152 The Animal Mind 



colors have no special influence in producing these simul- 

 taneous contrast phenomena, Ewald concludes that the 

 side regions of Daphnia's eye, like those of our own eye, are 

 totally color-blind. All of which seems a heavy weight of 

 inference to depend from rather slender evidence. 



That Daphnia seeks a region affected by the ultra-violet 

 rays of the spectrum in preference to darkness, although 

 the two look alike to our eyes, was shown by Lubbock (444) . 

 On the other hand, Loeb (431) and Moore (502) report 

 that the action of ultra-violet rays instantly makes Daphnia 

 avoid the light containing them, and it appears that these 

 rays, which are without effect on the human eye, exert 

 their influence through the eye of Daphnia, since individuals 

 without eyes are unaffected by them (300). 



Adaptation to darkness apparently takes place in the 

 eye of Daphnia, for individuals which have been a long 

 time in darkness will respond to a lower intensity of light 

 than those which have been long exposed to illumination 

 (174). Experiments on the effect of intermittent and 

 continuous lights of equal intensity on the movements of 

 the Daphnia eye indicate that the Talbot-Plateau Law, 

 according to which such lights are identical in effect, 

 holds for the vision of this crustacean as for the human eye 

 (212). It is this law which enables us to measure the grey 

 produced by a rapidly revolving disk of black and white 

 sectors as equal in brightness to the amount of light re- 

 flected by the sectors at rest. 



Experiments on the reactions of the crayfish, which has 

 a moderately strong tendency to avoid light, show that when 

 light falls vertically through red glass the animal seeks it 

 in preference to darkness : no marked preference is indi- 

 cated when the light is passed horizontally through red 

 glass (40). No clear evidence of color discrimination ap- 



