The subject of variations in the lower organisms and their dependence on conditions is a highly special one, and I have no knowledge which can justify me in offering any discussion of them, but I understand that hitherto little beyond empirical recognition of the phenomena has been attempted. A useful summary of observations made by many investigators was lately published by Hans Pringsheim,[1] who enumerates the different agencies which have been observed to produce modifications, and the various ways in which these changes are manifested. One of the most comprehensive studies of the subject from the genetic point of view is that made by F. Wolf.[2] In his extensive cultivations of _Bacillus prodigiosus_, _Staphylococcus pyogenes_ and _Myxococcus_ he succeeded in producing many strains with modified properties. In most of these the modifications arose in consequence of the application of high or low temperatures or of the addition of various chemical substances to the culture-media. Some of the variations, which are for the most part in the powers of pigment-formation, persisted when the strains were returned to normal conditions, and others did not. In reference especially to the variations witnessed in the Cocci the reader should consult the critical account of variation in that group published by the Winslows,[3] where much information on the subject is to be found. The authors attempted to determine the systematic relationships of the several forms, as far as possible, by the application of statistical methods. The result is interesting as showing that the problem of species in its main features is presented by these organisms in a form identical with that which we know so well in the higher animals and plants, whatever properties be selected as the diagnostic characters. There are many types perfectly distinct and others which intergrade. Some of the types change greatly with conditions while others do not. This is exactly what we encounter whenever we study the problem of species on an extended scale among the higher forms of life. There is now practically complete agreement among bacteriologists that the observations made first by Massini on the change in color of _Bacterium coli mutabile_ grown in Endo's medium, associated with the acquisition of the power to ferment lactose, are perfectly reliable and free from possibilities of mistake. The work has been extended and confirmed by many workers, especially R. Müller, who finds that this bacterium can similarly acquire and maintain the power to ferment other sugars. A careful account of the whole subject written by Müller for the information of biologists will be found in _Zts. für Abstammungsl._, VIII, 1912. After discussing the biological significance of the facts, he concludes with a caution to the effect that bacteria are so different from all other living things that generalizations from their behavior must not be indiscriminately applied to animals and plants.