of
biological evolution and are themselves the outcome of that evolution.

Darwin makes only passing reference to those modes of behaviour in
animals which go by the name of play. "Nothing," he says ("Descent of
Man", Vol. II. page 60; (Popular edition), page 566.), "is more common
than for animals to take pleasure in practising whatever instinct they
follow at other times for some real good." This is one of the very
numerous cases in which a hint of the master has served to stimulate
research in his disciples. It was left to Prof. Groos to develop this
subject on evolutionary lines and to elaborate in a masterly manner
Darwin's suggestion. "The utility of play," he says ("The Play of
Animals", page 76.), "is incalculable. This utility consists in the
practice and exercise it affords for some of the more important duties
of life,"--that is to say, for the performance of activities which will
in adult life be essential to survival. He urges (Ibid. page 75.) that
"the play of young animals has its origin in the fact that certain very
important instincts appear at a time when the animal does not seriously
need them." It is, however, questionable whether any instincts appear
at a time when they are not needed. And it is questionable whether
the instinctive and emotional attitude of the play-fight, to take
one example, can be identified with those which accompany fighting in
earnest, though no doubt they are closely related and have some common
factors. It is probable that play, as preparatory behaviour, differs in
biological detail (as it almost certainly does in emotional attributes)
from the earnest of after-life and that it has been evolved through
differentiation and integration of the primary tissue of experience,
as a preparation through which certain essential modes of 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. ("The Play of Animals" page 24.) 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 lin