LIFE ZONES 
BOTH physical geographers and naturah'sts divide the earth 
into hfe zones, from the Boreal Regions to the Tropics; 
and in the latter, in the mountainous portions, there is a 
corresponding division from the bases to the summits of 
the mountains, where, if the latter are sufficiently lofty, we 
have animals and plants representing the Arctic fauna and 
flora. It naturally follows that in the intermediate regions the 
different zones are also represented on the mountain sides. 
The distribution of animals and plants depends on tempera- 
ture, and Dr. Merriam has demonstrated that "the north- 
ward distribution of terrestial animals and plants is governed 
by the sum of the positive temperatures for the entire season 
of growth and reproduction, and that the southward distri- 
bution is governed by the mean temperature of a brief 
period during the hottest part of the year." He has divided 
the United States into Boreal (Northern), Austral (Southern), 
and Transition (Intermediate) climates, fauna, and flora. 
These are further subdivided into the following life zones: 
Arctic or Alpine, Hudsonian, Canadian, Transition, Upper 
Sonoran, Lower Sonoran, and Tropical. In Colorado 
we have the first five named zones, and possibly a very little 
Lower Sonoran in the lowest portions of western Colorado. 
The Alpine Zone is above the tree limit or timber-line which 
in Colorado is at about 12,000 feet or a little less. It is 
characterized by dwarf shrubs and plants. In Colorado 
there are no characteristic mammals, though in the Arctic 
regions there are the Polar Bear, Arctic Fox, and Reindeer. 
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