396 Evolution and Adaptation 



If a drop of weak acid be put into the water in which the 

 paramcecia are swimming, — for instance, in the water be- 

 tween a cover-slip and a slide, — it will be found, after a 

 time, that many individuals have collected in the drop. It 

 was at first supposed that the paramcecia are attracted by 

 the diffusion of the acid in the water, and turn toward the 

 source of the chemical stimulus ; but Jennings has shown that 

 this is not the way in which the aggregation is brought 

 about. If the individuals are watched, it will be found that 

 they swim forward in a spiral path without regard to the 

 position of the drop of acid. If one happens, by chance, to 

 run into the drop, there is no reaction as it enters, but when 

 it reaches the other side of the drop, and comes into contact 

 with the water on this side, it suddenly reacts. It stops, 

 backs into the middle of the drop, rotates somewhat toward 

 the aboral side (i.e. away from the vestibule), and then starts 

 forward again, only to repeat the action on coming into con- 

 tact with the edge of the drop again. The paramoecium has 

 been caught in a veritable trap. All paramcecia that chance 

 to swim into the drop will also be caught, until finally a large 

 number will accumulate in the region. The result shows, 

 that, in passing from ordinary water into a weak acid, no 

 reaction takes place ; but having once entered the acid, the 

 animal reacts on coming into contact with the water again. 



On the other hand, there are some substances to which the 

 paramoecium may be said to be negatively chemotropic. If 

 a drop of a weak alkaline solution be put into water in which 

 paramcecium is swimming, an individual that happens to run 

 against it reacts at once. It stops instantly, backs off, 

 revolving in the opposite direction, turns somewhat to one 

 side, and swims forward again. The chances are that it will 

 again hit the drop, in which case it repeats the same reaction, 

 turning again to one side. If it continues to react in this way, 

 it will, in the course of time, turn so far that when it swims 

 forward it will miss the edge of the drop, and then continue 



