Igrt] SHULL—REVERSIBLE SEX-MUTANTS 357 
In the second section of table I the hypothetical gene H for 
hermaphroditism is given limitations which make it fit all the 
empirical results of both the first and subsequent generations; 
but when the significance of the limitations is taken into account, 
it becomes evident that there is small advantage gained by the 
postulation of such a gene. Indeed the only advantage lies in the 
fact that in case the female is a positive homozygote, it keeps open 
the question whether or not there is a synaptic mate of F in the 
normal male; for a newly arisen hermaphrodite gene (H) might 
conceivably become a synaptic mate of F, even though the latter 
had had no synaptic mate in the normal male. 
If the female is a neutral homozygote, that is, if the female 
gene F has a male gene M as its synaptic mate in the male, the 
hermaphrodite gene (if it exist at all) must be absolutely coupled 
with this male gene. In like manner, if the female is a negative 
homozygote FFmm, the H (if present) must be coupled with the 
male gene M. It is simpler, however, to assume that the hermaph- 
rodite determiner is a modified form of the sex gene itself, than 
to suppose that it is a separate gene invariably coupled with the 
sex gene. This conception that hermaphroditism results from a 
mutative change in the sex gene, or in its homologue (?), the “Y- 
element,” is made the basis of the last section of table I, but can 
apply only to those cases in which a male gene is present, or if 
not a male gene, then its homologue, a sexually indifferent gene 
which takes the place of M in the male; for if the hermaphrodite 
character is assumed to be due to a change in the female gene 
(F), as it must be if the latter has no ‘‘synaptic mate,’’ the scheme 
will not work. 
It appears to me impossible at the present time to determine 
whether the females of Lychnis are positive, neutral, or negative 
homozygotes. The facts seem to be equally well met by any of 
these assumptions; but the definite limitations of the hermaphro- 
dite character to the males makes inapplicable the extreme form 
of the “presence and absence” hypothesis (that is, the hypothesis 
of unpaired genes) unless the female is a negative homozygote 
With reference to a male sex gene (M). While the possibility must 
be kept open that this is the relationship of the sexes in Lychnis, 
