The reader will wonder why the mouse should have any tendency to enter _B_, and why after so doing, it should trouble to go further, knowing, as it does from previous experiences, that entering one of the electric-boxes may result in discomfort. The fact is, a dancer has no very constant tendency to go from _A_ to _B_ at the beginning of the tests, but after it has become accustomed to the box and has learned what the situation demands, it shows eagerness to make the trip from _A_ to _B_, and thence by way of either the right or the left route to _A_. That the mouse should be willing to enter either of the electric-boxes, after it has experienced the shock, is even more surprising than its eagerness to run from _A_ to _B_. When first tested for brightness discrimination in this apparatus, a dancer usually hesitated at the entrance to the electric-boxes, and this hesitation increased rapidly unless it were able to discriminate the boxes by their difference in brightness and thus to choose the right one. During the period of increasing hesitancy in making the choice, the experimenter, by carefully moving from _I_ toward the entrances to the electric-boxes a piece of cardboard which extended all the way across _B_, greatly increased the mouse's desire to enter one of the boxes by depriving it of dancing space in _B_. If an individual which did not know which entrance to choose were permitted to run about in _B_, it would often do so for minutes at a time without approaching the entrance to the boxes; but the same individual, when confined to a dancing space 4 or 5 cm. wide in front of the entrances, would enter one of the electric-boxes almost immediately. This facilitation of choice by decrease in the amount of space for whirling was not to any considerable extent the result of fear, for all the dancers experimented with were tame, and instead of forcing them to rush into one of the boxes blindly and without attempt at discrimination, the narrowing of the space simply increased their efforts to discriminate. The common mouse when subjected to similar experimental conditions is likely to be frightened by being forced to approach the entrances to the boxes, and fails to choose; it rushes into one box directly, and in consequence it is as often wrong as right. The dancer always chooses, but its eagerness to choose is markedly increased by the restriction of its movements to a narrow space in front of the entrances between which it is required to discriminate. It is evident that the animal is uncomfortable in a space which is too narrow for it to whirl in freely. It must have room to dance. This furnished a sufficiently strong motive for the entering of the electric-boxes. It must avoid disagreeable and unfavorable stimuli. This is a basis for attempts to choose, by visual discrimination, the electric-box in which the shock is not given. It may safely be said that the success of the majority of the experiments of this