4 JOHN LINCK ULRICH 



This box has been used before in animal work in a similar 

 or modified form. All that was necessary for the rat to do to 

 solve the problem was to open the door of the box. The rats 

 usually opened, the box by raising the latch with their snouts. 

 The problem was not a difficult one to solve, for during learn- 

 ing, movements were made that were often far more complex 

 than were necessary to open the door. 



Young rats were used for experimentation. Usually, when 

 they were thirty-five days old, the mother and her litter were 

 separated. The young were then old enough to be self-depen- 

 dent. When forty to forty-five days old the rats were divided 

 into groups of two or four, the sexes being separated. At this 

 age their activities showed a ready adroitness that older animals 

 do not display; they were excitable and curious — characteristics 

 which are admirable for experimentation. No pronounced fixed 

 habits had likely been learned, and the simple environment of 

 the cage had not become monotonous, as was shown by the 

 fact that they indulged more rarely in sleep during the day 

 than the adults. 



The incentive to all activity in experimentation was food. 

 When it is used as a stimulus two considerations are present 

 that require attention in order to obtain the best results, viz., 

 the kind of food, and the amount of food. The usual food was 

 a preparation of bread and milk, and in addition occasional 

 pieces of fruit, carrot, and grain were added to maintain that 

 metabolic equilibrium which a variety of food produces and 

 which is so necessary to a healthy organism. Since the amount 

 of food also is quite as necessary a consideration, in view of the 

 fact that the activity of the rat is largely dependent upon this 

 factor, particular attention was as a consequence paid to its 

 control. It was early apparent how needful active metabolism 

 was to the production of responses which would secure success 

 in learning. Activity, conditioned by the need for food, is in 

 the nature of a normal life process. 



As far as practicable experimentation was done in the morn- 

 ing hours, though when several groups of rats were under con- 

 sideration the work extended into the afternoon. No time was 

 lost after the operator entered the room before actual work was 

 begun. Food was prepared for the rats, and experimentation 

 was usually continued without interruption. The reason for 



