2OO The Animal Mind 



in these animals. He experimented, it will be remem- 

 bered, with individuals belonging to three classes: land- 

 dwelling, water-dwelling, and amphibious. The first men- 

 tioned would crawl off the edge of a board 30 centimeters 

 above a net of black cloth only with much reluctance when 

 their eyes were uncovered ; when blindfolded they would not 

 move at all. The water tortoises plunged off without hesita- 

 tion from a height of 30 centimeters, but hesitated slightly 

 at 90 centimeters, although some individuals would take the 

 plunge at once even from a height of 180 centimeters. When 

 blindfolded, all of the water tortoises rushed off at any height. 

 The land-and-water-dwelling tortoises hesitated at 30 centi- 

 meters and at 90 centimeters showed a conflict of impulses, 

 trying to catch themselves, before launching off. When 

 blindfolded they would not leave the board at all, though they 

 moved about upon it freely (459). 



Some of the most important conditions of distance per- 

 ception in our own experience are lacking in the lower 

 vertebrates and in invertebrates. Stereoscopic vision, the 

 appearance of solidity given to objects by the fact that the 

 visual fields of the two eyes combine, thus producing blending 

 of two slightly different views of the object looked at, has 

 been held to be dependent on the partial crossing of the op- 

 tic nerves on their way to the brain, whereby each retina sends 

 nerve fibres to both hemispheres of the brain. This arrange- 

 ment does not appear in the animal kingdom below the birds ; 

 whatever function it plays in space perception is, then, 

 absent from reptiles, amphibians, fish, and invertebrates. 

 Certainly stereoscopic vision cannot exist in animals whose 

 eyes are so placed that the same object cannot be seen by 

 both, as is the case with most fishes. -In birds, whose eyes 

 are situated too far toward the sides of the head for the 

 same object to cast its images on the foveas or centres of the 



