2O Studies in the Theory of Descent. 



came warmer, a period must have arrived when 



Levana, in the most" northern portion of its area of distribution, 

 appears in two or only in one generation?" This question is 

 now answered by the Swedish Expedition to the Yenisei in 

 1876. Herr Philipp Trybom, one of the members of this 

 expedition, observed A. Levana at the end of June and 

 beginning of July, in the middle of Yenisei, in 60 63 N. 

 (Dagfjarilar fr&n Yenisei in Oversigt ap k. Vertensk. Akad. 

 Forhandlingon, 1877, No. 6.) Trybom* found Levana at 

 Yenisk on June 23rd, at Worogova (61 5') on July 3rd, at 

 Asinova (61 25') on July 4th, at Insarowa (62 5') on July 

 7th, and at Alinskaja (63 25') on July Qth. The butterflies 

 were especially abundant at the beginning of June, and were 

 all of the typical Levana form. Trybom expressly states, " we 

 did not find a single specimen which differed perceptibly from 

 Weismann's Figs, i and 2 (' Saison-Dimorphismus ' Taf. I.)." 



The Swedish expedition soon left the Yenisei, and con- 

 sequently was not able to decide by observations whether 

 a second generation possessing the Prorsa form appeared 

 later in the summer. Nevertheless, it may be stated with 

 great probability that this is not the case. The districts in 

 which Levana occurs on the Yenisei have about the same 

 isotherm as Archangel or Haparanda, and therefore the same 

 summer temperature. Dr. Staudinger, whose views I solicited, 

 writes to me: "In Finnmark (about 67 N.) I observed no 

 species with two generations ; even Polyommatus Phlceas, which 

 occurs there, and which in Germany has always two, and in 

 the south, perhaps, three generations, in Finnmark has only one 

 generation. A second generation would be impossible, and 

 this would also be the case with Levana in the middle of 

 Yenisei. I certainly have Levana and Prorsa from the middle 

 of Amur, but Levana flies there at the end of May, and the 

 summers are very warm." The middle of Amur lies, more- 

 over, in 50 N. lat., and therefore io 13 south of the 

 districts of the Yenisei mentioned. 



It must thus be certainly admitted that on the Yenisei A. 

 Levana occurs only in the Levana form, and that consequently 

 this species is at the present time, in the northernmost portion 



