COLOR CHANGES AND ADAPTATION IN FISHES. 20I 



as to what part the direct responses of the chromatophores to the stimulating agents 

 may play in the process of adaptation in normal animals. The results obtained in the 

 following experiments seem to give precise and conclusive answers to some of these, 

 as well as to other questions. 



A specimen of Paralichthys, 5 cm. long, in which adjustment to a black or a white 

 background occurred rapidly, was put into a glass dish on the stage of a binocular and 

 so arranged that the head was over a black surface and the tail over the opening in the 

 stage. The mirror was then adjusted so as to reflect strong diffused light up through 

 the opening, thus strongly illuminating the tail without affecting the head. The fish 

 was kept in this position an hour, which was much longer than was necessary for marked 

 changes in adjustment to either black or white. It was carefully examined under low 

 magnification from time to time and finally put entirely on a black surface and again 

 examined. No difference whatever was detected in the shade of the two ends of the 

 fish. This experiment was repeated several times with this specimen and also with two 

 others of about the same size. Those which were adapted to black remained uniformly 

 dark throughout the experiment and those which were adapted to white became uni- 

 formly dark over the entire surface of the body. There was no indication that the 

 intense reflected light had any direct effect on any of the chromatophores, although 

 many of them, especially those in the fins and the tail, were strongly illuminated and 

 all of them were unquestionably in stronger light than those on the opaque surface, 

 for much light penetrated the tissue even where it was thickest. If light of this inten- 

 sity does not under normal conditions appreciably affect the movement of the pigment 

 granules directly, there is no probability that light reflected from the bottom will do so. 

 This conclusion, moreover, is supported by the following observations and experiments. 



Specimens free in an aquarium, half of which was black and the other half white, 

 were repeatedly seen to come to rest with the head either on the black or on the white 

 and to remain long enough for adaptation to occur. In every instance observed in 

 which there was any response at all the entire fish assumed a shade corresponding to 

 that of the bottom under the head. Thus the posterior end of the animal in every 

 instance stood out in striking contrast against the background, while the anterior end 

 corresponded well with it. In no case was there any evidence of a line of demarca- 

 tion on the body corresponding with that in the background below. The same was 

 found to be true, no matter what position the fish had with respect to this line. If it 

 was parallel with the dividing line in the background so that one eye was over black 

 and the other over white, the skin assumed a gray shade of equal intensity on both 

 halves of the body. This also occurred if only the tip of the head was on the black 

 or white and in some instances even if the line between black and white was some little 

 distance in front of the anterior end. 



These statements are supported by the following detailed account of individual 

 observations and by the photographs reproduced on plate xxx. 



On July 3 a specimen of P. albigultus, 1 1 cm. long, was closely observed for several 

 hours in an aquarium one half of which was white and the other half black. It usually 

 moved about near the bottom very slowly for a period, then came to rest and remained 

 a variable length of time, and then moved away again, repeating this process. Adapta- 

 tion to black or white occurred remarkably rapidly in this specimen. So rapid were 

 the changes in the skin that in slowly swimming from black to white or vice versa the 



