40 KANSAS UNIVERSITY SCIENCE BULLETIN. 



may be some parallelism between the comparatively few suc- 

 cessful new types which I obtained and the sports which be- 

 come apparent in higher organisms. Whether there is any 

 innate connection generally between a temporarily diminished 

 vegetative activity and heterogenesis, as seemed to be the 

 case with certain of the micro-organisms in my experiments, 

 is, of course, only a matter of conjecture. 



We have further this important difference between cell 

 generations in unicellular and higher multicellular plants : 

 the possibility of isolating a single varying cell of the former 

 type alone, a matter which has a bearing on the question 

 whether new races arise among micro-organisms from single 

 varying cells under natural conditions. 



The comparative constancy of species of yeasts or bacteria 

 when kept under unvarying conditions argues against the 

 probability that varying cells of the sort which I isolated com- 

 monly produce progeny which successfully compete with the 

 parent culture. It will be remembered, too, that a large pro- 

 portion of these varying cells lack vitality, and in the case of 

 the yeast the growth of the new race is often very slow, until 

 it abandons the more filamentous condition and partially re- 

 verts to the type. But there are many chances of accidental 

 isolation of single cells of unicellular plants and in an en- 

 vironment which favors their development ; and, once started, 

 they may behave as most of my races did and become as 

 vigorous as the type, and, as in one race tested, they may be 

 capable of competing with the parent race in mixed cultures. 

 Again, there is evidence from the heating and drying experi- 

 ments conducted on a new race of Saccharomyces anomalus that 

 a greater resistance to unfavorable natural conditions may be 

 correlated with a morphological variation. This may be a 

 factor of weight in the origin of new races among micro- 

 organisms, subjected, as many of them are, to great vicissi- 

 tudes in environment. 



So, when we consider that physiological characteristics 

 may be correlated with the morphological, as in the case of 

 the increased power of fermentation in race A of B. coli com- 

 munis, and that we may well have variations characterized 

 by physiological characteristics alone, it seems well within 



