RELATION OF BURSARIA TO FOOD 



TABLE 25 

 Experiment XXIII 



41 



A difference is plainly evident. Other experiments show greater 

 or less difference, depending upon the conditions. 



2. The basis for and nature of food selection in Bursaria, as shown 

 hij the foregoing and other experiments 



It must be remembered that in any such experiments as the 

 foregoing the relation to food is in some ways an entirely new 

 one to the organism. Yet it must be insisted upon that the 

 yolk used in these experiments is assimilable by the organisms 

 (a fact which will be considered at length in a later paper) and 

 especially that whatever the mechanism of feeding and selection 

 in nature is, it must be the same one which is brought into action 

 in these experiments. Hence the criticism imagined above would 

 appear to have no importance for the question under consider- 

 ation here. In fact, it is to be believed that so far as these 

 expei'iments are concerned, they are only a more strongly empha- 

 sized condition of what we find in nature and that they picture 

 to us, so far as they go, the actual condition of the food relation 

 of Bursaria in its native culture. ' 



We may state the results briefly in the following way: First: 

 Yolk grains are rejected if the soluble adsorbed toxic substance 

 makes with the medium a sufficiently steep concentration gra- 

 dient. If this gradient is low relative to that of the yolk-soluble 

 substance, to which Bursaria reacts positively, then the organism 

 may eat the stained yolk, other conditions being equal. Second: 

 (a) Whether Bursaria will eat stained yolk grains or reject them 

 depends also, along with the steepness of the concentration gra- 

 dient, upon the specific chemical properties of the adsorbed sub- 



