120 ARBOREAL MAX 



arched at all. This is, of course, merely an adaptation 

 to posture, and as such it is seen in other, and non 

 arboreal, animals which tend to carry the trunk axis 

 vertical, no matter what may be the relation of the hind- 

 limb to the trunk. A flattened lumbar region is present 

 in the Kangaroos {3Iacropus), and it is the same in the 

 Jerboas (Dipus), which hop with the trunk axis nearly 

 vertical. Arboreal life brought about a lumbar flattening 

 early, since trunk uprightness is an easily attained out- 

 come of the climbing habit, but it also — in the Anthropoid 

 Apes — carried it a stage further than this. 



A lumbar flattening suffices for an animal which holds 

 its trunk upright upon the basis of its flexed lower limbs, 

 and it suffices for animals which sit and hop upright. 

 It will not suffice, however, when the trunk uprightness 

 has to be combined with extended lower limbs. If an 

 animal is to maintain its trunk and its lower limbs in one 

 continuous axis, something more than a lumbar flattening 

 is required, and a reversed lumbar curve is introduced. 

 In most monkeys the reversed lumbar curve is already 

 present in some slight degree. In Cercojiithecus jycilaiimis 

 it is perfectly definite, and the same may be said for all 

 thoroughly arboreal monkeys, when the vertebral column 

 is examined in its natural state; but the curvature dis- 

 appears altogether in the skeleton (see Fig. 42). The 

 reason for this is that the curvature is caused bv the 

 shaping of the soft intervertebral discs, rather than by 

 any change in the bones themselves, and when these 

 discs are lost in the preparation of the skeleton, the 

 presence of the curves is ignored in the subsequent 

 mounting of the specimen. If our knowledge be derived 

 from the actual animals, rather than from museum 

 skeletons, we cannot denv that a lumbar curve convex 

 forwards is already present in the monkeys. 



The straightening of the lower limb upon the trunk is 

 an extremely important factor in primate evolution, and 

 we will follow Professor Keith in regarding the habit of 



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