48 Heredity. 



forming work within they cannot, at the same time, perform work 

 without; and 'as the nerve cannot produce electricity without 

 using up something, the ultimate source of the forces which the 

 nerve transforms into electricity is the materials furnished by 

 the blood. The nerve is nourished with these materials, as the 

 pile is fed with zinc and acid.' l Thus perception that is to say, 

 the primary phenomena of consciousness comes under the general 

 law. It is impossible that it should come of nothing. We daily 

 experience thousands of perceptions, but none of these, however 

 vague and insignificant, can perish utterly. After thirty years some 

 effort some chance occurrence, some malady may bring them 

 back ; it may even be without recognition. Every experience we 

 have had lies dormant within us : the human soul is like a deep 

 and sombre lake, of which light reveals only the surface ; beneath, 

 there lives a whole world of animals and plants, which a storm or 

 an earthquake may suddenly bring to light before, the astonished 

 consciousness. 



Both theory and fact, then, agree in showing that in the moral, 

 no less than in the physical worlfl, nothing is lost. An impression 

 made on the nervous system occasions a permanent change in the 

 cerebral structure, and produces a like effect in the mind whatever 

 may be understood by that term. A nervous impression is no 

 momentary phenomenon that appears and disappears, but rather a 

 fact which leaves behind it a lasting result something added to 

 previous experience and attaching to it ever afterwards. Not, how- 

 ever, that the perception exists continually in the consciousness ; 

 but it does continue to exist in the mind, in such a manner that it 

 may be recalled to the consciousness. 



It is not easy to say what it is that survives our perceptions and 

 ideas. The least objectionable name for it is residuum, a term 

 which does not imply any theory, because it only indicates an 

 unquestionable fact of our mental life. It is not to be supposed 

 that these residua are always present to the mind, so that the 

 attention can at any moment be voluntarily directed to them. 

 But it may be assumed that every mental act leaves in our physical 

 and mental structure a tendency to reproduce itself, and that when- 



1 Wunclt, Menschen- und Thicrsede, 5th and 6th Lectures. 



