On the Seasonal Dimorphism of Butterflies. 75 



absolute temperature appears to give a tendency 

 to variation in a certain direction, the necessary 

 temperature being different for different species. 

 The latter statement is supported by the facts that, 

 in the first place, in different species there are 

 very different degrees of difference between the 

 summer and winter forms ; and secondly, many 

 digoneutic species are still monomorphic in Ger- 

 many, first becoming seasonally dimorphic in 

 Southern Europe. This is the case with P. 

 Machaon and P. Podalirius, as already mentioned, 

 and likewise with Polyommalus Phlceas. Zeller in 

 1846-47, during his journey in Italy, recognized 

 as seasonally dimorphic in a small degree a large 

 number of diurnal Lepidoptera which are not so 

 in our climate. 4 



In a similar manner the appearance of seasonal 

 dimorphism in species which, like Plebei^ls Amyn- 

 tas, do not hibernate as pupae, but as caterpillars, 

 can be simply explained by supposing that the 

 winter generation was the primary form, and that 

 the increase in the summer temperature since the 

 glacial period was sufficient to cause this particu- 

 lar species to become changed by the gradual 

 interpolation of a second generation. The dimor- 

 phism of P. Amyntas can, nevertheless, be ex- 

 plained in another manner. Thus, there may 



4 P. C. Zeller, " Bemerkungen iiber die auf einer Reise nach 

 Italien und Sicilien gesammelten Schmetterlingsarten." Isis, 

 1847, ii. xii. 



