To Kill A Mockingbird involves the maturation of the two Finch children, Jem and Scout. By looking at several quotations of Jem's, we can see how his character evolves and grows up throughout this novel.
One of the first times we see Jem stand up for himself and his family occurs in chapter 11. In response to Mrs. Dubose insulting Atticus for his defense of Tom Robinson, Jem can't help but respond even though Atticus had specifically instructed his son not to respond to the old woman's insults; Jem's pride won't allow him to take it on the chin. At this point in the novel, we can see that Jem is developing an identity of his own, but he still has a lot of growing up to do. Later, when Jem arrives at Mrs. Dubose's house to read to her as part of his punishment for losing his cool and destroying her flowers, Mrs. Dubose takes the opportunity to insult Scout's appearance. Despite the fearful shaking of his knees, which doesn't go unnoticed by Scout, Jem replies that
My sister ain't dirty and I ain't scared of you.
In chapter 23, we see just how much Jem is concerned with the influence of racism on the legal system. Jem and Atticus discuss this to great length and Jem grapples with some tough concepts. This is illustrated when he says that,
No sir, they oughta do away with juries. He wasn't guilty in the first place and they said he was.
At the start of chapter 25, Jem stops his sister from killing a roly-poly bug. Scout asks him why and he responds by saying,
Because they don't bother you.
This short reply illustrates that Jem has taken Atticus's message about killing innocent creatures, such as mockingbirds, to heart. Scout reckons that Jem is just going through a phase, yet it seems to the reader that Jem is maturing into a thoughtful and compassionate man, much like his father.