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 vertebræ 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
