On Twins:

‘And consider the confusing situation of the Dioscuri, the twins Castor and Pollux, who accompanied Jason on the Argo in the quest for the Golden Fleece. Mythographers could not decide whether the brothers were immortal or “half-mortal.” The uncertainty arose with good reason. Their mother, Leda, was human, but Pollux was fathered by Zeus, while Castor’s father was Tyndareus, a Spartan kin. The novel idea of twins with different fathers posed a puzzle for mortal versus immortal bloodlines for people to ponder in antiquity. Oddly enough, the notion of twins with different paternity was not just a  fantasy or plot contrivance. When two different males sire fraternal twins in the same ovulation cycle, the scientific term is heteropaternal superfecundation. It happens in dogs, cats, and other mammals, even including, albeit rarely, humans.’ – Adrienne Mayor “Medea’s Cauldron of Rejuvenation,” Gods and Robots