PLANT MIGRATION. 503 



along isothermal lines, since heat was then recognized as the 

 greatest factor in the distribution of plants and animals. But 

 since the mean temperature of arbitrary periods (annual or 

 monthly) does not coincide with the mean temperature of the 

 season for growth and reproduction, lines were established by 

 Merriam which coincide with the physiological constant of the 

 species. These lines form more accurate limits of the life 

 regions, and may be called biothermal lines, or biotherms. 



The Boreal region 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 nar- 

 row strip along the Pacific coast and the higher parts of the 

 three great mountain systems, the Sierra Cascade Range, the 

 Rocky Mountains, and the Alleghanies. 



The Austral region covers the whole of the United States and 

 Mexico, except the boreal mountain heights and the tropical 

 lowlands. 



The Tropical region covers the southern part of the peninsula 

 of Florida, the greater part of Central America, the lowlands of 

 southern Mexico south of the table-land, and a narrow strip on 

 each side of Mexico, which follows the coast northward into the 

 United States. 



970. Life zones and areas. The flora and fauna within each 

 of these great regions are not homogeneous, but present marked 

 differences, which have led to the subdivision of each region into 

 a number of minor belts or areas, characterized by particular 

 associations of animals and plants. Thus the Boreal region is 

 divided into three transcontinental belts or zones, known re- 

 spectively as the Arctic, Hudsonian, and Canadian; the Austral 

 region, into three transcontinental belts, known as the Transi- 

 tion, Upper Austral, and Lower Austral. The Tropical region 

 is likewise divisible, but the tropical areas within the United 

 States are of small extent. These transcontinental zones are 

 further divided into eastern, central, and western areas, because 

 of the influence and distribution of rainfall. The following 

 table will show the subdivisions: 



