skill may be acquired--those animals in
which the preparatory play-propensity was not inherited in due force
and requisite amount being subsequently eliminated in the struggle for
existence. In any case there is little question that Prof. Groos is
right in basing the play-propensity on instinctive foundations.[183]
None the less, as he contends, the essential biological value of play
is that it is a means of training the educable nerve-tissue, of
developing that part of the brain which is modified by experience and
which thus acquires new characters, of elaborating the secondary
tissue of experience on the predetermined lines of instinctive
differentiation and thus furthering the psychological activities which
are included under the comprehensive term "intelligent."

In _The Descent of Man_ Darwin dealt at some length with intelligence
and the higher mental faculties.[184] His object, he says, is to show
that there is no fundamental difference between man and the higher
mammals in their mental faculties; that these faculties are variable
and the variations tend to be inherited; and that under natural
selection beneficial variations of all kinds will have been preserved
and injurious ones eliminated.

Darwin was too good an observer and too honest a man to minimise the
"enormous difference" between the level of mental attainment of
civilised man and that reached by any animal. His contention was that
the difference, great as it is, is one of degree and not of kind. He
realised that, in the development of the mental faculties of man, new
factors in evolution have supervened--factors which play but a
subordinate and subsidiary part in animal intelligence.
Intercommunication by means of language, approbation and blame, and
all that arises out of reflective thought, are but foreshadowed in the
mental life of animals. Still he contends that these may be explained
on the doctrine of evolution. He urges[185] "that man is variable in
body and mind; and that the variations are induced, either directly or
indirectly, by the same general causes, and obey the same general
laws, as with the lower animals." He correlates mental development
with the evolution of the brain.[186] "As the various mental faculties
gradually developed themselves, the brain 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 gorill