CHROMOSOMES, HEREDITY AND SEX. 
513 
In general, therefore, the facts of sex-limited inheritance 
support the contention of a direct relation between chromo- 
somes and the transmission of inherited characters, although 
they do not as yet enable us to choose between particular 
theories of sex- determination. 
(4) Conclusion and Summary. 
The various classes of facts which have been described in 
the last two sections, if considered by themselves, provide 
strong evidence for the existence of an intimate relation 
between the presence or absence of a particular chromosome 
and the determination of sex. The case for the relation of 
chromosomes to Meudelian factors is less conclusive, but if 
the connection between chromosomes and sex-determination 
be admitted, the facts of sex-limited inheritance make it 
almost impossible to reject the belief in a similar connection 
with Mendelian factors. There ai-e, however, certain rather 
grave difficulties in the way of accepting the conclusion with 
regard to sex which demand a few words of explanation. 
In the first place, if sex is constantly determined by the 
presence or absence of a particular chromosome, it is difficult 
to understand why, within one group of animals, species of 
one order or class give indications that such a chromosome 
exists only in the male, although species of another order 
give exactly similar evidence that it is present only in the 
female. This difficulty disappears if the hypothesis is adopted 
that both sexes are heterozygous for sex-factors ($ = MF, 
S = Mf), and that there is selective fertilisation in such a 
way that M-bearing eggs are only fertilised by f-bearing 
spermatozoa, F-bearing eggs by M-bearing spermatozoa . It 
must be admitted, however, that in the absence of any direct 
evidence for selective fertilisation, this hypothesis is very 
speculative. A suggestion which is perhaps more probable 
is that there are two independent sex-factors, M and F, and 
that when both factors are homozygous (MMFF), in some 
forms M is epistatic over F, giving a male ; in others, F is 
