with a bit of pointed flint.  Yet during an entire geologic aeon 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.

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