ii6 THE FIRST PRINCIPLES OF HEREDITY 



always specific — i.e., it holds only for any given disease : 

 smallpox, diphtheria, etc. Immunity from one disease does 

 not protect against another. 



Immunity may be acquired or inborn. In the first case 

 the acquired immunity may be natural, as when a person 

 passes through an illness and acquires, in consequence 

 thereof, freedom from the same disease for a certain period 

 of his life {vaccinia or cowpox), or, may be, for the whole 

 remainder of his life (smallpox) ; or the immunity may be 

 artificially induced by various methods — vaccination, 

 serum treatment, etc. On the other hand, immunity 

 against certain infections may be an inborn characteristic 

 in some individuals, who, though exposed during an 

 epidemic to the contagion, yet never contract the disease. 

 It is a noteworthy fact that the various animal species 

 often show immunity from particular diseases, the diseases 

 differing in the different species. 



It would be going too far to enter here into details as to 

 what immunity exactly is, or in what way the different 

 means of artificial immunization act. Be it said shortly 

 that the immune person has the power to resist the influence 

 of the noxious microbes or their poisonous products by 

 antidotes which accumulate in his own system. 



The question at issue once more is : Is acquired immunity 

 inheritable, and how has the natural immunity from certain 

 epidemic diseases which seems to have arisen been brought 

 about ? Here again we have two possibilities. Either the 

 immunity acquired by the overcoming of the disease itself 

 has been transmitted to a greater and greater number of 

 individuals, or natural selection has been at work weeding 

 out those most susceptible to the disease, whilst leaving 

 those for propagation who have a natural resistance 

 against the disease. 



That acquired immunity is inherited is unlikely after all 

 we have already adduced against the inheritance of 

 acquired characters and diseases, nor has it so far been 

 proved to be a fact. Immunity may be transmitted by 



