CH. VI] VARIATIONS IN VIRULENCE 87 



tuberculin and modern vaccines against specific diseases 

 suggests that the r61e of the intracellular toxins is of greater 

 significance than this hypothesis would admit. 



(6) Another question that suggests itself is this: why, 

 if successive passages increase virulence, do not infectious 

 diseases, which are continually undergoing this process of 

 passage, become of deadly virulence? 



One explanation is that an obligatory parasite which kills 

 its host sinks the ship it is sailing in and thereby sacrifices its 

 own chance of survival, so that the most virulent organisms 

 are weeded out and destroyed. Many infectious diseases, 

 however, have not reached such a high degree of virulence 

 that this factor can have become operative in their case. 



(c) Yet a third difiiculty in accounting for the development 

 of virulence by natural selection is the occurrence of pheno- 

 mena such as that described by Eyre and Washbourn in which 

 an avirulent pneumococcus acquired a high degree of virulence 

 after a single passage through an animal. The virulence so 

 acquired was maintained for a couple of months on artificial 

 media, much longer, that is to say, than it was found possible 

 to maintain the same character when developed more slowly. 

 It is difiicult to conceive how, in the course of so few 

 generations comparatively, natural selection could cause the 

 character of virulence to predominate to such an extent. 

 Moreover, if it did so one would expect the character to be 

 lost with equal readiness outside the body. 



The explanation may be that when avirulent and virulent 

 pneumococci grow side by side on artificial media there is no 

 selective action and the avirulent, being the more robust as 

 these writers have shown, soon greatly outnumber the virulent ; 

 the latter, after subculture has been carried out several times, 

 may be so few in number that they give no evidence of their 

 presence. " Passage " in this case, by eliminating the avirulent, 

 would very soon select out a pure strain of virulent organisms. 

 Another explanation, however, suggests itself. The change 

 has more the aspect of an alteration in metabolism occurring 

 as a direct response to the change in the food material pro- 

 vided. It is easy to imagine that, once established, such 



