Self-preservation is the first law of nature

Self-preservation is the first law of nature.

Prov. Every living thing will fight to survive.; It is natural to think of yourself first. When foe's best friend was arrested, Joe pretended not to know him. "Perhaps it wasn't very loyal of me," he thought, "but self-preservation is the first law of nature."
See also: first, law, nature, of
References in classic literature ?
Although Oliver had been brought up by philosophers, he was not theoretically acquainted with the beautiful axiom that self-preservation is the first law of nature.