associated with the male and female sex glands, respectively, is
distinctive in each case, and a vast majority of individuals of each sex
conform nearly enough to the average so that classification presents no
difficulty.

The extreme as well as the average body types existing in the presence
of the respective types of sex-glands are different. For example, we
find an occasional hen with male spurs, comb or wattles, though she is a
normal female in every other respect, and lays eggs.[4] But we never
find a functional female (which lays eggs) with _all_ the typical
characteristics of the male body. Body variation can go only so far in
the presence of each type of primary sexuality (i.e., sex-glands).

The bodily peculiarities of each sex, as distinguished from the
sex-glands or gonads themselves, are known as _secondary_ sex
characters. To put our statement in the paragraph above in another form,
the primary and secondary sex do not always correspond in all details.
We shall find as we proceed that our original tentative definition of
sex as the ability to produce in the one case sperm, in the other eggs,
is sometimes difficult to apply. What shall we say of a sterile
individual, which produces neither? The problem is especially
embarrassing when the primary and secondary sex do not correspond, as is
sometimes the case.

Even in a fully grown animal, to remove or exchange the sex glands (by
surgery) modifies the bodily type. One of the most familiar cases of
removal is the gelding or desexed horse. His appearance and disposition
are different from the stallion, especially if the operation takes place
while he is very young. The reason he resembles a normal male in many
respects is simply that sexuality in such highly-organized mammals is of