THE UNIQUENESS OF THE INDIVIDUAL 



(though other micro-organisms will also serve). Streptococcal 

 infections are usually treated by the administration of sulphon- 

 amide drugs or of antibiotics of fungal or bacterial origin, such 

 as penicillin or streptomycin. It is a common observation of 

 clinical practice (and one which can be reproduced by experi- 

 ments in vitro) that the prolonged exposure of a population of 

 streptococci to penicillin, at concentrations which fall short 

 of bringing about its complete destruction, mai/ lead to the 

 evolution of a ''resistant strain\ i.e. a population which can 

 flourish unchecked at a concentration of penicillin that strongly 

 inhibited the growth of the parental organisms. The trans- 

 formation is heritable, for resistance once acquired long out- 

 lives the stimulus that originally called it forth. 



By disregarding all subtleties of interpretation, the ''training'' 

 process may be represented in alternative ways. The first is 

 illustrated by Fig. 4; the shaded circles represent resistant 



M>Q 



00° 



o 



00 



o 



fto*- 



O Q 



Fig. 4. The development of penicillin-resistance in 

 bacteria, according to a Darwinian interpretation. 

 For explanation see text. 



forms. It is presumed that the original bacterial population 

 was heterogeneous and contained genetic variants endowed 

 with a relatively high degree of resistance to the action of 

 penicillin. In the normal course of events — in a penicillin-free 



80 



