THE INHERITANCE OF DISEASE iii 



(c) Reappearance of Disease. 



A disease may be acquired by the parents, but the re- 

 appearance of the same disease in the children by no means 

 signifies that it is inherited. It was said of tuberculosis 

 that it is strongly inheritable, because it was found that 

 children of phthisical parents were very subject to the 

 ravages of that disease. Now, congenital tuberculosis is 

 very rare. From this alone it is evident that tuberculosis, 

 as such, is not easily transmitted. If the children of affected 

 parents show the disease so frequently, it may well be due 

 to the fact that they are constantly exposed to the con- 

 tagion, not forgetting that they may have inherited a bias 

 towards the tubercular infection. We must carefully dis- 

 tinguish between a disease re-acquired by each successive 

 generation and one that is inherited in the strict meaning of 

 the word. 



(d) Secondary Effects of Disease. 



A mistake similar to that last mentioned is often made 

 by confusing the inheritance of a disease with certain 

 secondary effects of the same in the offspring. Great dis- 

 turbances of the parents' health through chronic ailments, 

 cumulative slow poisons, etc., are bound to affect the germ- 

 cells unfavourably, so that the progeny arising from them 

 will grow up weak, or even diseased. Parents with a strong 

 neurotic taint or addicted to alcoholism will often beget 

 children liable to mental and nervous disorders. The re- 

 semblance between the condition of the parents and that 

 of the children would seem to point to an inheritance of the 

 disease ; but as the progeny of such parents may show a 

 variety of ill effects produced by the parents' diseased state, 

 the resemblance to the parental disease is only an accidental 

 one. An unstable condition of the germ will result very 

 often in a deranged mental and nervous equilibrium of the 

 growing individual. 



