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".