THE TROPICAL FOREST 99 
the knee turns outwards makes it even less fitted for 
the upright position than its allies, but the resultant 
inward position of the sole of the foot makes it even 
more skilful in climbing. The animals are indeed 
excellent climbers, but are somewhat deliberate in their 
movements. They do not appear to come voluntarily 
to the ground, and are fitted for dense primaeval forests. ° 
The gibbons are very much more active in their move- 
ments; so active, indeed, as to be able to catch birds on 
the wing. They mingle with their vegetable food a 
considerable amount of animal matter, especially eggs 
and nestling birds. With the other egg-eating animals 
of the tropical forests they have probably been one 
factor in determining the migration of so many birds 
every year to the northern forest, where they can rear 
their young in comparative security. 
The gibbons have even longer arms than the orang, 
but differ in that they can plant the foot flat upon the 
ground, and walk, though not rapidly, in the upright 
position. 
Of other forms we may note that the langurs, already 
mentioned (p. 77), are long-tailed, leaf-eating monkeys, 
widely distributed over South-East Asia. Though 
the arms are relatively short, the animals are exceed- 
ingly agile, making their way through the trees with 
great rapidity. In West Africa the langurs are re- 
placed by thumbless monkeys belonging to the genus 
Colobus, which are also leaf-eaters, and are remarkable 
for their long silky hair. 
West Africa, the region, it will be recollected, where 
the tropical forest is best developed, is also inhabited 
by the monkeys of the genus Cercopithecus, to which 
the monkey usually carried about by organ-grinders 
belongs. All have cheek-pouches, and the diet is very 
G2 
