Elective enrichment is an indispensable technique in bacterial physiology and genetics (van Niel, 1949). Specific biotypes are most readily isolated by the establishment of cultural conditions that favor their growth or survival. It has been repeatedly questioned, however, whether a selective environment may not only select but also direct adaptive heritable changes. In accord with similar discussions in evolutionary biology (Huxley, 1942), we may denote the concepts of spontaneous mutation and natural selection in contrast to specific induction as "preadaptation" and "directed mutation", respectively. Many lines of evidence have been adduced in support of preadaptation in a variety of systems (

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Genetic investigations with many different bacteria have revealed parallelisms and some contrasts with the biology of higher forms. The successful application of selective enrichment techniques to the study of gene recombination in Escherichia coli (Tatum and Lederberg, 1947; Lederberg et al., 1951) suggested that a similar approach should be applied to other bacteria. This paper presents the results of such experiments with Salmonella typhimurium and other Salmonella serotypes. The mechanism of genetic exchange found in these experiments differs from sexual recombination in E. coli in many respects so as to warrant a new descriptive term, transduction. MATERIALS AND METHODS Most of the strains of S. typhimurium were provided by Lilleengen (1948) as representative of his 21 "phage types", LT-1 through LT-22. Most if not all strains of S. typhimurium are lysogenic (Boyd, 1950), and these have provided 12 lines of bacteriophage. Other cultures were obtained from F. Kauffmann, E. K. Borman, and P. R. Edwards. All cultures were maintained on nutrient agar slants. Specific growth factor dependent mutants (auxotrophs) were obtained from ultraviolet irradiated cell suspensions subjected to the penicillin method for selective isolation (Davis, 1950a; Lederberg and Zinder, 1948). Similar mutants have been obtained in Salmonella by Plough et al. (1951) and Bacon et al. (1951). Other methods for the isolation and characterization of auxotrophic and fermentation mutants have been documented elsewhere (Lederberg, 1950; Lederberg and Lederberg, 1952). Streptomycin resistant mutants were selected by plating dense, unirradiated cell suspensions into agar containing 500 mg per L of dihydrostreptomycin. "Complete" indicator medium (EMB) was made up from the same formula as for E. coli (Lederberg, 1950). The defined eosin methylene blue medium ("EML agar") containedin g per L: sodium lactate, 2.5; (NH4)2SO4, 5; NaCl, 1; MgSO4, 1; K2HPO4, 2; methylene blue hydrochloride, 0.05; eosin Y, 0.3; and agar, 15. Difco products, penassay broth, and nutrient agar, were employed as "complete" media.

An antibody is a specific globulin which appears in the serum of an animal after the introduction of a foreign substance, an antigen (1). Each of the many globulins is specified by its reaction with a particular antigen (2). Our present concern is to formulate a plausible mechanism for the role of the antigen in evoking large amounts of a specific complementary globulin. An important element of any theory of antibody formation is its interpretation of selfrecognition, the means by which an organism discriminates its own constituents from the foreign substances which are valid stimuli of the immune-response.Recent speculation about antibody formation (3-8) has been dominated by instructive theories which suppose that the antigen conveys the instructions for the specificity of the globulin synthesized under its governance. Elective theories date from Ehrlich (9) and have been revived principally by Jerne (10), Talmage (2, 11), and Burnet (12).-These postulate that.the information required to synthesize a given antibody is already inherent in the organism before the antigenic stimulus is received, and the stimulus then functions to stimulate that mechanism electively. Jerne had proposed an elective transport of antibody-forming templates to functioning sites; Talmage and Burnet have explicitly proposed an elective-function based, on cellular selection. The details which distinguish the various proposals are pointed out in the following discussion.Immunology does not suffer from. a lack of experimental data, but still some of the most elementary questions are undecided, and it is not yet possible to choose between instructive and elective theories. However, the latter have had so little expression in the past few decades that a detailed exposition may serve a useful function, if only as a target for experimental attack. This article is an attempt to formulate an elective theory on the basis of genetic doctrines developed in studies of microbial populations.Of the nine propositions given here, only number 5 is central to the elective theory. The first four are special postulates chosen as an extreme but self-consistent set; however, they might well be subject to denial or modification without impairing the validity of the elective approach. The last four propositions are stated to account for the general features of antibody formation in cellular terms and may be equally applicable to instructive and elective theories. If this theory can be defended, and I know of no fatal refutation of it, then clearly elective theories of antibody formation perhaps less doctrinaire in detail should have a place in further experimental design, each proposition being evaluated on its own merits. I am particularly indebted to Burnet (13) for this formulation, but Burnet should not be held responsible for some elaborations on his original proposal, especially in propositions 1 through 4. A connected statement of the nine propositions is given in Table 1, and each one is discussed in detail in the following sections. Antibody GlobulinAl....

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