

ai 
‘ 
AS CAUSE OF THE APPARENT CONSTANCY OF THE MUTANT „SPREAD”. 323 
act constantly white-flowered and may easily be explained as 
arisen by mutation. 
It is hardly probable to suppose that a greater number of sup- 
posed mutations are owing to such a crossing-over between closely 
linked lethal factors, but on the other hand the number of known 
lethals increases both in Drosophila and in other organisms, so 
that one should not wonder if certain supposed mutants must be 
explained in this way. 


As to the quality itself: spread wings, it is, according to DEXTER }), 
due to a factor in the IIIrd chromosome, located at 61, and spread 
should be recessive towards normal wings. 
à I have not tried further to control the position of the spread 
factor, as I do not possess mutants fit hereto, but to my great as- 
tonishment I have found — as mentioned above — spread domi- 
nant to the normal type; but it cannot be doubted that my type with 
its horizontal and, in relation to the longitudinal axis of the body, 
very projecting wings really corresponds to the type originally 
described by DEXTER; as aforementioned, [ also received the original 
individual under the designation ,spread”. A typical spread indivi- 

1) J. DEXTER: The analysis of a case of continuous variation in Droso- 
phila by a study of its linkage relations. Amer. Natur., Vol. 48, 1914, p. 712. 
