Definitions: (1) adapted for a specific purpose; (2) in good, sound physical and mental condition; healthy; trained; qualified; (3) ready; prepared <fit for duty>

Idiom: In fine fettle.

• The beginning of love is to let those we love be perfectly themselves, and not to twist them to fit our own image. Otherwise we love only the reflection of ourselves we find in them. — Thomas Merton (1915–1968) American Trappist monk
• There are no extra pieces in the universe. Everyone is here because he or she has a place to fill, and every piece must fit itself into the big jigsaw puzzle. — Deepak Chopra (1946-) Indian-American author

Comment: Survival of the fittest only works for those species (or more likely subspecies) who are on the cusp of change. The Permian-Triassic extinction killed off about 83% of insects, 81% of all marine species, & 70% of terrestrial vertebrate species.