Mendel’s Discovery 93 
species contains. The further investigation of the species-problem 
must thus proceed by the analytical method which breeding experi- 
ments provide. 
In the nine years which have elapsed since Mendel’s clue became 
generally known, progress has been rapid. We now understand the 
process by which a polymorphic race maintains its polymorphism. 
When a family consists of dissimilar members, given the numerical 
proportions in which these members are occurring, we can represent 
their composition symbolically and state what types can be trans- 
mitted 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 redis- 
tributed 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’. It is not a little remarkable that on this 
point—which is the only one where observations of the nuclear pro- 
ceases 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 
1 In other words, the ova are each either female, or male (i.e. non-female), but the 
sperms are all non-female. 
