the apes gained their special power of grasping, that
characteristic to which they undoubtedly owe the partial freedom of
their hands and their tendency to assume the erect attitude.

The most distinguishing characteristic of the apes and of the nearly
related lemurs has not hitherto been definitely pointed out. This is
that they form the only group of strictly arboreal animals. The tree is
not alone their native habitat, but they are specially adapted to it in
their organs of motion, a fact which cannot be affirmed of any other
animal group. If we consider, for instance, the squirrels, one of the
best-known groups of tree-living animals, we find them to be members of
the great order of rodents, whose native habitat is the land surface.
Though the squirrels have taken to the trees, there has been no adaptive
change in the structure of their limbs and feet. The same may be said of
almost all tree-dwellers except the lemurs and apes. The sloth, indeed,
is specially adapted in organization to an arboreal residence, but this
change is individual, not tribal, this animal being an aberrant form of
the ground-dwelling edentata. In the apes and lemurs, on the contrary,
the ground-dwellers are the aberrant forms, stray wanderers from the
host. Nearly all the species live in trees, to which they are specially
adapted by the formation of their feet. It remains to inquire how this
deviation in structure arose, what were the steps of development of the
grasping foot and hand, the special characteristic of this group.

In considering this question, the first fact to appear is that the apes
and lemurs are plantigrade animals. Their natural tendency is to walk on
the sole of the foot, a habit which few other tribes of animals
possess. Most of the larger animals walk on the knuckles or the toes,
and develop claws or hoofs, but the ancestral form of the ape, ages in
the past, was doubtless a sole-walking quadruped, its toes apparently
provided with nails instead of claws. What the story of this very
ancient quadruped was we are quite unable to say. It may, in the
exigencies of existence, have come to a parting of the ways; a section
of the group, drawn by a love of fruit, developing the climbing habit;
the remaining section continuing on the ground and following a separate
line of evolution. Perhaps only a single species took to the trees; for
it is quite possible for a single form, in a new and advantageous
habitat, to vary in time into