246 BACTERIOLOGY. 



be acquired as the result of external changes. 

 An attack of illness due to " catching cold " or 

 of rheumatism affections causally related per- 

 haps increases the tendency to those diseases, 

 while an attack of any one of the more acute 

 infectious diseases, such as small-pox, scarlet 

 fever, or measles, confers immunity against an- 

 other attack. Susceptibility to disease is trans- 

 muted into protection against it. Such an in- 

 dividually acquired immunity can be handed 

 on from mother to child. I shall not in this 

 place broach the much mooted question whether 

 or not acquired characters can be inherited, 

 and shall not try to set forth here how, upon 

 the basis of my representation of the problem 

 of causation, the question really comes within 

 our reach. It may suffice at present to make 

 clear that, under certain circumstances, acquired 

 characters, among which may be reckoned ac- 

 quired tendency to disease or acquired immunity, 

 must be inherited. According to Kaltenbach, 

 twin-sisters, originating from two different 

 ova, were exposed in equal measure to in- 

 fection from scarlet fever ; one of them remained 

 entirely immune, the other succumbed imme- 

 diately. The latter resembled the father, the 

 immune child the mother, who, fourteen months 

 previously had experienced a severe attack of 

 scarlet fever. Here was a pronounced inborn 



