Because there are varying interpretations of Gaskell's character development and themes, I offer my basic understanding of North and South up front.
Gaskell skillfully blends a compelling personal journey with an insightful examination of the social and economic upheaval of Victorian England. North and South is a love story with a strong social and moral message. Margaret Hale's encounter with the industrial North and her evolving relationship with John Thornton elucidates and accentuates the struggle involved in true growth -- in opening our thought to new perspectives.
In Margaret and Thornton's initial arguments, Gaskell lays out the rigid idealism and assumptions of opposing viewpoints that lead to conflict and misunderstanding. Here, with the addition of Nicholas Higgins' antagonistic stance against the masters, Gaskell exposes the prejudices and basic lack of communication between people of differing classes and economic states.
The fervent dialogue and dramatic conflict of the first half of the novel threatens to overshadow the slower, subtle softening of the main characters' perspectives that unfolds after the climactic events of the riot. Margaret begins to recognize the depth of Thornton's integrity; Thornton is forced to acknowledge Higgins' admirable moral capacity; Higgins starts to respect and comprehend his employer.
Slowly, but with unmistakable details in silent reflection and behavior, Gaskell shows that Margaret is in love with Thornton well before she ever leaves Milton. John Thornton's love is constant, although he is sorely tempted at times to doubt her integrity. Their love is based on a deep attraction to the good in each other. They are both strong, intelligent, self-sacrificing, highly moral individuals who are devoted to their families. The only misunderstanding keeping them apart for much of the latter part of the novel is the belief that the other is not interested.
Gaskell's essential theme is one of finding harmony and unity among men. Her novels constantly challenge her readers to look beyond the surface of our outward differences to find our common humanity. Respect, compassion, and understanding naturally develop as we begin to see each other as individuals. And as we shed the imposed boundaries that separate us, humanity is ready to move forward on the path of progress.