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.




VII.

Change in the Direction of the Working of Natural Selection.


In the fresh light which these considerations throw upon the problem of