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…and in the guise of a swan he took refuge in the bosom of nemesis, and again in this form he espoused Leda; ever with artifice manifestly, and not with violence, does he pursue beauty in women. Isocrates
But Zeus in the form of a swan consorted with Leda, and on the same night Tynareus cohabited with her; and she bore Pollux and Helen to Zeus and Castor and Clytaemnestra to Tyndareus. Apollodorus
In part Affairs of the Gods: Zeus and Leda. Courtesy amaliadillin.com - Thank you Amalia Dillin