Saint Augustine on the unity of the human race in Adam

In Book XII of The City of God, Saint Augustine notes that, in the Genesis creation account, it would appear that for all animals, God created several at once.  Only for man, did he create only one and have the whole race derive from this one.  Augustine adduces an important meaning in this:

And therefore God created only one single man, not, certainly, that he might be a solitary bereft of all society, but that by this means the unity of society and the bond of concord might be more effectually commended to him, men being bound together not only by similarity of nature, but by family affection.  And indeed He did not even create the woman that was to be given him as his wife, as he created the man, but created her out of the man, that the whole human race might derive from one man…

With good cause, therefore, does the true religion recognize and proclaim that the same God who created the universal cosmos, created also the animals, souls as well as bodies.  Among the terrestrial animals, man was made by Him in His own image, and, for the reason I have given, was made one individual, though he was not left solitary.  For there is nothing so social by nature, so unsocial by its corruption, as this race.  And human nature has nothing more appropriate, either for the prevention of discord, or for the healing of it, where it exists, than the rememberance of that first parent of us all, whom God was pleased to create alone, that all men might be derived from one, and that they might thus be admonished to preserve unity among their whole multitude.  But from the fact that the woman was made for him from his side, it was plainly meant that we should learn how dear the bond between man and wife should be….

in this first man, who was created in the beginning, there was laid the foundation, not indeed evidently, but in God’s foreknowledge, of these two cities or societies, so far as regards the human race.  For from that man all men were to be derived–some of them to be associated with the good angels in their reward, others with the wicked in punishment; all being ordered by the secret yet just judgment of God.  Fir since it is writen, “All the paths of the Lord are mercy and truth,” neither can His grace be unjust, nor His justice cruel.