344 ADAPTATIONS TO SPACE AND MOTION 



greater or lesser saliency must go largely undiscussed since we know so 

 very little about them in man, and still less about them in the other 

 animals. 



Naturally, there is a rough correlation of saliency with feeding habits. 

 The well-armed carnivore does not need to be so fearful of unidentified 

 moving objects as does a timid and defenseless herbivore. For, in wild 

 nature, a moving object is generally another animal, and the observer's 

 responsiveness to it will depend upon the importance, to him, of reacting 

 in a motor way to another animal's approach. 



Movement may thus have unequal attention value and exciting power 

 for animals whose apparent objective basis for detecting movements is 

 about the same. Or, animals with vastly different eyes may respond to 

 moving and motionless objects in very similar ways. For instance, a frog 

 will snap only at small moving objects — which, in his natural surround- 

 ings, are ordinarily things which are good food for him. A penguin will 

 seize and eat only living, moving fishes. Though the penguin's visual 

 capacities (including those which we think have a bearing upon the detec- 

 tibility of movements) are vastly different from the frog's, either animal 

 could be perched on a mound of its natural food, fresh-killed, and would 

 proceed to sit there and starve to death. Such is the power of moving 

 matter over animal minds. 



In general, the less well developed the area centralis or fovea, the 

 more dependent is the animal upon the movements of objects for their 

 detection and evaluation. The penguin is probably an exception — 

 he sees a motionless fish well enough, but instinct tells him that a dead 

 (i.e., motionless) fish is not good to eat; and his olfaction, as in all 

 birds, is too poor to differentiate fresh-killed fish from stinking carrion. 

 Lacking sharp vision, an animal not only misses many sidewise move- 

 ments, but is readily stalked by an enemy which is careful to approach 

 in a straight line. Recognition of such toward-movements depends upon 

 appreciation of the 'growth' of the retinal image — which is poor where 

 acuity is low, both for direct reasons and also because poor accommo- 

 dation always accompanies poor resolving power. 



Not only amphibians, but most snakes, lizards, and many carnivorous 

 turtles appear not to see motionless prey. Motion is particularly impor- 

 tant to diurnal snakes, whose visual acuity is probably the lowest of any 

 diurnal vertebrates — Dryophis being a conspicuous exception in its abil- 

 ity to secure motionless prey solely by sight. Other diurnal snakes 'lose 

 contact' with the prey if it stops moving or freezes, and then attempt to 



