QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 273 



reacting animal but in the distribution of the stimulus in the 

 field. The term 'trial and error/ or better 'method of trial' 

 may, I believe, be more appropriately applied to the method of 

 aggregation by the kinetic reaction than to a method of orien- 

 tation. The 'varied movements' of the locomotion of forms like 

 Stentor or the blowfly larva take place whether or not the animals 

 are stimulated by light. Though they undoubtedly play a part 

 in orientation under some circumstances, it is by no means 

 certain that they are essential to the attaimnent of orientation. 

 In cases like that of a blowfly larva subjected to opposed beams 

 of light, they probably play a very insignificant part in orienting 

 the animal. It does not seem logical, therefore, to characterize 

 the orientation of such forms as orientation by the method of 

 trial, for even though there are some aspects of orientation 

 that might be 'explained' on this basis, there are others which 

 certainly cannot be so explained. An interpretation of orienta- 

 tion, to be acceptable, must accord not only with the details 

 of a special case, but with all the varying details appearing 

 under all the different manifestations of the phenomenon. 



In the case of aggregation by the kinetic reaction, light causes 

 the undirected locomotion which carries the animal into various 

 environmental conditions. The movements initiated by light 

 persist until 'selection' is accomplished with the cessation of 

 stimulation, in other words, until the animal 'happens' to move 

 into a non-photokinetic area. Such a reaction may be termed 

 'automatic distribution.' 



A significant fact in connection with the operation of a uni- 

 fonnly distributed stimulus is that animals, such as the blowfly 

 larva, which respond phototactically to horizontal light show a 

 simple kinetic response when subjected to uniforai illumination 

 from above. The reason for this is the fact that under such 

 conditions changes of axial position do not produce any difi"er- 

 ence in the relative amount of stimulation received by the sen- 

 sitive areas. There is no basis for orientation. This establishes, 

 as one of the critical factors in a directive response, a distribution 

 of the stimulus such that a change in axial position on the part 

 of the animal involves a, change in the distribution or intensity 



