BEHAVIOR OF UNICELLULAR ORGANISMS. 97 



We have in thigmotaxis one of the fundamental reactions of 

 Paramecium, not further analyzable into simpler component reac- 

 tions. As it seems to consist chiefly or entirely of a cessation 

 of a part of the usual ciliary motion, only the cilia in the 

 oral groove continuing to strike strongly backward, it may 

 be more philosophical to consider this partly resting condition 

 as the " normal" condition, the usual forward motion being 

 then considered a reaction to a stimulus, due to a change or 

 removal of the solid body against which the animal is resting, 

 or to some other change in the environment. There seems to 

 be no decisive reason for considering either the condition of 

 partial rest or of the usual forward motion as more "normal" 

 than the alternative condition ; taking either as a starting 

 point, the other may be considered a response to a stimulus. 



If the Paramecia are placed upon the slide in pure water, 

 containing no bacterial zoogloea, or any other solid, they do 

 not even then remain scattered uniformly throughout the prep- 

 aration. On the contrary, it is usually not long before the 

 animals are gathered into one or more close groups in some 

 part of the slide. Paramecia are usually found in the culture 

 jars also aggregated into groups ; this, taken together with the 

 above experimental demonstration that Paramecia, at first uni- 

 formly scattered, will soon collect into close groups without 

 evident external cause, might be held to indicate the existence 

 of a " social instinct" among these creatures. Another possi- 

 bility suggests itself that there may be some invisible chem- 

 ical substance in the region of these groups by which all the 

 Paramecia are attracted ; so that the fact that they come near 

 together would be a secondary result of the fact that all are 

 attracted by the same substance. 



The main results of the extended study of the conduct of the 

 Paramecia toward chemicals, to which this possibility led, may 

 be given in a few words. It was found that Paramecia tend to 

 gather together and form collections in drops of weakly acid 

 solutions, and in solutions of some salts, while they avoid alka- 

 line solutions and solutions of the salts of the alkali metals. 



Among the substances into solutions of which they gather 

 is carbon dioxide. If a bubble of carbon dioxide is introduced 



