Sensory Discrimination 65 



But the earliest distinction between a touch quality and a 

 quality that is other than touch seems to occur when food 

 sensation and contact sensation are differentiated. It is 

 possible that warmth and cold also appear as distinct sensation 

 qualities in the experience of low forms of animals, but we 

 have little real evidence of the fact. No organs of tempera- 

 ture sensation are definitely known even in human 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 sensa- 

 tion quality. In some cases the sensibility to thermal stimu- 

 lation 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 qualita- 

 tively distinct sensations. 



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

 tiation. Its importance, so far as we can judge, is rather on 

 the spatial than on the qualitative side. The sense quality 

 of pain we naturally think of as the accompaniment of the 

 negative reaction in its more violent forms, given to a stimulus 

 that is injuring the organism. Organic and kinaesthetic 

 sensations are hard to trace in the lower animals ; for animals 

 whose structure differs widely from our own, the qualities 

 of these two classes must remain beyond the power of our 

 imagination. That differences in physiological condition 

 such as are produced by hunger, satiety, or fatigue involve 

 differences of accompanying organic sensation in the con- 

 sciousness of the animal manifesting them is possible. 

 Kinsesthetic sensations, as we shall see, are apparently con- 



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