In highly organized forms like the mammals (including man), sex is linked up with _all_ the internal secretions, and hence is of the whole body.[3] As Bell [2, p.5] states it: "We must focus at one and the same time the two essential processes of life--the individual metabolism and the reproductive metabolism. They are interdependent. Indeed, the individual metabolism is the reproductive metabolism."
Here, then, is the reason men have larger, differently formed bodies than women--why they have heavier bones, tend to grow beards, and so on. The sex glands are only part of what we may call a well-organized chemical laboratory, delivering various products to the blood, but always in the same general proportions for a given sex. The ingredients which come from the sex glands are also qualitatively different, as has been repeatedly proved by injections and otherwise.
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.