3 1 



Having thus far briefly considered heredity in its physio- 

 logical aspect, I now purpose to glance at its psychological 

 bearings and influence, and shall first premise that between 

 physiology and psychology there exists not only an intimate 

 correlation, but an exact parallelism. The brain is the 

 organ of the mind, and thought is but the result of brain- 

 action ; the brain as regards its size, structure, and qualities 

 is hereditarily transmissible ; therefore, we are warranted 

 in asserting that every mental or intellectual state is con- 

 ditioned by a pre-existing physiological state, and that 

 psychological heredity has its source in physiological 

 heredity. That this is so is proved by the fact that the 

 nervous system is the physical condition and antecedent of 

 those phenomena denominated mental and moral ; and that 

 their interaction and interdependence is such that every 

 variation in the one necessitates a corresponding variation 

 in the other: in other words, between the phenomena, 

 arranged into two groups, as physiological and psychological, 

 there are intimate relations, whether of "invariable co- 

 existence or of invariable succession." The one set we 

 call physiological, when we regard them through the senses and 

 from without ; the other psychological, when we regard them 

 through the consciousness and from within. Just as it 

 would be impossible to think without a brain, so every 

 mental effort or condition necessitates a corresponding 

 state of the nervous system, and every state of the nervous 

 system re-acts proportionately on the great nerve-centres. 



Dr. Carpenter says : " Every kind of activity peculiar to 

 a living body involves a change of structure : and the 

 formation of the newly-generated tissue receives such an 

 influence from the conditions under which it originates, that 

 all its subsequent activity displays the impress. Hereditary 



