"To Enoch was born Irad, and Irad was the father of Mehujael, and Mehujael was the father of Methushael, and Methushael was the father of Lamech. Lamech married two women, one named Adah and the other Zillah." NIV translation
Now, we see the downward spiral as the disobedience of Cain has led to the practice of polygamy (having more than one wife). We have already seen God's plan for a man and his wife but, just a few generations of disobedience have led to this. This is the same type of thing that we see in many families today as generations of wickedness have led to a complete disregard for the ways of God.
"Adah gave birth to Jabal; he was the father of those who live in tents and raise livestock. His brother's name was Jubal; he was the father of all who play the harp and flute. Zillah also had a son, Tubal-Cain, who forged all kinds of tools out of bronze and iron. Tubal-Cain's sister was Naamah." NIV translation
We are introduced to the seventh generation of people and remember that seven is the number of completion throughout the Bible. Because of the fall, men were required to work and, here, we see the start of various occupations such as shepherd, musician, and ironworker. Although we are not told of Naamah's occupation, her name suggests that she was a singer.
"Lamech said to his wives, 'Adah and Zillah, listen to me; wives of Lamech, hear my words. I have killed a man for wounding me, a young man for injuring me. If Cain is avenged seven times, then Lamech seventy-seven times.'" NIV translation
The downward spiral has continued to the point of murder. Not only do we see that Lamech went so far as murder but he was even proud of the fact. He compared himself to God's promise of avenging those that would kill Cain and elevated himself above God.
"Adam lay with his wife again, and she gave birth to a son and named him Seth, saying, 'God has granted me another child in place of Abel, since Cain killed him.' Seth also had a son, and he named him Enosh. At that time men began to call on the name of the Lord." NIV translation
God gave Adam and Eve a third son named Seth. He had a son (Adam and Eve's grandson) who he named Enosh which means "man" or "mankind". And so begins the population of God's creation and men acknowledged God the Creator.