Darwin's former home, Down House, is now owned by English Heritage
An appeal has been launched to trace Charles Darwin's missing Galapagos notebook which provided crucial evidence for his theory of evolution.
English Heritage says the notebook, which helped him write On The Origin of Species, may have been stolen from his former Kent home in the 1970s or 80s.
In it he described encountering a giant tortoise and made notes on local birds.
English Heritage is putting Darwin's 15 notebooks online 150 years after On The Origin of Species was first published.
They will include highlights from a 1969 microfilm of the missing notebook. The books from Darwin's five-year voyage on HMS Beagle in the 1830s were all put on microfilm, but by the early 1980s the Galapagos book had vanished.
It is believed to have been stolen from Darwin's study at his former home, Down House, now owned by English Heritage.
The small, almost square notebook is bound in red leather with a brass clasp and labelled in Darwin's handwriting "Galapagos. Otaheite. Lima".
Darwin's great-great grandson, the author Randal Keynes, said: "Our family always felt that the best Darwin material should be at Down House so that the public could see it in his home.
Darwin's detailed notes will now be available online
"The Galapagos notebook is of outstanding value for the history of science. If Darwin had not posed the questions in that notebook, he might never have written On the Origin of Species."
English Heritage's chief executive, Simon Thurley, said: "There's a desperately sad gap on the Down House bookshelves and it's one that we hope will be filled."
The missing book contains entries from 1835 when Darwin was in Chile, Peru, the Galapagos and Tahiti.
The then 26-year-old noted: "met an immense Turpin; took little notice of me."
He was later told it was possible to tell from which of the Galapagos islands a tortoise came by the variation of its shell.