THE CAUSATION OF DISEASE. I 83 



as a vitiated reversion, and every complicated disease is a 

 bundle of vitiated reversions. 



The 'principle of Sexual Heredity is an important one. Ac- 

 cording to it, characters acquired by one sex at a particular 

 period of life tend, in the offspring, to appear in the same sex 

 and at the same period of life. This principle is not only true 

 as regards sexual characters, but applies in some degree to 

 each and every acquisition, whether sexual or not. Characters 

 acquired before the period of sexual life show a tendency to be 

 inherited by either sex indifferently, though at corresponding 

 ages ; hence, the diseases in children are in only a small degree 

 influenced by sex. Characters acquired during the post-sexual 

 period of life can obviously have no effect upon the offspring. 



It is to be noted that the male sex takes on structural 

 characters more readily than the female. This should prepare 

 us for a greater variety of disease in the male. 



Characters can pass potentially through one sex to the 

 opposite sex in the offspring. In all such cases it is certain that 

 the potentialities are kept under by a restraining influence, on 

 the removal of which they are ever ready to become actualities. 



The principle of heredity at corresponding ages asserts that 

 characters acquired by one parent at a particular period of life 

 tend to develop themselves in the offspring at the same period. 

 The truth of this principle is amply illustrated by disease. 



Such is a brief consideration of the principle of heredity. 

 It has prepared us for the fact that heredity plays an extensive 

 role in disease. In a later portion of this work we shall be in 

 a better position to establish this statement. 



Having considered the two factors of disease, S and E, 

 separately, we must next study them together, disease being a 

 mutual inter-action of the two ; and, although this subject has 

 been already trenched upon in this chapter, it is advisable to 

 make some further allusion to it. 



The S differs considerably in its manner of responding to 

 the E. An E which will have no evil effect upon one indi- 

 vidual, will in another produce the most dire results. 



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