Book XV. ———————————— Argument—Having treated in the four preceding books of the origin of the two cities, the earthly and the heavenly, Augustin explains their growth and progress in the four books which follow; and, in order to do so, he explains the chief passages of the sacred history which bear upon this subject.  In this fifteenth book he opens this part of his work by explaining the events recorded in Genesis from the time of Cain and Abel to the deluge.