Ever since his expulsion from Eden, man has lived by this ruthless pattern of land use: Cut, burn, plant, destroy, move on. But the planet is no longer big enough, says FAIRFIELD OSBORN, for this kind of plunder. With a world population of more than two billion we have barely enough forests, water sources, and arable soil for subsistence and no new lush lands to conquer. But suppose the population reaches three billion. President of the New York Zoological Society, Mr. Osborn is leading the drive for conservation on a national scale. His forthcoming book, Our Plundered Planet, is a shocking report on how far spoliation has gone the world over.