The constant heat and excessive moisture of the atmosphere in the torrid zone is apt to produce a feeling of lassitude among the dwellers in such regions, moreover, and great bodily activity is out of question. These conditions seriously affect the lives of the people, and, with few exceptions, tropical peoples are rarely noted for energy or enterprise. Great commercial enterprises are the exception rather than the rule, and they are usually carried on by foreigners who must live a part of the time in cooler localities. [Illustration: THE EFFECTS OF HIGH LATITUDE--TOO COLD TO PRODUCE BREAD-STUFFS] Polar regions are deficient both in the heat and light necessary for food-stuffs. Neither the grasses nor the grains fructify. As a result, but few herbivora can live there, and these are practically restricted to the musk-ox and the reindeer, which subsist on mosses and lichens. The native people are stunted in growth; their food consists mainly of raw blubber, and they are scarcely above savagery. The temperate zones are the regions of the great industries and activities of human life. The larger part of the land surface of the earth is situated in these zones; moreover, the people who dominate the world also live in them, and their supremacy is due largely to conditions of climate. The alternation of summer and winter causes a struggle for existence that develops the intellectual faculties and results in industrial supremacy. =Effects of Altitude.=--There is a decrease of temperature of 1 deg. F. for about every three hundred feet of ascent. But few people live at an altitude of more than six thousand feet above sea-level, and in many cases they depend on other localities for the greater part of their food-stuffs, because very few of such regions produce food-stuffs abundantly. The chief exceptions to this rule are found in tropical regions. The highlands of Mexico, the plateau-regions of Bolivia and Ecuador, and the highlands of southern Asia are habitable, but they are not densely peopled. Because of their altitude they are relieved of the enervating effects of tropical climate at the sea-level. Altitude likewise affects the amount of rainfall. Most plateaus are arid. As a rule, they are arid because of their altitude; and because of their aridity they are deficient in their power to produce food-stuffs. They are therefore sparsely peopled. =Effects of Rainfall.=--Regions having considerably more than one hundred inches of rain annually are very apt to be forest-covered, and therefore to be deficient in food-producing plants. Such localities have usually a sparse population, in spite of the profusion of vegetation. In some parts of India, lands that have been left idle for a few seasons produce such a dense jungle of wild vegetation that to reclaim them for cultivation is wellnigh impossible.