142 The Animal Mind 



to learn by experience, caused it to go through a simple 

 labyrinth leading to a tank of water. At the point where the 

 first choice between two paths occurred, a red card was 

 placed on one side and a white card on the other. When the 

 frog had learned to take the correct path, toward the white, 

 the cards were exchanged, without any other alteration in the 

 conditions; and the decided confusion of the animals in- 

 dicated that they had discriminated between the red and 

 white cards and had learned to react with reference to this 

 discrimination (454). 



Two species of frogs tested by Ellen Torelle showed posi- 

 tive phototropism, associated, as usual, with a tendency to 

 prefer blue to red light (401). The frog's phototropism, 

 moreover, persists even when the animal is blinded, although 

 in the normal animal the eyes are involved in the reaction, 

 since it occurs when the skin is covered and the eyes left 

 intact (224, 308). Skin sensitiveness to light has been 

 noted also in salamanders (103). The nature of the "dermal 

 light sensation" remains a mystery. It can hardly, in frogs, 

 be a painful irritation, since it produces a positive response ; 

 and it is not due to heat rays, for it occurs when these are 

 intercepted by passing the light through water. As Parker 

 says, radiant heat and light, " distinct as they seem to our 

 senses, are members of one physical series in that they are 

 both ether vibrations, varying only in wave length" (308). 

 While, then, the nerve endings in human skin are sensitive 

 only to the slowest of these vibrations, the heat rays, those in 

 the skin of the frog may respond to the whole series, with 

 what accompanying sensation qualities we cannot say. 



52. Vision in Other Vertebrates 



In some reptilian eyes, and in those of all birds, a few 

 fishes, and Ornithorhyncus, there are attached to the ends of 



