COLOR CHANGES AND ADAPTATION IN FISHES. 1 99 



The results of my observations are in harmony with the statements presented above. 

 The time required to produce adaptive changes in the skin, both in Paralichthys and 

 in Ancylopsetta, varies greatly. Under some conditions changes resulting in maximum 

 adjustment in shade to a given background occur in two minutes or less; under others 

 it requires several days. In general, the time required for adjustment is considerably 

 longer for large specimens than it is for small ones. It is much longer for individuals 

 kept continuously on a given background than it is for those frequently changed from 

 one background to another. That is, the time required for adaptation is greatly reduced 

 by practice. This is clearly shown in the following experiment. 



A P. albiguttus, 12 cm. long, after having been in a white granite pan continuously 

 for two weeks and long since maximum white, was transferred to a black pan August 

 18, 2.05 p. m. At 4.30 p. m. it was about one-half maximum black; August 19, 12 m., 

 about three-fourths maximum black; August 22, 10 a. m., nearly maximum black; 

 August 23, ID a. m., maximum black. This same individual, after having been fre- 

 quently transferred from white to black and vice versa, from August 23 to August 30, 

 was taken from the white background on which it was maximum white and put into the 

 black pan at 7.27 a. m. One minute later, 7.28, it was already five-sixths maximum 

 black, and after one minute more, 7.29, it was maximum black. The change from black 

 to white, however, was never observed to be so rapid as this; it was never observed to 

 occur in less than an hour. Thus, while it required five days to produce a complete 

 change in the skin from white to black, after continuous sojourn of two weeks on white 

 it required only two minutes, after repeated transfer, to change from one to the other. 

 This is a most remarkable change in the reaction to a given stimulus. Such a change in 

 the reaction of man would undoubtedly be called learning. Are the processes involved 

 in changes in reactions in these widely different organisms fundamentally the same as 

 they are in man? 



No specific observations were made on the effect of repetition on the time required for 

 adaptation in pattern and color, but judging from superficial observations on this point 

 made in connection with experiments on the degree of adaptation, it appears probable 

 that repetition has the same, or at least a similar, effect on adaptation to these character- 

 istics as it has on adaptation in shade. 



Changes in color require, in general, much more time than changes in shade or changes 

 in pattern. There is, however, much variation regarding this among the different 

 colors. Yellow, for example, is a color that the fish assume much more readily and 

 rapidly than green or blue. This may be due to the fact that yellow ordinarily predomi- 

 nates in their environment. I have seen specimens which had been kept on various 

 black and white backgrounds for weeks, showing no trace of yellow, become almost at 

 once distinctly yellowish when put on a yellow background. 



FACTORS INVOLVED IN THE PROCESS OF ADAPTATION. 



CHROMATOPHORES IN THE SKIN. 



The skin of the fishes contains several different sorts of colored cells or groups of cells 

 known as chromatophores. These cells are much branched, some contain melanin 

 granules, which are brown or black in color, others contain xanthine granules, which 

 vary from yellow to orange, and still others contain iridescent guanin crystals. Ballo- 



