152 'Jacques Loeh 



The reader will readily notice that I did not attempt to show 

 that all animal reactions are of the type of tropisms; on the con- 

 trary, I was, as far as I am aware, the first to point out that there 

 exists a type of reactions which are as different from tropisms as 

 are quantities of the dimension of an acceleration from those of 

 the dimension of a velocity. My aim was to analyze the behavior 

 of animals from a physico-chemical point of view and substitute 

 the methods of modern science for the anthropomorphisms of the 

 metaphysician. In this attempt it made no difference to me 

 whether the elementary components of the complex "Animal 

 Behavior" were found to be of the type / (z), {e. g., tropisms) or 



of the type / I _ - 1 (^. g., Unterschiedsempfindlichkeit) or of any 



other definite function. Moreover, I laid emphasis on the fact 

 that it is necessary to control the animal reactions before explain- 

 ing them, as only the control of the reactions offers a sufficient 

 test for the correctness of our analysis. From this point of view 

 I stated that for the control of heliotropic reactions the intensity 

 of the light may remain constant during the experiment, while 

 for the control of the reactions of the type of Unterschiedsemp- 

 findlichkeit the intensity of the light must change with a certain 

 rapidity during the time of the experiment. 



Whether it is due to mere carelessness or some other cause, a 

 number of American authors have disregarded this discrimination, 

 making their readers believe that the cases of Unterschiedsemp- 

 findlichkeit are represented by me as examples of tropisms and 

 then showing that the facts do not coincide with what they state 

 to be my theory of tropisms. I will give a definite instance of 

 this procedure. 



In 1893 I described a case of Unterschiedsempfindlichkeit in a 

 tubicolous worm at Naples, Serpula uncinata, showing that when 

 the rapidity of the decrease of the intensity of light reaches a cer- 

 tain value a contraction of this worm is caused, while an equally 

 rapid increase in the intensity of light causes no such reaction. 

 In later writings I especially singled out this reaction to illustrate 

 the typical difference between Unterschiedsempfindlichkeit and 



