Existence, for all organismic life, is a constant struggle to feed-a struggle to incorporate whatever other organisms they can fit into their mouths and press down their gullets without choking. Seen in these stark terms, life on this planet is a gory spectacle, a science-fiction nightmare in which digestive tracts fitted with teeth at one end are tearing away at whatever flesh they can reach, and at the other end are piling up the fuming waste excrement as they move along in search of more flesh.