THE MUTATED GENE 229 



up to vestigial, e.g., by drawing a typical series of these patterns 

 together into the contour of a normal wing (Fig. 41), we see a 

 way of understanding these patterns. If a definite amount of 

 the supposed growth substance diffuses into the wing from its 

 base — to use only one of the alternative explanations, the other 

 being its reciprocal — the path or bed of such a flow will be 

 determined by the structural, physical conditions of the organ, 

 and a more or less definite bed will be prescribed. If this flow 

 ceases at different times before the whole wing is covered (insuffi- 



Fig. 41. — Wings of a series of v^-alleles drawn together to demonstrate the path 

 of the destructive process. 



cient amount of the substance), the rest of the wing will degener- 

 ate, and the contour of the degenerating area, of the scallopings, 

 will represent the front line of the flow and the moment of its 

 being stopped. The whole series of stages of scalloping, then, 

 furnishes a cinematic picture of the bed along which the substance 

 in question spreads. The pattern produced is therefore a func- 

 tion of the amount of spreading growth substance controlled by a 

 mutant gene (or by a phenocopic effect) and of the structure 

 of the organ providing the drains and obstacles, shortly to be 

 described as the conditions of the system. In case of explanation 

 by spreading of a lytic substance, the moving picture would be 

 the reciprocal of the foregoing: the filling in of the wing area with 

 this substance, beginning at the tip. 



This interpretation recalls at once a case that we have already 

 described, the pattern upon the wings of intersexual males of the 

 gypsy moth (Fig. 35). We remember that here, too, the facts 

 led to the conclusion that the determination of scales in the male 



