On the Seasonal Dimorphism of Butterflies. 1 9 



The following is my explanation of the facts. 

 The form Levana is the original type of the 

 species, and Prorsa the secondary form arising 

 from the gradual operation of summer climate. 

 When we are able to change many specimens of 

 the summer brood into the winter form by means 

 of cold, this can only depend upon reversion to/ 

 the original, or ancestral, form, which reversion ap- 

 pears to be most readily produced by cold, that is, 

 by the same external influences as those to which 

 the original form was exposed during a long period 

 of time, and the continuance of which has preserved, 

 in the winter generations, the colour and marking 

 of the original form down to the present time. 



I consider the origination of the Prorsa from the 

 Levana form to have been somewhat as follows : 

 It is certain that during the diluvial period in \ 

 Europe there was a so-called 'glacial epoch,' which/ 

 may have spread a truly polar climate over our 

 temperate zone ; or perhaps a lesser degree of cold 

 may have prevailed with increased atmospheric 

 precipitation. At all events, the summer was then 

 short and comparatively cold, and the existing 

 butterflies could have only produced one genera- 

 tion in the year ; in other words, they were mono- 

 goneutic. At that time A. Levana existed only in 

 the Levana form. 16 As the climate gradually be- 



15 [Eng. ed. In the German edition, which appeared in 

 1874, I was not able to support this hypothesis by geographi- 

 cal data, and could then only ask the question "whether 



C 2 



