One thing the tour books don't tell you about London is that 2,000 of its residents are foxes. As native as the royal family, they fled the city about centuries ago after developers and pollution moved in. But now that the environment is cleaner, the foxes have come home, one of the many wild animals that have moved into urban areas around the world. 'The number and variety of wild animals in urban areas is increasing,' says Gomer Jones, president of the National Institute for Urban Wildlife, in Columbia, Maryland. A survey of the wildlife in New York's Central Park last year tallied the species of mammals, including muskrats, shrews and flying squirrels. A similar survey conducted in the 1890s counted only five species. One of the country's largest populations of raccoons(浣熊) now lives in Washington D.C., and moose(驼鹿) are regularly seen wandering into Maine towns. Peregrine falcons () dive from the window ledges of buildings in the largest U.S. cities to prey on pigeons. Several changes have brought wild animals to the cities. Foremost is that air and water quality in many cities has improved as a result of the 1970s' pollution-control efforts. Meanwhile, rural areas have been built up, leaving many animals on the edges of suburbia. In addition, conservationists have created urban wildlife refuges. The Greater London Council last year spent $750,000 to buy land and build 10 permanent wildlife refuges in the city. Over 1,000 volunteers have donated money and cleared rubble from derelict lots. As a result, pheasants now strut in the East End and badgers scuttle across lawns near the center of town. A colony of rare house martins nests on a window ledge beside Harrods, and one evening last year a fox was seen on Westminster Bridge looking up at Big Ben. For peregrine falcons, cities are actually safer than rural cliff dwellings. By 1970 the birds were extinct east of the Mississippi because the DDT had made their eggs too thin m support life. That year, ornithologist Tom Cade of Cornell University began rising the birds for release in cities, for cities afforded abundant food and contained none of the peregrine's natural predators. 'Before they were exterminated, some migrated to cities on their own because they had run out of cliff space,' Cade says. 'To peregrines, buildings are just like cliffs.' He has released about 30 birds since 1975 in New York, Baltimore, Philadelphia and Norfolk, and of the 20 pairs now living in the East, half are urbanites. 'A few of the young ones have gotten into trouble by falling down chimneys and crashing into window-glass, but overall their adjustment has been successful.' The first paragraph suggests that ______.