THE BEHAVIOR OF OTHER INFUSORIA 123 



strong salt solution collect in weak salt solutions or in tap water; 

 Paramecia in tap water collect in distilled water; Paramecia in dis- 

 tilled water collect in weak acids. In the same way, if two solu- 

 tions are open to any given infusorian, they tend to collect in that 

 one by which they are least repelled. Thus "attraction," as deter- 

 mined by the formation of collections, is a relative matter ; the infusoria, 

 like higher organisms, often have to put up with merely that by which 

 they are least repelled. To say that a certain infusorian gathers in a 

 given substance A, therefore, signifies little more than that it is less re- 

 pelled by this substance A , than by the substance in which it was found 

 at the time the experiment was tried. 



Most flagellates and ciliates are repelled by strong solutions of chem- 

 icals of almost all sorts. This is true even for strong solutions of the 

 same substances in which they collect when the solutions are weak. In 

 such substances we can therefore distinguish an optimum concentration. 

 Below the optimum the organisms are indifferent, while above the opti- 

 mum they are repelled. Expressing the facts more concretely, at the 

 indifferent concentration no reaction is caused when the organism passes 

 into the solution or out of it ; at the optimum concentration no reaction 

 is caused when the organism passes into the solution, but the avoiding 

 reaction is induced on passing out, while at concentrations above the 

 optimum the organisms react at passing inward. The result is then in 

 every case that they tend to gather in the optimum. 



The reaction is in each case caused by a change from one concentra- 

 tion to another. The amount of change necessary to cause the reaction 

 has been shown, in the case of fern spermatozoids (Pfeffer, 1884), to bear 

 a definite relation to the concentration of the solution in which the organ- 

 isms are immersed. In other words, the amount of change necessary 

 to cause the reaction varies according to Weber's law. Thus in the fern 

 spermatozoids the concentration of malic acid necessary to produce a 

 collection of the organisms must be about thirty times that in which the 

 organisms are already immersed. 



Massart (1891) found that specimens of Polytoma uvella in his cultures 

 were not repelled by chemicals even in the strongest solutions. Such 

 cases are very exceptional ; other investigators have found that even this 

 same organism (from other cultures) is repelled by various chemicals 

 (Pfeffer, 1904, p. 808, note). 



The variability and inconstancy of the reactions of infusoria to 

 chemicals deserves emphasis. Whether infusoria of a given species 

 react to a certain chemical or not, and how they react, depends upon 

 the past and present conditions of existence of the individuals. The 

 general outlines of the reactions can be determined for any species, but 



