578 
GEOFFKEY SMITH. 
investigators principally occupied themselves witli statistics, 
and with the supposed influence of various external factors, 
such as food or temperature, upon the production of one sex 
or the other. But the almost wholly negative result of such 
investigations, and the positive evidence afforded by such 
cases as that of the bee, and of the rotifer, Hydatina, in 
the latter of which two structurally different kinds of eggs 
are produced destined to give rise to males and females 
respectively, have influenced biologists to look for the cause 
of sex-determination in the mechanism of heredity. 
'Phe first to definitely formulate a Mendelian theory of sex 
was Castle in 1903 (2). He supposed that in all cases each 
sex IS a sex-hybrid or heterozygote of the composition $ , 
but that in the male sex maleuess was dominant, and in the 
female fenialeness. As evidence that in each sex the opposite 
sex-character is present in a latent state, he adduces the 
undoubted transmission of male characters through the 
female and vice versa, and the appearance of the secondary 
sexual characters of the opposite sex in animals, as the result 
of injury or disease of the gonads. 
fl’lie process of segregation and fertilisation he conceives as 
follows : Each sex being a heterozygote produces male and 
female gametes in equal proportions ; but among them there 
is selective fertilisation of such a kind that only male-bearing 
spermatozoa can fertilise female-bearing eggs and vice versa. 
Thus : 
Mule gametes (spermatozoa). Female gametes (ova). 
A ? ^ ? 
Both the zygotes produced have thus the composition J' ? , 
and we are given to understand that for some reason in half 
the zygotes maleness dominates, and in tlie other femaleness. 
It must be admitted that this theory, according to which 
every individual is potentially hermaphrodite, is sufficiently 
