238 A. A. SCHAEFFER 



bursaria selects its food on a chemical basis, I have nowhere 

 found a discussion in his paper which sets forth just what he 

 conceives to be a chemical basis as distinguished from a physical, 

 in so far as the selection of food is concerned. I have, therefore, 

 found it of interest to examine his paper in some detail, for 

 it is absolutely essential that the difference between selection 

 on a chemical and on a physical basis be clearly drawn, before 

 we can say that an animal selects its food on either basis. 



Lund fed bursaria with yolk of hen's eggs untreated, and 

 stained with various chemicals, employing essentially the methods 

 I used in my work on stentor ('10), and Metalnikow ('12), on 

 Paramecium. 



Yolk treated with various dyes is eaten less readily than 

 untreated yolk. If NaOH, HC1, saffranin, etc., are added to 

 the culture solutions in varying quantities, the yolk is eaten 

 in decreasing quantity in proportion as the toxic substance is 

 increased in concentration. Temperature also has a similar 

 effect; with a rise of temperature from 0° to 35° C, there is a 

 corresponding increase in the amount of yolk eaten. Beyond 

 35° C. the temperature becomes injurious. In addition to these 

 experiments, Lund mentions some observations to the effect 

 that carmine, india ink, aluminium, carbon black, cinnibar, etc., 

 are eaten by bursaria (p. 43). 



It appears then that bursaria stands between stentor and 

 Paramecium (and close to Paramecium) as far as concerns its 

 capacity to discriminate in feeding. 



From the results of his experiments, Lund expresses agreement 

 with Metalnikow ('12) in so far as the basis of selection is 

 concerned. 



But it was shown in the preceding pages that Metalnikow's 

 conclusion that food is selected on a chemical basis is not in 

 agreement with all the facts; and the same criticism to which 

 Metalnikow's conclusions are open, also apply to Lund's. It 

 will therefore not be necessary to go over this ground again. 

 It should be noted however, in this connection that Lund dis- 

 misses with a single paragraph, as if they did not bear at all 

 on the question of selection, all the observations he made on 

 the eating of chemically inert substances, such as aluminium, 

 carbon, etc. But it is precisely these observations which most 

 strikingly contradict his notion that selection is made on the 

 basis of the chemical composition of the substances. How can 



