La Genèse 10:1 Midrash : Bamidbar Rabbah, Midrash Tanchuma Buber, Midrash Tanchuma & Bereshit Rabbah

And these are the generations of Aaron and Moses on the day that God spoke to Moses on Mount Sinai (Numbers 3:1).
Anyone with whom the Omnipresent engages to establish a nation or a family tree from him, the Omnipresent would also engage with him to write generations in his regard, as you find twelve instances of generations in the verses. The first: “These are the generations of the heavens and the earth” (Genesis 2:4); “this is the book of the generations of Adam” (Genesis 5:1); “these are the generations of Noah” (Genesis 6:9); “these are the generations of the sons of Noah” (Genesis 10:1): “these are the generations of Shem” (Genesis 11:10); “these are the generations of Teraḥ” (Genesis 11:27); “these are the generations of Ishmael” (Genesis 25:12); “Isaac” (Genesis 25:19); “Esau” (Genesis 36:1); “Jacob” (Genesis 37:2). These are ten generations with which the Holy One blessed be He engaged to create the world and to establish nations. And there are two people that the Omnipresent wrote their generationst, one for the royal dynasty and one for the priestly dynasty: “These are the generations of Peretz” (Ruth 4:18), to establish the royal dynasty from him, and “these are the generations of Aaron and Moses"(Numbers 3:1), for the priestly dynasty.

Another interpretation (of Numb. 1:1-2): THEN THE LORD SPOKE UNTO MOSES IN THE SINAI DESERT…. This text is related (to Ps. 147:20): HE HAS NOT DONE SO FOR ANY NATION…., and also (to Ps. 148:14): HE HAS RAISED UP A HORN FOR HIS PEOPLE, PRAISE FOR ALL HIS SAINTS. <The situation> is comparable to a king who took <his> first wife and did not write her a marriage contract.³³Tanh., Numb. 1:5; Numb. R. 1:5. He divorced her and did not give her a bill of divorce. He did the same for the second and for the third, in that he did not write them either a marriage contract or a bill of divorce. {He did not act. However,} [In due time] when he saw a certain poor orphan, a woman of noble ancestry, he wanted to marry her. He said to his shoshevin (i.e., to his best man): Do not behave with her as with the previous ones. This is a woman of noble ancestry. She is modest in her actions and worthy. Write her a marriage contract <stating> in which week, in which year, in which month, on what day of the month, in which province,³⁴Gk.: eparchia. just as it is written in Esther (2:16): SO ESTHER WAS TAKEN UNTO KING AHASUERUS, UNTO HIS ROYAL PALACE IN THE TENTH MONTH, WHICH IS THE MONTH OF TEBETH, IN THE SEVENTH YEAR OF HIS REIGN. Thus the Omnipotent created the generation of the flood and did not write down when he created them. He removed them from the world and did not write down when he removed them. It is simply <stated> (in Gen. 7:11): ON THAT DAY ALL THE SPRINGS OF THE GREAT ABYSS BURST FORTH AND THE WINDOWS OF THE HEAVENS WERE OPENED.³⁵Note that the verse begins: IN THE SIX HUNDREDTH YEAR OF NOAH’S LIFE, IN THE SECOND MONTH, ON THE SEVENTEENTH DAY OF THE MONTH; but this dating is apparently not definite enough. Moreover, it was similar for the generation of the dispersion (Gen. 10:1-9); [similar for the Sodomites;] and similar for the Egyptians. For them it is not written either when they were created or when they died. {He did not act. However,} When Israel arose, the Holy One said to Moses: I will not behave with these as with those previous ones. These are people of noble ancestry, descendants from Abraham, Isaac, and Jacob. It is there written for them in which month, on what day of the month, in which year, in which province, in which metropolis I raised up their horn and gave them a head held high. It is therefore stated (in Numb. 1:1): THEN THE LORD SPOKE UNTO MOSES IN THE SINAI DESERT, i.e., the province, IN THE TENT OF MEETING, i.e., the metropolis. In which year? (Ibid., cont.:) IN THE SECOND YEAR.³⁶The citation of this section of the verse differs somewhat in the Masoretic text, which reads: ON FIRST DAY OF THE SECOND MONTH, IN THE SECOND YEAR. In which month? (Ibid., cont.:) IN THE SECOND MONTH. On what day of the month? (Ibid., cont.:) ON THE FIRST DAY OF THE MONTH. (Ibid., cont.:) [AFTER THEIR EXODUS FROM THE LAND OF EGYPT,] i.e, the era.³⁷Gk. hupateia (“consulship”). Why (in vs. 2) TAKE A CENSUS (literally: RAISE THE HEAD) OF THE WHOLE CONGREGATION OF THE CHILDREN OF ISRAEL…. To fulfill what is stated (in Ps. 147:20): HE HAS NOT DONE SO FOR ANY NATION…. What did he do? (Ps. 148:14): HE HAS RAISED UP A HORN FOR HIS PEOPLE, PRAISE FOR ALL HIS SAINTS, EVEN THE CHILDREN OF ISRAEL, A PEOPLE NEAR TO HIM. HALLELUJAH.

