He is supposed to know what to do after consulting some deep well of ancient instinct.

And yet, an instinct more ancient even than he made him hold his ground.

These creatures have an ancient instinct to return wherever they came from.

The ancient instinct to make things better for her had leapt to life, forced itself to the front of his mind.

It must be something to do with ancient territorial instincts.

But put them all together, in one place, and ancient instinct makes you uneasy.

As if something blind were stalking him with ancient instincts.

Some ancient instinct told him that eventually they were sure to approach their dead.

Not with ancient instincts still screaming at them to fight on.

Shelter building and group living were obviously an ancient instinct.