Sensory Discrimination: Methods of Investigation 59 



which to the more widespread effects of the operation. 

 Yet this much may be said for the combination of proof 

 from structure and behavior involved in the Method of 

 Extirpation, if we may so call it : where an animal reacts 

 to a certain stimulus, for instance light, when a sense organ 

 is intact, and fails to react to light, though otherwise nor- 

 mal, when the organ is removed, there arises a possibility 

 that light 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 nega- 

 tive 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 

 unpleasantness 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 reactions when applied elsewhere, then 

 the suggestion is given that different sense organs are 

 involved, and the same possibility arises of different sensa- 

 tion qualities. 



Two other forms of evidence whereby from behavior a 

 differentiation of sensory structures can be argued, and 

 from differentiation of sensory structures possible differ- 

 ences of sensation quality, may be mentioned. The first 

 of these consists in showing that reactions to different 



