If you could keep an endangered species from becoming extinct, should you?

There are those who say to do so would be to meddle in nature.

But increasingly, zoos see doing exactly that as one of their missions. After all, as one scientist says, since man's encroachment has been part of the problem, man should be part of the solution.

Hence, the title for the season premiere of "The Infinite Voyage" series: "The Keepers of Eden" (Monday at 8 on PBS).

No longer are zoos simply display sites for exotic animals, offering them behind bars like pictures in an exhibition. The approach has changed: If possible, animals are housed in environments as close to their native habitats as the zoo can create it. Some -- the San Diego Zoo is most notable -- encase the visitors (in a monorail) and let the animals roam free within the zoo's perimeters.

More and more, zoos also are interested in becoming protective environments for endangered species and laboratories for their perpetuation.

The Bronx Zoo in New York, for example, has the only breeding colony for the proboscis monkey in the United States. Washington's National Zoo breeds golden lion tamarins. The Cincinnati Zoo is involved in finding surrogate mothers for endangered species. A zoo along England's Jersey coastline tries to maintain unusual species that other zoos seem to have eschewed.

Of all the animals displayed in the 150 accredited zoos in this country, 80 percent were born in captivity. Unlike the animals captured in the wild by Frank "Bring 'Em Back Alive" Buck and others, these animals have always lived at the zoo or at a zoo breeding area.

If this seems disappointing to visitors who come to see exotic animals that are not native to the United States, it is nevertheless testimony to a healthy zoo environment. Clinton A. Fields, executive director of the Friends of the National Zoo, acknowledges that one of the proofs of a good zoo is the animals' rate of reproduction.

Here, the National Zoo is working to see that golden lion tamarins, native to the jungles of Brazil, will not die out in that country. The zoo breeds the small monkeys, then takes them back to Brazil to release them.

But first an animal behaviorist must teach the tamarins to survive in the jungle and even to climb on trees and vines, a skill that even the scientist thought they would have instinctively. We watch as a female tamarin known simply as 1772 is trained, then shipped to Brazil to be paired with Niko, a former National Zoo resident who successfully re-adapted to his species' native environment.

Tamarins mate for life, but because Niko's mate died, he is in the market for a new wife. The scientists hope Niko will show 1772 how to survive in their new home, and as we leave the newly "bonded" pair (the scientists' term), it appears that he will.

At the Cincinnati Zoo, scientists transplant embryos of threatened species into surrogate mothers who then give birth and nurture the babies, saving that species -- temporarily, at least -- from extinction.

Cross-species surrogate motherhood is the project of reproduction physiologist Betsy Dresser, who first transplanted a bongo antelope embryo into a common eland, also a type of antelope, in 1983.

Seven years later, she and her team at Cincinnati's Center for Reproduction of Endangered Wildlife had repeated the experiment six times with five different species, using both cryogenically frozen and fresh embryos, in vitro fertilization, and surgical and non-surgical transfer.

Among their successes: A Holstein cow gave birth to an endangered guar calf, and a domestic cat became the host-mother of an endangered species of desert cat.

Dresser believes that since man is part of the problem of animal extinction, man should not hesitate to save endangered animals when possible.

Last year, in an effort to save elephants from being killed by poachers for their ivory tusks, many nations joined in a world-wide ban on ivory trade. But among African animals, another species, the rhinoceros, also is declining rapidly. From about 10,000 a decade ago, rhinos number only a little more than 400 now.

Nonetheless, even the best efforts of humans may not be able to save the cheetah, a species so inbred that a computer display of the chromosomes of several cheetahs shows them all to be nearly identical genetically. Because they are so similar, they are all prey to the same diseases -- one Northwest zoo lost all of its cheetahs to a type of feline distemper -- and may either be sterile or reproduce poorly.

The message: Go to a zoo and see them while you can. You may have to tell your grandchildren about them.