More and more of the world's population are living in towns or cities.
The speed at which cities are growing in the less developed countries is alarming.
Between 1920 and 1960,big cities in developed contries increased two and a half times in size,
but in other parts of the world the growth was eight times their size.
The sheer size of growth is bad enough,
but there are now also very disturbing signs of trouble in the comparison of percentages of people living in towns and percentage of people working in industry.
During the 19th century,cities grew as a result of the growth of industry.
In Europe,the proportion of people living in cities was always smaller than that of the workforce working in factories.