Concernuig the Theory of Tropisnu 153 



tropism. To quote from a paper I published in 1897.'' "I noticed 

 in the course of my investigation that besides the heliotropic 

 effects there exists a second kind of mechanical effects of light 

 which is determined by the rapidity of the change of the inten- 

 sity of light and which I designated as Unterschiedsempfindlich- 

 keit * * * I found this type of reaction in tubicolous annelids- 

 e. g., Serpula uncinata. The gills of these animals protrude from 

 the tube. If we move our hand between the animal and the 

 source of light it rapidly withdraws into its tube as soon as the 

 shadow strikes it. In order to find out whether positive and nega- 

 tive changes in the intensity of light have the same effect I made 

 the following experiment: A glass aquarium which was covered 

 with a glass plate was placed on a table, about two meters from the 

 window. When I rapidly closed the shutters the worms rapidly 

 withdrew into their tubes, as a snail would upon a sudden touch. 

 The shutters did not close tightly and it was sufficiently light in 

 the room to observe the animals. If one waited a little the animals 

 again stretched their gills out of the tube. When now the shutters 

 were rapidly opened, no reaction on the part of the animals occurred . 

 When they were inside the tube the opening of the shutters did not 

 cause them to reappear. It is, therefore, only the decrease in 

 intensity which acts as a stimulus upon the animals." 



In the same paper I pointed out that in the physiological effect 

 of the galvanic current physiologists discriminate between the 

 effects dependent upon a constant current and the effects which 

 depend upon the rapidity of the changes in the intensity of a cur- 

 rent; and I showed that these differences correspond to the 

 differences between a tropism and Unterschiedsempfindlichkeit. 



Since inductive effects depend upon the value of — the funda- 



at 



mental importance of the discrimination between a tropism and 



Unterschiedsempfindlichkeit is at once obvious. Since the excised 



muscle is above suspicion of possessing a human soul it might be 



inferred that for the understanding of the corresponding type of 



reactions of lower animals physico-chemical data might suffice. 



^Pfliiger's Archiv. vol. 66, p. 439. 



