ACQUIRED IMMUNITY 127 



themselves under normal conditions of health ; that acquired 

 immunity is part of that general power of making useful 

 acquirements in particular directions which is possessed by 

 all the higher animals ; that the habituation is by way of 

 weak toxins to stronger toxins ; that the progressive weaken- 

 ing of the toxins is brought about by enzymes secreted by 

 certain cells as a normal part of their function. The last is 

 the principal assumption made, but the existence of digestive 

 ferments in the lymph has been proved beyond all doubt, as 

 has also the fact that snake-toxin digested and weakened by 

 the enzyme secreted by the stomach-cells causes, when it 

 enters the blood-stream, immunity against that poison in the 

 individual who has swallowed it. Finally, it supposes that 

 the reason why antitoxins are possible in the case of bacterial 

 and animal poisons as well as such vegetable poisons as abrin 

 and ricin, but not possible in the case of mineral poisons, is due 

 to the fact that the latter are of very simple composition and 

 cannot be altered without undergoing complete destruction 

 as poisons, whereas the former, being immensely more com- 

 plex chemically, can be digested in such a manner that they 

 are not destroyed as poisons, but merely weakened. 



205. But perhaps the most beautiful and, as it seems to 

 me, most convincing proof of the theory of habituation is sup- 

 plied by the phenomena of syphilis, a disease against which 

 immunity may be acquired, but which, owing to the feeble- 

 ness of its toxins (in this approaching leprosy and tuber- 

 culosis), yet persists long enough in a woman to cover the 

 entire period of several pregnancies. This evidence, notwith- 

 standing its interesting nature, has been curiously neglected 

 by bacteriologists. Normally, as we know, there is no con- 

 nection between the placental blood-vessels of a mother and 

 those of her foetus. But they are in close apposition, and 

 are very thin- walled, so that while solids (e. g. the parasites 

 of disease) are normally stopped, fluids, gases, and solids (e. g. 

 toxins) in solution pass from one to the other by diffusions. 

 Under abnormal circumstances (e. g. during disease) the con- 

 tinuity of the vessel- walls may be broken, and then the solids 

 as well as the fluids pass from one to the other. It can 

 hardly be doubted that this is what commonly happens in 

 syphilis ; and therefore a syphilitic mother very generally 

 bears a syphilitic child. But when, in rarer cases, the con- 

 tinuity of the vessel- walls is not broken, when a syphilitic 

 mother has a healthy child, or when, as more commonly 

 happens, the healthy partner of a syphilitic father is delivered 

 of an infected child, then is seen a most instructive pheno- 



