Four themes in The Jungle include the American Dream, class conflict, crime, and self-reliance.
The promise and elusiveness of the American Dream is the central theme of the book. The Rudkus family comes to the New World from Lithuania because they have been led to believe that in America,
"rich or poor, a man was free...he did not have to go into the army, he did not have to pay out his money to public officials - he might do as he pleased and count himself as good as any other men."
Sadly, the Rudkus family soon learns that they are not free at all in America because they must devote all their energies to staying afloat financially. The full impact of the reality of the Dream is brought to light when Ona Rudkus must work as a prostitute to ensure the survival of her family.
As poor laborers, the Rudkus family is constantly at odds with their employers, the ruling class which controls the wealth. Life is a never-ending struggle between the "haves" and "have nots," and after reaching the depths of poverty and desperation, Jurgis, the head of the Rudkus family, finally decides that he will do what it takes to become a "winner" in the perpetual class conflict that characterizes life in America.
Jurgis starts on a life of crime when he is himself a victim of a crime, a mugging. Jurgis decends into the criminal world, becoming involved in activities of ever increasing levels of immorality, including gambling and crooked politics. As his crimes become less defined in depravity, Jurgis finds that he has joined the ranks of the oppressors, almost without realizing what has happened to him.
A large part of the philosophy of the American Dream is self-reliance, the belief that the individual can achieve anything if he only works hard enough. Sadly, this idea is as false as the Dream itself, as illustrated by the experience of the Rudkus family; no matter how hard every member of the family tried, they were not able to achieve the elusive American Dream.