This section tells us that all the righteous in the world come from the lineage of Seth, who was born after Cain and Abel died. God increased Chesed in the world, and with each male a female was born to populate the world.
369. We have learned that after the passing of Cain and Abel, Adam returned to his wife. A different spirit enveloped him and he begot Seth. From this lineage, the righteous trace their line in the world. The Holy One, blessed be He, increased Chesed in the world, and with each was also born a female to populate the world, just as above, AS ZEIR ANPIN AND MALCHUT WERE BROTHER AND SISTER. The friends have established in the general Mishnah that it is written ABOUT THEM, "And if a man shall take his sister, his father's daughter, or his mother's daughter, and see her nakedness... It is a disgraceful (Heb. chesed, it is confusing but the word chesed can mean disgraceful or kindness) deed" (Vayikra 20:17). Assuredly it is Chesed, for after Chesed rests, offspring and roots come out underneath upwards, and branches spread, MEANING ZEIR ANPIN AND MALCHUT. What was near moved away. Then the branches grew, ZEIR ANPIN AND MALCHUT, and came to join into one in a tree. This was in the beginning, in the concealed state of the world. It is written, "For I have said, the world is built by love (chesed)" (Tehilim 89:3). THEREFORE, from now on, people in this situation THAT MARRY THEIR SISTER "shall be cut off in the sight of their people" (Vayikra 20:17).
370. We have learned that "the nakedness of your father's sister" is TO BE EXPLAINED as revealing in that which is hidden. It is written, "For the ways of Hashem are right, and the just do walk in them..." (Hoshea 14:10). How blessed is the lot of the just that know the ways of the Holy One, blessed be He, and walk in them. They are made known to them. Blessed is their share.