148 THE OLDEST BUTTERFLY 



northward, and whose brethren, thought by some 

 to be even forms of the same species, still cling to 

 the borders of the ice region of the north. They 

 were the first of their tribe to fly over the barren 

 fields of New England v/hen the earliest verdure 

 began to follow the withdrawing ice, and, moving 

 with it step by step, were at last, some of them, 

 beguiled by the local glaciers which remained in 

 the White Mountain region long after the main 

 glacial sheet had left these mountains far in its 

 rear, and until connection with the main body was 

 finally cut off. As one of our writers, Grote, has 

 expressed it : — 



" Return became at length impossible. They ad- 

 vanced behind the deceiving local glaciers step by step, 

 up the mountain side, pushed up from below by the 

 warm climate, which to them was uncongenial, until 

 they reached the mountain peak, now bare of snow in 

 the short summer. Here, blown sidewise by the wind, 

 they patiently cling to the rocks ; or in clear weather, 

 on weak and careful wing, they fly from flower of stem- 

 less mountain-pink to blue-berry, swaying from their 

 narrow tenure of the land. Drawn into the currents of 

 air that sweep the mountain's side, they are forced 

 downwards, to be parched in the hot vaUeys below. Yet 

 they maintain themselves ; they are fighting it out on 

 that line." 



