THE MEADOW-BROWNS OR SATYRS 223 



lower levels they were replaced by others migrating from 

 the south so that gradually there came about the dis- 

 tribution of plants and animals as we find them to-day. 



W^hen, however, the glaciers left the higher elevations 

 of the White Mountains and the Rocky Mountains there 

 were at the summits small areas in which the climatic 

 conditions were of very much the same arctic character 

 as prevailed along the margin of the ice cap. Conse- 

 quently conditions were here favorable for the continua- 

 tion of many of the arctic species which had disappeared 

 from the warmer, lower levels. It was as if we had a 

 great sea of air of a certain warmth and rising above this 

 the islands of the mountain tops, these islands retaining 

 the same arctic features as otherwise are found much 

 farther northward. 



Among the animals thus left stranded uy the retreat- 

 ing ice cap this White Mountain butterfly has perhaps at- 

 tracted the most attention from scientists. It is a butter- 

 fly of moderate size which shows in every phase of its 

 structure and its life-history the results of the long process 

 of adaptation to its unique environment. It has been 

 carefully studied by many observers and has been consid- 

 ered one of the most desirable trophies by every collector 

 of insects. As a result, notwithstanding its isolation and 

 the difficulty of studying it, its life-history is better known 

 than that of many a common and widely distributed 

 species. 



To appreciate the facts in regard to the structure and 

 life of this butterfly one must know that its habitat is 

 confined to a thousand feet or so at the summits of the 

 mountain, that in this area there are no trees or even 

 shrubs worth mentioning, and that the surface of the 



