474 FE,ANCIS B. SUMNER 



But aside from the evidence which they afford of the role played 

 by the eyes in these changes of color, the blinding experiments seem 

 to show that vision is necessary in order that the pigment cells 

 shall remain in a given state of tonus, exception being made to the 

 case of those fishes which are blinded in a uniformly dark state, 

 representing most nearly the resting condition of the chromato- 

 tophores. Continued adaptation to a less usual background, 

 e.g., a very pale one, may result in the new condition becoming 

 more or less fixed. The latter may persist for a time after loss of 

 sight, but the more habitual state of tonus finally reasserts itself. 

 The cases mentioned at the close of section 17 of the summary 

 might be explained by supposing that the pale condition had 

 become in considerable naeasure fixed, so as to reappear after the 

 stimuli responsible for the secondary dark condition had been 

 withdrawn by destruction of the sight. The ultimate return to 

 the dark state would be intelligible here as in the case of fishes 

 which are blinded when pale. But if the foregoing interpretation 

 is correct, it is hard to understand why any unusual state of the 

 chromatophores which has but recently been acquired should not 

 give place to the more habitual condition as soon as the light of 

 day is withdrawn (e.g., at night). But this was found not to be 

 the case. Here, as so often happens, the simple and obvious 

 explanation does not seem to contain the M^hole truth. 



Evidence has been offered which seems to show conclusively 

 that the plane in which a given surface lies with relation to the 

 fish determines in some cases, whether or not it shall be effective 

 in calling forth a given change. It was not made certain, however, 

 that even in such cases, the matter was not decided by purely 

 quantitative relations within the visual field. For, as was 

 pointed out, we must distinguish between the potential and the 

 actual visual fields. That the horizontal surface lying immedi- 

 ately about the fish is the one which is generally most potent in 

 determining the reactions of Rhomboidichthys, might be due 

 entirely to the fact that the animal's gaze is commonly turned in 

 this direction. In experiments upon Lophopsetta, we found (p. 

 454) that when the bottom, plus the upper half of the vertical 

 walls, were white, while the lower half of these walls was black, 



