Sensory Discrimination 63 



may produce in the animal's consciousness a specific sensation 

 quality, even although the animal ordinarily reacts to light 

 in a manner indistinguishable from that of its responses to 

 other stimuli. Though light and mechanical stimulation, 

 for example, both ordinarily produce a negative reaction, 

 yet if light brings about its effect only through the medium 

 of a specialized structure with which mechanical stimuli 

 are not concerned, then along with the probable un- 

 pleasantness accompanying the negative reaction there may 

 go a quality peculiar to the functioning of that special 

 structure. 



Another mode of combining evidence from structure with 

 evidence from behavior is by the use of localized stimuli. If 

 an animal gives a response, which in itself may have nothing 

 to mark it off from responses to other stimuli, when a special 

 kind of stimulation is applied to certain regions of the body, 

 and only then; while the other stimuli produce better re- 

 actions when applied elsewhere, then the suggestion is 

 given that different sense organs are involved, and the same 

 possibility arises of different sensation qualities. 



Two other forms of evidence whereby from behavior a 

 differentiation of sensory structures can be argued, and from 

 differentiation of sensory structures possible differences of 

 sensation quality, may be mentioned. The first of these con- 

 sists in showing that reactions to different stimuli may be 

 independently fatigued. The natural inference is that a 

 specific nervous apparatus belongs to each stimulus. The 

 second lies in demonstrating that the reactions to different 

 stimuli occur with different degrees of rapidity. If there is a 

 marked difference in the reaction times of an animal to different 

 forms of stimulation, each, again, may be supposed to affect 

 its own nervous pathway. A modification of this method 

 consists in noting the influence of a stimulus upon the time of 



