Both Uses

spontaneous
in
The History of Love
(Auto-generated)

Inside, a very old man with white hair was standing next to a filing cabinet under a poster that said: HENCE WITHOUT PARENTS, BY SPONTANEOUS BIRTH, RISE HE FIRST SPECKS OF ANIMATED EARTH—ERASMUS DARWIN.†

spontaneous = behaving in an instinctive, uninhibited manner OR happening naturally (without planning or external force)
The fifth time they met, under the spell of Rosa's youthful spontaneity—halfway through a heated discussion...—Litvinoff surprised himself by proposing they go hear a concert together.
*

spontaneity = behaving in an instinctive, uninhibited manner

Definitions:

(1) (spontaneous)
happening naturally, suddenly, or without outside planning or action
(2) (meaning too rare to warrant focus)