BEHAVIOR OF UNICELLULAR ORGANISMS. 103 



cause the reflex. Now, after passing across the drop it comes 

 again to the boundary where, if not stopped, it would pass out 

 again into the surrounding fluid. At this boundary there is, of 

 course, another change in the environment — a change in the 

 opposite sense from that experienced in passing into the drop. 

 Whether this second change will cause the reflex is of course 

 likewise impossible to predict, since it depends upon the nature 

 of the organism ; as a matter of fact we find that it does cause 

 the reflex. The Paramecium is, therefore, returned into the 

 drop and kept there in the manner already described. It 

 seems probable that the physiological condition of the Para- 

 mecium is changed by immersion in the drop of acid, so that 

 contact with the culture fluid now acts as a stimulus, though it 

 before did not. It seems not impossible to conceive, however, 

 that even without such a change in physiological condition, an 

 environmental change from b to a might cause a reaction, when 

 the opposite change, from a to b, would cause none. This has, 

 as is evident from the nature of a reflex, no necessary relation 

 to the comparative actual mechanical difficulty in passing in 

 one direction or the other. 



The one effect of a marked chemical stimulus on Paramecium 

 is, then, to produce the characteristic reflex already described, 

 and the apparent attraction or repulsion is determined by the 

 fact that some chemical substances or chemical changes cause 

 the reaction, while others do not. 



Now, experimentation with stimuli other than chemical leads 

 to the highly important observation that this same reflex is 

 produced by stimuli of the most varied nature. Substances 

 which seem to act upon Paramecium only through their osmotic 

 pressure, such as solutions of sugar, cause the same reflex ; 

 tonotaxis, then (to use the name employed by Massart), acts 

 through the same reflex as does chemotaxis. Mechanical stim- 

 uli, produced by jarring the preparation, cause the same reflex. 

 Heat and cold act in the same way, and the Paramecia avoid 

 hot or cold areas and collect in regions of optimum temperature 

 in exactly the same manner as they avoid certain chemicals and 

 collect in others. 



We are driven, therefore, to the conclusion that chemotaxis 



