What was the main fight that took place in "The Octopus"?
The main fight takes place early in the novel, between Magnus Derrick and S. Behrman, who represents the railroad. Behrman tells Derrick that he can not take possession of the ploughs that have been purchased. Derrick will have to wait until the ploughs pass through various cities before being returned. The railroad is doing this to increase their own profit. The fight between these two men actually represents the struggle between the people and the corporations (in this case, the railroad). Corruption was prevalent in corporations as this time - within 20 years, the Progressive Movement will retaliate against the injustice and seek for more fair working conditions and pricing.
The description of Behrman helps to highlight the theme at the heart of this conflict:
He was a real-estate agent. He bought grain; he dealt in mortgages. He was one of the local political bosses, but more important than all this, he was the representative of the Pacific and Southwestern Railroad in that section of Tulare County.
Behrman is described in terms of his businesses, not in terms of human relationships. This shows the corruption of the individual at the expense of the corporation's all-important goal - profits. The success of Behrman over Derrick shows the success of the capitalistic society over the humanity of the average person.