GEOGRAPHICAL DISTRIBUTION l6l 



continental glacier during the Ice Age. With the advance 

 of the ice all vegetation was either exterminated or com- 

 pelled to migrate southward. With the subsequent 

 retreat of the ice northward the glaciated region was 

 gradually re-occupied by the encroachment of vegetation 

 from the south, and of this flora the arctic species could 

 become permanently re-established only in what are now 

 the arctic regions, and in the arctic or sub-arctic climate 

 of the higher mountain tops, forming there what is known 

 as a relict flora. 1 It has been suggested that, in theory, 

 alpine plants on high mountain peaks south of the region 

 covered by the continental ice sheet, should not be related 

 to arctic and sub-arctic forms. In harmony with this 

 idea Wallace has cited the volcanic Peak of Teneriffe 

 (Pico de Teyde), in the Canary Islands, 12,000 feet high, 

 where, above the timber line, von Buch found only eleven 

 species of plants, eight of which appeared to be endemics ; 

 but all of them were related to the plants of the same 

 general region, growing at lower levels. 



However, seed-distribution by birds and winds and 

 other agencies has been going on continually since the 

 continental ice sheet began to recede, with the result 

 that arctic-alpine and subarctic-alpine plants are numer- 

 ous in the alpine zone of higher peaks below the southern 

 limits of continental glaciation. Thus the snowy cinque- 

 foil (Potentilla nivea) is found, not only throughout the 

 arctic regions, but also in the Alps, in alpine Asia, and 

 in the Rocky Mountains as far south as Utah and Colo- 



1 The effect of continental glaciation on the distribution of plants was 

 first noted by Edward Forbes, but was also worked out independently by 

 Darwin several years previous to the publication of Forbes's paper. 

 (Darwin, C. Life and Letters, I : 71-72, 372. New York, 1901. See also 

 The Origin of Species, 2: 152. New York, 1902.) 



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