subject to restrictions which continually recall the material
background on which it rests.

In the evolution of the nervous system of man as presented
in this brief outline we see a succession of steps rather than a
gradual ascent and at each step a fundamental change suddenly
appears, mutation-like in its character, to borrow a term from
the geneticists. Each step is a new phase in organization, but
not necessarily a change in the kind of elementary materials
involved, and with these changes in organization come changes
in the degrees of freedom of reaction which enable us to bridge
over the gulf that lies between the relatively circumscribed
activity in ordinary chemical operations and the greater free-
dom seen in the voluntary and responsible acts of human
beings. Something of this view of the nature and the possi-
bilities of living protoplasm has been put forward by Haldane
under the name of organicism, but with perhaps less reliance
on the material side of the problem than has been suggested
in this lecture. Yet interesting and important as it is to push,
to the extreme, speculation as to the relation of our mental life
to the materials of our body, it nevertheless must be remem-
bered that, with all our progress, we are still not far from the
position described by Vesalius in 1543 when he wrote, "How
the brain performs its functions in imagination, in reasoning,
in thinking, and in memory, I can form no opinion whatever."

-102-