o express in the simplest terms the
implications of the increase in size of the cerebrum. "In what," he
asks, "does the advantage of a larger cerebral mass consist?" "Man,"
he replies "is born with fewer ready-made tricks of the
nerve-centres--these performances of an inherited nervous mechanism so
often called by the ill-defined term 'instincts'--than are the monkeys
or any other animal. Correlated with the absence of inherited ready-made
mechanism, man has a greater capacity of developing in the course of
his individual growth similar nervous mechanisms (similar to but not
identical with those of 'instinct') than any other animal... The power of
being educated--'educability' as we may term it--is what man possesses
in excess as compared with the apes. I think we are justified in forming
the hypothesis that it is this 'educability' which is the correlative of
the increased size of the cerebrum." There has been natural selection
of the more educable animals, for "the character which we describe as
'educability' can be transmitted, it is a congenital character. But the
RESULTS of education can NOT be transmitted. In each generation they
have to be acquired afresh, and with increased 'educability' they are
more readily acquired and a larger variety of them... The fact is
that there is no community between the mechanisms of instinct and the
mechanisms of intelligence, and that the latter are later in the history
of the evolution of the brain than the former and can only develop in
proportion as the former become feeble and defective." ("Nature", Vol.
LXI. pages 624, 625 (1900).)

In this statement we have a good example of the further development
of views which Darwin foreshadowed but did not thoroughly work out. It
states the biological case clearly and tersely. Plasticity of behaviour
in special accommodation to special circumstances is of survival value;
it depends upon acquired characters; it is correlated with increase in
size and complexity of the cerebrum; under natural selection therefore
the larger and more complex cerebrum as the organ of plastic behaviour
has been the outcome of natural selection. We have thus the biological
foundations for a further development of genetic psychology.

There are diversities of opinion, as Darwin showed, with regard to the
range of instinct in man and the higher animals as contrasted with lower
types. Darwin himself said ("Descent of Man", Vol. I. page 100.) that
"Man, perhaps,