fit (adj.)youdaoicibaDictYouDict
"suited to the circumstances, proper," mid-15c., of unknown origin, perhaps from Middle English noun fit "an adversary of equal power" (mid-13c.), which is perhaps connected to fit (n.1). In athletics, "in condition, properly trained for action," from 1869. Related: Fitter; fittest. Survival of the fittest (1867) coined by H. Spencer.
survival (n.)youdaoicibaDictYouDict
1590s, "act of surviving; continuation after some event," from survive + -al (2). Phrase survival of the fittest (1864) was used by Spencer in place of Darwin's natural selection.