with, but independently of, Charles Darwin's works and were popular from the late 19th century to the end of World War I. These 19th-century unilineal evolution theories claimed that societies start out in a primitive state and gradually become more civilized over time; they equated the culture and technology of Western civilization with progress. Some forms of early sociocultural evolution theories (mainly unilineal ones) have led to much-criticised theories like social Darwinism and scientific racism, sometimes used in the past by European imperial powers to justify existing policies of colonialism and slavery and to justify new policies such as eugenics. Most 19th-century and some 20th-century approaches aimed to provide models for the evolution of humankind as a single entity. However, most 20th-century approaches, such as multilineal evolution, focused on changes specific to individual societies. Moreover, they rejected directional change (i.e. orthogenetic, teleological or progressive change). Most archaeologists work within the framework of multilineal evolution. Other contemporary approaches to social change include neoevolutionism, sociobiology, dual inheritance theory, modernisation theory and postindustrial theory. In his seminal 1976 book The Selfish Gene, Richard Dawkins wrote that "there are some examples of cultural evolution in birds and monkeys, but ... it is our own species that really shows what cultural evolution can do". Stadial theory Enlightenment and later thinkers often speculated that societies progressed through stages: in other words, they saw history as stadial. While expecting humankind to show increasing development, theorists looked for what determined the course of human history. Georg Wilhelm Friedrich Hegel (1770–1831), for example, saw social development as an inevitable process. It was assumed that societies start out primitive, perhaps in a state of nature, and could progress toward something resembling industrial Europe. While earlier authors such as Michel de Montaigne (1533–1592) had discussed how societies change through time, the Scottish Enlightenment of the 18th century proved key in the development of the idea of sociocultural evolution. In relation to Scotland's union with England in 1707, several Scottish thinkers pondered the relationship between progress and the affluence brought about by increased trade with England. They understood the changes Scotland was undergoing as involving transition from an agricultural to a mercantile society. In "conjectural histories", authors such as Adam Ferguson (1723–1816), John Millar (1735–1801) and Adam Smith (1723–1790) argued that societies all pass through a series of four stages: hunting and gathering, pastoralism and nomadism, agriculture, and finally a stage of