It could just be the unfamiliar heating system, or a door caught in a draught. At this stage he felt quite calm. The house had formidable security, which was one of the reasons he had bought it a few weeks earlier: apart from the floodlights, there was a three-metre-high perimeter wall with heavy electronic gates, a steel-reinforced front door with a keypad entry system, bulletproof glass in all the ground-floor windows, and a movement-sensitive burglar alarm, which he was sure he had turned on before he came up to bed. The chances that an intruder had got past all that and penetrated inside were tiny. Besides, he was physically fit: he had long ago established that high levels of endorphins enabled him to think better. He worked out. He jogged. An atavistic instinct to protect his territory stirred within him.

He slid out of bed without waking Gabrielle and put on his glasses, robe and slippers. He hesitated, and peered around in the darkness, but he could not recall anything in the room that might be useful as a weapon. He slipped his mobile into his pocket and opened the bedroom door – a crack at first, and then fully. The light from the lamp downstairs shed a dim glow along the landing. He paused on the threshold, listening. But the sounds – if there had ever been sounds, which he was beginning to doubt – had ceased. After a minute or so he moved towards the staircase and began to descend very slowly.

Perhaps it was the effect of reading Darwin just before he fell asleep, but as he went down the stairs he found himself registering, with scientific detachment, his own physical symptoms. His breath was becoming short, his heartbeat accelerating so rapidly it was uncomfortable. His hair felt as stiff as fur.

He reached the ground floor.

The house was a belle epoque mansion, built in 1902 for a French businessman who had made a fortune out of extracting oil from coal waste. The whole place had been excessively interior-designed by the previous owner, left ready to move into, and perhaps for that reason Hoffmann had never felt entirely at home in it.