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THE AMERICAN NATURALIST [Vol. XLVII 



not be a loss, and that a recessive may revert in the sense 

 that it may mutate. In chemical terms, the process is 

 reversible. 



Ill 



As I have pointed out, the presence and absence nomen- 

 clature, if properly understood, offers no practical diffi- 

 culties so long as only two changes in the same organ are 

 involved, but in experiments with Drosophila we have 

 passed beyond this stage and must have at command a 

 system by means of which more than two factors may be 

 easily and conveniently represented. How impossible it 

 becomes to use the presence and absence nomenclature 

 when new characters are appearing may be shown by the 

 following illustrations. 



As already stated, Mendel's method of representing 

 the allelomorphic pairs sufficed so long as one new char- 

 acter is contrasted with the original one. In this sense 

 the relation of a vermilion-eyed mutant to the red-eyed 

 fly could be fully represented by treating red (R) and 

 vermilion (V) as allelomorphs. But when another muta- 

 tion in eye color appeared the scheme was no longer fea- 

 sible. Now, in the same sense in which it became neces- 

 sary to supplant Mendel's scheme by another one, it be- 

 comes necessary to change the presence and absence 

 scheme when a third mutation appears in the same organ ; 

 for, the presence and absence scheme is not sufficiently 

 elastic to allow the introduction of a new term in the 

 series, unless a complete revision of the method is made 

 each time that a new mutation in kind occurs. 



For example, when it becomes desirable to compare the 

 eosin eye with the vermilion-pink (or orange eye already 

 known) it becomes puzzling to know what symbols to 

 adopt. If, as I assumed, the symbol in VPO is changed 

 to small o, then the formula for eosin becomes VPo. But 

 this is inconsistent with the scheme already adopted be- 

 cause the small letter o stands for a character called 

 eosin. If to avoid this ambiguity a letter E (or e) is in- 

 troduced for eosin the situation is even more puzzling. 



