the early 1900's, before the introduction of the principles of microbial genetics prerequisite to a solution of the problem. There is the added difficulty of the lack of any recent critical attempt at a reexamination of these early findings. It is, consequently, the purpose here to correlate these results with the more recent work of pleomorphism, including my own observations, and to see whether such principles as mutation, selection, and norm of reaction can be used in a reexamination of the phenomenon. Since my experience has been limited to a study of one species, Trichophyton mnentagrophytes, this paper will deal specifically with pleomorphism in this species and in general terms with pleomorphism in the dermatophytes as a whole. The original biological material in this study was one strain of T. mentagrophytes, the N form, first described by Robbins (1945, 1949) and kept in culture, apparently unchanged, up to the present time. It will be referred to here as the N, normal, or conidial form, to distinguish it from the pleomorphic variants derived from it. The nutrient medium used, a thiamine-neopeptone agar (McVeigh and Cambell,