Civilizations have flourished in certain climatic zones and not elsewhere. At one historical period civilizations were established in the tropics and semi-tropics. In the present period they are located chiefly in temperate climatic belts. Another source of differences between civilizations is the variation and the adaptability of certain peoples to the peculiar conditions out of which civilization grows. Still another explanation of the presence or absence of civilization in particular times and places is the "great man" theory of history. All human communities, pre-civilized and civilized, have had gifted leaders whose thoughts and actions have brought about social changes. These "greats" were the divinely, ideologically or sociologically inspired. Divine inspiration or revelation led to the founding of religious faiths. Ideological and sociological inspiration resulted in domestic cultural changes and the extension of economic, cultural and ideological activities into foreign lands, thus pushing the frontiers of nations, empires, and civilizations farther from the chief wealth-power centers. Thomas Carlyle wrote that history is the lengthened shadows of a few great men. Arnold Toynbee concluded from his _Study of History_ that religion has been a prime motive force in the building and preservation of civilizations. Technology has been a motive force of hard-to-define importance in revitalizing, changing, expanding and perpetuating civilizations. Increased productivity, expressing itself as increases in income, accumulated wealth and various forms of capital investment, have provided the economic basis for population growth and the more effective exploitation of natural resources and labor power, advances in the means for transportation and communication, accounting, planning management and "defense." Among the social motive forces responsible for the development of civilization is the accumulation of wealth in an impoverished world. The most important single factor in this connection was the development of a class of businessmen in a society dominated by landlords, churchmen and soldiers. Landlords, churchmen and soldiers lived during periods of animal husbandry and primitive agriculture on the very narrow margins produced during bountiful harvests. When harvests were bad, husbandmen and farmers were reduced to starvation levels. Lacking means of storage and refrigeration as well as facilities for transporting heavy materials such as food, fuel and building materials, pre-civilized society accumulated wealth slowly in mobile forms (precious metals and jewels) and made few productive investments.