In Possession's early chapters, we're led to believe that Maia Thomasine Bailey was Christabel LaMotte's niece. That's what Maud Bailey thinks, too, and she ought to know: after all, Maia Thomasine was her great-great-grandmother.
Imagine Maud's surprise, then, when she finally learns that Maia Thomasine was actually Christabel's daughter—the product of the love affair between Christabel and Randolph Henry Ash.
Talk about having to redraw the family tree.
We don't know much about little Maia Thomasine. We know that she had a physical resemblance to both Christabel and Randolph, and that she had Christabel's extraordinarily light golden hair. We also know that she never really liked her "aunt," and had no taste for poetry, either (28.113-15). As Christabel tells Randolph in her final letter to him:
"Me she did not love. To whom can I say this but to you? She sees me as a sorcière, a spinster in a fairy tale, looking at her with glittering eye and waiting for her to prick her poor little finger and stumble into the brute sleep of adult truth. And if my eye glittered with tears she saw them not. No, I will go on, I fill her with a sort of fear, a sort of revulsion—she feels, rightly, a too-much in my concern for her—but she misreads that, which is most natural, as something unnatural." (28.113)
As for the significance of Maia Thomasine's name, Randolph understands it right away when he comes across her unexpectedly in a field one day. In Greek mythology, Maia is the mother of the trickster god Hermes, and, in the real world of the novel, Thomasine is the name of one of the waterfalls that Randolph and Christabel visited during their trip to North Yorkshire, where their child was conceived (Postscript: 6).
By the time of Christabel's final letter to Randolph, Maia Thomasine was already grown up and married, and had already given birth to a child of her own. She never did learn the truth about her biological parents—that discovery would have to wait for generations yet to come.