48 THE BEHAVIOR OF LOWER ORGANISMS. 



and repeat the reaction. This is repeated many times until the organ- 

 isms are swimming either toward the end D or toward the end L. In 

 course of time it is found that the preponderance of movement is toward 

 the dark end /), so that the majority of the Stentors are gathered at D. 

 Why this should be so is explained by Holt & Lee as follows : 



The reason why the Stentors went eventually in greater numbers toward Z>, 

 and thus appeared oftener to choose e than f, is that such Stentors as went to e 

 progressed farther toward D than those which went to f could progress toward 

 L. These latter would soon strike the wall a second time, now pretty nearly at 

 right angles, and during the recoil the light stimuli would favor a return to d. 

 It appears then amply possible that the circumstance that the organism encoun- 

 ters the wall of the trough at an acute angle is sufficient to cause its farther 

 progress to be, in the long run, toward D. 



There is evidently nothing in this account which is inconsistent with 

 the method of light reaction which I have described. On the contrary, 

 the reason why the organisms finally swim toward the dark end and 

 gather there becomes much more evident when the reaction method 

 that I have described is taken into consideration. Let us suppose that 

 the Stentors, after striking the back of the trough, turn in equal numbers 

 toward D and toward L. In those swimming toward D the anterior 

 end is directed away from the source of strongest light (due to reflection 

 from the lighted end of the dish Z-), and the animals are passing into a 

 region of less intense light. There is thus nothing to cause the " motor 

 reaction," with its accompanying change in the direction of movement. 

 In the Stentors swimming toward Z,, on the other hand, the strongest 

 light falls on the anterior end, and the organisms are passing into a 

 region of more intense light. Either of these factors taken separately 

 may, as we have seen, cause the motor reaction (the turning toward 

 the right aboral side), thus changing the direction in which the Stentors 

 swim. The animals which start to swim toward L will therefore soon 

 be turned, and only when the direction of movement is toward D will 

 there be no cause for further change. 



The observations of Holt & Lee are thus quite in harmony with 

 the reaction method which I have described, and indeed receive 

 illumination when this reaction method is taken into consideration. 



In the " fourth case" discussed by Holt & Lee (loc. cit., pp. 475- 

 478), the two factors mentioned as determining the turning of the 

 Stentors away from the end L would work in opposite directions ; only 

 experience can tell which would be more effective. As Holt & Lee 

 do not state specially that they observed the reactions of Stentor under 

 these conditions no comment is required. Experiments of this character 

 will be further considered after we have described reactions to light 

 in flagellates. 



