Sensory Discrimination : Vision 143 



the cones in the retina transparent, colored globules like little 

 drops of oil. The significance of these colored drops is 

 wholly unknown. If they really transmit to the cones only 

 those colors which they seem to, then the color sensations of the 

 animals possessing them must be wholly different from ours. 



No experimental tests on color vision in reptiles have 

 been made, so far as the writer is aware. As for birds, in 

 the palmy days of the doctrine of sexual selection we should 

 have felt quite sure that the bright colored plumage of many 

 species indicated ability to distinguish colors. Some experi- 

 mental evidence of this power has been obtained. A chick 

 that had learned to pick out bits of the yolk of a hard-boiled 

 egg from the white was given bits of orange peel, which he 

 seized, but seemed to find exceedingly distasteful. After- 

 wards he was for some time suspicious of the bits of yolk. 

 On the other hand, after having learned to avoid bad-tasting 

 black and yellow caterpillars, he did not transfer his aversion 

 to black and yellow wasps ; probably their points of difference 

 from caterpillars were so numerous that the resemblance of 

 color was not attended to (281, pp. 40-41). 



Color vision in the English sparrow and the cowbird has 

 been tested by a method previously used on monkeys. A 

 number of glasses of like size and shape were covered inside 

 and out with differently colored papers, including red, yellow, 

 blue, green, dark and light gray. These glasses were placed 

 in a row on a board, and food was put always in the same 

 glass, the position of which, however, was changed in the 

 different experiments. The sparrow and cowbird learned 

 to pick out the right vessel under these conditions (345). 

 Somewhat similar tests were fairly successful with pigeons, 

 which were also experimented on by Graber's method of 

 allowing a choice between compartments illuminated through 

 differently colored glass. Although the pigeons showed no 



