Darwin liked Spencer’s term because it eliminated the troublesome implications in the word “selection,” and he adopted it in the 1869 fifth edition of Origin, wherein he wrote: “I have called this principle, by which each slight variation, if useful, is preserved, by the term natural selection, in order to mark its relation to man’s power of selection. But the expression used by Mr. Herbert Spencer, of the Survival of the Fittest, is more accurate, and is sometimes equally convenient.”
So Darwin did use the phrase. Just not in the first 4 editions.
