What is the theme in The Hairy Ape by Eugene O'Neill?

Class conflict is at the heart of this story. Yank is the epitome of the lower class, the working poor. He has the brawn but not the brain. He and his peers put their shoulders to the wheel and make the great capitalist machine run; they provide the sweat and muscle that will push Amenca to the forefront of the industrial age. The system exploits these efforts, reaping great profits for those who own the machines but offering little reward for those who operate them.