GRADES OF MOVEMENT 345 



regain rapport through olfactory exploring or trial-and-error tonguing. 

 Nocturnal snakes, many of which have superb olfactory powers, are 

 better able to locate and strike motionless prey, without need of vision. 



Though all birds have high visual acuity, hawks and insectivorous 

 forms are dependent upon motion for seeing prey at great distances. The 

 bird sitting on a fence-post may fly suddenly and directly to a point rods 

 away, pick up an insect, and return. This is a marvellous ability; but we 

 should not credit the bird with distinguishing a motionless bug at such 

 distances. In all probability the bug was moving or the bird would not 

 have seen it; and this is not entirely a matter of the saliency of the move- 

 ment, for, as will be brought out later, the same object can be distin- 

 guished about twice as far away, if it is in motion, as when it is still. 



Mammals in general are also quite dependent upon motion. The suc- 

 cessful use of the habit of 'freezing' by rodents and ungulates is in itself 

 an evidence that the carnivores which prey on them do not identify them 

 visually when they are still. Two or three breeds of dogs — the borzoi, the 

 greyhound, and to a less extent the dachshund — hunt by sight and must 

 keep the prey in sight or give up the chase; but such a situation is rather 

 artificial for a carnivore and is to be laid to the effects of breeding. Small- 

 eyed, nocturnal mammals are particularly dependent upon the move- 

 ments of their enemies for apprisal and escape. As we shall see, the eyes 

 of such animals as rats and mice have been called adapted to see motion; 

 but the truth is that they see motion better than form and color only by 

 a process of elimination — they are simply not good enough to see any- 

 thing except the gross movements of large objects. 



Grades of Movement — The most obvious basis upon which we might 

 classify movements is their speed. But speed is entirely relative, and is 

 related to the animal's own speed of movement. What may seem very 

 slow to a rabbit, may seem whizzingly fast to a snail. Obviously, the same 

 sensory and perceptual machinery is set in motion whether an object 

 moves past an animal or the animal moves past the object. What occurs 

 is a relative change of position of the two, and the animal's capacity for 

 maintaining a clear impression of an object must be adequate to cover 

 the speeds attained by natural objects important to him, as well as his 

 own locomotor speed among such objects when the latter are motionless. 

 The speed at which it is safe for an animal or a man to travel is largely 

 determined by his reaction time; but it is obviously not safe for an animal 

 to be unable to see an approaching enemy as anything more than a blur. 



