MEMORY IMAGES 165 



stimulus. Thus the image or the odor of a rose may call 

 up the memory of persons or surroundings which were 

 present on a former occasion when the image or odor of 

 the flower impressed us. Brain physiology shows that 

 this type of associative memory is the specific function 

 of definite parts of the brain, e.g., the cerebral hemispheres 

 which exist only in definite types of animals. We see 

 also that certain species among vertebrates, insects, Crus- 

 tacea, and cephalopods possess associative memory, while 

 to the knowledge of the writer no adequate proof for its 

 existence has ever been given for worms, starfish, sea 

 urchins, actinians, medusae, hydroids, or infusorians. 293 

 Claims for the existence of such memory in these latter 

 groups of animals have frequently been made, but such 

 claims are either plain romance or due to a confusion of 

 reversible physiological processes with the irreversible 

 phenomena of associative memory. The less a scientist 

 is accustomed to rigid quantitative experiments, the more 

 ready he is to confound the reversible after effects of a 

 stimulus — e.g., the after effects due to an increase in 

 hydrogen ion concentration — with indications of associa- 

 tive memory. Learning is only possible where there exists 

 a specific organ of associative memory, the physical 

 mechanism of which is still unknown. 



The manifestations of associative memory are gener- 

 ally discussed by the introspective psychologists, who as 

 a rule are not familiar with or do not appreciate the 

 methods of the physicist. There have been made repeated 

 attempts to develop methods for the analysis of associa- 

 tive memory, among which thus far only one satisfies the 

 demands of quantitative science, namely Pawlow's 

 method. As is well known even to the layman, eating 

 causes a flow of saliva. The quantity of saliva excreted 



