The major problem addressed in this volume concerns the origin of the New World monkeys. This problem is sometimes simplified to the question of whether they came from North America or from Africa. However, the biological problem is actually more complex and can be treated as four separate hypotheses: (1) descent of platyrrhines from an omomyid ancestor from North America, independent of catarrhines (Gazin, 1958; Orlosky and Swindler, 1975; Patterson and Pascual, 1972, Stirton, 1951; Stirton and Savage, 1950; Szalay, 1975; Wood, 1973, 1977); (2) descent from adapids, again independent of catarrhines (Gingerich, 1973); (3) origin of a monophyletic Anthropoidea, from either omomyids or adapids. Anthropoidea may have evolved in Holarctica, with later independent southern migrations of catarrhines into Africa and platyrrhines into South America (Gingerich, 1975, 1977; Gingerich and Schoeninger, 1977; Simons, 1969, 1976; Simpson, 1978). Alternately, anthropoids may have evolved in Africa, with the subsequent introduction of platyrrhines into South America from Africa, most likely via rafting (Genet-Varcin, 1974; Hoffstetter, 1972, 1974; Lavocat, 1974; see also Szalay, 1976, for the possibility of a South American origin with subsequent rafting of catarrhines to Africa). In either case, the closest relatives of platyrrhines would be the Catarrhini. (4) The fourth possibility is that platyrrhines have a Cretaceous, pre-rift, African origin and may even be polyphyletic (Hershkovitz, 1972, 1974, 1977). Should this be the case, platyrrhines would not be expected to show any close, unified relationship to other primate groups of later origin, e.g., omomyids, adapids, or catarrhines.