78 Studies in the Theory of Descent. 



the reversion of Pieris Napi and Araschnia 

 Levana have also shown. 



All the seasonally dimorphic butterflies known 

 to me are found in Hoffmann's list of Siberian 

 immigrants, with the exception of two species, 

 viz., Eiichloe Belemia, which is cited as an 

 African immigrant, and Pieris Krueperi, which 

 may have come through Asia Minor, since at the 

 present time it has not advanced farther west than 

 Greece. No considerable change of climate can 

 be experienced by migrating from east to west,' 

 so that the seasonal dimorphism of Pieris Krue- 

 peri can only depend on a cause similar to that 

 which affected the Siberian immigrants, that is, the 

 gradual increase of temperature in the northern 

 hemisphere since the glacial period. In this 

 species also, the winter form must be the primary 

 one. In the case of E. Belemia, on the other 

 hand, the migration northwards from Africa cer- 

 tainly indicates removal to a cooler climate, 

 which may have originated a secondary winter 

 form, even if nothing more certain can be stated. 

 We know nothing of the period of migration into 

 southern Europe ; and even migration without 

 climatic change is conceivable, if it kept pace with 

 the gradual increase of warmth in the northern 

 hemisphere since the glacial epoch. Experi- 

 ments only would in this case be decisive. If the 

 summer generation, var. Glauce, were the primary 

 form, it would not be possible by the action of 



