THE VITAL IMPETUS 



145 



thermal, or gravity fields. In all these cases the 

 factors affecting the movements of the organism are 

 directed ones. 



An electric field, for instance, (i), is produced by 

 placing the electrodes of a galvanic cell at opposite 

 extremities of a water-trough : we imagine the electrons 

 moving from one side of the trough to the other in 

 parallel lines, and in a certain direction. A light 

 field (2) would be produced by the radiation of light 

 travelling in straight lines through the water. 



The movements of the organism displaying a 



(2) 



FIG. 18. 



tropism or a taxis are not caused by the stimuli of the 

 field, but are only directed by it. In the absence of 

 these stimuli it would swim at random. In a field, 

 however, it will orientate itself in some direction with 

 reference to the lines of force. A " positively photo- 

 tactic ' animal swims towards the focus from which 

 the light radiation emanates, and a ' negatively 

 phototactic ' one swims in the other direction. On 

 the theory of tropistic and tactic movements this 

 orientation is produced by the differential stimulation 

 of the opposite sides of the organism. Let us take as 

 a concrete example the case of a caterpillar which 

 creeps up the stem of a plant to feed on the tender 

 shoots near the apex. The animal possesses an 



