THE ORGANISM AND ITS ENVIRONMENT 371 



Man and other advanced animals is commonly called intelligenxe. 

 Its very nature presupposes an internal self control involving ability 

 to utilize past experience with environmental conditions, and an 

 ability to exercise imagination, that is, to assemble hypothetical or 

 imaginary environmental situations and to imagine their solution in 

 advance of their actual occurrence. Just how these functions of the 

 central nervous system are derived from the metabolic processes of 

 the protoplasm of the human brain is wholly unknown. Instincts 

 reach their greatest complexity in the insects, while controlled 

 behavior, intelligence, is commonly assumed to reach its acme in 

 the human behavior; by some the assumption is made that intelli- 

 gence is exclusively a property of Man. At least one modern school 

 of philosophy holds that instincts and intelligence represent the two 

 possible functions of the nervous system, of equal order of com- 

 plexity, and that evolution has resulted in their parallel development 

 in insects and in Man. Fascinating as the discussion of the possible 

 relation between instinct and intelligence may be, further specula- 

 tion is out of place here. 



Not all reflex acts are instinctive. It will be recalled that a reflex 

 act is the result of a stimulus which passes more or less directly 

 from an incoming sensory path to an outgoing motor path, without 

 the necessity of involving the brain or other correlating nerve centre 

 (p. 222). Repetition or training often succeeds in converting an act 

 that at first requires the correlating effort of the brain into one that 

 becomes a reflex. Reflexes acquired in this fashion are known as 

 CONDITIONED REFLEXES. In rcccut ycats it has developed that very 

 many human acts are conditioned reflexes, and that only a very few 

 are instinctive. The training which results in the establishment of 

 human conditioned reflexes begins very early in life and soon be- 

 comes so firmly established that heretofore they have been thought 

 to be instinctive reflexes, that is to say, independent of previous 

 training. 



The nature of the external stimuli that induce more or less im- 



