60 The Animal Mind 



stimuli may be independently fatigued. The natural in- 

 ference is that a specific nervous apparatus belongs to 

 each stimulus. The second lies in demonstrating that the 

 reactions to different stimuli occur with different degrees 

 of rapidity. If there is a marked difference in the reaction 

 times of an animal to different forms of stimulation, each, 

 again, may be supposed to affect its own nervous path- 

 way. A modification of this method consists in noting 

 the influence of a stimulus upon the time of reaction to 

 another nearly simultaneous stimulus. If such an influ- 

 ence can be shown, it is evident that the force producing 

 it has some effect on the nervous system. By combining 

 this method with that of extirpating a sensory structure, 

 indications may be obtained that the nervous effect of the 

 auxiliary stimulus is dependent on a definite receptive 

 apparatus, and hence is probably accompanied by a special 

 sensation. This method was used by Yerkes to demon- 

 strate hearing in frogs (813). 



One further consideration offers itself to the student of 

 animal responses to stimulation. It has been the special 

 endeavor of Jennings to point out the fact that these re- 

 sponses, instead of being wholly accounted for by the 

 characteristics of the stimulus, are determined in part by 

 the internal, physiological condition of the animal (378). 

 We shall therefore note often in the course of the follow- 

 ing pages cases where difference of reaction is due to in- 

 ternal rather than to external causes. 



14. Evidence for Discrimination of Certain "Lower" 

 Sensation Classes 



Bearing all these points in mind, let us proceed to survey 

 the evidence for variety in the sensations of animals. In 



