CH. vi] VARIATIONS IN VIRULENCE 85 



is to attack, certain of these minute organisms acquire the 

 power of manufacturing toxins which weaken the defence and 

 counteract the opposition of the living tissues and so enable 

 them to gain a firmer foothold there. It is the individuals 

 which thus accommodate themselves to the exigencies of their 

 surroundings that are perpetuated by a process of natural 

 selection. 



That it is a case of particular adaptation to environment 

 and not merely a question of vitality or robustness is shown by 

 the following observations. 



(a) "Passage" through a certain species of animal while 

 increasing the virulence for that species may actually diminish 

 it for another species. If the process merely selected the 

 strongest, the strain of organisms resulting should show 

 heightened virulence for other animals also. 



(6) The most virulent organisms are not necessarily the 

 most robust. Eyre and Washbourn (1899) found, as Kruse and 

 Pansini had done previously, that in the case of the pneumo- 

 coccus the exact contrary was true. "The most virulent strains 

 were those which were most delicate and sensitive in artificial 

 cultivations and the less virulent ones were much less delicate 

 and could grow under conditions in which the virulent ones 

 were unable to flourish." The parasitic type required a certain 

 reaction and temperature and special media. It would not 

 grow if the reaction was even faintly acid or at a temperature 

 much below that of the body and rapidly died out on agar or 

 in broth. The saprophytic type grew luxuriantly either at 

 37 C. or at 20 C., in broth, agar, potato or gelatin, whether 

 acid or alkaline, and retained its vitality for many months. 



(c) Analogy with other processes of adaptation lends 

 further support to the view put forward. Thus, Rettger and 

 Sherrick (1911) have shown that by artificial selection a strain 

 of organisms can be made unusually resistant to the action of 

 an antiseptic such as corrosive sublimate. Penfold (1911 c) 

 showed that natural selection might in the same way develop 

 a special power of resistance to an antiseptic in this case 

 chlor-acetic acid. The last-named observer went further and 

 demonstrated that the particular strain of organisms which 



