ASSOCIATION AND COLOR DISCRIMINATION 451 



light which could be varied in intensity. When the fishes were 

 arranged evenly in both parts of the tank, Hess considered that 

 the two lights were equally bright. By measuring the intensity 

 of white lights which had the same effect on the fishes as various 

 colored lights, he obtained a luminosity curve which agreed very 

 well with that of the totally color-blind human eye. 



Hess offered the fishes imitation baits of various colors on 

 different backgrounds. If the brightness of the bait corre- 

 sponded with that of the background, it was not taken. 



Contrary to the observations of Frisch, Hess found no adap- 

 tation to brightness in the pigment cells of the epithelium of 

 Crenilabrus. 



For fishes living any distance below the surface, Hess insists 

 that a color sense would be useless because of the increasing 

 absorption of light by the water as greater depth is reached, 

 particularly the longer waves. Hence mating colors are value- 

 less. Hess admits the possibility, however, that fishes living 

 in shallow water may possess the ability to discriminate colors. 



Hess also attempts to account for the reactions of Daphnia 

 and some other animals by assuming that they react to the lumi- 

 nosity of the spectrum as it appears to the color-blind human 

 eye. Loeb and Wasteneys ('15) criticise this assumption on the 

 ground that there is no proof that the heliotropic effects of 

 light in lower animals are accompanied or determined by sen- 

 sations of brightness, and, furthermore, that color-blind human 

 beings do not show any positive heliotropism. Hess' further 

 contention that animals and plants are sensitive to different 

 parts of the spectrum — all animals to the yellowish-green, and 

 all plants to the blue — is shown to be incorrect. 



From the sources discussed considerable evidence has been 

 accumulated to show that fishes perceive colors. Adaptive 

 changes in the pigment cells of the skin of various species in re- 

 lation to backgrounds of different colors have been observed, 

 and when allowed a choice, fishes show preferences for back- 

 grounds of particular colors. Mating colors may furnish further 

 evidence of color discrimination. Thus far there seems to be 

 no valid case of warning coloration in animals serving as food 



