THE AUDITORY SENSITIVITY OF THE WHITE RAT 219 



Comparative psychologists are agreed, I believe, that, with 

 the higher animals, the ability to associate a stimulus with a 

 simple response shall be the criterion of sensitivity to that 

 stimulus. Exceptional cases may occur, as is suggested by the 

 studies of Yerkes upon the hearing of the frog. ■> Other factors 

 being equal, however, where an animal can learn an association 

 with one auditory or visual stimulus, inability to do so with 

 another auditory or visual stimulus is to be taken as evidence 

 of lack of sensitivity. Granting this, the above data prove 

 either that the rats cannot hear c' 512, under the conditions of 

 the present experiment, or (note 3 has some reference here) 

 that their sensitivity is extremely slight. The analogies between 

 the present conclusion and that reached by other students with 

 respect to color vision in animals are both striking and instruc- 

 tive. It is to be noted in particular that tone and color cor- 

 respond to periodic vibrations and that noise and the white- 

 black series correspond in general to heterogeneous vibrations. 

 The ability to react to periodic ether vibrations is apparently 

 a late acquisition in animals. Why then not expect, a priori, 

 the same to be true in sound, particularly when periodic vibra- 

 tions seem to be more artificial and hence rarer (at least in 

 the habitats of non-musical animals) than heterogeneous ones? 

 Further comment upon this must await a later presentation 

 of data. 



In addition to the above crucial evidence on the sensitivity 

 of the white rat to c' 512, much other material bearing upon 

 the same problem has been accumulated. All of this, while 

 not in and of itself decisive, is in harmony with the conclusion 

 above drawn. 



(1) All rats of set I ignored the tone and reacted on the basis 

 of noise and the absence of noise. There must be, then, some 

 fundamental difference in the effect of noise and tone on the rat. 

 Otherwise we should expect individual differences to appear. 



(2) Six rats of an untrained set (III) all failed to discrim- 

 inate a very intense sounding of c' from a faint sounding of 

 the same tone. These two intensities may be described with 

 sufficient accuracy as follows: One was as intense as could be 

 secured by striking the fork. The other was approximately of 



i Yerkes, R. M. " The Sense of Hearing in Frogs." Jour. Comp. Neur. and 

 Psych., 1905, 15, pp. 279-304. 



