462 'Journal of Comparative Neurology atid Psychology. 



order in which they were shifted during twenty-five series of ten 

 tests each, in addition to the preference series A and B, is given 

 in table i. In case a mouse required more than twenty-five series 

 of tests (250 tests), the same set of changes was repeated, beginning 

 with series i. In the table the letters r and / refer to the position 

 of the white cards; r indicates that they marked the electric box 

 which was on the right of the mouse as it approached the entrances 

 of the electric boxes from the nest-box; / indicates that it marked 

 the left electric box. 



The way in which this apparatus was used may be indicated 

 by a brief description of our experimental procedure. A dancer 

 was placed in the nest-box by the experimenter, and thence it was 

 permitted to pass into the entrance chamber, B. The experi- 

 menter then placed a piece of cardboard bet^\^en it and the door- 

 way between A and B and gradually narrowed the space in which 

 the animal could move about freely by moving the cardboard 

 toward the electric boxes. This, without in any undesirable way 

 interfering with the dancer's attempts to discriminate and choose 

 correctly, greatly lessened the amount of random activity which 

 preceded choice. When thus brought face to face with the en- 

 trances to the boxes the mouse soon attempted to enter one of them. 

 If it happened to select the white box it was permitted to enter, 

 pass through, and return to the nest-box; but if, instead, it started 

 to enter the black box the experimenter by closing the key, upon 

 which his finger constantly rested during the tests, caused it to 

 receive an electric shock which as a rule forced a hasty retreat 

 from the black passage-way and the renewal of attempts to dis- 

 cover by comparison which box should be entered. 



Each of the forty mice experimented with was given ten tests 

 every morning until it succeeded in choosing the w^hite box cor- 

 rectly on three consecutive days, that is for thirty tests. A choice 

 was recorded as wrong if the mouse started to enter the black box 

 and received a shock; as right if, either directly or after running 

 from one entrance to the other a number of times, it entered the 

 white box. Whether it entered the white electric box or the black 

 one, it was permitted to return to the nest-box byway of the white 

 box before another test given. Escape to the nest-box by w^ay of 

 the black box was not permitted. A male and a female, which 

 were housed in the same cage between experiments, were placed 

 in the experiment box together and given their tests turn about 



