These gibbons commonly go about in parties of six or seven; and while some individuals are black with white whiskers and a band on the sides and lower part of the face, in others the general colour is golden yellow with no white on the face. That the two phases are specifically identical is certain, and it does not appear that the light-coloured individuals eventually become dark, or _vice versa_. The natives, it is true, have an idea that the light-coloured individuals are the females, but this seems to be disproved by the fact that they are much less numerous than the black ones; and it thus appears probable that the species exhibits two distinct colour-phases. Gibbons are almost wholly arboreal apes; and in the trees they move about mainly with the aid of their long arms, by means of which they swing themselves from bough to bough, and thus from one tree to another, so that a whole troop will traverse the forest without descending to the ground. So active and agile indeed are these apes, that when confined in open cages they will catch birds on the wing, which apparently form part of their food. In addition to these, gibbons subsist on fruits of various kinds, leaves, young shoots, insects, and spiders. All gibbons have a remarkably powerful double-note cry, like _hu-lok_, which has a wailing sound, and awakes the echoes in the early morning and again at evening in the forests inhabited by these apes. At a distance the cry has some faint resemblance to the human voice; but at close quarters it is enough to drive the traveller, when resting in his tent, well-nigh distracted. THE ABYSSINIAN GREEN MONKEY (_Cercopithecus æthiops_) As a familiar example of a very large assemblage of exclusively African long-tailed monkeys collectively known as guenons (from a French word signifying to make grimaces), no better species could have been selected for illustration than the Abyssinian green monkey. This monkey, which like its cousin the common green monkey (_Cercopithecus callitrichus_, or _sabæus_) may be seen in almost all menageries and on many street-organs, is an East African species; whereas the true green monkey is West African, ranging from Senegambia through Sierra Leone to northern Liberia, but has been introduced into some of the Cape Verd and West Indian Islands, where it still occurs plentifully, at least in Barbados.