e members are occurring, we can represent their composition
symbolically and state what types can be transmitted by the various
members. The difficulty of the "swamping effects of intercrossing" is
practically at an end. Even the famous puzzle of sex-limited inheritance
is solved, at all events in its more regular manifestations, and we know
now how it is brought about that the normal sisters of a colour-blind
man can transmit the colour-blindness while his normal brothers cannot
transmit it.

We are still only on the fringe of the inquiry. It can be seen extending
and ramifying in many directions. To enumerate these here would be
impossible. A whole new range of possibilities is being brought into
view by study of the interrelations between the simple factors. By
following up the evidence as to segregation, indications have been
obtained which can only be interpreted as meaning that when many factors
are being simultaneously redistributed among the germ-cells, certain of
them exert what must be described as a repulsion upon other factors. We
cannot surmise whither this discovery may lead.

In the new light all the old problems wear a fresh aspect. Upon the
question of the nature of Sex, for example, the bearing of Mendelian
evidence is close. Elsewhere I have shown that from several sets of
parallel experiments the conclusion is almost forced upon us that, in
the types investigated, of the two sexes the female is to be regarded as
heterozygous in sex, containing one unpaired dominant element, while the
male is similarly homozygous in the absence of that element. (In other
words, the ova are each EITHER female, OR male (i.e. non-female), but
the sperms are all non-female.) It is not a little remarkable that on
this point--which is the only one where observations of the nuclear
processes of gameto-genesis have yet been brought into relation with
the visible characteristics of the organisms themselves--there should be
diametrical opposition between the results of breeding experiments and
those derived from cytology.

Those who have followed the researches of the American school will
be aware that, after it had been found in certain insects that the
spermatozoa were of two kinds according as they contained or did not
contain the accessory chromosome, E.B. Wilson succeeded in proving that
the sperms possessing this accessory body were destined to form FEMALES
on fertilisation, while sperms without it form males, the eggs