DEVELOPMENT OF MIND IN ANIMALS I33 



suppression continued even after the partition was removed. 

 Reptiles, according to Nissen, are better subjects for learn- 

 ing experiments than either fish or amphibia but they rate 

 much below most mammals. Electric shock given at the time 

 amphibia or reptiles snap at their food inhibits the eating be- 

 havior. Differential conditioning has been demonstrated in 

 arthropods, fish, amphibia, and reptiles. 



It hardly seems necessary to enumerate the many in- 

 stances of clear-cut learning experiments in the higher ver- 

 tebrates, but some consideration of the direct comparisons 

 between monkeys, apes and man will be brought out in 

 Chapter 12 wherein the possibility that conceptual thought 

 occurs below^ the level of man will be examined. 



In evaluating the phyletic record of behavior, Nissen 

 finds that the literature makes certain conclusions possible. 

 In perceptual organization (i.e., sensory correlation) evolu- 

 tion has proceeded along tw^o paths: (1) inherited percep- 

 tual patterns or instinct, and (2) individual acquisition of 

 perception or intelligence. Instinct reaches its highest lev- 

 els in the insects and birds; it has receded to a large extent 

 in the primates and man, which enjoy in turn a much wider 

 perceptual scope than any other life forms. In lower forms, 

 intelligence plays a very minor role but comes into its own 

 in some birds and, of course, the mammals. Intelligence is 

 organized in the course of experience by the inferred mech- 

 anisms of pattern identification, the selective process, and 

 symbolism. It is symbolism that makes possible the concep- 

 tual thought of man; it already arises to some extent at or 

 before the level of the primates. This evolution is very im- 

 portant to our thesis and will be reviewed later. 



Nissen finds that the sensory-response, connecting proc- 

 ess rises from lower to higher invertebrates, then is not sur- 

 passed until the higher primates appear. He thinks that sym- 

 bolization helps perceptual organization and makes possible 

 a quick and thorough connection of percepts and concepts 

 to specific responses. Central symbolic processes easily con- 



