The Swedish expedition soon left the Yenisei, and consequently 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 Phlæas_, 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, moreover, in 50° N. lat., and therefore 10°-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 of its area of distribution, in the same condition as that in which I conceive it to have been in mid Europe during the glacial period. It would be of the greatest interest to make experiments in breeding with this single-brooded _Levana_ from the Yenisei, i.e., to attempt to change its offspring into the _Prorsa_ form by the action of a high temperature. If this could not be accomplished it would furnish a confirmation of my hypothesis than which nothing more rigorous could be desired.] [18] See Exp. 10, Appendix I. [19] When Dorfmeister remarks that hibernating pupæ which, at an early stage “were taken for development into a room, or not exposed to any cold, gave dwarfed, weakly and crippled,” or otherwise damaged butterflies, this is entirely attributable to the fact that this able entomologist had neglected to supply the necessary moisture to the warm air. By keeping pupæ over water I have always obtained very fine butterflies. [20] [For other remarkable cases of sexual dimorphism (not _antigeny_ in the sense used by Mr. S. H. Scudder, Proc. Amer. Acad., vol. xii. 1877, pp. 150-158) see Wallace “On the Phenomena of Variation and Geographical Distribution, as illustrated by the Papilionidæ of the Malayan Region,” Trans. Linn. Soc., vol. xxv. 1865, pp. 5-10. R.M.]