HELIOTROPISM IN EUGLENA 417 



While these hypothetical changes that must be made in the 

 amphipod, to make it react like Euglena, are considerable they 

 concern only the details. The fundamental nature of the photo- 

 chemical substances, the nature of theu* stimulation and the 

 character of their connection with the locomotor organs have 

 none of them been modified. All that has been done is to make 

 an asymmetrical organism swimming in a spiral out of a bilateral 

 one. These changes are much less fundamental than those which 

 we would have to imagine in order to make an amphipod orient 

 to light by the selection of random movements. In order to 

 bring about this latter change the whole nature of the photochemi- 

 cal substances and their relations to the leg muscles would have 

 to be modified. In the one case the required changes are all of a 

 mechanical nature and so simple that the experiment might possi- 

 bly succeed. In the other case the required changes are largely 

 chemical, and so complex that we have no data for even imagining 

 what ought to be done in order to bring them about. 



Differences such as those between the intact amphipod, the 

 multilated amphipod and Euglena have always appeared as of 

 minor importance to Loeb and to most of the advocates of the 

 unportance of the tropisms. Their fundamental similarity has 

 appeared to them so self-evident that the special case of the 

 asymmetrical animal moving in a spiral has not usually been con- 

 sidered." To Jennings and Mast, on the other hand differences 

 such as these have appeared so important that they appear to 

 have prevented these two investigators from recognizing the 

 fundamental similarity of the reactions of Euglena and the amphi- 

 pods. 



6. THE NATURE OF THE STIMULUS 



In this section we have to consider whether the stimulus which 

 causes the heliotropism of Euglena consists in temporal changes 

 of light intensity or in some continuous action of the light which 

 is independent of changes in intensity. The difference between 



!■* Torrey, however, in a forthcoming paper in Science does consider this ques- 

 tion and shows that the orienting mechanism in Euglena is identical with that pro- 

 ducing the direct orientation in bilateral organisms. 



