CHANGES IN INTENSITY 97 



tions," and it seems as if at one time lie even intended 

 to deny the existence of tropisms and to maintain that all 

 animals were influenced only by rapidly changing intensi- 

 ties of light. It is needless to discuss such an idea (which 

 he probably no longer holds) in view of the contents of the 

 preceding chapters. He seems, however, to cling to it as 

 far as asymmetrical unicellular organisms are concerned. 

 When moving about Paramcecia often reverse the direc- 

 tion of their progressive motion for a moment, but then 

 do not return in the old direction, moving sidewise, on 

 account of the asymmetry in the arrangement of their 

 cilia. Jennings is probably right in assuming that this 

 factor can lead to collections of such infusorians, since 

 it may prevent their leaving a drop and going into the sur- 

 rounding medium. When, e.g., at the boundary of the two 

 media such a reversal of the action of the cilia occurs, the 

 organisms are prevented from crossing from one medium 

 into the other. 



But Jennings goes too far in this attempt, when he 

 tries to explain the heliotropic reactions of certain uni-i 

 cellular organisms, e.g., Euglena, in this way. He main- 

 tains 253 that unicellular organisms like Euglena go to the 

 light on account of shock movements produced by the 

 shading of the photosensitive region of the animal. 

 Euglena moves with a constant rotation around its longi- 

 tudinal axis and Jennings assumes that in a certain phase 

 of the rotation a photosensitive element (the eye spot) 

 of the organism is shaded. This he thinks causes a shock 

 movement, whereby the animal is swerved to the light 

 again during the next half of the spiral revolution, and 

 so on. Similarly in negatively heliotropic Euglena the 

 swerving away from the light is, according to Jennings, 

 the shock movement caused by the increased illumination 



