Sensory Discrimination 61 



motive. That is, the conditions of the experiment must be 

 so arranged that some already present tendency to act, 

 whether inborn in the animal or acquired by previous ex- 

 perience, shall be appealed to. 



This is increasingly the case, the higher the animal worked 

 with stands in the scale. The higher animals have what 

 might be called a large reserve fund of discriminations. That 

 is, they are capable of making many more selective reactions 

 to stimuli than they need at a given moment actually to use. 

 Hence in their case the experimenter must make a careful ad- 

 justment of conditions to bring out exactly the discrimination 

 wanted. He must either make the performance of the re- 

 action pleasant or its non-performance unpleasant to the ani- 

 mal. A monkey, for example, confronted by a set of glass 

 tumblers covered each with a differently colored paper, may 

 behave toward them all in precisely the same way; yet if 

 food be put regularly in the blue -tumbler, whose position in 

 the row is varied, it becomes worth the monkey's while to 

 make use of his discriminative powers, and he may show by 

 his different behavior toward the blue tumbler that it pro- 

 duces on him a different impression from the others. 



With simpler animals the problem is less difficult. If 

 an animal is capable only of a half dozen different ways of 

 responding to stimulation, we may with comparative safety 

 assume that it has less opportunity to hold them in reserve ; 

 and if such an animal invariably reacts in the same way to 

 two different forms of stimulus, or if the variations in its re- 

 sponse are not correlated with differences in the stimulation, 

 it becomes probable that the two stimuli produce in its as- 

 sumed consciousness identical sensation qualities. Thus it is. 

 not the number of stimuli to which an animal reacts that! 

 can be taken as evidence of the qualitative variety of itsf 

 sensations, but the number of stimuli to which it gives dif- 



