CHOICE OF FOOD IN AMEBA 239 



one explain the selection of insoluble substances such as carbon or 

 aluminium, which are eaten, on a chemical basis''! 



The experimental results which form the basis of Lund's 

 conclusion admit of another interpretation. The fact that more 

 and more yolk was eaten as less and less of NaOH, HC1, saffranin, 

 etc., was present, does not of course indicate active selection; 

 the presence of toxic substances affected the general bodily 

 condition so that feeding was to some extent suspended. It 

 cannot be inferred that the bursarias ' ' tasted ' ' with the food 

 selective mechanism the toxic substance, and on this account 

 refused to eat less as this substance was more and more con- 

 centrated. The experiments with temperature show a feeding 

 gradient that corresponds almost exactly with that obtained in 

 those experiments where toxic substances were employed in 

 varying concentrations. Clearly therefore, there must be other 

 evidence to show that selection is based on chemical constitution ; 

 for the observation that chemicals in varying concentrations 

 prevent feeding to a greater or less extent, does not mean that 

 these chemicals were 'tasted' by the mechanism of food selection 

 any more than that the various temperatures, or the electric 

 current, or the mechanical stirring, were tasted. In other words, 

 the feeding mechanism in bursaria, as in many other animals, 

 is affected by stimuli affecting the general bodily condition, as 

 seems to be the case when stimulated by agitating mechanically 

 the medium in which the bursarias live, or by passing electric 

 currents through the medium, etc. In these cases the inhibiting 

 impulses arise in other parts of the body, and are transmitted 

 to the feeding mechanism. 



Chemical solutions, when mixed with the medium, also affect 

 the sense organs of the ectoplasm generally, so that feeding may 

 be inhibited without involving the action of the food selective 

 mechanism at all (see Lund, pp. 41, 42). Whether Lund suc- 

 ceeded in localizing the effect of stains absorbed by the yolk 

 grains so that only the food selective mechanism was affected 

 and then not injuriously, is very doubtful; that he did succeed 

 may not be concluded from his experimental results. It is 

 probably superfluous to add that unless one is sure that the 

 food selective mechanism is at any time regulating the feeding 

 mechanism, discussion of the bases of food selection is beside 

 the point. 



The distinction between the feeding and the selective 



4 



