The second and third stages are commonly characterized, like the first, by established prohibitions and by clan exogamy; though with the advance in organization amicable relations with certain other groups are usually established, whereby the germ of tribal organization is implanted and a system of interclan marriage, or tribal endogamy, is developed.
"The Siouan Indians"
W. J. McGee
Natural selection has determined that exogamy produces fitter progeny than endogamy.
"The Kempton-Wace Letters"
Jack London Anna Strunsky
But the Binjhwars and Bharotias, who have a somewhat higher status than the others, refuse to admit Gonds, and are gradually adopting the strict rule of endogamy within the subtribe.
"The Tribes and Castes of the Central Provinces of India Volume II"
R. V. Russell