Passage Two Questions 51 to 55 are based on the following passage. A letter written by Charles Darwin in 1875 has been returned to the Smithsonian Institution Archives ( 档案馆) by the FBI after being stolen twice. "'We realized in the mid-1970s that it was missing,” says Effie Kapsalis, head of the Smithsonian Institution Archives . "I t wa s noted a s missin g and likel y take n by a n intern ( 实习生 ) , from wha t the FBI is telling us. Word got out that it was missing when someone asked to see the letter for research purposes,” and the intern put the letter back. "The intern likely took the letter again once nobody was watching it.” Decades passed. Finally, the FBI received a tip that the stolen document was located very close to Washington, DC. Their art crime team recovered the letter but were unable to press charges because the time of limitations had ended. The FBI worked closely with the Archives to determine that the letter was both authentic and definitely Smithsonian's property. The letter was written by Darwin to thank an American geologist, Dr Ferdinand Vandeveer Hayden, for sending him copies of his research into the geology of the region that would become Yellowstone National Park. The letter is in fairly good condition, in spite of being out of the care of trained museum staff for so long. "It was luckily in good shape,” says Kapsalis, "and we just have to do some minor things in order to be able to unfold it . It has some glue on it that has colored it slightly ; but nothing that w il 1 prevent us from using it . After it is repaired, we will take digital photos of it and that will be available online. One of our goals is to get items of high research value or interest to the public online." It would now be difficult for an intern, a visitor or a thief to steal a document like this. ''Archiving practices have changed greatly since the 1970s,” says Kapsalis, "and we keep our high value documents in a safe that I don't even have access to.”