22O THE BEHAVIOR OF LOWER ORGANISMS. 



on others, thus lowering the surface tension in the region acted upon. 

 Thereupon the drop sends out a projection on the side affected, and may 

 follow this up by moving in that direction. We may cause the drop to 

 send out projections and move in a certain direction by placing a 

 minute drop of alcohol near one side, or by heating one side ; move- 

 ment takes place toward the side affected. These agents act directly 

 on the surface of the drop, lowering the tension ; the movements are a 

 direct consequence of this change. Parallel phenomena maybe pro- 

 duced, as Bernstein (1900) has shown, with a drop of quicksilver. If 

 such a drop is placed in 10 per cent nitric acid, in which bichromate 

 of potash has been dissolved, the drop changes form and moves about. 

 If we place near such a drop, in vessel of 10 per cent nitric acid, a 

 crystal of potassium bichromate, the mercury drop moves rapidly over 

 to the crystal. Here, again, the chemical acts directly on the mercury, 

 lowering the surface tension at the region where it comes in contact 

 with it, thus producing the movement. 



Certain authors have held that the movements of Amoeba are pro- 

 duced in this way by the direct action of external agents decreasing 

 or increasing the surface tension of certain parts of the fluid mass. As 

 an example of the theory of direct action of external agents in control- 

 ling the behavior, we may take the view of the reaction of Amoeba to 

 chemicals recently given by Rhumbler (1902, p. 384). According to 

 Rhumbler, it is evident that when an Amoeba moves toward or away 

 from a certain chemical, the side directed toward the chemical has, in 

 the first case, a lessened surface tension, in the second case an increased 

 surface tension, as compared with the remainder of the body. 



The necessary differences of tension on the positive and negative sides may be 

 easily understood from our present standpoint, by holding that a positively 

 acting chemical decreases the surface tension both in the living alveolar system 

 of the cell and especially on the cell surface, upon which it must work most 

 strongly; that a negatively acting chemical, on the other hand, produces an 

 increase of surface tension in the alveoli of the cell, and especially, again, on the 

 cell surface; this increase is the greater, and from a physical standpoint must be 

 the greater, the more the molecules of the chemical affect or modify the tension 

 of the different cell alveoli or different parts of the cell surface (/. c., p. 384). 



The explanation of thermotaxis and electrotaxis would be, according 

 to Rhumbler, "exactly the same as for chemotaxis" (/. c., p. 385) ; 

 thus also as a result of the direct action of external agents. A fuller 

 explanation of the " tropisms " on this basis is given by Rhumbler in 

 an earlier paper (1898, pp. 183, iSS).* 



* Rhumbler emphasizes in the paper just cited (1898, p. 184) the importance 

 of " inner disposition " in deciding what effect shall be produced by external 

 agents, but in the tropisms, at least, he considers the action of the external agent 

 to be direct, the inner disposition deciding merely whether the substance of the 

 Amoeba is of such a character as to admit of the production of a given definite 

 change in surface tension by the outer agent. 



