I think

 I love that historical heroes are also full of doubt! 

"I think." Darwin's stolen notebooks, which included this crucial drawing of the tree of life that was a key step on his coming to recognize the reality of evolution, have been returned.

The BBC reports: Two "stolen" notebooks written by Charles Darwin have been mysteriously returned to Cambridge University, 22 years after they were last seen.

The small leather-bound books are worth many millions of pounds and include the scientist's "tree of life" sketch.

Their return comes 15 months after the BBC first highlighted they had gone missing and the library launched a worldwide appeal to find them.

"I feel joyous," the university's librarian Dr Jessica Gardner says.

The notepads date from the late 1830s after Darwin had returned from the Galapagos Islands. On one page, he drew a spindly sketch of a tree, which helped inspire his theory of evolution and more than 20 years later would become a central theory in his groundbreaking work On the Origin of Species.
"The theory of natural selection and evolution is probably the single most important theory in the life and earth environmental sciences and these are the notebooks in which that theory was put together," says Jim Secord, emeritus professor of history and philosophy of science at Cambridge University.

"They're some of the most remarkable documents in the whole history of science." https://www.bbc.com/news/entertainment-arts-60980288

From The Origin of Species:
The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connexion of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches; so with the species which lived during long-past geological periods, very few now have living and modified descendants. From the first growth of the tree, many a limb and branch has decayed and dropped off; and these lost branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only from having been found in a fossil state. As we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.

— Darwin, 1859

via {BBC}