214 PROBLEMS OF GENETICS 



restored is difficult to understand. Similar variations have been 

 observed in power of pigment production and other properties. 



These phenomena naturally raise the question whether any 

 cases of apparent loss of factors in higher forms may be com- 

 parable. 



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 pyo genes and 

 Myxococcus he succeeded in producing many strains with modi- 

 fied 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 pre- 

 sented by these organisms in a form identical with that which 

 we know so well in the higher animals and plants, whatever 



1 Pringsheim, H., Die Variabilitdt niederer Organismen, Berlin, 1910. 



2 F. Wolf, Modifikationen u. Mutationen von Bakterien, Zts. F. indukt. Abstam. 

 u. Vererbungslehre, II, 1909, p. 90. 



3 Winslow, C. E. A. and A. R. t Systematic Relationships of the Coccaceae. 

 New York. 1909. 



