ADJUSTMENT OF FLATFISHES 463 



for the changes just recorded, the case becomes decidedly puzzling. 

 For anyone with any knowledge of optics knows that gray — at 

 least a perfectly neutral gray — is not a color. Such a gray reflects 

 all the components of white light in their normal proportions. It 

 differs from white only in this, that it reflects a smaller fraction 

 of the total quantity of hght which falls upon its surface. Gray 

 is thus relatively darker than white, but not always absolutely 

 darker. When we ourselves judge of an object as being gray or 

 white we make an allowance for the degree of illumination to 

 which it is subjected, and this last is inferred from the totality 

 of the visual field. 



But how about the fish? It is not in the position of an outside 

 observer, with abundant standards of comparison at hand. This 

 tank, with its painted surfaces, would seem to constitute for the 

 time being its entire environment. How, then, if the walls of 

 the shaded white box reflect absolutely less light to the animal's 

 eyes than do those of the brightly lighted gray box, does the crea- 

 ture take on a lighter shade in the former than in the latter? 



So far as I can see, we are limited to two alternative explana- 

 tions: either (1) the fish takes into account the degree of illumina- 

 tion, just as we do, and makes due allowance for this in judging 

 of the paleness or darkness of the background; or (2) it makes a 

 direct visual comparison between its own surface and that of the 

 background and endeavors to bring the former into harmony 

 with the latter. ^1 In this second case, since the body of the fish 

 itself is lighted or shaded to an equal extent with the background, 

 it would have to become fully white in order to conform even to a 

 dimly lighted white background. 



Let us take up the latter of the foregoing alternatives first. 

 In order to test the question whether the fish compares its own 

 appearance with that of its background, I have tried the expedient 

 of concealing from the view of the animal its own skin color. For 



^1 Such hopelessly 'anthropomorphic' language may shock the sensibilities of the 

 ultra-mechanistic reader. I therefore hasten to explain that no consciously rea- 

 soned mental processes are here implied. The whole chain of events could doubt- 

 less be stated in purely physiological terms, were we more familiar with the facts, 

 but so, for that matter, might our own behavior. 



