76 
R. Ruggles Gates. 
facet number, and that by “partial non-disjunction,” the factor 
passes from one chromosome to another (presumably during 
meiosis) so that one chromosome is without a limiting factor while 
the other member of the pair has two. The egg retaining the 
latter chromosome would produce a bar-eyed individual. If in such 
a race a second non-disjunction occurs, separating the two factors, 
one chromosome would result having triple factors and one with 
a single factor. The latter would give a reversion to a full-eyed 
male or a heterozygous female. But it is doubtful if such a 
method of explanation by splitting up the factor could be applied 
to reversions in other factors which show no such variability as 
bar-eye. 
A chemical reversal seems more likely to supply a general 
explanation of reverse mutations, and this is important in its 
bearing on the nature of a mutational change. For if a recessive 
mutant factor can revert to the wild condition, then the mutation 
was not due to an irrevocable loss of a particle, but rather to the 
transformation of a particle or locus of a chromosome, first in one 
direction and afterwards back to the original condition. 
As regards the infrequency of simple Mendelian factor 
mutations in CEnothera, it would appear that this is partly due to 
their presence being masked by the greater number of lethal 
factors. And since lethal factors must produce non-viable gametes 
or zygotes, i.e., sterility of pollen, ovules or seeds, this is in accord 
with the large amount of sterility in CEnothera. For this reason, 
except in the case of mutations involving visible changes in the 
chromosomes, we can seldom be sure in CEnothera that the 
germinal change in a locus of a chromosome which marks the 
origin of a mutation, did not happen several or many generations 
previously. It is this contingency with regard to various mutations 
that has made it possible to suggest that the phenomena are merely 
the result of the splitting out of factors acquired in some previous 
hypothetical cross. This argument has been used, notwithstanding 
the fact that mutations occur in such well-authenticated and self- 
pollinated species as CE. biennis . 
It is an interesting ^and significant fact that although 
Drosophila , like most animals, can only be perpetuated by a 
crossing of two individuals in every generation, while in such 
species as CE. biennis natural crossing is a very rare event at best, 
yet the bogey of hybridization which is so often raised as a 
complete explanation of the mutations in CEnothera , has never, so far 
