228 EVIDENCE THAT DAPHNIAS 



placing the yellow in another trough, as before, for 

 comparison. The preference for the yellow was as 

 marked as ever. In the experiments with the red and 

 yellow the numbers were respectively 



Trough 1. TKotrcn 2. 



Under the In the Under the In the 



yellow. uncovered half. red. uncovered half. 



670 330 498 502 



When, therefore, the red solution was sufficiently 

 light, the Daphnias were indifferent to it. In the 

 experiments with light blue the numbers were — • 



Trough 1. Trough 2. Trough 3. 



Under In the Under In the Under In the 



the uncovered the uncovered the porcelean uncovered 



yellow. half. blue. half. plate. half. 



687 313 286 714 336 664 



One other possible objection also suggested itself to 

 me. I thought it might be said that the Daphnias 

 went under the yellow and the green not on account of 

 any preference for yellow or green light, but on account 

 of the shelter afforded by the covering. To test this, 

 I covered one half of a trough over with transparent 

 glass, leaving the other uncovered ; but after twenty 

 observations I found the number of Daphnias in each 

 half to be practically identical. The mere fact of the 

 covering, therefore, made no difference. In this way I 

 was able to test the preference of the Daphnias for 

 various colours, and the result made it abundantly clear 

 that Daphnias have the power of distinguishing between 

 light of different wave-lengths, and that they prefer 

 the light which we call yellow and green. Whether it 

 actually appears to them as it does to us is, of course, 



