CHOICE OF FOOD IN AMEBA \ 35 



only in two general ways: its molecules or ions may combine 

 chemically, temporarily or permanently, with a part of the sense 

 organ; or, some sort of physical energy radially propagated 

 from the sensed particles may produce physical changes in the 

 sense organ. If a substance affects a sense organ by. uniting 

 with it chemically, such sensations as might result therefrom 

 should be related in some way with the chemical constitution 

 of the substance, and the sensations should vary in some way 

 as the composition of the stimulating substance varies. Such 

 a sense organ would mediate true chemical sensations. It 

 would seem that all the elements of at least a few compounds 

 should be represented in consciousness, by a chemical sense 

 organ, when they are tasted or smelled. But this never happens 

 in man. The taste or smell of a substance of a given concen- 

 tration, on homogeneous sense organs, is always a simple sensa- 

 tion, such as proceeds from a simple stimulus. There is no 

 evidence from the psychological side, as it is recognized that 

 there is none on the physiological, that organs of taste or smell 

 sense the chemical composition of substances. But on the 

 other hand, there is some evidence that physical qualities are 

 sensed by the distinctive organs of taste and smell as is shown 

 by the possibility of stimulating the taste buds by electricity 

 so that there are produced sensations of sweetness, acidity, etc., 

 depending upon the particular buds stimulated. It is possible 

 therefore to stimulate a sense organ of taste by means of 

 chemicals in solution as well as by electricity, a physical stimulus. 

 But the effect of a chemical on a sense organ is not at all the 

 same thing as a chemical effect on a sense organ. 



But this distinction is not always clearly drawn. 



Metalnikow, in his very interesting and extensive paper on 

 the feeding habits of Paramecium, as an important example, 

 does not make very clear the distinction between reactions to 

 physical and to chemical qualities, although he asserts that 

 the chemical nature of a substance determines whether it will 

 be eaten by Paramecium. Let us therefore examine the exper- 

 imental results and arguments of Metalnikow, in the light of 

 what was said in the preceding paragraphs, to see whether he 

 is justified in stating that Paramecium selects its food upon a 

 chemical basis. 



Paramecia eat powdered glass, sulphur, chalk, aluminum, 



