372 CALVIN B. BRIDGES 



and another in 1916. On various other occasions dilute eosms 

 have been observed whose inheritance has not been followed in 

 detail.'^ 



Origin of modifiers. All of the modifiers were first detected 

 in stocks or experiments involving eosin. This -fact is not the 

 result of any influence of eosin upon mutation, but has a simple 

 explanation in the fact that the modifiers produce little or no 

 visible effect except when brought into coaction with eosin, and 

 hence they pass undetected no matter how numerous, or what 

 their origin, until this condition is satisfied. Only the last two 

 modifiers, 'pinkish' and 'cream c,' were found in stocks pure for 

 eosin, and in these cases the mutation might have occurred previ- 

 ously and have been incorporated with the eosin stock. The 

 other six modifiers were found in experimental cultures in which 

 only half of the flies were eosin; and in these cases it is usually 

 not possible to say whether the modifier originated in or was 

 introduced through the eosin or the not-eosin half of the experi- 

 ment. In the case of cream II, there is good evidence that the 

 mutation had occurred in a wild stock and had lurked there 

 undetected until the cross to eosin brought it out of hiding. 

 Cream b seems to have been present in both parent stocks. 

 Creams a and II, dark and whiting, may have arisen in either, 

 though the probability is on the side of the not-eosin parent, 

 since these modifications were likely to have been detected if 

 present in the eosin parent stocks. 



The scale of modifications. A graphic representation and com- 

 parison of the color differences produced by the modifiers is 

 given in diagram 1. 



In constructing this diagram two standards were chosen, 

 namely, the colorless eye produced by the interaction of eosin 

 and 'whiting', and the unmodified eosin. The modified eosins 

 were then spaced along the line connecting these grades in pro- 

 portion to their intensities of color. That is, homozygous cream 

 II is the lightest of the other modifications and 'dark' is the 

 darkest. Pinkish is the weakest of the dilutors of eosin. Creams 



'' In 1918 two other creams have been found, and the inheritance of one of these 

 presents features of exceptional interest for the chromosome theory of heredity. 



