The Deanes arrive, and Maggie and Tom come in to greet Lucy Deane. Mrs. Glegg speaks loudly to Mrs. Tulliver of the need to thin out Maggie's unruly hair. Maggie and Lucy get permission from Lucy's parents for Lucy to stay over. Maggie drags Tom upstairs with her to have him watch while she cuts her hair. Instead of joining in her rebellious triumph, Tom laughs and insults her new appearance. Tom goes downstairs, leaving Maggie feeling remorseful. First Kezia, the family servant, then Tom come upstairs and finally coax Maggie down to dinner. Everyone is shocked—then the women are reproachful and the men amused. Maggie begins to sob, and her father comforts her.
The children soon adjourn with their dessert, and Mr. Tulliver announces his plans for Tom's education. Mrs. Glegg in particular is skeptical and pessimistic about this plan. Mr. Tulliver and Mrs. Glegg quarrel, and Mrs. Glegg leaves, taking Mr. Glegg with her.
As the title of Book First, "Boy and Girl," suggests, much of these chapters are spent examining Maggie and Tom's childhood relationship. Eliot presents their relationship as close, with Maggie as dependent upon Tom. Imagery such as that used to describe Maggie's hugs—"Maggie hung on his neck in rather a strangling fashion"—lend an ominous tone to their relationship and foreshadow the final events of the novel. While Tom can be affectionate and loving toward Maggie, we also see that he does not encourage her cleverness, as her father does. Tom immediately cuts off Maggie's imaginings about Tom's hypothetical bravery when faced with a lion. Though Tom shares Maggie's fantasy of the two of them always living together happily, part of his fantasy involves exerting dominion over her by always "punish[ing] her when she did wrong."
Indeed, the depiction of Maggie and Tom's childhood relationship in Book First raises an important theme of forgiveness and justice. Tom is characterized as a stubborn boy who sticks to a code of fairness in his dealings and judgments of others. The administration of this code can cause pain to others, as with Maggie's confusion over proper conduct in the matter of the pastry in Chapter VI, or indirectly affect Tom adversely, as with the loss of Bob Jakin as a playmate. Yet, Tom always feels satisfied in the knowledge that he has acted correctly. Maggie, on the other hand, operates in relation to feelings. When she is deemed naughty by Tom or her mother or another, she does not reflect on the fairness or unfairness of the judgment against her but focuses instead on the misery of feeling unloved. Maggie craves forgiveness and offers forgiveness to others—she even inspires forgiveness in Tom in Chapter V through the overwhelming power of her own love and affection. The only twist on this is that Maggie does not easily forgive herself. Unlike Tom, who feels secure in his actions, "Maggie was always wishing she had done something different."
In her essay "Silly Novels by Lady Novelists" (1856), George Eliot disparaged several genres of novels, all of which violate realistic conventions by making their characters extraordinary beyond belief. One of the violations she focuses on is the tendency to sentimentalize child characters and put language in their mouths more befitting an adult. Eliot seeks to make Tom and Maggie seem realistic by focusing precisely on their immaturity. Through this, another theme of the section emerges—the lack of life perspective felt by children. Maggie's dramatic scenes of grief are connected repeatedly in the narrative to her inability to put her miseries in the context of past trials overcome or enjoy the experienced faith in the future.
The Tullivers and the Dodsons discuss Maggie's and Tom's physical and behavioral characteristics in relation to each of their parent's family line. This line of conversation, combined with the narrator's discussion of the faculties that distinguish humans from animals, reveal the historical context of Darwinism to George Eliot's novel. Darwin published his study On the Origin of the Species by Means of Natural Selection; or the Preservation of Favoured Races in the Struggle for Life in November 1859 as George Eliot was in the midst of writing The Mill on the Floss. Eliot was already interested and knowledgeable in the field of natural history and the language of natural history occurs throughout The Mill on the Floss.