He had the idea of the century. His plan for publishing it was to die first.

On the fifth of July 1844, in a village in Kent, a man of thirty-five finished a fair copy of a long essay and wrote a letter to his wife. It was written for one purpose: to tell her what to do with the essay if he died suddenly.

She was to spend four hundred pounds having it published. The letter named the men who might edit it, in order of preference, and told her which brown-paper portfolios held his notes.

The man was Charles Darwin, and the essay was his theory of how species come about. He did not die. He put it away and spent fourteen more years dissecting barnacles, breeding pigeons, sitting with fanciers in a Southwark gin palace and rotting seeds in brine in his cellar - building the case for something no one would ever see happen. Then a packet arrived from the Moluccas, sent by a collector he had never met, with the same idea in it.

He waited twenty-one years in all. Fear was part of it. The rest was that nobody would ever watch the thing happen, so before he said a word in public he meant to have the evidence to carry it.