HISTORICAL REVIEW 57 



the axis of its body in a definite position — with relation 

 to some external source of stimulation." 



It is evident from these statements that nearly every 

 reaction in living organisms comes under one or another 

 of the various definitions given to the term tropism. To 

 say that an organism is tropic or not tropic means but little 

 until the sense in which this term is used is defined. Failure 

 to do this has led to serious misunderstanding. I have no 

 objection whatever to the term tropism if used in its 

 original sense, or in any other definite sense. At present, 

 however, it conveys so many different meanings that it 

 inevitably leads to confusion. I shall therefore avoid using 

 it in the following analysis of reactions to light. 



5. Statement of Important Problems in the Study of 



Reactions to Light 



In this analysis we shall ai n to keep in mind the various 

 factors suggested as important in the different tropism 

 theories and other explanations of behavior. We shall ask 

 ourselves: is orientation direct, does the organism turn 

 directly toward or away from the source of stimulation, or 

 does it become oriented after a series of preliminary move- 

 ments? How is the stimulus causing orientation pro- 

 duced: by direction of rays through the organism in accord 

 with the theory of Sachs; by absolute difference of intensity 

 on symmetrically located points on the sensitive surface in 

 accord with the theories of Loeb and Verworn; or by changes 

 of intensity on the surface in accord with the ideas of Engel- 

 mann, Darwin, and Jennings? Does light act constantly 

 as a directive stimulation similar to the action of a constant 

 current of electricity in accord with Loeb's theory of trop- 

 ism, or does it act only when the organism turns out of its 

 course so as to produce changes of intensity, as suggested 

 by Jennings? Is orientation due to the direct effect of 

 light on the locomotor appendages in accord with the 

 theory of Verworn and the analysis of Torrey, to the indi- 



