I think that Darwin would himself acknowledge that "fittest" is a more accurate rendition than "strongest," but whether the quote can be rendered in this way without breaking the ten words constraint comes down to a question of whether "unfittest" counts as a legit word.
I think "fit" has become a free-floating standard rather than meaning "fitting into a particular environment".
Another month has passed and here is a new rationality quotes thread. The usual rules are: