Where are population growth and the size of families within the different natural areas of the city correlated with births and deaths, with marriages and divorces, with house rents and standards of living? _The neighborhood._—Proximity and neighborly contact are the basis for the simplest and most elementary form of association with which we have to do in the organization of city life. Local interests and associations breed local sentiment, and, under a system which makes residence the basis for participation in the government, the neighborhood becomes the basis of political control. In the social and political organization of the city it is the smallest local unit. It is surely one of the most remarkable of all social facts that, coming down from untold ages, there should be this instinctive understanding that the man who establishes his home beside yours begins to have a claim upon your sense of comradeship.... The neighborhood is a social unit which, by its clear definition of outline, its inner organic completeness, its hair-trigger reactions, may be fairly considered as functioning like a social mind.... The local boss, however autocratic he may be in the larger sphere of the city with the power he gets from the neighborhood, must always be in and of the people; and he is very careful not to try to deceive the local people so far as their local interests are concerned. It is hard to fool a neighborhood about its own affairs.[3] The neighborhood exists without formal organization. The local improvement society is a structure erected on the basis of the spontaneous neighborhood organization and exists for the purpose of giving expression to the local sentiment in regard to matters of local interest. Under the complex influences of the city life, what may be called the normal neighborhood sentiment has undergone many curious and interesting changes, and produced many unusual types of local communities. More than that, there are nascent neighborhoods and neighborhoods in process of dissolution. Consider, for example, Fifth Avenue, New York, which probably never had an improvement association, and compare with it 135th Street in the Bronx (where the Negro population is probably more concentrated than in any other single spot in the world), which is rapidly becoming a very intimate and highly organized community.