ADJUSTMENT OF FLATFISHES 441 



the rule. The dark tone assumed by the fish upon the dark sand 

 did not give way to a hghter shade when the animal was brought 

 into direct sunlight; while fishes on a white bottom acquired the 

 maximum degree of pallor, even though this white bottom was 

 heavily shaded. 



This state of affairs has already been pointed out by other observ- 

 ers,'"^ and indeed it would seem to be a necessary one in order 

 that the color should be adaptive or cryptic. For it is obvious 

 that the fish itself is shaded or lighted equally with the surface 

 on which it lies, so that the relation between the two remains 

 unaffected by the degree of illumination. 



A rather curious corollary may be drawn from this last principle. 

 Suppose that we have two aquaria side by side, one with its inner 

 surfaces painted white, the other painted a perfectly neutral 

 gray. Suppose, now, that the white aquarium is heavily shaded, 

 so as to admit comparatively little light, while the gray aquarium 

 is well illuminated. Under these conditions, the fishes in the white 

 tank should, according to hypothesis, assume the maximum degree 

 of pallor, while those in the gray tank should continue to display 

 some of their dark pigment, in amount depending upon the shade 

 of gray employed.*^ Now experiment shows that this is precisely 

 what happens. The fishes in the shaded white tank blanch to their 

 fullest extent, while those in the gray tank become gray. It is 

 obvious, however, that the dimly illuminated white bottom of the 

 one tank may be actually darker than the brightly illuminated gray 

 bottom of the other, in the sense that the former may reflect an 

 absolutely smaller amount of light to the observer than the latter. 

 The theoretical bearings of these facts will be discussed in a later 

 section. At present, I shall confine myself to an account of cer- 

 tain experiments upon Rhomboidichthys. 



Two glass jars, 20 cm. in diameter and 12 cm. deep, were used. 

 The bottom of one of these was painted gray of a shade nearly match- 

 es Most clearly by Keeble and Gamble for schizopod and decapod Crustacea 

 (Phil. Trans. Roy. Soc, Series B, vol. 196, 1904, pp. 353 et seq.) ; likewise by Bauer 

 for isopods (Centralblatt fur Physiologie, 1906, p. 459). 



" It is needless to point out that gray is not a color. Tf pure, it reflects white 

 light, though of reduced intensity. 



