52 The Animal Mind 



but differing degrees of a reaction whose essential features are 

 the same. 



While Paramecium definitely avoids by means of this neg- 

 ative reaction certain chemicals introduced into the water, it 

 shows a tendency to collect in the neighborhood of others. 

 Such is the case with weak acids, with a bubble of oxygen if 

 air has been long excluded from the slide, and with carbon 

 dioxide, which in water of course produces acid (214). Jen- 

 nings pointed out that the inclination of Paramecium to 

 gather in groups is very likely due to the attraction for them 

 of the carbon dioxide which they excrete. But he has also 

 shown that this "attraction" to certain chemicals does not 

 mean the presence of a special positive reaction. The fact 

 is that when the animals collect in a drop of weak acid, for 

 example, they are not drawn .toward the acid. They simply 

 happen, in their ordinary movements, to swim into it, and on 

 entering it show no disturbance whatever. But when they 

 come to the edge of the drop on their way out, they give the 

 negative reaction to the surrounding water. In this way they 

 are, as it were, trapped within the drop. 



The nearest analogue to a positive reaction in Paramecium 

 consists in the fact that sometimes, when they come into 

 contact with a solid, instead of darting backward, the animals 

 merely cease moving, and extending stiffly the cilia which 

 touch the object, remain at rest (Fig. 5). The utility of 

 this behavior is that around decaying vegetable matter, the 

 kind of solid oftenest found in the animal's ordinary environ- 

 ment, there is apt to be a supply of food in the way of bacteria ; 

 it is a good anchorage. What characteristics of the stimulus 

 determine that this "contact reaction," rather than the 

 negative reaction, shall be given? Does weak mechanical 

 stimulation occasion it, as happens with Amoeba's positive 

 reaction ? Evidence in favor of this is offered by the fact that 



