A hungry hamster will readily learn to dig, rear, or scrabble to acquire food, yet simply cannot learn to face-wash, scent-mark, or groom to achieve this result. An earthworm has no trouble learning to associate a taste with a hot, dry place, but cannot associate a tactile stimulus with electric shock. Cats eagerly approach and investigate localizable sounds that signal food, but such signals simply do not interest rats. Pigeons learn to peck a lighted key to avoid electric shock only with great difficulty, yet they will quickly learn a wing-flapping response to avoid shock. These examples represent some of the enigmas facing anyone attempting to construct a theory of learning; not only do different species differ in their abilities to cope with different learning tasks, but individual species also frequently exhibit paradoxical irregularities in their learning abilities. Demonstration of a learning ability in one situation (such as the ability to associate behavior with its consequences) is no indication that the animal will be able to learn such a thing in other situations. The theorist appears to be left with a heap of facts and phenomena which show no apparent species or task regularities. So what kind of order, if any, can be imposed on this literature? There are a number of often quite different approaches to this problem, some of which set out to extract general principles of learning from this melee; others take a different conceptual approach by placing emphasis on how learning abilities may have evolved as a form of behavioral adaptation. We shall discuss these different approaches in a moment. However, when setting out on a path to the understanding of learning there are clearly two distinct questions that need raising at the outset. The first asks what is learning for? The second asks how do animals learn? The first question asks why animals evolved learning abilities at all, in the sense of having to specify what selection pressures were important in the evolution of learning, and it is also concerned with what animals learn in terms of what problems they are faced with in their natural environments. 2The second question is concerned with the mechanisms of learning: given that an animal has to learn such-and-such in its natural environment, how does it do this? Different species may solve similar problems in completely different ways: evolution only selects for outcomes, not for specific means or processes, and it will select on the basis of the entire functional set of mechanisms available at the time (for example, some species may find Pavlovian associative mechanisms quite adequate for coping with a specific adaptive problem, whereas others may resort to instrumental learning mechanisms to solve the same functional problem).