4 TUBERCULOSIS, HEREDITY 



Almost every fact recorded can be interpreted without 

 cautious inquiry as illustrative of the importance of any 

 one of these three factors, according to the a priori 

 conviction of the interpreter. For example, let us look 

 at two or three examples of ' family phthisis '. 



Figs. I and II give two pedigrees due to Klebs. At first 

 sight Klebs' pedigrees seem to mark inheritance, but it 

 is equally open to the environmentalists to assert that 

 in such cases the family has occupied the same house or 

 lived in the same village. The infectionist can assert 

 equally well that the infection has been handed down 

 from parent to child, generation by generation. If you 

 examine Klebs' cases a little more closely you will find 

 i6 cases of marriage of phthisical individuals, and only 

 two cases of both husband and wife being phthisical. 

 In fact, 62% of the tuberculous members lived to adult 

 life, of whom 45 % married ; only 12-5 % of their mates, but 

 79 % of their offspring were tuberculous. If the phthisis 

 in the children be due in the first place to infection 

 from parents or to a common environment, we should 

 anticipate that more than | of the mates of phthisical 

 individuals would themselves be phthisical. This in 

 Germany is very nearly the number of phthisical 

 persons occurring in the population at large. In the 

 third pedigree (Fig. Ill, p. 6) there are no phthisical mates, 

 and the tuberculosis has skipped now one and now 

 two generations. It is less easy to believe that the 

 infection factor here is the principal source of the 

 heavy mortality, as the members of the stock have lived 

 in widely separated homes and under very diverse con- 

 ditions, flying from the family curse. The environmental 

 factor can hardly be of significance. I do not propose 

 to base any argument on these pedigrees— that would 



