n8 The Animal Mind 



of the fish; there is no cochlea. Yerkes has made an 

 interesting study of the reaction of frogs to sound. He 

 found that they occasionally ''straightened up and raised the 

 head as if listening" when other frogs croaked or made a 

 splash by jumping into the water. To no other sound did he 

 get any apparent response, nor was it possible to make frogs 

 in their native habitat jump or show any uneasiness by pro- 

 ducing any sort of noise, so long as the experimenter remained 

 invisible. "Apparently," Yerkes says, "they depend almost 

 entirely upon vision for the avoidance of dangers." It is of 

 course highly improbable that an organ should be adapted 

 only to the reception of the croaking of other frogs and the 

 splash of water, and not to noises made in imitation of these ; 

 and Yerkes suggests that the frogs may hear many sounds to 

 which they respond by inhibiting movement as a measure of 

 safety. This view is confirmed by the results of experiments 

 where the breathing movements of the frog's throat were 

 registered by means of a lever resting against it and recording 

 on smoked paper. Evidence from change of the breath- 

 ing rate was obtained of the hearing of sounds ranging from 

 fifty to one thousand single vibrations a second (456). Later, 

 it was shown that sounds, although they did not, when given 

 alone, cause the frogs to react, modified the responses to other 

 stimuli, reinforcing or inhibiting them according to the inter- 

 val between the sound and the other stimulus. This effect 

 was noticed both when the frogs were in the air and when they 

 were under water. It was more marked in the spring (the 

 mating season) than in the winter. That it concerned the 

 special auditory sense- apparatus, and hence may have been 

 accompanied by true auditory sensations, was shown by the 

 fact that it disappeared when the auditory nerves were cut. 

 Sounds ranging from fifty to ten thousand single vibrations 

 a second were effective (462, 464). This, of course, does not 



