A piece of paper signed by Charles Darwin in which he defends his theory of evolution is to be sold at a Sotheby’s auction in New York. The item is likely to fetch more than a million pounds, a record price for a Darwin manuscript.

The document was created by the English naturalist and biologist to be copied in a magazine of celebrities in 1865.

Darwin was not in the habit of archiving his writings and so very little original material survives. When he did he always signed himself only with the abbreviation ‘C Darwin’ or ‘Ch Darwin’. He very rarely wrote his full name, ‘Charles Darwin’.

Professor John van Wyhe, who curates the scholarly collection known as Darwin Online, told the BBC that the document is particularly special.

“He includes a passage that appears in the third edition of On the Origin of Species,” the senior lecturer at the National University of Singapore explained.

“It’s a really favourite passage, because he’s trying to make the point that people might find his theory unbelievable and outlandish, but they said the same about Newton and gravity, and nobody doubted the existence of gravity anymore. The same, he says, would be true eventually with evolution and natural selection”, the professor told BBC News.