Modification by Experience 299 



six boys of varying ages, one defective boy, five monkeys, 

 sixteen dogs, seven cats, and a horse. Only the human sub- 

 jects reached a stage of learning where they showed by 

 their behavior that they realized the impossibility of open- 

 ing a door that had been open in the preceding trial. The 

 monkeys always tried all four doors, but did not often push 

 repeatedly at the same door or persistently neglect a door ; 

 this lowest type of behavior was more frequent in the horse. 

 The fact may be noted for future reference that the behavior 

 of the horse in this situation was "stupider" than that of 

 any of the other subjects. 



Yerkes (826) developed the principle of this method 

 and generalized it as follows. The animal is offered the 

 choice among a number of compartments. The number 

 can be varied, and their position in space can be varied. 

 Thus, if there are ten compartments in the apparatus, 

 only three of them may be used in a certain experiment, 

 and these three may be situated in the middle or towards 

 either end, so that no associations will be formed with po- 

 sition in space. Or in another experiment five of the com- 

 partments, in any part of the series, may be used. The 

 compartments used in a given experiment have their en- 

 trance doors open. The problem may be varied in com- 

 plexity by making the "right" compartment, the one whose 

 entrance gives food, bear different relations to the rest. 

 It may be the first compartment on the left, the first com- 

 partment on the right, the second on the left, the second 

 on the right, the middle compartment, and so on. After 

 an animal has proved its ability to learn a simple problem, 

 such as "first on the right," it may be advanced to a more 

 complex one, such as "second on the left." The method 

 has been applied to crows (129), rats (113), pigs (826), 

 monkeys, and apes (824). The crow mastered the "first at 



