Peter Kareiva, chief scientist of the Nature Conservancy, the world's largest conservation group, has some hard truths for environmentalists. The movement is at a crossroads. Science has unraveled its founding ethic -- the protection of a pristine nature into perpetuity -- at both ends. If conservation is to survive, it will have to abandon its cherished myths. There will no return to Eden. Some species will not survive. It is time, Kareiva says, to enter the Age of Man. And he is not alone.