"Survival of the Fittest"

Who came up with that expression, and why?

First of all, who defines what is "fit"?

Could a cockroach be considered fittest?

Then comes "survival". This typically pertains to food, water, shelter, temporal endurance, and fulfillment of emotional needs (sex).

Cockroaches have lived long before mankind took over the planet. They will continue to endure.
Cockroaches will live under a rock. They think that is shelter.
Cockroaches will lap up water from a cesspool.
Which reminds me, cockroaches eat crap.

Cockroaches are vermin. What makes them fit to survive when we humans tell each other to go crawl under a rock, eat crap and die, and... well, I think you get the point by now.

One other thought... If it's to be fit to eat crap, who'll start a fast food franchise; put it in a bun? Imagine the adverts on television... I won't, thanks anyway.