completion of a subject’s reaction to the group of three keys, a group of seven keys at the opposite end of the keyboard may, for example, be presented.  Similarly, the subject is required to discover with the minimum number of trials the correct reaction-mechanism.  Thus, time after time, the experimenter presents a different group of keys so that the subject in no two successive trials is making use of the same portion of the keyboard.  It is therefore impossible for him to react to spatial relations in the ordinary sense and manner, and unless he can perceive and appropriately respond to the particular relation which constitutes the only constant characteristic of the correct reaction-mechanism for a particular problem, he cannot solve the problem, or at least cannot solve it ideationally and on the basis of a small number of observations or trials.

For the various infrahuman animals whose ideational behavior has been studied by means of this method, it has been found eminently satisfactory to use as reaction-mechanisms a series of similar boxes, each with an entrance and an exit door.  An incentive to the selection of the right box in a particular test is supplied by food, a small quantity of which is placed in a covered receptacle beyond the exit door of each of the boxes.  Each time an animal enters a wrong box, it is punished for its mistake by being confined in that box for a certain period, ranging from five seconds to as much as two minutes with various individuals or types of organism.  This discourages random, hasty, or careless choices.  When the right box is selected, the exit door is immediately raised, thus uncovering the food, which serves as a reward.  After eating the food thus provided, the animal, according to training, returns to the starting point and eagerly awaits an opportunity to attempt once more to find the reward which it has learned to expect.  With this form of the apparatus, the boxes among which choice may be made are indicated by the raising (opening) of the front door.

Since with various birds and mammals the box form of apparatus had proved most satisfactory, I planned the primate apparatus along similar lines, aiming simply to adapt it to the somewhat different motor equipment and destructive tendencies of the monkeys.  I shall now briefly describe this apparatus as it was constructed and used in the Montecito laboratory.

EXPLANATION OF PLATE IV

FIGURE 13.—­Multiple-choice apparatus, showing observer’s bench
   and writing stand. 
FIGURE 14.—­Apparatus as seen from observer’s bench. 
FIGURE 15.—­Entrances to multiple-choice boxes as seen from
   the response-compartment. 
FIGURE 16.—­Apparatus as seen from the rear, showing exit
   doors, food receptacles, and covers for same.