Drug Resistance in Bacteria 7 



it is very low. Since the phenomenon is neither one of com- 

 pletely stable changes, nor one to which the term hereditary 

 has a clear meaning, we consider arguments invoking the name 

 of Lamarck to be largely meaningless or irrelevant. The way 

 is thus opened for the consideration on its merit of experi- 

 mental evidence (which, it should be repeated, applies only 

 to the particular examples with which it is found — except in 

 so far as it illustrates what can happen as distinct from what 

 must happen). 



Some aspects of the mutation-adaptation controversy offer 

 an interesting analogy with the history of the phlogiston 

 theory in chemistry. This doctrine, it has been said, was 

 never formally abandoned by its supporters. But under the 

 pressure of facts they changed it, added to it, and buttressed 

 it with auxiliary hypotheses until it became indistinguishable 

 from its rival, except for some superfluous nomenclature 

 which was presently forgotten. 



Is something of the sort in process of happening with the 

 uncompromising version of the mutation theory? At first 

 mutations w^ere catastrophic, one-step events, occurring in a 

 purely random manner to an excessively minute proportion 

 of the population, and essentially during the hazards of 

 nuclear division. Gradually the lines of the picture have 

 softened. Mutations may occur in the absence of nuclear 

 division and may affect almost the whole population (Ryan, 

 1955; Szybalski, 1954-55); they may be induced by a drug 

 to which the mutant subsequently shows resistance, a marked 

 departure from randomness (Akiba, 1955; Szybalski, 1954-55) ; 

 they are sufficiently dependent on cytoplasmic events to be 

 associated with a considerable "phenotypic delay" (New- 

 combe, 1948, 1953); and as to their discreteness, so elaborate 

 a polygenic system is frequently assumed that the results of 

 recombination experiments become indistinguishable from 

 those which would be given by quantitative cytoplasmic 

 changes. 



This last fact would be even more commonly realized but for 

 the practice of creating an illusory impression of discreteness 



