Sensory Discrimination: Methods of Investigation 61 



the lowest forms, such evidence must be derived entirely 

 from behavior. That from the presence of a sense organ 

 is almost wholly lacking. And although various stimuli, 

 as we have seen, produce reactions in Amoeba, yet there 

 is only one case where these reactions are strikingly dif- 

 ferent according to the quality of the stimulus applied. 

 This instance consists in the distinction between food- 

 taking reactions, given to edible substances, and the 

 responses to mechanical stimulation. The sense of touch, 

 undoubtedly, must play a part in the mental life of the 

 lowest animals that have consciousness at all. But the 

 earliest distinction between a touch quality and a quality 

 that is other than touch seems to occur when food sensa- 

 tion and contact sensation are differentiated. It is possible 

 that warmth and cold also appear as distinct sensa- 

 tion qualities in the experience of low forms of animals, 

 but we have little real evidence of the fact. No organs of 

 temperature sensation are definitely known even in hu- 

 man beings. And the responses of low animals to thermal 

 stimulation are not specialized. They consist usually of 

 negative reactions, given when the animal is subjected to 

 a temperature either above or below, but especially above, 

 the "optimum"; and these reactions are not- different 

 from the ordinary negative type, suggesting unpleasantness 

 rather than a specific sensation quality. In some cases 

 the sensibility to thermal stimulation has been found to 

 be differently distributed from that to other classes of 

 stimuli. But in any case, sensations of warmth and 

 cold are probably in no member of the animal kingdom 

 differentiated into any greater number of qualitatively dis- 

 tinct sensations. 



The sense of touch, also, shows but little internal dif- 

 ferentiation. Its importance, so far as we can judge, is 



