THE VITAL IMPETUS 149 



that it adopts a method of " trial and error." Let us 

 suppose a number of infusoria (Paramecium) in a 

 film of water, at one part of which is a drop of acetic 

 acid slowly diffusing out into the surrounding medium. 

 There is a zone of changing concentrations round the 

 drop : if we draw imaginary contours through the 

 points where the concentration is approximately the 

 same (the concentric rings in the diagram), and then 

 draw straight lines normal to these rings (the radial 

 lines) we can construct a " field " analogous to an 

 electric or magnetic field. The 

 animal on approaching the field 

 ought to orientate itself and 

 take the direction of the " lines 

 of force." It does not, how- 

 ever, behave in this way, but 

 only enters the field at random. 

 Having entered, it remains 

 within a part where the con- 

 centration is within certain 

 limits. If it approaches the fig. 19. 



margin of this limited field it 



stops, swims backwards, revolves round its own axis, 

 and then turns to the aboral side ; and it repeats this 

 series of movements whenever it approaches (by 

 random) a region where the concentration is too 

 high, or one where it is too low. In this, and other 

 organisms we see then what Jennings has called a 

 typical " avoiding reaction," the precise nature of 

 which depends on the " motor-system " of the 

 animal. Its general movements are random ones, 

 but having found a region of " optimum conditions " 

 (conditions which are most suitable in its particular 

 physiological state), it remains there. 



Suppose (what indeed repeatedly happens) that an 



