very long duration of his infancy. These two things have gone hand in
hand. The increase of cerebral surface, due to the working of natural
selection in this direction alone, has entailed a vast increase in the
amount of cerebral organization that must be left to be completed after
birth, and thus has prolonged the period of infancy. And conversely the
prolonging of the plastic period of infancy, entailing a vast increase
in teachableness and versatility, has contributed to the further
enlargement of the cerebral surface. The mutual reaction of these two
groups of facts must have gone on for an enormous length of time since
man began thus diverging from his simian brethren. It is not likely that
less than a million years have elapsed since the first page of this new
chapter in the history of creation was opened: it is probable that the
time has been much longer. In comparison with such a period, the whole
recorded duration of human history shrinks into nothingness. The
pyramids of Egypt seem like things of yesterday when we think of the
Cave-Men of western Europe in the glacial period, who scratched pictures
of mammoths on pieces of reindeer-antler with a bit of pointed flint.
Yet during an entire geologic æon before these Cave-Men appeared on the
scene, "a being erect upon two legs," if we may quote from Serjeant
Buzfuz, "and wearing the outward semblance of a man and not of a
monster," wandered hither and thither over the face of the earth,
setting his mark upon it as no other creature yet had done, leaving
behind him innumerable tell-tale remnants of his fierce and squalid
existence, yet too scantily endowed with wit to make any written
disclosure of his thoughts and deeds. If the physiological annals of
that long and weary time could now be unrolled before us, the principal
fact which we should discern, dominating all other facts in interest and
significance, would be that mutual reaction between increase of cerebral
surface and lengthening of babyhood which I have here described.