in] THE FACTORS OF DISTRIBUTION 83 



some other stimulus, or by some regulatory action 

 effected by the nervous system of the organism. The 

 flight of a moth into a candle flame is an example of 

 phototaxis, and so is the action of a bird when it dashes 

 itself against the glass of a lighthouse lantern. Now 

 probably these are acts which can fairly well be 

 described as inevitable taxic movements which the 

 animal must perform under the irresistible stimulus of 

 the light. So also the movement of an inebriate 

 towards a public-house door, when he had just been 

 given a sixpence, would fittingly be described as an 

 inevitable taxis or tropism. But it seems to be rather 

 straining after generality to attempt to describe all 

 the movements and migrations of organisms as 

 tropisms. This, for instance, is the kind of physiology 

 that is often taught. A certain caterpillar feeds on 

 the tender shoots at the upper extremities of a plant, 

 and to reach these it must migrate upwards towards 

 the light, for the plant grows so that it always bends 

 towards the source of light. The caterpillar is said 

 to be 'orientated' by the light falling on its bodily 

 surface and it must place itself so that the light strikes 

 equally on both sides of its body. Since it moves 

 upwards it is said to be 'positively phototactic.' But 

 having fed it now moves downwards again and it is 

 now said to be 'negatively phototactic/ the act of 

 feeding having reversed its reaction to light. Obviously 

 one might describe the movements of a hungry man 



62 



