While urban settlements have existed for thousands of years, rapid urban growth is very recent. Until modem times, urban settlements attracted only a small percentage of the total population and rarely reached a population of more than a few thousand inhabitants. In the past two centuries, the world has experienced rapid urbanization. Relatively developed countries have been transformed from predominantly rural to predominantly urban societies. The process of urbanization involves two changes in the distribution of population in a country: an increase in the number of people living in urban settlements and an increase in the percentage of people living in urban settlements. These two types of increases occur for different reasons. The increase in the number of urban residents is a function of the total population increase in the society. As the population of the country grows, some of those additional people inevitably will live in urban settlements. Rome, the largest urban settlement of the ancient world, probably was no larger in population than the modern Des Moines, Iowa. Most urban historians estimate that the first settlement to exceed 1 million inhabitants was London in approximately 1810. Today, more than 100 cities exceed 2 million. At the same time, urbanization means an increase in the percentage of urban dwellers and a corresponding decrease in the percentage of rural residents. The increase in the percentage of urban residents is a function of the society's changing economic structure. A high percentage of urban residents reflects the fact that most of the jobs are in factories, offices, and services rather than on farms. In 1800, only 3 percent of the world's population lived in urban settlements, compared with more than 40 percent today. If current trends continue, by the end of the twentieth century, the population of urban settlements may exceed that of rural settlements for the first time in human history. The increase in the percentage of urban residents is related to ______.