The Writings of Charles Darwin on the Web is an amazing resource which allows you to search the texts–including, at an offshoot site, some of his correspondence! When I was writing my honours dissertation I had to stop myself reading the letters obsessively, seeing if I could glean some overlooked insight about language buried in amongst the natural history anecdotes and Darwin’s bleating about his poor health.
This letter (reference only) to his sister Caroline was a great find. In it, Charles used the hypothetical age of the Indo-European languages, and their differences from Chinese to agree with Herschel that yes, the earth must indeed be older than 6000 years as argued by the Bible.
I also love this one (below) that CD wrote to Thomas Henry Huxley. I confess to being a bit of a Huxley fan myself, but their mutual admiration society makes me smile…