July 7, 1916] 



SCIENCE 



29 



The Mental Life of Monkeys and Apes: A 

 Study of Ideational Behavior. By Robert 

 M. Yerkes. New York, 1916. Pp. 145. 

 This monograph reports the results of ad- 

 mirable experiments on two monkeys and an 

 orang-utan, first, by the multiple-choice 

 method of Yerkes, and second, by various 

 forms of the mechanical-adaptation method. It 

 also presents a plan for a research institute for 

 the study of the primates. 



The multiple-choice method is a means of 

 diagnosing and measuring an animal's ability 

 to respond correctly to relations of spatial 

 order, such as middle door of those open, or 

 right-hand door of those open, regardless of the 

 number of doors that are open or which doors 

 (of the entire nine possible to be open) they 

 are. Food was used as a reward for entering 

 the right door, and, irregularly, detention in a 

 box as a punishment for entering the wrong 

 door. 



The first problem was to learn to enter at 

 once the first door at the left end of whatever 

 doors were open. The following summary for 

 one monkey will give an idea of the sort of 

 facts obtained. In the course of 150 trials the 

 per cent, of successes rose from 30 to 90, for the 

 ten selections of doors open that were em- 

 ployed. In a test with one trial each of ten 

 still different selections of doors the number 

 of successes was 6. The monkey did not seem 

 to learn by a " free idea " of " door at left 

 end " ; for each selection of doors seemed to be 

 responded to by itself. 8 as the response to 

 8-9, 6 as the response to 6-7-8, 4 as the re- 

 sponse to 4-5-6-7-8, 7 as the response to 7-8-9 

 and 5 as the response to 5-6-7 were apparently 

 learned at a time when 1 as a response to 

 1-2-3, 3 as a response to 3-4-5-6-7 and 2 as a 

 response to 2-3—4-5-6, were not. There was 

 no sudden elimination of wrong responses. 

 " Stupid " responses appeared in connection 

 with the general behavior in the test. 



As a result of four such series of experi- 

 ments with one monkey and three with an- 

 other, Yerkes concludes that " the Pithecus 

 monkeys yielded relatively abundant evidence 

 of ideation but with Thorndike I must agree 

 that of ' free ideas ' there is scanty evidence, 



or rather, I should prefer to say, that although 

 ideas seem to be in play frequently, they are 

 rather concrete and definitely attached than 

 ' free.' Neither in my sustained multiple- 

 choice experiments nor from my supplementary 

 tests did I obtain convincing indications of 

 reasoning. What Hobhouse has called articu- 

 late ideas I believe to appear infrequently in 

 these animals. But on the whole, I believe that 

 the general conclusions of previous experi- 

 mental observers have done no injustice to the 

 ideational ability of monkeys." 



The orang-utan seemed to get " an idea of " 

 left-end-door of those open and use it to guide 

 his responses. He did not, however, appar- 

 ently get the idea of next-to-the-right-end-door 

 in 1,380 trials. Various phases of his behavior, 

 however, convinced Yerkes that he was re- 

 sponding to ideas or representations of experi- 

 ence. 



In the miscellaneous tests the orang-utan 

 showed great pertinacity and initiative. On 

 the whole "the orang-utan is capable of ex- 

 pressing free ideas in considerable number and 

 of using them in ways highly indicative of 

 thought-processes, possibly even of the rational 

 order. But contrasted with that of man, the 

 ideational life of the orang-utan seems poverty- 

 stricken." 



The experiments with the monkeys and the 

 ape are described with the author's customary 

 care and will be of service in many ways to 

 future workers in this field. For example, 

 they bear directly on the Smith-Watson-Carr 

 doctrine that frequency of connection, irre- 

 spective of the consequences of the connection 

 to the animal, is adequate to account for learn- 

 ing. Cases abound in the records where a cer- 

 tain wrong door is in the first few trials chosen 

 far oftener than the right door and yet even- 

 tually is never chosen. The multiple-choice 

 experiments should be widely used in studies 

 of both animal and human learning. 



The last division of the monograph presents 

 Yerkes' proposal for the provision of a spe- 

 cial institute for studying the monkeys and 

 apes. Porto Bico and Southern California are 

 suggested as satisfactory localities. The in- 

 direct value of such an institute for human 



