88 VARIATIONS IN VIRULENCE [CH. vi 



altered metabolism might persist outside the body for some 

 time if the substances provided for assimilation remained the 

 same. In the case quoted, Eyre and Washbourn found that 

 virulence was maintained only on media containing blood 

 (blood agar) just as Anne Williams found that the virulence of 

 diphtheria bacilli was maintained only on serum or ascitic 

 fluid. (The possible relationship between toxicity and 

 altered metabolism will be discussed later.) 



(d) Another difficulty is the development of the property 

 of virulence by organisms outside the living tissues, as, for 

 example, by saprophytic bacteria in the intestine or in the 

 cavity of the uterus during the puerperium. What part can 

 adaptation or selection play in the case of these ? 



It is true that a saprophyte which has acquired the power 

 of excreting toxins has thereby acquired the power also of 

 lowering the vitality of the living cells exposed to their 

 action, or even of killing these in cases where the superficial 

 tissues have been injured previously. The toxic saprophyte 

 by such means is enabled to procure fresh food-stuffs for its 

 own use, but since it is forced to share the spoils with its 

 non-toxic brethren this accomplishment is less a private gain 

 than a public advantage, and hardly conduces more to its 

 own survival than it does to theirs. It is evident, neverthe- 

 less, that a strain of saprophytes which developed toxic pro- 

 perties might survive and multiply under conditions in which 

 a strain of non-toxic saprophytes would die out, so that the 

 strain of saprophytes possessing the greatest toxicity would, 

 other things being equal, stand the best chance of being per- 

 petuated. 



A saprophyte, e.g. in the uterus during the puerperium, 

 may not only develop extreme toxicity but may actually 

 invade the living tissues and become parasitic. Here ob- 

 viously other questions are involved. One of these concerns 

 the part played by the food-stuffs of bacteria and the effects 

 of changes in these. 



The fortunes of an invading army depend as much upon 

 its successful victualling as upon its armament ; if the former 

 breaks down the latter is of no avail. A non-toxic saprophyte 



