The Differentiation of Populations | 223 



requiring two. Laboratory crosses indicate strong behavioral isola- 

 tion (females not responsive to males of the wTong form; copulation, 

 when induced, abnormally brief), and hybrids, when produced arti- 

 ficially, are wholly sterile and unlike any individuals found in na- 

 ture. In short, the two forms seem to be completely isolated from 

 each other genetically. 



Erebia cassioides, however, has a wider geographic distribution 

 than E. nivalis, and in regions where E. nivalis is absent it extends 

 to altitudes as high as those occupied by E. nivalis where both are 

 present. Conversely, in some areas where the mountains do not 

 reach great heights, E. nivalis lives at lower elevations, with E. 

 cassioides correspondingly lower or absent. 



Erebia tyndarus (n = 10) occurs in essentially the same life zone 

 as E. cassioides, but as one can see from Fig. 10.5 the two are not 

 sympatric. Erebia tyndarus occurs in the central Alps, with E. cas- 

 sioides on the east and west. Experimental crosses (£. tyndarus 

 females X £• cassioides males) showed little behavioral isolation, 

 and about 15 percent of the eggs from these crosses hatched with 

 25 percent survivorship among the young larvae. Although the 

 ranges of E. tyndarus and E. cassioides adjoin very closely, both in 



Fig. 1 0.5 1 Map showing distribution of butterflies of the Erebia tyndarus 

 group in central Europe. {From Lorkovic, 1957, Biolo'ski Glasnik 10.) 



