Some anthropologists are interested in the origins of human cultural forms and how these forms have changed over long periods of time. Just as Charles Darwin applied the notion of evolution to explain how biological species change over time, many 19th-century anthropologists used evolution to explain how cultures changed over time. This approach is called cultural evolutionism. Like Darwin, these anthropologists believed that simple forms evolved into more complex forms. Comparing different cultures of the world, they assigned the ones they considered more rudimentary to earlier evolutionary stages, while the ones they considered more complex were assigned to the more advanced stages. For example, British anthropologist Edward Tylor argued that human culture evolved from savagery through barbarism to civilization. He identified savagery with people who used gathering and hunting to meet their basic needs. The domestication of animals and plants was associated with barbarism. Civilization resulted from more advanced forms of farming, trade, and manufacturing as well as the development of the alphabet. Not surprisingly, British scholars identified their own culture as highly civilized. Elaborating on Tylor’s scheme, American anthropologist Lewis Henry Morgan subdivided each of these three stages into an even more elaborate model and proposed a mechanism for moving from stage to stage. Morgan focused on technology as the primary driver of cultural evolution. New and better ways of making things, according to Morgan, resulted in new patterns of social practice and thought. Advanced technology was associated with advanced civilization. But is technology the only measure of cultural accomplishment, or even the best one? Members of societies in which people gather and hunt for a living have vast stores of knowledge about their environments. Typically, they can name hundreds of plant species and tell when and where to find each of them. Many hunters can examine animal tracks to discern the species, sex, age, and condition of the animal as well as how long ago the tracks were laid. People in these societies also actively sustain and nurture diversity in their environments, careful to avoid depleting important resources. Is it really accurate to think of such cultures as simple? All cultures are complex, though in different ways. Technology is highly valued in American culture, while environmental knowledge and sustainability have historically been less valued. Is it any wonder that early American anthropologists ranked other cultures according to one of their own most cherished values? Perhaps people in more environmentally sustainable cultures might consider the United States to be an example of environmental savagery. Both Tylor and Morgan, like most anthropologists of their day, thought that all cultures passed through this single set of stages in the march toward civilization. This kind of theory is called unilineal evolution. Disagreeing with this way of thinking, anthropologists such as Franz Boas argued that there is no single line of cultural evolution but that each culture changes according to its own unique historical trajectory. Moreover, cultures evolve not in isolation but in constant interaction with one another. Rather than focusing on technological changes within a culture, Boas highlighted the diffusion of material objects, practices, and ideas among cultures in complex relations of trade, migration, and conquest. Though theories of unilineal cultural evolution have been largely abandoned, some anthropologists are still interested in discovering regular patterns that might govern how human cultures change over long periods of time. In the 1950s, American anthropologist Julian Steward developed an approach called cultural ecology, recognizing the importance of environmental factors by focusing on how humans adapt to various environments. Steward’s approach showed how humans in each environmental zone develop a set of core cultural features that enable them to make a living. Central to each cultural core are ways of getting or making all the resources necessary for human survival—in particular, food, clothing, and shelter. Similarly, anthropologist Marvin Harris developed a theory called cultural materialism, arguing that technology and economic factors are fundamental to culture, molding other features such as family life, religion, and politics. Though recognizing the importance of cultural change, many anthropologists reject the notion that all cultures change according to a general universal model, such as cultural materialism. Drawing from the Boasian notion that each culture follows its own historical path; many cultural anthropologists analyze change in terms of historical particularism. In this approach, contemporary processes are understood as products of the unique combination of internal and external forces unfolding over time in a particular culture.