HEREDITY 159 



vibratory movement or reaction in the nerves and 

 nerve-cells of the aforesaid organs ; indeed, it is 

 scarcely more, if at all, surprising that a transmission 

 of this kind should take place in the case of a part of 

 the nervous system which is normally hypersensitive 

 than in that of a part which under pathological 

 conditions is frequently so. It is true that reaction 

 in the latter instance may sometimes be caused 

 indirectly through the altered condition of the blood 

 or in other ways, but this does not apply to all cases. 

 Thus when, as sometimes happens, a sensitive person 

 will cough or sneeze at the mere opening of a door, 

 we must attribute this to an excess of sensibility or 

 readiness to vibrate in certain portions of the nervous 

 system — in other words, the effect produced is primarily 

 dynamic. So perfect a conducting medium is the 

 nervous system that no form of transmission need be 

 a cause for wonder. The well-known operation of 

 Paul Bert on a rat's tail and many other instances 

 prove the truth of this statement, and the conditions 

 being — as it is clear they are — permanently favour- 

 able, we may be sure that a channel for the passage 

 of what one may term hereditary impulses always 

 exists. 



Dynamic effects proceeding from various parts of 

 the body would each bear a certain character 

 depending on the conditions which gave rise to them, 

 since force is always influenced to some extent by the 

 matter through which it passes or from which it 

 issues. The study of sound furnishes us with some 



