436 Evolution and Adaptation 
of females, which require more food, but this still fails to 
account for the excess of females when more food is given, 
provided Yung’s experiments on tadpoles are correct. It 
may be, however, in the light of Pfitiger’s results, that there 
has been some mistake in the experiments themselves. 
We may now proceed to examine Castle’s argument, 
attempting to show in what way sex is predetermined in the 
embryo. His hypothesis rests on the three following prem- 
ises: “(1) the idea of Darwin, that in animals and plants of 
either sex the characters of the opposite sex are latent ; (2) the 
idea of Mendel, that in the formation of the gametes [germ- 
cells] of hybrids a segregation of the parental characters 
takes place, and when in fertilization different segregated 
characters meet, one will dominate, the other become latent 
or recessive; (3) the idea of Weismann, that in the matura- 
tion of egg and spermatozoon a segregation is attended by a 
visible reduction in the number of chromosomes in the 
germinal nuclei.” 
Expressed in a somewhat more general way, Castle suggests 
that each egg and each spermatozoon is either a male or a 
female germ-cell (and not a mixture of the two), and when a 
female egg is fertilized by a male spermatozoon, or vice versa, 
the individual is a sexual hybrid with one sex dominating 
and the other latent. The assumption that there are two 
kinds of. eggs, male and female, and two kinds of spermatozoa, 
male and female, is not supported by any direct or experi- 
mental evidence. Moreover, in order to carry out the 
hypothesis, it is necessary to make the further assumption 
that a female egg can only be fertilized by a male spermato- 
zoon, and a male egg by a female spermatozoon. While 
such a view is contrary to all our previous ideas, yet it must 
be admitted that there are no facts which disprove directly 
that such a selection on the part of the germ-cells takes place. 
If these two suppositions be granted, then Castle’s hypothesis 
is as follows :— 
