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.
As an optical instrument, the eye had well-nigh reached extreme
perfection in many a bird and mammal ages before man's beginnings; and
the essential features of the human hand existed already in the hands of
Miocene apes. But different methods came in when human intelligence
appeared upon the scene. Mr. Spencer has somewhere reminded us that the
crowbar is but an extra lever added to the levers of which the arm is
already composed, and the telescope but adds a new set of lenses to
those which already exist in the eye. This beautiful illustration goes
to the kernel of the change that was wrought when natural selection
began to confine itself to the psychical modification of our ancestors.
In a very deep sense all human science is but the increment of the power
of the eye, and all human art is the increment of the power of the
hand.[8] Vision and manipulation,--these, in their countless indirect
and transfigured forms, are the two coöperating factors in all
intellectual progress. It is not merely that with the telescope we see