Mutation of Phytomonas Stewartii by X-Ray Irradiation.

ARIATION in cultural characteristics, viability, and motility of bacV teria has been recognized almost from the beginning of their study in pure culture. Because of the influence of the Koch school, which emphasized cultural purity, early bacteriologists were prone to attribute changes in cultures to contamination. Thus, many of the earlier observations of bacterial variation probably were discounted or misinterpreted. Variations associated with virulence and antigenic types, however, have become so important, particularly from the medical standpoint, that in certain forms such as the pneumococci many variant types have been described, and many of the cultural conditions necessary for the full growth of the particular variant types have been made known. Lacking critical experiments to prove their cause and nature, these variant strains ordinarily have been referred to as dissociants, variants, and saltants arising by some process vaguely defined as “dissociation,” and, with the exception of certain workers, the terms “mutant” and “mutation” as ordinarily applied to variation in higher organisms such as maize, Drosophila, and man have been avoided. Superficially, if we neglect the differences in reproductive capacity of the mutants, the observed changes and the experimental conditions surrounding the changes suggest that variation is caused by the environment acting on the organism rather than by spontaneous changes within the organism. In a proper environment a parent culture may be replaced by one of its variants. In some cases the environment necessary to facilitate the appearance of these variants is known and may be controlled to some degree. In light of such facts it is not surprising that environmental differences have been interpreted as causative in initiating variation rather than correlative and that the mutation concept is often regarded as unnecessary to account for the variants observed. The problem of the origin of variation is of particular importance to the bacteriologists’ concepts of species and the geneticists’ outlook on the mech-