If we examine the physical phenomenon of sex as it manifests itself in the human creature, we find, in the first stages of the individual's existence, no difference discernible, by any means we have at present at our command, between those germs which are ultimately to become male or female.Later, in the foetal life, at birth, and through infancy though the organs of sex serve to distinguish the male from the female, there is in the general structure and working of the organism little or nothing to divide the sexes.

Even when puberty is reached, with its enormous development of sexual and reproductive activity modifying those parts of the organism with which it is concerned, and producing certain secondary sexual characteristics, there yet remains the major extent of the human body and of physical function little, or not at all, affected by sex modification.The eye, the ear, the sense of touch, the general organs of nutrition and respiration and volition are in the main identical, and often differ far more in persons of the same sex than in those of opposite sexes; and even on the dissecting-table the tissues of the male and female are often wholly indistinguishable.

It is when we consider the reproductive organs themselves and their forms of activity, and such parts of the organism modified directly in relation to them, that a real and important difference is found to exist, radical though absolutely complemental.It is exactly as we approach the reproductive functions that the male and female bodies differ; exactly as we recede from them that they become more and more similar, and even absolutely identical.Taking the eye, perhaps the most highly developed, complex organ in the body, and, if of an organ the term may be allowed, the most intellectual organ of sense, we find it remains the same in male and female in structure, in appearance, and in function throughout life; while the breast, closely connected with reproduction, though absolutely identical in both forms in infancy, assumes a widely different organisation when reproductive activity is actually concerned.