Since 2007, the American Museum of Natural History and the Darwin Manuscripts Project have been digitizing Charles Darwin's writings on evolution, and putting them online. Today, on the 155th anniversary of the publication of On the Origin of Species, the project is more than 50% complete.

According to AMNH:

The 16,000 [16,094, by my last tally -ed.] documents accessible on the site now cover the 25-year period inwhich Darwin became convinced of evolution; discovered natural selection; developed explanations of adaptation, speciation, and a branching tree of life; and wrote the Origin.

"Darwin's work in creating the Origin of Species encompassed much more than just setting pen to paper and writing the epochal book," Kohn said. "The Origin was the mature fruit of a prolonged process of scientific exploration and creativity that began toward the end of his Beagle voyage, which first kindled Darwin's interest in evolution, and that continued to expand in range and deepen in conceptual rigor through numerous well-marked stages."

The remainder of the manuscripts, which will be available in June 2015, will pick up in the year the Origin was published—1859—and will include the full record of Darwin's massive experimental research

A sampling of what you can already find online: