the hypothesis I shall propose, for there is, of course, no direct
final proof of the truth of even the doctrine of evolution.
It is idle to consider any experimental research into the cause
of phenomena that have developed by natural selection during
millions of years. Nature herself has made the experiments on
a world-wide scale and the data are before us for interpretation.
Darwin could do no more than to collect all available facts and then
to frame the hypothesis by which the facts were best harmonized.
Sherrington, that masterly physiologist, in his volume entitled
"The Integrative Action of the Nervous System," shows clearly how
the central nervous system was built up in the process of evolution.
Sherrington has made free use of Darwin's doctrine in explaining
physiologic functions, just as anatomists have extensively
utilized it in the explanation of the genesis of anatomic forms.
I shall assume, therefore, that the discharge of nervous energy is
accomplished by the application of the laws of inheritance and association,
and I conclude that this hypothesis will explain many clinical phenomena.
I shall now present such evidence in favor of this hypothesis as time
and my limitations will admit, after which I shall point out certain
clinical facts that may be explained by this hypothesis.

According to the doctrine of evolution, every function owes
its origin to natural selection in the struggle for existence.
In the lower and simpler forms of animal life, indeed, in our
human progenitors as well, existence depended principally upon
the success with which three great purposes were achieved:
(1) Self-defense against or escape from enemies; (2) the acquisition
of food; and (3) procreation; and these were virtually the only purposes
for which nervous energy was discharged. In its last analysis,
in a biologic sense, this statement holds true of man today.