Part 1 of To Kill A Mockingbird focuses mainly on the children's obsession with Boo Radley. In the novel's early chapters, readers come to recognize Scout, Jem, and Dill as curious children who entertain themselves by trying to get a glimpse at their mysterious neighbor. Despite Atticus's advice to leave Boo alone, the children keep at their games and fail to understand the fact that their behavior is hurtful and unfair.
In Part 2 of the novel, Harper Lee shifts her focus to the trial of Tom Robinson, which is a clearer example of the social (in this case racial) injustice that was all too common in the 1930s in the South. Watching their father defend Tom Robinson, who is obviously innocent of the crime of which he's accused, teaches the children that it is unfair to prejudge otherse and that a person's social/economic status does not dictate his or her worth as a person.
At the novel's end, and after having been saved by Boo Radley, Scout is able to finally understand Atticus's advice that we can never really understand others until we put ourselves in their situation. The novel's two distinct focuses (the children/Boo Radley in Part 1 and the Tom Robinson trial and its fallout in Part 2) serve to teach readers that injustice is present in all facets of society, and Lee successfully brings these ideas together at the end of the novel.
As was mentioned in the previous post, Part One of the novel deals with the children's obsession with Boo Radley, describes the unique experiences of growing up in a small town, and portrays Atticus's morally upright nature. In Part Two, Lee focuses more on Tom Robinson's trial and the ugly prejudice throughout the town of Maycomb. Atticus also takes the bold stance of defending Tom in front of a racist jury and the children lose their childhood innocence. In my opinion, Harper Lee chose to split the novel into two parts because it correlates with Scout's moral and intellectual development. In Part One, Scout is naive and blind to the overt racism that surrounds her. She also does not realize that Boo Radley is a peaceful, shy man. Atticus's role is also more of a father figure than a fierce defender of justice. In Part Two, Scout witnesses racial injustice firsthand and becomes aware of the prejudice throughout Maycomb. She matures into a morally upright individual like her father and no longer fears Boo. The subplot of the novel returns when Boo Radley rescues the children from Bob Ewell. The information included in Part One foreshadows Boo's selfless act in Part Two and reiterates the extended metaphor that "it is a sin to kill a mockingbird."