The Mind of the Simplest Animals 



41 



preceding one. If the whole of one side or end receives a 

 strong stimulus, if light falls on one side, or an electric 

 current is passed through the water, the side stimulated 

 — in the case of the electric current, the side toward the 

 positive pole — contracts as a whole, and the movement 

 takes place in the opposite di- 

 rection. These phenomena 

 constitute the negative reaction 

 (Fig. i). 



Secondly, the reaction to 

 solid bodies sometimes takes a 

 positive form. In this case a 

 pseudopodium is pushed for- 

 ward in the direction of the 

 stimulus, and the animal moves 

 toward the solid. As the neg- 

 ative reaction serves the pur- Fig- i- — Negative reaction of 

 _ c -J- -L i 1 Amoeba to stimulation by a glass 



pose of aVOldmg obstacles, so ,„d. a. Application of the stim- 



the positive reaction is useful ulus. h. change of direction 



in securing contact with a sup- "f^^^ovement. After Jemiings 



port on which to creep, and 



with food. It seems to be given in response to weak 

 mechanical stimuli, stronger ones producing the negative 

 reaction. No chemicals have been found to occasion 

 it, but weak chemical stimulation very likely cooperates 

 with mechanical stimulation when the positive reaction is 

 given to food. 



Schaefifer (659) has recently obtained evidence that 

 Amoeba can give the positive reaction to insoluble and 

 inedible objects before they come into contact with it. 

 The way in which such objects can act as stimuH is 

 ^till unexplained. It is possible that the movement of 

 the Amoeba produces water currents which are reflected 



