Technically, Darwin didn’t say (or write) either one; “Survival of the fittest” was Herbert Spencer’s misinterpretation of what Darwin’s “natural selection” was all about (that is, “natural” as contrasted with “breeders’ selection”). And it’s vulnerable to the charge of circularity. What Darwin was really driving at was, “survival of the form that will leave the most copies of itself to future generations”.
Technically, Darwin didn’t say (or write) either one; “Survival of the fittest” was Herbert Spencer’s misinterpretation of what Darwin’s “natural selection” was all about (that is, “natural” as contrasted with “breeders’ selection”). And it’s vulnerable to the charge of circularity. What Darwin was really driving at was, “survival of the form that will leave the most copies of itself to future generations”.