On the Seasonal Dimorphism of Butterflies. 47 



might be produced, which prove that quite constant 

 changes in a species may be brought about by the 

 direct action of changed climatic conditions. With 

 butterflies it is in many cases possible to separate 

 pure climatic varieties from other local forms, inas- 

 much as we are dealing with only unimportant 

 changes and not with those of biological value, so 

 that natural selection may at the outset be ex- 

 cluded as the cause of the changes in question. 

 Then again the sharply defined geographical 

 distribution climatically governed, often furnishes 

 evidence of transition forms in districts lying 

 between two climatic extremes. 



In the following attempt to make clear the 

 relationship between simple climatic variation and 

 seasonal dimorphism, I shall concern myself only 

 with such undoubted climatic varieties. A case 

 of this kind, in which the winter form of a 

 seasonally dimorphic butterfly occurs in other 

 habitats as the only form, i.e., as a climatic variety, 

 has already been adduced in a former paragraph. 

 I allude to the case of Pieris Napi> the winter 

 form of which seasonally dimorphic species occurs 

 in the temperate plains of Europe, whilst in Lap- 

 land and the Alps it is commonly found as a 

 monomorphic climatic variety which is a higher 

 development of the winter type, viz., the var. 

 Bryonicz. 



Very analogous is the case of Eiichloe Belia, 

 a butterfly likewise belonging to the Pierincz, 



