234 A. A. SCHAEFFER 



told at present there is no single quality possessed by all the 

 substances which ameba eats, which might be looked upon as 

 the basis of selection; nor do the refused substances possess a 

 common quality which causes amebas to leave these substances 

 alone. 



Excepting carmine, all the substances which ameba readily 

 eats are (presumably) digestible, though only two were actually 

 tested in this respect : globulin and grain gluten. But a number 

 of substances which are digestible, such as for example gelatin, 

 egg albumin, fibrin, etc., are eaten only very occasionally or 

 not at all. Of the lifeless solid substances, those which are the 

 more soluble (excepting gelatin and egg albumin, which are 

 very soluble, and globulin which is said to be insoluble) were 

 the more readily eaten. But it is possible that in the case of 

 globulin impurities were present, or that the salts in the water 

 caused slight solubility. The attractiveness of carmine and of 

 lead oxide, and the general indifference toward zein, fibrin, 

 keratin, etc., might be due to the solubility of the former and 

 the insolubility of the latter. Tyrosin is the only rapidly 

 dissolving substance eaten. It is very attractive, but it seems 

 that its rapid rate of solubility is slightly disagreeable or injuri- 

 ous. But there are still several items of behavior that cannot 

 be explained even if an essential part of the basis of selection 

 among lifeless substances should be the rate of solubility. Thus, 

 why should the soluble proteins: egg albumin and gelatin, be 

 passed by with indifference? Or what explanation could be 

 assigned for the fact that solids such as tyrosin and carmine 

 call forth feeding reactions very readily when these substances 

 are in the actual process of dissolving, but if a capillary tube 

 filled with solutions of these substances, is presented, the feeding 

 reaction is not produced? It is clear that neither solubility 

 as such nor the rate of solubility determines in all cases whether 

 a lifeless substance shall be eaten or rejected. 



The opinion has become quite general that the chemical con- 

 stitution of substances determines whether an animal will eat 

 them or not. This view, however, has no experimental support; 

 but on account of its general acceptance, especially as applying 

 to unicellular forms, it may be profitable to examine it in some 

 detail. 



Just how can a substance affect a sense organ? Apparently 



