624 
GENETICS: H. J. MULLER 
Oenothera produce their effect upon the gametes, rather than upon the 
zygotes. 
Double throwing stocks (Matthiola) present another case of balanced 
factors. This too differs in detail from the beaded case, for one of the 
factors acts very early, producing its lethal effect directly upon the 
gametes (pollen), as in Oenothera, whereas the other, although it affects 
the zygotes, does not act as a lethal to their soma, but merely causes 
their sterility. It is this factor which causes the double flower. 
7. The condition of balanced lethal factors must slowly lead to a par- 
tial degeneration of the chromosomes containing these lethals. For 
any new lethal recessive factors that arise in these chromosomes will 
never have the opportunity of becoming homozygous and producing 
their harmful effect, and so there will be no cause for natural selection 
to eliminate them. Lethal recessive mutant factors of all sorts (in- 
cluding 'deficiencies') will therefore gradually accumulate in the chromo- 
somes of the affected pair. Moreover, although Im i is the first lethal 
recessive which has been found in an autosome of Drosophila, theoreti- 
cal considerations and the experiments above reported lead to the con- 
clusion that in the course of time the number of such mutations will 
have been not inconsiderable, as they probably form a large proportion 
of all the mutations that occur. For similar reasons, chromosomes of 
stocks which are continually outcrossed, and the Y-chromosome in all 
species containing it, should undergo degeneration, because these 
chromosomes, too, are always protected, by heterozygosis, from the 
action of natural selection. 
8. Not only are mutants of an undesirable nature not eliminated 
from balanced chromosomes by natural selection, but recessive mutant 
factors of a desirable type also are prevented from becoming homozy- 
gous and producing their effects, and so they cannot be selected for. 
On account of this latter circumstance, evolution is hindered in these 
varieties. As each chromosome of the balanced pair degenerates, 
however, it must gradually lose the dominant normal factors that pre- 
vented recessive mutant allelomorphs in the opposite chromosome 
from manifesting themselves. The balanced races might, moreover, 
eventually return completely to a condition of normal genetic behavior, 
owing to the occurrence of doubling or non-disjunction, which might 
make two normal pairs of chromosomes out of one balanced pair. 
9. The inheritance of beaded is complicated not only by balanced 
lethal factors, but also by a modifiability of the character under the 
influence of environmental conditions, and by multiple factors. A 
number of the well known mutant factors for totally different characters 
