Polymazia is of much more frequent occurrence than is supposed. Julia, the mother of Alexander Severus, was surnamed "Mammea" because she had supernumerary breasts. Anne Boleyn, the unfortunate wife of Henry VIII of England, was reputed to have had six toes, six fingers, and three breasts. Lynceus says that in his time there existed a Roman woman with four mammae, very beautiful in contour, arranged in two lines, regularly, one above the other, and all giving milk in abundance. Rubens has pictured a woman with four breasts; the painting may be seen in the Louvre in Paris.
There was a young and wealthy heiress who addressed herself to the ancient faculty at Tubingen, asking, as she displayed four mammary, whether, should she marry, she would have three or four children at a birth. This was a belief with which some of her elder matron friends had inspired her, and which she held as a hindrance to marriage.
Leichtenstern, who has collected 70 cases of polymazia in females and 22 in males, thinks that accessory breasts or nipples are due to atavism, and that our most remote inferiorly organized ancestors had many breasts, but that by constantly bearing but one child, from being polymastic, females have gradually become bimastic. Some of the older philosophers contended that by the presence of two breasts woman was originally intended to bear two children.
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Indeed. And if you did, few would believe you.
> Some of the older philosophers contended that by the presence of two breasts woman was originally intended to bear two children.
Two breasts allows twins to survive better, but it also gives a survival advantage to one child in case one breast has trouble (which happens very often). I suspect it's a combination of redundancy and symmetry, rather like having two kidneys.