296 RYDBERG: Puy APHICAL NOTES 
the common transcontinental species that grow along the water- 
courses and species which are common to the Hudsonian-Subalpine 
Zone and the Canadian-Montane Zone. None of the conifers are 
common to the two regions, and among the trees which occur in 
common we find only a few species of Salix, Populus, and Betula. 
If we consider the relationship between the Montane plants of 
the Rockies and those of the Pacific Coast Mountains, however, 
we find the.conditions quite different. The Cascade Mountains 
are directly connected with the Rockies in the north and there 
are several mountain chains interposed between the Cascades and 
the Rockies in British Columbia, separated from one another only 
by narrow river-valleys. The Montane Zone of the Cascades, 
therefore, is practically continuous with that of the Rocky Moun- 
tains, and the two regions have many plants in common, the 
leading conifers not excluded. Many of the Pacific species have 
emigrated into the Rocky Mountains, especially into the northern 
Rockies, and many Rocky Mountain species into the Cascades. 
As has already been pointed out in earlier articles of mine, the 
Rockies may be divided into two principal parts, the Northern 
Rockies, extending from the Yukon Territory to northern Wyo- 
ming, and the Southern Rockies, of southern Wyoming, Colorado 
and northern New Mexico. Each of these main divisions may 
be subdivided, and the following districts may be distinguished: 
Northern Rockies 7. Big Horn District 
1. Canadian Rockies 8. Black Hills District 
2. Main Range, Montana Southern Rockies 
3. Selkirk-Bitterroot District 9. Main Range District 
4. Belt Mountains District 10. Uintah-Wasatch District 
5. Yellowstone District 11. Sevier District 
6. Sawtooth District 12. La Sal-Abajo District 
The main range of the Northern Rockies north of Butte, 
Montana, is rather homogeneous and the change in the Montane 
flora seen in proceeding northward is rather gradual, although 
many of the high northern species found in the Canadian Rockies 
(District 1) are not found in Montana. 
South of Butte the main range (District 2) becomes lower and 
less distinctly Montane until the neighborhood of Yellowstone 
Park is reached. Here it rises higher and the Alpine Zone 's 
present in the Bear Tooth, Shoshone, Teton, Gros Ventre, and 
