126 THE CAUSATION OF DISEASE. 



tend to be, for none of us can escape the inexorable law of 

 heredity. Though the influence of the Environment is great, 

 it may truly be said that we are what we are — great or small, 

 strong or weak — by virtue of heredity. But we have the 

 power to make ourselves better or worse. Wherefore, let 

 every potential parent remember that the strengthening or 

 weakening of his structure not only concerns himself, but 

 those who come after him — that his conduct of life may, in 

 other ways than by example and intellectual legacy, echo 

 through posterity. 



Seeing that disease is an abnormal inter-action of S and E, 

 and that S is almost entirely determined by heredity, it is 

 manifest that heredity must take an overwhelming share in 

 the causation of disease. But we shall the better be able to 

 appreciate this fact after certain other topics have been con- 

 sidered, and, therefore, the part played by heredity in disease 

 will be considered separately (see Part III). 



