530 ASA A. SCHAEFFER 



INTRODUCTION 



The diet of most of the higher animals includes a large variety 

 of materials. Many of the vertebrates extend or restrict their 

 diets on the basis of past experience; that is to say, they learn 

 that some of the materials which they have been eating are not 

 suitable for food, while some which they have been avoiding 

 may be eaten. The most striking example of the influence of 

 experience on the selection of food is found, of course, in man, 

 who utilizes an almost limitless variety of materials for food 

 purposes. Next to man in this respect comes perhaps the dog, 

 who has learned to eat many of the foods prepared by man. As 

 we pass down the scale of animals, we observe that learning or 

 experience plays a decreasing part in choice of food, and it ap- 

 pears that the sensations received by the organs of smell, taste 

 and touch determine the feeding reactions of the animal more 

 and more. Following out logically this line of thought there is 

 suggested the probability that the unicellular animals, which 

 stand at the bottom of the scale, select their food exclusively by 

 smell or taste or touch, and that learning or previous experience 

 plays no part in selection. It would be expected, therefore, that 

 the problems of choice of food would be reduced to its simplest 

 terms in the protozoa; and it was in the hope of throwing some 

 light on the relation existing between ameba and the materials 

 which it eats, that the experiments recorded in the following 

 pages were carried out. 



It is difficult to overestimate the importance of the relation 

 between a free living animal and its food. Every animal must 

 eat. And every animal must either choose its food, eat every- 

 thing it can get, or eat objects without regard to their chemical 

 and physical qualities. Now it happens that so far as known, all 

 animals choose their food, and the problem of food selection is 

 therefore one of fundamental importance. And yet in spite of 

 its importance, this problem has remained almost wholly unin- 

 vestigated. But, even though no extended studies have been 

 made on the relation between animals and the food which they 

 eat, we have in zoological literature a large number of 'notes' on 



