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,* 
I. 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; 
(6) Asiatic arctic-alpine or glacial plants, with the center in the 
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. 
