Lewis Henry Morgan is credited with being one of the precursors of twentieth century anthropology. He represents an earlier “evolutionary” approach to cultural difference. Many later scholars, though they had fully taken up the a different version of anthropology grounded in the concepts of culture and social structure, still wanted to reach back and recover Morgan's ideas from the evolutionary paradigm.
One of Morgan's better known contributions is the typology of society on a scale from primitive to civilized. He proposed that all societies could be classified according to stages of social evolution which all societies had to pass, starting in primitive state, moving to barbarism, and finally reaching civilization. Technological changes drove this process, leading people to adopt more complex forms of social organization, and creating more finely differentiated classes of kinship and membership in the society. In the period of civilization, societies exhibited a differentiation between the institutions of the state and society. In this sense, Morgan also argued that the path of development of a society was a movement from a state of societas to civitas, or a society organized in terms of groups of people united by kinship to one organized around specialized social institutions which functioned independently of people's kinship and group membership.
The evolutionary thesis is easy to criticize in retrospect. Societies do not follow the same path of change, and contemporary tribal societies, though different, are not living fossils or contemporary ancestors; these societies continue to change over time too. It is interesting to note Morgan's version of social evolution for several reasons. Morgan's synthesis of diverse cultural forms into a single theory of historical process based on technological change influenced Karl Marx and Friedrich Engels. Engels took up Morgan's analyses of kinship systems as a basis for his own analysis of the development of the nuclear family under capitalism. Morgan's theory was also a break from two leading alternative theories in his day. The first was a history of humanity based on a Biblical chronology, which was beginning to weaken in the face of a growing body of archaeological evidence. The second was a theory of human social differences based on race. This theory assumed that races had separate origins and separate destinies. Morgan, quite controversially, argued that all societies went through the same path of change, although at different times. The same basic human traits of intelligence could be seen in every society no matter how different. In the hands of E. B. Tylor and others, this same principle would be known as the “psychic unity of mankind.”
It's in these innovative aspects of Morgan's thought one can find what others wanted to retain. Morgan's distinction between societies and civitas became important for Meyer Fortes's theory of kinship as a relationship based on an “axiom of amity.” In Fortes's axiom, every society defines a set of people with whom one a norm of prescriptive altruism, one's relatives. One has morally neutral relationship with everyone else. The zone of people who are kin as opposed to strangers can be differently defined, bigger or smaller, in every society. For some societies, all members of the community are kin in one form or another. In some, all members of humanity are. Fortes' point is that all societies have a domain defined by societas and another domain defined by civitas, although the scope of each varies. Fortes tosses out the evolutionary ladder from Morgan's idea but keeps the emphasis on seeing kinship as basically a system of classification of all the members of a community. This too was what Claude Levi-Strauss wanted to recover from Morgan.