CHOICE OF FOOD IN AMEBA 243 



cups would be formed when the chemical substance was first 

 sensed, instead of after the source of diffusion was reached. 

 Then there is the further general observation that the ameba 

 moves directly toward a stimulating object, if positively attracted. 

 That is, the ameba moves parallel with lines of diffusion radiating 

 from the soluble object; or, if the object is not soluble, along 

 lines of force of another nature propagated in a similar manner. 

 It is clear therefore that for lifeless objects, choice cannot be 

 based on a single factor, nor can it be based exclusively on the 

 chemical composition of the sensed objects. 



The ameba seems to be stimulated at several points by a 

 sensed object, and locomotion then proceeds along the line of 

 intensest stimulation. This is a method of reaction directly 

 comparable to that of the higher animals, when stimulated 

 positively by heat from a definite source. The whole ectoplasm 

 (particularly the anterior portion) may be considered as a sense 

 organ capable of receiving stimuli at many points with local 

 reference. That the stimulus itself (for example, a change of 

 surface tension) directly conditions the path of movement seems 

 impossible; for the nature of the responses to stimuli is not at 

 all machine-like, as the experiments abundantly show, and as 

 they would necessarily be if the stimulus directly determined 

 the reaction. 



One of the chief difficulties in the way of accepting the 

 hypothesis that food is selected on a chemical basis is the dis- 

 covery that organismal and histonic selection are not synonymous. 

 This is very significant. Carmine, which is invariably readily 

 eaten, is nevertheless always speedily egested. It seems to make 

 little difference whether the ameba is hungry or nearly satiated, 

 speedy egestion follows the ready eating of carmine. Similar, 

 though not as striking results, are obtained in the behavior of 

 A. dubia toward lactalbu'min, uric acid, etc. 



Now, if the chemical nature of an object is thoroughly tested 

 at any time by an animal, it is just after it is eaten. The 

 digestive juices then begin to act upon it chemically, and its 

 constitution then determines what will happen to it. In a 

 general way, if the substance is digestible and harmless, it 

 remains in the body; but if indigestible or irritating, it is soon 

 excreted. Now the fact that organismal selection, in many 

 cases, leads to results directly opposed to those of histonic 



