THE FORESTS. 11 



been so profound as to affect severely all the forest growth within a 

 considerable radius. Arctic vegetation preceded the glaciers and 

 retired with them, but the descendants of some of the species are 

 still found upon mountain peaks. 



With the final recession of the ice, vegetation reoccupied the area 

 in successive waves. First the circumboreal plants, driven by the 

 rising temperatures, followed close upon the retreating feet of the 

 glaciers. In the hollows were lakes and ponds which were soon oc- 

 cupied by a swamp flora. Following these came the conifers, to 

 rehabilitate the land with a forest covering. 



About six regions of North America are now regarded as ancient 

 centers of plant migration. From these the northward movement 

 began at the close of the glacial age, and some of them furnished the 

 material for replanting the land denuded by the ice, TW T O of these 

 are important in the present discussion. The Arctic flora has already 

 been mentioned as having left its traces on the tops of the highest 

 mountains, but it is hardly represented by any woody species except 

 a few dwarf willows and heaths. 



Two centers of distribution remain to be considered. One of these 

 occupied a strip of the Pacific coast from northern California to 

 Washington west of the Cascades. From it species moved southward, 

 northward, and eastward. Some have now reached the Bitter Root 

 Mountains and the Flathead Valley, others have passed on as far as 

 the Black Hills. Those trees which have arrived from the Pacific 

 center are the grand fir, the Douglas spruce, the western larch, the 

 white pine, the hemlocks, the arbor vita?, the yew, the mountain ash, 

 the black cotton wood, the cascara, the flowering dogwood, several 

 willows, and many lesser plants. 



The other source of the Rocky Mountain flora lies in the far-off 

 Appalachians of the Carolinas and southward, where the flora is 

 one of great antiquity. Some of its species have stayed at home, but 

 others have wandered far afield. Those which have gone farthest 

 are those most easily aided by the wind in the distribution of their 

 seeds. From the Appalachian country some plants moved into Texas 

 and Kansas, others followed the Atlantic coast northward and oc- 

 cupied the country of the St. Lawrence and the region east of Hud- 

 son Bay. Still others wandered far to the Northwest, following the 

 streams across the plains or north into Canada and thence across the 

 continent, reaching the Pacific shores at Cook Inlet. In the Rocky 

 Mountains some of these moved southward, as the white spruce, 

 said to occur in a few places in Montana, but common throughout 

 Canada and the northeast. Among other trees from -this source 

 might be mentioned two species of dwarf juniper, the common cot- 

 tonwood of the Missouri Valley, the balsam cottonwood and the 



