84 DEVELOPMENT AND HEREDITY. 



the state of feeling, increasing or decreasing the 

 vitality or general nervous activity of the animal. 

 This statement holds true in so many cases, and 

 in such variety of cases, that it may be regarded 

 as a general law. It means that whether or not 

 the physical change results in any immediate action 

 on the part of the animal, there remains recorded 

 in its nervous mechanism some change that will 

 affect future action, either as memory or as the 

 beneficial result of practising some nervous co- 

 ordination, thus making future action easier, or else 

 as a generally weakening or intensifying effect on 

 the nervous activity. We know this statement to 

 be true, in cases where the nervous system itself 

 has been affected, and if we believe that the nervous 

 system has been evolved from undifferentiated 

 living matter, then we must accept the statement 

 in its broader application to all living matter. 



While we have no exact knowledge of the nature 

 of the molecular changes in the nervous system, nor 

 can we in any instance form a clear conception of 

 how they succeed each other in the sequence of 

 stimulation and reaction, yet still we may gain a 

 certain knowledge of the laws of their operation 

 by studying the psychical changes which accompany 

 the physical changes. We have in our own experi- 

 ence and daily observation a multitude of facts, 



