Richard Lydekker, 1911 Encyclopædia Britannica, Volume 11… (1911)
“ When they descend to the ground—which they must often do in order to obtain water—they frequently walk in the upright posture, either with the hands crossed behind the neck, or with the knuckles resting on the ground. Their usual food consists of leaves and fruits. Gibbons may be divided into two groups, the one represented by the siamang, Hylobates (Symphalangus) syndactylus, of Sumatra and the Malay Peninsula, and the other by a number of closely allied species. ”
