Throughout history man has changed his physical environment in order to improve his way of life. With the tools of technology he has altered many physical features of the earth. He has transformed wood- lands into farmland, and make lakes and reservoirs out of rivers for irrigation purposes or hydroelectric (水力发电的)power. Man has also modified the face of the earth by draining marshes(沼泽) and cutting through mountains to build roads and railways. However, man’s changes to the physical environment haven’t always had beneficial results. Today, pollution of the air and water is an increasing danger to the health of the planet. Each day thousands of tons of gases come out of the exhausts of motor vehicles; smoke from factories pollutes the air of industrialized areas and the surrounding areas of countryside. The air in cities is becoming increasingly unhealthy. The pollution of water is equally harmful. In the sea, pollution from oil is increasing and is killing enormous numbers of algae (水藻), fish and birds. The whole ecological balance of the sea is being changed. The same problem exists in rivers. Industrial wastes have already made many rivers lifeless. Conservationists believe that it is now necessary for man to limit the growth of technology in order to survive on earth.