CNOTHERA AND THE MUTATION THEORY 315 
(Enothera situation in other respects. If this stock 
is crossed to wild type flies the F'; individuals are 
beaded and wild-type in equal numbers, suggesting 
at once the ‘‘twin hybrids” (leta and velutina) of 
certain (Enothera crosses. If one of the “balanced” 
chromosomes contains one or more recessive mutant 
genes (and such have been purposely introduced into 
it by appropriate crosses), crossing over between 
any of the recessive genes and the lethal region may 
cause the appearance of a small number of specimens 
showing the corresponding recessive characters, and 
these will be produced in small numbers generation 
after generation, exactly as in the case of such 
“mutants” of O. Lamarckiana as nanella or oblonga. 
The frequency with which these types will appear 
will depend simply on the frequency with which 
crossing over occurs—.e., on the loci of the genes 
in question. 
Similar cases of balanced lethals have since been 
observed in other races of D. melanogaster, such as 
truncate and certain races of dichete. In at least 
one instance such a race has been artificially made 
up in order to save labor in keeping stock of certain 
lethal characters. There are strong indications that 
a somewhat similar case of balanced lethals occurs 
in stocks (Matthiola). 
This work of Muller’s, then, furnished definite 
information that permanent heterozygotes due to 
balanced lethals can and do exist, and furthermore 
correlated this phenomenon with the repeated ap- 
pearance of apparently new types in small numbers. 
