540 THE CHANGING WORLD 



The Downward Ascent of Man 



It is related that an anxious and somewhat ilhterate maiden lady 

 once inquired at a bookstore for a copy of a book of which she had 

 vaguely but hopefully heard, entitled The Decent Man. Her disap- 

 pointment was great when The Descent of Man by Charles Darwin 

 was finally identified as the probable book in question, confirming 

 her suspicions that her lifelong quest was hopeless as usual. The 

 "descent of man," however, remains another story. 



Probably the ancestral home of the primates was in tropical tree- 

 tops. The majority of living representatives of the Order still 

 retain the same arboreal headquarters, only a few kinds, among them 

 man, having subsequently taken to more insecure and adventurous 

 life on the ground. As previously suggested, it was doubtless tree- 

 shrews, in the primitive mammalian Order of the Insectivora, that 

 first broke away from the terrestrial habitat of ancestral reptiles, and 

 adventured into tree-tops. There are anatomical reasons that lead 

 us to suspect that the tree shrews, educated in their aerial manual 

 training school, gave rise in the course of time to lemurs and other 

 primates. Modern representatives of the tree shrews are still to be 

 found in the forests of Borneo. They are small, generalized, planti- 

 grade animals with five digits on each foot, and a long pointed snout, 

 housing well-developed organs of smell. 



Arboreal life wrought profound modifications in these primate 

 explorers of the new tree habitat. The poking insectivorous snout, 

 with its keen sense of smell, as its usefulness off the ground was 

 lessened, gradually retreated, while the eyes and the tactile sense, 

 indispensable in arboreal life, came into dominance. The front legs 

 were now lengthened and adapted for sustaining the hanging weight of 

 the body, while the hind legs became not only organs of support, 

 but also were fitted for springing and leaping from limb to limb. 

 When at rest in trees, the sitting posture was naturally adopted, so 

 that the originally horizontal quadruped became a more or less 

 vertical animal, a change entailing a long list of further anatomical 

 adjustments. Chief among the advantages accruing from sitting up 

 on end was the release of the front legs from the function of support. 

 Frogs, however, which are also famous sitters, still use their front legs 

 for bracing support, and so gain nothing new by assuming the semi- 

 vertical posture of contemplation. Generally speaking, the front legs 

 of sitting primates were transformed into reaching arms with grasping 



