BARBER: HEREDITY IN CERTAIN MICRO-ORGANISMS. 41 



the range of probabilities that mutations, if such they are, 

 have played some part in the evolution of species of micro- 

 organisms differing physiologically as well as morphologically 

 from the ancestral type. We may have here a factor in the 

 origin or increased virulence of some pathogenic types. 



It is suggested by Meyer* that such variations among 

 micro-organisms may be simply a matter of atavism, and 

 Will refers his yeast races to polymorphism. But in experi- 

 ments on higher organisms as well we often meet with the 

 same difficulty of deciding whether we have to do with the 

 appearance of a new character or the reappearance of a latent 

 one ; and in the case of both higher and lower plants only 

 long-continued experiments on many different types can de- 

 cide the matter. If mutations occur among the cells of 

 higher plants, we would expect, on a priori grounds, to find 

 them in the lower also, and perhaps more frequently in these 

 less differentiated and more plastic types. 



Fisher ( 1897 ) inclines to the view that new races among 

 micro-organisms are to be referred to degeneration. In favor 

 of this view is the diminished vitality of many cells similar 

 to those which originated new races in my experiments, and 

 the slow early growth and diminished spore production of 

 some of the yeast races. But, with the possible exception of 

 B. coli Y the now races described above are, when once begun, 

 as vigorous vegetatively as the type, and in the B. coli races A, 

 H and Y the offspring of the varying cells early became as 

 vigorous, or nearly as vigorous, as those of the type. Further, 

 these variations appear in relatively few cells and under ap- 

 parently optimum conditions of growth, so they can hardly 

 be referred to immediate conditions, unfavorable or other- 

 wise, acting on the whole culture. 



METHOD OF ISOLATION, f 



The essential parts of the apparatus used in the isolations 

 described above consist of an ordinary 1x3 inch glass slip, 

 to which are cemented pieces of glass in such a way as to 

 form a box open at the top and one end (see h, fig. 3). The 



*Vide supra. 



tA short preliminary description of this method was published in The Journal of the Kansas 

 Medical Society, of November, 1904. 



