244 A. A. SCHAEFFER 



selection, demonstrates conclusively that these two methods 

 of selection do not rest upon the same basis. 



Although Metalnikow did not seem to recognize the difference 

 between organismal selection (exercised while feeding) and 

 histonic selection (exercised when the process of digestion begins) , 

 his data clearly indicate that carmine, aluminium, powdered 

 glass, sulphur, etc., are as readily eaten as, but more speedily 

 ejected than food particles. The fixed habit of cyclosis in 

 Paramecium, and that of egestion at a definite point, serve to 

 obscure this difference somewhat, for it tends to equalize the 

 time during which digestible and indigestible substances are 

 retained in the body. But the fact nevertheless stands out 

 w T hen Metalnikow's tables are closely examined. Thus if tables 

 I and IV are compared, it is observed (table I, culture C) that 

 glass and sulphur are eaten more readily than milk, as readily 

 as starch, and nearly as readily as olive oil; while table IV shows 

 that (in other cultures, it is true) these substances are excreted 

 much sooner than starch, olive oil, or milk. It is quite evident, 

 then, that the ectoplasmic evaluation of substances differs from 

 the endoplasmic, in Paramecium as distinctly as in ameba, though 

 not as strikingly, and that choice of food is not made upon the 

 same basis by the ectoplasm and the endoplasm. 



Seeing then that organismal selection of food as observed in 

 ameba is not explicable on a chemical basis, one naturally looks 

 to physical properties for the explanation. Is selection based 

 upon physical properties? Unfortunately a complete answer is 

 not yet possible, nevertheless we may consider briefly the direc- 

 tion an explanation of selection on a physical basis would take. 



It is not necessary to suppose, in the first place, that each 

 substance to which ameba reacts, stimulates the ameba in a 

 specific way. It is possible that many of the substances as 

 sensed differ from each other only quantitatively in certain 

 physical characteristics. This is, for example, the case in man 

 with most tasting substances. The various sugars and saccharin 

 differ from each other only quantitatively in taste, supposing 

 that the taste buds for sweetness only are stimulated. The 

 same is true for various acids. And as a further illustration 

 one may refer to color vision. Here a large number of quan- 

 titatively varying ether vibrations produce specific sensations 

 which are then qualitatively interpreted. Attention may also 



