These chapters depict figure without names, for the most part, helping to demonstrate the idea that what is happening to the Joad family is happening to many other Americans. The plight of the farmers in the Dust Bowl is a generalized plight. The intercalary chapters also show that this plight is not attributable to nature primarily but can be attributed instead to corporate greed, banking policies and politics.
In defining the terms of the underlying conflict of the novel, these chapters also help to develop the novel's central theme of struggle against a faceless system (and/or struggle to maintain one's humanity in the midst of a dehumanizing system).
We can see this theme articulated in Chapter 5, where the mechanized tools of farming (tractors, etc.) are characterized as monsters and the men hired to run them are depicted as inhuman as well.
"The man sitting in the iron seat did not look like a man; gloved, goggled, rubber dust mask over nose and mouth, he was part of the...
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