82 GENETICS 



and of the powers of heredity to trace the origin of 

 the affliction back to a revolutionary ancestor who 

 acquired sciatic rheumatism by sleeping on the ground 

 at Valley Forge, yet this is quite as direct as many 

 alleged instances of the inheritance of disease. 



In the majority of instances, apparent cases of the 

 inheritance of disease are merely instances of reinfec- 

 tion. This reinfection of the offspring may occur very 

 early in embryonic life, even in the egg, in the case of 

 pebrine in silkworms (Pasteur) and in the tick which 

 transfers the protozoan parasite causing Texas fever. 

 Or it may happen after birth, provided the offspring 

 are exposed to the same environment as that in which 

 the parent acquired the disease, but in any case reinfec- 

 tion is not heredity. 



e. Immwrdty and the Effect of Drugs 



Ehrlich subjected mice to increasing doses of ricin 

 until they became immune to doses which are ordi- 

 narily fatal. When these ricin-immune mice were bred 

 to non-immune mates the offspring in turn showed some 

 degree of immunity if the immunized parent was a 

 female but not if the immunized parent was a male. 

 In other words, the immunity was transferred through 

 the female only, where the blood of the mother is for 

 a considerable period during foetal life in intimate rela- 

 tion with the blood of the offspring. Even here, just 

 as in the lifetime of an immunized individual, the im- 

 munity tended to fade out after a short time. 



As a matter of fact many of the instances that have 



