In 1992, Western scientists cataloged the ''discovery'' of the planet's largest new mammal in more than half a century, a forest-dwelling ox named the saola. Not merely a new species, it represented an entirely unrecorded genus of life. As of the publication of the fascinating Vietnam: A Natural History, scientists still had not sighted another free-ranging saola in the wild, although villagers sometimes kill an animal for meat.Since then, researchers in Vietnam have identified three new species of deer and a striking striped rabbit, 63 new terrestrial vertebrates and 45 fish. An animal once thought extinct on the Asian mainland, the lesser one-horned rhinoceros, was rediscovered. A wild pig, a monkey, a pheasant and at least two other varieties of birds have been re-sighted almost a century after they were identified and then vanished from scientists' view.The era of grand biological discovery pretty much ended long ago across most of the globe. Not so for Vietnam, which continues its struggle to emerge from the darkness of war. This natural history, compiled by three scientists from the Center for Biodiversity and Conservation at New York's American Museum of Natural History, is the latest chapter in the postwar development of one of the world's most remarkable, and mysteriously rich, landscapes.