Before considering what animals can do, without conscious thought, in the way of navigation, it would be useful briefly to review their ability to pilot themselves, to find their way around an area in which they have already had experience. Here landmarks play an obviously vital role, and in many cases they are visual landmarks which we can appreciate ourselves. But often the characteristics of the environment to which the animal reacts are non-visual, detected by senses similar to ours but of much higher acuity. The sense of smell is of dominating importance among the mammals, which will go so far as to provide their own ‘scent flags’ along the boundaries of their private preserves. Sounds are often used to guide animals, particularly to another member of their species or to their prey. The bats have gone further and rely on a form of echo-location for guidance of their fast erratic flight through the darkness. They emit high-pitched, if not supersonic squeaks and detect and analyse the reflections back from obstacles even so slight as a piano-wire stretched across their path. Animal ‘sonar’ of this kind has also been demonstrated in a bird and probably is also to be found in fish and porpoises. Certain snakes are believed to detect their warm-blooded prey by special heat-receptors in the head. Many other examples could be cited and our knowledge of the means by which animals can determine their course in familiar territory is probably not yet exhaustive. We are only just beginning to appreciate and understand what they can do when they are in completely unfamiliar areas or when, known landmarks are present but cannot be used.