FOREST GEOGRAPHY AND ARCHAEOLOGY. 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, ouee 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 ffreat continental interiors out of the 

 equatorial belt, except where cloud-compelling mountain- 

 chains coerce a certain deposition of moistiywrfrom air which 



