150 The Animal Mind 



obtained similar results with the use of an electric light 

 spectrum ; but besides throwing all the colors at once upon 

 the vessel, he allowed each color to act separately through 

 a narrow opening, and noted the speed of the positive 

 response produced. That the "preference" shown for 

 yellow-green light is not a matter of color vision, but of 

 response to the greater intensity of the light in this region 

 of the spectrum, was suggested by Bert (46), and Merej- 

 kowsky showed that the larvae of Balanus and Dias longi- 

 remis manifested no color preference when the colors were 

 made of equal intensity (484) . Lubbock attempted to prove 

 the existence of qualitative as distinguished from intensive 

 discrimination by various modifications of the experiment, 

 but without entirely conclusive results (444, pp. 221 ff.). 

 Yerkes, working on Simocephalus, a form closely related 

 to Daphnia, found that when a gaslight spectrum was 

 used, the animals collected in the red-yellow region, that 

 of greatest intensity for such light ; and that if this region 

 had its intensity diminished by a screen of India ink or 

 parafnne paper, the crustaceans moved out of it (799). 

 This seemed strong evidence that the apparent color 

 reactions of these animals were really responses to differ- 

 ences in the intensity of the light. Hess (306) studying 

 the movements of the eyes of Daphnia when subjected to 

 light of different colors, finds another case of total color- 

 blindness, and Erhard (207) gets similar results on Simo- 

 cephalus when the light is reflected from colored surfaces. 

 Nevertheless, there is evidence that colored rays have an 

 effect on these crustaceans that is not wholly dependent 

 on their intensity ; evidence, that is, in favor of color vision. 

 When Daphnias have been kept for some time in light of 

 a certain intensity, an increase in the intensity makes them 

 avoid light, while a decrease in intensity makes them seek 



