This Thing of Darkness

The year is 1828. Brilliant young naval officer Robert Fitzroy is given the captaincy of HMS Beagle, surveying the wilds of Tierra del Fuego, aged just twenty-three. But Fitzroy hides a dark secret: hereditary manic depression that can strike at any time. He is seized by two ambitions -- that he can prove, contrary to the spirit of the age, that black and white men are equal; and that he can prove the truth of the Book of Genesis. To this end, he takes a passenger: a young trainee cleric and amateur geologist named Charles Darwin. This is the story of a deep friendship between two men, and the twin obsessions that tore it apart, leading one to triumph and the other to disaster...

This Thing of Darkness is not just an epic historical novel. By turns gripping, funny, satirical and heartbreaking, it is also a novel about race, religion, science and colonialism that sheds many a light on the state of our world today. It is also one of history's great untold true stories, a tale of men under sail who were prepared to risk their very lives to get at the truth.

Synopsis from Open Library.