220 GENETICS AND EUGENICS 



the middle one of the series. He finds that in a pure culture 

 of eosin, the intensity of the pigmentation may vary from a 

 "deep pink darker than eosin" to a **pure white," through 

 the modifying action of eight other factors, '*in origin en- 

 tirely independent of one another" and located each at a 

 different genetic locus, four being in chromosome II and one 

 in chromosome III, the others not having been definitely 

 located. Seven of the eight modifying factors act as diluters 

 or lighteners of unmodified eosin, one only acting as a dark- 

 ener. They are in the order of their darkening (or lightening) 

 effects, (1) dark, (2) pinkish, (3) cream c, (4) cream b, 

 (5) cream a, (6) cream III, (7) cream II, and (8) whiting. 

 **Each of these genes arose by mutation," while the stocks 

 were under continuous study, ''by the transformation of the 

 materials of a particular locus into a new form having a 

 different effect upon the developmental processes." The 

 eye-color mutations observed to occur in Drosophila since 

 Morgan's discovery of white eye are so numerous that 

 Bridges classifies them in per cents, as 60 per cent general or 

 non-specific modifiers of eosin, such as vermilion and pink, 

 22 per cent specific modifiers of eosin, and 18 per cent allelo- 

 morphs of eosin. He continues, **It is probable that muta- 

 tion (change in single genes) is very much more frequent 

 than appears, since a great many mutations are of very 

 slight somatic effect and would pass undetected except that 

 certain characters such as eosin eye-color, truncate wings, 

 beaded wings, and a few others, are peculiarly sensitive dif- 

 ferentiators for eye-color and wing-shape genes, etc." Here 

 we have a picture of genetic mutability in the most carefully 

 studied of organisms, occurring contemporaneously, which 

 affords all the material needed for selection either natural or 

 artificial to act upon in either darkening or lightening the 

 eye-color by a series of progressive steps, if such change 

 should be found advantageous or desirable. 



Among organisms reproducing sexually, the evening- 

 primrose has probably been studied more intensively than 

 any other except Drosophila. But it is impossible to say in 



