The document was written in response to Hermann Kindt, the editor of the Autographic Mirror—a type of celebrity magazine of its day—who had requested a sample of Darwin’s handwriting to reprint in his publication.

It is listed with an estimated price of $600,000 to $800,000, but the BBC reported that the item is likely to fetch over a million pounds ($1.2 million) by the time bidding closes on Dec. 9. Even the achievement of the lowest estimation would set a world-record price for a Darwin manuscript.

Online bidding for the document opened on Friday, with the item being sold as part of a wider auction that also includes a first edition of the British scientist’s groundbreaking book On the Origin of Species, which itself is expected to sell for up to $400,000.

Describing the signed manuscript as “extremely rare,” auction house Sotheby’s—which is selling the item in New York—said the document contained a passage from Darwin’s revolutionary text and was “the most significant autograph manuscript by Darwin to ever appear at auction.”

Darwin was known to obsessively revise his publications, often discarding pages from working drafts, making any autographed manuscript a rarity. He also often signed documents with abbreviated versions of his name like Ch, Ch. Darwin or C. Darwin, so his full signature on the manuscript also adds to its value.

The correspondence dates to the autumn of 1865, four years after the publication of the third edition of Origin of Species—which Sotheby’s said makes the document all the more significant as it “encapsulates Darwin’s complete thinking on the subject [of evolution]” and includes “assertions that only appeared in the third, revised edition” of the text.

“I have now recapitulated the chief facts and considerations which have thoroughly convinced me that species have been modified, during a long course of descent, by the preservation or the natural selection of many successive slight favorable variations,” Darwin wrote in the manuscript for Autographic Mirror.

“I cannot believe that a false theory would explain, as it seems to me that the theory of natural selection does explain, the several large classes of facts above specified. It is no valid objection that science as yet throws no light on the far higher problem of the essence or origin of life.”

He added: “Who can explain what is the essence of attraction of gravity? No one now objects to following out the results consequent on this unknown element of attraction.”

Darwin’s works on evolutionary theory, while widely accepted and revered in the scientific community and beyond, have been a point of contention since their publication and are still rejected by some to this day because of their direct contradiction to creationism.

“Charles Darwin’s revolutionary text On the Origin of Species succeeded in creating a Book of Genesis for the modern age and it would be difficult to overstate the seismic impact it had not only on 19th century science and culture but also on subsequent thinkers and generations,” Richard Austin, Global Head of Books & Manuscripts at Sotheby’s, said in a statement. “We are proud to offer the most significant autograph manuscript by Darwin to come to auction.”