240 A. A. SCHAEFFER 



mechanisms is not merely academic. The distinction can readily 

 be made in the higher animals, and there are some observations 

 in my paper, viz., those where tyrosin was fed, which show that 

 the distinction is experimentally verifiable in ameba. But the 

 clearest illustration of this point among the lower forms is to 

 be found in stentor. 



It follows, then, that those experiments where various tem- 

 peratures, electric currents, toxic substances, were employed, 

 important as they are of course from other points of view, cannot 

 be admitted as evidence in support of the chemical theory of 

 food selection. And as to those experiments where the yolk 

 was stained and the superfluous dye washed out, it is really 

 doubtful if there is evidence here that bears on the problem 

 of choice of food. The adsorbed dye gradually washed out, 

 according to Lund, in the course of the experiments, and it is, 

 therefore, next to impossible to be sure that only the food 

 selective mechanism is stimulated when a response is obtained; 

 but this, as has been said, is the first essential in investigations 

 on choice of food. 



The food selective mechanism is the most delicate mechanism 

 affeited by food substances, and it is the last mechanism that 

 tests the particle before it is eaten or rejected. A particle of 

 any substance must not cause a general negative reaction before 

 it affects the food selective mechanism, otherwise the animal 

 cannot express choice as to its food qualities. If the body 

 reacts negatively to a particle (solid or in solution) before the 

 food selective mechanism is affected, the result is choice only 

 between indifferent and injurious substances. That is to say, 

 an injurious substance produces a negative reaction, while all 

 other substances are indifferent. The presence or absence of 

 food qualities is not the basis upon which choice is made under 

 such conditions. As a concrete case we may refer to Stentor 

 caeruleus. So far as we know, the cilia of the disk of stentor 

 are not a part of the food selective mechanism. But they are 

 a part of the feeding mechanism. Food materials, carmine, 

 starch grains, glass, sulphur, etc., brought to the disk by the 

 vortex produced by the membranellae, are all transferred to 

 the pouch and funnel leading to the mouth. But when an 

 injurious substance comes into contact with the disk, a negative 

 reaction sets in. The discal cilia accept indifferent or non- 



