ON THE FEEDING HABITS OF AMEBA 531 



the feeding habits of animals. These notes, though they are 

 fragmentary and incidental in character, nevertheless lead to 

 certain general conclusions, which point the way for investiga- 

 tion. We may observe, then, from the mass of these fragmentary 

 references, on the one hand, that some of the members of all the 

 great groups of animals have restricted diets ; only certain definite 

 substances are eaten. Thus the rhizopod Vampyrella spiro- 

 gyrae 'selects' only spirogyra filaments to feed upon; many in- 

 sects lay their eggs on certain definite species of plants which 

 serve as a special food for the larvae; frogs eat only moving ob- 

 jects; some birds eat only certain kinds of seeds, others feed only 

 on fish; and so on. In contrast to these animals may be named, 

 on the other hand, those which are omnivorous, representatives 

 of which are likewise found in all the great subdivisions of the 

 animal kingdom. This general comparison is interesting be- 

 cause it illustrates well how variable the basis is upon which food 

 substances are selected. Evidently discrimination cannot be 

 made upon the same basis by all animals. Perhaps, in general, 

 none but the most closely related species agree in having similar 

 bases of food selection, for it is well known that in at least some 

 of the higher forms there are definite individual differences in 

 this respect. 



It has been thought worth while to emphasize this fact of 

 diverse bases of food selection among animals, for the impression 

 has become quite general that an animal eats certain substances 

 because they contain certain chemical compounds possessing 

 food value, such as proteins, carbohydrates, or fats. But this 

 is true only if the end results of feeding — digestion — ^is consid- 

 ered. With the selection of food per se it has nothing to do. 

 For how can a frog in any way be sensible of the proteins in a 

 hard shelled beetle which it snaps up and swallows immediately? 

 Or how can a bird sense the food materials in hard seeds? 



With soluble or volatile substances the case is of course dif- 

 ferent. Meat extractives, some proteins, peptones, some carbo- 

 hydrates and some fats, may be selected by the senses of taste and 

 smell. If the assumption that there is a specific chemical rela- 

 tion between the substances and the end organ of taste or smell, 



