There is an interesting and subtle reference to the two lines of genealogy, mentioned in the Torah. There is the קין, or Cain line, and there is the שת, or Seth line.
At the end of Parshat Bereishit, the Torah tells us that בני האלוקים, literally, the Sons of G-d, saw the בנות האדם, the daughters of man. They saw that they were good, meaning, attractive, and they took them as wives, even if they were already married.
The Seth descendants, were referred to as, “Sons of G-d,” and the Cain line, were referred to as, “Sons of G-d.” The Torah does tell us that Cain married and fathered children. He married his twin sister, and because of murdering his brother, his descendants were tainted. They were not considered pious men, and the seventh generation, had Lemech, accidentally, kill Cain. There was also a Chanoch in this line, who was not considered anything special.
The Seth line, on the other hand, was more pure. He was Adam’s third son, who was not tarnished. His descendant Chanoch, “walked with G-d,” and was taken from this earth, at a relatively young age. There was also a second Lemech, that descended from Seth. He was the father of Noach.
The Kuzari wrote that each generation, had only one worthy offspring. This is the one mentioned in the Torah. The other siblings of that generation, were also considered not particularly special. The first person who had completely righteous offspring, was our father, Yaakov. Because all of his twelve sons were righteous, they were worthy to become a Family nation.
The Cain line did not survive, and the Seth line, ultimately, produced the Jewish people.