No. 556] SHORTER ARTICLES AND DISCUSSION 



237 



The question as to which of these views is the more probable 

 is closely bound up with the presence and absence hypothesis. 

 On a strict application of this idea there is of course no possibil- 

 ity of more than two members of any given allelomorphic group. 

 The presence and absence hypothesis as a universal principle 

 has been criticized by Morgan ('13) in a recent paper, on what 

 seem to me very strong grounds. It seems very unlikely that 

 protoplasm (chromatin?) is such a simple substance that the 

 only possible change in a given unit (molecule?) involves the 

 loss of that unit. On the other hand, if a slight change takes 

 place in a chemically complex gene, is it necessary to suppose 

 that its allelomorphic relations must be upset 1 That very slight 

 changes in the constitution of a gene might easily affect its be- 

 havior in ontogeny will, I think, be readily granted. 



It is to be noted that in all the cases cited above the supposed 

 three allelomorphs have similar ontogenetic effects. Thus the 

 three in rabbits, in Aquilegia, and in Papilio all affect the distri- 

 bution of pigment (and, in Papilio, also the shape of the wings), 

 those in Lychnis the sex, those in beans the production of the 

 same color in different organs, and those in Drosophila the pro- 

 duction of different colors in the same organ. This may perhaps 

 seem to be in favor of the view that we have here different modi- 

 fications of the same gene, rather than two distinct genes and 

 their absences. 



The history of the red-white-eosin group of eye colors in 

 Drosophila is interesting when considered from the viewpoint of 

 the presence and absence hypothesis. The first white-eyed fly 

 arose as a mutant in red stock. On presence and absence it must 

 have been caused by the simultaneous loss of two factors, which 

 were called C and by Morgan. Then, in white-eyed stock there 

 appeared an eosin-eyed fly. Here the factor called 0, just lost, 

 must have been put back again. Finally, in one of my own cul- 

 tures, eosin has given rise to white by mutation. 1 In both these 

 latter cases the flies had miniature wings, and in the white-to- 

 eosin case they also had black body color. These characters give 

 a check on the results, and make it extremely unlikely that any 

 contamination had occurred. Further evidence to this effect is 



1 After this paper went to press it was pointed out to me by Mr. H. J. 

 Muller that there is another possible explanation of this case, which does not 



entered into until certaiiTphenomena observed by Mr. C. B. Bridges have 

 been more fully investigated. 



