As it sometimes did when he was enhosted, his mind went back to old Peters, his instructor, who had taught "Project C" to the eager young zoologists. Project Contact had been mostly devoted to giving the student an open mind on metabolism and adaptability to environment. A Learner had to be able to reason out--and quickly--the metabolism of his host. It was little use knowing a Terran life-ecology; man lives on combustibles and oxygen, the oxygen combining with combustibles to provide heat, and plants live on carbon dioxide and water and sunlight, renewing the atmospheric oxygen. So old Peters had always stressed the student's learning their Basic Combinations. Basic Combinations prepared the student--or so the school board hoped--for a wide variety of chemical relationships between a host and its environment. The students had to know what to do to survive should the host, for instance, live in a chlorine atmosphere, and need large amounts of antimony in its diet for proper combustion and survival. There were a good many chemical elements in the universe; the student had to know how to deal with any combination of them in a host's metabolism. For the most part, the instincts of the host would carry a Learner through the Contact period. A species tended to keep its physical needs not only in its mind, but in its body as well. Mr. Peters had a saying he'd been fond of emphasizing to the students: "When in doubt, black out." The saying became a cliche to the student body, but they had the sense not to disregard it. A cliche is, after all, only a truth which has become trite because it is vitally necessary to use it often. "When in doubt, black out," meant simply that if a situation arose which seemed impossible to handle rationally, the enhosted Learner's last resort was reliance upon the instinctive behavior of the host. The only thing to be done was to pull the mind into a tiny knot bobbing in the host's own brain, and let the host itself, once more in control, take the Learner instinctively to environmental victory. Or defeat. * * * * * There were dangers, of course. A Learner enhosted in a chicken, for instance, would be a fool to trust the chicken's instincts regarding, say, a snake. A chicken confronted by a snake tends to become hypnotized by its deadly adversary, and to stand stupidly in place until it is killed. In cases of that sort, the Learner would be safer taking control and going clucking off to the nearest high ground. On the other hand, a Learner inhabiting something with the hairtrigger instincts of a bat would be much better off letting the animal's instincts take over in moments of grave risk, such as flying through the blades of a revolving fan. A bat could get through without a second thought about those whirling metal scythes, but a man's mind could not think fast enough to avoid a grim death by all-over amputation.