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 countries increased two and a half times in size, but in other parts of the world the growth was eight times their size.
The size of growth is bad enough. During the nineteenth 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. Now, however, the reverse is almost always true in the newly industrialized world : The percentage of people living in cities is much higher than the percentage working in industry.
Without a base of people working in industry, these cities cannot pay for their growth. There is not enough money to build enough houses for the people that live there, to say nothing of the new arrivals. There has been little opportunity to build water supplies or other facilities.

W: Isn't this wind lovely! Just look at the trees bending — I'm sure those big branches are going to break.
M: No, they're much too strong for that. These trees are over 200 years old, you know, so they must be tough. Anyway, ready to jog again?
W: In a couple of minutes. I'm enjoying this. Hey, Look! There is a kid up in this tree—there, over on the left. He is crying.
M: Oh, yes. He must be terrified in this wind! We'll have to help him if we can.
W: I'm terrified of heights. Let's call the fire service.
M: No. He may fall if we wait for them. Look, I'll climb up. Guide me, will you?
W: OK. Now, over to the left... gently does it... that's right—make him climb down after you. OK, lift him down to me... Now, then, young man, what's your name? ... Hey... he's run away!