Herbert Spencer was one of the most popular and influential 19th-century sociologists. It is estimated that he sold one million books in his lifetime, far more than any other sociologist at the time.[31] So strong was his influence that many other 19th-century thinkers, including Émile Durkheim, defined their ideas in relation to his. Durkheim's Division of Labour in Society is to a large extent an extended debate with Spencer from whose sociology Durkheim borrowed extensively.[32]
Also a notable biologist, Spencer coined the term survival of the fittest. While Marxian ideas defined one strand of sociology, Spencer was a critic of socialism, as well as a strong advocate for a laissez-faire style of government. His ideas were closely observed by conservative political circles, especially in the United States and England.