Front and back of the letter written by Charles Darwin to Ferdinand Vandeveer Hayden on May 2,1875. Photo courtesy of the FBI.

Four decades after a handwritten letter by Charles Darwin was swiped from the Smithsonian Institution Archives, the FBI says its agents have found and returned it.

The letter, written in 1875 by the British naturalist, was stolen in the mid-1970s, according to a release from the FBI. It was part of a collection of papers related to the history of North American geology and taken before it could be cataloged in the building. Earlier this year, the investigation bureau received a tip that helped agents find the artifact and place it in the care of the Smithsonian on May 26.

Darwin wrote the letter to American geologist Ferdinand Vandeveer Hayden, thanking him for sending two studies. It reads:

May 2nd
Dear Sir,

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. 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.

With much respect & my best thanks, I remain,

Dear Sir,

yours faithfully

Charles Darwin

Too much time has passed for officials to charge the thief, who wasn’t identified, according to the FBI.