Herbert Spencer was a prominent 19th century British philosopher who is considered one of the founders of social evolutionism and social Darwinism. He argued that societies evolve in a manner analogous to biological evolution, advancing from simple undifferentiated homogeneity to increasingly complex heterogeneity through differentiation and competition. He believed this social evolution was propelled by the "survival of the fittest" and that it would lead to continual social progress. Spencer applied evolutionary theory to develop a framework for understanding sociological concepts like social structure, functions, and social change over time.