ABSTRACT
The primate evolutionary trends are generally held to represent the gradual accretion of adaptations to an arboral way of life. The adaptations of several species of arboreal mammals appear to be intermediate between the broad adaptive categories, suggesting possible evolutionary pathways from one kind of arboreal adaptation to another. The allocation to the order Primates of forms showing none of the adaptively significant features distinctive of the living primates is warranted if it is assumed that these features have been produced by selection pressures which were already affecting the microsyopoid populations. Most of the living orders of mammals have arboreal representatives. The adaptations of some—bats, dermopterans, gliding marsupials and rodents, arboreal anteaters, and sloths—are highly specialized and not comparable to those seen in primates. The distribution of grasping extremities among arboreal mammals suggests that prehensile hands and feet represent an adaptation not to arboreal activity per se, but to activity on branches of relatively small diameter, whether in marginal undergrowth.
