230 THE TRESENT EVOLUTION OF MAN — PHYSICAL 



cholera — can not only exist but can multiply, though 

 apparently only to a limited extent, outside it, while 

 the microbes of yet others — e.g. malaria — can not only 

 exist and multiply outside it, but can multiply there to 

 an unlimited extent. 



Of the microbes of cholera, and probably also of 

 typhoid, it is interesting to remark, that though it 

 seems they are able to multiply for a period in con- 

 taminated fluids, yet in time they always perish, unless 

 they find a home again in a human host, as appears to 

 be proved by the fact that travellers are never infected 

 by them in long-deserted districts. That they are 

 truly parasitic when in man, in the sense that they 

 find their nutriment essentially in his living tissues, 

 not in the non-living contents of his bowel, is proved 

 by the circumstance that he is able to acquire immunity 

 against them ; or rather, it is proved by the fact that 

 after he has once acquired immunity the microbes of the 

 disease are no long^er able to flourish in the contents of his 

 bowel even should they gain fresh entrance. Reasoning 

 from the analogy of other diseases, acquired immunity 

 in this disease can only be due to the phagocytes acting 

 from a " position of advantage " on the microbes within 

 their reach, which of course would not be those that 

 lay free in the bowel, mixed with its contents ; the 

 only other rational theory here being one which supposes 

 that it is due to the secretion, by the cells lining the 

 intestinal tract of an immune person, of a substance 

 poisonous to the microbes ; an untenable theory when 

 we take into consideration the fact that immunity may 

 be acquired by individuals whose ancestry can have 

 had no experience of the disease, and therefore cannot 

 have developed the power of secreting this particular 

 complex chemical substance, this species of anti-toxin — 

 not only exactly when it was first wanted, i.e. when 

 infection first took place in the individual, but ever after. 



