Science on short notice

I got into a postition in my class I'm teaching the other day where, due to an interesting question raised by a student, I was, immediatly and with warning, called upon to explain Darwin's theory of evolution.

"OK," I thought silently. "Evolution." As often when I panic, a diagram seemed like the solution. The face of my highschool bio teacher swam before my vision. Students in my class sat unmoving, pens poised over their notebooks to take down the essential explanation of Darwin's theory.

I steeled myself. I drew two turtles on the blackboard. One had a normal shell, the other had a big curling spike on top of its shell.

I told how the turtle with the spike was a mutant. But--lo!--the turtles lived on an island where a giant bird was in the habit of attacking turtles by landing on their shells and then commencing an attack (of some sort, attack details not important, class). And the mutant turtle's spike foiled the attack bird! Thus, the mutant turtle would have many small turtle babies, while the "normal" turtle had fewer. Evolution.