Fantasy is a genre in which anything can happen — at least in theory. But in practice, fantasy realms are often as drearily limited as the realities they promise to transcend. Fantasy authors can magic dragons into being, but have trouble inventing dragon-riders who aren't white. Creators may believe a man can leap tall buildings, but having a black girl who can fly is usually a dream too far.

Ebony Elizabeth Thomas' new book "The Dark Fantastic: Race and the Imagination From Harry Potter to the Hunger Games,” is about the ways that black girls are excluded from or marginalized within fantasy. Black girls have little opportunity to see themselves in fun, exciting, imaginative pop culture story worlds. That may be part of the reason that the average 12th grade black student still placed only in the 22nd percentile for reading achievement, according to a 2016 analysis by EducationNext.

Black girls in fiction were mired in stories of realistic oppression, or else marginal secondary characters. There were few black girl equivalents of either Cinderella or Captain Kirk.

"In my own childhood," Thomas told me, "if you were a black girl in a story intended for children and teenagers, you were generally either (A) a slave, (B) fighting against and enduring Jim Crow, (C) marching for civil rights, (D) stuck in the ghetto and trying to get out, or (E) the suburban black best friend. There were almost no other options." Black girls in fiction were mired in stories of realistic oppression, or else marginal secondary characters. There were few black girl equivalents of either Cinderella or Captain Kirk.

This has begun to change to some degree. But progress is slow — and often stained by ongoing prejudice. In "The Dark Fantastic," for example, Thomas talks about Rue, a black girl in the hugely popular “Hunger Games” series. Rue is an important secondary character in the first novel. She's young, innocent, small and clever. Her death in combat is one of the most traumatic and painful events of the novel.

In the book, author Suzanne Collins describes Rue as having "dark brown skin and eyes." The creators of the “Hunger Games” film series, following the description, cast Amandla Stenberg for the part.

But even though this was true to Collins' vision, many fans were shocked and angry. "“Why is Rue a little black girl?” one typical tweeter asked in confusion. "Call me racist but when I found out Rue was black her death wasn’t as sad," another admitted.

"The ways in which we have been conditioned to read over time means that it's very difficult to see a black girl as innocent," Thomas says. "Rue is cute and innocent and little, three qualities that aren't generally applied to girls of African descent in stories, and especially not in fantasy." The history of tropes linking whiteness, innocence and delicacy were more powerful for many readers than the short description identifying Rue as black. Reader imaginations literally failed them. They couldn't see what Collins had put in front of them, and when they were forced to see it on screen, they rejected it.

Another example that Thomas discusses is the character of Bonnie in the television series "The Vampire Diaries," which ran on The CW from 2009 to 2017. In the novels on which the series is based, the character of Bonnie is white. But for the show the creators decided to cast Kat Graham, who is black.

Historically, teen soap operas like "Beverly Hills 90210" had few black characters. Thomas says she thinks that casting Graham came "out of an impulse to do good," and to push for change.

Historically, teen soap operas like "Beverly Hills 90210" had few black characters. Thomas says she thinks that casting Graham came "out of an impulse to do good," and to push for change.

However, both creators and fans had difficulties with a black Bonnie. The white Bonnie in the novels was a major character and had numerous romantic plotlines. On the show, though, Bonnie was relegated to a secondary role, and was never allowed to be a romantic rival to the white protagonists. "She just became sort of the bitchy black best friend," Thomas says. "A noble intent ended up creating a stereotypical, flat and problematic character."

These failures are disheartening, but Thomas sees reason for hope as well. The growth of online fan fiction communities has made it possible for black girls to reimagine or rework shows that exclude them. Thomas says. "If you don't see yourself in a story, you can write yourself into the narrative."

Thomas herself wrote a fan fiction epic centered on Angelina Johnson, a black Harry Potter character who is barely mentioned in the novels. And some of these reworkings can have influence beyond fan communities. Fan readings of Hermione as black, for example, may have influenced the decision to give the part to black actress Noma Dumezweni for the West End production of "Harry Potter and the Cursed Child."

"Social media has really amplified the voices of fans on the margins, including but not limited to black girls," Thomas says. "Queer fans, disabled fans, fans from cultures that are not Western. I feel as if the perspectives of those sitting around the edges of the audience are made more legible not just to creators or storytellers but also to the mainstream."

Perhaps partially as a result of greater visibility, we've begun to see more black women in fantastic narratives. "Black Panther’s” Nakia (Lupita Nyong'o), Okoye (Danai Gurira), and Shuri (Letitia Wright) are perhaps the most high-profile examples. But Thomas also points to Iris West (Candice Patton) on The CW show "The Flash," "She is subverting a lot of the usual black girl-in-a-teen-television-show tropes because she is the central character," Thomas says.

Examples like this make Thomas think that pop culture may be finally broadening its collective imagination enough to include black girls. That in turn may open a door to broader progress as well. You can't change society, after all, without first dreaming about how you want to change it.

"Resolving the crisis of race in our storied imagination," Thomas writes in "The Dark Fantastic," "has the potential to make our world anew."