His philosophy had taught him one thing above all others, that he was a survival of the fittest--only so long as he survived.

And he was always guarding against the end.

His brain was keen, his ears quick, and every fibre in him trained to its duty of watchfulness.

And he knew, without turning his head, that someone was standing in the doorway behind him.

There had come a faint noise, a shadowing of the fading sun-glow on the wall, the electrical disturbance of another presence, gazing at him quietly, without motion, and without sound.