COLOR CHANGES AND ADAPTATION IN FISHES. 219 



can not be accounted for solely on the basis of brightness, nor can they be accounted 

 for on this basis on the assumption that the brightness values for fishes, of the four 

 colors used, differ in any other way from their values in man; for in accord with all such 

 assumptions the reactions in each box should have been the same for all of the indi- 

 viduals tested. The reactions in the blue-green box, e. g., should have been the same 

 for the individuals adapted to blue as they were for those adapted to green. This, how- 

 ever, was not the case; the former were positive to blue, while the latter were positive 

 to green. 



VISION OF MOTION, SIZE, AND FORM. 



Much more work has been done on color-vision in fishes than on any other phase of 

 the subject. It is well known, however, that many species in securing food regulate 

 their movements by vision and that smell and taste play a minor role in the process, 

 if indeed they play any. This is true for Paralichthys and Ancylopsetta. It is also well 

 known that these fishes see other objects, especially such as may be injurious. It is 

 known, moreover, that the movement of the object seen is an essential factor in the 

 process of vision. Very little, however, is known as to what role in the process is played 

 by form, size, and surface characteristics. 



The following observations throw some light on these questions. A fiddler crab 

 of moderate size was thrown into an aquarium containing a number of hungry speci- 

 mens of Paralichthys. The crab was seized at once, partly swallowed, and then 

 thrown out. It was then taken by another specimen and again rejected. Neither of 

 these specimens nor any of the others in the aquarium was again seen to attack a crab, 

 although during the following few days specimens of various sizes were repeatedly 

 thrown in. Minnows of the same size, however, were invariably taken during this 

 time. Thus it is evident that the crabs were recognized, and this recognition, no doubt, 

 was made on the basis of form. 



Size, however, is also a distinguishing characteristic for these animals, as shown 

 by the fact that they rarely attack minnows which are so large that they can not be 

 swallowed. But if the simulation of patterns in the background in Paralichthys may 

 be used as a criterion of vision, we have evidence of far greater significance regarding 

 this matter. 



By referring to plates xxxi, xxxii, figures 49-55 and the legends accompanying 

 them, it will be seen that dots 0.5 mm. in diameter produce no specific effect, but that 

 those I mm. in diameter do. It will also be seen that the effect of dots 2 mm. in diam- 

 eter differs from that of those 3 mm. in diameter, and that the effect of these dots differs 

 from the effect of those 5 mm. in diameter. These results, as I have shown elsewhere, 

 are due to stimuli received through the eyes, and impulses passed through the brain 

 and the central and sympathetic nervous systems. These responses may be and prob- 

 ably are purely reflex. But, however that may be, they indicate as clearly as any other 

 responses can that these animals recognize the difference between spots 2 mm. and 

 3 mm. in diameter, and that they do not recognize spots 0.5 mm. in diameter. Regard- 

 ing subjectivity in these animals, we know next to nothing, if not actually nothing. It 

 is consequently evident that, until this state of affairs is changed, the term recognition 

 should be used strictly in an objective sense. 



