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Although a tremendous amount of work has been done on the subject of bacterial variation, the major part is concerned with the description of certain variants, interesting chiefly for their morphological, colonial, biochemical, or immunological peculiarities. There have been but few attempts to study the fundamental nature of bacterial variation and such hypotheses as have been advanced differ widely from each other with the result that practically every type of variation observed in higher forms of life has been suspected of occurring in bacteria. Neisser (1906) and Massini (t907) working with Escherichia coli-mutabile, stated that lactose-fermenting variants arose from the parent culture by a process resembling mutation. This is in agreement with the recent work of Lindegreen (1935 a and b; 1936). Penfold (1912) working with Twort's lactose-fermenting bacillus believed that it might become stabilized by constant selection. Stewart (1926) constructed a very interesting hypothesis of asexual character segregation, although he admitted that conjugation might occur in the colonial papillae. A similar hypothesis was presented by Reed (1933). Mellon (1925 a and b; 1927), in a large series of papers on this subject offered evidence that conjugation (chiefly autogamous) may occur. Other sexual phase and life cycle theories have been presented by Fuhrman (cited by Hadley, 1927), Hort (1917), Lohnis (1921), Leiske

Bacterial Variation as Studied in Certain Unstable Variants