MNEMIC THEOKY OF HEREDITY 191 



not only of receiving and responding immediately to stimuli of 

 various kinds, but also of storing up impressions or engrams for 

 future use. This is demonstrated quite clearly by the phenomena 

 known as " after-effects " in various plants and animals. One of 

 the best known examples of such after-effects is seen in the daily 

 periodicity of plant growth. The light of the sun acts as a 

 restraining or inhibiting stimulus upon the rate of growth of 

 ordinary plants. In consequence of this the plant grows most 

 rapidly in the early hours of the morning after a prolonged 

 exposure to darkness, and most slowly in the afternoon, after 

 long exposure to daylight. It has been shown that this daily 

 periodicity, or variation in rate of growth in correspondence with 

 the periodic variation in environment, is continued when the 

 plant is kept in perpetual darkness, and the direct stimulation of 

 changing environment thereby rendered impossible. In other 

 words, the plant, though entirely devoid of any nervous system 

 in the ordinary sense of the term, establishes a habit, which must 

 depend upon something analogous to memory on its part. 



The mnemic theory of heredity is based, as we have seen, upon a 

 comparison of the phenomena of inheritance with those of memory. 

 The latter are reasonably explained by supposing that impressions 

 received by certain cells of the brain may be stored up as 

 " engrams " in these cells for use on future occasions, when, 

 under appropriate stimulation, they give rise to mental condi- 

 tions corresponding to those produced by the original stimuli. 

 The stimulus which evokes a memory is commonly the 

 repetition of some stimulus which was originally associated 

 with the thing remembered. Thus the gight of a person whom 

 we have not seen for a long time, or even of his portrait, will 

 evoke a whole train of memories connected with that person. 



The degree of accuracy with which we remember any occurrence 

 depends partly upon the nature of the occurrence itself and 

 partly upon the frequency with which it has been brought under 

 our notice. The first time we take a walk we may have to pay 

 attention to every turning and every signpost in order to find our 

 way at all, but if we take the same walk many times we at length 

 become so familiar with it that we may be thinking of other 

 things the whole time and not consciously notice a single land- 

 mark from start to finish. As Samuel Butler has most forcibly 

 pointed out, the more perfect memory becomes the less is it 

 accompanied by consciousness of the things remembered, as when 



