Too often, I’ve seen people argue that “survival of the fittest” refers to the fittest organisms, and thus is perfectly in line with eugenics. However, this is not what Charles Darwin referred to in his book .
In Chapter IV of the book, entitled “Natural Selection; or the Survival of the Fittest,” Darwin clearly says that the phrase “survival of the fittest” is a rephrasing of natural selection. In fact, he argues it’s (emphasis my own):
Can it then be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should occur in the course of many successive generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest. Variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions.
Clearly, Darwin uses “survival of the fittest” to refer to the fittest traits, not organisms. And thus doesn’t argue for eugenics at all.
Edited to include more of Darwin’s passage for full context