THE FAILURES OF ARBOREAL LIFE 215 



and first finger are opposed to the other three digits. 

 The eutherian Sloths (Bradypodidce) show to perfection 

 the fatal effects^iTnere arboreal clinging. These animals 

 spend their lives for the most part among the branches 

 of trees, to which they cling hooked up in an inverted 

 position by a reduced and highly specialized series of 

 digits. The educational possibilities that the arboreal 

 habit offers to a Sloth are extremely limited; even the 

 range of its diet becomes restricted, and an animal that 

 has become an arboreal clinger is an animal entering upon 

 specific senility. With the phylogenetic history, and the 

 affinities of the Bradypodidoe we are not here concerned, 

 but perhaps they are not beyond the suspicion of having 

 certain Primate linkages, and it would be easy to point 

 the moral of the tendency to such a sloth-like condition 

 already manifested in Nycticebus tardigradus. This 

 Lemur may easily be appealed to as an example of a 

 tendency to arboreal clinging which may possibly be 

 exaggerated, and so lead astray from the line of true 

 Primate development. Nycticebus may also be pointed 

 to as showing possible tendencies to two other outcomes 

 of the arboreal habit which prove pitfalls of special- 

 ization in arboreal animals. We have noticed the 

 tendency shown by this animal to trust to the suspending 

 grasp of its feet rather than to that of its hands; it fre- 

 quently turns upside down, and hangs head downwards. 

 This is apparently a somewhat similar manifestation of 

 the trust to foot-grasp which has become so highly 

 elaborated in some New World Monkeys. It is a strange 

 feature of the South American arboreal animals that they 

 have assisted and perfected the foot-grasp by the develop- 

 ment of a prehensile tail. It might seem that the acquire- 

 ment of this new grasping organ, with all its beautiful 

 motor and sensory adaptations, would be a distinct 

 advance in the evolution of the Primates. Yet it has 

 proved to be a specialization which turned aside its 

 possessors from real progress. 



