238 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



the stimulation of the toxins, and acquire the power of 

 attacking the microbes — these latter, owing to a trait 

 which the survival of the fittest has developed in the 

 species, leave the host in the discharges, and maintain, 

 if they do not perish, a purely saprophytic existence 

 outside him till such time as, if fortune serve, they are 

 taken into the body of another and a susceptible host. 

 It should be remembered, moreover, since a human host 

 is essential to the persistence of the cholera bacilli, that 

 if they found their nutriment in the contents of the 

 bowel merely, if they were purely saprophytic, it would 

 be against their specific interest to secrete a poison 

 which would cause the death of the host that supplied 

 the means of subsistence. The law of the survival of 

 the fittest would, in that case, cause them to become as 

 innocuous to the host as the other micro-organisms 

 which normally inhabit his alimentary tract. We may 

 conclude then, that the cholera bacilli are not sapro- 

 phytic, in the sense that they find their nutriment solely 

 in the non-living contents of the bowel, but that they 

 are parasitic, in the sense that they find their nutriment 

 in the living tissues lining the bowel. The point, how- 

 ever, is capable of being tested experimentally, for if the 

 bacteria are saprophytic in the sense that they find 

 their nutriment in the contents of the bowel, then the 

 alvine discharges of an immune person should have 

 bactericidal powers as regards them, whereas the alvine 

 discharges of a susceptible person should form an 

 excellent culture medium ; but if they are parasitic, if 

 they find their nutriment in the living tissues lining the 

 bowel, then the alvine discharges from immune and 

 susceptible persons should have equal bactericidal or 

 nutritive powers. 



But while we may safely assert that the contents of 

 the bowel do not normally present all the conditions 

 under which the cholera bacilli are able to multiply to 



