MICROBIC DISEASES 283 



and reappeared in the offspring, it is probable that there was 

 behind the provocative stimulus an inborn predisposition, 

 and that the latter alone is transmitted. The case of alcoholism 

 has been discussed separately. 



Microbic Diseases. — In the strict sense there can be no in- 

 heritance of microbic diseases, for a microbe cannot form part 

 of the organisation of the germ-plasm. " No specific infective 

 disease is hereditary, if we use the term ' heredity ' in the sense 

 which Darwin and the biologists have given to it. If it appear 

 congenitally it is simply communicated to the foetus by infection " 

 (A. A. Kanthack, in Allbutt's System of Medicine, vol. i. p. 555). 

 Let us take two concrete cases — tuberculosis and syphilis. 



Tuberculosis. — As this familiar disease, in its many forms, is 

 always associated with the presence of a specific microbe, the 

 tubercle bacillus, it is not in itself transmissible. What is trans- 

 mitted is a predisposition making infection easy, a vulnerability of 

 epithelial surfaces, a weakness in the power of resisting and dealing 

 with the invading microbes. As Debierre puts it, " On ne nait pas 

 tuberculeux, on nait tuberculisable." 



Theoretically, it matters little when or where infection occurs, 

 but the various possibilities are of practical interest. 



( 1 ) It seems very unhkely that the spermatozoon is ever the bearer 

 of the tubercle bacillus. Out of sixteen guinea-pigs inoculated with 

 the sperm of tubercular males, six became tubercular, according 

 to Landouzy and Martin, but many have repeated this experiment 

 with negative results. Landouzy, quoted by Debierre, gives the case 

 of a phthisical officer who married a wife without any hereditary 

 taint in that direction. The five children all died of tubercular 

 disease ; but, of course, this may have been due to post-natal 

 infection. 



(2) Similarly, it seems very unlikely that the ovum is ever the 

 bearer of the tubercle bacillus. 



(3) In a few cases there is direct evidence that the mother may 

 infect her unborn offspring, the bacillus passing through the placenta. 

 In rabbits and guinea-pigs and some other animals this ante-natal 

 infection has been demonstrated ; but it is interesting to notice 

 that while tuberculosis is extremely common in cows (sometimes, 



