Sensory Discrimination: Methods of Investigation 57 



stimuli produce in its assumed consciousness identical 

 sensation qualities. Thus it is not the number of stimuli 

 to which an animal reacts that can be taken as evidence 

 of the quahtative variety of its sensations, but the number 

 o£jtisg^to^5sdW6hJit~.g^^ 4igeJfjeRL^g^c|w^s. When 

 Jennings, for instance, says that Amoeba "reacts to all 

 classes of stimuh to which higher aifimals react" (378, 

 p. 19), we cannot conclude that it possesses all classes of 

 sensations that higher animals possess, for its reactions 

 to these different stimuli are but little varied according to 

 the kind of stimulus.^ 



An ingenious way of getting evidence from' behavior is 

 the saHvary reflex method devised by the Russian physi- 

 ologist Pawlow (830). The sahvary ducts of the dog, 

 which lie near the surface, are operated on so that the 

 saliva can be discharged into a graduated tube. As is 

 weU known, the sight or smell of food increases the flow of 

 saliva. Now when any other stimulus, such as a sound, 

 regularly accompanies the sight or smell of food, this stimu- 

 lus, originally without effect on the sahvary flow, comes 

 to increase the flow eyen in the absence of food. If, now, 

 the stimulus that has thus acquired the power to affect 

 the salivary flow is given in irregular alternation with an- 

 other stimulus differing slightly from it, and the other 

 stimulus is found not to affect the flow of saliva, then the 

 inference can be drawn that sensory discrimination between 

 these two stimuh is possible for the animal. It is main- 

 tained by some investigators that when sensory discrimi- 

 nation can be studied through such simple types of behavior 



' One of many reasons for the unsatisfactoriness of an article by A. 

 Olzelt-Newin, entitled "Beobachtungen fiber das Leben der Protozoen" 

 (529), lies in the author's uncritical acceptance of the hypothesis that re- 

 action to a special kind of stimulus means a special kind of sensation. 



