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 reactions 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 before the period of auditory sensitiveness, the young dancer becomes extremely excitable and pugnacious. CHAPTER VII THE SENSE OF SIGHT: BRIGHTNESS VISION The sense of sight in the dancer has received little attention hitherto. In the literature there are a few casual statements to the effect that it is of importance. Zoth, for example (31 p. 149), remarks that it seems to be keenly developed; and other writers, on the basis of their observation of the animal's behavior, hazard similar statements. The descriptions of the behavior of blinded mice, as given by Cyon, Alexander and Kreidl, and Kishi (p.47), apparently indicate that the sense is of some value; they do not, however, furnish definite information concerning its nature and its role in the daily life of the animal. The experimental study of this subject which is now to be described was undertaken, after careful and long-continued observation of the general behavior of the dancer, in order that our knowledge of the nature and value of the sense of sight in this representative of the Mammalia might be increased in scope and definiteness. The results of this study naturally fall into three groups: (1) those which concern brightness vision, (2) those which concern color vision, and (3) those which indicate the role of sight in the life of the dancer.