40 The Animal Mind 



fluid where the tension of the surface is diminished in front, 

 i.e., at the point toward which the drop, in consequence of 

 the diminished tension there, rolls. Such movements, 

 Rhumbler shows, can be reproduced by placing, say, a 

 drop of clove oil under the proper conditions of surface 

 tension (632, 633). Jennings, on the other hand, has 

 observed, at least in certain species of Amoeba, that the 

 protoplasmic currents are all forward in direction, the 

 movement being really one of rolling, complicated by the at- 

 tachment of the lower part of the body to the solid object 

 on which the animal crawls. Mechanical conditions of 

 surface tension would not account for such currents (371, 

 373, 378). Bellinger rejects both the surface tension and 

 the "rolling" theories, and from a study of side views of the 

 moving Amoeba concludes that progression occurs through 

 the advancement of the front end freely through the water 

 and its subsequent attachment, the rest of the body follow- 

 ing through active contraction brought about by a con- 

 tractile substance (181). The problem is of great interest 

 to the student of vital phenomena, but its bearing on the 

 question of mind in the Amoeba is so obscure that we 

 need not consider it further, but may pass at once to the 

 study of the animal's reactions to special stimulation. 



These are, according to Jennings (373, 378), the foremost 

 authority on the behavior of the lowest organisms, three in 

 number ; namely, the negative, the positive, and the food- 

 taking reactions. First, if an Amoeba comes into strong 

 contact with a solid obstacle in its movements, or if a 

 solution of different composition from the water in which it 

 lives strikes against it, or if one side of it is heated, the 

 animal responds by contracting the part stimulated, re- 

 leasing it from the substratum, and moving in another 

 direction, usually one forming only a small angle with the 



