MENTAL EVOLUTION IN THE PRIMATES II7 



excellence, other animals also are intelligent and some of them 

 at least exhibit rational forms of intelhgence. For the 

 ancient inadequate formula, "Man is rational; brute, 

 instinctive," the present-day psychobiologist substitutes 

 the statement: Every living organism, by virtue of inherited 

 structures and developmental tendencies, is instinctive and 

 also in widely varying degrees capable of individual adapta- 

 tions which are more or less definitely intelhgent. Within 

 the order Primates, to which we shall confine our discussion 

 of the evolution of mind, we may not say that one* type is 

 more instinctive than another, although it is definitely 

 estabHshed that intelhgence differs both quantitatively and 

 quahtatively. To undertake our present task with the 

 conviction that man is mentally unique and therefore 

 without genetic relation to any existing or extinct type 

 of animal, would be quite as prejudicial to the discovery of 

 the truth as would be initial assumption that the human 

 mind has evolved from that of the gorilla. 



Ignoring technicahties of classification, it will serve our 

 purpose to group existing primates in five classes: lemurs, 

 tarsiers, monkeys, apes, and men. We shall make no reference 

 to extinct or fossil forms. The first two of our classes are 

 represented today by animals which strike the layman as 

 squirrel-hke rather than monkey-hke, and indeed they 

 resemble the flying mammals. Between these primitive and 

 often called pro-simian creatures and the distinctly manhke 

 primates are the New and Old World monkeys which, 

 differing extremely in appearance, exist in many genera 

 and species. Most closely resembhng man, and on the 

 whole differing httle if any more from him than from the 

 monkeys, are the four primate types assigned to the class apes, 

 or, more exphcitly, anthropoid apes: the gibbon, orang-outan, 

 chimpanzee and gorilla. 



As in turn we review what is known of the mode of fife, 

 adaptive capacity, and mental traits of each class of primate 

 beginning with the most primitive, we discover unmistakable 

 evidences of increasing resemblance to man in the progress 

 through lemur, tarsier, monkey and ape. To the speciafist 

 in psychobiology this is no less impressive and no less 

 strongly suggestive of genetic relations than are the struc- 



