Sensory Discrimination: Hearing 131 



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 move- 

 ments 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 breathing rate was obtained 

 of the hearing of sounds ranging from fifty to one thousand 

 single vibrations a second (807). 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 interval 

 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 (817, 815). This, of course, does not 

 mean that the frog perceives such sounds as differing in 

 pitch. 



37. Hearing in Higher Vertebrates 



The reptilian ear does not differ markedly from that of 

 amphibians. The writer knows of no experiments upon 

 the sense of hearing in reptiles. The cochlea, the organ 

 of hearing in mammals, is still imperfectly developed in 

 birds. But if we grant that animals which produce 

 sounds are capable of hearing them, some birds at 

 least must be able to make pitch discriminations of wide 

 range and great acuteness. The powers of imitation so 



