498 
I;. DONCASTER. 
covered that in a series of species all stages could be found 
between cases in which both sexes had equally paired idio- 
chroniosomes, through the condition in which they are 
unequally paired in the male, to the extreme case of total 
absence of the smaller one in the male. It therefore seemed 
natural to conclude that whether they were in appearance 
alike or unlike in the male, they were different in function, 
and that, since one of them showed all stages of disappearance 
in related species, this one is probably not functional as a 
sex- determiner, even when it is present. Two main hypotheses 
have been suggested to explain their action. AYilson first 
suggested^ that the two similar idiocliromosomes of the 
female bear respectively male and female determiners, that 
the single functional one of the male bears a male determiner, 
and that selective fertilisation occurs in such a way that 
male-bearing spermatozoa fertilise female- bearing eggs, and 
spermatozoa without sex-factor fertilise male-bearing eggs, 
thus : 
Male parent f. M. M.F. Female parent 
Spermatozoa f. M. M. F. Eggs 
Offspring f. M. M.F. 
Male. Female. 
Femaleness is supposed to be dominant over maleness, and 
therefore the zygote MF is a female, Mf a male (f repre- 
senting absence of sex-determiner). A second hypothesis, 
now more generally adopted, is due to Wilson and Castle (67, 
14), and is accepted by Morgan, who seems to have arrived at 
it independently : it is that sex is not due to specific male and 
female factors, but to the presence of one factor in greater or 
less amount. In the cases described, a “ single dose '' of this 
1 E. B. Wilson, ‘ Jonrn. Exp. Zool.,’ iii, 1906, p. 29. The same 
suggestion was made on independent grounds hy the writer (‘ Proc. 
Zool. Soc.,’ 1906, p. 132, and * Proc. Roy. Soc.,’ b, Ixxxii, 1910, p. 106). 
