THE BEHAVIOR OF AMCEBA 23 



ously, and always affect a certain side of the organism, while this is not 

 true of the other agents. 



If an intense stimulus acts on the entire surface of Amoeba at once, 

 the animal contracts irregularly and ceases to move. If the acting agent 

 is very powerful, the Amoeba may remain contracted till it dies ; other- 

 wise it usually soon begins locomotion again. 



We may classify the various changes in behavior due to stimulation 

 into three main types, which may be called the positive reaction, the 

 negative reaction, and the food reaction; these have already been de- 

 scribed in detail. These types are not stereotyped; each varies much 

 in details under different conditions. The movements in these reactions 

 are clearly not the direct results of the simple physical action of the 

 agents inducing them (see Jennings, 1904 g). As in higher animals, so 

 in Amoeba, the reactions are indirect. The effect of external agents is 

 to cause internal alterations, and these determine the movements. It 

 is therefore not possible to predict the movements of the organism from 

 a knowledge of the direct physical changes produced in its substance by 

 the agent in question. 



What decides whether the reaction to a given stimulus shall be posi- 

 tive or negative? This question touches the fundamental problem of 

 behavior. The nature of the physical or chemical action of an agent 

 does not alone determine the reaction, for to the same agent opposite 

 reactions may be given, depending on its intensity, or upon various 

 attendant circumstances. If we should make a chemical or physical 

 classification of the agents affecting movement in Amoeba, this would 

 not coincide with a classification based on the reactions given. But the 

 agents which produce a negative reaction are in general those which 

 injure the organism in one way or another, while those inducing the 

 positive reaction are beneficial. Any agent which directly injures the 

 animal, such as strong chemicals, heat, mechanical impact, produces 

 the negative reaction. The positive reaction is known to be produced 

 only by agents which are beneficial to the organism. It aids the animal 

 to find solid objects on which it can move, and is the chief factor in 

 obtaining food. Thus the behavior of Amoeba is directly adaptive ; it 

 tends to preserve the life of the animal and to aid it in carrying on its 

 normal activities. 



It may perhaps be maintained that certain reactions are not adap- 

 tive ; for example, that to the electric current. The reaction in this case 

 does not tend to remove the organism from the action of the stimulat- 

 ing agent. But it is instructive to imagine in such a case an organism 

 with possibilities of high intelligence — say even a human being — 

 placed under similar conditions, with similar limitations of sense and 



