Distribution of Insects in North America. 337 
enjoys cannot be meaningless. The question comes up, with 
regard to the White Mountain butterfly, as to the manner in 
which this species of Oeneis attained its present restricted geo- 
graphical area—How did the White Mountain butterfly get up 
the White Mountains? And it is this question that I am dis- 
posed to answer by the action attendant on the decline of the 
Glacial period. 
have before briefly outlined the phenomena attendant on 
the advance of the ice-sheet, and I now dwell for a moment on 
the action which must equally be presumed to have accompa- 
nied its retirement. Many of the features of its advance were 
repeated, in reverse order, on the subsidence of the main ice- 
sheet or glacial sea. The local glaciers appeared again, separate 
om the main body of ice, and filled the valleys and mountain 
. 
Tavines, thus running at variance with the main body of the 
the temperature shortened the winters and Tereshaned the sum- 
mers. ice-loving insects, such as our White Mountain butter- 
fly, hung on the outskirts of the main ice-sheet, where they 
found their fitting conditions of temperature and food. The 
the continuance of the Glacial period, the geographical distribu- 
on of the genus Oeneis had been changed from a high northern 
back again after itself by easy stages; yet not all of them. 
Some of these butterflies strayed by the way, detained by the’ 
*See Mr. Scudder’s article in the “ Geol of New Hampshire,” i, 342. Mr. 
Scudder first pointed out the existence of Alpine and sub-Alpine faunal belts on 
Mount Washington, and makes the ‘interesting remark, “that if the summit of 
Mount Washington’ were somewhat less than two thousand feet higher, it would 
a the limit of perpetual snow.” 
Am. Jour. Scr.—Tatep Sunine, Vou. X, No. 59.—Nov., 1875. 
