PSYCHOLOGY: R. M. YERKES 
641 
In addition to the method of multiple choices, the chiefly significant 
results of which cannot well be summarized, several supplementary 
methods of studying the adaptive behavior of monkeys and apes were 
employed. Chief among these are (1) a box stacking test; (2) a box 
and pole experiment; (v3) a form of draw-in experiment. 
The ape, although failing to stack boxes spontaneously in order to 
obtain a banana which was suspended from the roof of his cage, did so 
readily and skilfully when shown how to do it by the experunenter. 
His imitative activity v^as convincingly purposive. Previous to the 
opportunity to imitate the experimenter, he exhibited various methods 
of trying to get the banana. His attention was surprisingly constant, 
and his activity, although varied, was for the most part definitely di- 
rected toward the food. In the controlling influence of the prospec- 
tive reward and in the precision of execution of his various acts, Julius 
differed markedly from the monkeys. 
Neither monkey made systematic and sustained attempts to obtain 
the banana by the use of boxes. Neither imitated the experimenter 
and neither attended to the prospective reward more than a few seconds 
at a time. These statements indicate a vast gulf, psychologically, 
between monkey and ape. 
In the box and pole experiment, the banana was so placed in the middle 
of a long box that it could be obtained only by the use of a pole. The 
ape quickly, of his own initiative and v/ith few useless motions, suc- 
ceeded in obtaining the food. The monkeys never succeeded in 
obtaining it by any method and failed to use the pole at all as a tool. 
Similarly, in an experiment which gave the animals opportunity to 
obtain food by drawing it into the cage with a stick, the ape quickly 
and repeatedly adapted means to ends by using the stick, whereas the 
monkeys never once attempted to use it. 
The specimen of P. irus (Skirrl) had a penchant for the manipulation 
of objects as tools. It is therefore surprising that he failed in the 
above experiment. When given a board, hammer and nails, he drove 
the nails into the board skilfully and persistently only to draw them 
out again and repeat the performance, for the activity was its own 
reward. In all probability, this use of hammer and nails was not imi- 
tative, since no other monkey or ape under observation showed any 
inclination to use them as did Skirrl. Quite as assiduously and with 
evident satisfaction, he used lock and key, and saw, or any other ob- 
ject which happened to fall into his hands. In the use of a saw, he 
persistently refused or failed to imitate the experimenter, but finally 
hit upon a use for the instrument w^hich clearly gave him great satis- 
