the thighs commonly drawn up to the body, so
that by a sudden extension of the knee-joint, the
animals spring with great vigour and rapidity.
Their progression is not by walking on the ground,
though, as we shall see, some of them can effect
this awkwardly, but by climbing and bounding
among the branches of trees, in the dense forests
where they delight to dwell. It is to fit them for
these arboreal habits, that their whole structure is
modified. The development of the facial portion
of the skull throws the centre of gravity considerably forward of the point of its junction with the
spinal column, and this requires that the spinous
processes of the bones of the neck should be
enlarged, for the attachment of the muscles of the
back of the head. The lower vertebrae of the
spine are not gradually enlarged, as in Man, and
therefore possess not the power of perpendicular
support which a pyramidal form supplies, while
the more narrowed form and weaker structure of
the pelvis, the short and powerless thigh-bones, set
at an obtuse angle with the line of the trunk, the
leg-bones which can be brought into the same line
as the thighs only by muscular effort, while their
sustaining power is weakened by their capability
of rotatory motion, the weakness (arising from
the mobility) of the ankle, and the lack of the pedal arch, all manifest that the natural posture of even the most man-like of the Apes is considerably removed from the perpendicular. At the
same time these peculiarities of structure are most
beautifully adapted to the diagonal attitude, and
climbing habits which we have alluded to as proper to the animals of this Order.
But it is in the character of the extremities that
