MENTAL EVOLUTION IN THE PRIMATES 121 



crimination and of suitable adaptation to certain essential 

 features of the situation. Either the subject does not perceive 

 the differences in point or for some other reason it is incapable 

 of regulating its activities in accordance with them. From 

 lemur to man no less obviously than from infancy to maturity, 

 motivation becomes increasingly complex. New factors 

 appear and adjustments of behavior become more serviceable 

 and more nearly adequate. 



Behavioral adaptation, or as we should have called it a 

 few years ago, habit-formation, may occur with or without 

 insight and foresight. It is our immediate task to try to 

 trace in development and in evolution the appearance and 

 history of different types of adaptation. We shall begin with a 

 form which often is designated as "trial and error," but we 

 shall use the expression "bhnd trial" in order to contrast 

 it with insight and foresight. 



(a) Blind Trial. A box containing a bit of candy or a 

 rattle is presented to a primate subject. The only way to 

 obtain the object within is to open a door which is held by a 

 hidden mechanism whose release may be effected by pushing 

 a lever at one side of the box. This is a type of problematic 

 situation which many investigators have presented to 

 animals as a test of intelligence or of abihty to profit by 

 experience. Obviously, insight is precluded by the charac- 

 teristics of the situation. The subject, whether lemur, 

 monkey, ape, or man, manipulates the box and sooner or 

 later by happy accident operates the mechanism of release. 

 Thereupon it obtains the desired object, and thereafter when 

 the same problem is presented it may exhibit more or less 

 perfect adaptation. This type of experiment has been cited 

 as one in which solution by trial is inevitable. 



As contrasted with situations in which insight is either 

 impossible or highly improbable, there are those in which we 

 should naturally expect it to appear, were the animal 

 capable of it. Such, for example, is the milk-containing 

 glass bottle whose contents the primate desires. Observation 

 reveals that neither the human infant nor any of the other 

 primates, with the possible exception of certain of the 

 anthropoid apes, is likely to respond to this situation initially 

 with direct and perfect adaptation. Instead, a series of 



