in which they had hatched as larvae fly as butterflies. Only the 

 brood which proceeded from these last hybernated. There had come to 

 be a state of things in which the first generation grew up under very 

 different climatic influences from the second. So considerable a change 

 as now exists between the prorsa and levana forms could not have taken 

 place suddenly, but must have done so by degrees. If it did arise 

 suddenly, this would signify that every individual of this species possessed 

 the power to take two different shapes according as it was subjected to 

 warmth or cold. But the experiments have shown that this is not so, 

 that rather the last generation has an ineradicable tendency to take the 

 levana form which protracted heat will not alter, while both summer 

 generations have a preponderating tendency towards the prorsa form, 

 although they allow themselves frequently to assume the levana form in 

 various degrees by lengthened influence of cold. 



It seems to the author that the quoted result of his experiments may 

 not only easily be explained by the supposition of a gradual climatic 

 influence, but that this supposition is upon the whole the only admissible 

 one. While by the changes from the ice period to that of our present 

 climate, levana altered gradually from a monogoneutic to a digoneutic 

 species, at the same time a sharper dimorphism stamped itself gradually 

 upon it, which only arose through the changing of the summer genera- 

 tion, while the winter generation held fast to the primary shape and 

 marking of the species. When the summer became still longer, a third 

 generation could be interpolated, and the species became polygoneutic, 

 and in this manner, that two summer generations alternated with one 

 winter generation. 



The theory explains why at the same time the summer generation was 

 allowed to change, but not the winter one. The last cannot possibly 

 return to the prorsa form, because this is much younger than itself. But 



