Edward S. Ellis, Alden the Pony Express Rider (1909)
“ Shagbark had his silent moods, but not so often when the two were together. A peculiar result of nature and training shown by the veteran amused Alden. The guide never lost his habit of eternal alertness. No matter how deeply interested the two were in what was being said, Shagbark kept glancing ahead, on each side, and frequently behind him. Even when sitting on the ground or eating with the others this bird-like flitting of his eyes was kept up. It seemed impossible for such a man to be caught off his guard. ”
