Protagonist
Character Role Analysis
Adam, Cal
Even though there are arguably lots of different storylines happening in this novel—like those of the Hamiltons and Cathy, for instance—we are still mostly following them through Adam's plot. How do we know that Adam's plot is the dominant plot? This line:
Such a man was Adam Trask. (2.2.4)
Bam—Adam is our man. This line comes at the end of a chapter, and it's Steinbeck's way of saying Hey, this dude is important. Everyone else in the story matters because they are somehow related to Adam's plot. Characters like Samuel and Cathy might have their own separate stories going on, but the reason we care about them is because they have some bearing on Adam's journey throughout the novel.
But in Part Four, things shift and Adam takes a backseat to his son Cal. You might be wondering why Aron doesn't naturally succeed his father, since he's pretty much a copy of him (naïve, innocent, loved by his dad but really doesn't like his dad, idealizes the woman he's in love with, etc.). But you might say that, by switching the narrative to focusing on Cal, we finally get to see Cain's side of things.
Cain is also the one who ultimately has agency, after all, which means he has the power to change things. You'll notice that Adam has a lot of things done to him rather than doing it himself: being made to join the army, being beaten up by his brother, being shot by Cathy, and so on. So for the end of this novel, it makes sense that the new protagonist needs to be someone who can make things happen instead.