Sensory Discrimination : Vision 141 



the water. Food was always placed in the red pair of forceps, 

 which were made frequently to change places with the green 

 ones ; and the fish was caused to enter the compartment half 

 of the time on one side, and half of the time on the other. 

 This was to prevent identification of the food fork by its posi- 

 tion or the direction in which the fish had to turn. The animal 

 quickly learned to single out the red fork as the one important 

 to its welfare, and in forty experiments, mingled with others so 

 that the association might not be weakened, where there was 

 no food in either fork, and where the forceps and rubber bands 

 were changed so that no odor of food could linger, it never 

 failed to bite first at the red. Moreover, the probability that 

 its discrimination was based upon brightness was greatly 

 lessened by using, when we experimented without food, a 

 different red much lighter than that in the food tests. The 

 fish successfully discriminated red from blue paints in the 

 same way, and it was afterwards trained, by putting food 

 in the green fork, to break the earlier association and bite 

 first at the green (421), 



51. Vision in Amphibia 



The fact that the commonest form of color blindness in 

 human beings affects the qualities red and green, and that 

 these colors have the most restricted area of visibility, might 

 tempt one to the belief that ability to distinguish red and green 

 is a late acquisition in the animal kingdom. So far, com- 

 parative psychology offers no support for this view. The 

 fish whose behavior has just been described certainly made 

 some sort of distinction between the colors red and green. 

 And the only evidence of color vision in the Amphibia is 

 evidence that frogs discriminate, in some fashion, between 

 red and white, although the difference to the frog may be one 

 of brightness merely. Yerkes, in studying the frog's power 



