At this point Morgan's main work, "Ancient Society" (1877), inserts its lever. It is this work on which the present volume is based. Here we find clearly demonstrated what was only dimly perceived by Morgan in 1871. There is no antithesis between endogamy and exogamy; no exogamous "tribes" have been found up to the present time. But at the time when communal marriage still existed--and in all probability it once existed everywhere--a tribe was subdivided into a number of groups--"gentes"--consanguineous on the mother's side, within which intermarrying was strictly forbidden. The men of a certain "gens," therefore, could choose their wives within the tribe, and did so as a rule, but had to choose them outside of the "gens." And while thus the "gens" was strictly exogamous, the tribe comprising an aggregate of "gentes" was equally endogamous. This fact gave the final blow to McLennan's artificial structure. But Morgan did not rest here. The "gens" of the American Indians furthermore assisted him in gaining another important step in the field under investigation. He found that this "gens," organized in conformity with maternal law, was the original form out of which later on the "gens" by paternal law developed, such as we find it among the civilized nations of antiquity. The Greek and Roman "gens," an unsolved riddle to all historians up to our time, found its explanation in the Indian "gens." A new foundation was discovered for the entire primeval history. The repeated discovery that the original maternal "gens" was a preliminary stage of the paternal "gens" of civilized nations has the same signification for primeval history that Darwin's theory of evolution had for biology and Marx's theory of surplus value for political economy. Morgan was thereby enabled to sketch the outline of a history of the family, showing in bold strokes at least the classic stages of development, so far as the available material will at present permit such a thing. It is clearly obvious that this marks a new epoch in the treatment of primeval history. The maternal "gens" has become the pivot on which this whole science revolves. Since its discovery we know in what direction to continue our researches, what to investigate and how to arrange the results of our studies. In consequence, progress in this field is now much more rapid than before the publication of Morgan's book.