yS HUMAN BIOLOGY 



which retreat is usually impossible and in which extinction is 

 inevitable, yet it is an easily verifiable fact that in the tree- 

 shrews and lemurs the earlier stages of arboreal Kfe conserved 

 many skeletal characters which were very early lost by 

 related mammals that became speciahzed either for swift 

 running, or leaping on the ground, or digging, or swimming. 

 We are now in a position to consider some of the ways in 

 which the primitive mammahan skeleton became adapted 

 for arboreal habits (Morton). When, as described above, 

 the sacral portion of the vertebral column became attached 

 by ligament to the inner sides of the pelvis, the animal 

 acquired one of the first prerequisites for rearing up on its 

 hind legs, that is, by contracting the longitudinal dorsal 

 muscles the creature could, so to speak, raise the draw- 

 bridge and balance it upon the rear pier of the double 

 suspension bridge. It will readily be seen that arboreal 

 life put a premium upon this ability, as also upon the 

 possession of limbs that were equally well adapted for 

 pushing and for pulling. At first the Primates were little 

 more than quadrupeds that ran along the tops of the branches 

 and leaped like squirrels from branch to branch, differing 

 widely, however, from normal ground-living quadrupeds 

 in their grasping hands and feet. Such indeed are the tree- 

 shrews and lemurs of the present time and such were their 

 predecessors in Eocene times. Some of the leaping types, 

 such as the sifakas and indris of Madagascar and still more 

 the galagos and spectral tarsiers, specialized in leaping on the 

 long hind limbs, rearing the forepart of the body as described. 

 In these hopping forms as the backbone is reared upward, 

 the knee is bent and the femora are directed downward and 

 backward, the opposite condition to that which took place 

 in man (Morton). In another line of specialization leading 

 to the baboons, the animals started from a fully developed 

 monkey stage; spreading from the forests into more or 

 less open savannahs, they spent more and more time running 

 on the ground and gradually lost the typical monkey-like 

 configuration of the body and became more or less dog-like, 

 the fore and hind limbs being subequal in length, the hands 

 and feet becoming more or less paw-like, with somewhat 

 reduced thumb and great toe and slightly enlarged middle 



