MANY INSTINCTS ARE TRANSITORY.--Not only do instincts ripen by degrees,
entering our experience one by one as they are needed, but they drop out
when their work is done. Some, like the instinct of self-preservation,
are needed our lifetime through, hence they remain to the end. Others,
like the play instinct, serve their purpose and disappear or are
modified into new forms in a few years, or a few months. The life of the
instinct is always as transitory as is the necessity for the activity
to which it gives rise. No instinct remains wholly unaltered in man, for
it is constantly being made over in the light of each new experience.
The instinct of self-preservation is modified by knowledge and
experience, so that the defense of the man against threatened danger
would be very different from that of the child; yet the instinct to
protect oneself in _some_ way remains. On the other hand, the instinct
to romp and play is less permanent. It may last into adult life, but few
middle-aged or old people care to race about as do children. Their
activities are occupied in other lines, and they require less physical
exertion.