110 HEREDITY 



fairly stated — the exceptions being negligible — that 

 the new-born baby is "aseptic," "sterile," or "germ- 

 free," even though one or both of its parents may 

 be subjects of microbic disease. This assertion is 

 tantamount to a denial that the inheritance of disease 

 is a fact of any importance or frequency. 



But now we must proceed to make a most im- 

 portant qualification of this very important state- 

 ment — a qualification which enables us to answer 

 the very just criticism that, despite our theories, 

 germ-diseases are daily seen with most suggestive 

 frequency in parents and their children. It is true 

 that this criticism is largely irrelevant ; for the ex- 

 planation of the occurrence of the same disease in 

 parents and children often is that the children have 

 become infected by their parents. A recent writer 

 even inclines to the view that the apparent inherit- 

 ance of some factor — immediately to be considered 

 — in tuberculosis is largely to be accounted for by 

 the fact that the children of tuberculous parents 

 are so gravely exposed to infection. Nevertheless, 

 there doubtless is a factor in tuberculosis, and the 

 many diseases of which we have taken it as the type, 

 which is inherited — the factor of susceptibility. 



In all microbic diseases there are two cardinal 

 factors : the presence of the causative parasite, and 

 the receptivity of the host. Or, if we figure the 

 disease as a crop or harvest, we may say that it 

 depends upon the mutual fitness of the seed and the 

 soil. This becomes apparent directly we accept the 

 germ theory of disease. A cat may suffer from diph- 

 theria, and so may a baby: either may infect the 

 other. But no cat ever caught scarlet fever from 



