238 The Animal Mind 



it at one end of a narrow glass case sixteen inches long, at 

 the other end of which a grasshopper was placed. When 

 eight inches from its victim, the spider's movements 

 changed, and at four inches the leap was made ^ (571). 



Reactions of this character, where the animal makes a 

 single movement adapted to the distance of an object 

 from it, are almost the sole evidence we can get of accurate 

 perception of the third dimension. The alleged perform- 

 ance of the jaculator fish, which, as described by Romanes, 

 "shoots its prey by means of a drop of water projected from 

 the mouth with considerable force and unerring aim," 

 the prey being "some small object, such as a fly, at rest 

 above the surface of the water, so that when suddenly 

 hit it falls into the water," would involve distance per- 

 ception (640, p. 248). The catching of insects on the wing 

 by various amphibians, reptiles, and birds has the same 

 significance. A salamander cautiously stalking a small 

 fly will not strike until it gets within a certain distance. 

 In Necturus and in other animals the pause just before 

 snapping at food has been suggested to be for the purpose 

 of proper fixation (785). 



Training an animal to jump from one support to another 

 is a method that has been used to study distance percep- 

 tion in the mouse (775) and white rat (634). Waugh put 

 a mouse on a disk and raised it a certain distance above 

 a support ; he then measured the time the mouse hesitated 

 before jumping, when the height of the disk was varied. 

 From the fact that the mice hesitated longer, the greater 

 the height, he inferred some visual perception of distance. 



' Porter observed that the distance at which spiders of the genera Argiope 

 and Epeira could apparently see objects was increased six or eight times if 

 the spider was previously disturbed by shaking her web (612). This, of 

 course, does not refer to the power to jiidge distance. 



