CHAPTER XIX 



MEMORY IMAGES AND TROPISMS 



When a muscle is stimulated several times in succes- 

 sion, the effect of the second or third or later stimulation 

 may be greater than that of the first. A consistently 

 anthropomorphic author should draw the inference that 

 the muscle is gradually learning to react properly. What 

 seems to happen is that the hydrogen ion concentration is 

 raised by the first stimulations to a point where the effect 

 of the stimulation becomes greater. When the stimula- 

 tions continue and the hydrogen ion concentration be- 

 comes still greater, the response of the muscle declines and 

 finally becomes zero ; the hydrogen ion concentration has 

 now become too high. The writer observed that when 

 winged plant lice of a Cineraria were taken directly from 

 the plant, they did not react as promptly as after they 

 had gone through several heliotropic experiments. There 

 is nothing to indicate that this is a case of "learning," 

 since it may also be the result of a change in the hydrogen 

 ion concentration or of some other reaction product. It 

 may also be the result of some purely mechanical obstacle 

 to rapid locomotion being removed. 



We can speak of learning only in such organisms in 

 which the existence of associative memory can be proved. 

 By associative memory we mean that mechanism, by 

 which a stimulus produces not only the direct effects 

 determined by its nature, but also the effects of entirely 

 different stimuli which at some former period by chance 

 attacked the organism at the same time with the given 



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