Investigations have arisen from belief in many different theories of culture and have often given voice to new theoretical bases for approaching the elusive term. Many early anthropologists conceived of culture as a collection of traits and studied the diffusion, or spread, of these traits from one society to another. Critics of diffusionism, however, pointed out that the theory failed to explain why certain traits spread and others do not. Cultural evolution theory holds that traits have a certain meaning in the context of evolutionary stages, and they look for relationships between material culture and social institutions and beliefs. These theorists classify cultures according to their relative degree of social complexity and employ several economic distinctions (foraging, hunting, farming, and industrial societies) or political distinctions (autonomous villages, chiefdoms, and states). Critics of this theory argue that the use of evolution as an explanatory metaphor is flawed, because it tends to assume a certain direction of development, with an implicit apex at modern, industrial society. Ecological approaches explain the different ways that people live around the world not in terms of their degree of evolution but rather as distinct adaptations to the variety of environments in which they live. They also demonstrate how ecological factors may lead to cultural change, such as the development of technological means to harness the environment. Structural-functionalists posit society as an integration of institutions (such as family and government), defining culture as a system of normative beliefs that reinforces social institutions. Some criticize this view, which suggests that societies are naturally stable (see functionalism). Historical-particularists look upon each culture as a unique result of its own historical processes. Symbolic anthropology looks at how people's mental constructs guide their lives. Structuralists analyze the