''The Nature of Cities'' and urban geography in the last half century

On the 50th anniversary of the publication in 1945 of The Nature of Cities by Harris and Ullman, with its concentric zone, sector, and multiple nuclei models of internal patterns of cities, an additional peripheral model is suggested. Further observations are offered on central place, clustered, and linear patterns of distribution of cities. Developments in the geography of cities in the United States and Canada and the published contributions of urban geographers in North America in the last half century are summarized.