THE ANIMAL ANCESTRY OF MAN 83 



great toe inward. In such a position the gibbon is meeting 

 and solving the same problem of balancing the whole fore- 

 part of the body upon the pelvis and hind limbs that is 

 solved more completely by man. Sir Arthur Keith, in 

 searching for the early history of man's upright posture, 

 found that in the gibbon the arrangements of the diaphragm, 

 lungs, pericardium, and many other internal organs, manifest 

 many characteristically human adjustments to upright 

 posture, and he concluded that man had derived many of 

 his structural and functional adaptations to the upright 

 posture from an older brachiating stage. 



In conclusion, the annectant position of the gibbon 

 between the lower Old World Primates and the great-ape- 

 man series is fully documented by the monographic studies 

 of Tilney on the brain of Primates and of Keith on the 

 viscera; and if the inference were made that because the 

 gibbon is specialized in a few features his basic method of 

 brachiation may be ruled out of the line of advance leading 

 to man, such an inference would appear to be not in accord 

 with either the morphological or the palaeontological 

 evidence. Quite the reverse, while the brachiating gibbon 

 is a living witness of the ultimate derivation of man from 

 an arboreal quadrumanal monkey, he is also far more man- 

 like than monkey-like in many features of his viscera and 

 in his general adaptation to the upright posture. 



MAN EMERGES ON THE GROUND 



Up to the present point we have traced in outline the 

 general history of the vertebrate locomotor apparatus, 

 showing how the simplest fish-like forms contain the poten- 

 tiality and the ground-plan of the sequence of animals that 

 emerged from the swamps, invaded the dry land, ascended 

 into the trees and finally turned the backbone at right 

 angles to the plane of progression and gave rise to the noble 

 grade of brachiators. All the existing anthropoid apes 

 retain clear traces of derivation from a primitive brachiating 

 stem form, perhaps represented by some of the various 

 species referred to Dryopithecus and allied genera, which 

 roamed over^Europe and India during the Miocene and 

 Pliocene times. 



