"On a purely biological level, we must kill to survive," he writes.

The male you could have escaped, for it exists only on the biological level.

On a biological level, yes, men's and women's skin differ.

All this has a bearing on the biological level.

The treatment uses monitors that "feed back" certain biological levels.

We're trying to figure out what's going on at a biological level.

On the biological level, one has to consider the role of hormones and neurotransmitters in sexual responses.

Women can't get each other pregnant; so men on a deep biological level aren't threatened.

The merging had even taken place on a biological level.

It will therefore reach a safe biological level, to use the term recently used by the Commissioner.