A letter, seen front and back, by Charles Darwin that was stolen more than 30 years ago was recovered by the FBI’s Art Crime Team and returned to the Smithsonian on May 26, 2016. (Courtesy FBI)

More than 30 years ago, a Smithsonian intern came upon an 1875 letter written by British naturalist Charles Darwin — and pocketed it. Last month, the letter was returned.

The letter came into the possession of a good Samaritan who contacted the FBI earlier this year, bureau spokeswoman Katherine Zackel said. Working with the Smithsonian, FBI agents authenticated the letter and returned it to the Smithsonian Institution Archives.

The FBI, which announced the return Thursday, said the letter was among a collection of papers on North American geology given to the Smithsonian. The letter was stolen before it could be inventoried, Zackel said, so officials did not realize it had disappeared until researchers at Cambridge University asked for a copy. Smithsonian spokeswoman Effie Kapsalis said the FBI told the organization only that the thief was an intern.

Because the statute of limitations on the crime has run out, Zackel said, no one will be charged and thus the FBI will not reveal the name of the thief or how the letter got into the hands of the person who turned it in. She would say only that the letter was found in the Washington area and was remarkably well preserved.

“It’s in great condition. It’s surprising — it actually still looks really good,” she said.

This undated engraving released on Feb. 10, 1959, shows English naturalist Charles Darwin. (AFP/GETTY IMAGES)

In a statement, Smithsonian Institution Archives Director Anne Van Camp thanked the federal agents involved, saying that “this type of crime is not easily detected.”

Darwin penned the two-page letter, dated May 2, 1875, to American geologist Ferdinand Vandeveer Hayden, thanking him for sending two field studies of the American West. Part of the land Hayden surveyed soon after became Yellowstone National Park.

“I am much obliged to you for your kindness & for the honour which you have done me in sending your Geological Report of the Yellowstone River & your Preliminary Field Report on the Colorado & New Mexico,” Darwin wrote. “I had heard of your Geological researches on the Colorado & was anxious to see the conclusions at which you had arrived, & I am therefore especially obliged to you for having sent me your works.”

He signed it, “With much respect & my best thanks. I remain, Dear Sir, yours faithfully Charles Darwin.”

Kapsalis said the Smithsonian Institution Archives, which stores historical documents related to the organization’s history, has only two Darwin letters.

“The public is fascinated by anything related to Charles Darwin, and we’re happy to get it back,” she said. “The two letters are a pair once again.”

The letter was among papers collected by George Perkins Merrill, a geologist who worked for many years in the geology department of the U.S. National Museum, a precursor to the Natural History Museum.

Although it could not be verified that the author was the good Samaritan, a few months ago a poster asked about a Darwin letter on a website that provides experts to answer legal questions. “What is the statute of limitation on a theft from the smithsonian,” the person asked. “I recently found out something by a relative was stolen there about 30 years ago?”

The poster, who was anonymous, said the item was a letter by Darwin and “a piece of history” that should be returned. The poster said the person who took the letter had been an intern.

Tim Berra, a Darwin expert from Ohio State University, said the famous evolution theorist wrote about 7,000 letters in his life.

“Anything to or from or by Darwin is of historical significance and scientific significance,” he said. “Darwin was interested in what was going on in America. . . . Darwin was primarily trained as a geologist, so he was interested in geological formations all over the world.”

He noted that Cambridge has been assembling all of Darwin’s letters into a series of volumes; the 23rd, just published, includes the year this letter was written.