The city has been studied, in recent times, from the point of view of its geography, and still more recently from the point of view of its ecology. There are forces at work within the limits of the urban community—within the limits of any natural area of human habitation, in fact—which tend to bring about an orderly and typical grouping of its population and institutions. The science which seeks to isolate these factors and to describe the typical constellations of persons and institutions which the co-operation of these forces produce, is what we call human, as distinguished from plant and animal, ecology. Transportation and communication, tramways and telephones, newspapers and advertising, steel construction and elevators—all things, in fact, which tend to bring about at once a greater mobility and a greater concentration of the urban populations—are primary factors in the ecological organization of the city. The city is not, however, merely a geographical and ecological unit; it is at the same time an economic unit. The economic organization of the city is based on the division of labor. The multiplication of occupations and professions within the limits of the urban population is one of the most striking and least understood aspects of modern city life. From this point of view, we may, if we choose, think of the city, that is to say, the place and the people, with all the machinery and administrative devices that go with them, as organically related; a kind of psychophysical mechanism in and through which private and political interests find not merely a collective but a corporate expression. Much of what we ordinarily regard as the city—its charters, formal organization, buildings, street railways, and so forth—is, or seems to be, mere artifact. But these things in themselves are utilities, adventitious devices which become part of the living city only when, and in so far as, through use and wont they connect themselves, like a tool in the hand of man, with the vital forces resident in individuals and in the community. The city is, finally, the natural habitat of civilized man. It is for that reason a cultural area characterized by its own peculiar cultural type: “It is a quite certain, but never fully recognized, fact,” says Spengler, “that all great cultures are city-born. The outstanding man of the second generation is a city-building animal. This is the actual criterion of world-history, as distinguished from the history of mankind: world-history is the history of city men. Nations, governments, politics, and religions—all rest on the basic phenomenon of human existence, the city.”[2]