Sensory Discrimination : Vision 121 



Are they seen, or do the sensations accompanying them rather 

 resemble those produced by an irritating chemical? What 

 kind of sensation quality may we suppose exists in the con- 

 sciousness of an animal whose responses to light are mediated 

 by the skin, not by the eyes ? When an animal discriminates 

 in its reactions between rays that to our eye differ in color, is 

 the discrimination one of color qualities, or of differences in 

 brightness, such as the spectrum offers to a totally color-blind 

 person ? And if a colored ray does not produce a color sen- 

 sation in the consciousness of a given animal, that is, if the 

 animal is color-blind, does it produce the same brightness 

 sensation that it would produce in a color-blind human being ? 

 These questions will constantly suggest themselves, but in 

 most cases the evidence will be insufficient to settle them. 



41. Vision in Protozoa 



Many of the Protozoa, as we know, react to light. Amoeba 

 gives a negative response when light falls upon it from the 

 side ; that is, it moves away from the light, and Jennings con- 

 jectures that this probably occurs by the contraction of the 

 part of the body nearest the light, which is what would happen 

 if the light were a mechanical stimulus (211, p. n). Blue 

 light has the same effect as white light, and red light has no 

 effect at all ; but the reactions of Amceba to light of different 

 colors differ only in degree, and do not indicate any qualita- 

 tive difference of accompanying conscious processes (162). 

 Nor, if the reaction to light is really identical with the negative 

 reaction in general, can we conclude that any specific visual 

 sensation accompanies it. The same holds true of the re- 

 sponses of various ciliate and flagellate Protozoa to light. 

 These all, so far as observed, take place by the ordinary nega- 

 tive or avoiding reaction ; some of the animals give it on pass- 

 ing from a region of less to one of greater illumination, and 



