I am presently reading "The origin of species" by Charles Darwin. In the beginning of chapter III he is mentioning that Herbert Spencer coined the expression "survival of the fittest".
It does not seem plausible that Spencer was inspired by Darwins book (published 1859) to coin the expression in "Principles of Biology" published in 1864 as stated in the body text. It must have happened earlier. Does anyone have an earlier source?