122 PHYSIOLOGICAL GENETICS 



pletely upon a knowledge of the physiology of dominance. 



The former discussions make it clear that the facts that physio- 

 logical genetics have brought to light regarding dominance furnish 

 a basis for such views as Fisher holds. But the decisive problem 

 tor the phylogenetic side is, of course, not the underlying develop- 

 mental mechanism but the question of selection pressure, as 

 discussed in the papers of Fisher's opponents, but which do not 



belong here. 



Regarding the developmental system and its meaning for a 

 possible selection process, some additional views have been 

 derived, which are actually based upon the physiological facts. 



Muller (1932rf) realized that a theory of the phylogenetic 

 evolution of dominance of the Wild type by selection of modifiers 

 requires a physiological interpretation of dominance of the same 

 type as the one developed by Goldschmidt (1927c) and proposes 

 to look at the selection problem from a different angle. He 

 postulates that the mutations of modifiers favoring dominance 

 have been selected not so much for their specific protection 

 against heterozygosis at the locus in question as to provide a 

 margin of stability and security, to insure the organism against 

 weakening genetic or environmental variability. "These modi- 

 fiers must so affect the reaction set in motion by the primary 

 gene in question as to cause this gene, when in two doses, to be 

 near an upper limit of its curve of effectiveness, that is in a nearly 

 horizontal part of the curve, not so readily subject to variation 

 by influences in general, including reduction in the dosage of 

 the primary gene." This, of course, presupposes an explanation 

 of dominance of the type of our model (Fig. 23) and of the type 

 proved to exist in the vestigial case (Fig. 20). The phylogenetic 

 speculations (and calculations) on the selection of dominance 

 modifiers then fall in line with the results derived from physiologi- 

 cal genetics. 



Simultaneously with Muller, Plunkett (1932) derived the same 

 conclusion and expresses it in the same terms as those derived 

 above from the vestigial case. He points out that Wild type is 

 much less susceptible to environmental and other effects then 

 the mutants. This, he says, is due to differences in the distance 

 of the developmental process from its asymptote at the time when 

 it ends; processes controlled by Wild-type genes usually closely 

 approach their asymptotes, while those modified by mutant 



