290 JENNINGS! CHANGES IN HEREDITARY CHARACTERS 



one called "blood," near the extreme red end of the series, the 

 other, called "tinged," near the extreme white end; in fact, from 

 the descriptions it requires careful examination to distinguish 

 these two from red and white, respectively. Thus we have now 

 six grades of this unit. And in the same number of the same 

 journal, Safir (1916) adds another intermediate grade, lying 

 between "tinged" and esoin; this he calls "buff." All these 

 seven grades are diverse conditions of the single unit factor, 

 having its locus in a certain definite spot in the X-chromosome. 

 Such diverse conditions of a single actor are known as multiple 

 allelomorphs. 



So, up to date we know from the mutationists' own studies of 

 Drosophila that a single unit factor presents seven gradations of 

 color between white and red, each gradation heritable in the 

 usual Mendelian manner. These grades are the following: 

 (1) Red; (2) blood; (3) cherry; (4) eosin; (5) buff; (6) tinged; 

 (7) white. 



Three of these grades have been discovered in the ast five 

 months. It would not require a bold prophet to predict that as 

 the years pass we shall come to know more of these gradations, 

 till all detectible differences of shade have been distinguished, and 

 each shown to be inherited as a Mendelian unit. Considering 

 that the work on Drosophila has been going on only about seven 

 or eight years, this is remarkable progress toward a demon- 

 stration that a single unit factor can present as many grades as 

 can be distinguished; that the grades may give a pragmatically 

 continuous series. The extreme selectionist asks only a little 

 more than this. 



Besides showing that a unit factor may thus exist in numerous 

 minutely differing' grades, this case shows that a heritable varia- 

 tion may occur so small as to be barely detectible. Although the 

 variations do not usually occur in this way, the case presents the 

 conditions which would allow of a gradual transition from one 

 extreme to the other, by means of numerous intermediate con- 

 ditions. In a population in which were occurring such minute 

 changes as are here shown to be possible, we could get by selec- 

 tion such a continuous series of gradations as Castle describes in 



