ACTION OP THE NERVOUS SYSTEM. 77 



These latter experiments bring out a fact of 

 fundamental importance, namely, that in the con- 

 stitution of the plant there must arise an associa- 

 tion between the interval of time and the increase 

 of activity. This association can only be regarded 

 as of an elementary nervous or psychic nature. 



The facts in the case lead to the conclusion that 

 the greatest part of the molecular change, which 

 is brought about in living matter by the action 

 of external forces, is that change which occurs in 

 the nervous organisation. 



We have seen that the reactions of a piece of 

 metal, under a given strain, are governed by the 

 whole .previous history of the metal ; and the same 

 we see may be true of organisms. We see that 

 the changes in manner of growth, resulting from 

 changed conditions, have a tendency when long 

 continued, to become permanently ingrained in 

 the constitution of the organism. There are cases 

 where, the previous condition having been restored 

 after a long lapse of time, the changed manner 

 of growth still continued to show itself through 

 several generations : though finally, under the orig- 

 inal conditions of the race, in a few generations 

 the normal characteristics of the race reappeared. 

 It is well known that the same varieties of plants 

 — wheat, etc. — germinate and ripen in the short 



