264 Action of the Genetic Material 



development). Thus we shall have to look for a different explanation 

 of the Drosophila work, which probably does not involve the features 

 of the chemical basis of morphogenetic change. For the time being, 

 we may say frankly that the specific chemical phenocopies are not 

 yet understood, though the fact of their existence is very provocative. 

 But we may safely predict that a future explanation will shed light 

 on some phase of the problem of mutant action of the genie material. 



One fact which has been found in some of my unpublished experi- 

 ments may be relevant here. It is known that many mutants produce a 

 very variable phenotype (which might be made constant by selection 

 of proper modifier systems, or in the presence of "strong" alleles). 

 This is very conspicuous in mutants producing very large effects like 

 the homoeotic mutants (see my papers on podoptera). One mutant 

 may produce a whole gamut of effects from almost normal through 

 all intermediates up to the complete, extreme type. We shall discuss 

 these facts below in regard to their meaning for genie action. In one of 

 our experiments on chemical phenocopies, one mutant was specifically 

 copied (Scutenick), and always in a variable form involving a series 

 of effects from hardly visible ones through all intermediates affecting 

 more and more structural elements to the extreme pleiotropic expres- 

 sivity. I shall later propose that such variations in expressivity can only 

 be understood in terms of interplay of rates and thresholds. This 

 identity of the phenomenon in mutants and phenocopies is in favor of 

 the conclusions drawn for non-chemical phenocopies. 



In future discussions of the interpretation of phenocopies, all the 

 facts of chemically induced morphogenetic change in animals and 

 plants, since the classic work of Herbst (1901), will be included, 

 though it does not directly relate to phenocopy because in most cases 

 no identical mutant effect is known. However, as we might say safely 

 that all these effects are of the same general type as mutant effects, 

 they also become relevant for the problems of specific biochemical 

 phenocopy and, thus far, all of them seem to indicate general quanti- 

 tative effects upon rates of growth and differentiation, just as we 

 expect will be true of all phenocopies. 



It might be objected that it is hardly probable that all the diverse 

 manifestations of morphological mutants and phenocopies may be 

 reduced in the end to relative rates of individual processes of growth 

 and differentiation, but there are many examples which point in this 

 direction. We shall consider later Vogt's work on homoeotic mutants 

 which led to such conclusions. Here I mention only one example, the 

 seasonal dimorphism of butterflies, the classic example of which is 



