Explain why cities have grown so explosively in the western world (primarily Europe and North America) during the last century and a half or so
explain why cities have grown so explosively in the western world
I think that there is more that has to be taken into account when we look the phenomenon of urbanization. We cannot explain urbanization without mentioning the increase in farm productivity and the advances in transportation that allow excess farm produce to be moved to cities.
Before mid-1850s, it would have been hard for cities to grow very large. Farms were not productive enough to support huge urban populations and many people were needed in the rural areas to work on the farms. As mechanization and other technologies increased farm productivity, farms came to be able to feed larger urban populations and there was less need for people to live in rural areas and work on farms.
In addition, advances in transportation (notably the railroads) allowed urbanization to proceed. If it had not been for better transportation, the huge surpluses produced by farms in places like the Midwest US could not have been used to feed the huge urban populations of the East Coast.
Thus, we have to take into account the increasing ability of small farm populations to feed huge urban populations. This made urbanization possible.
The primary reason for the growth of cities was the Industrial Revolution and the beginnings of the factory system of manufacturing. The factory system--manufacturing of products from raw material to finished good under one roof--began in the nineteenth century with the textile industry in Lowell Massachusetts. At a time when farm income was quite uncertain and many families lived near poverty, factories offered a steady source of employment and income. The advent of the factory system also led to a tremendous increase in migration of people from Eastern and Southern Europe at a time when the frontier was closed. These people tended to stay in large cities and work in factories. Factories tended to be built in cities near rivers and streams where they had a source of water for steam and transportation. Railroad hubs were normally located near factories also. Thus the opportunity for labor led many people to move to cities and cause the city's population to explode.