The successive stages of social development--savage, pastoral nomadic, agricultural, and industrial--represent increasing density of population, increasing numerical strength of the social group, and finally increasing geographical area, resulting in a vastly enlarged social group or state. Increase in the population of a given land is accompanied by a decrease in the share which each individual can claim as his own. This progressive readjustment to a smaller proportion of land brings in its train the evolution of all economic and social processes, reacting again favorably on density of population and resulting eventually in the greatly increased social group and enlarged territory of the modern civilized state. Hence we may lay down the rule that change in areal relations, both of the individual to his decreasing quota of land, and of the state to its increasing quota of the earth's surface is an important index of social and political evolution. Therefore the rise and decline not only of peoples but of whole civilizations have depended upon their relations to area. Therefore problems of area, such as the expansion of a small territory, the economic and political mastery of a large one, dominate all history. [Sidenote: The Oikoumene.] Humanity's area of distribution and historical movement call the Oikoumene. It forms a girdle around the earth between the two polar regions, and embraces the Tropics, the Temperate Zones, and a part of the North Frigid, in all, five-sixths of the earth's surface. This area of distribution is unusually large. Few other living species so nearly permeate the whole vital area, and many of these have reached their wide expansion only in the company of man. Only about 49,000,000 square miles (125,000,000 square kilometers) of the Oikoumene is land and therefore constitutes properly the habitat of man. But just as we cannot understand a nation from the study of its own country alone, but must take into consideration the wider area of its spreading activities, so we cannot understand mankind without including in his world not only his habitat but also the vastly larger sphere of his activities, which is almost identical with the earth itself. The most progressive peoples to-day find their scientific, economic, religious and political interests embracing the earth. [Sidenote: Unity of the human species in the relation to the earth.]