MENDEL'S FIEST LAW 



35 



Marshall and Muller kept flies heterozygous for three re- 

 cessive mutant factor for about seventy-five generations, 

 and at the end of that time found that these factors had 

 not been weakened in any way as a result of juxtaposition 



Fia. 13. 



-Notch wings in the vinegar fly, extreme condition, 

 nearly normal condition, c. 



average condition. 



with their normal dominant allelomorphs. I have kept 

 a stock of notch-winged flies under selection for twenty- 

 five generations. Notch (Fig. 13) is a character varying 

 in the direction of normal wings (Fig. 13, c) ', in every 

 generation of notch, many notch flies have normal wings. 

 The character is dominant, and exists only in heterozy- 



