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in millions. It seems almost impossible to believe that we should have ever 
found such a mutant. 
5. The experimental evidence shows there are many factors arranged 
in a linear series on the X chromosome. Some affect wings, some body 
colors, others the shape of the eye, and so forth. Sturtevant has pointed out 
the significance of the fact in light of the above statement that the characters 
which behave as members of a multiple allelomorph system are closely 
related physiologically. 
6. If the mutants are the result of changes as shown in Fig. D it would 
seem as if a mutated stock would more readily give rise to subsequent mu- 
tations, since fewer simultaneous losses are necessary. As a matter of fact 
four of the members mutated directly from red while eosin came from white. 
7. Morgan has emphasized the idea that it is difficult to account for 
reverse mutations on the assumption of losses from a completely linked chain 
of loci, as the Presence and Absence theory postulates. On the other hand 
it is conceivable how such a reaction could come about if the mutant is the 
result of an expression of something analogous to an isometric change. 
8. Is chromatin such simple material that the only change conceivable 
is a loss? 
LITERATURE CITED. 
1. Morgan, Sturtevant, Muller and Bridges. Mechanism of Mendelian 
Heredity. Henry Holt. 1915. 
2. Bateson, William. The Address of the President of the British Asso- 
ciation for the Advancement of Science. Science. August 28, 1914. 
3. Hyde, Roscoe R. The new members of a Sex-Linked Multiple 
(Sextuple) Allelomorph System. Genetics. November 1916. Princeton 
Press. 
