There is now a class of experts whose sole occupation is to discover and locate, with something like scientific accuracy, taking account of the changes which present tendencies seem likely to bring about, restaurants, cigar stores, drug-stores, and other smaller retail business units whose success depends largely on location. Real-estate men are not infrequently willing to finance a local business of this sort in locations which they believe will be profitable, accepting as their rent a percentage of the profits. Physical geography, natural advantages and disadvantages, including means of transportation, determine in advance the general outlines of the urban plan. As the city increases in population, the subtler influences of sympathy, rivalry, and economic necessity tend to control the distribution of population. Business and industry seek advantageous locations and draw around them certain portions of the population. There spring up fashionable residence quarters from which the poorer classes are excluded because of the increased value of the land. Then there grow up slums which are inhabited by great numbers of the poorer classes who are unable to defend themselves from association with the derelict and vicious. In the course of time every section and quarter of the city takes on something of the character and qualities of its inhabitants. Each separate part of the city is inevitably stained with the peculiar sentiments of its population. The effect of this is to convert what was at first a mere geographical expression into a neighborhood, that is to say, a locality with sentiments, traditions, and a history of its own. Within this neighborhood the continuity of the historical processes is somehow maintained. The past imposes itself upon the present, and the life of every locality moves on with a certain momentum of its own, more or less independent of the larger circle of life and interests about it. The organization of the city, the character of the urban environment and of the discipline which it imposes is finally determined by the size of the population, its concentration and distribution within the city area. For this reason it is important to study the growth of cities, to compare the idiosyncrasies in the distribution of city populations. Some of the first things we want to know about the city, therefore are: What are the sources of the city’s population? What part of its population growth is normal, i.e., due to excess of births over deaths? What part is due to migration (_a_) of native stocks? (_b_) foreign stocks? What are the outstanding “natural” areas, i.e., areas of population segregation? How is distribution of population within the city area affected by (_a_) economic interest, i.e., land values? (_b_) by sentimental interest, race? vocation, etc.? Where within the city is the population declining? Where is it expanding?