392 FRANK W. BANCROFT 



Mast, however, instead of interpreting his observations in favor 

 of direct orientation, says ('11, p. 99) for the orientation of Eu- 

 glena in the crawling state which he considers 'in all essentials' 

 like that of the free-swimming individuals: ''When the organism 

 is not oriented every change from a position in which the light 

 strikes the ventral surface to one in which it strikes the dorsal, 

 and vice versa, due to rotation on the long axis, may be considered 

 a Hrial movement.'" 



To resume then: 'Trial and error' which at first was used to 

 designate the selection of chance movements, has now been 

 extended to include reactions in which the only elements of trial 

 consist in the normal spiral swimming in an unoriented position. 

 It might be -justifiable to call such unoriented spiral swimming 

 trial and error if it were not for the fact that in all cases trial and 

 error is contrasted with direct orientation, and the two used as 

 mutually exclusive terms. In this way trial and error has been 

 extended to include the very mechanism of locomotion through 

 direct orientation, if it were possible, would have to act; and the 

 only way that Euglena could orient directly according to this 

 view would be by growing some other locomotor mechanism that 

 would permit of progression in a straight line.^ I think that it is 

 evident that these objections to Jennings's view are conclusive 

 and that the criticism of Holmes, Torrey and Parker is well 

 founded. 



The comparison of galvanotropic with heliotrcpic orientation 

 has also been used by Jennings to show that the heliotropic orien- 

 tation is not direct. Thus, speaking of the infusoria in general 

 he says ('09, p. 314): 



In galvanotropism the organism usually does turn as directly as 

 possible into the position of orientation. It turns directly towards the 

 cathode (or anode as the case may be) — whatever the position of its 

 body when the electric current comes into action. But in phototropism 

 the position of orientation is attained by the same organisms in an entirely 



*It seems to me that this attitude of Mast is just as unjustifiable as to maintain 

 that a man who is walking towards some stairs does not ascend the stairs directly 

 but by 'trial and error,' because in bipedal locomotion his head bobs up and down 

 to a certain extent; and to maintain that this bobbing up and down represents a 

 series of trials and errors one of which is followed up. 



