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 vertebræ. 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 Asia in the
Miocene or early Pliocene were such as to compel the descent of the
pre-human ancestor from the trees, a step which was absolutely essential
to further human development." Continental elevation and consequent
aridity led to a dwindling of the forests, and forced the ape-man to
come to earth. "And at the last arose the man."
According to Lull, the descent from the trees was associated with the
assumption of a more erect posture, with increased liberation and
plasticity of the hand, with becoming a hunter, with experiments towards
clothing and shelter, with an exploring habit, and with the beginning of
communal life.
It is a plausible view that the transition from the humanoid to the
human was effected by a discontinuous variation of considerable
magnitude, what is nowadays called a _mutation_, and that it had mainly
to do with the brain and the vocal organs. But given the gains of the
arboreal apprenticeship, the stimulus of an enforced descent to terra
firma, and an evolving brain and voice, we can recognise accessory
factors which helped success to succeed. Perhaps the absence of great
physical strength prompted reliance on wits; the prolongation of infancy
would help to educate the parents in gentleness; the strengthening of
the feeling of kinship would favour the evolution of family and social
life--of which there are many anticipations at lower levels. There is
much truth in the saying: "Man did not make society, society made man."
A continuation of the story will deal with the emergence of the
primitive types of man and the gradual ascent of the modern species.
§ 4