112 HEREDITY 



For instance, medical men now believe, as we have 

 seen, that consumption is not hereditary, but, on the 

 other hand, that the " tendency " to consumption, or 

 the consumptive " diathesis," or the higher degree 

 of susceptibility to the attacks of the tubercle 

 bacillus, is hereditary. On every ground the dis- 

 tinction is well worth making. In the first place, 

 it now appears that there is no inevitable curse 

 pronounced upon the child of the consumptive. He 

 probably inherits from his parent that special degree 

 of susceptibility which proved fatal to that parent. 

 But he has not inherited the disease itself. And 

 let us recall the chapter on the relative importance 

 of heredity and environment. The exceptional sus- 

 ceptibility to tuberculosis is only a potentiality, and 

 the environment has the power of developing, or sup- 

 pressing, or modifying it. Thus the child of the 

 consumptive may live in such an environment — I 

 am tempted to say that environment in this place 

 practically means open bedroom windows — as to 

 suppress this potential susceptibility, and he may 

 indeed, though he has inherited this malign poten- 

 tiality, acquire a high degree of immunity. I speak 

 of the specific case, not of all microbic diseases. 

 But it is evident that the practical problem for 

 the child of the consumptive is not patiently to 

 await an inevitable doom, but to overcome heredity 

 by environment — in his case notably the atmospheric 

 environment. 



In the second place, the distinction between the 

 inheritance of a disease and the inheritance of a 

 susceptibility to the attacks of an external cause 

 of disease is worth making, because of its bearing 



