STRUCTURAL CULMINATION g8i 



metatarsal. The semi-eixtt and sitting postures were still tiirther emphasized 

 by these ehanges and ischial callosities appeared in consequence of habitual 

 squatting. The foot maintained digitigrade tendencies, although the heel 

 touched the supporting surface in locomotion, but did so after the toes made 

 their contact. The timing formula in this respect was toe-heel, as in the previ- 

 ous phases. The lower extremity despite its increased length still presented 

 its pelvic llexion. This stage is represented by the Cebidae and the old-world 

 monkeys except most of the baboons, whose aberrant cynomorphous spe- 

 cializations produced a quadrupedal reversion with shortening of the limbs, 

 fingers and toes in adjustment to li\ing on the ground. 



All of these modifications possibly occurred in the earlier portion of 

 the Oligoccne. Late in this geological era the primate stock took a great stride 

 forward. Locomotion was still predominantly i)rachiating in type. The 

 manus retained its suspension grip \\hich produced elongated digits, but a 

 rudimentary- thumb. The latter, however, was opposable. The tail disap- 

 peared and the foot developed a grasping grip. The legs became extended on 

 the pelvis, thus reducing the primitive pelvic thigh-fiexion, and rendering 

 the ancient type of quadrupedal locomotion impossible. This modification 

 at once advanced the cause of the erect posture. It did far more, howe\er, by 

 introducing the first phases of bipedal locomotion which, notwithstanding its 

 awkward and ineffectual nature at first, created the structural pattern 

 essential to the culminating diflerentiation of the human foot. 



Among living species the beginning of the Hylobate branch (gibbons) 

 clearly marks this critical departure. The orang-outang is probably an 

 oflshoot from this stage, while Dryopithecus carries the stage directly upward. 

 Early in the Miocene there came a great increase in body weight which particu- 

 larly aflected such primates as possessed this newly acquired orthograde 

 tendency. The greater weight brought the animals nearer to the ground, and 

 thus determined certain terrestrial specializations. Arboreal locomotion was 



