Two things especially are observable in this account of the posterity of Shem. The description of Shem, Genesis 10:21, we have not only his name, Shem, which signifies a name; but two titles to distinguish him by.
He was the father of all the children of Eber. Eber was his great grandson, but why should he be called the father of all his children, rather than of all Arphaxad's or Salah's? Probably because Abraham and his seed, not only descended from Heber, but from him were called Hebrews. Eber himself, we may suppose, was a man eminent for religion in a time of general apostasy; and the holy tongue being commonly called from him the Hebrew, it is probable he retained it in his family in the confusion of Babel, as a special token of God's favour to him. He was the brother of Japheth the elder; by which it appears, that though Shem be commonly put first, yet he was not Noah's first — born, but Japheth was elder. But why should this also be put as part of Shem's description, that he was the brother of Japheth, since that had been said before? Probably this is intended to signify the union of the Gentiles with the Jews in the church. He had mentioned it as Shem's honour, that he was the father of the Hebrews; but lest Japheth's seed should therefore be looked upon as shut out from the church, he here minds us, that he was the brother of Japheth, not in birth only, but in blessing, for Japheth was to dwell in the tents of Shem.
The reason of the name of Peleg, Genesis 10:25, because, in his days, (that is, about the time of his birth) was the earth divided among the children of men that were to inhabit it; either when Noah divided it, by an orderly distribution of it, as Joshua divided the land of Canaan by lot, or when, upon their refusal to comply with that division, God, in justice, divided them by the confusion of tongues.