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morphs. According to this conception there is a definite something (W) 
located at point 1.2 on the X chromosome which stands for the red eye of 
the wild fly. (Fig. A.) This gene underwent some kind of change and gave 
rise to white eyes (w). In another stock the same particle mutated and 
gave rise to eosin (we). In still another stock the same particle changed and 
gave rise to cherry (w‘). (W) is allelomorphic to (w), to (we) and to (we). 
each of these in turn is allelomorphie to each other; hence they form a 
system of Multiple Allelomorphs. This view is supported by a large amount 
of experimental data by Morgan and his co-workers, but strange as it may 
seem the numerical results can be interpreted in terms of the Presence and 
Absence theory provided the mutants are the result of losses of several factors 
that stand for red in a completely linked chain of loci. 
The assumption that these three mutants are the result of changes in 
loci lying very closely together on the chromosome as demanded by the 
Presence and Absence theory has been tested by Morgan and others by 
means of their linkage relations in three possible combinations as given 
in Fig. D. (Shown by the broken lines on the left.) The discovery of the two 
new mutants has made it possible to carry out the test in eight additional 
ways. The evidence which involves data from something like a half-million 
animals weighs heavily against the Presence and Absence theory and is 
entirely in accord with the assumption that something analogous to isomerism 
may change an hereditary factor resulting in the production of a new form. 
I have attempted to visualize this in Fig. E. If this is the correct inter- 
pretation the possibilities locked in a small amount of chromatin may be 
almost infinite, for a great many different arrangements are possible from 
a few things. 
There are some points worthy of consideration as tending to give weight 
to the Multiple Allelomorph theory. 
1. On the Presence and Absence theory, it is necessary to assume that in 
the region of 1.2 on the X chromosome there is a chain of five completely 
linked loci (very close together) upon which the color of the red eye of the 
wild fly depends. Multiple Allelomorphs accounts for all of the facts 
while postulating but one locus. 
2. Gratuitous to the Presence and Absence theory let us assume that the 
loci are in jutaposition. If we assume that blood, cherry, eosin, tinged and 
white have appeared as a result of successive losses as shown in Fig. C, we 
encounter a difficulty. When any two of these mutants are crossed the 
