128 Asa Arthur Schaeffer 



Now if Stentor selects its food upon a chemical basis we should 

 expect something like the following to occur, i. Living organ- 

 isms ought to call forth a different reaction from those cooked or 

 chemically treated, since it seems evident that the " taste" of these 

 two classes of food is very different. 2. When living organisms 

 are fed we should expect the behavior to differ from what occurs 

 when they are fed in sugar solution, etc. 3. We should not 

 expect carmine or ink to be eaten. 4. Starch grains would 

 probably be eaten if soaked in, and fed with, paramecium juice; 

 and the minute fragments of paramecia would also probably be 

 eaten if Stentor selected its food upon a chemical basis. But as 

 a matter of fact the reverse is true in each case. 



But let us see what Stentor would probably do if it selected its 

 food upon a tactual basis. Under tactual stimuli all those of 

 size, weight, form, and surface texture, and discrimination might 

 be made upon any one or all of these factors. 



It is clear from the outset that size plays httle or no part in the 

 selection, for all sizes of organisms from bacteria to paramecia 

 are ingested. So we have only to consider weight, form, and sur- 

 face texture. 



Selection upon the basis of weight alone would result in rejecting 

 all substances which differed in weight (specific gravity) from 

 living organisms. This would explain the rejection of glass, sand, 

 starch, etc., and the ingest'on of carmine and ink. But it prob- 

 ably would not explain selection obtaining between Phacus and 

 Euglena or between Euglena and Chilomonas, or between a 

 rapidly moving organism like Halteria and any other particle. 



Selection on the basis of form alone would also fail to explain 

 all the results obtained in the experiments. For a Phacus differs 

 more from a Euglena in form than from a starch grain, etc. 



Nor Would the factor of surface texture completely explain all 

 the results obtained in the experiments. 



It seems clear therefore that as far as my experiments go, no 

 single quality such as weight, or form, etc., is decisive for setting off 

 the Stentor's ingesting mechanism in all cases where discrimina- 

 tion occurs. It is probable that more than one factor serves as a 

 basis for discrimination. 



