The Simpsons
S2E4: Two Cars in Every Garage and Three Eyes on Every Fish
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Glad to, Mr. Burns. You see, every so often,
Mother Nature changes her animals, giving them bigger teeth,
sharper claws, longer legs...
or, in this case, a third eye.
And if these variations turn out to be an improvement,
the new animals thrive and multiply...
and spread across the face of the earth.