The single most famous European intellectual in the closing decades of the nineteenth century.

Herbert Spencer (1820 –1903) was an English philosopher, biologist, sociologist, and prominent classical liberal political theorist of the Victorian era.
Spencer developed an all-embracing conception of evolution as the progressive development of the physical world, biological organisms, the human mind, and human culture and societies. He was "an enthusiastic exponent of evolution" and even "wrote about evolution before Darwin did."
During his lifetime he achieved tremendous authority, mainly in English-speaking academia.

"The only other English philosopher to have achieved anything like such widespread popularity was Bertrand Russell, and that was in the 20th century."

Spencer was

"the single most famous European intellectual in the closing decades of the nineteenth century"

but his influence declined sharply after 1900.
Spencer is best known for coining the concept "survival of the fittest", which he did in Principles of Biology (1864), after reading Charles Darwin's On the Origin of Species.
While most philosophers fail to achieve much of a following outside the academy of their professional peers, by the 1870s and 1880s Spencer had achieved an unparalleled popularity, as the sheer volume of his sales indicate. He was probably the first, and possibly the only, philosopher in history to sell over a million copies of his works during his own lifetime. In the United States, where pirated editions were still commonplace, his authorized publisher, Appleton, sold 368,755 copies between 1860 and 1903. This figure did not differ much from his sales in his native Britain, and once editions in the rest of the world are added in the figure of a million copies seems like a conservative estimate. As William James remarked, Spencer

"enlarged the imagination, and set free the speculative mind of countless doctors, engineers, and lawyers, of many physicists and chemists, and of thoughtful laymen generally."