(Numb. 1:1-2) “Then the Lord spoke unto Moses in the Sinai desert […,] ‘Take a census of the whole congregation.” This text is related (to Ps. 147:20), “He has not done so for any nation, and they do not know His laws, Hallelujah.” [The situation] is comparable to a king who took [his] first wife and did not write her a marriage contract.²¹Numb. R. 1:5. He divorced her and did not give her a bill of divorce. He did the same for the second and for the third. [In due time] when he saw a certain poor orphan, a woman of noble ancestry, he wanted to marry her. He said to his shoshevin (i.e., to his best man), “I want to marry this one, and I shall not behave with her as with the previous ones. This is a woman of noble ancestry. She is modest in her actions and worthy. Write her a marriage contract [stating] in which week, in which year, in which month, on what day of the month, in which era.” [This is] just as it is written in Esther (2:16), “So Esther was taken unto King Ahasuerus, unto his royal palace in the tenth month, which is the month of Tebeth, in the seventh year of his reign.” Thus the Holy One, blessed be He, created the generation of the flood and did not write down when He created them. He removed them from the world and did not write down when He removed them. It is simply [stated] (in Gen. 7:11), “on that day all the springs of the great abyss burst forth and the windows of the heavens were opened.”²²Note that the verse begins, “In the six hundredth year of Noah’s life, in the second month, on the seventeenth day of the month.” But this dating is apparently not definite enough. And it was similar for the generation of the dispersion (Gen. 10:1-9); similar for the sodomites; and similar for the Egyptians. [However,] when Israel arose, the Holy One, blessed be He, said to Moses, “I will not behave with these as with those previous ones, as these are descendants from Abraham, Isaac, and Jacob. Therefore it is written in which month, on what day of the month, in which year, in which era, in which province I raised up their horn and gave them a head held high.” It is therefore stated (in Numb. 1:1), “Then the Lord spoke unto Moses in the Sinai desert,” i.e., the province; “in the tent of meeting,” i.e., the metropolis; “on the first day of the month,” i.e., the day; “in the second year,” i.e., the year.²³The citation of this section of the verse differs somewhat in the Masoretic text, which reads, “on the first day of the second month, in the second year. In which month? (Ibid., cont.) “In the second month.” [“After their Exodus from the land of Egypt,”] i.e., the era.²⁴Gk. hupateia (“consulship”). Why (in vs. 2) “Take a census (literally: raise the head) of the whole congregation of the Children of Israel…?” To fulfill what is stated (in Ps. 147:20), “He has not done so for any nation….” What did He do? He raised their heads, as stated (Ps. 148:14), “He has raised up a horn for His people, praise for all His saints, even the Children of Israel, a people near to Him. Hallelujah”.