somatic characters confined to one sex, proves that Mendelian conceptions,
however true up to a certain point, are by no means the whole truth about
heredity and development. For it is the essence of Mendelism as of
Weismannism that not only sex but all other congenital characters are
determined in the fertilised ovum or zygote. The meaning of a recessive
character in Mendelian terminology is one that is hidden by a dominant
character, and both of them are due to factors in the gametes,
particularly in the chromosomes of the gametes which come together in
fertilisation. For example, in fowls rose comb is dominant over single. A
dominant is something present which is absent in the recessive: the rose
comb is due to a factor which is absent from the single. The two segregate
in the gametes of the hybrid or heterozygote, and if a recessive gamete is
fertilised by another recessive gamete the single comb reappears. But
castration shows that the antlers of stags and other such characters are
not determined in the zygote when the sex is determined, but owe their
development, partly at least, to the influence of another part of the
body, namely, the testes during the subsequent life of the individual.
According to Mendelism the structure and development of each part of the
soma is due to the constitution of the chromosomes of the nuclei in that
part. The effects of castration show that the development of certain
characters is greatly influenced in some way by the presence of the testes
in a distant part of the body. The Mendelians used to say it was
impossible to believe in the heredity of somatic modifications due to
external conditions, because it was impossible to conceive of any means by
which such modifications could affect the constitution of the chromosomes
in the gametes within the modified body. It would have been just as
logical to deny the proved effects of castration, because it was
impossible to conceive of any means by which the testes could affect the
development of a distant part of the body.