PART II. 



LIFE ZONES OF THE UNITED STATES : BOUNDARIES, NATIVE 

 SPECIES, AND IMPORTANT CROPS. 



The continent of North America may be divided, accordinjj: to the 

 distribution of its animals and plants, into three primary transconti- 

 nental regions — Boreal, Austral, and Tropical. 



The Boreal region (coloi-ed green on the map) covers the whole of 

 the northern part of the continent, from the polar sea southward to 

 near the northern boundary of the United States, and farther south 

 occupies a narrow strij) along the Pacific coast and the higher parts 

 of the three great mountain sj'stems, the Sierra-Cascade Range, the 

 Rocky Mountains, and tlie Alleghanies. 



The Austrdl region (colored blue, yellow, and brown on the map) 

 coveis the whole of the United States and Mexico except the ]>oreal 

 mountains and Tropical lowlands. 



The Tropical region (colored red on the map) covers the southern 

 l)art of the peninsula of Floi-ida, the greater part of Central America, 

 the lowlands of southei-n INIexico soutli of the table-land, and a nar- 

 row strip on each side of iNfexico, wliicli follows the coast northward 

 into the United States. 



The fauna and flora within each of these three great regions are not 

 homogeiuH)us, l)ut present marked differences, wliich liave led to the 

 subdivision of each region into a number of minor belts or areas, char- 

 acterized by particular associations of animals and plants. Thus, the 

 Boreal region is divided into three transcontinental belts or zones, 

 known respectively as the Arctic, Iludsonian, and Canadian; the 

 Austral region, into three transcontinental belts, known as the Tran- 

 sition, IFpper Austral, and Lower Austral. Tlie Tropical region is 

 likewise divisible, but the tropical areas within the United States are 

 of such small extent that the divisions need not be here considered. 

 Hence, in North America north of southern Mexico, the total numl)er 

 of transcontinental life zones is seven, of which three are Boreal, 

 three Austral, and one Tropical. Beginning at the north, these zones 

 may be described as follows: 



1. THE ARCTIC-ALPINE ZONE. 



The Arctic or Arctic- Alpine zone lies above the limit of tree giowth 



and is characterized by such plants as the arctic poppy, dwarf willow, 



and various saxifrages and gentians. In the far north the snow l)unt- 



ing, snowy owl, ptarmigan, polar bear, arctic fox, polar hare, lemmings, 



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