LONDON - A first edition of Charles Darwin's seminal "On the Origin of Species" will be sold this week after it was found in a family's toilet in southern Britain, an auction house said Sunday.

The book, which was first printed in 1859, was bought by a family for just a few shillings in a shop about 40 years ago, Christie's auction house said.

The family has since kept the work on a bookcase in the guest lavatory at their home in the Oxford area, it said.

The book will go under the hammer in London on Tuesday, to coincide with the 150th anniversary of the publication of the father of the theory of evolution's famous work.

The book, about 1,250 copies of which were first printed, is expected to fetch 60,000 pounds (S$138,426).

Margaret Ford, head of books and manuscripts at Christie's, said the book would have been a bargain when it was bought by the present owner.

"It's incredibly important," Ford said, adding of the current owner: "He knew it (the book) was textually important.

"He maybe did not know how much of a bargain he was getting."

Christie's said the son-in-law of the current owners was at an exhibition on Darwin and spotted a picture of the spine of the work.

He realised the book in the toilet was something special, matching the binding of the work in the picture.

Darwin's "On the Origin of Species" set out his theory of evolution, by which species develop from generation to generation by a process of natural selection.