Practically the same results were obtained with another litter of four females. The change in their behavior was obvious on the eighteenth day, but at no time during the first month did they give any satisfactory indications of hearing. In the accompanying table, I have presented in condensed form the results of my auditory tests in the case of twelve litters of young dancers. TABLE 5 PERIOD OF AUDITORY REACTION IN YOUNG DANCERS PARENTS No. in Change in Ears Auditory Reactions Litter Behavior Open Appear Disappear 152+151 5 13th day 14th day 14th day 16th day 152+15l 8 (?) 13th day 13th day 17th day 152+151 5 13th day 13th day 13th day 17th day 152+151 4 10th day 12th day 13th day 15th day 410+415 5 14th day 15th day 15th day 19th day 410+415 6 13th day 14th day 14th day 18th day 420+425 2 12th day 14th day 14th day 17th day 210+215 5 17th day 13th day 17th day 19th day 210+215 6 11th day 14th day No reactions 220+225 6 16th day 14th day No reactions 220+225 6 17th day 13th day No reactions 212+211 4 15th day 14th day No reactions Certain of the litters tested responded definitely to sounds, others gave no sign of hearing at any time during the first four weeks of life. Of the twelve litters for which the results of auditory tests are presented in Table 5, eight evidently passed through an auditory period. It is important to note that all except one of these were the offspring of Nos. 151 and 152, or of their descendants Nos. 410 and 415 and Nos. 420 and 425. In fact every one of the litters in this line of descent which I have tested, and they now number fifteen, has given indications of auditory sensitiveness. And, on the other hand, only in a single instance have the litters born of Nos. 210 and 215, or of their descendants, given evidence of ability to hear. These two distinct lines of descent may be referred to hereafter as the 400 and the 200 lines. I have observed several important differences between the individuals of these 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.