ADJUSTMENT OF FLATFISHES 425 



upon the magnetite, following seven days upon the coarse dark 

 sand already referred to. As will be seen by comparison with 

 fig. la, the fish became considerably darker than it had been while 

 upon the preceding background. The general shade of the body 

 became darker than at any other time during my experiments, and 

 the intra-annular areas were little if an}^ paler than the rest of 

 the body. The annuli themselves, however, were still clearly 

 visible, the minute, much contracted white specks being in con- 

 spicuous contrast to the rest of the surface. Whether or not even 

 these specks would have finally disappeared remains problematic, 

 though I regard it as improbable, in the case of this specimen, at 

 least. After four days, white pebbles of about the size of a bean 

 were scattered at considerable intervals over the black surface. 

 I thought it likely that the pale spots of the fish would come more 

 distinctly into view under the circumstances. This did not happen, 

 however, at least in the course of two days — an interval w^hich was 

 usually more than sufficient in the case of this specimen. 



Two other fishes (no. 10 and no. 11) were also tested with the 

 magnetite sand, the former for a period of six days, the latter for 

 twelve days. The results are shown in figs. lOd and lie respect- 

 ively. It is probable that neither of these specimens became any 

 darker than upon the ordinary dark sand. It must be stated, 

 however, that prior to their transfer to the magnetite sand, one 

 of these specimens had been kept all of the time, and the other 

 most of the time, during a period of two months, upon white or 

 pale gray backgrounds. Owing to this cause, the pale condition 

 had become in a certain sense fixed, as later experiments showed 

 (p. 446). 



5. A very coarse sand (perhaps better, a fine gravel) composed 

 of white, gray and dark red particles. The latter gave the material 

 much more color than was possessed by any of the preceding 

 materials. This sand was taken from a beach, and was not, there- 

 fore, a submarine deposit. It was employed in only a few cases. 

 One of these was my fish no. 1, which was, as stated, a particularly 

 adaptable one. This specimen adjusted its skin pattern very well 

 to the texture and the general shade of the material Tfig. Id), but the 

 reddish color of the sand exerted little or no effect upon the animal, 



