50 PLANT-ANIMALS [CH. 



or accessary stimulus in the absence of the prin- 

 cipal. 



Applying this hypothesis to background reaction, 

 we assume that dark background, from constant 

 association with unilateral light, has come to suffice 

 to stir up the mneme and so to set going the nervous 

 apparatus which induces a precise muscular move- 

 ment. Thus, C. roscoffensis, placed on a dark back- 

 ground, begins to crawl about and continues to do 

 so. That this interpretation of the origin of back- 

 ground reaction, contains something of the truth 

 seems probable from the fact that the movements 

 performed by the animals on a dark background are, 

 compared with the business-like, phototropic move- 

 ments, aimless as to direction. They are non-directive, 

 chance movements ; but since they continue so long 

 as the dark background is there to call them forth, 

 they conduct the animals sooner or later to the Avhite 

 ground of the particoloured vessel. Arrived there, 

 the stimulus ceases from troubling and C. roscoffensis 

 is at rest. 



We conclude, therefore, that background, from 

 being a mere attendant circumstance, an environ- 

 mental accessory to unilateral light, has come itself 

 to serve as a stimulus to movements which, by the 

 devious paths of chance, direct the animals to the 

 lighter ground. 



In nature, under all ordinary conditions, back- 



