9. One of the latest phases of city development is the direct result of the invention of a new technique of building. Steel construction has made possible the skyscraper, the elevated railroad, and the subway, and thus introduced a new dimension into city growth. This new technique has made possible a density, per unit of ground surface, which has given the city an entirely new complexion. The full effects of this new invention are still not fully known. Holborn, I. B. S. “The City: The Outer Expression of an Inner Self,” _Art World_, III (December, 1917), 217–21. (III, 1; IX, 2.) Mumford, Lewis. _Sticks and Stones: A Study of American Civilization_ (New York, 1925). An evaluation and critique of the architectural aspects of American cities and their cultural significance. (V, 5; VI, 9; X, 2.) Nichols, C. M. (editor). _Studies on Building-Height Limitations in Large Cities_ (Chicago, 1923). Written from the point of view of the real-estate profession. Schumacher, Fritz. “Architektonische Aufgaben der Städte,” in Wuttke, _Die Deutschen Städte_, pp. 47–66. Discussion of the changing needs and methods in urban construction. The literature on the significance of the steel-construction technique is still very small. The professional engineers and architects have contributed some to their journals, but the interpretation of their contributions is still to be made. 10. Land values are the chief determining influence in the segregation of local areas and in the determination of the uses to which an area is to be put. Land values also determine more specifically the type of building that is to be erected in a given area—whether it shall be a tenement house, an office building, a factory, or a single dwelling—what buildings shall be razed, and what buildings are to be repaired. The technique of determining city land values has developed into a highly specialized and well-paid profession. Land values are so potent a selective factor that the human ecologist will find in them a very accurate index to many phases of city life. Aronovici, Carol. _Housing and the Housing Problem_ (Chicago, 1921). A study of the relation between rent, income, and housing. Arner, G. B. L. “Urban Land Economics,” in volume, _Urban Land Economics_, Institute for Research in Land Economics (Ann Arbor, Michigan, 1922). Gives a summary of land values in New York City and an outline of the subject. (VII, 1, 2.) George, W. L. _Labor and Housing at Port Sunlight_ (London, 1909). (III, 4, 6; V, 4, 5; IX, 1.) “Housing and Town Planning,” _Ann. Amer. Acad._, LI (January, 1914), 1–264. An excellent collection of authoritative articles on housing, city planning, city land values, transportation, and government. (III, 6; IV, 1, 2, 3; V; VII; VIII.) _Hull House Maps and Papers_ (New York, 1895).