But Darwin had a happy marriage, money, success. The action-packed life of the secretly manic-depressive captain who invited him onto The Beagle ended in suicide.

It is a brave, sad tale. Socially, practically, Robert Fitzroy was far ahead of his time. He championed the rights of people whom Victorian gentlemen and rapacious white settlers in New Zealand called savages, believing they should be treated equally. He brought Fuegian natives to England and returned them home at his own expense. He promoted meteorology and weather bulletins, which saved many lives but were halted in England by vested interests. Owners lost money not when fishermen died, but when boats did not go out because of forecasts.

Fitzroy constantly fought for ideals and lost. The Fuegians were damaged by coming to England. His young sailors died. And despite his scientific interests, he passionately defended biblical truth. Mutation of species, as they called it then, threatened belief in the literal truth of Genesis.

This became a dangerous issue on The Beagle. It later divided Darwin and Fitzroy. And this conflict drives Harry Thompson’s fictionalised picture of Fitzroy’s life. This Thing of Darkness does not work as a novel. Thompson has done fantastic research but cannot step back from it; the sentences are not alert enough. But as fictionalised biography, this is a fascinating read. Behind the rich textures, all those sailors, storms and places — Patagonia, Tierra del Fuego, Galapagos — is a grim battle about the origin of life.

Scientifically and intellectually, secure believers today are fine with evolution. It does not threaten sincere belief in God. But the mentally or intellectually insecure hate it. So a conflict which divided two brilliant young men in the 19th century still has significance in the 21st, and not just in America’s southern states. All marine life in the Galapagos is now threatened by the islands’ burgeoning population, lured from mainland Ecuador by fishing. Many, oddly enough, are evangelical fundamentalist Christians. They hate the islands’ scientific station, the Darwin Centre, not only because it wants to conserve what they want to fish, but also, quite simply, because of its name. Even in the very islands which provoked the theory of evolution Darwin inspires the same nervous fury that bit Fitzroy’s emotionally fragile mind and which ended only with the cutting edge of his morning razor.

Ruth Padel is chair of the Poetry Society, and a judge for the 2005 Science Aventis Prize. She is Charles Darwin's great-great-granddaughter. Her travel memoir Tigers in Red Weather is published by Little, Brown

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