easily explained by
supposing that the female is heterozygous for some factor that is not found
in the male. Femaleness is an additional character superposed upon a basis
of maleness, and as we imagine that there is a separate factor for each the
full constitutional formula for a female is FfMM, and for a male ffMM. Both
sexes are homozygous for the male element, and the difference between them
is due to the presence or absence of the female element F.

There are, however, other cases for which the explanation will not suffice,
but can be best interpreted on the view that the male is heterozygous for a
factor which is not found in the female. Such a case is that recently
described by Morgan in America for the pomace fly (_Drosophila
ampelophila_). Normally this little insect has a red eye, but white eyed
individuals are known to occur as rare sports. Red eye is dominant to
white. In their relation to sex the eye colours of the pomace fly {116} are
inherited on the same lines as the _grossulariata_ and _lacticolor_
patterns of the currant moth, but with one essential difference. The factor
which repels the red-eye factor is in this case to be found in the male,
and here consequently it is the male which must be regarded as heterozygous
for a sex factor that is lacking in the female.

[Illustration]

In order to bring these cases and others into line an interesting
suggestion has recently been put forward by Bateson. On this suggestion
each sex is heterozygous for its own sex factor only, and does not contain
the factor proper to the opposite sex. The male is of the constitution,
Mmff and the female Ffmm. Each sex produces two sorts of gametes, Mf and mf
in the case of the male, and Fm, fm in that of the female. But on this view
a further supposition is necessary. If each of the two kinds of spermatozoa
were capable of fertilising each of the two kinds of ova, we should get
individuals of the constitution MmFf and mmff, as well as the normal males
and females, Mmff and Ffmm. As the facts of ordinary bisexual reproduction
afford us no grounds for assuming the existence of these two classes of
individuals, whatever they may be, we must suppose that fertilisation. is
productive only between the spermatozoa carrying M and the ova without F,
or between the spermatozoa {117} without M and the ova containing F. In
other words we must on this view suppose that fertilisations between
certain forms of gametes, even if they can occ