56 The Animal Mind 



to determine their power of distinguishing stimuli or their 

 power of learning by experience, the first requisite is to 

 give the animal what we commonly call a 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 experience, 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 adjustment of conditions to bring out 

 exactly the discrimination wanted. He must either make 

 the performance of the reaction pleasant or its non-per- 

 formance unpleasant to the animal. A monkey, for ex- 

 ample, 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 dif- 

 ferent behavior toward the blue tumbler that it produces 

 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 response are not correlated with differ- 

 ences in the stimulation, it becomes probable that the two 



