ction 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
man's origin, we can now see more clearly than ever how great a
revolution was inaugurated when natural selection began to confine its
operations to the surface of the cerebrum. Among the older incidents in
the evolution of organic life, the changes were very wonderful which out
of the pectoral fin of a fish developed the jointed fore-limb of the
mammal with its five-toed paw, and thence through much slighter
variation brought forth the human arm with its delicate and crafty hand.
More wondrous still were the phases of change through which the
rudimentary pigment-spot of the worm, by the development and
differentiation of successive layers, gave place to the
variously-constructed eyes of insects, mollusks, and vertebrates. The
day for creative work of this sort has probably gone by, as the day for
the evolution of annulose segments and vertebrate skeletons has gone
by,--on our planet, at least. In the line of our own development, all
work of this kind stopped long ago, to be replaced by different methods