periods of advance and retreat respectively. As the glacier 

 dwindled, some of the New England peaks, such as Washington 

 and Ktaadn, soon projected above the surface. When sufficient 

 area was thus exposed, the accumulation of snowfields was again 

 permitted, and valley glaciers descended from the mountain tops. 

 In other words, following the withdrawal of the great continental 

 glacier, there came a period of glaciation. The northward 

 migrating flora was now met by these local ice sheets and tem- 

 porarily retarded. The continued shrinkage of these large cen- 

 ters finally gave rise to coalescing valley glaciers ; again permit- 

 ting the northward advance of plants toward these centers of 

 local glaciation. 



In this advance, most naturally, the Arctic forms were 

 the pioneers, following closely the melting ice front and 

 obtaining a foothold wherever morainic soil was exposed. This 

 advance was, as pointed out above, in unison and was also one 

 of latitudinal zonation ; the temperate plants, following closely 

 the progression of the Arctic forms. As the coalescing valley 

 glaciers gave way to isolated ones, the migrating Arctic species 

 came to the lowlands about these high peaks. As the snows 

 melted above, and as these now isolated valley glaciers retreated 

 in their cirques, a separation in the previously compact Arctic 

 flora took place. Some individuals pursued the receding snows 

 up the mountain slopes, occupying every inch of exposed ground, 

 while the main line of migration continued northward with the 

 ever shrinking glacier. As amelioration progressed and the 

 valley glaciers melted these Arctic forms ascended still higher. 

 The main body of migrants pushed onward in its northward 

 journey; while mingled temperate and pre-glacial alpine forms, 

 on the approach of normal climatic conditions, came to occupy 

 the intervening space between the mountains. Thus we have 

 Arctic products isolated upon Ktaadn. 



When these Arctic species began their mountain ascent they 

 were, because of migration in unison, practically unmodified and 

 identical with their northward journeying brethren, which, as 

 complete warmth returned, had once more come to occupy the 

 Arctics. As they ascended, unlike the compact body migrating 

 northward, they were subject to modifying influences in such 



