Each of these sex types, male and female, varies somewhat within itself, as is true of everything living.  The two are not so far apart but that they may overlap occasionally in some details.  For instance, some women are larger than are some men—­have lower pitched voices, etc.  The whole bodily metabolism, resting as it does upon a chemical complex, is obviously more like the male average in some women than it is in others, and vice versa.  But the average physical make-up which we find 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 the whole body, not of the sex-glands or organs alone.

Suppose this horse was desexed at two years old.  Nearly three years had elapsed since he was a fertilized egg.  During the eleven months or so he spent within his mother, he developed a very complicated body.  Beginning as a male, with a male-type metabolism (that is, as the result of a union between an X and a Y chromosome, not two X’s), all his glands, as well as the body structures they control, developed in its presence.  Not only the sex glands, but the liver, suprarenals, thyroid—­the whole body in fact—­became adjusted to the male type.  He had long before birth what we call a male sex complex.  Complex it is, but it is, nevertheless, easy enough to imagine its nature for illustrative purposes.  It is simply all the endocrine or hormone-producing organs organized into a balanced chemical system—­adjusted to each other.