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of which have been identified as the fons et origo mail of 

 other diseases, are so far influenced by heredity as to always 

 propagate themselves after their kind, each preserving its 

 own characteristic individuality, so that, as Tyndall observes, 

 from their respective viruses you may plant scarlet fever, 

 typhoid fever, small pox, et hoc genus omne ; that as surely as 

 a thistle rises from a thistle seed, as surely as the fig comes 

 from the fig, the grape from the grape, the thorn from the 

 thorn, so surely does the typhoid virus increase and multiply 

 into typhoid fever, and nothing else ; the scarlatina virus 

 into scarlatina, and into nothing else ; the small-pox virus 

 into small-pox, and nothing else ; so that to all intents and 

 purposes a virus may be regarded as a seed which, invariably 

 generates after its own kind. And here we see the all- 

 pervading influence of heredity that necessitates its recog- 

 nition as an universal law, for as in the highest forms of 

 organic life, from man himself down through the lower 

 animals, until we reach the lowest forms of all, there heredity 

 manifests itself in the reproduction of each after its kind. 



As in the two other groups of acute infectious diseases, 

 we find heredity acting similarly in this division viz., with 

 regard to predisposition and insusceptibility. It may, indeed, 

 be asserted as an axiom that individual predisposition plays 

 an important role not only in all epidemics, but in all cases, 

 and it may also be stated that this predisposition, while 

 differing in development in different individuals, is, if not 

 more, at least equally, as potent as all predisposing causes of 

 an external character. Let me take cholera as an illustration. 

 It is a well-known fact that many persons enjoy at least a 

 temporary immunity against the disease. During an epidemic 

 of cholera, should choleraic diarrhoea prevail, many escape it 

 entirely, others suffer only lightly and temporarily, while 



