448 FRANCIS B. SUMNER 



upon the observer was much Hke that which one experiences in 

 watching the development of a photographic plate. Indeed it not 

 infrequently happened, in those cases in which the maximum 

 adaptive effect was not displayed by the fish at the time of its 

 being disturbed, that this maximum effect appeared for a brief 

 period after the animal settled down, only to diminish again after 

 a few moments. 



3. EXPERIMENTS UPON RHOMBUS 



Although Rhombus maximus was the species which first 

 arrested my attention in the show aquarium by its extraordinary 

 adaptation to the gravel bottom, no striking results were obtained 

 in the laboratory from the single specimen which I used. More- 

 over, the species was too large for convenient manipulation. 



Two specimens of Rhombus laevis were, however, used with 

 some interesting results. Both of the specimens showed a high 

 degree of adaptation to the fine gravel, used in the foregoing 

 experiments, and one of them (the other was not tested) likewise 

 acquired a high degree of harmony with the dark sand. Both 

 specimens became much paler when placed upon the white marble 

 bottom of a large aquarium, though neither attained such an 

 extreme condition of pallor as did Rhomboidichthys. One of the 

 two, at the end of a stay of forty-six days, "harmonized pretty 

 well with the now much stained marble bottom, ' ' though the 

 maximum degree of adaptation had probably been brought about 

 long before this. Even after this extended sojourn upon the 

 marble, however, I note of this specimen, after transfer to gravel, 

 that ''within a short time, certainly in less than an hour, the spots 

 had come distinctly into view, and on the same afternoon the 

 fish harmonized pretty well with the gravel." 



After a short sojourn on the gravel, the corneas of this fish 

 were rendered opaque by the application of silver nitrate, the 

 animal being then returned to the same bottom. After the lapse 

 of a day, the fish was very much paler than before the operation, 

 and not far different from the condition when on marble. After 

 two days, however, the gravel condition {i.e. the darker, spotted 



