Sensory Discrimination: The Chemical Sense 67 



given stimulus acts more strongly upon them. On the 

 other hand, there is a possibility that qualitative as well 

 as intensive differences in the stimulus are responsible for 

 the contrasting reactions. "In general," says Jennings, 

 Paramecium "shows a tendency to come to rest against 

 loose or fibrous material ; in other words, it reacts thus to 

 material with which it can come in contact at two or 

 more parts of the body at once. To smooth, hard materials, 

 such as glass, it is much less likely to react in this manner" 

 (378, p. 61). Perhaps, then, the spatial distribution of the 

 stimulus over several points of the body surface increases 

 the probability of a contact rather than an avoiding re- 

 action. 



What, now, of the food-taking reaction in Paramecium : 

 does it show evidence of the existence of chemical dis- 

 crimination? When the animal finds itself in surround- 

 ings where certain presumably injurious chemicals, es- 

 pecially alkalis, are present, it gives its typical negative 

 reaction. If this should be called evidence of a special 

 chemical sense, we should be forgetting our general prin- 

 ciple that only unlike reactions constitute behavior indi- 

 cating sensory discrimination. Since Paramecium reacts 

 in the same way to strong mechanical stimulation and to 

 certain chemical stimulations, there is no reason for assum- 

 ing a discrimination between chemical and mechanical 

 stimuli. If it can be shown that the reaction is a localized 

 one, that the cilia which surround the mouth reverse the 

 direction of their beat when certain kinds of particles 

 strike upon them, with the result that these particles are 

 thrown out, then the question as to the existence of a 

 chemical discrimination would depend on whether the 

 rejected particles are chemically unlike those which are 

 accepted, or different only in size or mechanical consist- 



