It was the political philosopher Herbert Spencer who first coined the phrase ”survival of the fittest” in his book Principles of Biology, which was published in 1864. Spencer used it as a metaphor for Darwin’s idea of “natural selection”. Darwin was never completely satisfied with the term, but did see certain advantages. People could, for example think of a conscious intervention with Darwin’s own term of “natural selection”, and that was not the case at all.
On the other hand, the phrase also simplified the process and mechanisms in such a way that people got the wrong idea of how the development of life actually happened. Darwin’s theory was based on luck, coincidence, skills etc., where the statistic element is conclusive. In other words, there were a number of factors in the development of the species, which could not always be reduced to survival of the fittest.
Darwin adopted Spencer’s term as a synonym of ”natural selection” in the 5th edition of On the Origin of Species, which was published in 1869. The first Danish translation was published in 1871-1872. Here J.P. Jacobsen’s translation of “survival of the fittest” became “the most seemly stays alive”. It is almost impossible to understand today.
Darwin’s own interpretation of the term was that it referred to the survival of the best adjusted, not the strongest. It is also important to remember that even the most adjusted individual could unfortunately be eaten by a larger animal, get hit in the head by a rock or be killed under other circumstances. Evolution, in other words, offers no guaranties. It is a statistic theory.