MENTAL LIFE OF MONKEYS AND APES 17 



except number 5, the middle member of the group, be entered, 

 the entrance door is immediately lowered and both the exit and 

 entrance doors locked in position so that the animal is forced 

 to remain in the box for a stated period, say thirty seconds. 

 At the expiration of this time the entrance door is raised and 

 the animal allowed to retrace its steps and make another choice. 

 When the middle box is chosen, the entrance door is lowered and 

 the exit door immediately raised, thus uncovering the food, 

 which the animal eats. As a rule, by my monkeys and ape 

 the reward was eaten in the alleyway G instead of in the multiple- 

 choice box. As soon as the food has been eaten, the exit door is 

 lowered by the experimenter, and the animal returns by way of 

 G and H to runway D, where it awaits its next trial. 



As rewards, bananas and peanuts were found very satisfac- 

 tory, and although occasionally other foods were supplied in 

 small quantities, they were on the whole less constantly desired 

 than the former. 



Four problems which had previously been presented to other 

 organisms were in precisely the same form presented to the three 

 primates. These problems may be described, briefly, by defini- 

 tion of the right reaction mechanism, thus: problem 1, the first 

 mechanism at the subject's left; problem 2, the second mechan- 

 ism at the subject's right (that is, from the end of the series at 

 the subject's right) ; problem 3, alternately, the first mechanism 

 at the subject's left and the first at its right; problem 4, the mid- 

 dle mechanism of the group. 



It was my intention to present these four problems, in order, 

 to each of the three animals, proceeding with them as rapidly 

 as they were solved. But as it happened, only one of the three 

 subjects got as far as the fourth problem. When observations 

 had to be discontinued, Sobke was well along with the last, or 

 fourth problem; Skirrl was at work at the third problem; and 

 Julius had failed to solve the second problem. 



For each of the problems, a series of ten different settings of 

 the doors was determined upon in advance. These settings 

 differ from those employed in a similar investigation with the 

 pig only in that the numbering of the doors is reversed. In the 

 present apparatus, the boxes as viewed from the front (entrance) 

 are numbered from the left to the right end, whereas those of the 

 pig apparatus were numbered from the right end to the left end. 



