156 'Jacques Loeh 



without reference to the direction from which the light comes" 

 (Jennings p. 131). Neither in this nor in any other case in which 

 Jennings describes reactions which depend upon sudden changes 

 in the intensity of Hght (or any other form of energy) does he refer 

 to my previous experiments on this type of reaction.' 



I think, however, that those who are working in this field should 

 realize that reactions due to rapid changes in the intensity of light 

 or any other form of energy were first recognized as being typically 

 different from the cases of animal tropisms by the author of this 

 latter theory; and that if new cases of Unterschiedsempfindlichkeit 

 are found this does not contradict the existence of the reactions of 

 the type of tropisms any more than the existence of the make and 

 break contractions in a muscle contradicts the existence of electro- 

 tonic effects in the same organ; or the existence of accelerations 

 contradicts the existence of velocities. 



* Although Jennings has attacked my views for years he is certainly not familiar with my papers. 

 This is also evident from his erroneous statement of the theory of tropism on p. 94 of his paper, 

 published by the Carnegie Institution in 1904. 



