On the Seasonal Dimorphism of Butterflies. 77 



The reverse case is also theoretically conceiva- 

 ble, viz., that in certain species the summer form 

 was the primary one, and by spreading northwards 

 a climate was reached which still permitted the 

 production of two generations, the pupal stage of 

 one generation being exposed to the cold of win- 

 ter, and thus giving rise to the production of a 

 secondary winter form. In such a case hiberna- 

 tion in the pupal state would certainly give rise to 

 seasonal dimorphism. Whether these conditions 

 actually occur, appears to me extremely doubtful ; 

 but it may at least be confidently asserted that the 

 first case is of far more frequent occurrence. The 

 beautiful researches of Ernst Hoffmann 5 furnish 

 strong evidence for believing that the great 

 majority of the European butterflies have immi- 

 grated, not from the south, but from Siberia. Of 

 281 species, 173 have, according to Hoffmann, 

 come from Siberia, 39 from southern Asia, 

 and only 8 from Africa, whilst during the 

 greatest cold of the glacial period, but very few 

 or possibly no species existed north of the Alps. 

 Most of the butterflies now found in Europe have 

 thus, since their immigration, experienced a 

 gradually increasing warmth. Since seasonal 

 dimorphism has been developed in some of these 

 species, the summer form must in all cases have 

 been the secondary one, as the experiments upon 



1 " Isoporien der europaischen Tagfalter." Stuttgart, 1873. 



