Hill: Penobscot Vegetation 321 



glaciers was checked. Upon the withdrawal of the ice the 

 plants followed back to their old haunts, going in three 

 waves as Adams (1) conceives it. First the arctic-alpine 

 species, growing at the very edge of the melting ice, grad- 

 ually moved northward until they reached their present dis- 

 tribution in the Arctic Circle, leaving behind relic species 

 on the high mountain tops, the cold exposed shores and 

 other favorable situations. Next followed the trees of the 

 coniferous forest and the accompanying herbaceous vegeta- 

 tion, and these eventually attained the region of their pres- 

 ent widespread distribution in the acid Laurentian area of 

 eastern Canada. This group likewise left behind a much 

 greater proportion of relic forms in favorable places. 

 Finally the deciduous species began in their turn the north- 

 ward migration and, although the climate had sufficiently 

 ameliorated to permit their reaching the Maritime Pro- 

 vinces and the St. Lawrence valley, in many places they 

 found the ground already pre-empted by the conifers. In 

 favorable places they have been able to win out in the en- 

 suing competition, failing to do so only where physiographic 

 conditions were distinctly unfavorable for their growth. The 

 result is the present day mixture of northern and southern 

 forms, physiographic climax association-types, character- 

 ized by coniferous trees, occurring interspersed among ex- 

 amples of the regional climax association-type, in which de- 

 ciduous species predominate. The nature of the vegetation 

 in this zone and the successions leading to the climax asso- 

 ciation-type have been discussed in detail by Nichols (23) 

 for Cape Breton Island. 



Bearing in mind the statement of Harvey that ecologi- 

 cally a region should be classified upon the basis of the 

 highest ecological type which may be developed, it is evident 

 that the entire Transition region should be considered as 

 one which climatically is capable of developing a deciduous 

 climax, even though this potential climax fails of attain- 

 ment in many localities. Consequently, with this interpreta- 



