NORTH AND SOUTH. 369 



mals and reptiles, and the growth and development of many 

 different kinds of forest trees and wild flowers are thus defi- 

 nitely outlined. 



Topographical conditions exert an important influence in the 

 distribution of surface temperature. High mountain ranges, 

 running southward, carry the cooler or more temperate condi- 

 tions of the regions to the north along their crests far into the 

 warm zone which they penetrate. Likewise, lowland plains, ex- 

 tending northward, carry the conditions of greater warmth into 

 the cooler area of higher latitudes. It is not surprising, there- 

 fore, in passing from lowland to upland districts to find more or 

 less of a change in the character of the vegetation and in the 

 animal life. Certain species, quite abundant in the lowland 

 region, disappear on the higher ground, where other kinds, not 

 met with in the lowlands, make their appearance. 



Through the researches of biologists * it has been found that 

 the continent of North America may be divided north and south 

 into several great temperature belts or heat zones, each one of 

 which is characterized by peculiarities in its inhabitants. The 

 boundaries of these heat zones are marked by isotherms which 

 include a certain definite range of temperature that characterizes 

 the contained area of country and which is definitely related to 

 the reproductive functions of the animals and plants inhabiting 

 it. It is an interesting fact to note that the isotherm represent- 

 ing the boundary between two of these heat zones coincides with 

 the line that marks off the inland border of the Atlantic coast 

 plain from the interior uplands of the highland region. If we 

 turn our attention to the distribution of life in North America, 

 we shall find some facts that do not quite agree with our already 

 conceived ideas as to the divisions of the continent. An irregular 

 line drawn from the coast of northern New England northwest- 

 ward across the Great Lakes to the head waters of the Saskatche- 

 wan serves roughly to mark off a vast northern area known as 

 the "boreal zone. Its chief characteristic is the predominance of 

 coniferous forests which stretch away northward to the Barren 

 Grounds of arctic America. The inland border of the Atlantic 

 coast plain, after bending around the end of the Appalachian 

 highland region in northeastern Alabama, runs northward along 

 the western base of the mountains to Lake Ontario. Then, turn- 

 ing sharply westward, it pursues an irregular course across the 

 lower lake region and upper Mississippi Valley to the base of the 

 Rocky Mountains. This extremely irregular line marks off a 



* Especially those of Dr. C. Hart Merriam. See National Geographic Magazine, vol. vi, 

 p. 229. The Geographic Distribution of Life in North America. From Smithsonian Report 

 for 1891, p. 365. 



