1903-4. | How PLAntT LIFE IS DISTRIBUTED IN CANADA AND Why. 37 
reached 135 inches and at Clayoquot 146 inches, whilst in other places 
on the west coast of Vancouver Island this precipitation has averaged 
112 inches. On the mainland at Vancouver (city) it reaches sixty inches 
and at Port Simpson ninety-three inches. Whilst this humid climate 
prevails on the west side of the coast range, on the east side, especially 
in the valleys of the Fraser and Thompson Rivers north of Lytton, on- 
wards to about Alexandria near latitude 53°, and in the districts sur- 
rounding Okanagan Lake and to some extent in the eastern Kootenay 
and Upper Columbia River districts, the precipitation is deficient and 
irrigation is necessary. The annual rain and snow fall together vary on 
the average from ten inches at Midway and eleven inches at Kamloops 
to seventeen inches at Chilcotin and eighteen inches at Tobacco Plains. 
The dry climate has led to the appearance of some of the plants of the 
semi-arid sections of California and Nevada. 
That a large number of plants very familiar in Oregon and Washington 
and some in California should find their way to the immediate northward 
of the British Columbia boundary line, their range being facilitated by 
the direction of the mountain ranges and the river valleys, and the mild 
climate in these valleys, would be anticipated. One of the remarkable 
features, however, of, in great part, the southern half of the province, is 
the very large number of northern temperate plants which, in Canada, 
are exclusively British Columbian. They have originated on the milder 
and more humid plateaus and valleys west of the Rocky Mountains, and, 
with this great mountain range, as well as the treeless plains and dry 
climate of Alberta, as barriers to keep them there, they have remained 
a peculiarly western flora. Towering trees as well as shrubs and her- 
baceous plants are included in the group, and not a few of them may be 
regarded as representatives of kindred species in eastern Canada. In 
age, the species of this group are younger than the general Canadian 
flora and date their gradual evolution from a time when the raising of 
the great mountain ranges and inland plateaus created new conditions 
of humidity and dryness, light and shade, heat and cold, broad river 
valleys and high elevations above them, and the association of the smaller 
plants with forests of a more lofty type and so often amid rugged sur- 
roundings. 
Another interesting assemblage has been referred to as the Rocky 
Mountain Group. The general elevation, the rugged environment, and 
the very ample precipitation on the western sides of the Rocky Moun- 
tains and the Selkirk range, although falling very short of that on the 
British Columbia coast, has resulted in conditions on the lower slopes of 
these mountains favourable to very many plants which at the same time 
are never found near the more humid, milder coast. The precipitation 
