89 



On the decline of the ice-sheet the butterflies turned north- 

 ward, again returning to their native home. Some of the 

 specimens strayed by the way and were destined to plant 

 colonies apart from their companions as, for example, on the 

 White Mountains. 



Botanists entertain analogous views. Dr. Harshberger * 

 argues that the tundra vegetation and other arctic species 

 of plants occupied during the Glacial Epoch the southern 

 margin of the great ice-sheet, and that when most of them 

 migrated north, on the disappearance of the ice, some re- 

 mained behind to form the vegetation of sphagnum bogs and 

 alpine summits of the higher mountains. 



If these theories are correct, the Asiatic invasion and the 

 much more insignificant one from Europe, of which Helix 

 hortensis is one of the most striking representatives (p. 13), 

 should both be more recent than the flora and fauna of the 

 White Mountains, for the former have n6t penetrated beyond 

 the lower slopes of these mountains. 



Helix hortensis does not occcur nearer the White Mountains 

 than Portland in Maine, which is fully seventy miles to the 

 east. I have traced Oniscus asellus, a wood-louse probably 

 belonging to the same group, as far as the base of the White 

 Mountains. The earth-worm, Lumbricus castaneus, which 

 seems to have spread from continental Europe to the Faroes 

 and Iceland, reappearing on the other side of the Atlantic 

 in Canada and New England, may be a member of the same 

 dispersal. At any rate, I feel sure there are a great many 

 more of such species that have not spread to the higher parts 

 of the White Mountains, and therefore proclaim themselves as 

 more recent immigrants than those which are now in posses- 

 sion of the high plateau referred to. The latter are likewise 

 clearly older than the Asiatic immigrants, which will be more 

 fully described later on. 



But since Helix hortensis occurs in the lower Pleistocene 

 clays of Maine, it, as well as the whole group of European 

 immigrants, are pre-Glacial in age, and in this opinion I 

 concur with several of the authorities who have discussed, 

 this problem (p. 14). The members of this group arrived 



* Harshberger, John W., " North American Plant Dispersal," p. 2. . 



