THE MUTATED GENE 83 



ingredients; (/) by controlling the threshold above which one or 

 the other of these variables is effective. 



2. By controlling the method of diffusion of the ingredients of 

 pigmentation over the organ, which will show different grades of a 

 pigment pattern: (a) by controlling the time of onset and the time 

 of flow of the stream (see pattern, page 193) ; (6) by controlling 

 some process in the substratum which checks or enhances this 

 flow. 



It is not claimed that these are all possibilities. But if one of 

 them is realized here and another there, certainly a number of 

 different types of reactions are imaginable which may lead to 

 graded effects by change of rate. This example for pigmentation 

 may easily be varied to fit cases of growth pattern, of production 

 of bristle-forming substance, or of whatever morphogenetic 

 processes capable of a linear quantitative variation are imaginable. 



In a former chapter, we have already mentioned cases of 

 multiple allelomorphs which fulfill all the conditions of an 

 explanation by the concept of reactions of different velocities. 

 We point especially to the case of the vestigial wing in Drosophila, 

 where the different indications for this type of action of the genie 

 series were presented as follows: 



1. The orderly arrangement of the effects in a quantitative 

 series which included also the compounds at the proper place. 



2. The embryological proof that the actual process involved — 

 destruction of wing area — begins earlier and earlier in the 

 increasing series. 



3. The fact that at the beginning of the series a typical over- 

 lapping with the normal takes place with a regular increase of 

 percentage in the ascending series. 



4. The fact that the highest members of the series drift from 

 increasing in viability into lethality. 



5. The fact that change of temperature shifts the effect within 

 the same series. 



6. That effects of the same type and seriation may be produced 

 as phenocopies by temperature shocks or X rays. 



7. That effects of modifying genes may produce the same shifts. 



8. That certain phenomena of symmetry fit into the general 

 type of explanation (see page 229). 



Another type of series already mentioned, which presented facts 

 that could be conceived only in terms of different rates of one 



