90 The Dancing Mouse 



groups in addition to the one already mentioned. The 200 

 mice were sometimes gray and white instead of black and 

 white; they climbed much more readily and danced less 

 vigorously than those of the 400 group. These facts are 

 particularly interesting in connection with Cyon's descriptions 

 of the two types of dancer which he observed. 



In criticism of my conclusion that the young dancers are 

 able to hear certain sounds for a few days early in life, and 

 then become deaf, it has been suggested that they cease to 

 react because they rapidly become accustomed to the sounds. 

 That this is not the case, is evident from the fact that the re- 

 actions often increase in definiteness during the first two or 

 three days and then suddenly disappear entirely. But even 

 if -this were not true, it would seem extremely improbable 

 that the mouse should become accustomed to a sudden and 

 startlingly loud sound with so few repetitions as occurred in 

 these tests. On any one day the sounds were not made more 

 than five to ten times. Moreover, under the same external 

 condition, the common mouse reacts unmistakably to these 

 sounds day after day when they are first produced, although 

 with repetition of the stimulus at short intervals, the reactions 

 soon become indefinite or disappear. 



The chief results of my study of hearing in the dancer 

 may be summed up in a very few words. The young dancer, 

 in some instances, hears sounds for a few days during the 

 third week of life. The adult is totally deaf. Shortly be- 

 fore the period of auditory sensitiveness, the young dancer 

 becomes extremely excitable and pugnacious. 



