Food Selection Among Lower Animals. 41 



that selection is affected by more than a single quality affecting 

 the tactual senses, such as weight, size, surface texture, form, etc. 

 To stimulate the feeding mechanism, several of these factors must 

 be present in a certain degree or form of expression ; more than 

 one of these factors serves as a basis of discrimination. 



These experiments on stentor indicate that in the matter of food 

 selection stentor compares favorably with the higher animals. 

 The problem of food selection evidently does not begin with ani- 

 mals like stentor, for it is highly developed here. I was inter- 

 ested in knowing therefore whether in ameba, which as was said 

 above, is supposed by many to be the simplest animal living, the 

 problem of food discrimination begins ; that is, whether there is 

 any sign that the power of discrimination resides in ameba. 



The method of work was as follows : A single particle of the 

 substance which it was desired to test, was placed some distance 

 ahead of the ameba, by means of very fine glass needles. Camera 

 lucida drawings of the outline of the ameba were then made at 

 intervals of about half a minute, as the ameba moved ahead. In 

 this way a complete record of the behavior was obtained. 



Instead of using living organisms for feeding ameba, isolated 

 chemical substances were used. It was thought that by so doing 

 the interpretation of the behavior would be simpler, for instead of 

 having to do with a number of substances which are present in an 

 organism, only one substance can be the cause of whatever 

 changes are noticed. The number of variables and unknowns is 

 thus reduced to the minimum. 



The first question to be solved was : Does ameba possess the 

 power of discrimination in food? It did not require many ex- 

 periments to show that it does possess this power. In a general 

 way we may say that, excepting carmine, which is eaten, only 

 digestible substances are eaten. Not only does this hold true for 

 organisms, living or dead, but for isolated proteins, such as the 

 globulins and the albumins. Globulin, which is said to be in- 

 soluble, is readily eaten; but egg albumin, which is very soluble, 

 is very seldom, if ever, eaten. Feeding experiments with ameba 

 have not been carried as far as with stentor, but from the work 

 that has been done, it seems safe to say that the power of dis- 

 crimination is as highly developed in ameba as in stentor. 



