"Be fruitful and multiply..."

After reading Genesis 4-5 that phrase comes to mind.  Adam and Eve start a family, and their children start families of their own.  God had always intended for humanity to procreate, unfortunately, as Adam and Eve fell, so their children inherited a distorted view of God and His plan.  This is clearly evident in Cain and his family, which is noted for its murderers, and even though Seth's family is noted for men of faith, they are still under the curse of sin.  In the end, God sees that humanity as a whole and individually are fallen, and in muliplying have spread evil all over the earth, and He chooses in His sovreignty to flood the earth and start over with Noah and his family.