though toward evening, they descend in small troops to the open
ground, no sooner do they spy a man than they dart up the hillsides
and disappear in the darker valleys.

All observers testify to the prodigious volume of voice possessed by
these animals. According to the writer whom I have just cited, in one
of them, the Siamang, "the voice is grave and penetrating, resembling
the sounds gōek, gōek, gōek, gōek, goek ha ha ha ha haaāāā, and may be
easily heard at a distance of half a league." While the cry is being
uttered, the great membranous bag under the throat which communicates
with the organ of voice, the so-called "laryngeal sac," becomes
greatly distended, diminishing again when the creature relapses into
silence.

M. Duvaucel, likewise, affirms that the cry of the Siamang may be
heard for miles--making the woods ring again. So Mr. Martin describes
the cry of the agile Gibbon as "overpowering and deafening" in a room,
and "from its strength, well calculated for resounding through the
vast forests." Mr. Waterhouse, an accomplished musician as well as
zoölogist, says, "The Gibbon's voice is certainly much more powerful
than that of any singer I ever heard." And yet it is to be recollected
that this animal is not half the height of, and far less bulky in
proportion than, a man.

[Illustration: A GIBBON.]

There is good testimony that various species of Gibbon readily take to
the erect posture. Mr. George Bennett, a very excellent observer, in
describing the habits of a male _Hylobates syndactylus_ which remained
for some time in his possession, says: "He invariably walks in the