“The Descent of Man” 441 
would almost certainly become larger. No one, I presume, doubts 
that the large proportion which the size of man’s brain bears to his 
body, compared to the same proportion in the gorilla or orang, is 
closely connected with his higher mental powers.” “With respect to 
the lower animals,” he says}, “M. E. Lartet?, by comparing the crania 
of tertiary and recent mammals belonging to the same groups, has 
come to the remarkable conclusion that the brain is generally larger 
and the convolutions are more complex in the more recent form.” 
Sir E. Ray Lankester has sought to 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 trans- 
mitted, 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 ac- 
quired 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*” 
In this statement we have a good example of the further develop- 
ment 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. 
1 Descent of Man (Popular edit.), p. 82. 2 Comptes Rendus des Sciences, June 1, 1868. 
3 Nature, Vol. Lx. pp. 624, 625 (1900). 
