In evolutionary terms, “the survival of the fittest” refers not to physical fitness in the sense of vigor and strength, but to the ability to reproduce successfully. Rabbits and ants are fitter to survive than lions: that’s why there are so many more of them. If you use the phrase “survival of the fittest” as if it referred to a contest of brute strength, you will annoy biologists and some editors, who will judge your usage as unfit to survive.
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