446 FRANCIS B. SUMNER 



kept most of the time during a period of two months upon white 

 and pale gray bottoms, was transferred to the black magnetite 

 sand for a period of six days. At the end of this time the fish was 

 nearly or quite as dark (fig. lOd) as are most of these fishes when 

 adapted to a very dark bottom. The animal was then blinded by 

 searing the corneas with a red-hot platinum wire. The effect was 

 a conspicuous paling of the body (fig. lOe), which became evident 

 in a short time and persisted for some days, after which the darker 

 condition began to return. 



Substantially the same results were obtained from specimen 

 no. 11, which had been kept for an even larger proportion of 

 the preceding two months upon pale bottoms, and had been only 

 3 days upon the black sand at the time of blinding. Within a few 

 hours, the fish returned completely, or nearly so, to the extremely 

 pale condition which it had acquired upon its earlier backgrounds, 

 and remained in this condition for a day, after which the observa- 

 tions were brought to an end. 



These results are readily intelligible on the assumption that 

 the pale condition had, to a certain extent, become fixed in the 

 nervous mechanism during the long sojourn of the fish upon such 

 bottoms. The transfer to a dark bottom resulted, without much 

 delay, in the acquirement of a darker appearance, but this condi- 

 tion was not, at first, a stable one, and its maintenance seemed 

 to depend upon the continuance of visual stimuli. There are, it 

 is true, certain facts which seem irreconcilable with this hypoth- 

 esis, as we shall see later. But the foregoing experiments have been 

 repeated upon two other species, and the facts themselves are 

 beyond question. Moreover, fishes which had undergone no 

 extended sojourn upon a pale bottom did not (with a single excep- 

 tion) become pale when blinded. 



Changes having no relations to visual stimuli 



As stated in an earlier paragraph, decided changes occur in the 

 disposition of the skin pigment, which have no relation to the 

 background or, indeed, to any visual stimulus whatever. Cer- 

 tain very conspicuous phenomena of this sort were observed 



