Records of the direction of whirling of one hundred individuals have been obtained. For twenty of these mice the determination was made by counting the number of complete turns in five-minute intervals at six different hours of the day. For the remaining eighty individuals the direction was discovered by observation of the activity of the animals for a brief interval at five different times. Naturally, the former results are the more exact; in fact, they alone have any considerable quantitative value. But for the problem under consideration all of the determinations are sufficiently accurate to be satisfactory. The distribution of the individuals which were examined as to direction of whirling is as follows. RIGHT WHIRLERS LEFT WHIRLERS MIXED WHIRLERS TOTAL Males 19 19 12 50 Females 12 23 15 50 The frequency of occurrence of left whirlers among the females is unexpectedly high. Is this to be accounted for in terms of inheritance? In my search for an answer to this question I followed the whirling tendency from generation to generation in two lines of descent. These two groups of mice have already been referred to as the 200 line and the 400 line. The former were descended from Nos. 200 and 205, and the latter from Nos. 152 and 151. Individuals which resulted from the crossing of these lines will be referred to hereafter as of mixed descent. There were some striking differences in the behavior of the mice of the two lines of descent. As a rule the individuals of the 200 line climbed more readily, were more active, danced less vigorously, whirled less rapidly and less persistently, and were in several other respects much more like common mice than were the individuals of the 400 line. It is also to be noted (see Table 5) that few of the litters of the 200 line exhibited auditory reactions, whereas almost all of the litters of the 400 line which were tested gave unmistakable evidence of sensitiveness to certain sounds. These differences at once suggest the importance of an examination of the whirling tendency of each line of descent. The results for the several generations of each line which I had opportunity to examine are unexpectedly decisive so far as the question in point is concerned. INDIVIDUALS OF THE 200 LINE MALES FEMALES First generation No. 200, ? No. 205, ? Second generation No. 210, Mixed whirler No. 215, Left whirler Third generation No. 220, Mixed whirler No. 225, Mixed whirler Fourth generation No. 230, Right whirler No. 235, Mixed whirler Fifth generation No. 240, Right whirler No. 245, Left whirler INDIVIDUALS OF THE 400 LINE MALES FEMALES