62 THE BEHAVIOR OF LOWER ORGANISMS. 



the light is reduced by the same amount at both ends, so that the anterior 

 end is still more strongly lighted than the posterior. This case is 

 realized in the experiment in which a small screen is interposed between 

 the Euglenjfi and the window toward which they are swimming. The 

 light is thus somewhat decreased, but is still sufficient to cause orien- 

 tation. The anterior end is thus still lighted more than the posterior, 

 yet the organisms respond with the " motor reaction" at the moment 

 the light is decreased. The same thing is shown still more decidedly 

 in the experiment described on page 50, in which the " motor reaction " 

 is produced when the light is cut off from some other source than that 

 toward which the organisms are swimming. In this case the propor- 

 tion of light shining on the anterior end is greater after the change in 

 illumination than before, yet the " motor reaction" is produced at the 

 moment the change takes place. 



The explanation we have given is, therefore, the only one that is in 

 agreement with all the facts, and it accounts for every detail of the re- 

 actions to light. The cause of all the phenomena of light reaction in 

 Euglena is the fact that a sudden change in light intensity on the anterior 

 end induces a typical " motor reaction." It is noticeable that the 

 reaction is throughout due to a dynamic factor, to some change in the 

 relation of the organism to the light, a change due either to an active 

 alteration of the environment, or to a movement of the organism. To 

 static conditions, if not too intense, the organism may soon become 

 acclimatized, so that no farther reaction is caused. The absolute in- 

 tensity of the light affects the reaction only in so far as it determines 

 whether it shall be an increase or a decrease in intensity that causes 

 the " motor reaction." 



To sum up, the reaction of Euglena, from beginning to end, is ex- 

 plained by the fact that a sudden change in illumination, even though 

 slight, causes a definite motor reaction, the essential feature of which 

 is an increased swerving toward the dorsal side. Orientation is brought 

 about by the increased swerving in the next phase of the spiral course 

 when the illumination of the anterior end is diminished, and by the 

 decreased swerving in the next phase of the spiral when the illumination 

 of the anterior end is increased. In general terms we can say that the 

 reaction of Euglena to light is by the method of trial and error. The 

 organism tries turning in many directions ; when the turning is such 

 as to produce a decrease in the illumination of the anterior end it 

 "tries" other directions; when it is such as to produce increased 

 illumination of the anterior end, or when no change in illumination 

 results, the reaction ceases and the organism continues to swim forward 

 in that position. The result of this method of reaction is necessarily 

 orientation with the anterior end toward the source of light. 



