108 HEREDITY 



but were susceptible if the father alone had been 

 immunised. He further proved that this so-called 

 inheritance of immunity is not a true inheritance — 

 were it so, it would clearly be a case of the trans- 

 mission of an acquired character — but is an acquire- 

 ment of the young mouse, partly due to the passage 

 before birth, of certain substances into its blood from 

 the blood of the mother and partly due to a subse- 

 quent transference of these substances by means 

 of suckling. Plainly it is all one to the student 

 of heredity whether the young mouse acquired its 

 immunity from the blood with which the mother fed 

 it before birth or from the milk with which she fed 

 it after birth. The case is not one of heredity. 

 Similarly the human infant may acquire immunity 

 through its mother's milk. Ehrlich is inclined to 

 think that the very general immunity of the human 

 infant from a number of infectious diseases, such as 

 mumps, scarlet fever, and measles, during the first 

 year of life, is due to its constant imbibition in its 

 mother's milk of protective substances poured into 

 the mother's blood by her body-cells in consequence 

 of their acquired immunity to the diseases in ques- 

 tion. Plainly this also is no case of heredity ; for the 

 sequence of events could be paralleled by the pre- 

 cipitation of these protective substances from the 

 milk of any immunised animal, and their adminis- 

 tration to the child. Problems of heredity will arise 

 when we ask ourselves whether the immunity 

 acquired by a mother or a father can so aifect the 

 germ-plasm as to endow the offspring with a truly 

 inherited immunity. We have already seen that in the 

 case of the paternal mouse, immunised against certain 



