The locomotion of animals seems to be controlled by optical as well as by other kinds of stimulation for, in many species, locomotor behavior ceases when optical stimulation fails, as in total darkness. The starting point for a theory of locomotion in the higher animals would be the fact that they have evolved eyes which can register not merely light but the objects of an illuminated environment. Images are the necessary means by which an animal exploits the capacity of light to project reflecting surfaces to any given point in a transmitting medium. The center of the flow pattern during forward movement of the animal is the direction of movement. Fish, birds and arboreal animals live in a cluttered environment, that is, one whose open spaces are encroached upon by solid surfaces. There are plenty of invariant variables in the flux of optical stimulation on the basis of which an animal can identify classes of permanent objects.