10 For as by one man's disobedience sin entered, and death obtained [a place] through sin; so also by the obedience of one man, righteousness having been introduced, shall cause life to fructify in those persons who in times past were dead. And as the protoplast himself Adam, had his substance from untilled and as yet virgin soil (“for God had not yet sent rain, and man had not tilled the ground”), and was formed by the hand of God, that is, by the Word of God, for “all things were made by Him,” and the Lord took dust from the earth and formed man; so did He who is the Word, recapitulating Adam in Himself, rightly receive a birth, enabling Him to gather up Adam [into Himself], from Mary, who was as yet a virgin.
If, then, the first Adam had a man for his father, and was born of human seed, it were reasonable to say that the second Adam was begotten of Joseph. But if the former was taken from the dust, and God was his Maker, it was incumbent that the latter also, making a recapitulation in Himself, should be formed as man by God, to have an analogy with the former as respects His origin. Why, then, did not God again take dust, but wrought so that the formation should be made of Mary? It was that there might not be another formation called into being, nor any other which should [require to] be saved, but that the very same formation should be summed up [in Christ as had existed in Adam], the analogy having been preserved.
Source: Adversus haereses (New Advent)