342 ADAPTATIONS TO SPACE AND MOTION 



Some birds when walking (fowls, pigeons, doves) and others when 

 swimming (coots and gallinules) make perpetual forward-and-backward 

 oscillatory movements of the head. It has been claimed that the eyes 

 never actually move backward through space — the forward movement 

 of the body just cancels the backward movements of the head. Thus 

 although the body moves forward steadily, the head moves forward 

 through space by jerks and pauses. In effect, the eyes obtain a rapid 

 succession of previews of the surroundings from constantly new angles. 

 The forward movements of the head being so quick, each new parallac- 

 tic observation of the field is made almost simultaneously with the pre- 

 ceding one, and the exaggeration of the apparent relative motions of 

 objects at different distances furnishes a basis for the estimation of dis- 

 tance and relief. 



Many shore birds bob their heads vertically as they teeter along the 

 beach, and many snakes weave their heads from side to side during 

 scrutiny. Some birds and many lizards commonly have spells of nod- 

 ding periodically. These habits have been interpreted as devices for 

 producing an artificial relative motion in the surroundings. Many herpet- 

 ologists believe that the nodding of lizards is a sociological phenomenon 

 — the animals do it most when they are among their fellows, when they 

 are warmed up, well-fed, when they 'feel good' and so on. But this only 

 means that they nod most when they are in normal condition and on 

 the alert. The habit does not seem to be sexual; and if it is social at all 

 it is still not without visual importance. If a lizard nods mostly in the 

 presence of other lizards, that may merely signify that for a lizard noth- 

 ing so much merits close scrutiny and visual cogitation as does another 

 lizard. 



(E) Movement-Perception 



Human vision is such an enormously rich complex of experiences, and 

 human beings are so diversified in habits and interests, that no two of us 

 value our eyes for quite the same set of reasons. If asked what aspect of 

 vision means most to them, a watchmaker may answer "acuity", a night 

 flier, "sensitivity", and an artist, "color." But to the animals which in- 

 vented the vertebrate eye, and hold the patents on most of the features 

 of the human model, the visual registration of movement was of the 

 greatest importance. 



Any sense organ exists not simply to give its owner awareness of some 

 physical, environmental agency, but to provide a basis for awareness of 



