factors as the co-mingling with pre-glacial alpine forms, seek- 

 ing their original habitats, and new alpine climatic conditions, 

 x\gain, it is not to be presumed that identical species ascended 

 mountain peaks widely separated. We thus readily interpret 

 any varietal, specific, or even generic peculiarities which may 

 exist upon any mountain. However, the majority of Arctic and 

 alpine forms have, through the fixity of the specific type, come 

 down to us unchanged from glacial times. But we must not 

 neglect the possibility of subsequent distribution from a center 

 as the factor in maintaining this specific identity in Arctic and 

 alpine species. Lyco podium Selago is identical the world over 

 in Arctic and Alpine habitats. This identity may be due not to 

 the fixity of the specific type, for in Alpine conditions it has been 

 subjected to the modifying influences noted above, but to the fact 

 of frequent introduction, by wind dispersal, of individuals of 

 the specific type derived from the Arctic centers by distribution, 

 and the consequent commingling. 



The flora of Mt. Ktaadn is then glacial in origin, being 

 isolated, as a glacial relict, by the northward retreat of the con- 

 tinental ice sheet. We may now examine in particular its genesis 

 upon the mountain. We have seen that, as the isolated valley 

 glaciers, such as those occupying the North, South, and North- 

 west basins, retreated before "the increasing warmth, Arctic 

 plants approached the mountain base, and, as this local recession 

 continued, the flora arrived at the base of Ktaadn. 1 



1. With the establishment of the isolated valley glaciers, it is quite probable 

 that the higher peaks of the mountain arose as menataks above the local glacier; 

 and with the ever increasing exposure must have formed a foot hold for these 

 Arctic forms. It seems more probable, however, that the principal encroach- 

 ment was from base to summit. 



The first forms to reach the mountain were, in all probability, 

 lichens of the crustaceous type, such as Buellia geographica, 

 which found ready foothold on the increasing exposure of gran- 

 itic rock. Following closely the ascent of the pioneer lichen 

 society was that of the reindeer-iceland-moss combination, 

 encirling the mountain as a basal zone and ascending as the 

 pioneer society advanced. Encroaching upon this zone from 

 below came that of the Alpine tundra, extending out into the 

 lowlands, followed in turn by the Krummhols and passing grad- 

 ually into the Picea-Abies forest which doubtless covered the 



15 



