128 ROBERT M. YERKES 



animals would be rated as difficult by psychologists, for as a 

 rule the}^ involved definite relations and demanded on the part 

 of the subject both perception of a particular relation and the 

 ability to remember or re-present it on occasion. 



I was greatly surprised by the slow progress of the monkeys 

 toward the solution of these problems. It had been my suppo- 

 sition that they would solve them more quickly than any lower 

 type of mammal, but as a matter of fact they succeeded less 

 well than did pigs. Their behavior throughout the work proved 

 that of far greater significance for the experimenter than the 

 solution of a problem is definite knowledge of the modes of 

 behavior exhibited from moment to moment, or day to day. 

 This is true especially of those incidental or accidental modes 

 of response which so frequently appeared in connection with 

 my work that I came to look upon them, the surprises of each 

 day, as my chief means of insight. 



Evidences of Ideation in Apes 



Reliable literature of any sort concerning the behavior and 

 mental life of the anthropoid apes is difficult to find, and still 

 more rare are reports concerning experimental studies of these 

 .animals. There are, it is true, a few articles descriptive of tests 

 of mental ability, but even these are scarcely deserving of being 

 classed as satisfactory experimental studies of the psychology 

 of the ape. I have the satisfaction of being able to present in 

 the present report the first systematic experimental study of 

 any feature of the behavior of an anthropoid ape. 



Among the most interesting and valuable of the descriptions 

 ■ which may be classed among accounts of tests of mental ability 

 is Hobhouse's (1915) study of the chimpanzee. The subject was 

 an untrained animal, so far as stated, of somewhat unsatisfac- 

 tory condition because of timidity. Nevertheless, Hobhouse was 

 able to obtain from him numerous and interesting responses to 

 novel situations, some of which may be safely accepted as evi- 

 dences of ideation of a fairly high order. 



Similar in method and result to the work of Hobhouse is 

 that of Haggerty (unpublished thesis for the Doctorate of Philos- 

 ophy, deposited in the Library of Harvard University). Hag- 

 gerty's tests of the ability of young orang utans and chim- 



