Page:Popular Science Monthly Volume 51.djvu/381

Rh and reptiles, and the growth and development of many different kinds of forest trees and wild flowers are thus definitely outlined.

Topographical conditions exert an important influence in the distribution of surface temperature. High mountain ranges, running southward, carry the cooler or more temperate conditions of the regions to the north along their crests far into the warm zone which they penetrate. Likewise, lowland plains, extending northward, carry the conditions of greater warmth into the cooler area of higher latitudes. It is not surprising, therefore, 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 representing 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 northwestward across the Great Lakes to the head waters of the Saskatchewan 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, turning 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