530 ADAPTATIONS TO PHOTIC QUALITY 



off of the oxygen supply than to a deprivation of hormonic stimulation. 

 More significant, and suggesting a direct chemical influence of the retina 

 itself, are the recent experiments of Szepsenwol. He transplanted the 

 adult eyes of Fitzroya lineata to new locations in the body where they 

 could have no connection with the nervous system, and found that the 

 chromatophores would still perform. 



Response to Albedo — The physiological dermal changes of the aver- 

 age teleost consist of simple darkening on dark backgrounds and paling 

 on light or white ones. Both normal and eyeless animals become pale in 

 darkness, but eyeless animals mysteriously darken in the light. In some 

 species, as in the flounders lately examined by Osborn iPseudopleuro- 

 nectes americanus and Lophopsettd aquosa) , the blinded fish takes on 

 an intermediate shade, and the dark spots normal for the intact animal 

 disappear — this being the pattern which the intact fishes assume in dark- 

 ness. 



It may seem odd enough that a blinded fish should respond to light at 

 all, and we will consider the possible reason for this in a page or two; 

 but there is an even greater peculiarity about the responses of the intact 

 fish to light and dark backgrounds : it was Sumner who, years ago, first 

 noticed that in these responses the intensity of illumination is of little 

 consequence. This has been abundantly confirmed since, and has always 

 seemed remarkable. If the fish were responding merely to the amount of 

 light entering the eye, it should give the same dermal response to a 

 brightly illuminated dark background as to a dimly illuminated white 

 one — which would not adapt the fish at all! Instead however, the shade 

 assumed by the skin of the fish is always (unless the intensity of the in- 

 cident light is very low or extremely high) in accordance with the albedo 

 of the substrate — the percentage of incident light which the substrate 

 reflects. 



A response to albedo sounds impossible. It would be like a response 

 to specific gravity. The strange thing is that we do respond to specific 

 gravity — in the so-called size-weight illusion, wherein a pound of lead is 

 actually judged heavier than a pound of feathers. Analogous, also, is our 

 ability to recognize a melody as 'the same' after transposition to another 

 key. 



These phenomena have their counterpart, in human vision, in the one 

 which psychologists call brightness constancy. We see snow as white in 

 the evening, and see coal as black in noonday sunlight, even though the 



