FOREST GEOGRAPHY AND ARCHEOLOGY. 207 
very limited; and it cannot flee for shelter. But trees are 
social, and their gregarious habits give a certain mutual sup- 
port. A tree by itself is doomed, where a forest, once estab- 
lished, is comparatively secure. 
Trees vary as widely as do other plants in their constitu- 
tion; but none can withstand a certain amount of cold and 
other exposure, nor make head against a certain shortness of 
summer. Our high northern regions are therefore treeless ; 
and so are the summits of high mountains in lower latitudes. 
As we ascend them we walk at first under Spruces and Fir- 
trees or Birches; at 6000 feet on the White Mountains of 
New Hampshire, at 11,000 or 12,000 feet on the Colorado 
Rocky Mountains, we walk through or upon them ; sometimes 
upon dwarfed and depressed individuals of the same species 
that made the canopy below. These depressed trees retain 
their hold on life only in virtue of being covered all winter 
by snow. . At still higher altitudes the species are wholly dif- 
ferent ; and for the most part these humble alpine plants of 
our temperate zone — which we cannot call trees, because 
they are only a foot or two or a span or two high —are the 
same as those of the arctic zone, of northern Labrador, and of 
Greenland. The arctic and the alpine regions are equally 
unwooded from cold. 
As the opposite extreme, under opposite conditions, look to 
equatorial America, on the Atlantic side, for the wildest and 
most luxuriant forest-tract in the world, where winter is un- 
known, and a shower of rain falls almost every afternoon. 
The size of the Amazon and Orinoco — brimming throughout 
the year — testifies to the abundance of rain and its equable 
distribution. 
The other side of the Andes, mostly farther south, shows 
the absolute contrast, in the want of rain, and absence of for- 
est; happily it is a narrow tract. The same is true of great 
tracts either side of the equatorial regions, the only district 
where great deserts reach the ocean. 
It is also true of great continental interiors out of the 
equatorial belt, except where cloud-compelling mountain- 
chains coerce a certain deposition of moisture from air which 
