224 Transactions British Mycological Society. 



differing in some obscure serological reaction, and extremely 

 difficult to isolate. These correspond to the ultimate physio- 

 logically differentiated categories of fungi, and represent the 

 "species" of Lotsy(23) which is the genotype in the phanero- 

 gams. These serological races harmonise perfectly in their 

 growth and often exist as mixed populations. Many of the 

 pedigree cultures which have been used in laboratories for 

 years are such mixed populations, the component species 

 existing side by sid.e through numerous transplantations. It 

 is only the recently devised and still largely neglected technique 

 for the isolation of individual bacteria wherewith to initiate 

 pedigree cultures, that furnishes hope of solution for this most 

 embarrassing complication. The ordinary method of purifica- 

 tion by dilution is practically valueless in such circumstances. 



There can be little doubt that many of the "pure" cultures 

 of organisms which have formed the basis for experimental 

 researches on the induction of mutations, have not been abso- 

 lutely pure species but mixed populations with unknown 

 components. If therefore in these investigations the most 

 critical factor — the initial organism — may be a mixed and un- 

 known quantity, it does not seem wholly wise to build any funda- 

 mental and far reaching conclusions on the results obtained. 



Moreover it is an elementary principle in scientific methodo- 

 logy that of two hypotheses, that which may be expressed in 

 simpler terms, or in terms of more general applicability, should 

 be chosen ; and if one applies this to the phenomena in question, 

 one finds that they may be brought within the scope of present 

 hypotheses and paralleled by known facts. 



There are few of these results which may not be explained 

 in terms of the selection of components from a mixture, of 

 species from a population. The classical work of Johannsen(i9) 

 on pure lines shewed the immense importance of this concept 

 in the study of the higher organisms, and there are few aspects 

 of biology to which the pure line concept has not made its way, 

 although its influence is yet hardly appreciable in Mycology 

 and Bacteriology. As applied to the lower organisms this 

 concept may be shewn diagrammatically as follows: 



I. 



O O III 



I O I k o o 



N.F. o o o o 



II. 



Ill 



X ''^ o 



O II 



N.F. 



O I O II 



O I O O O > III 



I II 



O O I 



