Lengthening of Infancy, and Concomitant Increase of Brain-Surface.
The first appearance of infancy in the animal world thus heralded the
new era which was to be crowned by the development of Man. With the
beginnings of infancy there came the first dawning of a conscious life
similar in nature to the conscious life of human beings, and there came,
moreover, on the part of parents, the beginning of feelings and actions
not purely self-regarding. But still more, the period of infancy was a
period of plasticity. The career of each individual being no longer
wholly predetermined by the careers of its ancestors, it began to become
teachable. Individuality of character also became possible at the same
time, and for the same reason. All birds and mammals which take care of
their young are teachable, though in very various degrees, and all in
like manner show individual peculiarities of disposition, though in most
cases these are slight and inconspicuous. In dogs, horses, and apes
there is marked teachableness, and there are also marked differences in
individual character.
But in the non-human animal world all these phenomena are but slightly
developed. They are but the dim adumbrations of what was by and by to
bloom forth in the human race. They can scarcely be said to have served
as a prophecy of the revolution that was to come. One generation of dumb
beasts is after all very like another, and from studying the careers of
the mastodon, the hipparion, the sabre-toothed lion, or even the
dryopithecus, an observer in the Miocene age could never have foreseen
the possibility of a creature endowed with such a boundless capacity of
progress as the modern Man. Nevertheless, however dimly suggestive was
this group of phenomena, it contained the germ of all that is preëminent
in humanity. In the direct line of our ancestry it only needed that the
period of infancy should be sufficiently prolonged, in order that a
creature should at length appear, endowed with the teachableness, the
individuality, and the capacity for progress which are the peculiar
prerogatives of fully-developed Man.[7] In this direct line the manlike
apes of Africa and the Indian Archipelago have advanced far beyond the
mammalian world in general. Along with a cerebral surface, and an
accompanying intelligence, far greater than that of other mammals, these
tailless apes begin life as helpless babies, and are unable to walk, to
feed themselves, or to grasp objects with precision until they are two
or three months old. These apes have thus advanced a little way upon the
peculiar road which our half-human forefathers began to travel as soon
as psychical variations came to be of more use to the species than
variations in bodily structure. The gulf by which the lowest known man
is separated from the highest known ape consists in the great increase
of his cerebral surface, with the accompanying intelligence, and in the
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.
Thus through the simple continuance and interaction of processes that
began far back in the world of warm-blooded animals, we get at last a
creature essentially different from all others. Through the complication
of effects the heaping up of minute differences in degree has ended in
bringing forth a difference in kind. In the human organism physical
variation has well-nigh stopped, or is confined to insignificant
features, save in the grey surface of the cerebrum. The work of cerebral
organization is chiefly completed after birth, as we see by contrasting
the smooth ape-like brain-surface of the new-born child with the
deeply-furrowed and myriad-seamed surface of the adult civilized brain.
The plastic period of adolescence, lengthened in civilized man until it
has come to cover more than one third of his lifetime, is thus the
guaranty of his boundless progressiveness. Inherited tendencies and
aptitudes still form the foundations of character; but individual
experience has come to count as an enormous factor in modifying the
career of mankind from generation to generation. It is not too much to
say that the difference between man and all other living creatures, in
respect of teachableness, progressiveness, and individuality of
character, surpasses all other differences of kind that are known to
exist in the universe.