ution
in which the grasping power was gained and adaptation to arboreal life
completed. Whatever their relationship, they both possess the opposable
thumb as the hall-mark of their arboreal habitat, and whenever found
walking on the ground they may be looked upon as estrays from their
native place of residence.

Once the grasping power was gained, the first step of change from the
quadrupedal to the semi-erect attitude was completed. The process may
have begun in the effort to fit the sole of the foot to the rounded
surface of boughs; or its first stage may have been in the seizing of
overhead branches with the flexible hand; or both influences may have
acted simultaneously. We see the result only, we cannot trace the exact
process; but we have as an outcome the adoption of a method of
locomotion different from that of all other tree-dwellers, the forefoot
developing into the hand with its opposable thumb, and the hindfoot
gaining a similar grasping power in the toes.

The power of walking on a lower limb and grasping an upper one once
attained, a succeeding step in evolution quickly appeared, and one of
prime importance to our inquiry. The animal had ceased to be in a full
sense a quadruped, while not yet a biped, and a variation in the length
of its limbs was almost sure to take place. This is an ordinary result
when animals cease to walk on all fours. In the leaping kangaroo and
jerboa a shortening of the arms and lengthening of the legs appear. Here
the arms are relieved from duty and a double duty is laid on the legs,
with the consequence stated. In the ancient dinosaurian reptiles,
upright walkers, the same was the case. Those varied from quite small to
very large animals, but in all known instances the fore limbs were
greatly reduced in size. A similar condition may be seen in the birds,
the bones of whose fore-limbs have largely aborted from lack of
employment as walking organs.

In the case of the apes and lemurs, while a similar effect has taken
place, an interesting difference appears, due to the difference in
conditions. In these animals the fore limbs are not freed from duty as
organs of locomotion. In many cases, on the contrary, they have an extra
duty put upon them, with the result that they have grown longer instead
of shorter. Very likely these animals differed considerably in the past,
as they do to-day, in the degree of use of their legs and arms. Many of
them walk in the quadruped manner, either