66 



The Animal Mind 



cr 



positive reaction. The fact is that when the animafev. 

 collect in a drop of weak acid, for example, they are not \ 

 drawn toward the acid. They simply happen, tt\their 

 ordinary movements, to swim into it, and on leiiteriii^i^it 

 show no disturbance whatever. But when they ciame to 

 the edge of the drop on their way out, they give the nsga- 

 tive 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 some- 

 times, 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 decajdng vegetable matter, the kind of 

 solid oftenest found in the animal's ordinary 

 environment, there is apt to be a supply of 

 food in the way of bacteria; it is a good 

 What characteristics of the stimulus determine 

 that this "contact reaction," rather than the negative re- 

 action, 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 the con- 

 tact reaction is more likely to occur if the animal comes 

 against the solid when swimming rather slowly. Jennings re- 

 ports also that individuals vary. "Often all the individuals 

 in a culture are thus inclined to come to rest, while in an- 

 other culture all remain free-swimming, and give the avoid- 

 ing reaction whenever they come in contact with a solid" 

 (378, p. 60). This would suggest that some individuals 

 are in a state of greater excitability than others, so that a 



Fig. 5.— 



Positive 



thigmotaxis 



in Parame- 



dum. After 



Jennings 



(378). 



anchorage. 



