64 The Animal Mind 



reaction to another nearly simultaneous stimulus. If such an 

 influence 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 demonstrate 

 hearing in frogs (456, 462, 464). 



One further consideration offers itself to the student of 

 animal responses to stimulation. It has been the special en- 

 deavor of Jennings to point out the fact that these responses, 

 instead of being wholly accounted for by the characteristics 

 of the stimulus, are determined in part by the internal, 

 physiological condition of the animal (211). We shall 

 therefore note often in the course of the following pages cases 

 where difference of reaction is due to internal rather than to 

 external causes. 



17. 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 

 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 different according to the 

 quality of the stimulus applied. This instance consists in the 

 distinction between food-taking reactions, given to edible sub- 

 stances, 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. 



