108 ARBOREAL MAN 



activity, a vertebral column possessing a uniformly 

 sloping series of spinous processes. The question t hat- 

 presents itself is, Does this spinal arrangement represent 

 the inherited handicap of these arboreal animals, an 

 ancestral birthright which has determined and limited 

 their peculiar climbing habits ; or have their individualities 

 as tree-clingers modified a spinal column which may at 

 one time have possessed the doubly sloping series of 

 spines indicative of greater activity ? The present-day 

 arboreal Sloths possess a backbone of the lumbering- 

 terrestrial walkers. Are they derived from a lumbering - 

 walking stock of which some of the smaller, lighter 

 members ha^e taken to the trees and become lumbering 

 tree-clingers, since that was the limit of their arboreal 

 possibilities ? The evidence of paleontology certainly 

 points towards the last conclusion as being nearer the 

 truth. 



The extinct relations of the Sloths are well known. 

 On the strength of the evidence afforded by Megatherium, 

 Mylodon, and other well-studied gigantic fossil Edentates, 

 it seems justifiable to regard the modern Sloths as diminu- 

 tive descendants of lumbering animals, and to look upon 

 their restricted arboreal activities as the necessary result 

 of their ancestry. We may assume the correctness of 

 Owen's conclusion that Mylodon robustus reared itself 

 against the trees in tripod fashion, and pulling down 

 the branches, browsed upon their leaves. 



From such a beginning we would picture some smaller 

 members of the same stock going farther than this, and 

 clinging to the branches in their search for food; and in 

 this manner we would picture the Sloths becoming 

 arboreal. 



Even when they had reached the tree-tops, and had 

 definitely made their homes among them, they were still 

 limited by the handicap which their ancestral terrestrial 

 shuffling gait had imposed upon them; and though no 

 animals are more thoroughly arboreal than the existing 



