en movement, though probably stimuli from the interior of the
body first arouse these responses. They are most apt to occur during
the organic state of "euphoria", and tend to disappear during fatigue.

There is a counter-tendency to this tendency towards general activity,
and that is _inertia_, the tendency towards inactivity or _economy of
effort_. Most pronounced in fatigue, this also appears in lassitude
and inert states that cannot be called fatigue because not brought on
by excessive activity. After sleep, many people are inert, and require
a certain amount of activity to "warm up" to the active condition. As
the child grows older, the {152} "economy of effort" motive becomes
stronger, and the random activity motive weaker, so that the adult is
less playful and less responsive to slight stimuli. He has to have
some definite goal to get up his energy, whereas the child is active
by preference and just for the sake of activity.

During the first year or so of the child's life, his playful activity
takes shape in several ways. First, out of the great variety of the
random movements certain ones are picked out and fixed. This is the
way with putting the hand into the mouth or drumming on the floor with
the heels, and these instances illustrate the important fact that many
learned acts develop out of the child's random activity. Without play
activity there would be little work or accomplishment of the
distinctively human type. Second, certain specific movements, those of
locomotion and vocalization, appear with the ripening of the child's
native equipment, and take an important place in his play. Third, his
play comes to consist more and more of responses to external objects,
instead of to internal stimuli as at first. The playful responses to
external objects fall into two classes, according as they manipulate
objects or simply examine them.

We have, then, a small group of instincts that is very closely related
to the fundamental instinct of random activity.

Locomotion.

Evidence has already been presented [Footnote: See p. 95.] indicating
that walking is instinctive and not learned, so that the human species
is no exception to the rule that every species has its instinctive
mode of locomotion. Simpler performances which enter into the very
complex movement of walking make their appearance separately in the
infant before being combined into walking proper. Holding up the head,
sitting up, kicking with an alte