HELIOTROPISM IN EUGLENA 391 



In his reply to Torrey, Jennings does not meet Torrey's objec- 

 tion squarely, but still makes his position very plain. In his 

 explanation of the figure to which Torrey refers, Jennings says 

 that ''At 3[=c] the normal amount of swerving is restored." 

 Now it is the part of the curve from c to 5 in which the normal 

 amount of swerving obtains to which Torrey expressly refers, but 

 Jennings in reply to him says ('08, p. 705) : ''The most direct way in 

 which the organism, swimming in a spiral, could become oriented to 

 the light would be by an increase in the swerving to the right and a 

 decrease in the swerving to the left, and this is what the tropism 

 theory would lead us to expect. But the fact is that there is an 

 increase in the swerving both to the left and to the right, the spiral 

 becoming a wider one; the increase to the right being, however, 

 greater than that to the left, the organism becomes gradually 

 pointed to the right. The increased swerving to the left is not 

 •accounted for by the tropism theory, and is indeed squarely op- 

 posed to it, while it is to be expected if the analysis I gave is cor- 

 rect. " Here we see that Jennings in attempting to meet Torrey's 

 objection concerning the part of the course from c to 5 points to 

 the behavior during the previous part of the course from a to c 

 the additional swerving in which Torrey has interpreted as due to 

 Unterschiedsempfindlichkeit and not in accordance with the 

 tropism schema. It is evident, however, that this does express 

 clearly and fairly Jennings's views. He thinks that the part of 

 the course from c to 5, in which there is the normal amount of 

 swerving, is not direct orientation because in the previous part of 

 the course there was additional swerving in both directions. Since 

 this additional swerving was considered a motor reaction the later 

 swervings which are only towards the light are interpreted as motor 

 reactions also. 



But even this evidence in favor of the trial and error concep- 

 tion has been removed by observations of Mast ('11, p. 104) 

 who finds for Euglena that the orientation "takes place just as 

 Jennings represents (fig. 12 [ = fig. 1]) with the exception that if the 

 direction of the rays is changed without any change of the inten- 

 sity, orientation may take place without an increase in the diam- 

 eter of the spiral course represented in fig. 12 ( = fig. 1) a-c.'' 



