No. 604] SHOETER ARTICLES AND DISCUSSIONS 



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in cases of apparent multiple allelomorphism (Morgan, iluller, 

 Sturtevant, and Bridges, 1915, chap. 7). In fact, this added as- 

 sumption probably permits a more direct and therefore prac- 

 tically simpler representation of the actual course of segregation. 

 Whether the corresponding factor-to-factor notations now used 

 for Drosophila (Castle, 1913; Morgan, Sturtevant, Muller, and 

 Bridges, 1915, p. 233) are everywhere adequate and convenient 

 is another question, as Emerson (1913) has shown. 



Cases of multiple allelomorphism involve no special difficulty in 

 principle for the presence-and-absence scheme. They can of 

 course be represented only by linkage formulae, in which the 

 "presence" of one factor of the set is linked with the absence of 

 the rest — but their very occurrence suggests that we might, as is 

 suggested above, correctly enough represent single factorial dif- 

 ferences in the same way. All this can affect only the conveni- 

 ence of the notation, and not at all its logical applicability. 



If we adopt a factor-to-faetor system of notation, it is natural 

 to conceive of the opposed "factors" not as mere potentialities, 

 but as physical units responsible for genetic potentialities. When 

 we have taken this viewpoint, we have begun to use the w^ord 

 factor in the second sense mentioned above ; we are thinking of 

 assumed physical units of segregation, not merely of observed 

 potentialities of development. Morgan (1915, p. 419), in dis- 

 cussing "presence and absence," uses factor in this sense, as do 

 Morgan, Sturtevant, Muller, and Bridges (1915, pp. 220-222). 



No doubt what has been said above is an old story to experi- 

 enced geneticists in general, in view of such discussions as those 

 of East (1912) and Morgan (1915). The distinction is so funda- . 

 mental, however, and the double use of the term factor so in- 

 creases the difficulties of the case, that consideration of the gen- 

 eral problem from the present terminological vie\^^)oint seems 

 highly desirable. Perhaps greater precision in the use of several 

 terms could be attained. 



Johannsen (1909, pp. 124-125), in defining the term Gen 

 (gene), makes it perfectly plain that he means the material 

 basis^ or cause (the Anlage), of whatever sort, of a "unit char- 

 acter," defining a unit character as one dependent on a special 

 kind of gene. He is evidently inclined to consider the gene as 

 the material unit of segregation, holding that the sum of the 

 genes constitutes the genn-plasm, and there is a widely prevalent 



