74 PHYSIOLOGICAL GENETICS 



stance or forming a bristle-suppressing substance; or by a decrease 

 relative to the other developmental processes in the rate of such 

 reactions. (Note the parallel to the vestigial case). It is known, 

 of course, that the general rate of development increases with 

 increase of temperature, and therefore the bristle-forming reac- 

 tion must have a specific temperature coefficient (as had been 

 assumed by Krafka, 1920a; Goldschmidt, 1920b; and others in 

 their cases). The fact of increase of bristle-destroying effect 

 with temperature points to the second alternative — decrease 

 of rate relative to general development. It is then assumed that 

 the concentration of the bristle substance is proportional to 

 the mean number of bristles produced and that the rate of the 

 decisive reaction that reduces the bristles also is always propor- 

 tional to that concentration. The total time of this reaction is 

 calculated to be proportional to the total time of development at 

 all temperatures within the vital optimum. Now, the difference 

 between Wild type and Dichaete is that in the latter a reaction 

 takes place that may be described as bristle destroying. This 

 reaction of the Dichaete gene, then, is to produce a catalyst which 

 catalyzes the bristle-destroying (or obstructing) substance with 

 a definite velocity proportional to its concentration. (Note, 

 again, the parallel to the vestigial case and the phage.) From 

 these premises, Plunkett proceeds to calculate the thermal 

 increment for the bristle-reducing reaction on the basis of an 

 irreversible monomolecular reaction, using the data for tempera- 

 ture-bristle effect and those for time of development. Further- 

 more, he calculates the effect of different temperatures as expected 

 if the equation that was used actually applies. The calculated 

 expectations fit very closely the data of the foregoing table. The 



