116 THE PRINCIPLES OF HEREDITY 



immunity under the stimulus of disease has been evolved in 

 the species by the same means as the power of making other 

 " use-acquirements" has been evolved for instance, the power 

 that a muscle possesses of getting used to fatigue, or the power 

 which tendons, glands, and other structures possess of de- 

 veloping in response to the stimulus of increased use. In 

 all these instances the organism (or given parts of it) reacts 

 in a definite and highly beneficial way to given stimuli. But 

 most well-known theories assign an almost magical origin to 

 immunity. We are expected to believe that it arises through 

 processes which have no known analogy in nature. It will 

 be instructive to note a few of these theories, and then make 

 an attempt to bring this particular class of acquirements into 

 line with other adaptive acquirements. Such an attempt, 

 however great, through lack of knowledge or discernment, the 

 ultimate failure, has at any rate the merit of seeking to 

 unify our knowledge. 



188. Some zymotic diseases, as compared to others, are 

 short and sharp. The sufferer perishes or recovers from the 

 actual disease within well-defined time-limits, though he may 

 suffer from certain after-effects the sequelae for a longer 

 period. Before he falls ill of any of these diseases he is free 

 from the specific pathogenetic micro-organisms of it, as is 

 obvious from the fact that he does not infect his fellows. 

 During his illness he swarms with them, since he is then 

 infective. During or after recovery he rids himself of his 

 enemies, since he is no longer infective. Something banishes 

 or destroys the parasites in his system. Recovery implies, 

 in fact, their banishment. A profound change befalls the 

 sufferer. From being a soil on which parasites flourish, his 

 body becomes poisonous to them. At the least it becomes a 

 soil in which they cannot exist easily. He acquires immunity. 

 Other diseases run a prolonged indefinite course. Immunity 

 cannot be acquired against them. 



189. Now it is very noteworthy that it is only against 

 diseases the micro-organisms of which produce more or less 

 powerful toxins as judged by the systemic effects that 

 immunity can be acquired. When the toxins are feeble or 

 non-existent e.g. in tuberculosis and leprosy when the 

 onset of the disease is unmarked by symptoms of poisoning, 

 when the pathogenetic organisms, instead of engaging at a 

 long range, enter at the outset into what may be termed an 

 actual physical struggle with the phagocytes, which, undeterred 

 by toxins, at once attack the disease germs, and taking them 

 into their substance attempt to destroy them, then immunity 



