342 PHYSICAL GEOGRAPHY. 



interchanging their stations, and yet which never pass their impercepti- 

 ble barriers. Many butterflies from the valley occasionally struggle to 

 the extremest summits, and one or two, such as Polygonia Faunus and 

 Aglais Milberti, are frequently found within the sub-alpine region. In 

 all, the capabilities of flight are unlimited, yet I have but two or three 

 times taken CEneis semidea more than a mile and a quarter from the 

 summit ; and the appearance of the valley butterflies upon the heights 

 may easily be accounted for, from the fact that all insects with reasonable 

 powers of flight seem to delight in seeking the most elevated situations. 

 Their scanty numbers in these parts is in marked contrast with their often 

 astonishing profusion in their proper haunts below. 



The results we have reached, by our study of the faunas of these 

 mountain slopes, are what might be expected from a comparison of the 

 elevation of these mountains with that of the European Alps, at the same 

 time taking into consideration the difference in climate between the two 

 countries. In the Alps the lower limit of the sub-alpine zone is placed 

 by different writers at from 4000 to 4500 feet above the sea, and that of 

 the alpine zone at from 6000 to 6500 feet. Now, although Mt. Blanc lies 

 in a latitude (45 45') north of Mt. Washington (44 15') by a degree and 

 a half, yet a comparison of the isothermal and isochimenal lines, which 

 pass respectively through these two points, would show that a mountain 

 elevation in Europe, which should have climatic conditions similar to 

 those of the White Mountains, ought to be placed north of the Alps,' and 

 would be found between the mountains of Switzerland and Norway at 

 just such a proportionate distance from them as the heights of the alpine 

 and sub-alpine zones of the White Mountains were found to be related 

 respectively to those of the Alps and Scandinavian mountains. By the 

 same comparison we may also judge, that if the summit of Mt. Wash- 

 ington were somewhat less than two thousand feet higher, it would 

 reach the upper limits of the alpine district, or the region of perpetual 

 snow. 



An attempt to institute a rigid comparison between the alpine and 

 sub-alpine regions of our White Mountains and those of the Alps is not 

 so easy as would be imagined. If we examine their physical features 

 alone, we shall discover important differences. In New Hampshire these 

 regions are confined solely to the summits of the very highest moun- 



