466 FRANCIS B. SUMNER 



mining factor in the reaction. I believe, however, that the inhibi- 

 tory influence of the mask, in all these experiments, was due not 

 only to the injury inflicted by needle and thread, but to the inter- 

 ference of the cloth with the respiratory movements of the opercu- 

 lum. This interference would of course cease with the removal of 

 the overlying portion of the mask. 



One test of the hypothesis in question, which was made uninten- 

 tionally with Rhomboidichthys at Naples, must be referred to at 

 this point, for I regard it as of greater significance than all of 

 these experiments with artificial masks and stains. Specimen no. 

 10, which had been kept for fifteen or sixteen days in a marble- 

 bottomed tank, and had in consequence assumed a high degree of 

 pallor, was transferred to the coarse dark sand used in so many of 

 my experiments. The fish immediately buried itself with great 

 rapidity, and remained so, with only its eyes protruding, during 

 its entire sojourn upon this bottom. It is probable that it never 

 emerged (in the datyime, at least) except when forced to do so 

 by myself, and at such times it concealed itself with extreme rapid- 

 ity. Nevertheless, after two days, this specimen was nearly as 

 dark as the sand, and after five days it was described in my notes 

 as harmonizing almost perfectly with this material. After another 

 extended sojourn (twenty-six days) upon white and pale gray 

 backgrounds, the fish was placed upon the jet-black magnetite 

 sand. In this, it displayed the same tendency toward conceal- 

 ment, remaining buried, except when forced to leave cover. Never- 

 theless, the fish was quite plainly darker after the lapse of a 

 single day, and of a very dark shade after the lapse of six days 

 (fig. lOd), although my notes state that ''when placed here, the 

 fish seemed almost white in comparison with the sand." There 

 surely had been little opportunity in this case for the fish to ob- 

 serve the appearance of its own body. 



The foregoing experiments with these two species of fish, al- 

 though not free from contradictions, certainly do not bear out the 

 visual comparison hypothesis, but rather come very near to refut- 

 ing it altogether. A really satisfactory test of the alternate hypoth- 

 esis seems likewise very difficult in practice, and, although I have 

 devoted much time to the matter, I have, at the present writing, 



