THE INHERITANCE OF DISEASE 497 



in the various species but these, however significant, must 

 still be regarded as differences in detail. It would seem 

 proper to consider therefore that the results of the animal 

 experiments may safely be applied to the interpretation 

 of what has been observed of the inheritance of the human 

 disease to the extent, at least, that we should for the future 

 be ready to accept the statistics and familial observations 

 at the significance thay carry on their face, rather than 

 straining all points of possible criticism and reservation. 

 In other words, it seems established in all reasonable 

 probabihty that important factors influencing the incidence 

 of tuberculosis and the development of the disease in the 

 individual are inherited. 



The studies of human material from the pathological 

 standpoint show, as has been said, that most individuals 

 become infected with tuberculosis at one time or another 

 and it may therefore be concluded that neither in kind nor 

 degree are the inherited factors capable of preventing 

 infection. They must, therefore, be exerted on the progress 

 of the disease after the body is invaded by Bacillus tuber- 

 culosis. The direct evidence at present available from human 

 sources does not carry us beyond this point. 



What we know of the pathology of human tuberculosis, 

 experience derived from animal experimentation with this 

 disease, and consideration of our knowledge of other infec- 

 tious diseases enables us to set up a series of surmises or 

 hypotheses with regard to the possible nature of the inher- 

 itable factors involved but it would be difficult if not impos- 

 sible to check these effectively by direct studies of the human 

 disease. It has been possible to make a beginning in this direc- 

 tion on the basis of the guinea-pig experiments just mentioned. 



It is found in the first place that there are a number of 

 inherited factors involved. At least three and possibly four 

 separately inherited factors or factor groups are indicated 

 by the results with the available guinea-pig families. It 

 cannot be assumed that these families assembled by chance 

 for other purposes present all the possible variants. Nor 

 can it be assumed that the most complete collection of 

 guinea-pig material would accurately portray and relatively 

 evaluate the human factors. What is presented is a minimum 



