2 86 HEREDITY AND DISEASE 



This is evident from the successful results of modern preventive 

 medical practice in regard to consumption. 



Statistics showing that in one sanatorium 35 per cent, of the 

 tubercular cases belonged to tubercular families, in another 38 

 per cent., and so on, are not of great theoretical interest. The 

 reappearance is due, in the first place, to the inheritance of the 

 constitutional predisposition — i.e. of a bodily soil very open to the 

 entrance of the weed, very suitable for its culture, very weak in 

 the power of resisting its ravaging growth. The reappearance is 

 due in the second place to the too common persistence of functional 

 and environmental conditions favourable both to infection and to 

 the enfeeblement which means defeat. It is enough to allude to 

 the lack of fresh air and exercise. It is an old story, told in many 

 forms and very true, that one boy of a tubercular family went to 

 sea and alone escaped the doom which befell his brothers and 

 sisters. Nor are cases unknown where a return in imagined security 

 to the old home in the town, and to the sedentary life of a clerk, 

 has resulted in belated but fatal infection. In the third place, we 

 have to bear in mind the likelihood of one member of a family 

 infecting another with the tubercle bacillus. 



But besides the transmission of a constitutional vulnerability, 

 besides the rare occurrence of ante-natal infection, besides the 

 likelihood of household infection, besides the persistence of con- 

 ditions of life which favour the disease — are there any other factors ? 

 There are probably two others. On the one hand, a seriously 

 tubercular mother may be unable adequately to nourish her offspring 

 before and after birth, and the ill-nourished offspring becomes the 

 more readily the prey of disease. On the other hand, it seems 

 likely that the bodily disturbances induced by tubercular disease 

 in the parents may prejudicially affect the vigour of the germ-cells 

 themselves, and thus lead to the production of inferior offspring. 



Syphilis. — As this disease appears to be due to a specific microbe, 

 its reappearance in the offspring of syphilitic parents is not strictly 

 a fact of inheritance. The father may infect his offspring without 

 the mother being affected, and it is possible that the microbe may 

 enter the ovum with the spermatozoon. The father may affect 

 his offspring indirectly by first infecting the mother — that is, the 

 microbe may pass through the placenta into the child. In certain 

 cases — e.g. when conception occurs soon ^fter the date of the primary 

 disease — the probabilities of the offspring being infected are greatj 



