The overcrowding
of the teeth that followed the shortening of the snout was one of the
taxes on progress of which modern man is often reminded in his dental
troubles.

Another acquisition associated with arboreal life was a greatly
increased power of turning the head from side to side--a mobility very
important in locating sounds and in exploring with the eyes.
Furthermore, there came about a flattening of the chest and of the back,
and the movements of the midriff (or diaphragm) came to count for more
in respiration than the movements of the ribs. The sense of touch came
to be of more importance and the sense of smell of less; the part of the
brain receiving tidings from hand and eye and ear came to predominate
over the part for receiving olfactory messages. Finally, the need for
carrying the infant about among the branches must surely have implied an
intensification of family relations, and favoured the evolution of
gentleness.

[Illustration: _Photo: New York Zoological Park._

THE GIBBON IS LOWER THAN THE OTHER APES AS REGARDS ITS SKULL AND
DENTITION, BUT IT IS HIGHLY SPECIALIZED IN THE ADAPTATION OF ITS LIMBS
TO ARBOREAL LIFE]

[Illustration: _Photo: New York Zoological Park._

THE ORANG HAS A HIGH ROUNDED SKULL AND A LONG FACE]

[Illustration: _Photo: British Museum (Natural History)._

COMPARISONS OF THE SKELETONS OF HORSE AND MAN

Bone for bone, the two skeletons are like one another, though man is a
biped and the horse a quadruped. The backbone in man is mainly vertical;
the backbone in the horse is horizontal except in the neck and the tail.
Man's skull is mainly in a line with the backbone; the horse's at an
angle to it. Both man and horse have seven neck vertebrae. Man has five
digits on each limb; the horse has only one digit well developed on each
limb.]

It may be urged that we are attaching too much importance to the
arboreal apprenticeship, since many tree-loving animals remain to-day
very innocent creatures. To this reasonable objection there are two
answers, first that in its many acquisitions the arboreal evolution of
the _humanoid_ precursors of man prepared the way for the survival of a
_human_ type marked by a great step in brain-development; and second
that the passage from the humanoid to the human was probably associated
with _a return to mother earth_.

According to Professor Lull, to whose fine textbook, _Organic Evolution_
(1917), we are much indebted, "climatic conditions in A