Why cities grow
Only about 10 million people existed at the beginning of the Neolithic Period. The population of the world reached about 500 million by A.D. 1650 and about 5 1/3 billion by 1990. This population explosion led to an increase in both the size and number of cities.
Two other trends also have aided the development of cities. One trend-sometimes called population implosion or population urbanization-is the increasing concentration of people in small parts of the earth. These parts are the cities and their surrounding areas. The other trend-sometimes called population displosion or population diversification-is the movement to cities by people of a variety of cultural backgrounds. Through the years, cities came to include people of different racial, religious, national, and language groups. This mixing of people brought about cultural diffusion, a process by which people of different backgrounds learn from each other by exchanging ideas. Cultural diffusion ranks among the most important factors in the development of civilization.
Excerpt adapted from the "City" article, The World Book Encyclopedia © 1999