These racist theories put forth on scientific hypothesis were combined with unilineal theories of social progress, which postulated the superiority of the European civilization over the rest of the world. Furthermore, they frequently made use of the idea of "survival of the fittest", a term coined by Herbert Spencer in 1864, associated with ideas of competition, which were named social Darwinism in the 1940s. Charles Darwin himself opposed the idea of rigid racial differences in "The Descent of Man" (1871), in which he argued that humans were all of one species, sharing common descent. He recognised racial differences as varieties of humanity, and emphasised the close similarities between people of all races in mental faculties, tastes, dispositions and habits, while still contrasting the culture of the "lowest savages" with European civilization.