640 PSYCHOLOGY: R. M. YERKES 
Errors 
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FIG. 1 
as correct boxes only those to the left of the middle box of the series. 
The nearest box, in such case, was never the correct box. Conse- 
quently, this modification of method greatly increased, as the curve of 
errors shows, the number of mistakes. 
For a few days after this change was made, no improvement in reac- 
tion appeared. On May 10, in a series of 10 trials, 7 were incorrect, 
but the following day and thereafter only correct choices appeared. 
Thus, suddenly and without warning, the ape solved his relational 
problem. 
Is this the result of ideation? If not, what happened between the 
poor performance on May 10 and the perfect series on May 11? Be- 
cause of varied results obtained in other experiments with this ape, 
I suspect that ideational processes developed. 
The two monkeys succeeded in solving problem 2; the P. irus after 
1070 trials, and the P. rhesus after 400 trials. The orang utan failed 
utterly, although he was given 1380 trials. Ultimately, he ceased to 
try to select the right box and followed the path of least resistance. 
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