

420 THE SYLVA OF MONTANA. 
most of them grow largest on its western slope, only half 
of them reached the eastern rim along our route, and sev- 
eral of these were merely stragglers. ‘This accords with the 
general rule that the most trees, both in number and species, 
grow where the most heat and moisture are combined. The 
forests of the western rim are far denser than those of the 
eastern, though the soil cannot be considered generally go 
good on aecount of the kinds of rocks from which it is dis- 
integrated. 
An exactly parallel case is presented by the Cascade and 
Coast Ranges, as described in the Natural History of Wash- 
ington Territory (Pacific R. R. Reports), but there the spe- 
cies, though mostly the same, are somewhat differently ar- 
ranged to correspond with differences in climate, consequent 
on the much lower elevation of those ranges and their near- 
ness to the ocean. Yet we there find Pinus contorta, Thuja 
gigantea, Abies Menziesii, A. Mertensiana and Taxus brevi- 
Jolia among the prevailing species at the level of the ocean, 
while here they do not oceur lower than 2000 feet above it, 
showing that they require moisture rather than coolness of 
climate, for at the coast the rains are heavier while the mean 
temperature is far more mild than here. But Pinus pon- 
derosa, Acer glabrum, Betula occidentalis, Crategus rivula- 
ris, Larix occidentalis, Pinus monticola, Pyrus fraxinifolia 
and Abies Williamsonii, here characteristic trees, scarcely, 
if at all, cross the Cascade Range, while Abies Douglassii, 
and several peculiar species not found here, replace them 
between that and the Coast Range. ; 
It is therefore much safer to- assume a similarity in the 
moisture of the climate and soil of two regions thus widely 
separated, from comparison of their forests, than similarity 
in temperature. Iam here comparing portions of two re- 
gions included between the same degrees of latitude, but 
according to another rule dependent on the climate of the 
| western regions, all the above species of Rocky Mountain 
trees are found, or probably will be found to reach the ¢ 




