The serpent seed idea, which ascribes the ancestry of legendary monsters such as Grendel to Cain, was somewhat widespread in the Middle Ages. It also appears in early Gnostic Christian texts as well as in some Jewish texts, for example a 9th-century book titled "Pirke De-Rabbi Eliezer". In "Cain: Son of the Serpent" (1985), David Max Eichhorn traces the idea back to early Jewish Midrashic texts and he identifies many rabbis who taught the belief that Cain was the son of a union between the Serpent and Eve.