CAUSES OF VARIATIONS. 67 



closely related nuclei originating in the same individual or the same 

 cell could add to the essential characters of the organism in any way. 



Individuals originating from single spores of Glomerella must be 

 regarded as homozygous so far as it is possible to ascertain at present, 

 i. e., no union of nuclei or gametes between different individuals, 

 races, or strains has been proved. 



Whatever differences the progeny of such individuals might show 

 would evidently be due to mutation or some other internal cause, 

 such as the renewed expression of latent hereditary characters, or 

 else to some external or environmental action. 



As it has been impossible to trace any causal relation or connection 

 between most of the phenomena of variation observed and the con- 

 ditions of environment to which the organisms were subjected, it must 

 be concluded that the inducing causes are internal or else so obscure 

 as to escape observation. In any case the evidence accumulated by 

 others as well as by the writers appears sufficient to justify the con ■ 

 elusion that many of the variations observed and reported here are 

 not entirely due to any effect of simple environmental factors. It 

 must be remembered, however, that it is difficult if not impossible at 

 present to standardize media and methods with sufficient accuracy to 

 positively determine the exact effect of environmental factors on 

 these fungi. Gorham (38) and others have pointed out the apparent 

 impossibility of standardizing media containing agar and other organic 

 compounds. Synthetic media of pure inorganic chemicals can not be 

 substituted for many fungi, as they do not usually fruit on such 

 media. More work is needed to verify the conclusions of Stevens and 

 Hall (87) and others in regard to the direct effect of modifications of 

 the chemical constituents of culture media on the behavior of para- 

 sitic fungi. The problem is evidently far more complex than some 

 investigators have appreciated, and its complete solution can scarcely 

 be hoped for in the near future, but the most hopeful line of attack 

 seems to be that of pedigreed cultures of asexual or unisexual organ- 

 isms grown and observed under the most exactly determined and 

 controlled conditions possible and in sufficiently large numbers and 

 through enough generations to reduce probable errors from accidental 

 causes to a minimum. 



The work of Jennings (49) with Paramecium and that of Barber 

 (6), Will (96), Beijerinck (11), and Hansen (44) on yeasts, as well as 

 that of other authors cited by Pringsheim (65), demonstrate at 

 least one thing, and that is the actual existence of rather distinct 

 races or strains within species. These races possess more or less 

 distinctive and constant morphological or physiological character- 

 istics which are generally inherited by their progeny and are appar- 

 ently not primarily dependent upon environmental conditions. 



252 



