440 THE ICE AGE IN NORTH AMERICA. 



attained its present restricted geographical area. . . . How 

 did the White Mountain butterfly get up the White Mount- 

 ains ? I am disposed to answer, by the action attendant on 

 the decline of the Glacial period. ... 



The main ice-sheet had pushed them insensibly before it, 

 and, during the continuance of the Glacial period, the geo- 

 graphical distribution of the genus CEneis had been changed 

 from a high northern region to one which may well have in- 

 cluded portions of the Southern States. And, on its decline, 

 the ice-sheet drew them back again after itself by easy stages ; 

 yet not all of them. Some of these butterflies strayed by the 

 way, detained by the physical nature of the country, and 

 destined to plant colonies apart from their companions. When 

 the main ice-sheet left the foot of the White Mountains, on its 

 long march back to the pole, where it now seems to rest, some 

 of these wayward, flitting CEneis butterflies were left behind. 

 These had strayed up behind the local glaciers on Mount 

 Washington, and so became separated from the main body of 

 their companions, which latter journeyed northward, follow- 

 ing the course of the retirement of the main ice-sheet. They 

 had found in elevation their congenial climate, and they have 

 followed this gradually to the top of the mountain, which they 

 have now attained, and from which they can not now retreat. 

 Far off in Labrador the descendants of their ancestral com- 

 panions fly over wide stretches of country, while they appear 

 to be in prison on the top of a mountain. I conceive that in this 

 way the mountains may generally have secured their Alpine 

 animals. The Glacial period can not be said strictly to have 

 expired. It exists even now for high levels above the sea, 

 while the Eskimos find it yet enduring in the far north. Had 

 other conditions been favorable, we might now find arctic 

 man living on snow-capped mountains within the temperate 

 zone. 



At a height of from 5,600 to 6,200 feet above the level of 

 the sea, and a mean temperature of about 48° during a short 

 summer, the White Mountain butterflies (CEneis semidea) yet 

 enjoy a climate like that of Labrador within the limits of New 

 Hampshire. And in the case of moths an analogous state of 

 things exists. The species Anarta melanopa isfoui ' on Mount 



