350 BRAINS OF RATS AND MEN 



of thing that most sharply differentiates men from 

 rats. These powers of imagination, of "free associa- 

 tion," of invention, of idealization, depend upon the 

 fluidity of the intracortical organization and the pres- 

 ervation of its plasticity. Just in proportion as this 

 organization becomes fixed in stable patterns and 

 the behavior congeals in habitual molds do "con- 

 structive thinking" and originality of behavior re- 

 gress, and we revert to lower levels of more stable 

 conduct. 



The human type of plasticity of behavior is the 

 very antithesis of habit; it is a new vital mode, though 

 its rudiments are readily discernible in the rat*s capac- 

 ity to form new habits. New vital patterns have come 

 in at every turn of the road of evolutionary progress, 

 for evolution is creative in the sense that the elements 

 of organization are recombined in original ways at 

 every transition from type to type. This is a natural 

 process and demands no appeal to mystical or super- 

 natural agencies. 



Somewhere in the history of primate evolution, 

 during the course of progressive elaboration of the 

 apparatus of cortical associations, sufficient com- 

 plexity of tissue and plasticity of organization was at- 

 tained to facilitate rapid learning, the retention of 

 memories of single experiences and the abstraction 

 from these of certain features common to all of them, 

 and finally the integration of these common features 

 into symbolic patterns. Symbolic thinking is a new 



