The Senses and the Nervous System 97 



And if the disks were reversed, the blue being moved to 

 where the red had been and vice versa, two mass migrations 

 took place. The little jewels swam across the tank to their 

 red foster-parent, and the cichlasomas took the place where 

 the jewels had been, now in front of the blue disk. 



Why, in the face of such evidence, is there any question 

 that fish can tell color? Because of the possibility that it is 

 brightness, rather than color, which is distinguished. If a 

 certain red gives off more light than a certain blue, then it 

 may be difference in brightness which enables the fish to tell 

 one from the other, rather than difference in color. And 

 even if apparatus is used to provide colors of equal bright- 

 ness-value, the die-hards are not satisfied, maintaining that 

 there is no justification for believing that two colors have 

 the same brightness-value to the fish eye just because they 

 have to the human eye. 



However, recent experiments have offered convincing 

 proof that actual color rather than brightness is the deter- 

 mining value for at least one species — the large-mouth black 

 bass. These experiments were of the conditioned response 

 type, the fish being trained to associate one color with reward 

 — food — and another color with punishment — in this case a 

 mild electric shock. Young black bass one to two inches long 

 lived for weeks in white enameled basins six inches across, 

 one to each basin. They were fed on daphnia ("water fleas") 

 and mosquito larvae, which were presented to them in what 

 scientists call pipettes and normal people call eye-droppers. 

 Opaque pipettes were used of different colors: the red pi- 

 pette, let us say, always contained daphnia, the yellow one 

 did not J if the fish approached the red he got food, if he 

 approached the yellow he got a warning shock. The question 

 was: could the fish distinguish between the two colors, and 

 learn to keep away when the yellow pipette entered the 

 water? Of course, other colors were used alsoj and to take 



