Morgan began his anthropological career by studying the Iroquois, writing one of the earliest modern ethnographies. He was a strong advocate for the tribe, into which he was formally adopted. Initially, he argued for the essential unity of mankind, but after several years of cross-cultural research, he changed his views and created a new theory of social evolution. In Ancient Society (http://www.marxists.org/reference/archive/morgan-lewis/ancient-society/), Morgan subdivided human cultures into three broad categories:
Unlike Tylor, who continued to believe in the essential unity of humanity, Morgan injected a racist element into his evolutionary model by arguing that from "the Middle Period of barbarism, however, the Aryan and Semitic families seem fairly to represent the central threads of this progress, which in the period of civilization has been gradually assumed by the Aryan family alone." His theory thus contributed to the popular idea of social Darwinism. His ideas were taken up by archaeologists like William Matthew Flinders Petrie and V. Gordon Childe. Morgan's unilinear evolutionary scheme held a particular appeal to Karl Marx and Friedrich Engels, who added Communism to Morgan's three stages as the culminating phase in the evolution of human society. While his evolutionary scheme and social Darwinist bent have been rejected by anthropologists since the early 20th century, Morgan's emphasis on the connection between society and technology still has great appeal among many archaeologists interested in a more materialist, Marxian, or openly Marxist approach (his theories were official academic policy in the former Soviet Union because of their incorporation within Marxist ideology).