Why is the novel "To Kill a Mockingbird" divided into two parts and how does this relate to the overall theme of the book?
The novel is indeed divided into two parts: Part 1 consists of chapters 1-11 and Part II consists of chapters 12-31.
There are a few perspectives as to why the novel is divided into two parts. Part 1 delineates all the intricacies of small town society, all the little courtesies and aggravations of life, and all the many social customs common to a cautiously segregated South. In essence, Harper Lee uses the first part of the book to describe the innocence and rich vivacity of Southern living. The freshness of youth is depicted in the adventures of Jem, Scout, and Dill. Their quirky imaginations allow them to concoct fantastic stories about their boogeyman nemesis, Boo Radley. Like all young children, they are fascinated by what they do not understand.
Yet, all young children must learn certain life lessons eventually. Thus, Lee balances moments of sweet guilelessness with solemnity in the first part of the book. When Jem cuts down Mrs. Dubose's flowers in a fit of frustration at her harsh words about Atticus, he finds himself consigned to reading to the cantankerous old lady every afternoon. It is this first lesson in compassionate understanding which lends such import to how both Jem and Scout approach important events in the second part of the book.
The major events in the second part of the book involve Tom Robinson's trial, his eventual death, and Bob Ewell's disgraceful attack on both Jem and Scout. Part II is darker, with good reason. Beneath the surface hospitality and warmth of small town Maycomb lies all the peculiarities and ugliness that imperfect human nature often occasions. Despite the best of intentions, this fact of life cannot be ignored, and Harper Lee focuses her authorial voice on these matters with precision and grace.
In Part Two, Jem and Scout learn the importance of the lessons Atticus painstakingly teaches them in Part One. The children come to realize that they must try to look at matters from different perspectives if they hope to gain some measure of understanding and compassion for others during difficult situations. Yet, they also know that there is great store to be set by the courage of one's convictions. In this, Atticus' courage and resolve in the face of extreme societal pressure from Maycomb's white populace is demonstrated clearly in Part Two.
The Tom Robinson trial is the catalyst for strife in Maycomb; racism and bigotry creates division and conflict between the white and black citizens of Maycomb. As an example, Jem and Scout have to endure unfair scrutiny from their white neighbors as well as contemptuous rejection from a black parishioner in Calpurnia's church. In the meantime, Aunt Alexandra argues with Atticus about releasing Calpurnia from the household.
The overall theme of the book centers around the mockingbird. To kill a mockingbird is to kill senselessly. After all, the mockingbird does nothing but bring pleasure to everyone. So, the events in Part One converge in Part Two to support this basic theme: apart from the grievances between the white and black communities in Maycomb, the fact still remains that an innocent, black man's life was senselessly sacrificed to the altar of bigotry and injustice. Tom Robinson was a mockingbird figure, cut down in the prime of his life. In Part Two, Scout and Jem realize that the sin of killing a mockingbird isn't just literal; it's also symbolic. Good people come in all shapes and sizes.
This can be seen in Scout's eventual change in perspective regarding Boo Radley. By the end of the novel, she comes to understand Sheriff Tate's reluctance to expose Boo Radley's heroism to the whole town.
“Scout,” he said, “Mr. Ewell fell on his knife. Can you possibly understand?”
“Yes sir, I understand,” I reassured him. “Mr. Tate was right.” Atticus disengaged himself and looked at me. “What do you mean?” “Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it?