LONDON – Twenty years ago, two notebooks belonging to the renowned naturalist Charles Darwin were found missing from the archives at Cambridge University Library. The books recorded Darwin's thoughts after he returned to England from his famed voyage aboard the HMS Beagle, as he grasped toward ideas that would form the foundations of modern evolutionary biology.

Now, the library said it believes the notebooks were most likely stolen, and it appealed to the public for any information about them. Police say they have asked Interpol to place the items on its Stolen Works of Arts Register.

The library, which houses the largest collection of Darwin's writings, has described the missing notebooks as "priceless" but nevertheless estimated their value at "millions of pounds." One page, reproduced around the world, gives his first sketch of a "Tree of Life," mapping out how related species could diverge from a common ancestor.

The notebooks were first listed as missing in January 2001, and staff had long thought they had simply been misplaced among its vast collection of around 10 million books and objects. But after an exhaustive search, curators concluded that they were likely stolen.