Both Uses of

spontaneous
in
Moby Dick

The latter was the eternal, living principle or soul in him; and in sleep, being for the time dissociated from the characterizing mind, which at other times employed it for its outer vehicle or agent, it spontaneously sought escape from the scorching contiguity of the frantic thing, of which, for the time, it was no longer an integral.†
Chpt 43-45 *

spontaneously = happening in a natural manner without planning or external force

You might almost say, that this strange uncompromisedness in him involved a sort of unintelligence; for in his numerous trades, he did not seem to work so much by reason or by instinct, or simply because he had been tutored to it, or by any intermixture of all these, even or uneven; but merely by a kind of deaf and dumb, spontaneous literal process.†
Chpt 106-108

spontaneous = behaving in an instinctive, uninhibited manner OR happening naturally (without planning or external force)

Definition:

happening naturally, suddenly, or without outside planning or action