What is the theme in The Iron Heel by Jack London?

The Struggle between Socialism and Capitalism is the book's most apparent theme, the anchor of both its narrative and philosophical explorations; in other words, characters and events in the book's plot are driven and defined by this struggle in the same way as its consideration of socially relevant ideas and ideals. The most important thing to note about this central theme is that its relevance to the book is built upon its relevance to the world at the time the book was written.