Chapter V 

 MENTAL EVOLUTION IN THE PRIMATES 

 Robert M. Yerkes 



MANY of us doubtless would be profoundly impressed 

 if by watching a cinema record, run backward, we 

 were able to trace to their prenatal beginnings 

 the personality, character, temperament, intellectual traits, 

 mannerisms and other characteristic modes of reaction of one 

 of our intimate friends. Let us assume that the pictorial record 

 adequately represents the psychological and psychobiologi- 

 cal characteristics of our friend from day to day throughout 

 a half century of existence, and, further, that in the picture 

 the physical appearance of the individual is so far con- 

 ventionalized that the mental appearances, as we may call 

 them, dominate the attention and interest of the observer. 

 Under these circumstances what is likely to be the experience 

 of the open-minded, well-informed layman who eagerly 

 watches the retracing of mental development? With con- 

 siderable assurance we predict the following: 



The first few years of the story will yield him an agreeable 

 sense of famiharity and intelhgibihty. The friend will not 

 only be definitely recognized in the pictorial representation, 

 but there may appear a delightful sense of intimacy and 

 understanding. Gradually, as the years and then the decades 

 are retraced, the feeling of familiarity will lessen and finally 

 it will give place to one of strangeness; the individual is no 

 longer identified or even identifiable as one's friend, save 

 perhaps by the speciahst in psychobiology. Instead, the 

 representation is that of a more or less obviously typical 

 human mind and personality. By this time the initial 

 agreeable glow of understanding has given place to surprise, 

 doubt, and the observer probably looks and feels puzzled. 

 But even more surprising experiences are in store for him, as 

 reaching back a few more years the representation gradually 

 loses its resemblance to what the layman knows as human 

 behavior, mentality, and personality, and comes to suggest 



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