It is probable that there was a family book for the house of Pharez, in which their genealogies were preserved, and important bits of history were recorded; and that the Book of Ruth was compiled from it. (See the note at Genesis 2:4)

Now these are the generations - The Targum gives a copious paraphrase on this and the following verses, I shall insert the principal parts in their proper places.

Now these [are] the generations of (k) Pharez: Pharez begat Hezron,
(k) This genealogy is brought in to prove that David by succession came from the house of Judah.

Now these are the generations of Pharez,.... The son of Judah, by Tamar before mentioned, Ruth 4:12, for the intention of this genealogy is to confirm the truth of Jacob's prophecy, of Shiloh the Messiah coming from the tribe of Judah, Genesis 49:10 and therefore it begins with Pharez, well known to be the son of Judah, and ends with David, whose son the Messiah was to be, as is owned by all Jews and Gentiles that believe the divine revelation:
Pharez begat Hezron; who was one of those that went down with Jacob into Egypt, being born in the land of Canaan, Genesis 47:12 called Esrom in Matthew 1:3.

these are the generations of Pharez--that is, his descendants. This appendix shows that the special object contemplated by the inspired author of this little book was to preserve the memory of an interesting domestic episode, and to trace the genealogy of David. There was an interval of three hundred eighty years between Salmon and David. It is evident that whole generations are omitted; the leading personages only are named, and grandfathers are said, in Scripture language, to beget their grandchildren, without specifying the intermediate links.

"These are the generations of Perez," i.e., the families descended from Perez in their genealogical order (toledoth: see at Genesis 2:4). The genealogy only goes back as far as Perez, because he was the founder of the family of Judah which was named after him (Numbers 26:20), and to which Elimelech and Boaz belonged. Perez, a son of Judah by Tamar (Genesis 38:29), begat Hezrom, who is mentioned in Genesis 46:12 among the sons of Judah who emigrated with Jacob into Egypt, although (as we have shown in our comm. on the passage) he was really born in Egypt. Of this son Ram (called Aram in the Sept. Cod. Al., and from that in Matthew 1:3) nothing further is known, as he is only mentioned again in 1-Chronicles 2:9. His son Amminidab was the father-in-law of Aaron, who had married his daughter (Exodus 6:23), and the father of Nahesson (Nahshon), the tribe-prince of the house of Judah in the time of Moses (Numbers 1:7; Numbers 2:3; Numbers 7:12). According to this there are only four or five generations to the 430 years spent by the Israelites in Egypt, if we include both Perez and Nahesson; evidently not enough for so long a time, so that some of the intermediate links must have been left out even here. But the omission of unimportant members becomes still more apparent in the statement which follows, viz., that Nahshon begat Salmah, and Salmah Boaz, in which only two generations are given for a space of more than 250 years, which intervened between the death of Moses and the time of Gideon. Salmah (שׂלמה or שׂלמא, 1-Chronicles 2:11) is called Salmon in Ruth 4:21; a double form of the name, which is to be explained form the fact that Salmah grew out of Salmon through the elision of the n, and that the terminations an and on are used promiscuously, as we may see from the form שׁריה in Job 41:18 when compared with שׁרין in 1-Kings 22:34, and שׁריון in 1-Samuel 17:5, 1-Samuel 17:38 (see Ewald, 163-4). According to the genealogy of Christ in Matthew 1:5, Salmon married Rahab; consequently he was a son, or at any rate a grandson, of Nahshon, and therefore all the members between Salmon and Boaz have been passed over. Again, the generations from Boaz to David (Ruth 4:21, Ruth 4:22) may possibly be complete, although in all probability one generation has been passed over even here between Obed and Jesse. It is also worthy of notice that the whole chain from Perez to David consists of ten links, five of which (from Perez to Nahshon) belong to the 430 years of the sojourn in Egypt, and five (from Salmon to David) to the 476 years between the exodus from Egypt and the death of David. This symmetrical division is apparently as intentional as the limitation of the whole genealogy to ten members, for the purpose of stamping upon it through the number ten as the seal of completeness the character of a perfect, concluded, and symmetrical whole.
The genealogy closes with David, an evident proof that the book was intended to give a family picture form the life of the pious ancestors of this great and godly king of Israel. But for us the history which points to David acquires a still higher signification, from the fact that all the members of the genealogy of David whose names occur here are also found in the genealogy of Jesus Christ. "The passage is given by Matthew word for word in the genealogy of Christ, that we may see that this history looks not so much to David as to Jesus Christ, who was proclaimed by all as the Saviour and Redeemer of the human race, and that we may learn with what wonderful compassion the Lord raises up the lowly and despised to the greatest glory and majesty" (Brentius).