The Sense of Hearing 87 



the same manner daily up to the time of the appearance of 

 the reactions. To argue that the mice heard before the 

 fourteenth day, but were unable to react because the proper 

 motor mechanism had not developed sufficiently would be 

 short-sighted, for if the response depended upon the develop- 

 ment of such a mechanism, it is not likely that it would 

 disappear so quickly. I am therefore satisfied that these 

 reactions indicate hearing. 



With another litter the following results were obtained. 

 On the thirteenth day each of the eight members of the litter 

 responded definitely and uniformly to the Galton whistle, 

 set at 5 (probably about 8000 complete vibrations), and to a 

 Konig steel bar of a vibration rate of 4096 Vs. The largest 

 individuals, for almost always there are noticeable differences 

 in size among the members of a litter, appeared to be most 

 sensitive to sounds. 



On the fifteenth day and again on the seventeenth unmis- 

 takable responses to sound were observed ; on the eighteenth 

 the responses were indefinite, and on the nineteenth none 

 were obtained. I continued the tests up to the twenty-eighth 

 day without further indications of hearing. 



Certain individuals in this litter reacted so vigorously to 

 the loud sound produced by striking the steel bar a sharp 

 blow and also to the Galton whistle, during a period of five 

 days, that I have no hesitation in saying that they evidently 

 heard during that period of their lives. Other members of 

 the litter seemed to be less sensitive; their reactions were 

 sometimes so indefinite as to leave the experimenter in doubt 

 about the presence of hearing. 



A third litter, which developed very slowly because of lack 

 of sufficient food, first showed unmistakable reactions to 

 sound on the twenty-first day. On this day only two of the 

 five individuals reacted. The reactions were much more 



