340 PHYSICAL GEOGRAPHY. 



sub-arctic into the arctic zone, and contains one representative, which is 

 the most northern butterfly known, B. polaris* Our White Mountain 

 butterfly belongs to the second category, having its representatives on 

 this continent in the Hudsonian fauna. It is very closely allied to two 

 Hudsonian species ( B. Boisdiivalii and B. Cltariclca), and at first sight 

 might be taken for them, especially for the former ; but repeated exami- 

 nations of many individuals have confirmed my first impression that 

 they were distinct. The genus Pezotettix, to which the grasshopper 

 belongs, is not so strictly northern as Brenthis, but has several represent- 

 atives at least in the Hudsonian fauna, and, like Brenthis, is also found in 

 the alpine elevations of Europe. 



But even the narrow limit of the alpine zone of the White Mountains 

 claims for its own a single butterfly, which probably has a more restricted 

 range than any other in the world. One may search the season through 

 over the comparatively vast and almost equally barren elevations within 

 the sub-alpine district of the White Mountains, and fail to discover more 

 than here and there a solitary individual whirled by fierce blasts down 

 the mountain slopes, while, a few hundred feet above, the butterflies swarm 

 in great numbers. Every passage of the sun from behind a cloud brings 

 them out in scores, and they may often be captured as fast as they can be 

 properly secured. The contrast between the occasional and unwilling 

 visitor in the sub-alpine region, and the swarms which flutter about the 

 upper plateaus, is most significant. Yet the Carices, the food-plant of the 

 caterpillar, are quite as abundant in the lower regions as in the upper, 

 even to the species C. rigida, upon which I found the larva feeding. Now 

 this butterfly, CEncis sanidca, belongs to a genus which is peculiar to 

 alpine and arctic regions ; in fact, it is the only genus of butterflies which 

 is exclusively confined to them. It has numerous members, both in this 

 country and in the old world. One is confined to the Alps of Europe ; 

 most of the European species, however, are found only in the extreme 

 north. The genus extends across the whole continent of America, and 

 several of its species occur on the highest elevations of the Rocky Moun- 

 tains. Several species are common to Europe and America ; and it is to 

 one of these that (2?. sanidca is most closely allied. A few species 

 descend into the Hudsonian fauna ; but, as a whole, the genus has its 



* This was taken by the Polaris expedition at Polaris bay, their extreme northern station, lat. 82° 16' N. 



