DISCUSSION. 



What sort of mechanism is necessary to account for the reactions of the chrotnato- 

 phores resulting in adaptation in shade, in pattern, and in color? Let us consider this 

 question with reference to these different characteristics in the order given: 



Adaptation in shade. — Different shades in the skin of flounders depend upon the 

 degree of concentration of the melanin granules in the melanophores. The movement 

 of these granules, which regulates the degree of concentration, is controlled by stimuli 

 received through the eyes and the nervous system. 



If this movement were proportional to the light received by the eyes from the back- 

 ground, and if each nielanophore were connected with receptors in only one of the ret- 

 inas, one could readily account for adaptation in shade, but such is not the case. The 

 movement of the melanin granules is not proportional to the light received from the 

 background, for the degree of concentration of these granules on a background of a 

 given shade is the same, no matter how strong or how weak the light is; and each melan- 

 ophore is probably connected with receptors in both eyes, for if one eye receives Hght 

 from a black and the other from a white background the skin assumes an intermediate 

 shade. To account for the regulation of the movement of the granules it is consequently 

 necessary to assume a mechanism, such that the effect of the light reflected from the back- 

 ground on the movement of the melanin, is modified by the action of light received from 

 other sources ; and to account for the intermediate shades reproduced when one eye receives 

 stimuli from a background of a given shade and the other from one of a different shade, 

 it seems necessary to assume that the effective stimulus bears a definite relation to the 

 arithmetical means of the light received from the background by each of the two eyes, 

 modified by light received from other sources. It can not be proportional to the sum- 

 mation of the light received by both eyes, for if this were true a greater concentration 

 of the melanin and a more rapid reaction would be expected, under given conditions, 

 if both eyes are functional, than would be if only one is functional. This, however, 

 does not occur; consequently it must be further assumed that the effect on a melano- 

 phore of a given amount of light received by a retina from the background modified 

 by light recieved from other sources is twice as great if only one eye is functional as it 

 is if both are functional. 



Without attempting any further analysis, it is evident that to account for the 

 observed phenomena concerning adaptation in shade it is necessary to postulate a 

 rather complex mechanism, the working of which is as yet not altogether clear. The 

 difficulties encountered in attempting to elucidate the physiology of adaptation in this 

 respect, however, are much less perplexing than are those encountered in attempting 

 mechanically to explain adaptation in pattern and color. In these we have to account 

 for all of the phenomena regarding the compromise in the effect of direct and reflected 

 light and in the action of the two eyes and others as well. In other words, adaptation 

 in shade is always present. 



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