The Hairy Ape explores two common, interrelated themes in modern literature: personal isolation and alienation. Eugene O’Neill masterfully translates these themes into Darwinian terms, choosing a protagonist so low on the evolutionary scale as to be almost indistinguishable from the real apes. Yank has found his place, his womb, as it were, in the protected environment of the stokehold, a place where no more developed human would care to be.

As a simple stoker, Yank makes his meager contribution to society, and it, in turn, takes care of him until the fateful day that Mildred Douglas, a do-gooder, a not-so-liberal liberal, leaves her protected world to enter Yank’s for a brief moment. In the twinkling of an eye, Mildred makes Yank see what he really is. Having already been shown to be prone to fixations, he develops an obsession about Mildred and her kind that ultimately destroys him. He wants to destroy all that Mildred is and stands for, but her society is stronger than his. It is organized against the likes of Yank, and he does not stand a chance against it.

The great irony in the play is that Mildred’s society barely knows that the Yanks of the world exist. A day after she sees Yank and faints from the shock, Mildred will be back in her normal routine; the dirty, sweating, half-naked stoker will remain but a vague, mildly unpleasant memory for her. Meanwhile, the poison of Yank’s hatred for the Mildreds of the world boils within him, and he swears a vengeance that society—Mildred’s society—will not allow him to execute.

The Fifth Avenue scene represents total rejection. Yank is an unpleasant distraction to be overlooked, rather than confronted, by the people he affronts. When he finally oversteps the limits and hits one of the pillars of this well-dressed, Sunday-strutting society, he is removed officially but still ignored. Yank has no way to command the recognition he craves, and without doing so he cannot wreak his vengeance on society.

Similar rejection at the IWW office reinforces Yank’s alienation and confirms his suspicion that he does not belong. He then seeks from his ancestral simian cousins the recognition his more highly evolved fellow humans deny him, only to realize that he belongs in neither world. In an early version of the play, O’Neill did not have Yank die in the monkey house but had him return to his ship, which became his prison. O’Neill decided, however, that the best dramatic possibility for this play was to have Yank die, to have the life crushed out of him by a creature not quite of his own kind but at least very much like him.

Class Conflict
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 America 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.

Although Yank initially envisions himself above the first-class passengers on the ship—reassuring himself with the knowledge that without people like him the ship would not run—he comes to realize that the rich are getting richer from his efforts while his own rewards remain paltry. It is Mildred's father who owns the steel works and the ship line. And it is people like Mildred who can afford the furs and diamonds on Fifth Avenue. They are living the good life by exploiting the workers.

It is this realization that he is only a cog in the machine and not the center of the industrial universe that plants the first seeds of Yank's disillusion. Before Mildred's appearance in the stokehole, Yank had not been directly exposed to the upper class. While his perception of himself was one of elevated status, he is confronted with the fact that the true mark of high status—money—is in the hands of others. His illusions of importance in question, Yank begins to ponder his exact place in society.

Meaning in Life
Although it is a pose at direct odds with his mental capacities, Yank is seen several times throughout the play in the pose of the ''Thinker'' (a famous sculpture by Auguste Rodin depicting a man in deep, contemplative thought). What provokes these ponderous episodes is his struggle to understand his role in life. It is a role that he thought he understood. He worked hard, providing the human energy that enabled the massive ship to run its engines. For these efforts he felt he should be viewed as a kind of superhuman, a creature upon whom the rest of society depended. Yet when Mildred nearly faints at his brutish appearance, he is confronted with the possibility that others do not see him in this light.

While his initial reaction to being called an animal—a hairy ape—is one of pleasure, he comes to realize that the distinction is not a positive one. Far from being considered a superman, he is an outcast and an oddity. He is not like his fellow workers, and he is certainly not like the first-class passengers.

His first realization that he does not have the social standing he believed provokes growing self-reflection in Yank. Prior to Mildred's visit, he had a firm ideal of his place in society, the meaning of his life. Learning that others do not see him as he sees himself poses the question: where does he fit in? Lacking even the most basic social tools, Yank is an outcast even among the other firemen. Where he had previously seen this alienation as proof of his superiority, he now begins to question his place among humanity. At the start of the play, Yank is happy—or at least content—with his station in life. The knowledge that reality is far from his perception marks the start of his downfall, his search for a place to belong, and eventually his death.

Socialism and Society in the Industrial Age
While the FBI feared that The Hairy Ape would be used as a propaganda tool for those with socialist/ communist agendas, the play came to be known more for its study of human nature than for its politics. Socialism as voiced by the character of Long, argues that the only fair economic system is one that allows ownership by the workers and a more even distribution of wealth among all citizens. While The Hairy Ape makes some arguments in favor of better working conditions and an equitable share of profits (it is clear from the play that the firemen are not well compensated for toiling under extreme conditions), it does not aspire, like Clifford Odets's Waiting for Lefty, to present an overview of the injustices wrought on the working class.

What O'Neill sought to illustrate was how America's rapid evolution into an industrial nation created personality types that were suited for the necessary tasks. In a form of Darwinian adaptation, those with physical prowess became the workers while those with a sense for money and planning became the upper class. This evolution also created rigid ideals for each social class. O'Neill's interest lay in the development of an extreme social persona such as Yank. Yank's strength and skill as a menial worker allowed him to develop and excel at one thing—stoking an engine furnace. Yet his advancement as a firemen came at a cost to his humanity. He has evolved to an ultra-refined state in which he is as much a machine as human. He can no longer interact with his peers. Beyond criticizing or embracing one system, the play condemns a society— socialist, capitalist, or other—that would allow such an extreme disassociation to take place in the name of progress.