PERCEIVE DIFFERENCES OF COLOR. 229 



another and a more difficult question — one, moreover, 

 not yet solved even for the higher animals. Nor would 

 I necessarily claim for them any sesthetic sense of 

 beauty; it must be remembered that they feed on 

 minute algoe and other minute vegetables, the prevalent 

 colors of which are yellow, yellowish green, and green. 

 There is, therefore, nothing improbable, a priori, but 

 rather the reverse, in their preference for these colors. 



It will be observed that though in these vessels the 

 Daphnias made their preference unmistakable, there 

 were always a certain number in the least popular 

 part. This is natural, because, as the position of the 

 light half was reversed every observation, the Daphnias 

 had to swim across the vessel, and some naturally did 

 not find their w^ay to the favourite part. Then, again, 

 in any considerable numbers of Daphnias some are 

 changing, or have recently changed, their skin, and 

 are, therefore, more or less inactive. Moreover, in pure 

 water the desire for food must often overpower any 

 preference for one colour over another. To such causes 

 as these we must, I think, attribute the presence of so 

 many Daphnias in the first vessel at the opaque end, and 

 in the second in the uncovered part. 



Still, it was of course not impossible that the pre- 

 sence, for instance, of a certain number under the red 

 and blue was due to a difference of taste ; that, though 

 the majority preferred yellow, there might be some 

 preferring blue or red. To test this I tried the follow- 

 ing experiment. I placed, as before, fifty Daphnias in 

 three of the vessels, covering one half of one with the 

 yellow, of a second with blue, and the third with red. 

 I then from time to time, at intervals of not less than 

 half au hour, removed these which were in the un- 



