英文原文：

Good morning and welcome to today's lecture.Today we're going to discuss the mass urbanization of the world's population, which is an unprecedented trend worldwide. First, I'll focus on two major reasons why people are moving to cities. Well, the first reason is economic. People are moving to the cities because that's where they can find jobs and earn money. Until the 20th century, the major source of employment, full- and part-time, was farming. Now, no more than 15% of all jobs are connected to farming. Jobs now are being created in information techonology, manufacturing and service areas such as tourism and financing. And all of these new jobs are in or around major cities. The second reason for the move to the cities has to do with quality of life issues--comfort and convenience. For example, most of us would like our children to receive a good education and cities often offer better schools. And then for many, city life is just more comfortable. There are transportation networks, shops, and places of entertainment. An interesting consequence of urbanization is that the average age of people in the countryside is increasing while that of the cities' is falling. More old people stay in the countryside than young people, and the opposite is true in the cities. This is of course connected to the fact that it's the young people who want jobs.

Now I'd like to identify three key changes in our cities. First of all, they're getting bigger and bigger. For the first time in history, there will soon be more people living in urban areas than in rural environments. Most cities are bigger now than ever before. In 1950, New York City was the only city with a population of 10 million. For the first time in the history of society, we now have many cities with populations of over 10 million people--what we call megacities. Tokyo, Mexico City, Shanghai, New York City and Sao Paulo are just a few examples of today's megacities. And experts say that the number of megacities will increase in the future. Cities are not just getting bigger, they're also changing shape. They're getting taller because land is getting more and more expensive. So instead of having a few big houses on a piece of land, we can have a taller apartment building that a thousand people can live in. All of you can probably think of buildings or parks or stores that have been torn down to make room for bigger, taller or more modern buildings. Skyscrappers have become a symbol of modern cities. Things are also changing shape in other ways. The concentric zone model represents the structure of some cities built at the beginning of the 20th century. The business district is in the center, surrounded by the other zones. But the sector model and the multiple nuclei model are probably more typical of the cities we know today. They show the urban sprawl that's occuring in contemporary cities. Urban sprawl basically means that cities are spreading out often in an uncontrolled way. Notice that in these two models, the business districs is close to all the other districs. These sprawls often occurs in random and unpredictable ways and has a huge impact on the quality of life of city residents.

The third change is that our cities are breaking up into smaller communities often by ethnic group or income level. Of course, many cities do have a kind of identity or personality, but the city is not homogeneous. For example, migrates to the cities often want to live in their own communities, or with people from a similar cultural background. Another example is that if you're moving from a smaller community to a city, you will be most likely to move close to friends or family members who will help you get a job or give you support. However, this often means that people stay within their community and do not come into contact with others from different backgrounds. The biggest challenge facing us now is to improve the quality of life in cities because, sadly, they don't always offer economic security, the safety, or the comfort they promise. Many cities have slum areas or ghettos where people live in dangerous or destitute conditions. The beautiful architecture and vibrant night life are one face of the city, but cities also have problems of inequality, crowding and poverty.