486 ADAPTATIONS TO PHOTIC QUALITY 



simultaneously offered. Reeves drew the natural conclusion that the fishes 

 saw the red and the gray as qualitatively different things, since the two 

 equalled the same thing (the blue) for the human in brightness. There 

 is an obvious fallacy here, but it is of no importance to the main con- 

 clusion that hue discrimination occurs in the two species. 



Both species were much more sensitive to lights when dark-adapted, 

 but Reeves could find no evidence that there was any change in the 

 relative brightness of red and blue or of red and white. She therefore 

 questioned the occurrence of a Purkinje phenomenon, though on quite 

 different grounds from those of Hess. The importance of the presence 

 or absence, or demonstrability, of a Purkinje shift for the certainty of 

 hue discrimination has been greatly overemphasized as explained above 

 (pp. 474-5). 



Reeves came close to the conditioned reflex technique — a very modem 

 tool of research on animal color-vision — in her observations on untrained 

 mud-minnows (Umbra litni) and shiners (Notropis cornutus). She 

 noticed that the respiratory rate of mud-minnows was the same (30/ 

 min.) in daylight, and in daylight plus tungsten-lamp illumination. 

 When she slipped a ruby glass plate under the lamp the fishes settled 

 to the bottom, had fits of trembling, and more than doubled their 

 breathing rate. Shiners breathed 60 times per minute in diffuse daylight, 

 85 times per minute when a carbon filament lamp was turned on in 

 addition, and 150 times per minute when a ruby filter was placed over 

 the lamp. In this experiment it was perfectly clear that the response was 

 to redness as such, since the respiration rose with an increase of bright- 

 ness, but rose still higher when that brightness was somewhat reduced 

 by a filter which introduced hue. 



Untrained Hyborhynchus notatus would readily approach blue and 

 gray patches of light equated in brightness for humans and offered side 

 by side. But when a red, equated in brightness with the other two stimuli 

 for the light-adapted human, was substituted for the gray, the minnows 

 stayed away. Untrained Semotilus behaved identically. When, after the 

 blue and red had been offered for two hours, the gray was returned in 

 place of the red, they approached the patches promptly; but the red- 

 shyness reappeared when, three hours later still, the red was once more 

 exchanged for the gray. Strangely enough, trained dace were just as shy 

 of gray as wild ones were of red. 



Differential behavior of fishes toward blue, red, and gray matched in 

 brightness for the human eye, either light- or dark-adapted, is suggestive 



