In letters to his closest friend, the botanist Joseph Hooker, he pours out his grief over the death of his daughter-in-law, Amy. He also speaks of his ideas on evolution for the first time - something he writes was like "confessing to a murder".

Of the many letters that Darwin wrote and received in his life, among the most important were his correspondence with his friend of 40 years, Joseph Hooker. As well as tracking the development of Darwin's scientific ideas, the letters give an intimate insight into a Victorian friendship.

Almost the entire collection - more than 1,400 letters - has been published by Cambridge University's Darwin Correspondence Project.