274 BRADLEY M. PATTEN 



of the stimulus on the sensitive surfaces. For brevity, we may 

 call a stimulus which fulfils these conditions a directive stimulus 

 and one which does not, a non-directive stimulus. 



Not all animals that are sensitive to light respond by orien- 

 tation when they are subjected to the action of directive light. 

 There must, therefore, be a second critical factor, a factor inherent 

 in the responding organism. In the case of the blowfly larva, 

 it was pointed out that orientation might be explained as a re- 

 sult of differential stimulation of bilateral sensitive surfaces 

 inclined- to each other at an angle, and the proportional trans- 

 mission of that stimulation to the bilateral musculature. The 

 explanation in this form will not apply to the case of an animal, 

 like Stentor, which has but a single sensitive area. The two 

 organisms, however, have in common a factor which I believe is a 

 fundamental one in all directive reactions, a definite response 

 to stimulation which is proportional to the intensity of the 

 stimulus. In the blowfly larva there are bilateral sensitive 

 areas, the stimulation of which produces a reaction bilaterally 

 proportional to the amount of stimulation. In Stentor the spiral 

 path of locomotion makes the single sensitive area perform the 

 functions of two. There is a definite response to stimulation in 

 the form of a swerving toward the aboral pole, and the amount of 

 swerving is proportional to the stimulus. The operation of the 

 response first on one side and then on the other brings about the 

 orientation of the animal. Each animal has a method of react- 

 ing which is dependent on its structural peculiarities, but the 

 animals respond in a definite and consistent way to stimulation 

 by light, and both give a reaction proportional to the intensity 

 of the stimulus. 



When an animal does not respond to a directive light by 

 orientation but merely by undirected activity, it is because the 

 second critical factor of phototaxis is not present, the animal 

 has no mechanism for producing a definitely directed response 

 proportional to the intensity of the stimulus. We may assume 

 that even in forms which do not respond directly, stimulation is 

 proportional, to the intensity of the stimulus, since the experi- 

 mental evidence clearly indicates that stimulation, in its final 



