survival of the fittest
survival of the fittest {n. phr.}
The staying alive or in action of the best prepared; often: idea that those living things best able to adjust to life survive and those unable to adjust die out.
Life in the old West was often a case of survival of the fittest.
With changes in the world's climate, dinosaurs died but many smaller animals lived on. It was survival of the fittest.
On the 50-mile hike it was survival of the fittest; only 12 out of 25 Scouts finished.
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Source: A Dictionary of American Idioms
