DOUBTFUL PHENOMENA OE HEREDITY 389 



not concerned in the process, and that the transmission is in this 

 case due to infection of the germ with the microbes by which 

 the disease is induced.* 



The ' transmission ' of carcinoma might be accounted for in 

 a similar way. — if, as has recently been supposed, this disease 

 is really due to microbes. 



It is, however, also conceivable that both causes — the trans- 

 mission of abnormal predispositions, and infection of the 

 germ — might combine to bring about the transference of a 

 disease from one generation to another. Without desiring to 

 encroach upon the domain of pathology, I am inclined to sup- 

 pose that this is the case as regards ' hereditary' tuberculosis : 

 there is no doubt about the occurrence of a 'tuberculous 

 habit,' — that is,, a certain complication of structural peculiarities 

 which is commonly connected with the disease, such as a 

 narrowness of the chest, for instance. These peculiarities 

 must result from the structure of the germ-plasm, in which a 

 definite variation of certain determinants and groups of deter- 

 minants must have taken place, and they are therefore certainly 

 transmissible. The disease itself, however, is not due to this 

 ' habit,' but is caused by the presence of specific parasites, the 

 tubercle-bacilli, which have a harmful effect upon the various 

 living tissues. They may be introduced artificially into the blood, 

 and then produce the disease even in perfectly normal individuals. 

 They may, moreover, enter the body ' spontaneously,' e.g., by 

 some natural means, and will then also give rise to the disease. 

 But in the latter case the probability of infection seems largely 

 to depend upon the susceptibility or power of resistance of the 

 individual, and at the present day pathologists are of opinion 

 that persons exhibiting the • tuberculous habit ' already referred 

 to have a much slighter power of resistance to the parasites 

 which have passed into the body than strongly-built people. 

 The inheritance of the disease would accordingly depend on the 

 transmission of a constitution very liable to infection. 



Without wishing to deny the existence of such a predispo- 

 sition to infection, I do not believe that the transmission of 

 tuberculosis is due merely to the inheritance of a greater degree 



* A more detailed account and proof of this view concerning the infec- 

 tious nature of traumatic epilepsy in guinea-pigs is contained in my essay 

 on ' The Significance of Sexual Reproduction,' Appendix iv. 



