250 A. A. SCHAEFFER 



is based upon physical characters of the food objects rather than 

 upon their chemical constitution. Thus, among amebas, move- 

 ment of an object is in a very large number of cases an adequate 

 stimulus; stentors seem to react to a composite stimulus formed 

 of several qualities such as weight, surface texture, shape, size, 

 solubility, etc. ; while paramecia seem to be sufficiently stimulated 

 by the solidity of particles of small size. 



Not only has the extension of the sensing range of food objects 

 been based on the physical characters in these protozoa, but 

 also in most if not all of the higher animals as well. Thus all 

 seeing animals depend more or less on the physical characters 

 of objects as a basis for choosing their food. After objects are 

 selected by sight as suitable for food, another test is made of 

 them by the sense of taste, or touch, or smell, or all of them 

 together. But there is at least one group of animals that depend 

 on sight almost, if not quite exclusively — the anura. Only 

 moving objects are snapped up, and these are usually snapped 

 up with such vigor that even if they are found disagreeable 

 in the mouth, such as hairy caterpillars are, for example, they 

 often cannot be ejected. A frog would starve in the presence 

 of motionless food objects. It is likely also that in many seed 

 eating birds, taste and smell play a very minor part in food 

 discrimination. There has, in short, been a tendency in animals 

 generally to increase their sensing range of food by reacting 

 to distinctive physical qualities, especially such as can be sensed 

 at a distance, until in some forms, according to experimental 

 evidence, none but physical qualities are concerned in selection. 



SUMMARY 



1. Ameba is capable of exercising very nice discrimination 

 in feeding between two particles of different composition — one 

 digestible, the other not — lying very close together. When the 

 particles stick together slightly, the food cup in some way 

 separates them so that the food particle comes to lie inside the 

 food cup while the other particle is actively pushed to the 

 outside. The ameba not only expresses choice between particles 

 of different composition under special conditions therefore, but 

 if the conditions are such that a choice cannot at once be made, 

 the ameba can change the conditions (separate the particles) 

 so that a choice may then be made. 



2. The experiments with two particles of diverse composition 



