COLOR CHANGES AND ADAPTATION IN FISHES. 



207 



c 



that adaptation can occur with the skin entirely concealed and that light reflected from 

 the skin or received by it plays no appreciable part in the process. 



THB DIRECTION OF THE LIGHT AS A FACTOR IN ADAPTATION. 



The fact that flounders and Crustacea become maximum white on a white back- 

 ground and gray on a gray background even if the former, owing to weak illumina- 

 tion, reflects less light than the lat- 

 ter, shows conclusively that the 

 shade assumed by these creatures 

 is not proportional to the absolute 

 amounts of light received from below. 

 Keeble and Gamble (1904, p. 354) 

 maintain, for Crustacea, that it bears 

 a specific relation to the ratio be- 

 tween the light received by the eyes 

 direct from above and that received 

 from below after reflection from the 



, . , ,. . direct 

 background— I.e., the ratio — g^^^ 



light." Sumner inclines to the same 

 view with reference to fishes. He 

 holds that adaptation in shade can 

 not be regulated by a "direct visual 

 comparison [by the fish] between its 

 own body surface and the bottom 

 on which it lies" (p. 476); and he 

 further says: 



May not, then, the ratio between the 

 light reflected from the near-by surfaces 

 within the tank and the light which enters 

 the latter from above be that factor of the 

 total stimulus which renders possible these 

 acciuate adjustments of the shade of the 

 fish's body to that of its background? I 

 think that this is the true solution of the 

 problem. 



This hypothesis seems to meet all 

 the requirements of the phenomena 

 in question; but neither Keeble and 

 Gamble nor Sumner succeeded in es- 

 tablishing it experimentally, although 

 Sumner says that he constructed ap- 

 paratus for this purpose, but was un- 

 able to make the necessary tests, 

 owing to lack of material. The fol- 

 lowing experimental results throw some light on the problem in hand : 



If the shade of the animal depends upon the ratio between the amount of light 

 received by the eyes direct from the source above and that received by the reflection 



CD"-' 



Fig. 2. — Vertical section of apparatus used in testing, in flounders, 

 the effect of abnormally increasing the relative amount of light 

 received by the eyes from below. A. glass aquarium; CD. crystal- 

 lizing dish; S, siphon; I. inlet for water; O, outlet; GP, glass plate; 

 C, opaque cylinder; M, mirror; L. light. 



