138 B. E. Fernow—The Battle of the Forest. 
it. Somewhat corresponding, yet not quite, to this distribution 
of moisture, the western slopes are found to be better wooded 
than the eastern, and the greater difficulty of establishing < 
forest cover here must be admitted; yet since the forest has the 
capacity of creating its own conditions of existence by increasing 
the most important factor of its life, the relative humidity, the 
extension of the Same may only be a question of time. 
Temperature extremes, to be sure, also set a limit to tree- 
growth, and hence the socalled timber line of high mountains, 
which changes in altitude according to the latitude. 
If, now, we turn our attention from the phyto-topographic - 
consideration of the forest cover to the phyto-geographic and 
botanical features, we may claim that the North American forest, 
with 425 or more arborescent species, belonging to 158 genera, 
many of which are truly endemic, surpasses in variety of useful 
species and magnificent development, any other forest of the tem- 
perate zone, Japan hardly excepted. In addition there are prob- 
ably nowhere to be seen such extensive fields of distribution of 
single species. 
These two facts are probably explained by the north-and- 
south direction of the mountain ranges, which permitted a rees- 
tablishment after the Ice age of many species farther northward, 
while in Europe and the main part of Asia the east-west direc- 
tion of the mountains offered an effectual barrier to such rees- 
tablishment, and reduced the number of species and their field 
of distribution ; nor are the climatic differences of different lati- 
tudes in North America as great as in Europe, which again 
predicates greater extent in the fields of distribution north and 
south. On the other hand, the differences east and west in floral 
composition of the American forest are greater than if an ocean 
had separated the two parts instead of the prairie and plains. 
This fact would militate against our theory that the intermediate 
forestless region was or would be eventually forested with species 
from both the established forest regions, if we did not find some 
species represented in both regions and a junction of the two 
floras in the very region of the forestless areas. 
In the sand hills which traverse Nebraska from east to west 
there are now found in eastern counties the sand-drowned trunks 
of the western bull pine, and the same pine belonging to the 
Pacific flora is found associated with the black walnut of the 
eastern region along the Niobrara river. 
