Sensory Discrimination: Vision 163 



Watson (771) does not think these observations suffi- 

 ciently prove color discrimination on the part of the fish. 

 He says, "Ordinarily we mean when we say that an animal 

 is sensitive to difference in wave-length that such stimuli 

 play a role in the adjustment of the animal to food, sexual 

 objects, shelter, escape from enemies, etc. i.e., that such 

 stimuli initiate activity in arcs which end in the striped 

 muscles." Because the changes of color are produced 

 not by such arcs, but by the sympathetic nervous system, 

 Watson thinks color vision not proved ; "we can easily con- 

 ceive," he says, "of mimicry of this kind taking place in 

 an animal whose retina does not contain the physico-chemi- 

 cal substances . . . necessary to initiate response to differ- 

 ences in wave-length." Since the changes of color are in- 

 duced by differences in wave-length and induced through 

 the retina, we may reply that it does not seem easy, or in 

 fact at all possible, to conceive the absence of such photo- 

 chemical substances from the fish's retina. Moreover, 

 Mast finds that fish which have thus become adapted to 

 a given color will seek that color : this is an activity in- 

 volving the striped muscles. 



On the whole, the weight of evidence is at present in 

 favor of the possession of color vision by fish. 



42. The Problem of Visual Qualities: Reptiles and 

 Amphibia 



Skin sensitiveness to light has been demonstrated in 

 certain amphibians. The response of the frog to light 

 persists when the animal is blinded, although in the nor- 

 mal animal the eyes are involved in the reaction, since it 

 occurs when the skin is covered and the eyes left intact 

 (405, 538). The skin of salamanders also is sensitive to 



