336 | T. H. Morgan 
sion, in both cases. On the Mendelian scheme any sperm may 
fertilize any egg, but if this applied to sex, as Castle points out, 
there would be three, not two sorts of individuals, but selective 
fertilization while avoiding this difficulty runs foul of another one, 
because all the fertilized eggs will be sex hybrids, 1.e., all alike each 
having resulted from the combination of male and female. In 
other words, while the theory accounts for the combination of 
male and female characters in every individual, it fails to account 
for those factors that determine sex, 1. e., the sex of the individual 
that arises from the egg. It is this problem that is the primary 
requirement of a theory of sex determination. It was obviously 
impossible at that time to account for the sex of the individual by 
referring the results to the egg alone or to the sperm alone, for, 
besides being inconsistent with some of the views adopted for 
special cases, this assumption would be directly opposed to the 
Mendelian theory of heredity which assumes that the dominance 
or recessiveness 1s due to the intrinsic properties of each character 
and does not depend on whether it is at the time carried by the 
sperm or the egg. 
Castle argues that “the strongest evidence of the latency of each 
sex in the other is afforded by the transmission through one sex of 
the characters of the other. ‘Thus, as Darwin states when the 
domestic cock is crossed with the hen pheasant, the male offspring 
have the secondary sexual characters of the male pheasant; these, 
manifestly, must have been inherited through the female pheasant.” 
There is possibly a fallacy in this argument. ‘The facts only show 
however that the secondary sexual characters of plumage, etc., of 
the pheasant are transmitted as dominant characters to the hybrids 
both male and female. 
SEX DETERMINATION AS THE RESULT OF A QUANTITATIVE RELATION 
In attempting to sum up in 1903 the evidence, then recent, 
as to the factors of sex determination, I pointed out that while the 
newer facts showed that sex was early determined in the germ 
cells, they did not warrant the conclusion that “the male and 
female primordia” in the germ cells are separated, but that at first 
