64 THE BEHAVIOR OF LOWER ORGANISMS. 



organism reacts to a decrease in illumination by a sudden turn to one 

 side, by an increase in the width of the spiral, and by a change in the 

 course, just as happens in Euglena and Cryptomonas. The unoriented 

 organism becomes oriented in a manner which is similar to that de- 

 scribed above for the two organisms just named. Since, however, I 

 am unable to give the precise relations of these movements to struc- 

 tural differentiations of the body, a further account of details would 

 not be of interest. 



GENERAL RESULTS. 



In summing up our results on reactions to light in the organisms 

 studied, there are two points of especial interest which should be con- 

 sidered separately. The first relates to the nature of the reaction 

 produced, the second to the nature of the agent causing the reaction. 



NATURE OF REACTION PRODUCED BY LIGHT. 



As to the nature of the reaction produced by light there has been 

 much discussion. The orthodox tropism theory is perhaps that which 

 has the greatest number of adherents. It is set forth in detail in the 

 paper of Holt & Lee (1901). According to this theory the light acts 

 directly on the motor organs of the side on which it impinges ; supra- 

 optimal light causes increase of the backward stroke (in the case of 

 cilia or other swimming organs) ; suboptimal light causes a decrease 

 in the backward stroke. The result is that the organism is turned 

 directly toward or from the more intensely lighted side, and hence 

 toward or from the source of light. The diagrams given in the pre- 

 ceding paper (Figs, i and 2) can be applied directly to the elucidation 

 of this theory. 



In the experiments on the ciliates and flagellates set forth in the 

 present paper the precise method of reaction was determined by obser- 

 vation. It is not in accordance with the tropism theory above set 

 forth. This has been emphasized in detail in the account of the 

 reactions of Stentor, so that it need not be reiterated here. The reac- 

 tion to light is of the same character as that to other stimuli, and takes the 

 form of a motor reaction in which the organism performs a definite 

 set of actions. It first usually stops or swims backward, then turns 

 toward a structurally defined side, then continues forward. The 

 result is to change the course of the organism. As a result of the con- 

 tinual rotation on the long axis, together with the swerving toward a 

 certain side, the organism comes to be pointed successively in every 

 direction. In continues to swim forward in that direction which does 

 not induce a stimulus to further swerving. The whole reaction is a 

 strongly marked example of the type of behavior which may be called 

 the " method of trial and error." 



