18 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



lines of diffusion of the acetic acid orient the Paramecia 

 positively according to their direction, and that thus the 

 Paramecia reach the solution by simply swimming forward 

 after the orientation is completed. But that would be far 

 from the truth. Jennings found, on the contrary, by 

 observing the single individuals, that all the Infusoria swim 

 at random and enter the solution at random also, but that 

 then they are kept within the limits of a certain concentra- 

 tion of the diffusing acid by a very strange feature : as soon 

 as they reach those limits the passing of which would bring 

 them out of the region of the acid, they give a certain very 

 typical motor reaction, which makes them remain in the 

 region where they were. The reaction consists in a swim- 

 ming backward, combined with a revolution round the long 

 axis and a turning to the aboral side. 



And quite the same holds for " negative chemotaxis," as 

 happening, for instance, in the presence of a solution of 

 ordinary salt. All of the animals which by their ordinary 

 forward motion would reach the region of a certain con- 

 centration of the diffusing chloride of sodium, perform the 

 reaction just named in the very moment of entering this 

 region. Thus they never really penetrate to this region, 

 for the reaction may be repeated as often as necessary ; but 

 the few organisms which were in the region of the salt at 

 the beginning of the experiment may freely leave it. In 

 the end, of course, all the animals are out of range of the 

 solution, just as in " positive chemotaxis " all the animals 

 were in range. 



It must be granted that Loeb, in establishing what he 

 called " Unterschiedsempfindlichkeit," i.e. the reactions of 

 animals to differences of intensity say of light, came very 



