Zoos are among mankind's oldest institutions, dating back at least 4,500 years, and probably more. Across the world they have brought together and displayed live wild animals for people to look at and over the years hundreds of millions have. Any institution with so long a history and so universally attended must reach something in people deeper than idle curiosity. Since it is fashionable to speak of roots today, it might be suggested that zoos allow us to stay in touch with our most primitive roots in a primeval world where human survival depended on knowing the shapes and habits of wild animals. So important were wild creatures to our distant ancestors that they were the most frequent subjects of paintings on cave walls, formed the basis for virtually all early religions, and were in numerous instances worshipped as gods.

Now our survival is threatened more by what we ourselves have worked, and by the stresses of living among these creations, than it is by wild animals to whom we relegate less and less living space with each passing year. In this world the need for good zoological gar-dens is urgent. The exponential growth of human population and the ever-increasing sprawl of cities does more than rob land from wildlife: it pushes the animals father away from city dwellers. People live in brick, concrete, and glass environments where they lose all touch with wilderness; children grow up who have never tried to catch a frog, never seen a hawk soar or a deer step daintily into a forest clearing—let alone watched a herd of elephants amble across the river or a pride of lions stalk prey.

People who have the time and money can take an occasional trip to the remaining wilderness and find, in places where wild animals still live, the renewal of spirit that comes from prolonged visits to wild country. For millions of others who are unable to leave the cities or can't afford to, good zoos laid out among plants and trees can bring what conservationist Lan Player calls "a taste of wilderness''. Perhaps more important in the long run, zoos can help give deprived people an awareness that we share the world with many other animals and should have a decent regard for their worth and right to live. If zoos did no more than accomplish these two ends, they would serve a noble purpose.

As it happens, however, today's zoos can do far more. They can become breeding centers for those wild species whose continued existence has become precarious. The team "captive breeding" has been used to describe this new role of zoos, and this book describes the effort the most important task that zoos have yet undertaken.

In the second sentence of the first paragraph, "hundreds of millions" refers to the great number of_____ .

A．mankind's various institutions

B．zoos across the world

C．live wild animals displayed

D．people who have visited zoos