If by suburb is meant an urban margin1 that grows more rapidly than its already developed interior, the process of suburbanization2（郊区化） began during the emergence3 of the industrial city in the second quarter of the nineteenth century.
Before that period the city was a small highly compact cluster（群，蔟） in which people moved about on foot and goods were conveyed by horse and cart. But the early factories built in the 1840's were located along waterways and near railheads（铁路末端） at the edges of cities, and housing was needed for the thousands of people drawn4 by the prospect5 of employment. In time, the factories were surrounded by proliferating6（增生的） mill towns of apartments and row houses that abutted7（毗邻） the older, main cities. As a defense8 against this encroachment9（侵入） and to enlarge their tax bases, the cities appropriated their industrial neighbors. In 1854, for example, the city of Philadelphia annexed10 most of Philadelphia County. Similar municipal maneuvers11（军事演习） took place in Chicago and in New York. Indeed, most great cities of the United States achieved such status only by incorporating the communities along their borders.
With the acceleration12 of industrial growth came acute urban crowding and accompanying social stress-conditions that began to approach disastrous13 proportions when, in 1888, the first commercially successful electric traction14 line was developed. Within a few years the horse-drawn trolleys15（手推车） were retired16 and electric streetcar networks crisscrossed and connected every major urban area, fostering a wave of suburbanization that transformed the compact industrial city into a dispersed17（分散的） metropolis18. This first phase of mass-scale suburbanization was reinforced by the simultaneous emergence of the urban Middle Class, whose desires for homeownership in neighborhoods far from the aging inner city were satisfied by the developers of single-family housing tracts19.
