74 TROPISMS 



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 photochemical sub- 

 stances and their relations to the leg muscles would have to be modified. 

 In the one ease the required changes are all of a mechanical nature and 

 so simple that the experiment might possibly 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 (Bancroft 21 ). 



The asymmetry of organisms only modifies the geo- 

 metrical character of the path but not the mechanism of 

 the reaction. 



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