ADJUSTMENT OF FLATFISHES 477 



by surfaces within the tank and the light which enters the latter 

 from above be that factor of the total stimulus which renders pos- 

 sible these accurate adjustments of the shade of the fish's body to 

 that of its background?^' I think that this is the true solution ol 

 the problem, and I hope, with apparatus already constructed, to put 

 the question to experimental test, as soon as material is available. 

 That such a relation between the light intensities of two por- 

 tions of the visual field may form an integral part of the immediate 

 perception,without the necessity of rational mental processes, I 

 think will be granted by all. Now naturally the fish knows noth- 

 ing of the distinction between the source of the light and that part 

 of the environment which is illuminated by it. There is for the 

 animal but one continuous visual field, though this may not all 

 be apprehended at once. The latter is constituted by various 

 areas, differing in luminosity or in color. Those portions of this 

 field which lie below, or but little above, a horizontal plane passing 

 through the animal itself are the ones to which the appearance of 

 the latter is adjusted. It is these which, as already seen, probably 

 occupy the focus of attention most of the time. Those portions 

 which lie more nearly overhead, and thus ordinarily beyond the 

 focus of attention, must, however, serve in some way as a criterion 

 by which the shade of the rest of the visual field is apprehended. 

 With a given amount of light from outside the tank, a greater or a 

 less amount of reflected light from the bottom would of course 

 imply a lighter or a darker shade in the latter. On the other hand, 

 with a given (absolute) amount of fight reflected from the bottom, 

 the occurrence of a low degree of illumination overhead would 

 lead the animal to attribute a paler shade to this bottom {i.e., 

 to see it as paler) than if the source of light were a brilliant one. 



*i These words, and in fact my entire discussion of this problem, down to the 



end of the next paragraph, were written before I had any knowledge of the almost 



identical hypothesis which was put forward some years ago by Keeble and Gamble 



(Phil. Trans. Roy. Soc, Series B, vol. 196, 1904). Under these circumstances, such 



a verbal coincidence as is to be noted in comparing their statement with mine is 



rather surprising. On p. 358, these authors state: ". on the white and 



black grounds the animal appeals for pigment-guidance to the amount 



of light scattered or absorbed from the ground; or, as we put it previously, it is a 



direct ,. , ,, 



reaction to the ratio — ; light. 



reflected 



