120 ACQUIRED CHARACTERS SEC. 



female of Prorsa (Fig. 6). But as there are many more 

 intermediate forms than those figured by Weismann, this 

 explanation requires further confirmation. Afterwards Prorsa 

 chrysalids were placed by Weismaun in a temperature of 

 0-1.25 C. in the ice-chamber : out of twenty butterflies, fifteen 

 now changed into Porima, and among these were three which 

 were almost indistinguishable from Levana. Five remained 

 unaffected by the cold and emerged as the summer form Prorsa. 



Dorfmeister had never applied such low temperatures, and 

 had only succeeded in obtaining Porima. 



The converse experiment of Weismann to rear Prorsa from 

 Prorsa by raising the temperature succeeded, among forty 

 chrysalids which were kept in a forcing-house at 15- 31 

 C., only with four, of which three were Prorsa and one Porima ; 

 all the rest produced Levana in the next spring. 



The majority of the species of our white butterflies 

 (Pieridse) show very strikingly a winter and a summer form. 

 In the winter form of Pieris Napi the roots of the wings are 

 much powdered on the upper side. In this case Weismann, 

 by keeping the chrysalids of the progeny of the winter form 

 three months in the ice -cellar and allowing them to emerge 

 in the hothouse, obtained perfect winter forms. But there is 

 a variety of P. napi which lives in the Swiss Alps, in the Jura, 

 and in the polar regions, and which can be described as a 

 very dark form of the winter variety of Napi : P. Bryonioe. 

 The male of Bryonise is almost exactly similar to the ordinary 

 winter form of Napi, from which the female differs by the 

 gray -brown powdering of the entire upper surface. In the 

 polar regions Bryonise is the only form of Napi ; in the Alps 

 (excepting some isolated regions) it is mixed with the or- 

 dinary Napi. I agree with Weismann that Bryonise 

 must be the ancestral form of Napi, basing my opinion on 

 the fact that, as Weismann's figures show, a series exists 

 from the female of the former as starting-point through 



