In a flash Howland saw a huge form leap from the gloom and caught the gleam of an uplifted knife.

There was no time for him to leap aside, no time for him to reach for the revolver which he carried in his pocket.

In such a crisis one's actions are involuntary, machine-like, as if life, hovering by a thread, preserves itself in its own manner and without thought or reasoning on the part of the creature it animates.
For an instant Howland neither thought nor reasoned.

Had he done so he would probably have met his mysterious assailant, pitting his naked fists against the knife.

But the very mainspring of his existence--which is self-preservation--called on him to do otherwise.