The group of marsupials is an element the origin of which presents a most difficult problem. Some belong to the opossum series which could well have been developed from some remnant of the Mesozoic marsupial fauna that had a world wide distribution; but the presence of diprotodonts, which are characteristic of Australia, and of the _Borhyaenidae_ which are closely related to the _Thylacinidae_ of Australia, suggests a migration from that continent as late as Tertiary times; but to my mind this involves a connection which is most too difficult to postulate. There is no evidence that they came to South America in company with other faunas, for they have not been found associated with any other fauna outside of Southern Patagonia. The explanation of the affinities of the Patagonian marsupials with the Australian marsupials is a problem which is not yet in position to be settled. The birds probably came from Africa with the invasion of the ancestors of the Notungulates. The idea of an invasion from Africa in Upper Cretaceous times, and possibly another at a later time is correlated with the other evidence of a land bridge between these two continents, as deduced by students of other groups. Eigenmann, working on the freshwater fishes,[6] Lydekker, studying the hystricomorphs,[7] Von Ihering, studying the freshwater mussels,[8] Ortmann, studying the freshwater crabs,[9] not to mention several others studying mullocks, insects, plants, etc., have all postulated a land connection from Brazil to northern Africa during Cretaceous time to explain the distribution of their various groups. The divergence is in the time when this land bridge sank, some believing it to have lasted into Tertiary times, most feeling that it sank in Upper Cretaceous times. Another body of evidence is presented to show that a land bridge connected the West Indies with the Mediterranean regions.[10] There was presumably but one such transatlantic connection. Its position further to the south would seem to me to explain the distributional facts found in the West Indies, but the striking resemblances between the faunas of Africa and South America require a connection from the South Upper American Continent and Africa. [Footnote 6: Princeton Expeditions to Patagonia, vol. 3, p. 310, 1905-11.] [Footnote 7: History of Mammals, p. 127, 1896.] [Footnote 8: Archhelenis and Archinotis, p. 125-145, 1907.] [Footnote 9: Proc. Amer. Philos. Soc. Philadelphia, vol. 41, p. 350, 1902.] [Footnote 10: See Scharff, Distribution and Origin of Life in America, Ch. 11, 1912.] It was along this land bridge which the ancestors of the _Notungulata_ traveled, and when in South America, due to their isolation, developed all the peculiarities of the group. This must have been not later than the latter part of the Cretaceous.