. Geographers, historians, economists, and political scientists have incorporated the points of view of their respective disciplines into diverse definitions of the city. . . . A sociologically significant definition of the city seeks to select those elements of urbanism which mark it as a distinctive mode of human group life. The characterization of a community as urban on the basis of size alone is obviously arbitrary. No definition of urbanism can hope to be completely satisfying as long as numbers are regarded as the sole criterion