The fact is, however, that the city is rooted in the habits and customs of the people who inhabit it. The consequence is that the city possesses a moral as well as a physical organization, and these two mutually interact in characteristic ways to mold and modify one another. It is the structure of the city which first impresses us by its visible vastness and complexity. But this structure has its basis, nevertheless, in human nature, of which it is an expression. On the other hand, this vast organization which has arisen in response to the needs of its inhabitants, once formed, imposes itself upon them as a crude external fact, and forms them, in turn, in accordance with the design and interests which it incorporates. Structure and tradition are but different aspects of a single cultural complex which determines what is characteristic and peculiar to city, as distinguished from village, life and the life of the open fields. _The city plan._—It is because the city has a life quite its own that there is a limit to the arbitrary modifications which it is possible to make (1) in its physical structure and (2) in its moral order. The city plan, for example, establishes metes and bounds, fixes in a general way the location and character of the city’s constructions, and imposes an orderly arrangement, within the city area, upon the buildings which are erected by private initiative as well as by public authority. Within the limitations prescribed, however, the inevitable processes of human nature proceed to give these regions and these buildings a character which it is less easy to control. Under our system of individual ownership, for instance, it is not possible to determine in advance the extent of concentration of population which is likely to occur in any given area. The city cannot fix land values, and we leave to private enterprise, for the most part, the task of determining the city’s limits and the location of its residential and industrial districts. Personal tastes and convenience, vocational and economic interests, infallibly tend to segregate and thus to classify the populations of great cities. In this way the city acquires an organization and distribution of population which is neither designed nor controlled. The Bell Telephone Company is now making, particularly in New York and Chicago, elaborate investigations, the purpose of which is to determine, in advance of its actual changes, the probable growth and distribution of the urban population within the metropolitan areas. The Sage Foundation, in the course of its city-planning studies, sought to find mathematical formulae that would enable them to predict future expansion and limits of population in New York City. The recent development of chain stores has made the problem of location a matter of concern to different chain-store corporations. The result has been the rise of a new profession.