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 jioison 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 saprophytic, 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, however, 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
