THE EVOLUTION OF INTELLIGENCE 105 



essentially a function of the nervous system and that its char- 

 acteristics vary with the development of the latter. 



Turning, therefore, for our clew to the behavior of living 

 organisms, we find this ranging all the way from the very simple 

 series of movements executed by the unicellular organisms like 

 amoeba, which is quite devoid of a specialized nervous system, 

 up to the complicated behavior of the primates and the phe- 

 nomenal intellectual achievements of man himself. At once 

 we are confronted with a distinction which, much transformed 

 in modern times, goes back to the period of the earliest specu- 

 lations upon mind and behavior. All the early writers known 

 to us stress the distinction between instinct and reason, ac- 

 crediting to the animals instinctive behavior of a far more 

 highly developed type than that disclosed in the behavior of 

 man, and assigning to man powers of reasoning either wholly 

 or largely denied to animals. Modern scientific analysis has 

 been disposed greatly to qualify the rigidity of this distinction. 

 Man certainly has a very definite equipment of instincts and 

 some of the operations of animals contain in them the begin- 

 nings at least of rational conduct. 



Broadly speaking, actions are designated intelligent when 

 they disclose the ability to adjust quickly and successfully to 

 new and variable conditions. They are judged unintelligent 

 or mechanical when the same action is elicited again and again 

 regardless of changes in the situation which calls it forth, or 

 of the possibly disastrous nature of the reaction itself. From 

 this point of view, the great group of actions, known as tro- 

 pisms, must be regarded as non-intelligent, even though they 

 may at times benefit their possessors. Here, for example, is a 

 group of organisms which at once seek the darkest corner of 

 any area within which they are confined. Here is another 

 group which seeks the lightest place. Again, here is a group 

 which, if possible, takes up a position where the body may 

 press, or be pressed upon by, surrounding objects. In the case 



