INSECTS. 157 



rado ami Xew Mexico. In the Wliite 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 Mount 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 <~ew 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 when 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, upon lofty mountains only r 

 being unable to exist in the lowlands. la the case of this butterfly,, 

 such a climate was found at or near the snow-line in the Eocky Mount- 

 ains, and upon the summits of the White Mountains. 



