THE M ( TA TED GENE 223 



threshold present ; distribution, 3:2; active minimum, 2.5.) With 

 further increase of the effect, i.e., still more lowering of the avail- 

 able percentage, the probability of asymmetric distribution 

 with one wing above the threshold will decrease rapidly, and 

 absence of the character on one side will become very rare, 

 though asymmetrical expression on the two sides will continue. 



For a general understanding of the process it is rather unim- 

 portant whether or not the foregoing interpretation is literally 

 true. The decisive point is that such conditions as quantities of 

 a growth-promoting or retarding substance, thresholds of its 

 action, time and place of its liberation, path of its spreading will 

 provide a system in which a definite constellation will auto- 

 matically lead to an asymmetrical result. Any gene controlling 

 the master process, which in a given constellation w r ill enforce 

 asymmetric procedure, has, then, an asymmetric action. If 

 this explanation is correct, it follows that in cases such as those 

 just described, other genes (modifiers) may impress symmetry 

 upon the same development by shifting one of the integrating 

 processes, e.g., the value of the threshold or the time of onset 

 of the spreading of the substance. This is actually the case. 

 In the vestigial-wing series, the intermediate phenotypes are 

 rather asymmetrical on both wings. But it was possible to 

 select a symmetrical type from a few rare more symmetrical 

 specimens. 1 



A special study of such a case has been made by TimofeefT 

 (1934). The character, already discussed on page 71, is the 

 formation of cross veins in the wing of Drosophila funebris. In 

 the case of the scalloping of the vestigial wing, and of the inter- 

 sexuality in Lymantria, there was an asymmetry for both wings, 

 but no large independent variation, because the average amount 

 of effect (scalloping, pigmentation) was about constant for a 

 given genetic constitution. In the D. funebris case, Timofeeff 

 thinks that variation was independent in both wings, and 

 asymmetry therefore a statistical result of two independent 

 variables. By selection it was possible to establish lines with 

 a larger right-left correlation, more symmetry. There are, then, 

 genes that may control the phenomenon, just as in the case 

 of the vestigial wings. Timofeeff remarks that one might think 

 of humorally controlled processes and points to the fact that in 



1 Unpublished work. 



