"[I]t is not inherently absurd to suppose that there is an obligation to protect animals from natural predators, even if this obligation has limited practical application. Nor (except, perhaps, in the case of ethical holists) does it conflict with our deepest moral convictions. If one of these convictions is that we should strive to reduce the amount of suffering in the world, then assisting prey animals, in some cases at least, is one way in which this might be accomplished."