72 PHYSIOLOGICAL CKNETICS 



are necessary for the cross-vein format ion, e.g., the formation of 

 morphogenetic Bubstances, are speeded up by temperature action. 

 In the second sensitive period, coinciding with the actual forma- 

 tion of the veins, the amount of these substances is determined, 

 but the time of development may be differentially changed by 

 temperature action. This is, of course, the same type of interpre- 

 tation thai has always been found for gene-controlled processes, 

 following Goldschmidt's theory of timed velocities. 



But the decisive point has not been touched here: why is the 

 effect produced in different percentages of individuals? This 

 part of the problem requires, no doubt, some form of the threshold 

 concept combined with the conception of a certain fluctuation 

 of the effect, as derived above for the cases studied by Gold- 

 schmidt. If this fluctuation takes place near the level of the 

 threshold, a more or less considerable part of the range of fluctua- 

 tion may fall above the threshold. We have already emphasized 

 this point in the last section. This shift of the curve of fluctua- 

 tion of some decisive process beyond the threshold may, however, 

 be brought about by an action of temperature or of so-called 

 modifying genes, e.g., as a shift of the mean of fluctuation toward 

 or away from the threshold value; or as an increase or decrease 

 of the range of fluctuation of this reaction product; or as a shift 

 of the threshold value for the action of that substance. Then 

 penetrance, as far as it is not hidden segregation, is a phenomenon 

 of threshold combined with statistic variability of the products 

 of gene action. In a recent paper on the scute gene, Child 

 (1936) applies the same concept to the process of bristle formation 

 and illustrates it by the same type of diagram that Goldschmidt 

 used in other cases. 



4. Further Evidence. — We begin with work that was under- 

 taken to find out whether an insight into the action of the mutant 

 gene could be obtained by the use of quantitative methods. 

 Some of this work has been reported in earlier chapters (see 

 pages 31-34), or parts of it have been mentioned in other con- 

 nections. 



The first author who had derived the rate concept from experi- 

 ments on somatic characters produced by a series of multiple 

 allelomorphs was Wright (1916). Almost simultaneously Gold- 

 schmidt derived a rate concept from his work on the multiple 

 allelomorphs of sex genes (19176) and pigmentation (1917d), 



