1 62 The Animal Mind 



studied this phenomenon in the case of the fish Phoxinus 

 laems. If two fish whose skins are, at the time, of equal 

 brightness, are placed one on a yellow ground, the other 

 on a gray ground, and these grounds are properly chosen as 

 to brightness, the fish will not alter their own brightnesses, 

 although if either ground is made lighter or darker a cor- 

 responding change occurs in the skin of the fish lying on 

 the altered ground. When the fish remain at the same 

 brightness, then, it may be inferred that the "brightness 

 values" of the two grounds are identical. But after a few 

 hours, it will be found that the fish on a yellow ground 

 shows a yellow stripe which does not appear on the other 

 fish (see also 277). Mast (475) has made a very thorough 

 study of this phenomenon in the case of the flounders 

 Paralichthys and Ancylopsetta. These fishes become 

 strikingly bluish on blue grounds, greenish on green grounds, 

 and so forth, adapting themselves to blue, green, yellow, 

 orange, pink, and brown, and less successfully to red. The 

 color changes are brought about by certain pigment-con- 

 trolling mechanisms in the skin, which are connected with 

 the sympathetic nervous system. But the color stimulus 

 acts through its effect on the eyes : the changes do not 

 occur if the eyes are covered. Moreover, the effect of the 

 stimulus received by one eye is modified by that of the 

 stimulus received from the other eye : if one eye is on a 

 black ground and the other on a white ground, the skin 

 becomes gray. Mast succeeded in showing that the rate 

 at which alternating black and white sectors must follow 

 each other in order to fuse into a continuous gray is the 

 same for the eye of the flounder as for that of the human 

 being: he placed the fish over a rotating black and 

 white disk and noted the speed of rotation required for the 

 fish to become gray instead of mottled black and white. 



