THE CAUSE OF INFECTIOUS DISEASE. 247 



quality, but this quality was inherited, and the 

 character thus transmitted was originally ac- 

 quired through disease. An interesting ob- 

 servation has been made in certain cases of 

 twins and triplets. When, by infection of the 

 mother with small-pox opportunity was given 

 for placental, intra-uterine infection of all the 

 offspring, one or another of the children re- 

 mained free from the disease, although the pos- 

 sibility of infection taking place by means of the 

 placenta was proved by its occurrence in the 

 case of one or more of the children. But it has 

 also been observed that a pregnant woman who 

 had been successfully vaccinated gave birth to 

 a heal thy child who nevertheless sickened three 

 years later with small-pox, and in another 

 case small-pox was observed in a foetus 

 whose mother had previously suffered with the 

 disease. Whether an acquired disposition is 

 transmissible depends upon the kind and dur- 

 ation of the influence. On this point it is easy 

 to deceive oneself. In Ehrlich's experiments 

 in regard to rendering animals resistant to 

 poisons, at first sight it seemed as if the young 

 inherited immunity ; another interpretation 

 proved, however, to be the correct one. The 

 young of a non-immune mother, when fed on 

 the milk of an immunized mother, acquired 

 immunity, thereby showing that the immunity 



