56 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



or stimuli cease to operate an influence showing 

 itself in the growth of habit. A good example of 

 this is seen in the behavior of Convoluta roscoffensis, 

 a small, wormlike animal living on the coast of 

 Brittany and leading a life closely dependent on the 

 tides. With ebb tide the Convoluta show themselves 

 at the surface, where they form large green patches. 

 As the flood tide covers them, they seek protection 

 by subsidence in the sand. The effect of this 

 rhythmical stimulation shows itself in the singular 

 fact that when kept in an aquarium, away from 

 all tidal action, they continue for a short period to 

 rise and sink in the sand in tune with the ebb and 

 flow of the tide. Similar examples of habit or 

 mnemic action are seen in plants. In sleeping 

 plants the leaves take a distinctly different position 

 at night from that which they take by day; they 

 have, in fact, a habit or memory based on the alter- 

 nation of night and day. But if the sleeping plant 

 be placed in a dark room after its leaves are in the 

 sleeping position, the leaves will take their light 

 position despite the fact that they are screened from 

 the stimulus of light. Here, then, we have clear 

 illustrations of habits or memory traces which may 

 be revived in the absence of the stimuli which 

 originally formed these engrams, or, if not in the 

 complete absence of these stimuli, when they are 

 much feebler than before. Habit is, in fact, nothing 

 more than a faculty acquired by frequent repetition 

 of responding to a mere fraction of the original and 



