:;r_> 



morphs. According to this conception there is a definite something (\V) 

 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 (w e ) and to (wc). 

 each of these in turn is allelomorphic 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 Avhich 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 



