14 DEVELOPMENT AND HEREDITY. 



evolution among animals and plants, yet there is 

 still a large class of phenomena in the nervous 

 activity and psychic life of organisms, which is barely 

 taken into account in summing up the conclusion 

 as to the essential nature of evolution. That this 

 latter class of phenomena should not contradict the 

 conclusions drawn from the bodily structure, was 

 apparently all that could be expected from it. But 

 the vast importance which intelligence, instinct, and 

 reflex nervous activity have for organisms, demands 

 for these phenomena a more careful study, — 

 especially in regard to the light they shed upon the 

 ultimate nature of living matter, and consequently 

 upon its developmental activity. 



It is really just among the phenomena of this 

 class that we shall find the clearest proof of the 

 inheritance of acquired characters, as I shall en- 

 deavour to show. 



From the results obtained by the science of 

 physiological psychology it is evident that what 

 we know as psychical development must have its 

 correlative physical development. Nor must we 

 forget that even in the higher realms of psychical 

 life these two terms merely express different views 

 of the same thing. It has been sometimes supposed 

 that psychic activity is the antecedent cause of 

 growth changes in the nervous matter of the brain. 



