Recognizing of course the qualifications inherent in so broad a statement the answer is:  the handling of the lime salts.  Life originated, or at least lived and worked for long ages in sea water.  During these eras the salts of the sea have come to play a dominant role in its being.  The lime salts, because of their peculiar properties of dissolving or precipitating themselves according to electrical conditions in their medium, have come to occupy a central position in all the processes of growth, metabolism and sex differentiation.  So it is that masculinity may be described as a stable, constant state in the organism of lime salts, and the feminine as an unstable, variable state of lime salts.  The male skeleton contrasts with the female as the stronger, larger, heavier and straighter because it is an expression of a greater capacity to utilize, store and keep lime in the system.  Women throughout their reproductive period are liable to rapid and pendulum-like fluctuations of their lime content.

Menstruation, pregnancy, lactation, all draw upon the stores of lime, sometimes depleting them to the point of softening of the bones and wrecking the whole skeleton.  The endocrines control the transport, and course, combinations and permutations in the history of lime’s progress among the cells, and are in turn themselves affected by it.  Man is relatively free of these liabilities, and so remains man by his freedom from the recurrent crises involving the lime salt reserve which constitute the essence of the life story of woman.

THE SEX INDEX

It follows from these considerations that when it becomes necessary to size the sex composition of a man or woman, a measurement becomes establishable which may be spoken of as the sex index.  To be able to say of Mr. Llewylln Jones that he is sixty per cent masculine and forty per cent feminine, or of Mrs. Worthington that she is seventy per cent feminine and thirty per cent masculine would be of the utmost value under all kinds of circumstances.  Unfortunately, lacking as we do the exact figures of an advanced blood chemistry (yet in its most infantile infancy) a direct indexing of the sort is impossible.  But it is certainly conceivable, along the lines of measurement suggested by the Binet tests and others, that a scale of evaluation of the secondary sex traits may be elaborated, which would turn out as valuable in understanding the frictions of the individual, and more concretely, that aspect of it to which pathologists of the mind are tracing so much needless misery and suffering:  maladjusted sexuality, expressed and suppressed.  Nothing will contribute more to harmonious adjustment for these sufferers than recognition of the fact that we are all, more or less, partial hermaphrodites.

THE FUNCTIONAL HERMAPHRODITE