COLOR CHANGES AND ADAPTATION IN FISHES. 209 



2. Gray background. — On August 16, at 10.30 a. m., the sheet of white paper with 

 black spots was replaced with one which had been made uniformly gray of a medium 

 shade (Ridgway's pale neutral gray). The light from the mirror was intercepted, and 

 the fish was fully illuminated from above. At i p. m. the entire surface was fairly 

 uniformly gray, and the fish was quite inconspicuous. At this time the cylinder was 

 put in place and covered and the light from the mirror turned on. The background, as 

 seen from the top of the cylinder, appeared distinctly gray, not white. At 2 p. m. the 

 fish was nearly maximum white. There were no dark spots and the three ocelli were 

 very light gray. The following morning, 6 a. m., the fish was fully as white as it had 

 been at any time on a pure white background. The cylinder was now removed and the 

 light from the mirror intercepted. At 10 a. m. the fish was well adapted to the gray 

 again. The background was now again illuminated from below, and also left exposed 

 to light from above. The fish seemed to turn a shade lighter, although there was some 

 question concerning this. It was left until 3 p. m., then a sheet of gray tracing paper 

 and another sheet of white paper were added to the gray paper under the dish, making 

 the background much darker and nearly opaque. The cylinder was then put in place, 

 covered, and the light from the mirror turned on. The background, as seen from above, 

 appeared nearly black. At 5 p. m. the fish was nearly if not quite maximum white, 

 and the following morning it was unquestionably maximum white. 



Thus we see that if the light from above is abnormally low in comparison with that 

 from below, Paralichthys may become white on a background which appears dark gray 

 to the human eye. This strongly supports the idea that the shade assumed by the 

 skin in these creatures under normal conditions depends upon the amount of light 

 received by the eyes from above, as well as upon the amount received from below ; for 

 under such conditions animals on a gray background always become gray. This idea 

 is also supported by the results obtained in the following experiment, in which the 

 background contained numerous black spots: 



3. White background containing black spots. — On August 11, 2 p. m., the sheet of 

 white paper under the crystallizing dish in the first experiment was replaced by one of 

 the same kind and size containing dense black dots 5 mm. in diameter. These dots were 

 made with India ink, and they were such a distance apart that they covered one-half 

 of the entire surface. The opaque cylinder was removed and the light from the mirror 

 was not obstructed. Thus the background was illuminated both with light from below 

 and from above, but it appeared much as it did under normal conditions of illumination, 

 the spots being merely relatively somewhat darker. Numerous black spots appeared 

 almost at once in the skin of the fish, just as under normal conditions, except that the 

 spots were slightly more conspicuous. At 2.15 p. m. the cylinder was put in place and 

 closed so as to cut off all light from above. Under these conditions of illumination the 

 spots in the background, as seen from above, were still very prominent. At 2.30 p. m. 

 no change had taken place in the skin. At 8 p. m. all of the black spots except three, 

 the oceUi, had disappeared, and the fish was nearly if not quite as pale as it had been 

 in the preceding test on a pure white background. The three ocelli were, however, 

 somewhat darker. This shows that simulation of the pattern in the background is 

 dependent upon light received from above; and it shows again that decrease in the 

 illumination from above tends to cause the skin of the fish to become abnormally light. 



