466 Jounial of Comparative Neurology and Psychology. 



1904, p. 625). The corresponding quantitative relations have 

 not been worked out for Paramecium. 



The fact tliat change is the essential feature in causing re- 

 action is of course correlated witb. the fact that organisms 

 become acclimatized, so far as reaction is concerned, to a cer- 

 tain strength of stimulus. To say that the organism becomes 

 thus acclimatized is indeed little more than to say that it reacts 

 only to changes. 



The change which produces stimulation may be a direct 

 alteration in the environment, as when a chemical is brought 

 near a specimen, or when it is touched at the anterior end with 

 a glass rod, or when the temperature is raised or lowered from 

 without. But under natural conditions the change is more usu- 

 ally produced h)' the movements of the animal itself. In its 

 rapid swimming the animal passes from one region to another, 

 the conditions in one region changing to those in the next, and 

 thus causing reaction. Further, as we ha\'e seen, the spiral 

 course gives opportunity for frequent changes to act upon the 

 organism; the anterior end is pointed successively in many di- 

 rections, receiving "samples" of water from each direction. 

 The greater the swerving in the spiral course the greater the 

 opportunity for frequent changes to affect the animal. The 

 avoiding reaction, with its swerving in many directions, may in- 

 deed be looked upon as a method of subjecting the organism 

 successively to many changes. 



It is, however, not mere change per se that causes the re- 

 action, but change of a certain kind or in a certain direction. 

 Of two opposite changes, one usually produces the reaction, 

 while the other does not. Paramecium reacts when it passes 

 out of a weak acid, not when it passes in ; it reacts when it 

 passes into an alkali, not when it passes out. A Paramecium 

 at 28° reacts at passing to a higher temperature, not at passing to 

 a lower one ; a Paramecium at 20° shows the opposite relations. 

 The direction of change which produces the avoiding reaction 

 may be briefly characterized as that leading aivay fnini the opti- 

 mum, while change leading toward the optimum produces none. 

 It is thus clear that in most cases the actual determining: factor 



