Until the end of the 15th century America had remained the “lost continent.” Her inhabitants, neglected by all, lived removed from the other populations of the globe. The accidental arrival of a few Icelandic boats in Canada and the northeastern United States, or of occasional Asiatic junk boats on the California coast had had no influence on the culture or the physical type of the American Indian. And a real immigration from Polynesia is doubtful and has never been satisfactorily demonstrated.

Some new proof of the extended biological isolation of these populations has been offered by the study of their blood composition. Although most of them show Mongolan traits more or less markedly, almost the totality of South American Indians belong to the blood group O, and all are likewise Rh negative. The group A, largely represented in Asia, is found with certain frequency only in the northwest tribes of the continent.