MUTATION 



265 



of balanced lethal stocks like beaded that they repeat 

 precisely this phenomenon. For instance, if a beaded 

 male is crossed to wild female, two kinds of offspring are 

 produced, vis., beaded and normal. A similar process 

 would account for twin hybrids in (Enothera crosses. 

 There is another peculiar phenomenon that has been 

 described for crosses in the evening primroses, viz., the 

 occurrence in F^ of four types. This phenomenon, too, 

 can be imitated in Drosophila by crossing balanced lethal 

 dichete to balanced lethal beaded (Fig. 117). 



Both stocks breed tru«. Fovir types In Pj l:l:l!l 



FiQ. 117. — Diagram illustrating lethals and four types in Fi. 



Other parallels might be cited, but these, I think, will 

 suffice to indicate that the discovery of balanced lethal 

 stocks may solve some at least of the outstanding difficul- 

 ties of mutation and inheritance in (Enothera, and bring 

 it into line with other groups. There are, of course, other 

 peculiarities of the evening primrose that such zygotic 

 lethals will not explain; such, for instance, as the 15-chro- 

 mosome type, and 0. gigas. But these cases are already 

 on the road to solution. 



The occurrence of other lethals, called gametic lethals, 



