396 Evolution and Adaptation 
If a drop of weak acid be put into the water in which the 
parameecia 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 parameecia 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 (z.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 paramcecium 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 
paramcecium 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 
