The phrase "survival of the fittest" was coined by Herbert Spencer, a British philosopher and sociologist, in his 1864 book *Principles of Biology*.

However, it was Charles Darwin who developed the theory of evolution by natural selection. While Darwin didn't use the exact phrase "survival of the fittest," his theory is fundamentally based on the idea that individuals with traits better suited to their environment are more likely to survive and reproduce, passing on those advantageous traits to their offspring.

So, while Spencer popularized the phrase, Darwin's theory of natural selection is the foundation of the concept of survival of the fittest.