iv PIERIS NAPI AND PIERIS BRYONI& 121 



its male to the female of the ordinary winter form of Napi, 

 through the male of the latter to the female, and so to the 

 male, of the summer form. Here also the males always show 

 the beginning of the new characters which distinguish the 

 next variety, which is one degree higher. The common Napi 

 must therefore, according to this interpretation, be regarded 

 as a variety which has arisen from Bryoniae through the 

 influence of a warmer climate, as the commencement of a 

 new, with us the only, species. 



Weismann placed the chrysalids of Bryonise in a hot- 

 house at 15- 30 C., but only Bryoniae emerged; he failed 

 to convert Bryoniae into Napi, as he had conversely, by the 

 application of cold, changed the summer form of Napi into the 

 winter form. 



Thus in Levana, as in Napi, the progeny of the cold form 

 are converted by cold again into the cold form, but only ex- 

 ceptionally the progeny of the warm form into the warm 

 form. In other words, the application of cold had an effect, 

 the application of warmth generally none. 



But in summer two generations of Prorsa occur. The first 

 takes flight in July, the second in August. The latter sup- 

 plies the chrysalids which form the Levana after the winter, 

 and which Weismann could only exceptionally, through 

 warmth, change again into Prorsa. Weismann concludes 

 from this that different reactions follow the same stimulus, 

 and that this difference can only lie in the physical nature of 

 the generation operated on, not outside it. According to him, 

 cold and warmth are not the direct causes of the origin of the 

 Levana and Prorsa forms, but only the indirect. The change 

 of the progeny of the cold form back again into the cold form 

 through cold he refers to reversion to the ancestral form. 



The origin of Prorsa is explained as having occurred 

 gradually through the gradual rise of the warmth of the 

 climate, i.e. of the summer. As the Levana is the ancestral 



