The Problem of Orientation 89 



would tend rather to neutralize one another than 

 to give rise to any continuous efforts in one direc- 

 tion." 



It is not possible, I think, to construe phototaxis 

 entirely in terms of differential sensibility. Re- 

 sponses to the shock due to a sudden increase or 

 decrease of light may play a part in the orientation 

 of many forms, but the continuous stimulating in- 

 fluence of the rays is, in many cases at least, and 

 very probably in most, the factor of greater im- 

 portance. 



The same conclusion is strengthened by the re- 

 cent excellent investigations of Bancroft on the 

 tropisms of the protozoan Euglena. The phototaxis 

 of this simple organism has been the subject of more 

 or less contention on the part of Jennings, Mast, 

 Torrey, Parker, Bancroft, and to a small extent the 

 present writer, Jennings and Mast having endeav- 

 ored to prove that orientation takes place by the 

 trial and error method, the other participants main- 

 taining that orientation is direct. Bancroft has 

 shown that orientation is in large measure independ- 

 ent of the reactions to sudden increase or decrease 

 of illumination and that it apparently depends upon 

 a different mechanism. It is not probable there- 

 fore that orientation is effected through responses 

 to variations in light intensity as Jennings and Mast 

 have contended. 



As Loeb has pointed out if it can be shown that 

 the Bunsen-Roscoe law applies to phototaxis it 



