INSECTS. 157 



rado aDd New Mexico. In the White Mountains it is abundant on the 

 summit of Mount Washington ; but in the territory between this region 

 and Labrador it is unknown, as also between INIount Washington and 

 the Eocky Mountains. How far to the northwest of the continent it 

 flies is not known to me. It has not appeared in collections from 

 Alaska, in which Frcya was represented in considerable numbers. The 

 peculiar distribution of this species, C. Semidca, by which it inhabits^ 

 mountain summits thousands of miles apart and not the intervening- 

 country, and in the White Mountains of New Hampshire is thoroughly 

 isolated and restricted to a very small area, is explained as in the case 

 of plants similarly distributed and isolated (address of Prof. Asa. 

 Gray, Dubuque, 1872). The advance to the southward of the glacial 

 ice pushed before it multitudes of plants and animals, forcing them 

 along very distant lines of longitude in many cases; and wlien the re- 

 ceding of the ice took place, and a milder temperature began to prevail, 

 some species which had obtained a foothold at the south remained there,, 

 finding a climate in which they could live, ui)on lofty mountains ouly, 

 being unable to exist in the lowlands. In the case of this butterfly^ 

 such a climate was found at or near the snow-Une in the Eock.>' ■Mount- 

 ains, and upon the summits of the White Mountains. 



