or, what is more usual, he keeps his arms uplifted in nearly an erect position, with the hands pendent ready to seize a rope, and climb up on the approach of danger or on the obtrusion of strangers.  He walks rather quick in the erect posture, but with a waddling gait, and is soon run down if, while pursued, he has no opportunity of escaping by climbing....  When he walks in the erect posture, he turns the leg and foot outward, which occasions him to have a waddling gait and to seem bow-legged.”

Dr. Burrough states of another Gibbon, the Horlack or Hooluk: 

“They walk erect; and when placed on the floor, or in an open field, balance themselves very prettily by raising their hands over their head and slightly bending the arm at the wrist and elbow, and then run tolerably fast, rocking from side to side; and, if urged to greater speed, they let fall their hands to the ground, and assist themselves forward, rather jumping than running, still keeping the body, however, nearly erect.”

Somewhat different evidence, however, is given by Dr. Winslow Lewis: 

“Their only manner of walking was on their posterior or inferior extremities, the others being raised upward to preserve their equilibrium, as rope-dancers are assisted by long poles at fairs.  Their progression was not by placing one foot before the other, but by simultaneously using both, as in jumping.”  Dr. Salomon Mueller also states that the Gibbons progress upon the ground by short series of tottering jumps, effected only by the hind limbs, the body being held altogether upright.

But Mr. Martin, who also speaks from direct observation, says of the Gibbons generally: 

“Pre-eminently qualified for arboreal habits, and displaying among the branches amazing activity, the Gibbons are not so awkward or embarrassed on a level surface as might be imagined.  They walk erect with a waddling or unsteady gait, but at a quick pace, the equilibrium of the body requiring to be kept up, either by touching the ground with the knuckles, first on one side then on the other, or by uplifting the arms so as to poise it.  As with the Chimpanzee, the whole of the narrow, long sole of the foot is placed upon the ground at once, and raised at once, without any elasticity of step.”

After this mass of concurrent and independent testimony, it cannot reasonably be doubted that the Gibbons commonly and habitually assume the erect attitude.

But level ground is not the place where these animals can display their very remarkable and peculiar locomotive powers, and that prodigious activity which almost tempts one to rank them among flying, rather than among ordinary climbing mammals.

Mr. Martin has given so excellent and graphic an account of the movements of a Hylobates agilis, living in the Zooelogical Gardens, in 1840, that I will quote it in full: