ACQUIRED IMMUNITY 129 



reason the children of a married couple who have the disease, 

 as a rule, even in the absence of treatment, show less and 

 less signs of it; whereas a woman not previously infected 

 may have grievously diseased children to a father who has 

 long ceased to exhibit signs of the disease. Moreover, if a 

 non-infected mother bear infected children, who live, to a 

 diseased father, her children also show less and less signs of 

 it, until, at length, they show no signs at all. 



207. If we are right in the foregoing portion of this chapter, 

 acquired immunity to syphilis, like acquired immunity to 

 other diseases, must depend on acquired power of tolerating 

 the toxins ; the cells concerned (especially those concerned in 

 the elaboration of the digestive bodies) being able to perform 

 their functions in spite of the presence of the toxins ; in 

 consequence of which the micro-organisms must gradually be 

 destroyed ; or, if some of them survive, they must survive 

 in the unfavourable environment in a highly resistant, and, 

 therefore, judged by analogy, a very harmless form e.g. as 

 resting spores. Now we have evidence that pathogenetic 

 organisms do not induce disease in susceptible organisms 

 when the number in which they find entrance sinks 

 below a certain minimum a minimum which varies with 

 the species of the invading parasites, with the species of 

 the animals attacked, and with each individual of the latter. 

 Thus it has been found that to kill a rabbit by artificial in- 

 fection at least sixteen thousand virulent anthrax bacilli must 

 be introduced, a lesser number perishing and conferring on 

 the rabbit increased resisting powers. (Lubarsch.) Again, one 

 cubic centimetre of a fresh broth culture of bacillus pyocya- 

 neus will infallibly cause fatal disease in a rabbit, which, 

 however, will survive and acquire increased resisting power 

 from the injection of a quarter of that quantity. But 

 immature animals are less resistant than adult animals of the 

 same species ; for instance, young guinea-pigs succumb more 

 easily to attenuated anthrax bacilli than do older animals. 

 Young animals, therefore, are capable of being infected by a 

 smaller quantity of microbes than adults ; and for this reason 

 it probably is that a father may infect his offspring with 

 syphilis at a time when few microbes survive in him, and long 

 after he has ceased to be infective to a woman ; and this 

 readier infectibility of the young suffices also to explain the 

 fact that while it is rare for a diseased woman to bear a non- 

 infected child, it is much more common for a diseased child 

 to be born of a healthy mother. Moreover, both ovum and 

 spermatozoon are unicellular animals; in them there is no 



E 



