650 Mast, Orientation in Euglena with some Remarks on Tropisms. 



increase or to sudden decrease of intensity; and (4) positive orien- 

 tation in individuals which responded with shock-reactions to in- 

 crease instead of to decrease of intensity. All of these reactions 

 he maintains are contrary to expectations based on the change-of- 

 intensity theory of orientation. He consequently concludes that 

 this theory fails to account for the observed phenomena and that 

 tHe continuous-action theory must be valid. He says (p. 425), 

 "We have no evidence against the view that in Eiiglena the 

 gradual orientation is a function of the continuous action of the 

 light". 



To what extent do Bancroft's results actually warrant these 

 conclusions? Let us consider first the results recorded in diagram B 

 together with the methods used in obtaining them. 



According to this diagram the expected correspondence between 

 orientation and shock-reaction holds in the lower light intensity, 

 but as the intensity increases, Bancroft maintains, there is no 

 orientation although there is still a shock-reaction. Is this result 

 necessarily in opposition to the change-of-intensity theory? It would 

 be if the shock-reactions had been induced by changes in illumina- 

 tion in the field, of the same nature and degree as were produced 

 on the sensitive tissue within the unoriented organism due to its 

 rotation, for only such changes could function in the process of 

 orientation; but this was in all probability not true. In these tests 

 Bancroft placed an opaque screen in front of the light practically 

 cutting it entirely off. Thus the reduction of intensity was no 

 doubt very much greater than that caused by the rotation on the 

 axis. If this is correct then all of the reactions described in this 

 diagram with the possible exception of the last, may be, for all 

 that is known to the contrary, in full accord with the change-of- 

 intensity theory; and the same may be said with reference to the 

 reactions represented in the two succeeding diagrams. In all of 

 the experiments in which the results given in these diagrams were 

 obtained there is every indication that changes of intensity of 

 such a degree were used in testing for the shock-reaction as could 

 not possibly have been produced in the process of rotation by the 

 movement of the shadows of the translucent bodies in the organism. 

 The change-of-intensity theory demands shock-movements in negative 

 Eiiglena only when the intensity of the field is increased, but if 

 the intensity is greatly decreased the negative individuals may 

 become positive, and respond with the shock-reaction. This is 

 especially probable if they are near the neutral region as was true 

 according to the diagrams in Bancroft's experiments. 



I consider it inexpedient to attempt further analysis of these 

 results, for, strange as it may seem, if they are fatal to the change- 

 of-intensity theory, as Bancroft holds, they are equally fatal to 



