Two of Charles Darwin's most famous notebooks, including one that contains his "Tree of Life" sketch, have "likely been stolen," officials said.

According to Cambridge University Library in England, the books were last seen in September 2000, but were declared missing when a routine check was performed in January 2001. It was believed the books had been misplaced, but now it's likely that the books were taken by thieves.

“I am heartbroken that the location of these Darwin notebooks, including Darwin’s iconic ‘Tree of Life’ drawing, is currently unknown, but we’re determined to do everything possible to discover what happened and will leave no stone unturned during this process," Dr. Jessica Gardner, University Librarian and Director of Library Services, said in a statement.

Charles Darwin's Tree of Life Sketch. (Credit: University of Camrbidge Library)

Charles Darwin's Tree of Life Sketch. (Credit: University of Camrbidge Library)

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Gardner continued: “This public appeal could be critical in seeing the notebooks safely return, for the benefit of all, and I would ask anyone who thinks they may be able to help to get in touch. We would be hugely grateful to hear from any staff, past or present, members of the book trade, researchers, or the public at large, with information that might assist in the recovery of the notebooks."

The university declined to put an exact value on the notebooks, but "given their unique nature," they are worth millions of dollars.

Perhaps not coincidentally, the news coincided with "Evolution Day," the anniversary that Darwin published his iconic "On the Origin of Species" in Nov. 1859, the book that includes the famous "Tree of Life" sketch, first drawn in 1837 aboard the HMS Beagle.

“Cambridge University Library is one of the world’s great libraries and home to globally important collections, assembled and cared for over six centuries, and encompassing thousands of years of human thought and discovery," Professor Stephen J Toope, Vice-Chancellor of the University, explained. “As a result of this appeal for help, we hope to locate the missing Darwin notebooks and restore them to their rightful place alongside the University Library’s other treasures, making them available to scholars and researchers in the centuries to come.”

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Included in its works are the "most significant collections of Darwin material anywhere in the world," containing 182 volumes, 189 archive boxes, 170 plans and drawings and more than 8,000 letters.

In 2016, a letter from Darwin to a marine biologist was sold at auction, Fox News previously reported.

The two notebooks have had their contents previously digitized, including the Tree of Life sketch, the university library added.

Anyone with information about the notebooks can email the library at ManuscriptAppeal@lib.cam.ac.uk  or contact the website of the Cambridgeshire Police for more information.

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