Rydberg: Phytogeographical notes 91 



glaciers increased and extended downwards and the forests re- 

 ceded down the mountain sides. At last the foothills and even 

 the plains of northern Europe, Asia and America received an 

 alpine-arctic climate and bore mostly arctic vegetation. As the 

 ice sheet receded, the climate became more temperate, the forest 

 again took possession of the land and the arctic plants were partly 

 driven towards the pole, partly up the mountains, until in our 

 days they are confined to the arctic regions and the highest 

 mountain tops, where they have become isolated. In America 

 the glacial drifts did not reach as far south as Colorado, but evi- 

 dently the temperature during the glacial epoch must have been 

 low enough, so that the local glaciers of the Rockies may have 

 covered most of the mountains, and the great plains and the foot- 

 hills surrounding them undoubtedly had an arctic or subarctic 

 climate at that time. Many of the arctic plants had originated 

 before the glacial period and had spread over the two continents. 

 Some of these still exist on both, others might have died out 

 on one of the continents, but remain on the other. Some 

 might become exterminated in the mountain regions while con- 

 tinuing their existence in the arctic regions, or vice versa. 

 Some, after isolation, might have changed in the course of time 

 and developed into new species. This is perhaps more common 

 in the case of the alpine than of the arctic plants. The arctic- 

 alpine plants may therefore be classified in the following cate- 

 gories.* 



1. Circumpolar arctic-alpine or glacial plants, found in the 

 arctic as well as in the mountain regions of both continents. 



2. Eurasian arctic-alpine or glacial plants, found only in the 

 mountains of the Old World and the arctic regions. Some of 

 these may be: 



(a) European arctic-alpine or glacial plants, with the center of 

 distribution in the Alps; 



(b) Asiatic arctic-alpine or glacial plants, with the center in the 

 Altai. 



3. American arctic-alpine or glacial plants, confined to the 

 mountains of the New World and the arctic. 



* Here I do not include the alpine plants of tropical regions, which must be 

 considered altogether by themselves. 



