Herbert Spencer did not misappropriate the phrase he invented "survival of the fittest" for his own works on the parallels between evolution and economics in 1864, because he did not think natural selection sounded aggressive enough.
Darwin never used the term prior to this, though Darwin did quote it in "The Variation of Animals and Plants Under Domestication" published in 1868 describing its use as being "as clear as daylight". However, Alfred Russel Wallace as an editor replaced natural selection with Survival of the fittest in posthumous editions of "origin of the species"