He was followed by Abel, whom the elder brother slew, and who was the first to show by a kind of foreshadowing of the sojourning city of God, what iniquitous persecutions that city would suffer at the hands of wicked and, as it were, earth-born men, who love their earthly origin, and delight in the earthly happiness of the earthly city. But how old Adam was when he begat these sons does not appear. After this the generations diverge, the one branch deriving from Cain, the other from him whom Adam begot in the room of Abel slain by his brother, and whom he called Seth, saying, as it is written, "For God hath raised me up another seed for Abel whom Cain slew."  These two series of generations accordingly, the one of Cain, the other of Seth, represent the two cities in their distinctive ranks, the one the heavenly city, which sojourns on earth, the other the earthly, which gapes after earthly joys, and grovels in them as if they were the only joys. But though eight generations, including Adam, are registered before the flood, no man of Cain's line has his age recorded at which the son who succeeded him was begotten. For the Spirit of God refused to mark the times before the flood in the generations of the earthly city, but preferred to do so in the heavenly line, as if it were more worthy of being remembered. Further, when Seth was born, the age of his father is mentioned; but already he had begotten other sons, and who will presume to say that Cain and Abel were the only ones previously begotten? For it does not follow that they alone had been begotten of Adam, because they alone were named in order to continue the series of generations which it was desirable to mention. For though the names of all the rest are buried in silence, yet it is said that Adam begot sons and daughters; and who that cares to be free from the charge of temerity will dare to say how many his offspring numbered? It was possible enough that Adam was divinely prompted to say, after Seth was born, "For God hath raised up to me another seed for Abel," because that son was to be capable of representing Abel's holiness, not because he was born first after him in point of time. Then because it is written, "And Seth lived 205 years," or, according to the Hebrew reading, "105 years, and begat Enos,"  who but a rash man could affirm that this was his first-born? Will any man do so to excite our wonder, and cause us to inquire how for so many years he remained free from sexual intercourse, though without any purpose of continuing so, or how, if he did not abstain, he yet had no children? Will any man do so when it is written of him, "And he begat sons and daughters, and all the days of Seth were 912 years, and he died?"  And similarly regarding those whose years are afterwards mentioned, it is not disguised that they begat sons and daughters.
Consequently it does not at all appear whether he who is named as the son was himself the first begotten. Nay, since it is incredible that those fathers were either so long in attaining puberty, or could not get wives, or could not impregnate them, it is also incredible that those sons were their first-born. But as the writer of the sacred history designed to descend by well-marked intervals through a series of generations to the birth and life of Noah, in whose time the flood occurred, he mentioned not those sons who were first begotten, but those by whom the succession was handed down.
Let me make this clearer by here inserting an example, in regard to which no one can have any doubt that what I am asserting is true. The evangelist Matthew, where he designs to commit to our memories the generation of the Lord's flesh by a series of parents, beginning from Abraham and intending to reach David, says, "Abraham begat Isaac;"  why did he not say Ishmael, whom he first begat? Then "Isaac begat Jacob;" why did he not say Esau, who was the first-born? Simply because these sons would not have helped him to reach David. Then follows, "And Jacob begat Judah and his brethren:" was Judah the first begotten? "Judah," he says, "begat Pharez and Zara;" yet neither were these twins the first-born of Judah, but before them he had begotten three other sons. And so in the order of the generations he retained those by whom he might reach David, so as to proceed onwards to the end he had in view. And from this we may understand that the antediluvians who are mentioned were not the first-born, but those through whom the order of the succeeding generations might be carried on to the patriarch Noah. We need not, therefore, weary ourselves with discussing the needless and obscure question as to their lateness of reaching puberty.