Both neontological and phylogenetic studies are necessary to interpret primate locomotion. Reference to palaeoprimatology and palaeocology, for instance, will lead to a fuller understanding of the roots of such gaits as the vertical clinging and leaping of Tarsius, Indri and Propithecus.
Evolutionary trends in posture and locomotion are discussed. The postural trend has been towards maintenance of trunk verticality and the locomotor trend towards an increasing dependence on the forelimbs among arboreal primates. Three stages are recognized in the phylogenetic course of arboreal locomotor adaptation: Stage A. Vertical clinging and leaping; Stage B. Quadrupedalism; Stage C. Brachiation.
The role of prehensility of the hand in the evolution of locomotor types is discussed in relation to forest morphology and, in particular, to stratification. Finally a scheme of evolution, set in the framework of ecology, for Old World Monkey groups is presented.