Classical ensembles continue, as they long have, to emphasize bedrock works by composers like Beethoven and Brahms, and many carve out a growing niche for contemporary works. But what often go missing are masterworks by modern American composers such as Roger Sessions, Walter Piston and William Schuman or Europeans such as Luigi Dallapiccola, Goffredo Petrassi and Michael Tippett.

“The music particularly of the mid- to late 20th century has pretty much disappeared,” said Pulitzer Prize-winning composer John Harbison, who studied with Sessions and Piston. “There are really only two exceptions that I’ve noticed: Shostakovich and Messaien.”

Musicians of the Chicago Symphony Orchestra will perform Harbison’s Little Fantasy on The Twelve Days of Christmas, a 1988 work for brass quintet, in Episode 10 of CSO Sessions, a holiday-themed program premiering Dec. 17 as part of the ongoing series of small-ensemble concerts streamed on the CSOtv portal.

Harbison theorizes that part of the disappearance of mid- and late 20th-century music can be attributed to the heightened attention and excitement that ensembles believe they can garner through premieres and the thrill of the new.

There have been examples of orchestras showcasing some of these increasingly forgotten composers such as the Great American Composers Series, a six-CD set released in 2008 on the Delos label. It featured conductor Gerard Schwartz leading the Seattle Symphony and New York Chamber Symphony performing works by the likes of Roy Harris, Howard Hanson and Alan Hovhaness.

But Harbison believes that these isolated projects are not enough. “It would need a much more broadly conceived follow-up than that,” he said.

Of course, the profiles of some composers rise and fall and rise again based on changing tastes and revolving musical fashions. Will some of these 20th-century composers experience such a revival? Harbison has no idea: “I certainly would not have guessed that we would lose track, as we have, of Tippett and Sessions and Piston, people like that, so I have no guess about whether they will come back.

“I’m just glad that we still have Messiaen very strongly, and we have [Morton] Feldman. There are some survivors, and we have to cherish that.”